a test track evaluation of light vehicle brake assist

DOT HS 811 371 September 2010

A Test Track Evaluation of Light Vehicle Brake Assist

DISCLAIMER

This publication is distributed by the U.S. Department of Transportation, National Highway Traffic Safety Administration, in the interest of information exchange. The opinions, findings, and conclusions expressed in this publication are those of the authors and not necessarily those of the Department of Transportation or the National Highway Traffic Safety Administration. The United States Government assumes no liability for its contents or use thereof. If trade names, manufacturers’names, or specific products are mentioned, it is because they are considered essential to the object of the publication and should not be construed as an endorsement. The United States Government does not endorse products or manufacturers.

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Technical Report Documentation Page 1.

Report No. DOT HS 811 371

2. Government Accession No. 3. Recipient's Catalog No.

4. Title and Subtitle A Test Track Evaluation of Light Vehicle Brake Assist

5.

Report Date September 2010

6. Performing Organization Code

NHTSA/NVS-312 7. Author(s)

Garrick J. Forkenbrock, NHTSA Andrew Snyder and Robert E. Jones, TRC Inc.

8. Performing Organization Report No.

9. Performing Organization Name and Address National Highway Traffic Safety Administration Vehicle Research and Test Center 10820 SR 347; Bldg. 60 East Liberty, OH 43319

10. Work Unit No. (TRAIS)

11. Contract or Grant No.

12. Sponsoring Agency Name and Address National Highway Traffic Safety Administration 1200 New Jersey Avenue, SE Washington, DC 20590

13. Type of Report and Period Covered

Draft Final Report 14. Sponsoring Agency Code

15. Supplementary Notes The authors acknowledge the efforts of Bryan O’Harra and Thomas Gerlach Jr. for assistance with vehicle preparation, data collection, and data processing; and Larry Jolliff for assistance with data collection and test driving. 16. Abstract

The objectives of the work described in this report were twofold: (1) to objectively identify the BA activation thresholds of five contemporary test vehicles, and (2) to evaluate braking performance of the test vehicles using BA activation threshold-based brake applications. The study’s second objective was broken down into two parts. First, the braking performance of the vehicles with BA enabled was compared to that achieved with the system disabled. For these tests, only brake applications believed to be representative of the vehicles’ BA activation thresholds were used. Next, the braking performance achieved via use of threshold-based brake applications was compared to the maximum braking capability of the vehicle. Three braking maneuvers were used in this study: two straight line stops (initiated from 45 and 65 mph), and a brake in-a-turn maneuver initiated from 45 mph. For each test, a programmable brake controller was used to insure the maneuvers were executed as accurately and repeatably as possible. To evaluate whether the manner in which the brakes were applied influenced the BA activation threshold, two brake controller feedback loops were used. When “displacement feedback” was utilized, the brake controller used a control feedback loop capable of modulating brake pedal force to maintain constant pedal position. Similarly, when “force feedback” was used, the brake controller used a control feedback loop capable of modulating brake pedal position to maintain constant application of force. Generally speaking, use of displacement feedback-based application thresholds allowed the effect of BA to be successfully evaluated, and demonstrated the technology is capable of producing substantial reductions in stopping distance for some vehicles—provided the right combination of brake pedal displacement and high application rate are applied. For four of the five vehicles evaluated in this study, use of threshold-based applications with BA enabled were able to achieve mean stopping distances within approximately 16 ft of the vehicles’ maximum braking capability. The brake controller’s force feedback-based applications were generally unable to distinguish the stopping performance realized with BA enabled from that achieved with the systems disabled. Therefore, the authors believe this feedback strategy is unsuitable for evaluating BA performance. The use of displacement feedback control logic is recommended.

17. Key Words Brake Assist, Brake Testing, Stopping Safety

Distance, Advanced Technology, 18.

Distribution Statement Document is available to the public from

the National Technical Information www.ntis.gov

19. Security Classif. (of this report)

Unclassified 20. Security Classif. (of this page)

Unclassified 21. No.

211 of Pages 22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

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CONVERSION FACTORS

Approximate Conversions to Metric Measures Approximate Conversions to English Measures

Symbol When You Know Multiply by To Find Symbol Symbol When You Know Multiply by To Find Symbol

LENGTH LENGTH

in in ft mi

inches inches

feet

miles

25.4 2.54 30.48 1.61

millimeters centimeters

centimeters kilometers

mm cm cm km

mm cmm km

millimeters centimeters

meters kilometers

0.04 0.39 3.3 0.62

inches inches feet miles

in in ft mi

AREA AREA

in2

ft2

mi2

square inches square feet square miles

6.45 0.09 2.59

square centimeters square meters square kilometers

cm2

m2

km2

cm2

m2

km2

square centimeters square meters

square kilometers

0.16 10.76 0.39

square inches square feet

square miles

in2

ft2

mi2

MASS (weight) MASS (weight)

oz lbounces

pounds 28.35 0.45

grams kilograms

g kg

gkg grams

kilograms 0.035 2.2

ounces pounds

oz lb

PRESSURE PRESSURE

psi psi

pounds per inch2

pounds per inch2 0.07 6.89

bar kilopascals

bar kPa

bar kPa

bar kilopascals

14.50 0.145

pounds per inch2

pounds per inch2 psipsi

VELOCITY VELOCITY

mph miles per hour 1.61 kilometers per hour km/h km/h kilometers per hour 0.62 miles per hour mph

ACCELERATION ACCELERATION

ft/s2 feet per second2 0.30 meters per second2 m/s

2 m/s2 meters per second2 3.28 feet per second2 ft/s2

TEMPERATURE (exact) TEMPERATURE (exact)

F Fahrenheit 5/9[(Fahrenheit) - 32°C] Celsius C C Celsius 9/5 (Celsius) + 32F Fahrenheit F

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NOTE REGARDING COMPLIANCE WITH

AMERICANS WITH DISABILITIES ACT SECTION 508

For the convenience of visually impaired readers of this report using text-to-speech software,

additional descriptive text has been provided for graphical images contained in this report to

satisfy Section 508 of the Americans With Disabilities Act (ADA).

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TABLE OF CONTENTS CONVERSION FACTORS ........................................................................................................................................ ii NOTE REGARDING C OMPLIANCE WITH AMERICANS WITH DISABILITIES ACT SECTION 508 ............................................................................................................................................................ iii LIST OF FIGURES ..................................................................................................................................................... x LIST OF TABLES ..................................................................................................................................................... xv EXECUTIVE SUMMARY ...................................................................................................................................... xix

1.0 INTRODUCTION ............................................................................................................................................... 1

1.1 Other NHTSA Brake Assist Programs ......................................................................................................... 1

1.1.1 Crash Data Analyses ......................................................................................................................... 1

1.1.2 Human Factors Research .................................................................................................................. 2

1.2 The Need For Objective Characterization .................................................................................................... 2

1.3 Test Objectives For The Work Described In This Report ............................................................................ 3

1.3.1 Objective #1: Identifying Brake Assist Activation Thresholds ........................................................3

1.3.2 Objective #2: Stopping Distance Measurement ...............................................................................3 2.0 TEST METHODOLOGY .................................................................................................................................. 4

2.1 Test Maneuvers ............................................................................................................................................ 4

2.2 Test Conduct ................................................................................................................................................. 5

2.2.1 Part 1: Brake Assist Activation Threshold Determination ...............................................................5

2.2.1.1 Brake Pedal Displacement Threshold .................................................................................6

2.2.1.2 Brake Pedal Force Threshold ..............................................................................................6

2.2.2 Part 2: Evaluation of Brake Assist Effectiveness ............................................................................. 6

2.2.2.1 Tests Performed with Maximum Commanded Inputs ........................................................6

2.2.2.2 Brake Assist Effectiveness Comparison Groups ................................................................7

2.3 Stopping Distance Corrections ..................................................................................................................... 7

2.4 Test Vehicles ................................................................................................................................................ 8

2.4.1 Vehicle Manufacturer Brake Assist Descriptions .............................................................................8

2.4.2 Brake Conditioning ........................................................................................................................... 9

2.4.3 Tires ................................................................................................................................................ 9

2.4.3.1 Tire Conditioning .............................................................................................................. 10

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TABLE OF CONTENTS (continued)

2.4.3.2 Use of Inner Tubes ............................................................................................................ 10

2.4.3.3 Frequency of Tire Changes ...............................................................................................11 3.0 INSTRUMENTATION .................................................................................................................................... 12

3.1 Programmable Brake Controller ................................................................................................................. 12

3.2 Programmable Steering Machine ............................................................................................................... 12

3.3 Sensors and Sensor Locations .................................................................................................................... 13

3.3.1 Steering Wheel Angle ..................................................................................................................... 13

3.3.2 Brake Pedal Force ........................................................................................................................... 13

3.3.3 Brake Pedal Position ....................................................................................................................... 14

3.3.4 Inertial Sensing System ................................................................................................................... 14

3.3.5 Ultrasonic Sensors .......................................................................................................................... 15

3.3.6 Vehicle Speed ................................................................................................................................. 15

3.3.7 Brake Line Pressure ........................................................................................................................ 15

3.4 Data Acquisition ......................................................................................................................................... 15

3.5 Post Processing Filters ................................................................................................................................ 16 4.0 DETERMINING BRAKE ASSIST ACTIVATION THRESHOLDS ..........................................................17

4.1 Displacement Feedback Based Threshold Determination ..........................................................................19

4.1.1 Determining Pedal Displacement Magnitude Thresholds (Step 1 of 4) ..........................................21

4.1.2 Determining Pedal Displacement Magnitude Thresholds (Step 2 of 4) ..........................................21

4.1.3 Determining Pedal Application Rate Thresholds (Step 3 of 4) .......................................................22


4.1.5 BMW 330i Displacement Feedback Threshold Determination ......................................................22

4.1.5.1 BMW 330i Displacement Feedback Threshold Determination Attempt #1 .....................23

4.1.5.1.1 BMW 330i Step 1 ............................................................................................23

4.1.5.1.2 BMW 330i Step 2a ..........................................................................................24

4.1.5.1.3 BMW 330i Step 3a ..........................................................................................24

4.1.5.2 BMW 330i Displacement Feedback Threshold Determination Attempt #2 .....................24

4.1.5.2.1 BMW 330i Step 2b ..........................................................................................24

4.1.5.2.2 BMW 330i Step 3b ..........................................................................................25

4.1.5.2.3 BMW 330i Step 4 ............................................................................................25

4.1.6 Chrysler 300C Displacement Feedback Threshold Determination .................................................26

4.1.6.1 Chrysler 300C Step 1 ........................................................................................................26

4.1.6.2 Chrysler 300C Step 2a ......................................................................................................27

4.1.6.3 Chrysler 300C Step 2b ......................................................................................................27

4.1.6.4 Chrysler 300C Step 3 ........................................................................................................28

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4.1.6.5 Chrysler 300C Step 4 ........................................................................................................29

4.1.7 Cadillac STS Displacement Feedback Threshold Determination ...................................................29

4.1.7.1 Cadillac STS Step 1 ..........................................................................................................29

4.1.7.2 Cadillac STS Step 2a ........................................................................................................30

4.1.7.3 Cadillac STS Step 2b ........................................................................................................31



4.1.8 Toyota 4Runner Displacement Feedback Threshold Determination ..............................................33

4.1.8.1 Toyota 4Runner Step 1 .....................................................................................................33

4.1.8.2 Toyota 4Runner Step 2a ...................................................................................................33

4.1.8.3 Toyota 4Runner Step 2b ...................................................................................................34


4.1.8.5 Toyota 4Runner Step 4a ...................................................................................................35

4.1.8.6 Toyota 4Runner Step 4b ...................................................................................................36

4.1.8.7 Toyota 4Runner Step 4c ...................................................................................................36

4.1.9 Volvo XC90 Displacement Feedback Threshold Determination ....................................................36

4.1.9.1 Volvo XC90 Step 1 ...........................................................................................................36

4.1.9.2 Volvo XC90 Step 2 ...........................................................................................................37

4.1.9.3 Volvo XC90 Step 3 ...........................................................................................................37

4.1.9.4 Volvo XC90 Step 4 ...........................................................................................................38

4.2 Force Feedback Based Threshold Determination ....................................................................................... 38

4.2.1 Determining Pedal Force Magnitude Thresholds (Step 1 of 3) .......................................................39

4.2.2 Determining Pedal Force Magnitude Thresholds (Step 2 of 3) .......................................................40


4.2.4 BMW 330i Force Feedback Threshold Determination ...................................................................41

4.2.4.1 BMW 330i Step 1 .............................................................................................................41

4.2.4.2 BMW 330i Step 2 .............................................................................................................42

4.2.4.3 BMW 330i Step 3 .............................................................................................................42

4.2.5 Chrysler 300C Force Feedback Threshold Determination ..............................................................43

4.2.5.1 Chrysler 300C Step 1 ........................................................................................................43

4.2.5.2 Chrysler 300C Step 2 ........................................................................................................44

4.2.5.3 Chrysler 300C Step 3 ........................................................................................................45

4.2.6 Cadillac STS Force Feedback Threshold Determination ................................................................45



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4.2.7 Toyota 4Runner Force Feedback Threshold Determination ...........................................................48




4.2.8 Volvo XC90 Force Feedback Threshold Determination .................................................................50

4.2.8.1 Volvo XC90 Step 1 ...........................................................................................................50

4.2.8.2 Volvo XC90 Step 2 ...........................................................................................................51

4.2.8.3 Volvo XC90 Step 3 ...........................................................................................................52

4.3 Brake Assist Threshold Summary .............................................................................................................. 52

4.3.1 Application Rate ............................................................................................................................. 53

4.3.2 Nominal Brake Pedal Displacement ............................................................................................... 55

4.3.3 Brake Pedal Force ........................................................................................................................... 56

4.3.3.1 Peak Brake Pedal Force ....................................................................................................56

4.3.3.2 Nominal Brake Pedal Force (Static Data) .........................................................................57

4.3.3.3 Nominal Brake Pedal Force (Dynamic Data) ...................................................................57

4.3.4 Longitudinal Acceleration (Vehicle Deceleration) .........................................................................58

4.3.4.1 Peak Longitudinal Acceleration ........................................................................................58

4.3.4.2 Overall Longitudinal Acceleration ...................................................................................58

4.3.5 Stopping Distance ........................................................................................................................... 59

4.3.6 The Need for Comparative Tests .................................................................................................... 59

5.0 BRAKE ASSIST EFFECTIVENESS COMPARISIONS ..............................................................................65

5.1 Brake Assist Effectiveness at the Activation Threshold (Displacement Feedback) ...................................66

5.1.1 Straight Line Stopping Distances from 45 mph – Brake Assist Enabled vs. Disabled ...................66

5.1.1.1 Chrysler 300C................................................................................................................... 66

5.1.1.2 BMW 330i ........................................................................................................................ 66

5.1.1.3 Cadillac STS ..................................................................................................................... 69

5.1.1.4 Toyota 4Runner ................................................................................................................ 70

5.1.1.5 Volvo XC90 ...................................................................................................................... 71

5.1.2 Brake In-A-Turn Stopping Distances from 45 mph – Brake Assist Enabled vs. Disabled .............71

5.1.2.1 Chrysler 300C................................................................................................................... 71

5.1.2.2 BMW 330i ........................................................................................................................ 73

5.1.2.3 Cadillac STS ..................................................................................................................... 75

5.1.2.4 Toyota 4Runner ................................................................................................................ 77

5.1.2.5 Volvo XC90 ...................................................................................................................... 78

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5.1.3 Straight Line Stopping Distances from 65 mph – Brake Assist Enabled vs. Disabled ...................78

5.1.3.1 Chrysler 300C................................................................................................................... 78

5.1.3.2 BMW 330i ........................................................................................................................ 80

5.1.3.3 Cadillac STS ..................................................................................................................... 81

5.1.3.4 Toyota 4Runner ................................................................................................................ 82

5.1.3.5 Volvo XC90 ...................................................................................................................... 83

5.2 Brake Application Comparisons (Brake Assist Enabled, Displacement Feedback) ...................................83

5.2.1 Straight Line Stops from 45 mph – Activation Threshold vs. Maximum Displacement ................84

5.2.1.1 Chrysler 300C................................................................................................................... 84

5.2.1.2 BMW 330i ........................................................................................................................ 84

5.2.1.3 Cadillac STS ..................................................................................................................... 86

5.2.1.4 Toyota 4Runner ................................................................................................................ 89

5.2.1.5 Volvo XC90 ...................................................................................................................... 89

5.2.2 Brake In-A-Turn Stops from 45 mph – Application Threshold vs. Max Displacement .................89

5.2.2.1 Chrysler 300C................................................................................................................... 89

5.2.2.2 BMW 330i ........................................................................................................................ 90

5.2.2.3 Cadillac STS ..................................................................................................................... 90

5.2.2.4 Toyota 4Runner ................................................................................................................ 91

5.2.2.5 Volvo XC90 ...................................................................................................................... 91

5.2.3 Straight Line Stops from 65 mph – Activation Threshold vs. Maximum Displacement ................93

5.2.3.1 Chrysler 300C................................................................................................................... 93

5.2.3.2 BMW 330i ........................................................................................................................ 94

5.2.3.3 Cadillac STS ..................................................................................................................... 94

5.2.3.4 Toyota 4Runner ................................................................................................................ 96

5.2.3.5 Volvo XC90 ...................................................................................................................... 99

5.3 Brake Assist Effectiveness Evaluation using Force Feedback ...................................................................99

5.3.1 Straight Line Stopping Distance Comparisons from 45 mph ........................................................100

5.3.1.1 Chrysler 300C................................................................................................................. 100

5.3.1.2 BMW 330i ...................................................................................................................... 101

5.3.1.3 Cadillac STS ................................................................................................................... 102

5.3.1.4 Toyota 4Runner ..............................................................................................................104

5.3.1.5 Volvo XC90 .................................................................................................................... 105

5.3.2 Brake In-A-Turn Stopping Distance Comparisons from 45 mph ..................................................106

5.3.2.1 Chrysler 300C................................................................................................................. 106

5.3.2.2 BMW 330i ...................................................................................................................... 108

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5.3.2.3 Cadillac STS ................................................................................................................... 109

5.3.2.4 Toyota 4Runner ..............................................................................................................111

5.3.2.5 Volvo XC90 .................................................................................................................... 112

5.3.3 Straight Line Stopping Distance Comparisons from 65 mph ........................................................113

5.3.3.1 Chrysler 300C................................................................................................................. 113

5.3.3.2 BMW 330i ...................................................................................................................... 115

5.3.3.3 Cadillac STS ................................................................................................................... 116

5.3.3.4 Toyota 4Runner ..............................................................................................................118

5.3.3.5 Volvo XC90 .................................................................................................................... 119

5.4 Summary of Brake Application Comparisons .......................................................................................... 120

5.4.1 Chrysler 300C ............................................................................................................................... 121

5.4.2 BMW 330i .................................................................................................................................... 122

5.4.3 Cadillac STS ................................................................................................................................. 124

5.4.4 Toyota 4Runner ............................................................................................................................ 126

5.4.5 Volvo XC90 .................................................................................................................................. 128

5.5 Effect of Threshold Determination on Stopping Distance Mean Differences ..........................................130

5.6 Inability of Force Feedback to Identify Brake Assist Activation Thresholds...........................................131

6.0 CONCLUSIONS ............................................................................................................................................. 133

6.1 Identifying Brake Assist Activation Thresholds....................................................................................... 133

6.2 Brake Assist Performance Evaluation ...................................................................................................... 134

6.2.1 Displacement Feedback-Based Tests ............................................................................................134

6.2.2 Force Feedback-Based Tests ......................................................................................................... 136

6.3 Utility of Findings .................................................................................................................................... 138

7.0 REFERENCES ................................................................................................................................................ 139

APPENDIX .............................................................................................................................................................. 140

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10

15

20

25

LIST OF FIGURES

Figure 3.1 ATI Programmable Brake Controller installed in a 2006 BMW 330i. ............................................... 12

Figure 4.1 Brake pedal force observed as a function of time for three subjects during a crash-imminent driving scenario .................................................................................................................................. 17

Figure 4.2 Brake pedal force observed as a function of time during an automated brake stop performed with a programmable brake controller using a position feedback algorithm ......................................18

Figure 4.3 Programmable brake controller command module.............................................................................20

Figure 4.4 Brake pedal travel presented as a function of brake controller stroke ................................................20

Figure 4. Stopping distance as a function of command module magnitude (Step 1, BMW 330i) .....................23

Figure 4.6 Stopping distance as a function of command module magnitude (Step 3b, BMW 330i) ...................25

Figure 4.7 Stopping distance as a function of command module magnitude (Step 1, Chrysler 300C)................27


Figure 4.9 Stopping distance as a function of command module magnitude (Step 1, Cadillac STS) ..................30

Figure 4. Stopping distance as a function of command module magnitude (Step 3, Cadillac STS) ..................32

Figure 4.11 Stopping distance as a function of command module magnitude (Step 1, Toyota 4Runner) .............33


Figure 4.13 Stopping distance as a function of command module magnitude (Step 1, Volvo XC90)...................37


Figure 4. Stopping distance as a function of command module magnitude (Step 1, BMW 330i) .....................42

Figure 4.16 Stopping distance as a function of command module magnitude (Step 3, BMW 330i) .....................43



Figure 4.19 Stopping distance as a function of command module magnitude (Step 1, Cadillac STS) ..................47

Figure 4. Stopping distance as a function of command module magnitude (Step 3, Cadillac STS) ..................48





Figure 4. Brake pedal application rate as a function of commanded brake controller magnitude (force feedback).................................................................................................................................. 53

Figure 4.26 Brake pedal application rate as a function of commanded brake controller magnitude (displacement feedback) ..................................................................................................................... 55

Figure 4.27 BMW 330i brake assist threshold comparison ...................................................................................60

Figure 4.28 Chrysler 300C brake assist threshold comparison ..............................................................................61

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LIST OF FIGURES (continued)

Figure 4.29 Cadillac STS brake assist threshold comparison ................................................................................ 62

Figure 4.30 Toyota 4Runner brake assist threshold comparison ...........................................................................63

Figure 4.31 Volvo XC90 brake assist threshold comparison .................................................................................64

Figure 5.1 Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph. ...................................................................................................................................... 67

Figure 5.2 Test outputs produced during four straight line stops performed from 45 mph with the 2006 BMW 330i. ................................................................................................................................ 68

Figure 5.3 Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph. ...................................................................................................................................... 69

Figure 5.4 Test outputs produced during four straight line stops performed from 45 mph with the 2005 Cadillac STS .............................................................................................................................. 70

Figure 5.5 Stopping distances observed during brake in-a-turn stops performed with the Chrysler 300C from 45 mph. ...................................................................................................................................... 72

Figure 5.6 Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2005 Chrysler 300C ...................................................................................................................... 73

Figure 5.7 Stopping distances observed during brake in-a-turn stops performed with the BMW 330i from 45 mph ....................................................................................................................................... 74

Figure 5.8 Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2006 BMW 330i ........................................................................................................................... 75

Figure 5.9 Stopping distances observed during brake in-a-turn stops performed with the Cadillac STS from 45 mph ....................................................................................................................................... 76

Figure 5.10 Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2005 Cadillac STS ........................................................................................................................ 77

Figure 5.11 Stopping distances observed during straight line stops performed with the Chrysler 300C from 65 mph ....................................................................................................................................... 79

Figure 5.12 Test outputs produced during three straight line stops performed from 65 mph with the 2005 Chrysler 300C ...................................................................................................................... 80

Figure 5.13 Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph. ...................................................................................................................................... 81

Figure 5.14 Test outputs produced during three straight line stops performed from 65 mph with the 2005 Cadillac STS ........................................................................................................................ 82

Figure 5.15 Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph using two application techniques .................................................................................. 85

Figure 5.16 Test outputs produced during four straight line stops performed from 45 mph with the 2006 BMW 330i ................................................................................................................................. 86

Figure 5.17 Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph using two application techniques .................................................................................. 87

Figure 5.18 Test outputs produced during four straight line stops performed from 45 mph with the 2005 Cadillac STS .............................................................................................................................. 88

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Figure 5.19 Stopping distances observed during brake in-a-turn stops performed with the Volvo XC90 from 45 mph using two application techniques .................................................................................. 92

Figure 5.20 Test outputs produced during four brake in-a-turn stops performed from 45 mph with the Volvo XC90 .................................................................................................................................. 93

Figure 5.21 Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph using two application techniques. ................................................................................. 95

Figure 5.22 Test outputs produced during four straight line stops performed from 65 mph with the 2005 Cadillac STS. ............................................................................................................................. 96

Figure 5.23 Stopping distances observed during straight line stops performed with the Toyota 4Runner from 65 mph using two application techniques. ................................................................................. 97

Figure 5.24 Test outputs produced during three straight line stops performed from 65 mph with the 2003 Toyota 4Runner. ........................................................................................................................ 98

Figure 5.25 Stopping distances observed during straight line stops performed with the Chrysler 300C from 45 mph (force feedback). ......................................................................................................... 101

Figure 5.26 Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph (force feedback). ......................................................................................................... 102

Figure 5.27 Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph (force feedback). ......................................................................................................... 103

Figure 5.28 Stopping distances observed during straight line stops performed with the Toyota 4Runner from 45 mph (force feedback). ......................................................................................................... 105

Figure 5.29 Stopping distances observed during straight line stops performed with the Volvo XC90 from 45 mph (force feedback). ......................................................................................................... 106

Figure 5.30 Stopping distances observed during brake in-a-turn stops performed with the Chrysler 300C from 45 mph (force feedback) .......................................................................................................... 108

Figure 5.31 Stopping distances observed during brake in-a-turn stops performed with the BMW 330i from 45 mph (force feedback) .......................................................................................................... 109

Figure 5.32 Stopping distances observed during brake in-a-turn stops performed with the Cadillac STS from 45 mph (force feedback) .......................................................................................................... 110

Figure 5.33 Stopping distances observed during brake in-a-turn stops performed with the Toyota 4Runner from 45 mph (force feedback). ......................................................................................................... 112

Figure 5.34 Stopping distances observed during brake in-a-turn stops performed with the Volvo XC90 from 45 mph (force feedback). ......................................................................................................... 113

Figure 5.35 Stopping distances observed during straight line stops performed with the Chrysler 300C from 65 mph (force feedback) .......................................................................................................... 115

Figure 5.36 Stopping distances observed during straight line stops performed with the BMW 330i from 65 mph (force feedback) .......................................................................................................... 116

Figure 5.37 Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph (force feedback) .......................................................................................................... 117

Figure 5.38 Stopping distances observed during straight line stops performed with the Toyota 4Runner from 65 mph (force feedback) .......................................................................................................... 119

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Figure 5.39 Stopping distances observed during straight line stops performed with the Volvo XC90 from 65 mph (force feedback) .......................................................................................................... 120

Figure 5.40 Mean stopping distances produced during displacement (left) and force (right) feedback tests performed with the Chrysler 300C. .......................................................................................... 121

Figure 5.41 Mean stopping distances produced during displacement (left) and force (right) feedback tests performed with the BMW 330i................................................................................................. 123

Figure 5.42 Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Cadillac STS ..................................................................................... 125

Figure 5.43 Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Toyota 4Runner.................................................................................127

Figure 5.44 Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Volvo XC90 ...................................................................................... 129

Figure A1.1 2006 BMW 330i brake assist threshold determination Step 1a (displacement feedback) ................141

Figure A1.2 2006 BMW 330i brake assist threshold determination Step 2a (displacement feedback) ................142

Figure A1.3 2006 BMW 330i brake assist threshold determination Step 3a (displacement feedback). ...............143

Figure A1.4 2006 BMW 330i brake assist threshold determination Step 2b (displacement feedback). ...............144

Figure A1.5 2006 BMW 330i brake assist threshold determination Step 3b (displacement feedback). ...............145

Figure A1.6 2006 BMW 330i brake assist threshold determination Step 4 (displacement feedback). .................146

Figure A1.7 2006 BMW 330i brake assist threshold determination Step 1 (pedal force feedback) .....................147

Figure A1.8 2006 BMW 330i brake assist threshold determination Step 2 (pedal force feedback) .....................148

Figure A1.9 2006 BMW 330i brake assist threshold determination Step 3 (pedal force feedback). .................... 149

Figure A2.1 2005 Chrysler 300C brake assist threshold determination Step 1 (displacement feedback) .............151

Figure A2.2 2005 Chrysler 300C brake assist threshold determination Step 2a (displacement feedback) ...........152

Figure A2.3 2005 Chrysler 300C brake assist threshold determination Step 2b (displacement feedback)...........153

Figure A2.4 2005 Chrysler 300C brake assist threshold determination Step 3 (displacement feedback) .............154

Figure A2.5 2005 Chrysler 300C brake assist threshold determination Step 1 (pedal force feedback)................155



Figure A3.1 2004 Cadillac STS brake assist threshold determination Step 1 (displacement feedback) ...............159

Figure A3.2 2004 Cadillac STS brake assist threshold determination Step 2a, Part 1 (displacement feedback) ................................................................................................................... 160

Figure A3.3 2004 Cadillac STS brake assist threshold determination Step 2a, Part 2 (displacement feedback) .................................................................................................................. 161

Figure A3.4 2004 Cadillac STS brake assist threshold determination Step 2b (displacement feedback).............162


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Figure A3.7 2004 Cadillac STS brake assist threshold determination Step 1 (pedal force feedback) ..................165



Figure A4.1 2003 Toyota 4Runner brake assist threshold determination Step 1 (displacement feedback) ..........169

Figure A4.2 2003 Toyota 4Runner brake assist threshold determination Step 2a (displacement feedback) .................................................................................................................. 170

Figure A4.3 2003 Toyota 4Runner brake assist threshold determination Step 2b (displacement feedback) ................................................................................................................... 171

Figure A4.4 2003 Toyota 4Runner brake assist threshold determination Step 3 (displacement feedback) ..........172

Figure A4.5 2003 Toyota 4Runner brake assist threshold determination Step 4a (displacement feedback) ................................................................................................................... 173

Figure A4.6 2003 Toyota 4Runner brake assist threshold determination Step 4b (displacement feedback) .................................................................................................................. 174

Figure A.4.7 2003 Toyota 4Runner brake assist threshold determination Step 4c (displacement feedback) ................................................................................................................... 175

Figure A4.8 2003 Toyota 4Runner brake assist threshold determination Step 1 (pedal force feedback) .............176



Figure A5.1 2004 Volvo XC90 brake assist threshold determination Step 1 (displacement feedback) ................180




Figure A5.5 2004 Volvo XC90 brake assist threshold determination Step 1 (pedal force feedback)...................184



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Table 2.1 Brake Assist Research Test Maneuver Overview .................................................................................5

Table 2.2 Brake Assist Enabled vs. Disabled Comparisons Performed with Identical Commanded Inputs ........7

Table 2.3 Brake Assist Enabled Comparisons Performed with Different Commanded Inputs ............................7

Table 2.4 Light Vehicle Brake Assist Research Test Vehicles ............................................................................. 8

Table 2.5 Test Vehicle Tire Descriptions ........................................................................................................... 10

Table 3.1 Test Vehicle Sensor Information ........................................................................................................ 13

Table 4.1 Brake Assist Activation Threshold Information (Pedal Displacement Control Feedback) ................54

Table 4.2 Brake Assist Activation Threshold Information (Pedal Force Control Feedback) .............................54

Table 5.1 Chrysler 300C Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph) ........66

Table 5.2 BMW 330i Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph) ..............67

Table 5.3 Cadillac STS Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph) ...........69

Table 5.4 Toyota 4Runner Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph) ......71

Table 5.5 Volvo XC90 Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph) ...........71

Table 5.6 Chrysler 300C Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph) ..................................................................................................... 71

Table 5.7 BMW 330i Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph) ........74

Table 5.8 Cadillac STS Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).....75

Table 5.9 Toyota 4Runner Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph) ..................................................................................................... 77

Table 5.10 Volvo XC90 Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph) .....78

Table 5.11 Chrysler 300C Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph) ........78

Table 5.12 BMW 330i Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph) ..............80

Table 5.13 Cadillac STS Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph) ...........81

Table 5.14 Toyota 4Runner Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph) ......83

Table 5.15 Volvo XC90 Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph) ...........83

Table 5.16 Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................84

Table 5.17 BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................84

Table 5.18 Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................87

Table 5.19 Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................89

Table 5.20 Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................89

LIST OF TABLES

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LIST OF TABLES (continued)

Table 5.21 Chrysler 300C Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph)..........................90

Table 5.22 BMW 330i Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................90

Table 5.23 Cadillac STS Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................91

Table 5.24 Toyota 4Runner Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph)..........................91

Table 5.25 Volvo XC90 Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph) ..............................................91

Table 5.26 Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph) ..............................................94

Table 5.27 BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph) ..............................................94

Table 5.28 Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph) ..............................................94

Table 5.29 Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph) ..............................................97

Table 5.30 Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph) ..............................................99

Table 5.31 Chrysler 300C Straight Line Stopping Distance Summary (Force Feedback; 45 mph) ...................100

Table 5.32 Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................100

Table 5.33 BMW 330i Straight Line Stopping Distance Summary (Force Feedback; 45 mph).........................101

Table 5.34 BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................102

Table 5.35 Cadillac STS Straight Line Stopping Distance Summary (Force Feedback; 45 mph) .....................103

Table 5.36 Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................103

Table 5.37 Toyota 4Runner Straight Line Stopping Distance Summary (Force Feedback; 45 mph) .................104

Table 5.38 Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................104

Table 5.39 Volvo XC90 Straight Line Stopping Distance Summary (Force Feedback; 45 mph) ......................105

Table 5.40 Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................106

Table 5.41 Chrysler 300C Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph) .............107

Table 5.42 Chrysler 300C Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).....................................107

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Table 5.43 BMW 330i Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph) ..................108

Table 5.44 BMW 330i Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................109

Table 5.45 Cadillac STS Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph) ...............110

Table 5.46 Cadillac STS Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................110

Table 5.47 Toyota 4Runner Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph) ..........111

Table 5.48 Toyota 4Runner Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).....................................111

Table 5.49 Volvo XC90 Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph) ................112

Table 5.50 Volvo XC90 Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph) ........................................................113

Table 5.51 Chrysler 300C Straight Line Stopping Distance Summary (Force Feedback; 65 mph) ...................114

Table 5.52 Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph) ........................................................114

Table 5.53 BMW 330i Straight Line Stopping Distance Summary (Force Feedback; 65 mph).........................115

Table 5.54 BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph) ........................................................116

Table 5.55 Cadillac STS Straight Line Stopping Distance Summary (Force Feedback; 65 mph) .....................117

Table 5.56 Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph) ........................................................117

Table 5.57 Toyota 4Runner Straight Line Stopping Distance Summary (Force Feedback; 65 mph) .................118

Table 5.58 Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph) ........................................................118

Table 5.59 Volvo XC90 Straight Line Stopping Distance Summary (Force Feedback; 65 mph) ......................119

Table 5.60 Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph) ........................................................120

Table 5.61 Chrysler 300C P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications .......................................................122

Table 5.62 BMW 330i P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications .......................................................123

Table 5.63 Cadillac STS P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications .......................................................125

Table 5.64 Toyota 4Runner P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications .......................................................127

Table 5.65 Volvo XC90 P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications .......................................................129

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1

2

3

4

5


Table 6. Brake Assist Activation Thresholds ................................................................................................. 133

Table 6. Displacement Feedback BA Mean Stopping Distance Improvement (ft) .........................................135

Table 6. Displacement Feedback BA Effectiveness Assessment (ft) .............................................................135

Table 6. Force Feedback BA Mean Stopping Distance Differences (ft) ........................................................136

Table 6. Force Feedback BA Effectiveness Assessment (ft) ..........................................................................137

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EXECUTIVE SUMMARY

Brake assist (BA) technology endeavors to improve safety by automatically increasing the gain of a vehicle’s brake system in certain driving situations. Although the manner in which this is accomplished varies among vehicle and brake manufacturers, the intent of these systems is the same: to allow the driver to better utilize the potential of their vehicle’s brake system, and to ultimately provide shorter stopping distances. Brake assist is expected to be particularly beneficial to drivers who do not apply enough brake pedal force during panic or emergency situations. Without BA intervention, insufficient pedal force limits the driver to a fraction of the vehicle’s maximum braking potential and contributes to extended stopping distances.

Brake assist systems monitor a driver’s braking inputs using sensors that measure brake pedal position and application rate. If the driver applies a combination of pedal displacement and application rate in a manner that is interpreted by the BA system as “panic,” the system will intervene by increasing the gain of the vehicle’s foundation brakes (e.g., the amount for brake line pressure realized for a particular amount of pedal displacement), thereby reducing stopping distance.

Study Objectives

The objectives of the work described in this report were twofold: (1) to objectively identify the BA activation thresholds of five contemporary test vehicles, and (2) to evaluate braking performance of the test vehicles using BA activation threshold-based brake applications.

The study’s second objective was broken down into two parts. First, the braking performance of the vehicles with BA enabled was compared to that achieved with the system disabled. For these tests, only brake applications believed to be representative of the vehicles’ BA activation thresholds were used. Next, the braking performance achieved via use of threshold-based brake applications was compared to the maximum braking capability of the vehicle.

Test Conduct

Three braking maneuvers were used in this study: two straight line stops (initiated from 45 and 65 mph), and a brake in-a-turn maneuver initiated from 45 mph. All tests were initiated at a target maneuver entrance speed, and were concluded when the vehicle had come to a complete stop.

For each test, a programmable brake controller was used to insure the maneuvers were executed as accurately and repeatably as possible. To evaluate whether the manner in which the brakes were applied influenced the BA activation threshold, two brake controller feedback loops were used. When “displacement feedback” was utilized, the brake controller used a control feedback loop capable of modulating brake pedal force to maintain constant pedal position. Similarly, when “force feedback” was used, the brake controller used a control feedback loop capable of modulating brake pedal position to maintain constant application of force.

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Test Results

Test results were obtained in two stages. First, the BA activation thresholds were determined. Using these results, tests performed to quantify BA stopping performance were conducted.

Brake Assist Activation Thresholds

Using the brake controller, a series of stops were performed with brake inputs ranging from large to small. The data produced during these tests were reviewed and inspected for trends believed to indicate the presence of BA intervention. Throughout this iterative process, brake application rate was held constant, taken to be the maximum capability of the brake controller.

Once the BA threshold magnitude (i.e., brake pedal displacement or application force) had been identified, tests to isolate the minimum application rate capable of activating BA were performed. This process was very similar to that used to determine the threshold magnitude, except the commanded magnitude was held constant (i.e., at the threshold magnitude) and the application rates were varied from fast to slow. The subsequent data were analyzed for trends believed to indicate the presence of BA intervention. Results of these iterative processes are provided in Table 1.

Table 1. Brake Assist Activation Thresholds.

Vehicle

Displacement Feedback Force Feedback

Brake Pedal Displacement

(inches)

Brake Pedal Rate

(in/sec)


(inches)

Brake Pedal Rate

(in/sec)

2006 BMW 330i 1.9 23.0 2.1 15.0

2005 Chrysler 300C 2.0 19.7 3.1 19.2

2004 Cadillac STS 2.6 18.6 3.3 19.1

2004 Volvo XC90 2.0 23.9 3.2 22.0

2003 Toyota 4Runner 2.8 26.9 3.2 25.9

Quantifying the Effect of Brake Assist

For each of the three maneuvers, tests performed with brake applications taken to represent the vehicles’ respective BA activation thresholds were nominally repeated ten times per configuration. These tests were performed with BA enabled and disabled. For those tests performed with maximum braking, five tests per configuration were performed. To assess whether differences in mean stopping distance were statistically significant, the data were analyzed with SAS software using a generalized linear model (GLM).

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Displacement Feedback

Use of displacement feedback was well suited to finding the BA activation threshold shortening of stopping distances in 13 of the 15 test conditions (five vehicles x three test maneuvers = 15 test conditions). It can also be said, with a high degree of certainty, that the BA systems evaluated in this study significantly shortened straight line stopping distances, with mean reductions ranging from 2.2 to 177.4 ft from 65 mph, and from 1.5 to 90.3 ft from 45 mph. Three of the five vehicles produced shorter stopping distances using BA during the brake in-aturn tests from 45 mph, where mean reductions ranged from 3.5 to 98.0 ft.

In 11 of the 15 test conditions, use of maximum pedal displacement provided significantly shorter stopping distances than comparable tests using the BA activation threshold-based applications: 6.5 to 32.2 ft for the straight line tests initiated from 65 mph, 4.9 to 37.4 ft brake in-a-turn stops performed from 45 mph, and 4.0 to 11.8 ft for the 45 mph straight line tests.

The authors believe the displacement feedback-based results discussed in this report are encouraging. Generally speaking, use of displacement feedback-based application thresholds allowed the effect of BA to be successfully evaluated, and demonstrated that the technology is capable of producing substantial reductions in stopping distance for some vehicles—provided the right combination of brake pedal displacement and high application rate are applied. Although the mean stopping distances achieved with displacement feedback-based BA activation thresholds were significantly longer than those produced with maximum pedal displacement, for four of the five vehicles evaluated in this study the threshold-based inputs were able to achieve mean stopping distances within approximately 16 ft of the vehicles’ full braking capability.

Force Feedback

Use of force feedback was unable to consistently differentiate the mean stopping distances produced with BA enabled versus disabled, despite relying on nearly equivalent methodology and analysis techniques successfully used during the displacement feedback-based tests. With force feedback, only 4 of the 15 test conditions had a statistically significant difference in mean stopping distances, three of which had minor increases attributed to BA.

The lack of significantly different stopping distance means can be attributed to limitations imposed by the force feedback control logic. For each vehicle, there exists a range of command module settings for which stopping distance was not affected by the commanded magnitude. If command module inputs contained within this range are used, there is a high likelihood the maximum, or near maximum braking capability of the vehicle will be realized with or without BA activation. On the other hand, use of command module settings below this range will not allow the brake controller to realize high application rates.

Since low application rates were unable to activate BA for the vehicles evaluated in this study (i.e., those associated with low command module magnitudes), it was necessary for the authors to use alternative command module settings. It is believed these settings were responsible for the small (and mostly insignificant) mean stopping distance differences. In other words, the

xxi

similarity of the force feedback-based BA enabled versus disabled results was not the result of BA intervention. Rather, it was achieved by virtue of the respective command module settings.

In 11 of the 15 test conditions, use of maximum pedal force provided significantly shorter stopping distances than comparable tests using the applications taken to represent the force feedback-based BA activation threshold: 5.4 to 10.7 ft for the straight line tests initiated from 65 mph, 2.4 to 7.8 ft brake in-a-turn stops performed from 45 mph, and 1.7 to11.3 ft for the 45 mph straight line tests. These results indicate that while the command module settings taken to represent the force feedback-based BA activation threshold were large enough to make distinction between BA enabled versus disabled stopping distances insignificant (i.e., the application magnitude dominated any measureable effect of BA activation on stopping distance), these settings were not large enough to consistently realize the maximum braking capability of the respective vehicles. Given the similarity of the enabled and disabled stopping performance realized with force feedback, the authors believe this point has little to do with BA. Rather, it is simply believed to pertain to differences in brake application magnitude and foundation brake system gain. In other words, the settings taken to represent the force feedback-based BA activation threshold were large enough to nearly, but not fully, realize maximum braking even without BA activation.

Given its inability to distinguish the stopping performance realized with BA enabled versus disabled, while simultaneously contributing to physically similar stopping distances being realized with maximum and threshold-based brake applications, the authors believe that force feedback unsuitable for evaluating BA performance. Alternatively, use of displacement feedback control logic is recommended.

Utility of Findings

In this report, the authors have provided information about the brake pedal input magnitudes and rates required to activate the BA systems of five contemporary vehicles. When used in conjunction with knowledge of the human driver’s physical brake application capability, as well as their willingness or reluctance to access it, the data presented in this report may be useful for estimating the extent to which BA activations may occur in real-world driving scenarios. The BA activation thresholds discovered during conduct of the displacement feedback-based tests are considered to be particularly relevant for consideration in such research.

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1.0 INTRODUCTION

Brake assist (BA) technology endeavors to improve safety by automatically increasing the gain of a vehicle’s brake system in certain driving situations. Although the manner in which this is accomplished varies among vehicle and brake manufacturers, the intent of these systems is the same: to allow the driver to better utilize the potential of their vehicle’s brake system and ultimately provide shorter stopping distances. Brake assist is expected to be particularly beneficial to drivers who do not apply enough brake pedal force during panic or emergency situations. Without BA intervention, insufficient pedal force limits the driver to a fraction of the vehicle’s maximum braking potential and contributes to extended stopping distances.

Brake assist systems constantly monitor the driver’s braking inputs using sensors that measure brake pedal position and application rate, either directly (e.g., by using a sensor installed on the brake pedal assembly) or indirectly (e.g., by using a pressure sensor that relates the brake pressure gradient within the master cylinder to brake pedal application rate). If the driver applies a combination of pedal displacement and application rate in a manner that is interpreted by the BA system as “panic,” the system will intervene by increasing the gain of the vehicle’s foundation brakes (i.e., the amount for brake line pressure realized for a particular amount of pedal displacement), thereby reducing stopping distance.

1.1 Other NHTSA Brake Assist Programs

In addition to the test track based work described in this report, NHTSA is presently performing additional BA research. These programs include a field effectiveness study using crash data and human factors work performed with subjects from the general public.

1.1.1 Crash Data Analyses

Careful evaluation of crash data is one of the most important ways to assess whether BA is a truly effective countermeasure to crash scenarios in which the driver applies the brakes. Recognizing this, NHTSA performed a rear-end crash analysis based on data from 26 states. This data set presently contains 8,059 Mercedes cars, model years 1996-1997, involved in rear-end crashes. The Mercedes models used in this analysis were selected because BA was not available in model year 1996, but was standard equipment in model year 1997.

Of the pairs of vehicles considered in each crash, Mercedes cars were the striking vehicles in approximately a third of the cases, regardless of whether they were equipped with BA or not:

MY1996 Mercedes vehicles without BA were striking vehicles in 32.11% of the crashes

MY1997 Mercedes vehicles with BA were striking vehicles in 31.99% of the crashes

Given the similarity of the crash rates associated with these populations, it is not surprising that the most recent data analysis indicates the effect of BA was not statistically significant (p=0.91).

1

1.1.2 Human Factors Research

To better understand how the driver interacts with a vehicle equipped with BA, and to determine whether they can realize the improved braking believed to be offered by BA, NHTSA worked with the Virginia Tech Transportation Institute (VTTI) to perform human factors based research on the topic. The study was conducted within the confines of a closed course VTTI facility located in Blacksburg, Virginia, using a 2006 Mercedes-Benz R350 and a 2007 Volvo S80. To facilitate comparison of tests performed with and without BA, each vehicle was modified in a way that allowed the researchers to enable or disable the respective BA systems without adversely affecting the vehicles’ antilock brake system (ABS) or electronic stability control (ESC) functionality.

Sixty-four participants, balanced for age and gender, were subjected to a driving scenario configured to encourage the use of panic braking (an unexpected barricade appeared in the driver’s path). Using this scenario, none of the drivers were able to activate the BA, regardless of which vehicle was driven.

In an attempt to understand whether training could improve the ability of the participants to activate BA, a second study was performed. Following a debriefing performed after the first test trials, the drivers were instructed how to perform a “panic” stop. Using the same barricade, the educated participants were then told to use panic braking to avoid the same barrier used in the first test trials. In these later trials, 28 percent of drivers were able to activate BA. However, the stopping distances produced with BA were only found to be an average of 1.8 ft shorter than those produced with BA disabled.

As part of a comprehensive literature review performed during the planning stages of the study, VTTI researchers prepared an excellent summary of all known BA publications [1]. Highlights include:

Brake Assist was developed after simulator research demonstrated that many drivers fail to realize a vehicle’s maximum braking potential during panic stops. (Feigel & Schonlau, 1999; Sorniotti, 2006; H. Yoshida et al., 1997)

When placed in a driving scenario that required panic braking as the only available avoidance maneuver, the brake applications used by 47% of drivers failed to engage ABS or produce wheel skid. (Hara, Ohta, Yamamoto, and Yoshida (1998) and Yoshida, Sugitani, Ohta, Kizaki, Yamamoto and Shirai (1998))

Significant differences in initial pedal speed exist between panic and normal braking. (Hara, Ohta, Yamamoto, and Yoshida (1998))

1.2 The Need For Objective Characterization

The research programs mentioned in Sections 1.1.1 and 1.1.2 describe two ways NHTSA is attempting to quantify BA effectiveness: via crash data analysis and by observation of the

2

human driver in panic driving scenarios. Both of these programs will produce results from driver-vehicle interactions. Although it can be argued that such interactions will ultimately define BA effectiveness with the best face validity (i.e., real-world relevance), these results inherently contain confounding factors that make it very difficult to decouple a driver’s ability to perform well in critical driving scenarios from the crash avoidance capability specifically offered by a vehicle’s BA system.

To better understand how BA influences the true braking capability of the vehicle, use of objective tests performed under carefully controlled conditions is required. Such tests must be designed in a way that eliminates the effect of test driver skill to the greatest extent possible. For this reason, the objective tests were performed with a programmable brake controller and the vehicles were driven on a test track.

1.3 Test Objectives For The Work Described In This Report

The objectives of the work described in this report were twofold: (1) to objectively identify the BA activation thresholds of five contemporary test vehicles, and (2) to evaluate braking performance of the test vehicles using BA activation threshold-based brake applications.

1.3.1 Objective #1: Identifying Brake Assist Activation Thresholds

Brake assist technology exists to increase the likelihood a driver will be able to realize a vehicle’s full braking capability in emergency or panic situations. Therefore, in order for BA to be effective in the real world, the activation thresholds must be attainable by a large population of drivers. However, since BA can summon maximum braking, it is also important that the activation threshold not be too low (i.e., to prevent BA activation in situations where the driver does not wish to experience maximum braking). Part 1 of this study (Chapter 4) describes how the BA activation thresholds were identified.

1.3.2 Objective #2: Stopping Distance Measurement

Once these thresholds were identified, a series of tests comprising straight line and brake in-aturn tests were performed. Stopping distance results from tests performed with BA enabled and disabled were compared using two input classifications. First, stops were performed with inputs equal to the BA intervention thresholds determined in Part 1. Second, stops were made with inputs equal to the brake controller’s maximum capability. The BA-enabled results from these two groups were compared to determine whether brake inputs performed at the BA activation thresholds were able to realize maximum stopping ability1 for the respective vehicles.

1 In the context of this report, brake applications performed with inputs of approximately 199-lbf and 24 in/sec (measured at each vehicle’s respective brake pedals), were taken to be “maximum” inputs. These magnitudes represent the maximum capability of the programmable brake controller used to command all brake applications used in this study.

3

2.0 TEST METHODOLOGY

Common processes were used to evaluate the brake performance of each test vehicle discussed in this report. This chapter provides a general description of the test maneuvers, presents an overview of the processes used to identify BA activation thresholds, and discusses the methods used to evaluate BA effectiveness. Additionally, descriptions of the test vehicles and vehicle preparation are provided.

2.1 Test Maneuvers

Two types of tests were used in this study: (1) straight line stops, and (2) braking in-a-turn maneuvers. Both types of tests were initiated at a target maneuver entrance speed and were concluded when the vehicle had come to a complete stop. To insure these maneuvers were executed as accurately and repeatably as possible, a programmable brake controller (described in Section 3.1) was used for each test. Using this controller, various combinations of brake pedal force, displacement, and application rate were commanded. A more comprehensive description of the individual conditions is provided in Section 2.2.

To perform the tests, the driver was instructed to bring the vehicle to a speed greater than the target maneuver entrance speed, release the throttle, and coast down to the target maneuver entrance speed. When the vehicle reached the target maneuver entrance speed, the driver triggered the brake controller with a remote actuator, typically positioned on the steering wheel.

During the straight line stops, the driver was instructed to use the minimum amount of steering necessary to maintain lane position2. However, in the case of the brake in-a-turn maneuvers, a programmable steering machine was used to input a steering wheel angle capable of producing a lateral acceleration of approximately 0.6 g3 prior to initiation of the braking event. In other words, the brake in-a-turn maneuvers required the driver to carefully time the sequence of steer and brake inputs such that a steady-state lateral acceleration of approximately 0.6 g was nominally achieved at the instant the brake controller application was commanded (i.e., at the instant the vehicle had coasted down to its target maneuver entrance speed). So as to reduce the overall number of brake in-a-turn tests performed for the work described in this report, only clockwise steering inputs were used (i.e., right turns).

The straight line stops used two maneuver entrance speed targets: 45 and 65 mph. Due to facility size limitations, braking in-a-turn maneuvers were performed with 45 mph entrance speeds only. Table 2.1 provides an overview of the test maneuvers, maneuver entrance speeds, and the steering wheel angles used during the braking in-a-turn tests.

2 Only minor steering inputs were needed to maintain lane position during the straight line stops. For this reason, these inputs are not discussed in this report. 3 The steering angle required to achieve a lateral acceleration of 0.6 g, corrected for the effect of roll angle, was calculated using data collected during a series of three clockwise Slowly Increasing Steer maneuver tests performed at 50 mph.

4

Table 2.1. Brake Assist Research Test Maneuver Overview.

Test Maneuver Maneuver Entrance

Speed (mph)

Commanded Steering Input

(degrees)

Straight Line Stop 45, 65 n/a

Chrysler 300C: 82 BMW 330i: 60

Braking In-A-Turn 45 Cadillac STS: 87 Toyota 4Runner: 70 Volvo XC90: 66

2.2 Test Conduct

The work described in this report was performed in two parts, both of which used a programmable brake controller for all brake applications. In Part 1, BA activation thresholds were determined. Identification of these thresholds facilitated conduct of Part 2, where the braking performance of the vehicles observed with BA enabled and disabled was evaluated.

2.2.1 Part 1: Brake Assist Activation Threshold Determination

In this report, the authors use the term “activation threshold” to describe the minimum brake application magnitude and rate capable of activating a vehicle’s BA. “Application magnitude” refers to the amount of brake pedal displacement or applied force used during an input to the brake pedal. In the case of the tests where application magnitude was quantified by brake pedal displacement, the brake controller used a control feedback loop capable of modulating brake pedal force to maintain constant pedal position. Similarly, when application magnitude was quantified by the amount of force applied to the brake pedal, the brake controller used a control feedback loop capable of modulating brake pedal position to maintain constant application of force. An overview of these processes is described in Sections 2.2.1.1 and 2.2.1.2.

Note: For the sake of brevity, the authors use the term “displacement feedback” to describe tests performed with the brake controller’s brake pedal displacement control feedback loop. Similarly, the term “force feedback” is used to describe tests performed with the controller’s brake application force control feedback loop.

Regardless of whether inputs based on displacement or force feedback was used, the process used to determining the BA activation thresholds was nearly equivalent. For these tests, the driver placed the transmission in drive, accelerated to a speed slightly greater than the desired target speed, released the throttle, and coasted down to the desired target speed. When the target speed of 45 mph was achieved, the driver commanded the brake controller to initiate a braking maneuver. The controller application was released shortly after the vehicle had come to a complete stop.

5

2.2.1.1 Brake Pedal Displacement Threshold

Determining the activation threshold with displacement feedback began by first identifying the pedal displacement magnitude required to activate BA. Using the brake controller, a series of stops were performed with brake inputs ranging from large (i.e., using the maximum capability of the controller) to small (i.e., low commanded magnitudes that produced very long stopping distances). The data produced during these tests were reviewed and inspected for trends believed to indicate the presence of BA intervention. Throughout this iterative process, brake application rate was held constant, taken to be the maximum capability of the brake controller.

Once the BA displacement threshold magnitude (i.e., brake pedal displacement) had been identified, tests to isolate the minimum application rate capable of activating BA were performed. This process was very similar to that used to determine the threshold magnitude, except the commanded brake pedal displacement was held constant (i.e., at the threshold magnitude) and the application rates were varied from fast to slow. The subsequent data were analyzed for trends believed to indicate the presence of BA intervention.

2.2.1.2 Brake Pedal Force Threshold

The process used to identify the BA activation threshold with force feedback was conceptually equivalent to that used with displacement feedback. First, the pedal force magnitude required to activate BA was identified by performing a series of stops with brake inputs ranging from large to small. The data produced during these tests were reviewed and inspected for trends believed to indicate the presence of BA intervention. Throughout this iterative process, brake application rate was held constant, taken to be the maximum capability of the brake controller.

Once the BA force threshold magnitude (i.e., brake pedal force) had been identified, tests to isolate the minimum application rate capable of activating BA were performed. During this process, the commanded brake pedal force was held constant (i.e., at the threshold magnitude) and the application rates were varied from fast to slow. The subsequent data were analyzed for trends believed to be indicative of BA activation.

2.2.2 Part 2: Evaluation of Brake Assist Effectiveness

2.2.2.1 Tests Performed with Maximum Commanded Inputs

In addition to the tests performed at the displacement and force thresholds described in Section 2.2.1, many tests were also performed at the maximum capability of the brake controller. When maximum displacement was commanded, the brake controller attempted to apply and maintain approximately 5 inches of actuator stroke by automatically modulating the amount of applied force. Similarly, when maximum force was commanded, the controller attempted to apply and maintain approximately 200 lbf by automatically modulating pedal position. Regardless of which control feedback loop was used, all tests performed with maximum commanded inputs were also performed using the maximum application rate attainable by the brake controller.

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2.2.2.2 Brake Assist Effectiveness Comparison Groups

Brake assist effectiveness was evaluated via comparison of two groups: tests performed with BA enabled versus disabled using the same application magnitudes and rates (i.e., threshold based inputs), and tests performed with BA enabled, but with different application magnitudes (i.e., threshold and maximum input magnitudes). More detailed descriptions of these comparisons are provided in Tables 2.2 and 2.3, respectively.

To provide a reasonable sample size for the comparisons discussed in this report, multiple trials per test condition were performed. Ten trials per condition were nominally performed during threshold based test series. Test series performed with maximum commanded inputs nominally included five trials per condition due to concerns that a large number of repeated brake applications performed at the brake controller’s maximum capability could potentially damage the vehicles’ respective brake systems.

Table 2.2. Brake Assist Enabled vs. Disabled Comparisons Performed with Identical Commanded Inputs.

Test Maneuver Brake Assist Enabled versus Disabled Comparisons

Straight Line Stops (performed at 45 and 65 mph)

(1) Displacement Threshold (2) Force Threshold

Braking In-A-Turn (performed at 45 mph only)

(1) Displacement Threshold (2) Force Threshold

Table 2.3. Brake Assist Enabled Comparisons Performed with Different Commanded Inputs.

Test Maneuver Threshold versus Maximum Application Comparisons

(Brake Assist Enabled Only)

Straight Line Stops (performed at 45 and 65 mph)

(1) Displacement Threshold vs. Maximum Displacement (2) Force Threshold vs. Maximum Force

Braking In-A-Turn (performed at 45 mph only)

(1) Displacement Threshold vs. Maximum Displacement (2) Force Threshold vs. Maximum Force

2.3 Stopping Distance Corrections

The target maneuver entrance speeds used in this study were chosen to reflect real world applicability and available testing real estate. When small differences between the target and actual entrance speeds were observed for an individual test, the stopping distance of that test was normalized using the following equation, recommended in SAE J299 [2]:

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s' corrected stopping distance

v2 vtarget target maneuver entrance speedtargets' s whereactual2 vactual actual maneuver entrance speedvactual

sactual actual stopping distance

2.4 Test Vehicles

The vehicles used in this study were selected for three reasons: (1) collectively, they provided a relatively diverse fleet of contemporary vehicles; (2) they were owned by NHTSA and located at the Agency’s Vehicle Research and Test Center (VRTC), and (3) it would be possible, working with brake suppliers and/or the vehicle manufacturers, to disable the vehicle’s respective BA systems without also disabling or modifying the antilock brake and/or electronic stability control systems. Table 2.4 provides a listing of the test vehicles used in this study, the manufacturer of the respective BA systems, and the name used by the vehicle manufacturer to describe the vehicle’s BA system.

Table 2.4. Light Vehicle Brake Assist Research Test Vehicles.

Model Year Vehicle Brake Assist

Make Model Make Name

2005 Chrysler 300C Continental Teves Brake Assist System (BAS)

2006 BMW 330i Continental Teves Dynamic Brake Control (DBC)

2005 Cadillac STS Delphi Panic Brake Assist

2003 Toyota 4Runner Aisen Brake Assist System

2004 Volvo XC90 Continental Teves Electronic Brake Assistance (EBA)

2.4.1 Vehicle Manufacturer Brake Assist Descriptions

The owner’s manuals provided with the vehicles used in this study each contained language describing the respective BA systems and how they may be expected to operate. In each description, it is stated a sudden or rapid brake application is used to activate BA. Note that the ability of BA to provide full braking action is not specifically mentioned in three of the five descriptions (only the BMW 330i and Volvo XC90 manuals contain such language).

2005 Chrysler 300C; Brake Assist System (BAS)

The BAS is designed to optimize the vehicle’s braking capability during emergency braking maneuvers. The system applies optimum pressure to the brakes in emergency braking conditions that might otherwise be afforded solely by the driver’s braking style. This can reduce braking distances. The BAS complements the antilock brake system (ABS). Applying the brakes very quickly results in maximum BAS assistance. To receive

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the benefit of the system, you must apply continuous braking power during the stopping sequence. Do not reduce brake pedal pressure.” [3]

2006 BMW 330i; Dynamic Brake Control (DBC)

“When you apply the brakes rapidly, this system automatically produces the maximum braking force boost and thus helps to achieve the shortest possible braking distance during full braking.”[4]

2005 Cadillac STS; Panic Brake Assist

“Your vehicle has a panic brake assist system that monitors the intention of the driver while braking. If the system senses that the driver has applied hard/fast pressure to the brake pedal, the system will generate additional pressure, making it easier for the driver to maintain brake application.” [5]

2003 Toyota 4Runner; Brake Assist System

“When you slam the brakes on, the brake assist system judges as an emergency stop and provides more powerful braking for a driver who cannot hold down the brake pedal firmly.” [6]

2004 Volvo XC90; Emergency Brake Assistance (EBA)

“The EBA function is an integrated part of the Dynamic Stability Traction Control (DSTC) system. EBA is designed to provide full brake affect immediately in the event of sudden, hard braking. The system is activated by the speed with which you depress the brake pedal.” [7]

2.4.2 Brake Conditioning

New brake components (i.e., brake pads, rotors, and fluid) were installed on each test vehicle prior to the tests described in this report. These components were of original equipment specification and were installed at the vehicles’ respective dealerships. Once the vehicles returned from receiving brake work, new original equipment tires were installed and the brakes were burnished using the procedures specified in Federal Motor Vehicle Safety Standard (FMVSS) No. 135 [8].

2.4.3 Tires

Prior to testing, new original equipment tires were installed on each vehicle evaluated in this study. A list of the tires installed on these vehicles is provided in Table 2.5.

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Table 2.5. Test Vehicle Tire Descriptions.

Model Year Make Model Size Load Rating/ Speed Index

Inflation Pressure

(psi)

Chrysler 300C Continental ContiTouring Contact P225/60R18 99H Front: 30

Rear: 30

BMW 330i Bridgestone Potenza RE050A (runflat)

Front: 225/40ZR18 Rear: 255/35ZR18

Front: 88W Rear: 90W

Front: 32 Rear: 36

Cadillac STS Michelin Pilot HX MXM4 Front: P235/50ZR18 Rear: P255/45ZR18

Front: 97W Rear: 99W

Front: 30 Rear: 30

Toyota 4Runner Bridgestone Dueler HT 840 P265/70R16 111S Front: 32 Rear: 32

Volvo XC90 Michelin 4x4 Synchone P235/65R17 104H Front: 36 Rear: 39

2.4.3.1 Tire Conditioning

Prior to actual testing, the tires were “scrubbed in” to wear away mold sheen and be brought up to operating temperature. Although removal of the mold sheen was partially accomplished during the 200-stop brake burnish, additional tire conditioning was performed using a two-step process similar to that used by NHTSA for the Agency’s Rollover NCAP and FMVSS 126 ESC compliance testing [9, 10].

First, the vehicle was driven around a circle 100 ft in diameter at a speed that produced a lateral acceleration of approximately 0.5 to 0.6 g. Using this circle, three clockwise laps were followed by three counterclockwise laps. Once these six laps were complete, the driver commanded sinusoidal steering using a steering wheel angle capable of producing a lateral acceleration of 0.5 to 0.6 g (ss) at a frequency of 1 Hz for 10 cycles while maintaining a vehicle speed of 35 mph. A programmable steering machine was used to input the sinusoidal steering used during the break-in procedure. A total of four passes using the sinusoidal steering inputs were used.

2.4.3.2 Use of Inner Tubes

Regardless of vehicle classification (i.e., passenger car or MPV), inner tubes were not used for any of the vehicles evaluated in this study4. Since the brake in-a-turn tests were to be performed under only moderate lateral acceleration, the authors believed the risk of rim-to-pavement contact and/or debeading was negligible.

4 For vehicles with GVWRs less than 10,000 lbs, NHTSA frequently installs inner tubes in each of a vehicle’s four tires prior to performing limit handling tests, particularly if the vehicle has a high center of gravity and/or narrow track width. This is to reduce the possibility of rim-to-pavement contact and/or tire debeading.

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2.4.3.3 Frequency of Tire Changes

All tests performed with a given vehicle used the same tire set. This was possible since the amount of wear incurred while testing was quite low, less than that observed during the conduct of maneuvers capable of imposing high lateral loads (i.e., the Sine with Dwell and NHTSA Fishhook maneuvers). Furthermore, the tread wear resulting from the repeated stops was more uniform across the width of the tire.

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3.0 INSTRUMENTATION

All vehicles evaluated in this study were similarly instrumented with sensors, data acquisition system, a programmable steering machine, and a programmable brake controller. This chapter briefly describes the test equipment and how it was utilized.

3.1 Programmable Brake Controller

Accurately and repeatably achieving specific combinations of brake pedal force, displacement, and/or application rate is very difficult, even for highly skilled test drivers. For the work performed in this study, use of a programmable brake controller specifically addressed these concerns. This controller uses PID (proportional, integral, derivative) control feedback logic to operate in one of three modes: (1) commanded brake pedal position is achieved and maintained by modulating brake pedal force, (2) commanded brake force is achieved and maintained by modulating brake pedal position, and (3) commanded deceleration is achieved and maintained by modulating brake pedal force and/or displacement. Of these three modes, only the first two were used for the work described in this report.

The brake controller was essentially comprised of four components: an electronically controlled actuator assembly, the mounting apparatus, a driver-operated command module, and an electronics box. The actuator and mounting apparatus were easily installed in the vehicles via attachment to the seat and seat tracks, and did not require modification to the vehicles (i.e., holes, alteration of interior trim, etc.). The electronics box was secured to the rear seat or positioned on the passenger-side floor of the rear seat compartment. The command module (presented later in Figure 4.3) allowed the driver to independently specify application magnitude (pedal displacement or applied force) and application rate via use of two rotary dials. Figure 3.1 shows the brake controller installed in a 2006 BMW 330i.

Electronics Box

Actuator Assembly

Mounting Apparatus

Figure 3.1. ATI Programmable Brake Controller installed in a 2006 BMW 330i.

3.2 Programmable Steering Machine

A programmable steering machine was used to provide steering inputs for tests that required braking in-a-turn. Descriptions of the steering machine, including features and technical specifications, have been previously documented and are available in [11, 12].

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3.3 Sensors and Sensor Locations

Table 3.1 provides an overview of the sensors used for the work described in this report. Sensors are listed with the data channel measured in the first column of the table. Additional columns list the sensor type, sensor range, sensor manufacturer, and sensor model number.

Table 3.1. Test Vehicle Sensor Information.

Data Measured Type Range Manufacturer Model Number

Steering Wheel Angle Angle Encoder ±720 degrees Automotive Testing, Inc. (ATI)

Integral with ATI Steering Machine

Brake Pedal Force (applied by the brake controller)

Load Cell 0-200 lbf Automotive Testing, Inc. (ATI)

Integral with ATI Brake Controller

Brake Pedal Force (measured at the brake pedal)

Load Cell 0-300 lbf GSE 3100A

Brake Controller Stroke Linear Distance Potentiometer 0-5 inches Automotive Testing,

Inc. Integral with ATI Brake Controller

Brake Pedal Travel Linear String Potentiometer 0-20 Space Age Control 160-1215

Longitudinal, Lateral, and Vertical Acceleration Roll, Yaw, and Pitch Rate

Multi-Axis Inertial Sensing System

Accelerometers: ±2 g Angular Rate Sensors: ±100°/s

BEI Technologies, Inc. Systron Donner Inertial Division

MotionPak Multi-Axis Inertial Sensing System MP-1

Left, Right, Front, and Rear Vehicle Ride Height

Ultrasonic Distance Measuring System 4-40 inches Massa Products Corp. M-5000 / 220 kHz

Vehicle Speed Radar Speed Sensor 0.1-125 mph B+S Software und Messtechnik GmbH DRS-6

Longitudinal and Lateral Speed Non-Contact Optical Sensor

0.3-155 mph (long.) ± 30.0 mph (lat.)

Corrsys-Datron Sensorsystems, Inc. Correvit S-400

Stopping Distance Direct Contact 5th

Wheel infinite Labeco 606007-1

Brake Line Pressure Millivolt Output Pressure Transducer 0-2500 psi Tronix PSI-100

Wheel Speed DC Tachometer 0 – 60 mph Servo-Tek Products Company SA – 7388F – 1

3.3.1 Steering Wheel Angle

Steering wheel position was recorded with an angle encoder integrated with the programmable steering machine.

3.3.2 Brake Pedal Force

The application force produced by the brake controller was measured with a small load cell positioned between the end of the actuator assembly and the mounting apparatus (i.e., near the

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driver seat, not the brake pedal). However, static and dynamic geometric offsets experienced during brake applications often caused this measurement to be higher than that of the force acting perpendicular to the brake pedal. For this reason, a second load cell was attached to the face of the brake pedal.

3.3.3 Brake Pedal Position

Brake actuator stroke was measured with a linear displacement transducer installed within the actuator assembly. As was the case for application force, there were some geometric differences between the actuator’s motion and actual brake pedal displacement since the actuator’s motion was linear whereas the brake pedal moved in an arc. For this reason, simply utilizing brake controller stroke would not accurately represent true brake pedal position. Therefore, two alternative methods were employed.

First, linear potentiometers were installed between a fixed under-dash location to the respective brake pedal arms. Although the data produced by these sensors were still confounded by geometric effects to a degree, they provided an important way to determine if the vehicle’s brake pedal was being displaced due to brake controller stroke or due to movement of the brake controller assembly during a period of high vehicle deceleration and very low brake pedal application force.

Ultimately, the authors decided the most realistic (and practical) assessment of brake pedal travel was to calculate the arc length of the brake pedal during each stop. These calculations required:

1. The distance from the brake pedal arm pivot point, located under the dash, to the center of the brake pedal (where the force of the brake controller was applied).

2. The angle between the brake controller actuator and the brake pedal, measured over a range of brake pedal displacements (i.e., brake controller command module settings). This was directly measured by a machinist protractor.

3. The brake controller stroke, measured over a range of brake controller command module settings

Using these data, the arc length of the brake pedal travel, measured from a zero position, was calculated. Brake pedal arc length was then plotted as a function of brake controller stroke and a second order curve was fit to the data. Since the authors were confident the brake controller stroke data channels were configured and validated correctly, it was believed the subsequent pedal arc length calculations were as accurate as reasonably possible, provided the brake controller stroke data channels were not clipped and the brake controller assembly did not move during the braking event.

3.3.4 Inertial Sensing System

A multi-axis inertial sensing system was used to measure accelerations and angular rates. The system was placed near the vehicle’s center of gravity (C.G.) so as to minimize roll, pitch, and

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yaw effects. Since it was not possible to position the accelerometers precisely at every vehicle’s C.G. for each loading condition, sensor outputs were corrected to translate the motion of the vehicle at the measured location to that which occurred at the actual C.G. during post-processing of the data. The equations used for these corrections were derived from equations of general relative acceleration for a translating reference frame and use the SAE Convention for Vehicle Dynamics Coordinate Systems. The sensing system did not provide inertial stabilization of its accelerometers. Therefore, lateral and longitudinal accelerations were also corrected for vehicle roll and pitch angles, respectively, during post processing using the techniques explained in [13].

3.3.5 Ultrasonic Sensors

An ultrasonic distance measurement system was used to collect left, right, front, and rear vehicle ride heights for the purpose of calculating vehicle roll and pitch angles. One ultrasonic ranging module was mounted on each side of a vehicle, each positioned at the vehicle’s longitudinal center of gravity. Front and rear mounted modules were attached along the vehicle’s lateral centerlines, typically to the front and rear bumper assemblies. Vehicle roll angle was computed with data output from the two side-mounted sensors used in conjunction with roll rate data measured by the multi-axis inertial sensing system. Similarly, vehicle pitch angle was computed with data output from the front and rear mounted sensors used in conjunction with pitch rate data measured by the multi-axis inertial sensing system. Reference [13] presents the technique used.

3.3.6 Vehicle Speed

Vehicle speed (i.e., longitudinal velocity) was measured with a direct-contact fifth wheel assembly attached to the rear bumper of each vehicle. Sensor outputs were transmitted not only to the data acquisition system, but also to a dashboard display unit. This allowed the driver to accurately monitor vehicle speed.

3.3.7 Brake Line Pressure

The brake line pressure of each wheel’s hydraulic circuit was measured downstream of the ESC/ABS hydraulic control unit with pressure transducers installed in one of two ways: (1) via the bleeder screw, when clearance from suspension, fenders, and brakes was available; or (2) via use of T-fittings installed between the vehicles rigid and flexible brake lines.

3.4 Data Acquisition

In-vehicle data acquisition systems, comprised of ruggedized industrial computers, recorded outputs from the previously mentioned sensors during the conduct of test maneuvers. All NHTSA data were sampled at a rate of 200 Hz.

The computers employed the DAS-64 data acquisition software developed by VRTC. Analog Devices Inc. 3B series signal conditioners were used to condition data signals from all transducers listed in Table 3.1. Measurement Computing Corporation PCI-DAS6402/16 boards digitized analog signals at a collective rate of 200 kHz. The test drivers armed the trigger for data collection prior to each test; however, actual data collection was automatically initiated when the brake controller stroke was greater than or equal to 0.5 inches. To provide the initial

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conditions just prior to execution of each test maneuver, a short period of pre-trigger data was recorded.

Signal conditioning consisted of amplification, anti-alias filtering, and digitizing. Amplifier gains were selected to maximize the signal-to-noise ratio of the digitized data. Filtering was performed with two-pole low-pass Butterworth filters with nominal cutoff frequencies selected to prevent aliasing. At a nominal cutoff frequency of 15 Hz, the calculated breakpoint frequencies were 18 and 19 Hz for the first and second poles respectively. A higher nominal cutoff frequency of 1800 Hz (1800 Hz at pole 1 and 1900 Hz at pole 2) was used on the steering wheel angle channel.

3.5 Post Processing Filters

Most sensor data were filtered in post processing with 6-Hz 12-pole, 2-pass, phaseless digital Butterworth filters using Matlab software. The filtered and zeroed data were then used to calculate roll and pitch angle, as previously described in Section 3.3.4.

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Figure 4.1. Brake pedal force observed as a function of time for three subjects during a crash-imminent driving scenario.

4.0 DETERMINING BRAKE ASSIST ACTIVATION THRESHOLDS

Results from the tests performed in this study are discussed in Chapters 4 and 5. In this chapter, the methods used to identify the BA activation thresholds for each vehicle are described. Although an overview of the methods used to identify these thresholds was previously presented in Section 2.2.1, a more detailed description of these methods is presented in this chapter. In Chapter 5, the effect of BA activation on stopping distances is discussed.

When contemplating how best to identify the smallest brake application required to elicit BA, it became apparent the term “activation threshold” can be interpreted in different ways. Much of the literature published about BA, from technical reports to owner’s manuals, imply it exists to provide better braking to drivers who can input quick pedal applications with low force. However, given the infinite combinations of application techniques potentially utilized by the human driver, it becomes important to clarify what “low force application” really means.

Consider, for example, the three brake applications shown in Figure 4.1. These data were collected during a human factors study that contained a surprise, crash imminent driving scenario in which a full-size Styrofoam vehicle was pulled directly into the path of the subject’s vehicle [14]. If the driver did not brake and/or steer, there was a 100 percent probability they would collide with the artificial vehicle. Results from three different drivers are shown. Each driver operated the same vehicle, equipped with ABS, and was presented with the same driving scenario.

Despite the common scenario, the three subjects used three very different braking strategies. In the case of Subject 1, a large initial application was quickly applied, but was immediately

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followed with reduced force throughout the remainder of the braking event. Conversely, Subject 2 quickly applied a lesser initial application, but used higher force later in the maneuver. Different still, Subject 3 applied a rapid initial application followed by a period of constant magnitude.

Of these three examples, the authors consider results from Subjects 1 and 3 to be of the greatest interest. In the case of Subject 1, the signature of the brake force data is similar to that produced when the brake controller is programmed to rapidly achieve and maintain a desired brake pedal position (i.e., via use of displacement-based control feedback), as shown by the blue data traces in Figure 4.2. A large initial force spike is required to rapidly accelerate the brake pedal to the target displacement from the initial resting position. Once at the target displacement, much less force is generally required to maintain it5.

Figure 4.2. Brake pedal force observed as a function of time during an automated brake stop performed with a programmable brake controller using a position feedback algorithm.

In the case of Subject 3, the signature of the brake force data is similar to that produced when the brake controller is programmed to rapidly achieve and maintain a desired brake pedal force magnitude, as shown by the red “Force Feedback” data traces in Figure 4.2. Note the absence of substantial overshoot during the first part of the braking event.

5 There are some exceptions to this trend. For some vehicles, ABS operation can cause the brake pedal to push back against the driver’s foot. If an attempt to maintain pedal position is made while this occurs, brake pedal force will increase.

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The results contained in Sections 4.1 and 4.2 evaluate how two brake application techniques, similar to those shown for Subjects 1 and 3 in Figure 4.1, can be used to evaluate BA application thresholds. In Section 4.1, the authors endeavor to identify the minimum combinations of pedal displacement magnitude and rate of application capable of activating BA. Section 4.2 explains how the authors attempted to isolate the minimum combinations of brake pedal force magnitude and rate of application capable of activating BA.

Note: For the sake of brevity, “brake pedal displacement magnitude” will generally be referred to as “displacement magnitude” for the duration of this report. Similarly, “brake pedal force magnitude” will simply be referred to as “force magnitude.”

Within these discussions, two definitions of “low application force” are explored. By controlling for pedal displacement (described in Section 4.1, where inputs similar to those produced by Subject 1 were used), low application force refers to the inputs that occur during most of the braking maneuver (i.e., after the initial force spike). By controlling for pedal force (described in Section 4.2, where inputs similar to those produced by Subject 3 were used), low application force refers to the minimum sustained brake force magnitude capable of activating BA.

4.1 Displacement Feedback Based Threshold Determination

Identifying the minimum combinations of pedal displacement and rate of application capable of activating BA required four steps. Steps 1 and 2 were used to isolate the displacement magnitude, whereas Steps 3 and 4 were used to isolate the application rate.

The processes described in Section 4.1 document the specific manner in which the iterative adjustment of displacement magnitude and application rate was used to identify the BA activation threshold for each vehicle using the brake controller’s displacement-based control feedback. The driver made these adjustments via a handheld “command module,” the interface with the brake controller. The brake controller command module is shown in Figure 4.3.

The command module’s two rotary dials allow the controller actuator stroke and application rate to be independently adjusted. However, it is important to realize that the values specified on the command module refer to the percent of maximum controller capability, not to a specific pedal displacement or application rate. For example, consider the command module settings shown in Figure 4.3. When these settings are used in conjunction with the displacement feedback control loop, the “amplitude” setting of “50” corresponds to a brake controller stroke nominally equal to 2.5 inches, or 50 percent of the controller’s maximum stroke capability of 5 inches. Similarly, the commanded application rate shown in Figure 4.3, a value of “75,” corresponds to a brake controller stroke rate nominally equal to 22.5 in/sec, or 75 percent of the controller’s nominal maximum stroke capability of 30 in/sec.

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Figure 4.4. Brake pedal travel presented as a function of brake controller stroke.

Figure 4.3. Programmable brake controller command module.

Although the response of the brake controller to the respective command module settings was consistent across each vehicle evaluated, the relationship between brake controller stroke and actual brake pedal position was somewhat vehicle-dependent, as shown in Figure 4.4. To keep the descriptions of how the authors determined the BA activation thresholds as consistent as possible, the processes described in this section indicate what command module settings were used throughout the various steps. The physical descriptions of the displacement magnitude (inches) and application rates (inches per second) are not provided until the end of each vehicle’s respective section.

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4.1.1 Determining Pedal Displacement Magnitude Thresholds (Step 1 of 4) Step 1 was to iteratively decrease pedal displacement from large to small while maintaining a constant commanded application rate. Beginning with a command module setting of 99/996, displacement magnitude was first reduced from 99 to 90. Generally speaking, subsequent reductions were performed using incremental reductions of “10” on the command module until a setting of 10/99 had been utilized. Once the entire sweep of pedal displacements had been performed, the subsequent data were reviewed and inspected for trends believed to indicate the presence of BA intervention using four evaluation considerations. This process was used to narrow the range of commanded pedal displacements down to an interval bounded by two input increments spaced 10 command module increments apart. Specifically, the four evaluators were as follows:

Stopping Distance. Large differences in stopping distance resulting from small changes to the brake controller command module settings were believed to provide evidence that the vehicle was being evaluated near a BA intervention threshold. Longitudinal Acceleration. Pronounced differences in peak and/or overall deceleration during the braking event resulting from small changes to the brake controller command module settings were believed to provide evidence that the vehicle was being evaluated near a BA intervention threshold. Deceleration magnitude, rise time, and ability to maintain high deceleration during the stop duration were important considerations. Brake Line Pressure. To achieve the shortest possible stopping distances, the brake pedal inputs should be great enough to activate each vehicle’s respective antilock brake system (ABS) during the stop. These interventions were detected by observing the brake line pressure of each wheel circuit. Of particular interest was determining whether small changes to the brake controller command module settings result in pronounced differences in the peak brake line pressure magnitude before, during, and/or after ABS activation. Brake Pedal Force Data Traces. For some vehicles, an indication of BA activation was seen by comparing the amount of force required to maintain brake pedal position during a stop. After the initial spike in pedal force required to rapidly establish brake pedal position, the pedal force magnitudes observed during the tests performed with BA activation were less than those performed without activation.

4.1.2 Determining Pedal Displacement Magnitude Thresholds (Step 2 of 4) Within the reduced range of commanded magnitudes output from Step 1 (i.e., as specified in Section 4.1.1), additional tests were performed using finer increments. Beginning with the upper bound of a desired range, the command module settings were decremented by one. Using these

6 Maximum magnitude and rate is achieved with command module settings of 99/99, since the user cannot specify “100” on either of the module’s two-digit rotary dials.

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finer command module increments, in conjunction with the four evaluation considerations specified in Step 1, the pedal displacements believed to best represent the BA activation thresholds were identified.

4.1.3 Determining Pedal Application Rate Thresholds (Step 3 of 4)

Once the displacement magnitude threshold had been determined, the command module was used to perform an iterative reduction of commanded application rates. Conceptually, the process used for Step 3 was equivalent to that used in Step 1, with the primary difference being the use of fixed application magnitude (i.e., the BA pedal displacement threshold setting) while varying application rates.

To begin Step 3, the command module value believed to best coincide with the vehicle’s BA activation threshold was selected and the command module rate was set to the maximum. Beginning with these settings, rate magnitude was iteratively reduced from 99 to 90. Subsequent rate magnitude reductions were performed using incremental reductions of “10” on the command module until a rate setting of “10” had been utilized. For most vehicles, the final rate reduction lowered the commanded setting to “05.”

Once the entire sweep of application rates had been executed, the subsequent data were reviewed and inspected for trends believed to indicate the presence of BA intervention using the four evaluation considerations specified in Step 1. This process was used to narrow the range of commanded rates down to an interval bounded by two input increments spaced 10 command module increments apart.


Step 4 was comprised of one additional test performed with a commanded rate midway within the reduced range of increments used in Step 3. Using the four evaluation considerations specified in Step 1, the rate value believed to best indicate the BA activation threshold for a particular vehicle was identified. Note that unlike the small iterative increments used to isolate the application rate, only one rate was typically used for Step 4. This was because the brake controller used in this study was not very sensitive to changes in commanded rates until settings less than “40” were used. However, for the vehicles evaluated in this study, command module settings less than 40 were unable to activate BA during tests performed with displacement feedback.

4.1.5 BMW 330i Displacement Feedback Threshold Determination

Figures A1.1 through A1.6, presented in Appendix A1, illustrate the iterations used to identify the displacement feedback-based BA activation threshold for the BMW 330i. Each figure presents test outputs collected from eight channels:

Vehicle speed Longitudinal acceleration Brake pedal force (measured at the brake pedal)

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Figure 4.5. Stopping distance as a function of command module magnitude (Step 1, BMW 330i)

Brake pedal displacement Brake line pressures measured at each of the four brake calipers

4.1.5.1 BMW 330i Displacement Feedback Threshold Determination Attempt #1

Identifying the BMW 330i displacement threshold magnitude required two attempts by the authors. Results from both attempts have been documented and are reported to describe the entire process used to ultimately determine the threshold magnitude for this vehicle. The processes used during the first attempt are described as Step 1, Step 2a, and Step 3a. Step 4, the final step used to isolate the application rate threshold after the displacement threshold had been identified, was not performed during the first attempt to determine the displacement threshold.

4.1.5.1.1 BMW 330i Step 1

The BMW 330i “Attempt #1” evaluation began with Step 1, the iterative reduction of pedal displacements magnitudes previously described in Section 4.1. The test outputs from Step 1 are shown in Figure A1.1. Due to the differences in front brake line pressures and overall vehicle deceleration observed between the tests performed with command module settings of 50/99 and 60/99, tests that produced stopping distances of 69.7 and 79.1 ft, respectively, the authors surmised the BA activation threshold magnitude resided within, or close to, an interval bounded by these two commanded inputs.

Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.5. Interestingly, the stopping distance data alone provided little indication as to where the BA activation threshold magnitude may reside for the BMW 330i. Stopping distances produced with command module settings of 50/99 through 99/99 differed by a maximum of 13.7 ft, however those distances produced with command module settings of 60/99 through 90/99 differed by a maximum of only 4.3 ft.

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4.1.5.1.2 BMW 330i Step 2a

The BMW 330i was the first vehicle evaluated for the work described in this report. As such, some of the later, more refined threshold isolation techniques used for the other vehicles had not yet been formalized. For this reason, Step 2a only consisted of one additional test for the BMW 330i, performed with a command module setting of 55/99. Later vehicles used finer magnitude increments for Step 2.

Figure A1.2 presents the tests used for the Step 2a analysis of the BMW 330i. Here, outputs of the 55/99 test, which produced a stopping distance of 70.9 ft, were compared with Step 1 tests performed with command module settings of 50/99 and 60/99. Since there was higher deceleration and much more ABS activity at the front axle during the 60/99 tests (i.e., more pronounced brake line pressure modulation), the authors surmised the activation threshold magnitude was realized with this command module setting. Therefore the displacement magnitude selected for use in Step 3a was taken to be “60.”

4.1.5.1.3 BMW 330i Step 3a

Figure A1.3 shows the brake controller settings used to iteratively reduce application rate during Step 3a tests performed with the BMW 330i. Overall, the suite of Step 3a tests produced vehicle deceleration levels higher than expected; particularly for the tests performed with application rate settings not expected to be capable of initiating BA intervention. Although the reasons for this are not entirely understood, the authors believe the high decelerations were likely the result of the vehicle’s high brake gain and the selection of an inappropriate displacement magnitude setting (too high). These data implied the displacement threshold was incorrectly identified. Therefore, the authors returned to the data collected during Step 1, and selected an alternative interval. This alternative, defined by the lower displacement magnitudes, was used for the BMW 330i “Attempt #2” evaluation.

4.1.5.2 BMW 330i Displacement Feedback Threshold Determination Attempt #2

As previously mentioned, identifying the BMW 330i displacement threshold magnitude required two attempts by the authors. This section describes the second (final) attempt. Note that the previously mentioned Step 1 data, that shown in Figure A1.1, was used for both attempts at identifying the BMW 330i brake pedal displacement threshold.

4.1.5.2.1 BMW 330i Step 2b

Figure A1.4 shows the BMW 330i Step 2b tests performed with command module settings ranging from 40/90 to 48/907. Of these tests, two produced much higher deceleration and brake line pressure at the front axle: during one of the two tests performed with command module settings of 45/90, and during the test performed with a 48/90 setting. Given that such different results were obtained during the two 45/90 tests (an indication the brake application was performed at or near the BA activation threshold), and that the 45/90 tests for which BA was

7 The rate magnitudes were set to “90” rather than “99” for this test series, but the two settings produced nearly identical application rates and are believed to be directly comparable

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Figure 4.6. Stopping distance as a function of command module magnitude (Step 3b, BMW 330i).

believed to have occurred produced brake line pressures and vehicle decelerations very similar to those produced with the larger 48/90 settings, the authors decided to use “45” as the commanded displacement magnitude setting for use in Step 3b.

4.1.5.2.2 BMW 330i Step 3b

Figure A1.5 shows the brake controller settings used to iteratively reduce application rate during Step 3b tests performed with the BMW 330i. Due to the substantial differences in brake line pressures and initial vehicle decelerations observed between the tests performed with command module settings of 45/60 and 45/70, the authors surmised the BA activation threshold resided within an interval bounded by these settings. Note that when compared with the Step 3a data presented in Figure A1.3, differences between the tests believed to have initiated BA and those that did not were much more pronounced.

Step 3b stopping distances observed as a function of command module magnitude are provided in Figure 4.6. As was the case in Step 1, the Step 3b stopping distance data alone provided little direct indication as to where the BA activation threshold rate may reside for the BMW 330i, given the noise in the data observed when commanded rate settings between 50 and 70 were used. That said, this noise, particularly the spike observed during the test performed with a commanded rate setting of 60, did at least support the possibility of the activation threshold being realized when commanded rates between 40 and 70 were used, since the population of data produced very near the threshold is expected to contain values above and below those associated with the actual threshold.

4.1.5.2.3 BMW 330i Step 4

Based on the positive output from Step 3b, the authors were more confident the BA application rate threshold for the BMW 330i could be isolated. Since substantial differences were seen during the tests performed with command module settings of 45/60 and 45/70, Step 4 testing

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consisted of one additional test performed with settings of 45/65. As shown in Figure A.1.6, comparison of data produced with command module settings of 45/60, 45/65, 45/70, and 45/99 revealed no significant differences between the 45/65, 45/70, and 45/99 tests. Each of these tests differed from the lesser 45/60 inputs by revealing much higher brake line pressures and vehicle deceleration. Therefore, the authors believe the BMW 330i BA displacement threshold was realized when command module settings of 45/65 were utilized. This corresponds to a quasi steady state application of 1.9 inches8, performed at 23.0 inches per second.

4.1.6 Chrysler 300C Displacement Feedback Threshold Determination

Figures A2.1 through A2.4, presented in Appendix A2, illustrate the iterations used to identify the displacement feedback-based BA activation threshold for the Chrysler 300C. In a manner similar to the process used for the BMW 330i, identifying this threshold required multiple attempts by the authors, although to a lesser extent. Where the BMW 330i required two Step 2 and 3 test series, the Chrysler 300C only needed the Step 2 series repeated. Results from both Step 2 attempts are discussed in this section. The first attempt is described as Step 2a, the second as Step 2b.

4.1.6.1 Chrysler 300C Step 1

The results produced during the Chrysler 300C’s Step 1 tests are shown in Figure A2.1. Given the substantial differences in brake line pressures and vehicle deceleration observed between the tests performed with command module settings of 40/99 and 50/99, and the 57.1 ft difference in stopping distance between these two tests, the authors surmised the displacement threshold magnitude resided within, or close to, an interval bounded by these two commanded inputs. Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.7. Stopping distances produced with command module settings of 50/99 through 99/99 differed by a maximum of 26.4 ft, however the distances produced with command module settings of 70/99 through 99/99 differed by a maximum of only 4.4 ft.

8 The brake applications observed in this study generally had some overshoot (even those that used the pedal displacement feedback control loops). In the case of the pedal displacement based tests, this overshoot was quickly dampened, typically followed by a period of constant pedal position until the vehicle had completed the stop. As used in this report, the term “quasi steady state” is used to describe the magnitude of the constant pedal position. These values do not describe or account for any overshoot.

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Figure 4.7. Stopping distance as a function of command module magnitude (Step 1, Chrysler 300C).

4.1.6.2 Chrysler 300C Step 2a

For the Chrysler 300C, Step 2a tests consisted of iterative adjustments to the displacement magnitude using command module settings of 49/99 to 41/99. Unlike the Step 2 tests performed with the BMW 330i, the adjustments performed with this vehicle used finer commanded displacement increments. This provided a Step 2a data set comprised of nine additional tests, rather than the single midpoint increment used during evaluation of the BMW 330i. Figure A2.2 presents the tests used for the Step 2a analysis of the Chrysler 300C.

Based on comparison of the Step 2a data to the trends in brake line pressure and vehicle deceleration observed during the Step 1 tests, the authors did not believe BA activation occurred during any of the Chrysler 300C tests performed in the Step 2a series. As such, additional tests were deemed necessary. A description of these additional tests is provided in Section 4.1.6.3.

4.1.6.3 Chrysler 300C Step 2b

The Chrysler 300C Step 2b tests featured iteratively adjusted displacement magnitudes using command module settings from 50/99 to 55/99. As was the case for the Step 2a tests, fine command module increments were used. Figure A2.3 presents the test output of the Step 2b tests.

Unlike the Step 2a tests performed with the Chrysler 300C, differences in brake line pressure and vehicle deceleration were observed during the Step 2b series, particularly between the tests performed with command module settings less than or equal to 52/99 and those using settings of 53/99 or more. These results implied a command module setting of 53/99 should provide the correct displacement magnitude threshold setting on the command module, however a second test was performed with the 53/99 settings for confirmation. Unlike the first 53/99 test, the authors do not believe the confirmation test was able to activate BA since the vehicle deceleration and brake line pressures of the repeated 53/99 test were in much better agreement

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with the Step 2b tests performed with lesser command module settings (i.e., those ranging from 50/99 to 52/99).

To minimize the potential for BA activation variability to confound the ability to isolate the application rate threshold in the subsequent tests performed with the Chrysler 300C, the authors decided the use of a conservative displacement magnitude setting would be prudent. For this reason, and the fact that BA intervention was believed to have occurred during tests performed with command module settings of 54/99 and 55/99, the displacement magnitude selected for use in Step 3 was taken to be “55.”


Figure A2.4 shows the incremental brake controller settings used to iteratively reduce application rate during Step 3 tests performed with the Chrysler 300C. Due to the substantial differences in brake line pressures and initial vehicle decelerations observed between the tests performed with command module settings of 55/50 and 55/60, the authors surmised the BA activation threshold resided within an interval bounded by these settings.

Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.8. Although noise in the data produced during the test performed with command module settings of 55/50 resulted in a small spike in stopping distance relative to those distances produced with 55/40 and 55/60 settings, the authors believe the 6.8 ft difference between the tests performed with command module settings of 55/50 and 55/60, and the subsequent stabilization of the distances achieved with larger commanded application rates, further supported the possibility of the vehicle’s application rate threshold being bounded by command module settings 55/50 and 55/60. Stopping distances produced with command module settings of 55/60 through 55/90 differed by a maximum of 1.1ft.

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In the case of the Chrysler 300C, no Step 4 tests were performed. This vehicle was the second evaluated in this study, and at the time these tests were performed it was not clear whether finer application rate increments were necessary (command module rate settings of “50” and “60” produced very similar application rates). Therefore, the Chrysler 300C BA pedal displacement threshold was best realized when a command module setting of 55/60 was utilized. This corresponds to a quasi steady state application of 2.0 inches, performed at 19.7 inches per second.

4.1.7 Cadillac STS Displacement Feedback Threshold Determination

Figures A3.1 through A3.6, presented in Appendix A3, illustrate the iterations used to identify the displacement feedback-based BA activation threshold for the Cadillac STS. In a manner similar to the process used for the Chrysler 300C, identifying this threshold required multiple attempts by the authors, comprised of two Step 2 iterations. Results from both of the Step 2 attempts are discussed in this section. The first attempt is described as Step 2a, the second as Step 2b.

Generally speaking, identifying the BA activations described in this report required the authors to rely solely on the test data output during the sequence of stops performed with a particular vehicle (e.g., brake line pressure, deceleration, stopping distances, etc.). However, in the case of the Cadillac STS, the vehicle also provided an aural cue that BA intervention had occurred. According to the vehicle’s BA supplier, the driver could expect to hear the ABS pump operate very early in the braking maneuver if BA was in operation9. Although this information was not directly recorded by the data acquisition system installed in the vehicle, the test driver recorded instances of audible ABS pump operation on log sheets maintained throughout each test series.

4.1.7.1 Cadillac STS Step 1

The results produced during the Cadillac STS Step 1 tests are shown in Figure A3.1. Note that the vehicle deceleration and brake line pressures produced with the 60/99 and 70/99 inputs were very close to those of the 99/99 input despite the much larger pedal application force and displacement values of the maximum input. Also apparent was that the vehicle deceleration and brake line pressures produced with the 50/99 input were much less than those of the other inputs presented in this figure. Finally, the driver log sheets indicated tests performed with command module settings of 60/99 or more contained pronounced ABS pump activity throughout the entire braking maneuver (although “some pump operation” was indicated for the test performed with a command module setting of 50/99).

Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.9. Note that there was a 20.3 ft difference between the stopping distances recorded during the tests performed with command module settings of 50/99 and 60/99. Use of larger command module magnitudes resulted in little further stopping distance reduction. Stopping

9 This is also reflected in the owner’s manual.

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Figure 4.9. Stopping distance as a function of command module magnitude (Step 1, Cadillac STS).

distances produced with command module settings of 60/99 through 99/99 differed by a maximum of 3.1 ft.

4.1.7.2 Cadillac STS Step 2a

For the Cadillac STS, Step 2a tests consisted of iterative adjustments to displacement magnitude using command module settings of 51/99 through 59/99. Data output from these tests are shown in Figure A3.2. As shown in this figure, differences in brake line pressure and vehicle deceleration were observed during this test series, particularly between the tests performed with command module settings less than or equal to 55/99 and those using settings of 56/99 or more. Additionally, the driver’s log sheets indicated tests performed with command module settings of 56/99 or more contained pronounced ABS pump activity throughout the entire braking maneuver. These results implied a command module setting of 56/99 should correspond to the correct displacement magnitude threshold for the Cadillac STS, and that the displacement magnitude selected for use in Step 3 should be “56.” However, to verify the ability of this setting to consistently activate BA, five additional 56/99 tests, one 57/99 test, and one 60/99 test were performed. Data output from these tests, tests that were performed five calendar days after the previously mentioned Step 2a series, are presented in Figure A3.3. For the sake of comparison, the 60/99 test performed as part of Cadillac STS Step 1 is also included in Figure A3.3.

Although the data output from the repeated 56/99 tests shown in Figure A3.3 are very consistent, the authors believe there is little indication BA intervention occurred during these tests, or during the test performed with the 57/99 settings. When compared to the data output from the 60/99 Step 1 tests, the deceleration magnitude and initial rate of change observed during the 56/99 and 57/99 tests were less, an effect likely due to the lower front brake line pressures and an absence of significant ABS activity. Furthermore, the driver did not detect the pronounced sound of ABS pump activity early in the maneuver (expected when BA was activated with this vehicle). These

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factors indicated 56/99 and 57/99 settings were not robust enough to be used for the Cadillac STS Step 3 and Step 4 tests.

Comparison of the two 60/99 tests shown in Figure A3.3 appear to indicate the displacement magnitude associated with a command module setting of “60” was closer to the BA activation threshold than previously believed (i.e., based on the Step 1 data alone). Although the repeat of the 60/99 test did not produce vehicle decelerations as large as those observed during Step 1, they were greater than those associated with the five repeated 56/99 tests and the single 57/99 test. While it is possible the higher brake line pressures of the repeated 60/99 test may be due to BA activation, the driver did not detect the pronounced sound of ABS pump activity early in the maneuver, implying such activation did not occur. This further supported the likelihood that inputs commanded by the 60/99 setting resided very near the vehicle’s BA activation threshold, since 60/99 tests performed during Step 1 test did produce the pronounced sound of ABS pump activity early in the maneuver, and the resulting brake line pressures, deceleration rate, and deceleration magnitudes (for a majority of the stop) were greater than the repeated 60/99 tests.

4.1.7.3 Cadillac STS Step 2b

Since the data output during the Step 2a tests did not indicate a consistent BA intervention threshold, a series of Step 2b tests were performed with larger pedal displacement magnitudes. Figure A3.4 presents data from key Step 2b tests performed with the Cadillac STS. For the sake of clarity, only data produced with command module settings of 60/99 to 65/99 are shown (i.e., the lower half of the Step 2b range). Overall, the data output during these tests were quite close, with the most notable differences, albeit relatively small, being in mid-maneuver brake line pressure and deceleration. This is where the tests performed with settings of 60/99 and 61/99 were generally less than those of the other tests. For this reason, a commanded pedal displacement setting of “62” was selected for use in the Step 3 test series.


Figure A3.5 shows the brake controller settings used to iteratively reduce application rate during Step 3 tests performed with the Cadillac STS. For the sake of clarity, only data produced from tests performed at the maximum rate (i.e., 62/99) and those near what was later determined to be the displacement-based intervention threshold rate are shown. Like those shown in Figure A3.3, the data produced with these command module settings were very similar. However, the brake pedal force data indicated differences existed between the tests performed with settings of 62/30 and 62/40, and those performed with larger commanded rates. While the 62/30 and 62/40 settings have lower initial force peaks, the pedal forces observed during a majority of the respective stops were higher than those of the tests performed with command module settings of 62/50 to 62/99. Other differences include higher initial brake line pressure peaks during the tests performed with inputs settings of 62/50 or more, an effect that resulted in earlier peak deceleration, albeit by a small margin. Based on these findings, the authors surmised the BA activation threshold resided within an interval bounded by the 62/40 and 62/50 tests.

Step 3 stopping distances observed as a function of command module application rate are provided in Figure 4.10. Note that while there was only a 1.9 ft difference between the stopping

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distances recorded during the tests performed with command module settings of 62/40 and 62/50, the stopping distances produced with command module settings of 62/40 through 62/99 differed by a maximum of 3.3 ft (influenced by the small dip in stopping distance observed during the 62/80 test; if this test was omitted, the range is reduced to only 1.1 ft).



Unlike the small iterative increments used to isolate the displacement threshold magnitude, only one increment was used for isolating the application rate. For the Cadillac STS Step 4 test series, this increment was 62/45, positioned midway between the 62/40 and 62/50 settings revealed in Step 3. Finer increments were not deemed necessary, as tests performed with the BMW 330i and Chrysler 300C indicated the brake controller was not very sensitive to changes in commanded rates until settings of “40” or less were specified.

The results observed during the Step 4 Cadillac STS tests are shown in Figure A3.6. Comparison of the data produced by the 62/40 and 62/45 tests revealed no substantive differences. However, both of these tests differed from those performed with 62/50 command module settings, where differences in the brake pedal force data (despite using nearly equivalent pedal displacement), higher front brake line pressure peaks, and higher vehicle deceleration were observed. Therefore, the authors concluded the Cadillac STS BA displacement threshold was best realized when a command module setting of 62/50 was utilized10. This corresponds to a quasi steady state application of 2.6 inches, performed at 18.6 inches per second.

10 Although the authors believe they were able to identify the displacement feedback-based activation threshold for the Cadillac STS, the presence of BA appeared to be less obvious than observed during tests performed with the other vehicles evaluated in this study.

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4.1.8 Toyota 4Runner Displacement Feedback Threshold Determination

Figures A4.1 through A4.4, presented in Appendix A4, illustrate the iterations used to identify the displacement feedback-based BA activation threshold for the Toyota 4Runner. Identifying this threshold required multiple attempts by the authors, including two Step 2 and three Step 4 evaluations. Results from each of these evaluation steps are discussed in this section. Adopting the labeling convention used earlier, the multiple Step 2 iterations are referred to as Steps 2a and 2b. Similarly, the multiple Step 4 iterations are referred to as Steps 4a, 4b, and 4c.

4.1.3.1 Toyota 4Runner Step 1

The results produced during the Toyota 4Runner’s Step 1 tests are shown in Figure A4.1. Given the substantial differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 40/99 and 50/99, and the 62.8 ft difference in stopping distance between these two tests, the authors surmised the displacement magnitude threshold resided within, or close to, an interval bounded by these two commanded inputs. The Toyota 4Runner Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.11. Stopping distances produced with command module settings of 50/99 through 99/99 differed by a maximum of 3.3 ft.

Figure 4.11. Stopping distance as a function of command module magnitude (Step 1, Toyota 4Runner).

4.1.8.2 Toyota 4Runner Step 2a

Step 2a tests performed with the Toyota 4Runner consisted of six fine adjustments to the displacement magnitude using command module settings ranging 50/99 to 45/99. Figure A4.2 presents the output of these tests.

In a manner very similar to the Step 2a tests performed with the Chrysler 300C, the brake line pressures and vehicle deceleration produced during the Step 2a Toyota 4Runner tests did not

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indicate BA activation, although they were expected to have done so. As such, additional tests were performed as described next in Section 4.1.8.3. Interestingly, there was a pronounced difference between the vehicle’s response to the commanded inputs of 50/99 used during Step 1 and Step 2a, an indication that the BA displacement threshold may reside very near that produced with a commanded displacement magnitude setting of “50.”

4.1.8.3 Toyota 4Runner Step 2b

In the case of the Toyota 4Runner, Step 2b tests featured iteratively adjusted displacement magnitudes bounded by command module settings from 50/99 to 55/99. As was the case for the Step 2a tests, fine command module increments were used. Figure A4.3 presents the test output of the Step 2b tests.

With the exception of the tests performed with command module settings of 51/99 and 52/99, the authors believe each of the six Step 2b tests exhibited signs of BA intervention, as evidenced by the high decelerations and pronounced ABS activity recorded at each wheel. Therefore, the authors cautiously surmised the activation threshold likely resided within a range of inputs bounded by command module settings 50/99 and 53/99.


Since the results presented in Step 2b indicated that accurately isolating the BA pedal displacement threshold might be difficult (highly variable outputs were being produced within a range of very similar inputs), the authors expected that multiple Step 3 and/or Step 4 iterations would be necessary. With that in mind, Step 3 began with the smallest application magnitude the authors believed was capable of activating BA (50/99). Test data produced during the sequence of Step 3 tests performed with the Toyota 4Runner are shown in Figure A4.4.

As shown in Figure A4.4, each test produced strong deceleration and pronounced ABS activity, even those performed with command module settings as low as 50/10. That said, the authors did not believe the results clearly revealed a BA activation threshold rate. Therefore, multiple Step 4 tests were performed: Steps 4a, 4b, and 4c.

Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.12. Stopping distances produced with command module settings of 50/40 through 50/99 differed by a maximum of 2.5 ft.

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4.1.8.5 Toyota 4Runner Step 4a

Step 4a was comprised of a comparison of four tests performed at moderate application rates. Two tests, those performed with commanded inputs of 50/50 and 50/40, were performed as part of the Step 3 sequence and were previously presented in Figure A4.4. The other two tests were new.

The Step 3 50/50 and 50/40 tests were selected for use in Step 4a due to the large differences observed in post-maneuver brake line pressure, where the 50/40 tests produced the highest of the Step 3 post-maneuver brake line pressures and the 50/50 tests produced nearly the lowest (second only to the 50/90 tests). For this reason, the authors surmised it was possible the application rate associated with the BA activation threshold could reside near that achieved by a 50/50 input.

With the Step 3 findings in mind, the authors performed two Step 4a specific tests. The 50/40 test was intended to assess the robustness of the 50/40 setting’s ability to repeatably activate BA. Similarly, the 50/35 test was performed to determine whether the threshold may reside lower, with a rate slightly less than that produced with a 50/40 setting.

Although the 50/40, 50/30, 50/20, and 50/10 tests performed in Step 3 all resulted in pronounced ABS activity during their respective stops, the 50/35 Step 4a test did not. The effect of this test’s low brake line pressures translated into a much lower deceleration rate than those produced by the other tests shown in Figure A4.5. The reasons for this are unclear, especially since the authors expected the lower Step 3 application rates (particularly those associated with the slow 50/20 and 50/10 inputs) to be less capable of eliciting BA intervention than the faster rates. Therefore, more tests were performed with commanded input rates between 50/40 and 50/50, as described as Step 4b tests in the next section.

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4.1.8.6 Toyota 4Runner Step 4b

Step 4b tests were performed to assess the ability of the 50/40 command module setting to repeatably activate BA with the Toyota 4Runner. Step 4b also included three tests performed with a command module setting of 50/45, and one with a commanded input of 40/45.

As shown in Figure A4.6, tests performed with the 50/40 inputs produced less ABS activity and lower post-maneuver brake line pressures than those performed with the 50/45 inputs. These tests also resulted in less deceleration. Interestingly, despite the use of identical brake controller stroke magnitudes, the amount of brake force recorded during the 50/40 test was less than that observed during the 50/45 test.

Despite the use of an application rate believed to have been capable of eliciting BA intervention during the 50/45 tests, the 40/45 command module setting produced brake line pressures and vehicle deceleration values much lower than the other Step 4b tests. These results indicated a larger displacement magnitude should be used during the Step 4c tests.

4.1.8.7 Toyota 4Runner Step 4c

The final Step 4 evaluation performed with the Toyota 4Runner, Step 4c, compared output from tests believed to be comprised of input commands very near the BA activation threshold. Each setting considered for analysis in Step 4c provided strong braking, as evidenced by the pronounced ABS activity and high vehicle deceleration shown in Figure A4.7.

While the authors were comfortable with the commanded displacement magnitude of “50,” the disparity in the effect of application rate shown with the Step 3, 4a, and 4b data made isolating the best application rate less certain. That said, each of these steps did indicate BA activation was possible with commanded application rates greater than or equal to “40.” Ultimately, the authors believed the Toyota 4Runner BA displacement threshold was best realized when command module settings of 50/50 were utilized. This corresponds to a quasi steady state application of 2.8 inches, performed at 26.9 inches per second. Although it is possible lower commanded application rates could be used successfully, the authors believe a setting of 50/50 offers the best combination of low application rate and ability to repeatably activate BA.

4.1.9 Volvo XC90 Displacement Feedback Threshold Determination

Figures A5.1 through A5.4, presented in Appendix A5, illustrate the iterations used to identify the displacement feedback-based BA activation threshold for the Volvo XC90. Of the five vehicles evaluated in this study, identifying the activation threshold of the Volvo XC90 was the most straightforward; it repeatably produced substantially different braking behavior when brake applications near the threshold were used.

4.1.9.1 Volvo XC90 Step 1

Test outputs produced during the Volvo XC90 Step 1 tests are shown in Figure A5.1. Given the substantial differences in brake line pressures and vehicle decelerations observed between the

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tests performed with command module settings of 40/99 and 50/99, and the 104.2 ft difference in stopping distance between these two tests, the authors surmised the BA displacement threshold magnitude resided within, or close to, an interval bounded by these two commanded inputs. Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.13. Stopping distances produced with command module settings of 50/99 through 99/99 differed by a maximum of 2.0 ft.

Figure 4.13. Stopping distance as a function of command module magnitude (Step 1, Volvo XC90).


For the Volvo XC90, Step 2 tests initially consisted of command module settings iteratively increased from 40/99 to 50/99. However, as shown in Figure A5.2, the resulting outputs of each of these tests were nearly equivalent. Therefore, an additional test was performed with a command module setting for 39/99.

Comparison of the brake line pressure and vehicle deceleration produced with a command module setting of 39/99 to those using settings of 40/99 or more produced profound differences. However, to minimize the potential risk of BA activation variability confounding the ability to isolate the application rate threshold in the Step 3 and 4 tests, the authors decided the use of a conservative displacement magnitude setting was desirable. For this reason, the displacement magnitude selected for use in Step 3 was taken to be “41,” one command module increment greater than the smallest able to produce BA intervention.


Figure A5.3 shows the brake controller settings used to iteratively reduce application rate during Step 3 tests performed with the Volvo XC90. Due to the substantial differences in brake line pressures and initial vehicle deceleration observed between the tests performed with command module settings of 41/60 and 41/70, and the 89.8 ft difference in stopping distance between these two tests, the authors surmised the BA activation threshold resided within an interval bounded by

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these settings. Step 3 stopping distances observed as a function of command module application rate are provided in Figure 4.14. Stopping distances produced with command module settings of 41/70 through 41/99 differed by a maximum of 0.5 ft.



For the Volvo XC90, Step 4 was comprised of three additional tests, performed with command module settings of 41/60, 41/65, and 41/70. Outputs from these tests are shown in Figure A5.4. Here, use of the 41/70 command module setting produced substantially higher brake line pressures and vehicle decelerations than were observed during the tests using settings of 41/60 and 41/65 (whose output were nearly equivalent). Therefore, the authors concluded the Volvo XC90 BA pedal displacement threshold was best realized when a command module setting of 41/70 was utilized. This corresponds to a quasi steady state application of 2.0 inches, performed at 23.9 inches per second.

4.2 Force Feedback Based Threshold Determination

Identifying the minimum combinations of brake pedal force and application rate capable of activating BA required iterative steps conceptually similar to those used to isolate the displacement feedback-based thresholds. Steps 1 and 2 were used to identify force threshold magnitudes. Step 3 was used to identify the application rate threshold.

The processes described in Section 4.2 document the specific manner in which the iterative adjustment of force magnitude and application rate was used to identify the BA activation threshold for each vehicle using the brake controller’s force-based control feedback loop. The driver made these adjustments via the command module interface with the brake controller.

The command module’s two rotary dials allowed the controller actuator force and application rate to be independently adjusted. As with the displacement feedback loop, the values specified

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on the command module refer to the percent of maximum controller capability, not to a specific pedal force magnitude or application rate. Consider, for example, the command module settings previously shown in Figure 4.3. When these settings were used in conjunction with the force feedback, the “amplitude” setting of “50” corresponded to a brake pedal force nominally equal to approximately 100 lbf, 50-percent of the controller’s maximum brake force capability of approximately 200 lbf. Similarly, the brake controller stroke rate shown in Figure 4.3 would nominally be 1500 lbf/sec, 75-percent of the nominal maximum stroke capability of 2000 lbf/sec.”

Note: When used in conjunction with the force feedback, the brake controller will attempt to achieve an application rate target specified in pounds of force per second (using the percentage of maximum capability indicated on the command module). However, since these units have little practical meaning, the authors provide descriptions of application rate magnitude in terms of inches per second in this report, calculated by using measured pedal displacement over time.

Although the force produced by the brake controller actuator for a given command module setting was consistent (i.e., as measured at the controller), differences in controller installation and brake pedal geometry among the test vehicles caused the force, measured at the brake pedal and acting perpendicular to its face, to be somewhat vehicle-dependent. To keep the descriptions of how the authors attempted to determine the BA activation thresholds as consistent as possible, the processes described in Section 4.2 indicate what command module settings were used throughout the various steps. The physical descriptions of the brake pedal force magnitudes (pounds) and application rates (expressed in inches per second, rather than pounds per second) are not provided until the end of each vehicle’s respective section.

4.2.1 Determining Pedal Force Magnitude Thresholds (Step 1 of 3)

Step 1 was to iteratively decrease pedal force from large to small while maintaining a constant commanded application rate. Beginning with a command module setting of 99/99, application force magnitude was first reduced from 99 to 90. Generally speaking, subsequent force magnitude reductions were performed using incremental reductions of “10” on the command module until a setting of 10/99 had been utilized.

Once the entire sweep of pedal force magnitudes had been performed, the subsequent data were reviewed and inspected for trends believed to indicate the presence of BA intervention using three evaluation considerations. This process was used to narrow the range of commanded pedal forces down to an interval bounded by two inputs spaced 10 command module increments apart. Specifically, the three evaluators were as follows:

Stopping Distance. Large differences in stopping distance resulting from small changes to the brake controller command module settings were believed to provide evidence that the vehicle was being evaluated near a BA intervention threshold.

Longitudinal Acceleration. Pronounced differences in peak and/or overall deceleration during the braking event resulting from small changes to the brake controller command module settings were believed to provide evidence that the vehicle was being evaluated

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near a BA intervention threshold. Deceleration magnitude, rise time, and ability to maintain high deceleration during the stop duration were important characteristics.

Brake Line Pressure. To achieve the shortest possible stopping distances, the brake pedal inputs should be great enough to activate each vehicle’s respective antilock brake system (ABS) during the stop. These interventions were detected by observing the brake line pressure of each wheel circuit. Of particular interest was determining whether small changes to the brake controller command module settings result in pronounced differences in the peak brake line pressure magnitude before, during, and/or after ABS activation.

4.2.2 Determining Pedal Force Magnitude Thresholds (Step 2 of 3)

Within the reduced range of commanded magnitudes output from Step 1, additional tests were performed using finer increments. Beginning with the upper bound of a desired range, the command module settings were typically decreased by increments of one. Using these finer command module increments, in conjunction with the three evaluation considerations specified in Step 1, attempts were made to identify the pedal force magnitude believed to best represent the BA activation threshold.


Once the pedal force magnitude threshold had been determined, the command module was used to perform an iterative reduction of commanded force rates. The process was conceptually equivalent to that used in Step 1, with the primary difference being that the application magnitude was fixed at the BA force threshold while the application rates were varied.

To begin Step 3, the force magnitude was set to the value believed to best coincide with the vehicle’s BA activation threshold and the commanded rate set to the maximum. Rate magnitudes were then iteratively reduced from 99 to 90. Subsequent rate magnitude reductions were performed with incremental reductions of “10” on the command module until a rate setting of “10” had been utilized. For most vehicles, the final rate magnitude reduction lowered the commanded setting from “10” to “5.” Once the entire sweep of application rate magnitudes had been executed, the subsequent data were reviewed and inspected for trends believed to indicate the presence of BA intervention using the three evaluation considerations presented in Section 4.2.1.

In contrast to the data output by Step 3 during the displacement feedback tests, the authors do not believe the combination of Step 3 with force feedback was able to successfully identify application rate intervals containing BA thresholds for any of the vehicles evaluated in this study. Generally speaking, large differences in commanded application rate did not produce profound differences in the vehicle responses used to indicate the presence of BA intervention— a trend that was in contrast to that observed when displacement feedback was used.

Note that since the coarse increments used in the Step 3 iterative reduction of application rate failed to identify the BA activation thresholds when force feedback was used, the authors

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forewent use of the finer application rate increments. For this reason, no Step 4 tests were performed in conjunction with force feedback.

4.2.4 BMW 330i Force Feedback Threshold Determination

This section provides a detailed account of how Steps 1, 2, and 3 were used in the attempts to identify the BA pedal force threshold of the BMW 330i. These descriptions are supplemented by Figure A1.7 through A1.9, presented in Appendix A1, which provide an illustrative account of key vehicle responses collected during these steps. In a manner consistent with that previously used in Section 4.1, each figure presents data collected from eight channels:

Vehicle speed Longitudinal acceleration Brake pedal force (measured at the brake pedal) Brake pedal displacement Brake line pressures measured at each of the four brake calipers

4.2.4.1 BMW 330i Step 1

Test outputs produced during the iterative adjustment of pedal force magnitude are presented in Figure A1.7. As shown in this figure, tests performed with very low pedal force applications, particularly those performed with command module settings of 5/99 and 10/99, produced substantially lower brake line pressures and vehicle decelerations than did the tests performed with settings of 20/99 and higher. Interestingly, use of command module settings equal to or greater than 20/99 produced very consistent brake line pressures and decelerations during the respective stops, including pronounced ABS activity until the vehicle stopped.

Due to the differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 10/99 and 20/99, and the 14.0 ft difference in stopping distance between these two tests, the authors surmised the BA activation threshold magnitude resided within, or close to, an interval bounded by these two commanded inputs. The BMW 330i Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.15. Stopping distances produced with command module settings of 20/99 through 99/99 differed by a maximum of 3.2 ft.

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Figure 4.15. Stopping distance as a function of command module magnitude (Step 1, BMW 330i).

4.2.4.2 BMW 330i Step 2

Step 2 tests performed with the BMW 330i used an iterative reduction of command module magnitudes from 15/99 to 10/99. In this test series, each brake application produced rear ABS activity, however only one (i.e., when the command module was set to 15/99) appeared to produce some slight ABS activity at the front of the vehicle, particularly at the left front wheel. Overall, the vehicle decelerations produced during tests performed with command module settings of 13/99, 14/99, and 15/99 were quite consistent and notably higher than those produced with the lesser 10/99 and 11/99 commanded inputs. The output of the Step 2 tests performed with the BMW 330i are presented in Figure A1.8.

Ultimately, the authors selected the force application magnitude achieved with a command module setting of “15” for use in Step 3. Note that since the BMW 330i was the first vehicle evaluated for the work described in this report, and some of the later, more refined threshold isolation techniques used for the other vehicle had not yet been formalized. For this reason, Step 2 only consisted of five tests for this vehicle. In the case of the Toyota 4Runner and Volvo XC90, Step 2 tests performed with the force feedback control loop used a more complete suite of individual trials.

4.2.4.3 BMW 330i Step 3

Figure 4.44 shows the brake controller settings used to iteratively reduce application rates during Step 3 tests performed with the BMW 330i. Unlike the results of the displacement feedback- based tests described in Section 4.1, substantial changes in application rate had little effect on the vehicle’s braking response, although the tests performed with command module settings of 15/80, 15/90, and 15/99 did produce some ABS activity during the later stages of their respective stops. Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.16. Stopping distances produced with command module settings of 15/05 through 15/99 differed by a maximum of only 5.0 ft.

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Figure 4.16. Stopping distance as a function of command module magnitude (Step 3, BMW 330i).

Based on the outcome of Step 3, presented in Figure A1.8, the authors believed the BMW 330i BA activation threshold was best realized when a command module setting of 15/99 was used with force feedback. This corresponds to a quasi steady state application of 27.8 lbf and 2.1 inches, performed at 15.0 in/sec. Note that the application rate of 15.0 in/sec is less than half of the brake controller’s maximum capability, as described in previous sections. This was due to a brake controller limitation experienced when small force magnitudes were used in conjunction with high application rates. This limitation, and the influence the authors believe it had on the results of this study, is discussed in Section 4.3. However, since the application rate was 34.8 percent lower than that required to realize the BA activation threshold during the displacement feedback-based tests with this vehicle, the authors question whether the BA activation threshold of the BMW 330i can be accurately identified using force feedback-based control logic. This point is further discussed in Chapter 5 where, using the threshold-based inputs, the braking performance of the vehicle achieved with BA is compared to that observed with the system disabled.

4.2.5 Chrysler 300C Force Feedback Threshold Determination

This section provides a detailed account of how Steps 1, 2, and 3 were used in the attempts to identify the BA pedal force threshold of the Chrysler 300C. These descriptions are supplemented by Figures A2.5 through A2.7, presented in Appendix A2, which provide an illustrative account of key vehicle responses collected during these steps.


Test outputs produced during the iterative adjustment of pedal force magnitude are presented in Figure A2.5. As with the BMW 330i, tests performed with very low pedal force, particularly those performed with command module settings of 5/99 and 10/99, produced substantially lower brake line pressures and vehicle decelerations than did the tests performed with settings of 20/99 and higher. Furthermore, use of command module settings equal to or greater than 20/99 also

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produced very consistent brake line pressures and decelerations during the stops performed with the Chrysler 300C, including pronounced ABS activity until the vehicle stopped.

Due to the differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 10/99 and 20/99, and the 47.0 ft difference in stopping distance between these two tests, the authors surmised the BA activation threshold resided within, or close to, an interval bounded by these two commanded inputs. The Chrysler 300C Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.17. Stopping distances produced with command module settings of 20/99 through 99/99 differed by a maximum of 6.1 ft.



Step 2 tests performed with the Chrysler 300C used an iterative reduction of command module magnitudes from 19/99 to 15/99. In this test series, ABS activity and mid-maneuver vehicle decelerations were more pronounced during tests performed with command module settings of 17/99 and 19/99, although the pressure modulation was more apparent during the 19/99 test, as shown in Figure A2.6. Both tests resulted in a stopping distance of 80.6 ft. The fact that a command module setting of 18/99 did not produce vehicle responses in agreement with those observed during the 17/99 and 19/99 tests (specifically the brake line pressures, vehicle deceleration profiles, and a 5.9 ft longer stopping distance) seems to indicate that while 17/99 may produce brake pedal applications closest to the lowest BA activation threshold, test variability may not necessarily allow for BA activation to be repeatably realized. Since BA activation repeatability is an attribute important to the comparative analyses discussed in Chapter 5, the authors conservatively selected the force application magnitude achieved with a command module magnitude of “19” for use in Step 3.

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Figure A2.7 shows the brake controller settings used to iteratively reduce application rates during Step 3 tests performed with the Chrysler 300C. Unlike the results of the displacement feedback tests described in Section 4.1, substantial changes in application rate had little effect on the stopping distance, although some changes in mid-maneuver brake line pressure and vehicle decelerations were observed. Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.18. Stopping distances produced with command module settings of 19/05 through 19/99 differed by a maximum of 7.4 ft, whereas those produced with command module settings 19/10 through 19/99 differed by a maximum of only 3.6 ft.


The authors believe the Chrysler 300C BA activation threshold was best realized when a command module setting of 19/99 was used with force-feedback. This corresponds to a quasi steady state application of 37.8 lbf and 3.1 inches, performed at 19.2 in/sec. Although the maximum application rate of 19.2 in/sec realized during the 19/99 tests was inhibited by the brake controller limitation previously described in Section 4.2.4.3, this rate was only 2.5 percent lower than the 19.7 in/sec required to realize the BA activation threshold during the displacement feedback based tests performed with this vehicle. For this reason, best determining whether the BA activation threshold had been accurately established required comparison of tests performed with and without the vehicle’s BA enabled. A discussion of these tests is provided in Chapter 5.

4.2.6 Cadillac STS Force Feedback Threshold Determination

This section provides a detailed account of how Steps 1, 2, and 3 were used in the attempts to identify the BA pedal force threshold of the Cadillac STS. These descriptions are supplemented by Figures A3.7 through A3.9, presented in Appendix A3, to provide an illustrative account of key vehicle responses collected during these steps.

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Recall that in the case of the Cadillac STS, the vehicle’s BA supplier indicated that the driver can expect to hear the ABS pump operating very early in the braking maneuver if a BA intervention had occurred. Although this information was not directly recorded by the data acquisition system installed in the vehicle, the test driver did record incidents of audible ABS pump operation on log sheets maintained between each individual trial.


Test outputs produced during the iterative adjustment of brake pedal force magnitudes are presented in Figure A3.7. Consistent with the previous vehicles, the Cadillac STS tests performed with very low pedal force, particularly those performed with command module settings of 5/99 and 10/99, produced substantially lower brake line pressures and vehicle decelerations than did the tests performed with settings of 20/99 and higher. Furthermore, use of command module settings equal to or greater than 20/99 also produced similar ABS activity and vehicle decelerations during the respective stops.

The differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 10/99 and 20/99, and the 14.3 ft difference in stopping distance between these two tests, seemed to indicate the activation threshold magnitude resided within, or close to, an interval bounded by these two commanded inputs. That said, although Figure A3.7 clearly indicates the vehicle’s ABS was modulating brake line pressures during the test performed with brake controller command module settings of 20/99, the driver did not hear the “tell-tale” sound of the ABS pump motor precharging the vehicle’s brake system at the onset of the maneuver. As indicated by the BA supplier of the Cadillac STS, this would imply the system did not activate during the 20/99 test. However, when the next application increment of 30/99 was applied, the sound of the ABS pump motor precharging the vehicle’s brake system was detected. Therefore, although the brake line pressure and vehicle deceleration data indicated the Step 2 tests should be comprised of commanded inputs within a interval bounded by the 10/99 and 20/99 tests, inclusion of the driver’s auditory cues implied an interval between the 20/99 and 30/99 tests was more appropriate.

The Cadillac STS Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.19. Stopping distances produced with command module settings of 20/99 through 99/99 differed by a maximum of 2.5 ft. Stopping distances produced with command module settings of 30/99 through 99/99 differed by a maximum of 2.4 ft.

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Step 2 tests performed with the Cadillac STS used an iterative reduction of command module magnitudes from 29/99 to 21/99. The Step 2 test outputs are presented in Figure A3.8. In this test series, brake line pressures, ABS activity, and vehicle deceleration were quite consistent, revealing little distinction between the individual test outputs (although some small differences early in the maneuver were observed during the 21/99 test). In fact, the only distinguishing characteristic present during the Cadillac STS Step 2 tests was seen in the brake pedal travel data, where pedal displacements observed during the tests performed with command module settings between 21/99 and 25/99 were in good agreement, differing (as a group) from the larger displacements produced with command module settings between 26/99 and 29/99. Assuming these differences were attributable to BA activation, the results seemed to indicate the potential for the vehicle’s activation threshold magnitude to be realized when a command module of setting of 26/99 was used. For this reason, the authors selected this force application magnitude for use in Step 3.


Figure A3.9 shows the brake controller settings used to iteratively reduce application rates during Step 3 tests performed with the Cadillac STS. Unlike the results of the displacement feedback tests described in Section 4.1, substantial changes in application rate had little effect on the vehicle’s overall response to the brake applications. Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.20. The entire range of stopping distances produced with command module settings of 26/05 through 26/99 differed by a maximum of only 2.8 ft.

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The authors believed the Cadillac STS BA activation threshold was best realized when a command module setting of 26/99 was used with force-feedback. This corresponds to a quasi steady state application of 40.4 lbf and 3.3 inches, performed at 19.1 in/sec. Interestingly, this rate was 2.7 percent greater than the 18.6 in/sec application rate associated with the displacement feedback-based threshold; a relationship not seen for the other vehicles discussed in this report11. That said, the authors still believed the final step in determining whether the BA activation threshold had been accurately established required comparing outputs from tests performed with the vehicle’s BA enabled and disabled. A discussion of these tests is provided in Chapter 5.

4.2.7 Toyota 4Runner Force Feedback Threshold Determination

This section provides a detailed account of how Steps 1, 2, and 3 were used in the attempts to identify the BA pedal force threshold of the Toyota 4Runner. These descriptions are supplemented by Figures A4.8 through A4.10, presented in Appendix A4, to provide an illustrative account of key vehicle responses collected during these steps.


Test outputs produced during the iterative adjustment of brake pedal force magnitudes are presented in Figure A4.8. Consistent with the previous vehicles, the Toyota 4Runner tests conducted with very low pedal force applications, particularly those performed with brake controller command module settings of 5/99 and 10/99, produced substantially lower brake line pressures and vehicle decelerations than did the tests performed with settings of 20/99 and higher. Furthermore, use of command module settings equal to or greater than 20/99 also

11 Although the minimum application rate believed to activate BA with the Cadillac STS was greater with force feedback than for displacement feedback, it is important to realize the magnitudes of these rates were actually quite close. The Cadillac STS was one of three vehicles for which the threshold application rate was essentially identical (i.e., within 3.7 percent) to that determined from displacement feedback.

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produced pronounced front and rear ABS activity and very consistent decelerations during the respective stops.

Due to the differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 10/99 and 20/99, and the 25.7 ft difference in stopping distance between these two tests, the authors surmised the BA activation threshold resided within, or close to, an interval bounded by these two commanded inputs. The Toyota 4Runner Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.21. Stopping distances produced with command module settings of 20/99 through 99/99 differed by a maximum of only 1.8 ft.



Step 2 tests performed with the Toyota 4Runner used an iterative reduction of command module magnitudes from 19/9912 to 10/99. The Step 2 test outputs are presented in Figure A4.9. In this series, the brake line pressures and vehicle decelerations produced during tests performed with command module settings of 10/99, 11/99, and 12/99 differed considerably from those observed during tests performed with higher commanded magnitudes. These differences, and the fact that the stopping distance achieved with a command module setting of 13/99 was 10.1 ft shorter than that produced with the lesser setting of 12/99, suggested a commanded magnitude of “13” was near the Toyota 4Runner’s BA activation threshold when force feedback was utilized. As such, this magnitude was chosen for use in Step 3.


Figure A4.10 shows the brake controller settings used to iteratively reduce application rates during Step 3 tests performed with the Toyota 4Runner. Unlike the results of the displacement

12 The 19/99 data file collected during the Toyota 4Runner evaluation was found to be corrupt during processing, and is therefore not shown in Figure A4.9.

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feedback based tests described in Section 4.1, substantial changes in commanded application rate resulted in only minor changes in the vehicle’s overall response to the brake applications, although the effect of application rate on the Toyota 4Runner’s stopping distance was the most apparent of the vehicles evaluated in this study. Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.22. Stopping distances produced with command module settings of 13/05 through 13/99 differed by a maximum of 10.9 ft, whereas those distances produced with command module settings of 13/10 through 13/99 differed by a maximum of 6.3 ft.


The authors believe the BA activation threshold for the Toyota 4Runner was best realized with a command module setting of 13/99 when force feedback was used. This corresponds to a quasi steady state application of 28.0 lbs and 3.2 inches, performed at 25.9 in/sec. For this vehicle, the application rate associated with the 13/99 input using force feedback was identical to that needed to activate BA during the displacement feedback based tests described in Section 4.1.8. That said, the authors continue to believe that accurately determining whether the BA activation threshold had been ascertained requires comparing tests performed with and without the vehicle’s BA enabled, as discussed in Chapter 5.

4.2.8 Volvo XC90 Force Feedback Threshold Determination

This section provides a detailed account of how Steps 1, 2, and 3 were used in the attempts to identify the BA pedal force threshold of the Volvo XC90. These descriptions are supplemented by Figures A5.5 through A5.7, presented in Appendix A5, to provide an illustrative account of key vehicle responses collected during these steps.


Test outputs produced during the iterative adjustment of brake pedal force magnitudes are presented in Figure A5.5. Consistent with the previous vehicles, the Volvo XC90 tests performed with very low pedal force applications, particularly those performed with a brake

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controller command module setting of 10/99, produced substantially lower brake line pressures and vehicle deceleration than did the tests performed with settings of 20/99 and higher. Furthermore, use of command module settings equal to or greater than 20/99 also produced considerable front and rear ABS activity and very consistent vehicle decelerations during the respective stops.

Due to the differences in brake line pressures and vehicle decelerations observed between the tests performed with command module settings of 10/99 and 20/99, and the 26.8 ft difference in stopping distance between these two tests, the authors surmised the BA activation threshold resided within, or close to, an interval bounded by these two commanded inputs. The Volvo XC90 Step 1 stopping distances observed as a function of command module magnitude are provided in Figure 4.23. Stopping distances produced with command module settings of 20/99 through 99/99 differed by a maximum of only 3.4 ft.



The Step 2 test outputs are presented in Figure A5.6. In the case of the Volvo XC90, these tests used an iterative reduction of command module magnitudes from 20/99 to 10/99. As observed during the Step 2 tests performed with the Toyota 4Runner, the brake line pressures and vehicle decelerations produced during tests performed with command module settings of 15/99 or less differed from the responses observed during tests performed with higher commanded magnitudes. Additionally, while the test performed with a command module setting of 16/99 produced substantial ABS activity at each corner of the vehicle, the overall vehicle deceleration produced during this test was lower than that realized with larger command module magnitudes. These observations provided some indication that a commanded magnitude of “17” best coincided with the Volvo XC90 BA activation threshold when the brake controller’s force feedback control logic was utilized, despite the fact the stopping distance observed during the 16/99 test was only 2.6 ft shorter than that achieved by the 17/99 command module setting (recall the consistency of the stopping distances shown in Figure 4.23).

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Figure A5.7 shows the brake controller settings used to iteratively reduce application rates during Step 3 tests performed with the Volvo XC90. In agreement with the trend established by other vehicles evaluated in this study, substantial changes in commanded application rate had a negligible effect on the vehicle’s overall response to the force feedback-based brake applications. Step 3 stopping distances observed as a function of command module magnitude are provided in Figure 4.24. Stopping distances produced with command module settings of 19/05 through 19/99 differed by a maximum of only 5.4 ft with the Volvo XC90.


The authors believe the Volvo XC90 BA activation threshold was best realized when a command module setting of 17/99 was used with force feedback. This corresponds to a quasi steady state application of 32.0 lbf and 3.2 inches, performed at 22.0 in/sec. Although the maximum application rate of 22.0 in/sec realized during the 17/99 tests was inhibited by the brake controller limitation previously described in Section 4.2.4.3, this rate was only 7.9 percent lower than the 23.9 in/sec required to realize the BA activation threshold during the displacement feedback tests performed with this vehicle. For this reason, determining whether the BA activation threshold had been accurately established required comparing tests performed with the vehicle’s BA enabled and disabled. A discussion of these tests is provided in Chapter 5.

4.3 Brake Assist Threshold Summary

The processes described in Sections 4.1 and 4.2 were designed to evaluate how two brake controller feedback loops, those based on a constant commanded pedal displacement or pedal force, can be used to identify BA application thresholds. In Section 4.1, the authors endeavored to identify the minimum combinations of pedal displacement magnitude and rate of application capable of activating BA. Section 4.2 describes how the authors attempted to isolate the minimum combinations of pedal force and rate of application capable of activating BA. Tables

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4.1 and 4.2 summarize key input and output data pertaining to theses processes. Comparison of the data in these tables provides insight into some interesting trends.

4.3.1 Application Rate

With the exception of the Cadillac STS13, the application rates of the tests believed to best coincide with the BA activation thresholds were higher when the brake controller’s displacement feedback was used. This was in spite of the fact that all force feedback-based results presented in Table 4.2 were performed with the brake controller configured to output it’s maximum application rate. However, the extent to which these differences were in evidence depended strongly on the vehicle being considered. Overall these differences ranged from -0.5 to 8.0 in/sec (-2.7 to 34.8 percent).

Why was the maximum application rate realized during the force feedback tests generally less than that produced during the displacement feedback tests? Recall that for both feedback loops, Steps 1 and 2 were used to identify the minimum commanded input magnitudes believed to be capable of activating BA. When force feedback was used, the command module settings believed to best coincide with these activation thresholds were of low magnitude, ranging from “13” for the Toyota 4Runner to “26” for the Cadillac STS. This ultimately proved to be a problem for the brake controller, as it was generally unable to realize maximum application rate capability when low force magnitudes were commanded, as shown in Figure 4.25. Note that in this figure, all tests were performed with maximum commanded application rate (i.e., “99” was specified on the brake controller’s command module).

Figure 4.25. Brake pedal application rate as a function of commanded brake controller magnitude (force feedback).

13 The application rates believed to be most applicable to the Cadillac STS brake assist thresholds were 18.6 in/sec (displacement feedback) and 19.1 in/sec (force feedback). Therefore, the application rate associated with the displacement feedback loop was 0.5 in/sec, or 2.7 percent, less that that used by the force feedback loop.

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Table 4.1. Brake Assist Activation Threshold Information (Pedal Displacement Control Feedback).

Vehicle

Command Module Settings Brake Pedal

Rate (inches/sec)

Nominal Brake Pedal

Displacement1

(inches)

Peak Brake Pedal Force

(lbf)

Nominal Brake Pedal Force (lbf) Peak

Longitudinal Acceleration

(g)

Stopping Distance

(ft)Magnitude Rate Static1 Dynamic2

2006 BMW 330i 45 65 23.0 1.9 85.7 13.1 7.0 -1.152 72.7

2005 Chrysler 300C 55 60 19.7 2.0 100.0 28.0 19.9 -0.997 82.3

2004 Cadillac STS 62 50 18.6 2.6 111.2 26.9 18.0 -1.136 72.6

2004 Volvo XC90 41 70 23.9 2.0 45.5 6.8 1.1 -1.009 76.9

2003 Toyota 4Runner 50 50 26.9 2.8 65.9 32.7 43.6 -1.033 78.1

1Measured after the vehicle had completed the stop, so as to avoid any confounding effect of ABS activity. 2Measured over a brief interval that began 250 ms after peak brake force was achieved, and was concluded when vehicle speed reached 2.5 mph.

Table 4.2. Brake Assist Activation Threshold Information (Pedal Force Control Feedback).

Vehicle

Command Module Settings Brake Pedal

Rate (inches/sec)

Nominal Brake Pedal

Displacement1

(inches)

Peak Brake Pedal Force

(lbf)

Nominal Brake Pedal

Force1

(lbf)

Peak Longitudinal Acceleration

(g)

Stopping Distance

(ft) Magnitude Rate

2006 BMW 330i 15 99 15.0 2.1 34.1 27.8 -1.167 68.0

2005 Chrysler 300C 19 99 19.2 3.1 44.8 37.8 -1.069 80.1

2004 Cadillac STS 26 99 19.1 3.3 47.6 40.4 -1.080 70.3

2004 Volvo XC90 17 99 22.0 3.2 38.8 32.0 -1.074 73.8

2003 Toyota 4Runner 13 99 25.9 3.2 34.5 28.0 -1.009 80.0

1Measured after the vehicle had completed the stop, so as to avoid any confounding effect of ABS activity (comparable to the “static” measurements shown in Table 4.1)

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Figure 4.26. Brake pedal application rate as a function of commanded brake controller magnitude (displacement feedback).

Incidentally, the phenomenon seen in Figure 4.25 also occurred when displacement feedback was utilized. However, since the command module magnitudes associated with the BA activation thresholds were higher during the displacement feedback-based tests (the settings ranged from 41 to 62), the brake controller had more time to successfully achieve the desired application rates. Figure 4.26 presents the brake pedal application rate as a function of commanded brake controller magnitude for the displacement feedback loop.

4.3.2 Nominal Brake Pedal Displacement

In Tables 4.1 and 4.2, the “Nominal Brake Pedal Displacement” column contains brake pedal displacements measured after the vehicle had completed the stop. Although these values do not necessarily represent the position of the brake pedal during the respective stops14, they do provide a coarse way to compare the average amount of pedal displacement associated with what was believed to be the vehicles’ respective BA activation thresholds. The authors consider these data to be of interest since they quantify one of the two primary application magnitudes used by the driver (with the second being application force). The interaction of how much and how fast the driver must displace the brake pedal, and the application forces associated with different application strategies, provide better insight into what “minimum” brake applications are necessary to realize the benefits offered by BA activation.

For each vehicle evaluated in this study, the nominal brake pedal displacements observed during tests performed at the BA activation thresholds were greater when the force feedback control loop was used. The extent to which these differences were in evidence was vehicle-dependent. Overall differences of 0.2 to 1.2 inches (10.3 to 60 percent) were observed.

14 Brake pedal position was modulated by the brake controller to maintain constant pedal force during the force feedback tests.

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Note: Due to the conceptual differences inherent to the displacement and force feedback loops, these differences are not surprising. In order for the force feedback control loop to function (i.e., be able to achieve a commanded force magnitude), the brake pedal must be displaced enough to provide some resistance. In other words, compliance in the brake system (pedal free play, bringing the brake pads in contact with the brake rotors, etc.) had to be overcome before the brake pedal was able to support a reaction force counter to that applied by the brake controller.

4.3.3 Brake Pedal Force

Table 4.1 contains three columns pertaining to brake pedal force: “Peak Brake Pedal Force” and two “Nominal Brake Force” categories. The authors believe this breakdown is appropriate because the application forces associated with the BA activation thresholds observed during the displacement feedback tests were ultimately found to be comprised of three stages: (1) a brief spike of high pedal force, (2) a period of lesser pedal force, often comprised of force modulation (i.e., periods of high and low forces were commonly observed during this time), and (3) a period of constant force, where the vehicle had come to a stop but the brake controller was still applying force to the pedal to maintain constant pedal position.

Since Table 4.2 contains data collected during tests performed with force feedback, the applied force realized early in the maneuver would, ideally, be expected to remain consistent throughout the stop. For this reason, Table 4.2 only contains two columns of brake force data: (1) peak brake force to quantify brake force application overshoot, and (2) nominal brake force data, recorded after the vehicles had completed their respective stops.

4.3.3.1 Peak Brake Pedal Force

On a per-vehicle basis, peak brake pedal forces observed during tests believed to be associated with the respective BA activation thresholds differed substantially, although the extent to which these differences were in evidence was vehicle-dependent. When considering the peak brake force values in Tables 4.1 and 4.2, it is helpful to understand how the peaks are produced and when they occur. In the case of the displacement feedback tests, the brake pedal had to be rapidly accelerated from the zero position to a target magnitude. This required a brief period of high force be used until the target magnitude had been realized. The peak force observed during a particular stop occurred during this period. In the case of the force feedback tests, peak force was used to describe the brake force overshoot present after the force target was first satisfied.

For the vehicles evaluated in this study, the peak brake pedal forces observed during stops performed at the BA activation threshold with displacement feedback ranged from 45.5 to 111.2 lbf. These peak values were 6.7 to 63.6 lbf (14.7 to 60.2 percent) greater than comparable peaks produced with force feedback (i.e., when results from the same vehicles were compared).

Peak brake force data is important because, when combined with the nominal brake forces discussed in Section 4.3.3.2, the data indicate that although reduced pedal displacement and lower sustained brake pedal force can be associated with stops performed at the BA activation threshold, actually achieving the threshold criteria appears to require brief periods of sufficiently

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high application rate and pedal force. This is an important distinction since previous NHTSA research has indicated drivers do not typically apply high pedal forces [14].

For this reason, the authors anticipate that the output of the NHTSA/VTTI human factors work described in Section 1.1.2 will be extremely important, as the study should provide data demonstrating whether actual drivers are capable of applying the combinations of pedal displacement, application rate, and high initial force believed to be necessary to realize the shorter stopping distances provided by BA activation.

4.3.3.2 Nominal Brake Pedal Force (Static Data)

In a manner conceptually similar to the data presented in the “Nominal Brake Pedal Displacement” columns of Tables 4.115 and 4.2, the “Nominal Brake Pedal Force” column contains brake forces measured after the vehicles had completed their respective stops. Although these values do not necessarily represent the forces applied to the brake pedal during the stops16, they do provide a coarse way to compare the pedal forces associated with what was believed to be the vehicles’ respective BA activation thresholds.

With the exception of the Toyota 4Runner, the nominal brake pedal force observed during the displacement feedback threshold stops was 9.8 to 25.2 lbf (35.0 to 370.6 percent) less than comparable pedal forces applied during force feedback tests. Conversely, the nominal brake pedal force applied during the displacement feedback tests performed with the Toyota 4Runner were 4.7 lbf (14.4 percent) greater than the force feedback test performed at what was believed to the vehicle’s BA activation threshold.

4.3.3.3 Nominal Brake Pedal Force (Dynamic Data)

In the case of the displacement feedback tests, brake pedal force was automatically modulated by the brake controller, as necessary, to keep pedal displacement constant during each stop. As previously discussed in Section 4.1, the application forces during the stop often provided clues as to whether BA activation had occurred during the stop. In Table 4.1, the “dynamic” force data was measured over a brief interval that began 250 ms after peak brake force was achieved and was concluded when vehicle speed reached 2.5 mph.

The dynamic force data shown in Table 4.1 were not only found to be much less than the peak values for each vehicle, but were less than the later static values for four of the five vehicles evaluated in this study. For these vehicles, the dynamic force values were 44.4 to 93.2 lbf (80.1 to 97.6 percent) less than the peak values, and 5.7 to 8.9 lbf (28.9 to 83.8 percent) less than the static values. The only exception observed was the Toyota 4Runner, whose dynamic force data was 22.3 lbf (33.8 percent) less than the peak value, but was 10.9 lbf (33.3 percent) greater than

15 In Table 4.1, Nominal Brake Pedal Force has been broken down into two sub-columns containing “static” and “dynamic” data. The static data were collected after the vehicle had come to a complete stop and are directly comparable to the data shown in Table 4.2. The dynamic values represent the average forces measured during the brake stops. The dynamic force values are discussed in Section 4.3.3.3. 16 Brake pedal force was modulated by the brake controller to maintain constant pedal position during the displacement feedback tests.

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the static value. Also noteworthy are the very low dynamic force magnitudes realized with the BMW 330i (7.0 lbf) and Volvo XC90 (1.1 lbf). Although the reasons for why these forces were so low is not fully understood, the authors believe the interaction between the constant pedal position commanded by the displacement feedback loop and the behavior of vehicles’ respective brake pedals during the brake assist enhanced stops are suspect. For example, if a byproduct of BA activation is a subsequent increase in pedal displacement, the force required to maintain a constant pedal position would be reduced. In an extreme case where the pedal is being pulled away from the commanded position (i.e., towards the floor), it is possible that the force required to maintain the desired position could be negative; the load cell providing the measurement would be in tension rather than compression.

The dynamic force data shown in Table 4.1 was of interest for the comparative tests discussed in Chapter 5, where the dynamic forces observed during tests performed at the BA activation threshold with BA enabled are compared with forces recorded during the same tests performed with BA disabled. Consideration of these forces is believed to provide some insight as to whether BA intervention can actually reduce the amount of force required by the driver to maintain high deceleration and ultimately produce shorter stopping distances. Comparison of the static and dynamic forces shown in Table 4.1 provided some indication this may be the case.

4.3.4 Longitudinal Acceleration (Vehicle Deceleration)

4.3.4.1 Peak Longitudinal Acceleration

The authors included peak vehicle deceleration in Tables 4.1 and 4.2 as a coarse way of comparing the severity of the brake stops performed during applications believed to be most associated with the vehicles’ respective BA activation thresholds. Although informative, the authors caution the reader that these values represent single data points collected at one instant in time and are not necessarily indicative of the average deceleration magnitude produced during a particular stop.

Overall, the peak deceleration values produced with the two feedback loops at the BA activation threshold were quite similar. In the case of the BMW 330i, Chrysler 300C, and Volvo XC90, the peak decelerations produced during the displacement feedback tests were only 0.02 to 0.07g (1.3 to 7.2 percent) less than those produced during the force feedback tests. Conversely, the peak decelerations produced during the displacement feedback tests performed with the Cadillac STS and Toyota 4Runner were 0.02 to 0.06g (2.3 to 4.9 percent) greater than those produced during the force feedback tests.

4.3.4.2 Overall Longitudinal Acceleration

Figures 4.27 through 4.31, presented at the end of this chapter, compare tests performed with displacement and force feedback using inputs believed to reside at the BA activation thresholds. These comparisons, shown on a per-vehicle basis, clearly demonstrate the profound differences in brake pedal displacement and application force associated with the two feedback loops. Despite these differences, the overall vehicle deceleration profiles produced with the Cadillac STS, Toyota 4Runner, and Volvo XC90, at their respective displacement and force feedback

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thresholds, were nearly equivalent (i.e., on a per-vehicle basis). A similar trend was observed with results from the BMW 330i and Chrysler 300C, however some divergence of the vehicle's respective deceleration profiles was observed midway through the stops.

4.3.5 Stopping Distance

Overall, the stopping distances associated with the displacement feedback thresholds were similar to those produced during the force feedback tests. Discounting the Toyota 4Runner, use of force feedback resulted in stopping distances 2.2 to 4.7 ft (2.7 to 6.5 percent) shorter than those achieved by the same vehicles during the displacement feedback tests. Conversely, the displacement feedback tests performed with the Toyota 4Runner resulted in stopping distances 1.9 ft (2.4 percent) longer than that achieved when force feedback was used.

That stopping distances produced by the two control feedback loops were in seemingly good agreement was encouraging. As one would expect, if each application strategy were to realize the minimum BA activation threshold then comparable stopping distances should be produced. However, to truly assess how BA activation affected the vehicles’ braking capability, additional tests were required. The outputs of these tests, including comparative analyses of tests performed with BA enabled and disabled, are discussed in Chapter 5.

4.3.6 The Need for Comparative Tests

To better determine whether the BA thresholds were truly realized, additional comparisons were deemed necessary, specifically those performed with BA enabled and disabled. If the thresholds were actually realized during the work described in Section 4.1 and 4.2, significant differences in stopping distance would be expected. Also of interest is comparing the stopping performance of the vehicles operating at the BA activation threshold with the maximum capability of the vehicles. These comparisons are presented in Chapter 5.

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Figure 4.27. BMW 330i brake assist threshold comparison. Data used to calculate static nominal forces, dynamic nominal forces, and nominal pedal displacements are shown in black, green, and cyan, respectively.

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Figure 4.28. Chrysler 300C brake assist threshold comparison. Data used to calculate static nominal forces, dynamic nominal forces, and nominal pedal displacements are shown in black, green, and cyan, respectively.

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Figure 4.29. Cadillac STS brake assist threshold comparison. Data used to calculate static nominal forces, dynamic nominal forces, and nominal pedal displacements are shown in black, green, and cyan, respectively.

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Figure 4.30. Toyota 4Runner brake assist threshold comparison. Data used to calculate static nominal forces, dynamic nominal forces, and nominal pedal displacements are shown in black, green, and cyan, respectively.

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Figure 4.31. Volvo XC90 brake assist threshold comparison. Data used to calculate static nominal forces, dynamic nominal forces, and nominal pedal displacements are shown in black, green, and cyan, respectively.

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5.0 BRAKE ASSIST EFFECTIVENESS COMPARISIONS

In Chapter 4, the authors explained how the BA activation thresholds associated with the brake controller’s displacement and force feedback loops were identified. All tests performed in the activation threshold determination phase of this study were performed with BA enabled. The tests discussed in Chapter 5, however, were performed with BA enabled and disabled. The data produced during these tests were used to quantify BA effectiveness. In the context of this report, effectiveness was quantified via a series of stopping distance comparisons.

The analyses discussed in this chapter are broken down into two components: (1) comparison of tests performed with BA enabled and disabled at the vehicles’ respective BA activation thresholds, and (2) comparison of tests performed at the vehicles’ respective BA activation thresholds to tests performed with inputs equal to the maximum capability of the brake controller.

The first two sections of this chapter discuss results from tests performed with displacement feedback. The third section presents results from tests performed with force feedback. Within each section, results from the straight line stops performed from 45 and 65 mph, and the brake in-a-turn tests performed from 45 mph, are described individually.

Tests performed with brake applications believed to represent the vehicles’ respective BA activation thresholds were nominally repeated ten times per condition (i.e., with BA enabled and disabled). For those tests performed with maximum braking, five tests per condition were performed. To assess whether differences in mean stopping distance were statistically significant, the data were analyzed with SAS software using a generalized linear model (GLM).

In the tables presented in Sections 5.1 through 5.3, standard deviation is provided as a measure of stopping distance variability about the mean for a particular test series. A small standard deviation is indicative of stopping distance consistency. Although standard deviations are not directly compared or discussed in this report, they were used by SAS (as variance), in conjunction with the means and group sample sizes, to create the p-values presented in the tables shown in this chapter. No overlap in the ranges and small standard deviations will invariably lead to highly significant p-values. In the context of this report, p-values were used to quantify whether differences in mean stopping distances produced by two test series were statistically significant and if so, to what extent. The following is provided to assist the reader in relating terms that use “significance” to a value produced by SAS.

By definition, a variable is considered “significant” if it has a p ≤ 0.05. This four-place decimal will appear under the column heading “Pr > F”.

As used frequently in the descriptions of the data, a variable is said to be “highly significant” if it has a p ≤ 0.0009. A p-value within this range indicates a high degree of confidence that the mean stopping distances associated with the variables being compared are different.

P-values within the range 0.05 < p ≤ 0.10 were taken to be “marginally insignificant.”

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Beyond the value of p 0.1000, where there is only 90 percent probability that the means of the variable are different, the variable is said to be “insignificant.” Any similar phrase, such as “was not significant,” should be interpreted likewise.

In addition to the p-values and simple statistics, the tables in this chapter also contain the variable name used by SAS to test for significance. For the tests conducted using only the BA activation threshold, the variable name used was BA, and it had two conditions: Disabled and Enabled. For the tests using two application strategies, the variable name used was Apply, and it had these two conditions: Max and Threshold.

5.1 Brake Assist Effectiveness at the Activation Threshold (Displacement Feedback)

All tests described in this section were performed with brake applications believed to be at the vehicles’ respective BA activation thresholds. These tests were performed using displacement feedback.

5.1.1 Straight Line Stopping Distances from 45 mph – Brake Assist Enabled vs. Disabled

5.1.1.1 Chrysler 300C

When BA was disabled, the average stopping distance for the Chrysler 300C from 45 mph was 99.6 ft. When BA was enabled, the average stopping distance was reduced to 87.4 ft. The p-value associated with the difference between the stopping distance means is <.0001, shown in Table 5.1. This is a highly significant result and indicates that the means are undeniably different. In other words, BA was responsible for reducing the mean stopping distance of the Chrysler 300C by 12.2 ft under the circumstances of this test series.

In addition to quantifying the significance of the different means, Table 5.1 also presents the ranges of stopping distances recorded during each BA configuration test series. In the case of the Chrysler 300C, there was no overlap of the individual stopping distances. As shown in this table, the longest stopping distance produced when BA was enabled was 92.1 ft, whereas the shortest stopping distance observed with BA disabled was 95.9 ft. For the sake of brevity, the presence of this trend in later sections will be reported as “there was no overlap in the stopping distances.”

Table 5.1. Chrysler 300C Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph).

300C Threshold Braking, Straight Line Stops From 45 mph (ft)

BA N Mean Std Dev Minimum Maximum Pr > F

Disabled 12 99.6 3.25 95.9 105.4 <.0001

Enabled 12 87.4 2.77 82.3 92.1

5.1.1.2 BMW 330i

For the BMW 330i, the mean stopping distances with BA disabled versus enabled were 87.1 and 78.8 ft, respectively. The p-value associated with the difference between these means is 0.0141,

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shown in Table 5.2. While the magnitude of this p-value is greater than that previously presented for the Chrysler 300C, it still indicates the mean stopping distances produced with the BMW 330i’s BA were significantly different than those without. Therefore, for the straight line tests performed at 45 mph with the BMW 330i, the data indicate BA contributed to an average stopping distance reduction of 8.3 ft.

Table 5.2. BMW 330i Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph).

330i Threshold Braking, Straight Line Stops From 45 mph (ft)


Disabled 10 87.1 1.54 83.8 89.4 0.0141

Enabled 10 78.8 9.48 67.6 95.7

Unlike the stopping distances observed during tests performed with the Chrysler 300C, there was substantial overlap of the BMW 330i stopping distances produced with BA enabled and disabled. In fact, the entire range of stopping distances produced during tests performed with BA disabled was contained within that of the enabled range. Those results were surprising and provided an indication of brake output disparities present when operating the BMW 330i at its BA activation threshold. Use of these inputs produced shorter and longer stopping distances than those produced with BA disabled, despite the nearly equivalent brake inputs. Figure 5.1 clearly illustrates the stopping distance overlap observed for this vehicle.

451

457

453

465

Figure 5.1. Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph.

In an attempt to better understand why such a large range of stopping distances was produced during the BMW 330i BA enabled tests, outputs from the four tests highlighted in Figure 5.1 were plotted in a manner similar to that used in Chapter 4. In Figure 5.2, the authors believe the

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outputs of Test 453 indicated the presence of BA activation. During this test, brake line pressures and vehicle deceleration were high and ABS modulation was observed. The reduction of brake pedal force following an initial application pulse was in agreement with the trends previously presented in Figures 4.8, 4.9, and 4.10.

Although Test 451 was performed with BA enabled, and initially produced brake line pressures and vehicle deceleration nearly identical to those of Test 453, brake line pressure was released midway through the maneuver, resulting in a pronounced reduction of deceleration and increased stopping distance (longer than any test performed with disabled BA in this test condition).

Finally, comparison of Tests 457 (BA enabled) and 465 (BA disabled) revealed nearly equivalent outputs, indicating BA intervention was not realized during Test 457.

Figure 5.2. Test outputs produced during four straight line stops performed from 45 mph with the 2006 BMW 330i.

The stopping distance data shown in Figure 5.1 and the overall output data shown in Figure 5.2 indicate the brake applications selected by the authors for use in these tests resided at the BMW 330i BA activation threshold. However, use of these inputs produced large output disparities, most likely due to the BA activation not being realized during each test performed in the enabled configuration. In general, when the averaging of long and short stopping distances reduces the difference in mean stopping distances, and data overlaps, statistical certainty tends to be lowered.

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5.1.1.3 Cadillac STS

When BA was disabled, the mean stopping distance for the Cadillac STS was 72.0 ft. When the system was enabled, the mean stopping distance was reduced slightly to 70.5 ft. The p-value associated with the difference of the stopping distance means is 0.0083, as shown in Table 5.3. This indicates the 1.5 ft stopping distance reduction can be attributed to BA activation.

Table 5.3. Cadillac STS Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph).

STS Threshold Braking, Straight Line Stops From 45 mph (ft)


Disabled 11 72.0 1.27 69.9 74.3 0.0083

Enabled 11 70.5 1.13 68.1 72.1

Like those of the BMW 330i, Cadillac STS tests performed with BA enabled produced a range of stopping distances that overlapped those produced during the disabled system tests, albeit to a lesser extent. This overlap, in conjunction with the small mean difference, implies braking the Cadillac STS with inputs believed to be at the vehicle’s BA activation threshold will produce stopping distances that are, practically speaking, quite similar regardless of whether the system is enabled or disabled. Figure 5.3 presents the range of stopping distances observed for the Cadillac STS.

1119/1120 1132

1130

Figure 5.3. Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph.

To better understand why the overlap of stopping distances shown in Figure 5.3 was observed, key outputs from the four Cadillac STS tests highlighted in Figure 5.3 were plotted. In Figure 5.4, the extremes of the BA enabled group (Tests 1130 and 1132) were compared to two tests

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that produced stopping distances close to the disabled BA series mean (Tests 1119 and 1120). Based on review of these data, the authors believe the outputs of Tests 1130 and 1132 do indeed indicate the presence of BA activation, although only the reduction of brake pedal force following the initial application pulses and differences in the initial peak vehicle deceleration were particularly apparent.

Figure 5.4. Test outputs produced during four straight line stops performed from 45 mph with the 2005 Cadillac STS.

5.1.1.4 Toyota 4Runner

When BA was disabled, the mean stopping distance produced with the Toyota 4Runner was 101.9 ft. When BA was enabled, the mean stopping distance was reduced to 79.4 ft. The p-value associated with the difference of the stopping distance means is <.0001, as shown in Table 5.4. This is a highly significant result that indicates the 22.5 ft stopping distance reduction realized in this comparison can be attributed to BA intervention. Unlike the comparison of straight line stops performed from 45 mph with the BMW 330i and Cadillac STS, there was no overlap in the range of stopping distances produced during the tests performed with BA enabled versus disabled.

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Table 5.4. Toyota 4Runner Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph).

4Runner Threshold Braking, Straight Line Stops From 45 mph (ft)


Disabled 6 101.9 5.63 96.0 111.7 <.0001

Enabled 10 79.4 3.63 76.5 89.1

5.1.1.5 Volvo XC90

When BA was disabled, the average stopping distance for the Volvo XC90 was 166.8 ft. When BA was enabled, the average stopping distance was substantially reduced to 76.5 ft. The p-value associated with the difference between the stopping distance means is <.0001, shown in Table 5.5. This is a highly significant result that indicates the 90.3 ft stopping distance reduction is attributable to BA intervention. As with the Toyota 4Runner, there was no overlap in the range of stopping distances produced during the tests performed with BA enabled versus disabled.

Table 5.5. Volvo XC90 Straight Line Stopping Distance Summary (Displacement Feedback; 45 mph).

XC90 Threshold Braking, Straight Line Stops From 45 mph (ft)


Disabled 10 166.8 5.66 157.5 177.0 <.0001

Enabled 10 76.5 2.46 74.6 82.5

5.1.2 Brake In-A-Turn Stopping Distances from 45 mph – Brake Assist Enabled vs. Disabled

In a manner similar to that presented in Section 5.1.1, this section discusses results from stops performed with BA enabled and disabled initiated at 45 mph. However, the tests discussed in this section utilized the brake in-a-turn maneuver, previously described in Section 2.1.


When BA was disabled, the mean stopping distance for the Chrysler 300C was 106.9 ft. When the system was enabled, the mean stopping distance was reduced to 103.4 ft. The p-value associated with the difference in stopping distance means is 0.0193, as shown in Table 5.6. This is a significant result that indicates the 3.5 ft stopping distance reduction realized in this comparison can be attributed to BA intervention.

Table 5.6. Chrysler 300C Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).

300C Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 8 106.9 2.90 103.6 111.4 0.0193

Enabled 11 103.4 2.97 95.7 106.5

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Unlike the straight line stops performed at 45 mph, the Chrysler 300C brake in-a-turn tests performed with BA enabled and disabled exhibited substantial stopping distance overlap. Figure 5.5 presents these data. Note that the shortest stopping distance performed with BA disabled (103.6 ft) was nearly equivalent to the average distance produced of the enabled test series (103.4 ft).

273

268 247

271

Figure 5.5. Stopping distances observed during brake in-a-turn stops performed with the Chrysler 300C from 45 mph.

In a manner consistent with that used in Section 5.1.1, key outputs from the four tests highlighted in Figure 5.5 are presented in Figure 5.6 to examine why the stopping distance overlap occurred. In this figure, results from three tests performed with BA enabled (i.e., the longest, shortest, and closest to the series mean) are compared to the disabled BA test that produced a stopping distance closest to the enabled series mean.

In Step 3 of the threshold determination discussion previously presented in Section 4.1.6.4, the authors observed that the BA enabled tests performed with BA in operation exhibited different brake line pressures and vehicle deceleration profiles than those for which activation did not occur. In addition to these differences, shown in Figure A2.4, the brake pedal force characteristics also differed. Specifically, once the peak force used to establish the target pedal position had subsided, the force required to maintain the target pedal position was lower during tests with BA activation for much of the stop. Looking to Figure 5.6, a similar trend is revealed. In this figure, the brake pedal force for each test performed with BA enabled revealed lower application forces for much of the respective braking events.

Another indication of differences between the tests performed with BA enabled versus disabled in Figure 5.6 are the brake line pressure data, specifically those present at the left front wheel17.

17 Since the vehicle was turning to the right during each brake in-a-turn maneuver, the left front wheel was the outside front. This wheel received the highest normal forces during the braking events.

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The pressures of the tests performed with BA enabled were greater than those of the disabled system tests for much of the maneuver. Interestingly, the effect of the elevated brake line pressures was not particularly apparent in the vehicle deceleration data presented in Figure 5.6, as the overall decelerations associated with the BA enabled tests were quite similar to those produced during the disabled system tests.

Figure 5.6. Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2005 Chrysler 300C.

5.1.2.2 BMW 330i

For the BMW 330i, mean stopping distances with BA disabled and enabled were 107.5 and 109.1 ft, respectively. The p-value associated with this difference is 0.2714, as shown in Table 5.7 and indicates the 1.6 ft difference between the two means is not significant. That said, the fact the mean stopping distance of the BA enabled tests was greater than that of the disabled system tests was surprising and required additional investigation as to why this phenomenon was observed.

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Table 5.7. BMW 330i Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).

330i Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 107.5 2.93 101.4 109.9 0.2714

Enabled 10 109.1 3.29 103.9 114.0

The brake in-a-turn tests performed with the BMW 330i produced substantial stopping distance overlap. Four of the ten tests performed with BA enabled produced stopping distances longer than the longest stopping distance observed with the system disabled. Figure 5.7 illustrates the overlap observed for this vehicle.

602

564 611

610

Figure 5.7. Stopping distances observed during brake in-a-turn stops performed with the BMW 330i from 45 mph.

In an attempt to better understand the large stopping distance overlap and to determine why some of the BA enabled test series had longer stopping distances than the disabled, outputs from the four tests highlighted in Figure 5.7 were compared.

Based on the results of this comparison, shown in Figure 5.8, the authors question whether BA was activated during the enabled test series. Despite producing the shortest stopping distance with BA enabled, the vehicle deceleration produced during Test 564 was nearly identical to that observed during Test 602, a BA disabled test that had lower overall brake line pressures and a stopping distance very near the disabled series mean.

Given the nearly identical steering inputs, brake applications, and maneuver entrance speeds, the source of the stopping distance differences presented in this section remains in question. The authors believe test output variability, and the ability to repeatably activate BA at the brake application taken to represent the BA activation threshold, are two probable explanations.

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Figure 5.8. Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2006 BMW 330i.


For the Cadillac STS, the mean stopping distance with BA disabled was 100.4 ft. With BA enabled, the mean stopping distance was slightly less at 100.3 ft. As shown in Table 5.8, the p-value associated with the difference between these means is 0.7610, indicating the 0.1 ft difference between the two means is not significant.

Table 5.8. Cadillac STS Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).

STS Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 100.4 1.03 98.9 102.05 0.7610

Enabled 10 100.3 1.53 96.7 102.15

Like the straight line stops performed with the Cadillac STS, there was substantial overlap of the stopping distances produced with BA enabled and disabled. However, the overlap of the brake in-a-turn data was more pronounced. In fact, nearly the entire range of brake in-a-turn stopping

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distances produced during the BA enabled series was within the range defined by the disabled system tests. Figure 5.9 shows that only one BA enabled test resulted in a stopping distance shorter than those of the disabled series.

1174

1167 1176

1171 1180

Figure 5.9. Stopping distances observed during brake in-a-turn stops performed with the Cadillac STS from 45 mph.

To better understand why the Cadillac STS stopping distances shown in Figure 5.9 overlapped, key outputs from the five highlighted tests were plotted in Figure 5.10. In this figure, the two tests associated with the shortest BA enabled stops (Test 1171 and 1174) are compared to two tests that produced stopping distances close to their respective series means (Tests 1167 and 1176), and to the test producing the shortest BA disabled stop (Test 1180).

Of the data presented in Figure 5.10, the BA enabled Tests 1171 and 1174 produced test outputs markedly different than the others. Lower brake pedal force (after the initial spike needed to rapidly achieve the target pedal position), higher front brake line pressures, and greater vehicle deceleration were all in evidence during these tests. In contrast, BA enabled Test 1167 produced test outputs nearly identical to those of the disabled system tests, including that which produced the shortest stopping distance of the disabled system series. The authors believe the similarity of these outputs indicate it is unlikely BA activated during Test 1167. Furthermore, since the vehicle deceleration of Test 1167 is representative of the remaining BA enabled tests (i.e., other than Tests 1171 and 1174), the authors do not believe BA activation occurred during eight of the ten tests in this series.

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Figure 5.10. Test outputs produced during four brake in-a-turn stops performed from 45 mph with the 2005 Cadillac STS.


When BA was disabled, the average stopping distance for the Toyota 4Runner was 133.0 ft. When BA was enabled, the average stopping distance was substantially reduced to 83.9 ft. The p-value associated with this difference in stopping distance means is <.0001, shown in Table 5.9. This is a highly significant result and indicates the 49.1 ft stopping distance reduction realized in this comparison can be attributed to BA intervention. There was no overlap of the individual stopping distances produced during the tests performed with BA enabled versus disabled.

Table 5.9. Toyota 4Runner Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).

4Runner Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 9 133.0 21.2 96.2 160.3 <.0001

Enabled 10 83.9 2.91 80.5 91.4

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5.1.2.5 Volvo XC90

When BA was disabled, the mean stopping distance produced with the XC90 was 201.7 ft. When BA was enabled, the mean stopping distance was substantially reduced to 103.7 ft. The p-value associated with this difference in stopping distance means is <.0001, as shown in Table 5.10. This is a highly significant result and indicates the 98.0 ft reduction in average stopping distance can be attributed to BA intervention. Like the Toyota 4Runner, there was no overlap of the individual stopping distances during the Volvo XC90 tests performed with BA enabled versus disabled, despite the broad range of stopping distances observed during the BA enabled test series.

Table 5.10. Volvo XC90 Brake In-A-Turn Stopping Distance Summary (Displacement Feedback; 45 mph).

XC90 Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 201.7 5.7 195.0 213.9 <.0001

Enabled 10 103.7 30.8 88.2 172.0

5.1.3 Straight Line Stopping Distances from 65 mph – Brake Assist Enabled vs. Disabled

In Section 5.1.1, straight line stopping distances performed from 45 mph with BA enabled and disabled were compared. The straight line stopping distance comparisons in this section are based on tests initiated at 65 mph.


When BA was disabled, the mean stopping distance for the Chrysler 300C was 176.4 ft. When the system was enabled, the mean stopping distance was reduced to 165.4 ft. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.11. This is a significant result, indicating that the 11.0 ft reduction in average stopping distance can be attributed to BA intervention.

Table 5.11. Chrysler 300C Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph).



Disabled 11 176.4 2.73 172.0 182.0 <.0001

Enabled 13 165.4 7.01 158.3 176.0

Unlike the straight line stops initiated at 45 mph with the Chrysler 300C, the 65 mph BA enabled and disabled stops produced overlapping stopping distances. These data, whose overlap resembled that produced during the brake in-a-turn test series, are presented in Figure 5.11.

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207

215 220

Figure 5.11. Stopping distances observed during straight line stops performed with the Chrysler 300C from 65 mph.

Key outputs from the tests highlighted in Figure 5.11 are presented in Figure 5.12 to examine why the stopping distance overlap occurred. These tests represent the shortest BA enabled stop, the shortest stop performed with BA disabled, and the BA enabled stop closest to the shortest BA disabled stop.

The test outputs shown in Figure 5.12 reveal clear differences between the two featured BA enabled tests. Furthermore, when compared to the outputs of the shortest disabled BA stop, these data indicate it is unlikely BA activation was realized for each stop performed with BA enabled. Indeed, the brake pedal force, brake line pressure, and vehicle deceleration data traces of Test 215 (BA enabled) and Test 220 (BA disabled) were nearly identical. Conversely, Test 207 (the shortest stop of the enabled series) produced notably higher brake line pressures and vehicle decelerations, despite requiring substantially less brake application force for much of the stop. The authors believe BA activation is responsible for the stronger braking produced during this test.

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Figure 5.12. Test outputs produced during three straight line stops performed from 65 mph with the 2005 Chrysler 300C.

5.1.3.2 BMW 330i

For the BMW 330i, the mean stopping distances with BA disabled and enabled were 200.1 and 169.1 ft, respectively. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.12. The magnitude of the p-value is highly significant and indicates BA activation was responsible for the 31.0 ft difference between the two stopping distance means. In contrast to the stops initiated at 45 mph, there was no overlap of the individual stopping distances produced during the BMW 330i tests performed with BA enabled and disabled from 65 mph.

Table 5.12. BMW 330i Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph).



Disabled 10 200.1 2.41 197.6 204.9 <.0001

Enabled 12 169.1 13.60 145.7 191.6

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When BA was disabled, the mean stopping distance for the Cadillac STS was 142.9 ft. When the system was enabled, the mean stopping distance was reduced to 140.7 ft. The p-value associated with the difference in the stopping distance means is 0.0028, as shown in Table 5.11. This is a significant result, indicating that the 2.2 ft stopping distance reduction realized in this comparison can be attributed to BA intervention.

Table 5.13. Cadillac STS Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph).



Disabled 10 142.9 1.34 140.2 144.6 0.0028

Enabled 10 140.7 1.48 139.0 143.3

Similar to the tests performed at 45 mph, the straight line, BA enabled stops initiated at 65 mph produced stopping distances that overlapped those produced when the system was disabled. The overlap of the 65 mph tests, in conjunction with the small mean difference, suggests that braking the Cadillac STS with the inputs taken to represent the vehicle’s BA activation threshold (using displacement feedback) will produce stopping distances that are, practically speaking, quite similar regardless of whether the system is enabled or disabled. Figure 5.13 presents the range of stopping distances observed for the Cadillac STS.

1153 1143

1144

Figure 5.13. Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph.

Key outputs from the tests highlighted in Figure 5.13 are presented in Figure 5.14 to examine why the stopping distance overlap occurred. In this figure, results from two tests performed with BA enabled are compared to one performed with disabled BA.

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Figure 5.14. Test outputs produced during three straight line stops performed from 65 mph with the 2005 Cadillac STS.

Overall, the data shown in Figure 5.14 present trends in agreement with those observed during Cadillac STS stops performed in the other test conditions. Most of the test outputs reveal little discrimination among the tests presented in this figure; brake line pressures indicate similar magnitudes and ABS activation for each test, and vehicle deceleration is very similar throughout the stop regardless of whether BA was enabled or disabled (although the initial peak vehicle deceleration was slightly greater during the tests performed with BA enabled). That said, once the initial application force peak used to establish the target pedal position had subsided, the brake application forces realized with BA enabled were lower (overall) than those observed with the system disabled for much of the stop. As previously discussed, the authors believe this characteristic is indicative of BA activation for many of the vehicles evaluated in this study, including the Cadillac STS.


When BA was disabled during the straight line stops initiated at 65 mph, the mean stopping distance produced with the Toyota 4Runner was 229.7 ft. When BA was enabled, the mean stopping distance was reduced to 161.2 ft. The p-value associated with the difference in average stopping distance is 0.0002, as shown in Table 5.14. This is a highly significant result and indicates the 68.5 ft stopping distance reduction realized in this comparison can be attributed to

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BA intervention. Unlike the straight line stops initiated at 45 mph, there was slight overlap of the individual stopping distances produced during the Toyota 4Runner tests performed with BA enabled and disabled.

Table 5.14. Toyota 4Runner Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph).



Disabled 10 229.7 43.17 176.0 300.0 0.0002

Enabled 10 161.2 14.58 145.3 181.3

5.1.3.5 Volvo XC90

For the Volvo XC90, the mean stopping distances with BA disabled and enabled were 331.0 and 153.6 ft, respectively. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.15. This is a highly significant result and indicates the substantial 177.4 ft stopping distance reduction realized in this comparison can be attributed to BA intervention. In agreement with the straight line stops initiated at 45 mph, there was no overlap of the individual stopping distances produced during the Volvo XC90 tests performed with BA enabled and disabled.

Table 5.15. Volvo XC90 Straight Line Stopping Distance Summary (Displacement Feedback; 65 mph).



Disabled 10 331.0 10.95 317.9 348.3 <.0001

Enabled 10 153.6 1.78 150.4 156.1

5.2 Brake Application Comparisons (Brake Assist Enabled, Displacement Feedback)

Quantifying the enabled versus disabled braking performance of the test vehicles at the BA activation threshold was one of the primary objectives of work documented in this report. However, comparing the stopping ability achieved with BA enabled at the activation threshold versus that achieved with maximum pedal displacement (i.e., inputs intended to realize the maximum braking capability of the vehicle) was also of interest.

In this section, the statistical significance between maximum and threshold-based stopping distances is used to evaluate how braking performance differed between the two application techniques. For the tables in this section, a small difference between the means, or a numerically higher p-value (insignificant), implies braking the vehicle with the inputs taken to represent the BA activation threshold are capable of attaining the vehicle’s maximum braking capability.

As previously explained in Sections 4.1, “maximum” braking was achieved by setting the brake controller command module to 99/99. With displacement feedback, this setting commanded the

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maximum stroke and application rate from the brake controller. Nominally, these values were 5 inches and 30 in/sec, respectively.

5.2.1 Straight Line Stops from 45 mph – Activation Threshold vs. Maximum Displacement


With maximum pedal displacement, the mean stopping distance for the Chrysler 300C was 76.6 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 87.4 ft. The p-value associated with the difference of the stopping distance means is <.0001, as shown in Table 5.16. This is a highly significant result, indicating that differences in the two application techniques were responsible for a 10.8 ft difference in stopping distance. Note that the Table 5.16 data shows no overlap of the stopping distance ranges produced with the two brake application techniques.

Table 5.16. Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

300C Max versus Threshold, Straight Line Stops From 45 mph (ft)

Apply N Mean Std Dev Minimum Maximum Pr > F

Max 5 76.6 1.15 74.9 77.6 <.0001

Threshold 12 87.4 2.77 82.3 92.1

5.2.1.2 BMW 330i

For the BMW 330i, using maximum pedal displacement produced a mean stopping distance of 67.0 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 78.8 ft. The p-value associated with the difference in stopping distance means was 0.0170, as shown in Table 5.17. This is a significant result, indicating that differences in the two application techniques were responsible for the 11.8 ft difference in stopping distance.

Table 5.17. BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

330i Max versus Threshold, Straight Line Stops From 45 mph (ft)


Max 5 67.0 0.87 65.8 67.8 0.0170

Threshold 10 78.8 9.48 67.6 95.7

As shown in Table 5.17, the BMW 330i tests performed with maximum and threshold-based application techniques produced a range of stopping distances that had slight overlap. Figure 5.15 presents the individual stopping distances observed during these tests.

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536

533

459

453

Figure 5.15. Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph using two application techniques.

As previously described in Section 5.1.1.2, the 45 mph straight line tests performed with displacement feedback and BA disabled produced stopping distances that were completely within the range established by the enabled system tests (recall Figure 5.1). In this comparison, the combination of brake output disparity and the inability of displacement threshold-based applications to repeatably activate BA were believed to be responsible for the inconsistent stopping distances produced with the BA enabled tests. Unfortunately, the same inconsistency that confounded the BA enabled versus disabled comparison made in Section 5.1.1.2 also affects the maximum versus threshold-based braking comparison discussed in this section.

Key outputs from the tests highlighted in Figure 5.15 are presented in Figure 5.16 to examine the stopping distance overlap. In this figure, results from the shortest and longest tests produced with maximum pedal displacement (Tests 533 and 536, respectively) are compared to two of the better performing displacement feedback-based threshold tests (Tests 453 and 459). The stopping distances produced within the range of tests bounded by Tests 453 and 459 are believed to be the result of BA activation. The similarity of the stopping distances produced within this range and those achieved with maximum pedal displacement indicate that had a displacement feedback-based command module setting capable of more consistently activating BA been selected, the ability of the system to achieve near-maximum braking may have been better realized.

Overall, the brake line pressures and vehicle decelerations produced during the four tests shown in Figure 5.16 were very similar, although the larger application magnitudes associated with the maximum pedal displacement inputs produced somewhat higher brake line pressures and decelerations very early in the braking maneuvers (i.e., for approximately 250 ms after initiation of the stops). However, the most striking comparisons pertain to brake pedal force and displacement.

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Figure 5.16. Test outputs produced during four straight line stops performed from 45 mph with the 2006 BMW 330i (brake assist enabled; tests performed at the activation threshold and with maximum pedal displacement and application rate).

In Figure 5.16, the tests performed with maximum pedal displacement required substantially higher peak force to establish and maintain brake pedal position for the duration of the respective stops, despite producing very similar stopping distances to those achieved with the lesser applications. The mean force applied from 0.5 to 2.0 seconds after the initial application was 265.6 and 241.1 lbf for maximum pedal displacement Tests 533 and 536, respectively. Conversely, displacement feedback-based threshold Tests 453 and 459 only required 7.4 and 6.2 lbf to maintain the respective target pedal positions over the same interval.


With maximum pedal displacement, the mean stopping distance for the Cadillac STS was 69.9 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased slightly to 70.5 ft. The p-value associated with the difference in stopping distance means is 0.3753, as shown in Table 5.19. This was a non-significant result, indicating the different application techniques had no measureable effect on stopping distance. In other words, braking the vehicle with inputs at the displacement feedback-based threshold produced

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stopping distances nearly equivalent to those achieved by the maximum displacement applications.

Table 5.18. Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

STS Max versus Threshold, Straight Line Stops From 45 mph (ft)


Max 5 69.9 1.46 67.7 71.5 0.3753

Threshold 11 70.5 1.13 68.1 72.1

The authors were surprised to see two very different brake applications produce such similar stopping distances with the Cadillac STS. Although these data imply that braking with inputs at the displacement feedback-based threshold can realize the maximum braking potential of the vehicle, it is important to recall the findings previously presented in Section 5.1.1.3. In that section, the authors indicated that braking the Cadillac STS with inputs believed to reside at the BA activation threshold produced stopping distances that were quite similar regardless of whether the system is enabled or disabled. Practically speaking, this implies there may be little difference between stopping distances achieved with maximum pedal displacement and those produced with inputs at the displacement feedback-based threshold with BA disabled. Given the intent of BA, this finding is perplexing and suggests the BA activation threshold associated with displacement feedback was not correctly identified for the Cadillac STS. This point is supported by considering the substantial overlap in individual stopping distances produced with the two application techniques, shown in Figure 5.17. Specifically, the extensive overlap in stopping distance data supports the author’s belief that the pedal displacement magnitude of the threshold-based applications was large enough to nearly realize the full braking capability of the vehicle.

961 1135

1130

965

Figure 5.17. Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph using two application techniques.

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Key outputs from the tests highlighted in Figure 5.17 are presented in Figure 5.18 to better understand why the stopping distance overlap occurred. In this figure, results from two tests performed with maximum pedal displacement (Tests 961 and 965) are compared to two tests performed with two displacement feedback-based threshold applications (Tests 1130 and 1135).

Figure 5.18. Test outputs produced during four straight line stops performed from 45 mph with the 2005 Cadillac STS (brake assist enabled; tests performed at the activation threshold and with maximum pedal displacement and application rate).

Overall, the front brake line pressures and vehicle decelerations produced during the four tests shown in Figure 5.18 were very similar. The rear brake line pressures were also quite similar, however those produced during the tests performed with inputs believed to reside at the BA activation threshold were, generally speaking, somewhat greater over the duration of the stop. However, the most striking comparisons shown in Figure 5.18 pertain to brake pedal force and displacement.

In Figure 5.18, the Cadillac STS tests performed with maximum pedal displacement required substantially higher peak force to establish and maintain brake pedal position for the duration of the respective stops. The mean force applied from 0.5 to 2.0 seconds after the initial application was 143.9 and 144.2 lbf for Tests 961 and 965, respectively. Conversely, displacement

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feedback-based threshold Tests 1130 and 1135 only required 22.0 and 15.9 lbf to maintain the respective target pedal positions over the same interval.


Using maximum pedal displacement, the mean stopping distance for the Toyota 4Runner was 72.2 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 79.4 ft. The p-value associated with the difference in stopping distance means is 0.0008, as shown in Table 5.19. This is a highly significant result, indicating that differences in the two application techniques were responsible for a 7.3 ft difference in stopping distance. Note that there was no overlap in the individual stopping distances produced with the two brake application techniques.

Table 5.19. Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

4Runner Max versus Threshold, Straight Line Stops From 45 mph (ft)


Max 5 72.2 0.61 71.3 72.8 0.0008

Threshold 10 79.4 3.63 76.5 89.1

5.2.1.5 Volvo XC90

For the Volvo XC90, use of the maximum pedal displacement produced a mean stopping distance of 72.5 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 76.5 ft. The p-value associated with the difference in stopping distance means is 0.0036, as shown in Table 5.20. This is a significant result, indicating that differences in the two application techniques were responsible for a 4.1 ft difference in stopping distance. Note that there was no overlap in the individual stopping distances produced with the two brake application techniques.

Table 5.20. Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

XC90 Max versus Threshold, Straight Line Stops From 45 mph (ft)


Max 5 72.5 0.83 71.2 73.5 0.0036

Threshold 10 76.5 2.46 74.6 82.5

5.2.2 Brake In-A-Turn Stops from 45 mph – Application Threshold vs. Max Displacement


With maximum pedal displacement, the mean stopping distance for the Chrysler 300C was 88.0 ft. When displacement feedback-based threshold applications were used, the mean stopping

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distance increased to 103.4 ft. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.21. This is a highly significant result, indicating differences in the two application techniques were responsible for the 15.4 ft difference in mean stopping distance. Note that there was no overlap in the individual stopping distances produced with the two brake application techniques.

Table 5.21. Chrysler 300C Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

300C Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 88.0 0.86 87.4 89.4 <.0001

Threshold 11 103.4 2.97 95.7 106.5

5.2.2.2 BMW 330i

Using displacement feedback, the maximum and threshold-based mean stopping distances for the BMW 330i were 71.7 and 109.1 ft, respectively. The p-value associated with the difference in means is <.0001, as shown in Table 5.22. The magnitude of the p-value indicates the 37.4 ft difference is highly significant. In agreement with the trend observed for most of the vehicles evaluated in this study, the data shown in Table 5.22 indicates there was no overlap in the individual stopping distances produced with the two brake application techniques.

Table 5.22. BMW 330i Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

330i Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 6 71.7 0.53 70.9 72.3 <.0001

Threshold 10 109.1 3.29 103.9 114.0


In the case of the Cadillac STS, the mean stopping distance produced with maximum pedal displacement was 84.1 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 100.3 ft. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.23. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 15.4 ft difference in mean stopping distance. As with most of the test vehicles, there was no overlap in the individual stopping distances observed with the two brake application techniques.

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Table 5.23. Cadillac STS Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

STS Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 6 84.1 3.19 80.7 90.1 <.0001

Threshold 10 100.3 1.53 96.7 102.1


With displacement feedback, the mean stopping distances produced with maximum and threshold-based pedal displacements were 79.0 and 83.9 ft, respectively, for the Toyota 4Runner. The p-value associated with the difference in means is 0.0029, shown in Table 5.24. The magnitude of the p-value indicates the 4.9 ft difference is significant. As with most of the other test vehicles, there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.24. Toyota 4Runner Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

4Runner Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 79.0 0.67 78.2 79.8 0.0029

Threshold 10 83.9 2.91 80.5 91.4

5.2.2.5 Volvo XC90

Using maximum pedal displacement, the mean stopping distance for the Volvo XC90 was 89.5 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 103.7 ft. The p-value associated with the difference in stopping distance means is 0.3314, as shown in Table 5.25. The magnitude of the p-value indicates that differences in the two application techniques were not responsible for the 14.2 ft mean stopping distance difference.

Table 5.25. Volvo XC90 Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 45 mph).

XC90 Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 89.5 1.3 87.9 91.4 0.3314

Threshold 10 103.7 30.8 88.2 172.0

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In contrast with the stopping distances produced by the other vehicles discussed in Section 5.2.2, the data shown in Table 5.25 indicate there was overlap in the individual stopping distances produced with the two brake application techniques. Figure 5.19 presents the individual stopping distances observed during these tests.

396

394

404 398

Figure 5.19. Stopping distances observed during brake in-a-turn stops performed with the Volvo XC90 from 45 mph using two application techniques.

Key outputs from the tests highlighted in Figure 5.19 are presented in Figure 5.20 to examine the stopping distance overlap. In this figure, results from the two longest tests performed with displacement feedback-based threshold applications (i.e., Tests 396 and 394) are compared to those observed during Tests 398 and 404. Test 404 was performed using maximum pedal displacement, while Test 398 used an application performed at the displacement feedback-based BA activation threshold.

Overall, the brake line pressures, vehicle decelerations, and stopping distances produced for Tests 398 and 404 were quite similar, despite considerable differences in brake application force and displacement magnitudes. The mean force applied from 0.5 to 2.0 seconds after the initial application was 241.1 lbf to establish and maintain brake pedal position for the duration of the Test 404 stop, while Test 398 required only 6.2 lbf to maintain the target pedal position over the same interval.

In contrast to the outputs shown for Test 398, the substantially lower brake line pressures and absence of ABS activity observed during Tests 396 and 394 indicate use of the same displacement feedback-based threshold applications were unable to activate BA.

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Figure 5.20. Test outputs produced during four brake in-a-turn stops performed from 45 mph with the Volvo XC90 (brake assist enabled; tests performed at the displacement feedback-based activation threshold and with maximum pedal displacement and application rate).

5.2.3 Straight Line Stops from 65 mph – Activation Threshold vs. Maximum Displacement

In Section 5.2.1, results from straight line stops performed with displacement feedback and maximum versus threshold-based brake applications were compared. These tests were initiated from 45 mph. The straight line stops discussed in this section used the same commanded brake applications, however the tests were initiated from 65 mph.


With maximum pedal displacement, the mean stopping distance for the Chrysler 300C was 149.4 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 165.4 ft. The p-value associated with the difference of the stopping distance means is 0.0001, as shown in Table 5.26. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 16.0 ft difference in stopping distance. Note that there was no overlap of the individual stopping distances produced with the two brake application techniques.

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Table 5.26. Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph).



Max 5 149.4 1.28 147.8 151.2 0.0001

Threshold 13 165.4 7.01 158.3 176.0

5.2.3.2 BMW 330i

Using displacement feedback, the maximum and threshold-based mean stopping distances for the BMW 330i were 136.9 and 169.1 ft, respectively. The p-value associated with the difference between these means is 0.0001, shown in Table 5.27. The magnitude of the p-value indicates the 32.2 ft difference between the two means is highly significant. Like that produced with the Chrysler 300C there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.27. BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph).



Max 5 136.9 2.68 133.9 139.8 0.0001

Threshold 12 169.1 13.60 145.7 191.6


With maximum pedal displacement, the mean stopping distance for the Cadillac STS was 140.9 ft. When displacement feedback-based threshold applications were used, the mean stopping distance decreased slightly to 140.7 ft. The p-value associated with the difference of the stopping distance means is 0.8195, as shown in Table 5.28. This was not a significant result, indicating the different application techniques produced nearly equivalent stopping distances.

Table 5.28. Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph).



Max 5 140.9 0.94 140.0 142.3 0.8195

Threshold 10 140.7 1.48 139.0 143.3

In Section 5.2.1.3, the authors surmised the pedal application magnitude believed to represent the BA activation threshold may have been too large, thus confounding a meaningful comparison of

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the stopping distances produced with inputs based on maximum pedal displacement. As noted in the previous section, the substantial overlap in stopping distances from 45 mph occurred because the inputs used to define the activation threshold magnitude were large enough to nearly realize the maximum braking capability of the vehicle. Since the 65 mph tests used the same pedal application magnitudes as the 45 mph tests, it was not surprising to find that the 65 mph test results also had substantial overlap in stopping distances, as shown in Table 5.28.

Figure 5.21 presents the stopping distances observed for each test represented in Table 5.28. For each application technique, the tests that produced the two shortest stopping distances are identified; Tests 966 and 968 for the stops performed with maximum pedal displacement, and Tests 1144 and 1145 for the stops performed with displacement feedback-based threshold applications.

966/968

1144/1145

Figure 5.21. Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph using two application techniques.

Key outputs from the tests highlighted in Figure 5.21 are presented in Figure 5.22 to examine the stopping distance overlap. Overall, the brake line pressures and vehicle decelerations produced during the four tests shown in Figure 5.22 were quite similar, despite considerable differences in brake application force and displacement magnitudes.

Like the test output data previously shown in Figure 5.18 (test outputs produced during stops initiated from 45 mph), the data presented in Figure 5.22 show the tests performed with maximum pedal displacement required substantially higher peak force to establish and maintain brake pedal position for the duration of the respective stops. The mean force applied from 0.5 to 2.0 seconds after the initial application was 155.1 and 160.3 lbf for maximum displacement Tests 966 and 968, respectively. Conversely, displacement feedback-based threshold Tests 1144 and 1145 only required 13.9 and 19.7 lbf to maintain the target pedal position over the same interval.

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Figure 5.22. Test outputs produced during four straight line stops performed from 65 mph with the 2005 Cadillac STS (brake assist enabled; tests performed at the activation threshold and with maximum pedal displacement and application rate.


Using displacement feedback, the maximum and threshold-based mean stopping distances for the Toyota 4Runner were 148.9 and 161.2 ft, respectively. The p-value associated with the difference between these means is 0.1843, as shown in Table 5.29. The magnitude of the p-value indicates the 12.3 ft difference between the two means is not significant. Like the Cadillac STS comparison presented in Section 5.2.3.3, the data shown in Table 5.29 also reveal overlap of the individual stopping distances produced during the tests performed with the two brake application techniques.

Note: If not considered carefully, this overlap can have the effect of misguiding the reader. Although the stopping distance means of the two data sets discussed in this section are not statistically different, the individual data sets differ considerably as shown later in Figure 5.24.

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Table 5.29. Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph).



Max 3 148.9 1.01 147.8 149.8 0.1843

Threshold 10 161.2 14.58 145.3 181.3

In Section 5.2.3.3, the entire range of Cadillac STS stopping distances produced with maximum displacement applications were within the range established by the lesser inputs. However, unlike the Cadillac STS, the authors are more comfortable that the brake applications associated with the displacement feedback-based activation threshold were better identified for the Toyota 4Runner18. This, and the fact the 45 mph stops made with maximum displacement were significantly shorter than those performed with the threshold-based applications, make the results from the Toyota 4Runner’s 65 mph straight line tests particularly interesting.

Figure 5.23 presents the stopping distances observed for tests represented in Table 5.29. In this figure, three key tests are indicated. Test 448 was chosen to represent a typical stop performed with maximum pedal displacement. Tests 522 and 523 are representative of those producing the shortest and longest stopping distances, respectively, of the displacement feedback-based threshold applications used during the straight line stops performed from 65 mph.

448

523

522

Figure 5.23. Stopping distances observed during straight line stops performed with the Toyota 4Runner from 65 mph using two application techniques.

18 Recall that for each of the BA enabled versus disabled comparisons made in Section 5.1, the Toyota 4Runner was able to produce shorter stopping distances with BA enabled than when the system was disabled (with no stopping distance overlap), and that the differences in the mean stopping distances made for each of the three comparison groupings were statistically significant. These findings clearly indicated that for tests performed with applications believed to be associated with the BA activation threshold, BA activation reduced stopping distances.

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What the authors find particularly interesting about the data shown in Figure 5.23 is that the applications believed to reside at the BA activation threshold produce stopping distances longer and shorter than those produced during the tests performed with maximum pedal travel applications. This implies the inputs selected to correspond to the BA activation threshold were capable of realizing the maximum braking capability of the vehicle during the stops initiated from 65 mph, but the variability associated with braking the vehicle at this threshold prevented the maximum braking from being repeatably recognized (the range of stopping distance produced in this conditioned spanned 36.0 ft).

Key outputs from the highlighted tests shown in Figure 5.23 are presented in Figure 5.24 to examine the stopping distance overlap produced by the two application techniques, and to show how two maneuvers performed with nearly equivalent brake applications can produce vastly different outcomes.

Figure 5.24. Test outputs produced during three straight line stops performed from 65 mph with the 2003 Toyota 4Runner (brake assist enabled; tests performed at the activation threshold and with maximum pedal displacement and application rate.

In Figure 5.24, Tests 522 and 523 were performed using displacement feedback-based threshold applications. Despite nearly identical inputs of pedal displacement and force, Test 523 produced

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much lower brake line pressures and vehicle deceleration than those observed one stop earlier during Test 522. Conversely, Test 522 produced much higher brake line pressures and vehicle deceleration; attributes that facilitated a stopping distance that was not only the shortest of the related test series, but also shorter than any of the five stops performed with maximum pedal displacement. The mean force applied from 0.5 to 2.0 seconds after the initial application was 195.8 lbf19 for maximum displacement Test 488. Conversely, displacement threshold-based Tests 522 and 523 only required 16.8 and 18.5 lbf to maintain the target pedal position over the same interval.

5.2.3.5 Volvo XC90

With maximum pedal displacement, the mean stopping distance for the Volvo XC90 was 147.1 ft. When displacement feedback-based threshold applications were used, the mean stopping distance increased to 153.6 ft. The p-value associated with the difference of the stopping distance means is <.0001, as shown in Table 5.30. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 6.5 ft difference in stopping distance. Note that there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.30. Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Displacement Feedback; Brake Assist Enabled; 65 mph).



Max 5 147.1 1.06 146. 148.7 <.0001

Threshold 10 153.6 1.78 150.4 156.1

5.3 Brake Assist Effectiveness Evaluation using Force Feedback

Sections 5.1 and 5.2 discussed displacement feedback-based test results from two different perspectives. In Section 5.1, stopping distances and test outputs from stops performed with BA enabled and disabled, using threshold-based brake applications, were compared. Section 5.2 compared the data produced during tests performed with maximum and threshold-based brake applications with BA enabled. Rather than present the data produced from the force feedback stops in two sections, Section 5.3 combines all discussions pertaining to force feedback into one section.

In agreement with the displacement feedback test methodology, force feedback testing was comprised of three maneuvers: straight line and brake in-a-turn stops initiated from 45 mph, and straight line stops initiated from 65 mph. When analyzed with the processes used for the displacement feedback tests, the mean stopping distance differences produced by comparable BA enabled versus disabled tests rarely produced significantly different results when force feedback

19 The actual force magnitude was larger. The force applied during Test 488 exceeded the measureable range of the 200 lbf load cell used during tests performed with the Toyota 4Runner. The load cells used for the other vehicles had greater force measuring capacities.

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was utilized. However, comparison of the mean stopping distances produced with maximum and threshold-based brake applications with BA enabled were significantly different for some vehicles.

5.3.1 Straight Line Stopping Distance Comparisons from 45 mph

For each vehicle, this section discusses two analyses performed with straight line stopping distance data from stops initiated at 45 mph. First, the stopping distances produced with BA enabled and disabled are compared using threshold-based applications. Next, the stopping distances produced during two different application techniques are discussed.


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Chrysler 300C was 85.9 ft when BA was disabled. When BA was enabled, the mean stopping distance was nearly identical at 86.0 ft. The p-value associated with the difference between the stopping distance means is 0.9234, shown in Table 5.31, meaning the 0.1 ft difference between the two means is not significant. Note that the entire range of stopping distances achieved during the BA disabled test series were contained within that established by the enabled system tests.

Table 5.31. Chrysler 300C Straight Line Stopping Distance Summary (Force Feedback; 45 mph).



Disabled 10 85.9 2.55 81.7 89.1 0.9234

Enabled 10 86.0 2.49 81.6 90.4

With maximum pedal force, the mean stopping distance for the Chrysler 300C was 74.7 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 86.0 ft. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.32. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 11.3 ft difference of the stopping distance means. Note that there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.32. Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).



Max 5 74.7 3.93 67.7 77.1 <.0001

Threshold 10 86.0 2.49 81.6 90.4

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Figure 5.25 presents the distances produced during each straight line stop initiated from 45 mph with the Chrysler 300C using force feedback. Although these data have been summarized in Tables 5.31 and 5.32, Figure 5.25 provides a useful way to consider the distribution of the individual stopping distances using four brake input conditions (i.e., disabled or enabled, with maximum or threshold-based input magnitudes), including the identification of outliers capable of influencing the respective stopping distance summaries20.

Figure 5.25. Stopping distances observed during straight line stops performed with the Chrysler 300C from 45 mph (force feedback).

5.3.1.2 BMW 330i

For the BMW 330i, using brake inputs at the BA activation threshold with the system disabled produced a mean stopping distance of 67 ft. When BA was enabled, the mean stopping distance was 68.1 ft. The p-value associated with the difference between these means is 0.0159, shown in Table 5.33, meaning the 1.1 ft difference is significant.

Table 5.33. BMW 330i Straight Line Stopping Distance Summary (Force Feedback; 45 mph).



Disabled 10 67.0 1.03 65.3 68.4 0.0159

Enabled 10 68.1 0.77 66.5 69.1

20 Unless there was a problem with test conduct (e.g., data acquisition malfunction or inappropriate maneuver entrance speed), individual test trials were not discarded simply because outlying data were produced. Given that the test input conditions were carefully controlled, the authors consider it appropriate to consider all valid test trials for inclusion in the respective analyses.

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Using force feedback, the mean stopping distances for BMW 330i tests performed with maximum force and threshold-based applications were 65.2 and 68.1 ft, respectively. The p-value associated with the difference between these means is <.0001, shown in Table 5.34, meaning the 2.9 ft difference is highly significant.

Table 5.34. BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).



Max 5 65.2 1.25 63.8 66.8 <.0001

Threshold 10 68.1 0.77 66.5 69.1

Figure 5.26 presents the distances produced during each force feedback-based straight line stop initiated from 45 mph with the BMW 330i. When interpreting the data summaries shown in Tables 5.33 and 5.34, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.26. Stopping distances observed during straight line stops performed with the BMW 330i from 45 mph (force feedback).


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Cadillac STS was 71.6 ft with BA disabled, and 70.9 ft when BA was enabled. The p-value associated with the difference between the stopping distance means is 0.0539, shown in Table 5.35, meaning the 0.7 ft difference is marginally insignificant.

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Table 5.35. Cadillac STS Straight Line Stopping Distance Summary (Force Feedback; 45 mph).



Disabled 10 71.6 0.77 70.6 73.1 0.0539

Enabled 10 70.9 0.85 69.6 72.1

With maximum pedal force, the mean stopping distance for the Cadillac STS was 69.2 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 70.9 ft. The p-value associated with the difference in stopping distance means is 0.0014, as shown in Table 5.36. This is a significant result, indicating that differences in the two application techniques were responsible for the 1.7 ft difference of the stopping distance means.

Table 5.36. Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).



Max 5 69.2 0.43 68.7 69.8 0.0014

Threshold 10 70.9 0.85 69.6 72.1

Figure 5.27 presents the distances produced during each straight line stop initiated from 45 mph with the Cadillac STS using force feedback. When interpreting the data summaries shown in Tables 5.35 and 5.36, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.27. Stopping distances observed during straight line stops performed with the Cadillac STS from 45 mph (force feedback).

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For the Toyota 4Runner, the mean stopping distances with BA disabled and enabled were 77.8 and 77.2 ft, respectively, when threshold-based applications were used. The p-value associated with the difference between these means is 0.4062, shown in Table 5.37, meaning the 0.6 ft difference is not significant.

Table 5.37. Toyota 4Runner Straight Line Stopping Distance Summary (Force Feedback; 45 mph).



Disabled 10 77.8 1.73 75.8 80.5 0.4062

Enabled 9 77.2 1.34 75.3 79.3

The mean stopping distances for Toyota 4Runner tests performed with maximum and threshold-based applications were 75.8 and 77.2 ft, respectively. The p-value associated with the difference between these means is 0.0753, shown in Table 5.38, meaning the 1.4 ft difference is marginally insignificant.

Table 5.38. Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).



Max 4 75.8 0.70 74.9 76.4 0.0753

Threshold 9 77.2 1.34 75.3 79.3

Figure 5.28 presents the distances produced during each straight line stop initiated from 45 mph with the Toyota 4Runner using force feedback. When interpreting the data summaries shown in Tables 5.37 and 5.38, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

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Figure 5.28. Stopping distances observed during straight line stops performed with the Toyota 4Runner from 45 mph (force feedback).

5.3.1.5 Volvo XC90

When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Volvo XC90 was 75.7 ft with BA disabled. When BA was enabled, the mean stopping distance increased to 79.2 ft. The p-value associated with the difference between the stopping distance means is 0.0009, shown in Table 5.39, meaning the 3.5 ft is significant.

Table 5.39. Volvo XC90 Straight Line Stopping Distance Summary (Force Feedback; 45 mph).



Disabled 10 75.7 1.63 72.8 78.1 0.0009

Enabled 12 79.2 2.44 74.4 83.3

With maximum pedal force, the mean stopping distance for the Volvo XC90 was 73.8 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 79.2 ft. The p-value associated with the difference in stopping distance means is 0.0005, as shown in Table 5.40. This is a significant result, indicating that differences in the two application techniques were responsible for the 5.4 ft difference of the stopping distance means.

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Table 5.40. Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).



Max 5 73.8 1.84 70.9 76.0 0.0005

Threshold 12 79.2 2.44 74.4 83.3

Figure 5.29 presents the distances produced during each straight line stop initiated from 45 mph with the Volvo XC90 using force feedback. When interpreting the data summaries shown in Tables 5.39 and 5.40, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.29. Stopping distances observed during straight line stops performed with the Volvo XC90 from 45 mph (force feedback).

5.3.2 Brake In-A-Turn Stopping Distance Comparisons From 45 mph

For each vehicle, this section discusses the two analyses performed using the 45 mph brake in-aturn stopping distance data. First, the BA enabled and disabled stopping distances are compared using inputs believed to represent the respective BA activation thresholds. Next, the stopping distances produced using the force feedback-based activation threshold and those with maximum inputs are compared.


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Chrysler 300C was 93.8 ft with BA disabled. When BA was enabled, the mean

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stopping distance increased to 97.0 ft. The p-value associated with the difference between the stopping distance means is 0.0159, shown in Table 5.41, meaning the 3.2 ft difference is significant.

Table 5.41. Chrysler 300C Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph).

300C Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 93.8 2.48 88.4 96.5 0.0159

Enabled 10 97.0 2.76 92.8 100.4

With maximum pedal force, the mean stopping distance for the Chrysler 300C was 89.2 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 97.0 ft. The p-value associated with the difference in stopping distance means is <.0001, as shown in Table 5.42. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 7.8 ft difference of the stopping distance means. Table 5.42 also shows there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.42. Chrysler 300C Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).

300C Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 89.2 1.16 87.7 90.7 <.0001

Threshold 10 97.0 2.76 92.8 100.4

Figure 5.30 presents the distances produced during each brake in-a-turn stop initiated from 45 mph with the Chrysler 300C using force feedback. Although these data have been summarized in Tables 5.41 and 5.42, Figure 5.30 provides a useful way to consider the distribution of the individual stopping distances using four brake input conditions.

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Figure 5.30. Stopping distances observed during brake in-a-turn stops performed with the Chrysler 300C from 45 mph (force feedback).

5.3.2.2 BMW 330i

For the BMW 330i, the mean stopping distances with BA disabled and enabled were 80.1 and 80.0 ft, respectively, when force feedback-based activation threshold applications were used. The p-value associated with the difference between these means is 0.9231, shown in Table 5.43, meaning the 0.1 ft difference is not significant. The entire range of stopping distances achieved during the BA-enabled test series were contained within that established by the disabled system tests.

Table 5.43. BMW 330i Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph).

330i Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 11 80.1 2.88 74.7 85.7 0.9231

Enabled 13 80.0 1.89 77.2 83.4

Using force feedback, the mean stopping distances for BMW 330i tests performed with maximum and threshold-based applications were 72.4 and 80.0 ft, respectively. The p-value associated with the difference between these means is <.0001, shown in Table 5.44. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 7.6 ft difference of the stopping distance means. Table 5.44 also shows there was no overlap of the individual stopping distances produced with the two brake application techniques.

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Table 5.44. BMW 330i Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).

330i Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 72.4 1.27 71.3 74.6 <.0001

Threshold 13 80.0 1.89 77.2 83.4

Figure 5.31 presents the distances produced during each brake in-a-turn stop initiated from 45 mph with the BMW 330i using force feedback. When interpreting the data summaries shown in Tables 5.43 and 5.44, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.31. Stopping distances observed during brake in-a-turn stops performed with the BMW 330i from 45 mph (force feedback)


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Cadillac STS was 84.4 ft with BA disabled. When BA was enabled, the mean stopping distance was 83.5 ft. The p-value associated with the difference between the stopping distance means is 0.3593, shown in Table 5.45, meaning the 0.9 ft difference is not significant. Note that the entire range of stopping distances achieved during the BA-disabled test series were contained within that established by the enabled system tests.

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Table 5.45. Cadillac STS Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph).

STS Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 84.4 0.94 82.6 85.6 0.3593

Enabled 11 83.5 2.65 79.6 88.2

With maximum pedal force, the mean stopping distance for the Cadillac STS was 81.0 ft. When applications at the force feedback-based BA activation threshold were used, the mean stopping distance was 83.5 ft. The p-value associated with the difference in stopping distance means is 0.1195, as shown in Table 5.46, meaning the 2.5 ft difference is not significant.

Table 5.46. Cadillac STS Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).

STS Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 6 81.0 3.57 78.1 88.0 0.1195

Threshold 11 83.5 2.65 79.6 88.2

Figure 5.32 presents the distances produced during each brake in-a-turn stop initiated from 45 mph with the Cadillac STS using force feedback. When interpreting the data summaries shown in Tables 5.45 and 5.46, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.32. Stopping distances observed during brake in-a-turn stops performed with the Cadillac STS from 45 mph (force feedback)

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For the Toyota 4Runner, the mean stopping distances with BA disabled and enabled were 82.8 and 82.6 ft, respectively, when force feedback-based activation threshold applications were used. The p-value associated with the difference between these means is 0.7912 shown in Table 5.47, meaning the 0.2 ft difference is not significant. Note that the entire range of stopping distances achieved during the BA-disabled test series were contained within that established by the enabled system tests.

Table 5.47. Toyota 4Runner Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph).

4Runner Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 82.8 1.29 80.9 85.1 0.7912

Enabled 10 82.6 1.83 80.0 85.6

The mean stopping distances for Toyota 4Runner tests performed with maximum and threshold-based applications were 80.2 and 82.6 ft, respectively. The p-value associated with the difference between these means is 0.0136, shown in Table 5.48, meaning the 2.4 ft difference is significant.

Table 5.48. Toyota 4Runner Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).

4Runner Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 80.2 0.54 79.5 80.7 0.0136

Threshold 10 82.6 1.83 80.0 85.6

Figure 5.33 presents the distances produced during each brake in-a-turn stop initiated from 45 mph with the Toyota 4Runner using force feedback. When interpreting the data summaries shown in Tables 5.47 and 5.48, this figure provides a convenient way to view the individual stopping distance distributions within each of the four brake input conditions.

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Figure 5.33. Stopping distances observed during brake in-a-turn stops performed with the Toyota 4Runner from 45 mph (force feedback).

5.3.2.5 Volvo XC90

When force feedback was used, brake inputs applied at the BA activation threshold with BA disabled and enabled produced stopping distances of 85.5 and 85.6 ft, respectively, for the Volvo XC90. The p-value associated with the difference between these means is 0.9148, shown in Table 5.49, meaning the 0.1 ft difference is not significant.

Table 5.49. Volvo XC90 Brake In-A-Turn Stopping Distance Summary (Force Feedback; 45 mph).

XC90 Threshold Braking, Ay=0.6g Stops From 45 mph (ft)


Disabled 10 85.5 3.64 82.1 92.1 0.9148

Enabled 10 85.6 2.91 80.8 90.9

With maximum pedal force, the mean stopping distance for the Volvo XC90 was 89.4 ft. When applications at the BA activation threshold were used, the mean stopping distance decreased to 85.6 ft. The p-value associated with the difference in stopping distance means is 0.0167, as shown in Table 5.50, meaning the 3.8 ft difference is significant.

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Table 5.50. Volvo XC90 Brake In-A-Turn Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 45 mph).

XC90 Max versus Threshold, Ay=0.6g Stops From 45 mph (ft)


Max 5 89.4 1.28 88.1 91.3 0.0167

Threshold 10 85.6 2.91 80.8 90.9

Figure 5.34 presents the distances produced during each brake in-a-turn stop initiated from 45 mph with the Volvo XC90 using force feedback. When interpreting the data summaries shown in Tables 5.49 and 5.50, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.34. Stopping distances observed during brake in-a-turn stops performed with the Volvo XC90 from 45 mph (force feedback).

5.3.3 Straight Line Stopping Distance Comparisons from 65 mph

For each vehicle, this section discusses the two analyses performed using 65 mph straight line stopping distance data. First, the BA enabled and disabled stopping distances are compared using inputs believed to represent the respective BA activation thresholds. Next, the stopping distances produced using the force feedback-based activation threshold and those with maximum inputs are compared.


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Chrysler 300C was 157.1 ft when BA was disabled. When BA was enabled, the

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mean stopping distance was 158.5 ft. The p-value associated with the difference between the stopping distance means is 0.1223, shown in Table 5.51, meaning the 1.4 ft difference is not significant.

Table 5.51. Chrysler 300C Straight Line Stopping Distance Summary (Force Feedback; 65 mph).



Disabled 10 157.1 1.78 154.0 160.0 0.1223

Enabled 10 158.5 2.07 156.3 161.6

With maximum pedal force applied at a nominal speed of 65 mph, the mean stopping distance for the Chrysler 300C was 151.2 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 158.5 ft. The p-value associated with the difference in stopping distance means is 0.0003, as shown in Table 5.52. This is a significant result, indicating that differences in the two application techniques were responsible for the 7.3 ft difference of the stopping distance means. Table 5.52 also shows there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.52. Chrysler 300C Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph).



Max 5 151.2 3.73 145.5 155.5 0.0003

Threshold 10 158.5 2.07 156.3 161.6

Figure 5.35 presents the distances produced during each straight line stop initiated from 65 mph with the Chrysler 300C using force feedback. Although these data have been summarized in Tables 5.51 and 5.52, Figure 5.35 provides a useful way to consider the distribution of the individual stopping distances using four brake input conditions.

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Figure 5.35. Stopping distances observed during straight line stops performed with the Chrysler 300C from 65 mph (force feedback)

5.3.3.2 BMW 330i

For the BMW 330i, the mean stopping distances with BA disabled and enabled were 137.3 and 138.6 ft, respectively, when threshold-based applications were used. The p-value associated with the difference between these means is 0.3525, shown in Table 5.53, meaning the 1.3 ft difference is not significant.

Table 5.53. BMW 330i Straight Line Stopping Distance Summary (Force Feedback; 65 mph).



Disabled 10 137.3 2.26 133.9 140.9 0.3525

Enabled 6 138.6 2.87 135.4 143.1

From a nominal speed of 65 mph, the mean maximum and threshold-based stopping distances for the BMW 330i were 132.5 and 138.6 ft, respectively. The p-value associated with the difference between these means is 0.0132, shown in Table 5.54, meaning the 6.1 ft difference is significant.

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Table 5.54. BMW 330i Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph).



Max 5 132.5 3.62 128.3 137.7 0.0132

Threshold 6 138.6 2.87 135.4 143.1

Figure 5.36 presents the distances produced during each straight line stop initiated from 65 mph with the BMW 330i using force feedback. When interpreting the data summaries shown in Tables 5.53 and 5.54, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.36. Stopping distances observed during straight line stops performed with the BMW 330i from 65 mph (force feedback)


When brake inputs performed at the activation threshold were utilized, the mean stopping distance for the Cadillac STS was 142.1 ft with BA disabled. When BA was enabled, the mean stopping distance was 141.5 ft. The p-value associated with the difference between the stopping distance means is 0.2352, shown in Table 5.55, meaning the 0.6 ft difference is not significant.

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Table 5.55. Cadillac STS Straight Line Stopping Distance Summary (Force Feedback; 65 mph).



Disabled 10 142.1 1.32 140.0 144.0 0.2352

Enabled 10 141.5 1.15 139.8 143.2

With maximum pedal force, the mean 65 mph stopping distance for the Cadillac STS was 140.7 ft. When applications at the BA activation threshold were used, the mean stopping distance was 141.5 ft. The p-value associated with the difference of the stopping distance means is 0.2943, as shown in Table 5.56, meaning the 0.8 ft difference is not significant.

Table 5.56. Cadillac STS Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph).



Max 5 140.7 1.42 138.9 142.3 0.2943

Threshold 10 141.5 1.15 139.8 143.2

Figure 5.37 presents the distances produced during each straight line stop initiated from 65 mph with the Cadillac STS using force feedback. When interpreting the data summaries shown in Tables 5.55 and 5.56, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

Figure 5.37. Stopping distances observed during straight line stops performed with the Cadillac STS from 65 mph (force feedback)

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For the Toyota 4Runner, the mean stopping distances with BA disabled and enabled were 156.6 and 157.5 ft, respectively, when threshold-based applications were used. The p-value associated with the difference between these means is 0.2585, shown in Table 5.57, meaning the 0.9 ft difference is not significant.

Table 5.57. Toyota 4Runner Straight Line Stopping Distance Summary (Force Feedback; 65 mph).



Disabled 10 156.6 1.88 152.2 158.7 0.2585

Enabled 10 157.5 1.64 155.1 160.5

From a nominal speed of 65 mph, the mean maximum and threshold-based stopping distances for the Toyota 4Runner were 152.1 and 157.5 ft, respectively. The p-value associated with the difference between these means is <.0001, shown in Table 5.58. This is a highly significant result, indicating that differences in the two application techniques were responsible for the 5.4 ft difference of the stopping distance means. Table 5.58 also shows there was no overlap of the individual stopping distances produced with the two brake application techniques.

Table 5.58. Toyota 4Runner Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph).



Max 5 152.1 0.61 151.3 153.0 <.0001

Threshold 10 157.5 1.64 155.1 160.5

Figure 5.38 presents the distances produced during each straight line stop initiated from 65 mph with the Toyota 4Runner using force feedback. When interpreting the data summaries shown in Tables 5.57 and 5.58, this figure provides a convenient way to view the individual stopping distance distributions using four brake input conditions.

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Figure 5.38. Stopping distances observed during straight line stops performed with the Toyota 4Runner from 65 mph (force feedback)

5.3.3.5 Volvo XC90

When brake inputs performed at the force feedback-based activation threshold were utilized, the mean stopping distance for the Volvo XC90 was 161.1 ft when BA was disabled. When BA was enabled, the mean stopping distance was reduced to 157.7 ft. The p-value associated with the difference between the stopping distance means is 0.0239, shown in Table 5.59, meaning the 3.4 ft difference is significant.

Table 5.59. Volvo XC90 Straight Line Stopping Distance Summary (Force Feedback; 65 mph).



Disabled 10 161.1 1.66 158.0 163.5 0.0239

Enabled 10 157.7 4.00 150.8 165.3

With maximum force applied at a nominal speed of 65 mph, the mean stopping distance for the Volvo XC90 was 147.0 ft. When applications at the BA activation threshold were used, the mean stopping distance increased to 157.7 ft. The p-value associated with the difference in stopping distance means is 0.0002, as shown in Table 5.60. This is a significant result, indicating that differences in the two application techniques were responsible for the 10.7 ft difference of the stopping distance means. Table 5.60 also shows there was no overlap of the individual stopping distances produced with the two brake application techniques.

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Table 5.60. Volvo XC90 Straight Line Stopping Distance Summary For Two Brake Application Techniques (Force Feedback; Brake Assist Enabled; 65 mph).



Max 4 147.0 0.88 146.3 148.2 0.0002

Threshold 10 157.7 4.00 150.8 165.3

Figure 5.39 presents the distances produced during each straight line stop initiated from 65 mph with the Volvo XC90 using force feedback. When interpreting the data summaries shown in Tables 5.59 and 5.60, this figure provides a convenient way to view the individual stopping distance distributions within each of the four brake input conditions.

Figure 5.39. Stopping distances observed during straight line stops performed with the Volvo XC90 from 65 mph (force feedback)

5.4 Summary of Brake Application Comparisons

In Sections 5.1 and 5.2, results from stops performed with displacement feedback were discussed. Similarly, Section 5.3 discussed results from force feedback-based testing. These sections provided two key analyses: (1) comparisons of mean threshold-based stopping distances produced with BA enabled versus disabled, and (2) comparisons of mean maximum and threshold-based stopping distances. The rationale for performing the tests described in Sections 5.1 through 5.3 was twofold: (1) to better understand whether the BA thresholds identified in Sections 4.1 and 4.2 were truly realized, and (2) to compare the stopping performance of the vehicles operating at the BA activation thresholds with their maximum braking capability.

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For each comparison presented in Sections 5.1 through 5.3, stopping distance means, standard deviations, minimums and maximums were provided. Additionally, p-values were provided to quantify whether differences in the stopping distance means produced by two test series were statistically significant and if so, to what extent. To concisely summarize the mean stopping distances observed during this study, Sections 5.4.1 through 5.4.5 provide charts with results from the displacement and force feedback tests on a per-vehicle basis. However, since it is often unclear whether the differences seen in these visual representations are statistically significant, summaries of the p-values for each stopping distance comparison are also provided.

Note: For the sake of brevity, this section refers to the maximum inputs produced by the brake controller for a given feedback loop as “maximum” applications. All maximum applications discussed in this section were performed with BA enabled21. Similarly, inputs believed to reside at the BA activation thresholds for a given feedback loop are referred to as “threshold” or “threshold-based” applications.

5.4.1 Chrysler 300C

Figure 5.40 presents the mean stopping distances produced with the Chrysler 300C using displacement and force feedback-based brake applications. Note the similarity of the stopping distances associated with the maximum inputs. This is not particularly surprising given the nearly identical input conditions (i.e., a command module setting of 99/99 was used for each feedback loop). However, it is a good demonstration of test output consistency among two individual test series.

Figure 5.40. Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Chrysler 300C.

To better quantify whether the stopping distance differences seen in Figure 5.40 are meaningful, Table 5.61 summarizes the significance of each Chrysler 300C comparison.

21 Whether BA is enabled or disabled is not believed to have a significant effect on stopping distance since the magnitudes and application rates associated with “maximum” inputs were so large.

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Table 5.61. Chrysler 300C P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications.

Entrance Speed (mph)

Stopping Maneuver


BA Enabled vs. BA Disabled

Max vs. Threshold (BA Enabled)



65 Straight <0.0001 0.0001 0.1223 0.0003

45 Curve 0.0193 <0.0001 0.0159 <0.0001

Straight <0.0001 <0.0001 0.9234 <0.0001

When displacement-based threshold applications were used with the Chrysler 300C, the mean stopping distances produced with BA enabled were significantly shorter than those observed when the system was disabled. However, the stopping distances achieved with displacement feedback-based threshold applications were still significantly longer than those produced with maximum pedal displacement. These trends indicate that although BA generally improved the stopping performance of the Chrysler 300C when displacement feedback was used, it did not realize the vehicle’s full braking potential when activated.

The force feedback-based straight line stopping distances performed from 45 and 65 mph with BA enabled were not significantly different than those observed when it was disabled. Despite the exhaustive efforts of the authors to identify the most appropriate BA activation threshold using force feedback, ultimately these inputs were unable to consistently elicit system operation.

Brake in-a-turn stops performed with force feedback and BA enabled were found to be significantly longer than those performed with the system disabled for the Chrysler 300C. The authors believe this is attributable to a combination of factors, including an inability of the force feedback-based threshold inputs to consistently elicit BA activation and test output variability. Recall that in the previously presented Figure 5.30, there were two outlying stopping distances present in the disabled BA tests series. These outliers, the two shortest stops, lowered the disabled threshold mean. As a result, the difference between the stopping distance means (those of the BA enabled and disabled test series) was found to be significant. Had these two data points not been considered, the entire range of stopping distances produced during brake in-aturn stops performed with force feedback and BA disabled would have resided within the range of the threshold-based BA enabled data set and the significance would not exist.

5.4.2 BMW 330i

Figure 5.41 presents the mean stopping distances produced with the BMW 330i using displacement and force feedback-based brake applications. The stopping distances associated with the maximum inputs were nearly equivalent, further demonstrating the potential for realizing consistent braking via automation and careful test conduct.

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Figure 5.41. Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the BMW 330i.

Visual inspection of the data shown in Figure 5.41 clearly shows that the type of feedback loop used, the presence or absence of an active BA system, and brake application technique can each affect stopping distance. Table 5.62 summarizes whether these effects were significant by providing the p-values associated with each of the BMW 330i comparisons.

Table 5.62. BMW 330i P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications.


Stopping Maneuver






65 Straight <0.0001 0.0001 0.3525 0.0132

45 Curve 0.2714 <0.0001 0.9231 <0.0001

Straight 0.0141 0.0170 0.0159 <0.0001

The data presented in Table 5.62 indicate that when displacement feedback-based threshold applications were used, the mean straight line stopping distances of the 45 and 65 mph tests with BA enabled were significantly different (shorter) than those produced when the system was disabled22. This was not the case for the brake in-a-turn tests, where the mean stopping distances observed during with BA enabled and disabled were not significantly different. As indicated in Section 5.1.2.2, the displacement feedback-based threshold applications used in this study were unable to consistently elicit BA intervention during this maneuver.

22 The results from the 45 mph straight-line tests are somewhat surprising given that all BA disabled stopping distances observed in this test series were contained within the range established by the enabled stops, as previously shown in Figure 5.1.

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In the case of the force feedback-based tests, the BA enabled and disabled mean stopping distances for the brake in-a-turn and 65 mph straight line maneuvers were not significantly different. Interestingly, this was not true for the 45 mph straight line stops with force feedback. While the difference between the means was significant, the mean BA enabled stopping distance was longer than that produced with the system disabled. The reason for this is not presently known, but recalling the distribution of stopping distances previously shown in Figure 5.26, test output variability and the strong likelihood that force feedback-based threshold inputs did not activate BA are suspect.

Overall, stops performed with threshold-based inputs were significantly longer than those attained with maximum braking, regardless of stopping maneuver or feedback type. For the stops performed with displacement feedback, the trend indicates that BA activation can significantly shorten the stopping distances of the BMW 330i. However, the combination of BA enabled and threshold-based applications used in this study were unable to consistently produce BA activation, thereby confounding an accurate assessment of BA’s effect on braking capability. This was particularly true for the 45 mph straight line, maximum versus threshold stops (as explained in Section 5.2.1.2) and the brake in-a-turn maneuvers, where the mean BA-enabled stopping distance was not significantly different than that observed with the system disabled. While a different combination of activation threshold inputs may have reduced brake output disparities, one of the goals of this study was to evaluate BA performance using threshold-based inputs.

Based on the results summarized in Figure 5.41 and Table 5.62, the authors do not believe that, for this vehicle, quantifying the manner and extent to which BA changes stopping distances can be accurately assessed using force feedback comparisons. As previously indicated, the force feedback-based threshold applications used during evaluation of the BMW 330i were unable to effectively differentiate brake in-a-turn and 65 mph straight line stops performed with BA enabled and disabled. Such quantification is further confounded by the 45 mph straight line test results, where the stopping distances produced with BA disabled were significantly shorter than when the system was enabled. These findings impede a meaningful assessment of maximum versus threshold-based input performance.

5.4.3 Cadillac STS

Figure 5.42 presents the mean stopping distances produced with the Cadillac STS using displacement and force feedback-based brake applications. In agreement with the trend seen for the other vehicles, the mean stopping distances associated with the maximum inputs remained quite comparable between application techniques.

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Figure 5.42. Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Cadillac STS.

In the case of the Cadillac STS, the mean stopping distances produced within each maneuver type were quite similar regardless of whether displacement or force feedback was utilized. The most obvious exception to this trend was found during displacement feedback testing, where threshold-based mean distances produced during the brake in-a-turn stops were significantly longer than the mean distance produced with maximum inputs, as indicated in Figure 5.42. The p-value associated with this comparison is provided as part of the Cadillac STS overall significance comparison shown in Table 5.63.

Table 5.63. Cadillac STS P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications.


Stopping Maneuver






65 Straight 0.0028 0.8195 0.2352 0.2943

45 Curve 0.7610 <0.0001 0.3593 0.1195

Straight 0.0083 0.3753 0.0539 0.0014

The summary presented in Table 5.63 indicates four of the six maximum versus threshold-based application comparisons produced stopping distances that were not significantly different. Although this appears to imply using threshold-based inputs may have allowed BA to achieve stopping performance similar to that produced when maximum inputs were used, consideration of the enabled versus disabled stopping distance means indicates this may not necessarily be the case. For the force feedback tests, none of the stopping distance means produced with BA enabled were significantly shorter than those produced with BA disabled. Since the force feedback-based threshold applications were unable to effectively differentiate stops performed

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with BA enabled and disabled, it cannot be assumed that BA activation was responsible for the insignificant differences produced during the 65 mph straight line and 45 mph brake in-a-turn maneuvers (i.e., performed with force feedback).

Despite the similarity of the stops produced during the BA enabled and disabled tests performed with displacement feedback (i.e., as seen in Figure 5.42 and the previously shown Figures 5.3 and 5.13), the data shown in Table 5.63 indicate there was a significant difference between the mean stopping distances produced with the straight line maneuvers performed at 45 and 65 mph, and that BA intervention was responsible for these differences. Further evidence of BA effectiveness during displacement feedback tests can be found by considering the maximum versus threshold-based comparisons. Here, the mean stopping distances produced with BA enabled with threshold-based inputs were not significantly different than those achieved with maximum inputs. Therefore, these data indicate the threshold-based applications used in this study, in conjunction with displacement feedback, were able to successfully activate BA during tests performed with the Cadillac STS, and that the braking performance realized with these small brake pedal inputs was quite similar to that achieved using much larger brake pedal applications.

However, two threshold-based stopping distance means did produce significantly different results from those produced with maximum inputs: the brake in-a-turn tests with displacement feedback and the straight line stops from 45 mph with force feedback. This is best explained by considering the BA enabled versus disabled comparisons made for the same brake maneuvers. Here, the differences in the mean stopping distances were not significant and marginally insignificant, respectively. Since there was little difference between the means with BA enabled versus disabled, it is unlikely the effect of BA was adequately captured with the threshold based inputs. Since the inputs were unable to reduce stopping distance via BA activation, it is quite possible that the threshold-based inputs were simply unable to achieve stopping distances as short as those produced with the larger maximum inputs by virtue of their lesser magnitudes and the vehicle’s fundamental brake gain.

5.4.4 Toyota 4Runner

Figure 5.43 presents the mean stopping distances produced during tests performed with the Toyota 4Runner using displacement and force feedback-based brake applications. As seen with the other vehicles, the stopping distances associated with the maximum inputs continued to be nearly equivalent.

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Figure 5.43. Mean stopping distances produced during displacement (left) and force (right) feedback loop tests performed with the Toyota 4Runner.

As the trends in Figure 5.43 show, the authors’ ability to effectively detect the presence of BA during tests performed with the Toyota 4Runner was strongly dependent on which feedback loop was utilized. With displacement feedback, the mean stopping distances produced with BA enabled were significantly different (shorter) than their respective disabled means for each of the three stopping maneuvers, as shown in Table 5.64. This was not true when force feedback was used, where there were no significant differences between the mean stopping distances produced with BA enabled and disabled, regardless of stopping maneuver.

Table 5.64. Toyota 4Runner P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications.


Stopping Maneuver






65 Straight 0.0002 0.1843 0.2585 <0.0001

45 Curve <0.0001 0.0029 0.7912 0.0136

Straight <0.0001 0.0008 0.4062 0.0753

Comparison of the mean stopping distances produced with maximum versus threshold-based inputs produced disparate results for the Toyota 4Runner. For this vehicle, the significance of the differences between the two application types depended on stopping maneuver and feedback loop.

Two straight line stops, those performed from 45 mph with displacement feedback, and from 65 mph with force feedback, produced significantly shorter mean stopping distances than those produced with threshold-based inputs. As seen in Table 5.64, a similar trend was present for brake in-a-turn tests performed with displacement feedback, and to a lesser extent the brake in-a

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turn tests performed with force feedback. These results imply that for some test conditions, threshold-based inputs were unable to realize the vehicle’s maximum braking.

Conversely, the threshold-based straight line stops performed from 65 mph with displacement feedback, and from 45 mph with force feedback, did not produce stopping distances significantly longer than those achieved with maximum braking. In the case of the 65 mph displacement feedback-based comparison, the authors believe this can be attributed to high BA effectiveness, where threshold-based inputs produced BA activations that nearly realized the full braking capability of the vehicle.

However, for the 45 mph force feedback-based comparison, the lack of significance is most likely attributable to the magnitude of the threshold application being high enough to nearly realize the full brake capability of the vehicle (due to the gain of the foundation brakes). This is further supported by considering two points: (1) the similarity of the mean stopping distances produced with BA enabled and disabled straight line tests performed from 45 mph using force feedback-based threshold applications, and (2) the marked similarities of the mean stopping distances produced using maximum pedal force, noted earlier in Figure 5.43.

Finally, the authors believe the data presented in Figure 5.43 and Table 5.64 indicate that of the two feedback loops used in this study, displacement feedback provided a far superior method for evaluating the BA system installed on the Toyota 4Runner, regardless of maneuver type. Recall that the goals of this research were to objectively identify the BA activation thresholds, and to quantify what effect the use of these thresholds has on stopping distances (see Section 1.3). Having a test method that detects the presence of BA (enabled versus disabled) largely depends on whether experimenters are able to observe differences in braking performance via stopping distance. The displacement feedback results for the Toyota 4Runner clearly demonstrated a reduction in mean stopping distance between the BA enabled and disabled test conditions for all three test maneuvers.

Note: The fact there was a significant difference between the maximum and threshold-based stopping distance means when displacement feedback was used during straight line and brake ina-turn stops from 45 mph is less of a concern than the fact none of the maneuvers performed using force feedback with BA enabled produced stopping distances significantly different from those performed with BA disabled (i.e., an indication BA activation was likely not produced).

5.4.5 Volvo XC90

Figure 5.44 presents the mean stopping distances produced during tests performed with the Volvo XC90 using displacement and force feedback-based brake applications. In agreement with the trend observed with the other vehicles used in this study, the stopping distances produced with the maximum inputs continued to be essentially equivalent.

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Figure 5.44. Mean stopping distances produced during displacement (left) and force (right) feedback tests performed with the Volvo XC90.

The mean stopping distance trends produced with the Volvo XC90 (shown in Figure 5.44) were quite similar to those associated with the Toyota 4Runner (previously shown in Figure 5.43). However, the significance of the mean stopping distance comparisons, presented in Table 5.65, differed somewhat.

Table 5.65. Volvo XC90 P-value Summary for Stopping Distance Comparisons Made With Displacement And Force Feedback-Based Brake Applications.


Stopping Maneuver






65 Straight <0.0001 <0.0001 0.0239 0.0002

45 Curve <0.0001 0.3314 0.9148 0.0167

Straight <0.0001 0.0036 0.0009 0.0005

The mean stopping distances produced with BA enabled were significantly shorter than those achieved with the system disabled for five of the six Volvo XC90 comparisons used in this study. As shown in Table 5.65, the one comparison for which there was not a significant difference involved the force feedback brake in-a-turn tests, where the two ranges of stopping distances had substantial overlap (see Figure 5.34).

Interestingly, and in a manner similar to that previously described for the 45 mph straight line stops performed with the BMW 330i using force feedback, the Volvo’s BA enabled mean stopping distance was significantly longer than that produced with the system disabled. As before, the reasons for this phenomenon are not fully understood. However, recalling the

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distribution of stopping distances previously shown in Figure 5.26, test output variability and the likelihood force feedback-based threshold inputs did not activate BA are suspect.

Generally speaking, the mean stopping distances produced with maximum brake applications were significantly different than those achieved with threshold-based inputs. The one exception was the insignificant difference between stopping distance means produced with the displacement feedback brake in-a-turn maneuver. Given that the BA enabled mean stopping distance was significantly shorter than the disabled condition for this comparison, the authors believe that BA activation was responsible for the near-maximum stopping distances observed.

Each of the three stopping maneuvers performed with maximum pedal application and force feedback produced significantly different stopping distances than those performed with the threshold-based brake applications. The fact that each of these differences was significant agrees with results produced with the Chrysler 300C and BMW 330i. However, for the Volvo XC90, it must be pointed out that the threshold-based mean stopping distance from the brake in-a-turn maneuver was significantly shorter than that produced with maximum applications. The reasons for this are not fully understood.

5.5 Effect of Threshold Determination on Stopping Distance Mean Differences

It is important to recognize that at the end of the iterative process described in Chapter 4, the authors had identified brake applications believed to best represent the minimum combination of magnitude and rate necessary to activate BA. In Chapter 5, these threshold-based inputs were used for the various brake performance comparisons, some of which indicated that BA activation was not consistently achieved. In other words, test series containing individual trials performed with threshold-based inputs often contained both activations and what appeared to be non-activations. Additionally, given the stopping distance output disparity seen during some test series performed with these inputs, it is possible partial BA activations were present (i.e., the grouping of stopping distances were not clustered in a manner indicative of binary distinction).

This observation imposed an interesting dilemma. On one hand, the authors strongly believed that considering all braking performance observed during the respective threshold-based tests was meaningful and relevant. If a BA system was unable to achieve consistent performance, and that lack of consistency contributed to extended stopping distances (i.e., more closely related to those achieved with BA disabled), that was believed to be an important finding. On the other hand, including stopping data from BA enabled tests for which there was evidence activation was not fully realized confounds the ability to perform a “pure” analysis of BA performance. In other words, by including tests for which BA activation was not fully apparent in calculations intended to quantify BA enabled performance, the maximum extent to which BA can actually reduce stopping distances with threshold-based inputs may not be accurately represented.

The authors would have preferred that the threshold-based tests performed with BA enabled produce consistent and repeatable BA activations, so as to show the maximum effect of the technology. However, it is implicitly understood that by virtue of using threshold based inputs, both activation and non-activations were expected to occur. Increasing the input magnitudes slightly beyond those selected as threshold values was considered (i.e., after the tests discussed

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in Chapter 5 were performed), however the data produced by tests performed with the Cadillac STS indicated that seemingly small adjustments in input magnitude could cause the “threshold” based applications to become large enough that the maximum, or near maximum, braking capability of the vehicle could be realized by simply having the right combination of foundation brake system gain and a sufficiently large brake application.

Ultimately, the authors decided against attempting to produce a collection of threshold-based test series comprised solely of stops for which BA activation had been absolutely realized. Given that one of the primary objectives of this study was to identify the minimum combination of application magnitude and rates capable to activating BA, and that such inputs conservatively reside at the activation threshold, inclusion of all stopping data collected during the threshold-based test series was deemed appropriate. The authors concede that by choosing this approach, it is possible the maximum BA braking performance attainable within a given threshold-based test series may be diluted should non-activations occur.

Having said that, it should be emphasized there are important differences in the ability of the displacement and force feedback loops to evaluate BA performance at the activation threshold.

5.6 Inability of Force Feedback to Identify Brake Assist Activation Thresholds

Despite the use of common test methodology and data analysis techniques, the authors believe accurately identifying the force feedback-based BA activation threshold was not successful. Unlike the tests performed with displacement feedback, the iterative adjustment of input magnitude used during the force feedback tests provided little indication BA had an affect on stopping distance. Rather, the authors believe the differences in stopping distance initially attributed to BA activation (i.e., throughout the processes described in Chapter 4) were actually the result of interactions between application magnitude and foundation brake system gain.

When contemplating the problems of using force feedback, it is important to consider the relationship of brake controller command module magnitude to stopping distance, shown in the figures presented in Section 4.2. In these figures, there exists a range of command module settings for which stopping distance was not affected by commanded magnitude. The point where this first occurs is when the commanded brake application magnitude is able to realize, or nearly realize, the full braking capability of the vehicle. From this point, the authors do not believe BA activation is meaningful (i.e., capable of further reducing stopping distance). Therefore, the only opportunity for BA activation to reduce stopping distance during tests performed with force feedback is to intervene in response to commanded magnitudes less than those capable of producing full braking. Therein lies a problem, as the brake controller was generally unable to realize maximum application rate capability when low force magnitudes were commanded, as shown in Figure 4.25.

To understand why this phenomenon occurs, one must recognize that the controller must accelerate the brake pedal from zero position to the desired force target in a fixed amount of time, as configured by the command module settings. With low command module magnitude settings, the amount of time required to accelerate the vehicle’s brake pedal from zero position to the target is very short, particularly when high application rates are commanded. Since the brake

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controller requires a short period of time to “ramp up,” (i.e., bring the brake pedal from rest to the desired application rate), and a balance exists between being able to accelerate the pedal from rest as quickly as possible while still realizing the magnitude target with minimal overshoot, there is simply not enough time for the controller to achieve the commanded rate target before it must slow down. For these reasons, it appears there is little that can be done to remedy the problems associated with using force feedback to identify BA activation thresholds.

Note: Since the displacement feedback-based BA activation thresholds used higher commanded magnitudes than those associated with force feedback, higher application rates were generally attainable. This allowed the iterative processes designed to isolate the application rate associated with the BA activation threshold to be successfully executed with displacement feedback.

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6.0 CONCLUSIONS

The objectives of the work described in this report were twofold: (1) to objectively identify the BA activation thresholds of five contemporary test vehicles, and (2) to evaluate braking performance of the test vehicles using BA activation threshold-based brake applications. A programmable brake controller was used to perform each stop conducted in this study.

The study’s second objective was broken down into two parts. First, the braking performance of the vehicles with BA enabled was compared to that achieved with the system disabled. For these tests, only brake applications believed to be representative of the vehicles’ BA activation thresholds were used. Next, the braking performance achieved via use of threshold-based brake applications was compared to the maximum braking capability of the vehicle.

6.1 Identifying Brake Assist Activation Thresholds

Using the brake controller and two of its control feedback loops (closed loop control logic intended to monitor and maintain brake pedal displacement or applied force), a suite of test series were performed to iteratively isolate the minimum combinations of pedal displacement (or brake force), and rate of application capable of activating BA. Consideration of factors such as stopping distance, longitudinal acceleration, brake line pressure, and brake pedal force data traces were used throughout this process. The BA activation thresholds identified for each feedback loop are provided in Table 6.1.

Table 6.1. Brake Assist Activation Thresholds.

Vehicle

Displacement Feedback Force Feedback*


(inches)

Brake Pedal Rate

(in/sec)


(inches)

Brake Pedal Rate

(in/sec)

2006 BMW 330i 1.9 23.0 2.1 15.0

2005 Chrysler 300C 2.0 19.7 3.1 19.2

2004 Cadillac STS 2.6 18.6 3.3 19.1

2004 Volvo XC90 2.0 23.9 3.2 22.0

2003 Toyota 4Runner 2.8 26.9 3.2 25.9

*The inputs taken to represent the force feedback-based BA activation thresholds were deemed non-relevant after the conduct of subsequent tests.

Considering only the data used to derive the thresholds shown in Table 6.1, the authors were generally pleased with the results produced with displacement feedback. Use of this feedback loop provided the authors with brake pedal displacement magnitudes and application rates appropriate for use in evaluating BA effectiveness.

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Despite the use of common test methodology and data analysis techniques, the authors believe accurately identifying the force feedback-based BA activation threshold was not successful. Unlike the tests performed with displacement feedback, the iterative adjustment of input magnitude used during the force feedback tests provided little indication BA had an affect on stopping distance. Rather, the authors believe the differences in stopping distance initially attributed to BA activation were actually the result of interactions between application magnitude and foundation brake system gain.

6.2 Brake Assist Performance Evaluation

Once the thresholds believed to reside at the respective BA activation thresholds had been identified, BA performance was evaluated with three braking maneuvers: two straight line stops (initiated from 45 and 65 mph), and a brake in-a-turn maneuver initiated from 45 mph. For each maneuver, tests were performed with BA enabled and disabled. Two brake application severities were used: stops performed at the BA activation threshold and with maximum inputs.

Tests performed with threshold-based brake applications were nominally repeated ten times per test condition. For those tests performed with maximum braking, five tests per configuration were nominally performed. To assess whether differences in mean stopping distance were statistically significant, the data were analyzed with SAS software using a generalized linear model (GLM).

6.2.1 Displacement Feedback-Based Tests

Table 6.2 summarizes the differences in mean stopping distances from the BA enabled versus disabled tests performed with displacement feedback. In this table, a positive value indicates the mean stopping distance achieved with BA enabled was shorter than that produced with BA disabled. Conversely, a negative value indicates the mean stopping distance produced with BA enabled was longer than the comparable BA disabled series mean.

Beneath the respective mean stopping distance difference, Table 6.2 also features significance descriptors to indicate whether the change in stopping distance resulting from BA activation was significant and to what degree. Highly significant, significant, and significant findings confirm that BA was responsible for the stated change in stopping distance. An insignificant finding indicates that BA performance was indistinguishable from the foundation brakes while using the BA threshold input.

The summary presented in Table 6.2 provides strong evidence that use of displacement feedback was well suited to finding the BA activation threshold, as evidenced by the shortening of stopping distances in 13 of the 15 test conditions. It can also be said, with a high degree of certainty, that the BA systems evaluated in this study significantly shortened straight line stopping distances, with mean reductions ranging from 2.2 to 177.4 ft from 65 mph, and from 1.5 to 90.3 ft from 45 mph. Three of the five vehicles produced shorter stopping distances using BA during the brake in-a-turn tests from 45 mph, where mean reductions ranged from 3.5 to 98.0 ft.

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Table 6.2. Displacement Feedback BA Mean Stopping Distance Improvement (ft)*

Enabled vs. Disabled

Chrysler 300C

BMW 330i

Cadillac STS

Toyota 4Runner

Volvo XC90

65 mph straight line 11.0 31.0 2.2 68.5 177.4

45 brake in-a-turn 3.5 Insignificant Insignificant 49.1 98.0

45 mph straight line 12.3 8.3 1.5 22.5 90.3

*All brake applications based on inputs believed to reside at the vehicle’s respective brake assist activation threshold. The magnitudes of the stopping distance differences shown were significant at the p ≤ 0.05 level.

Table 6.3 summarizes the mean stopping distances differences observed during displacement feedback tests performed with maximum and threshold-based applications. Here, BA was enabled for each comparison. In agreement with the convention used in Table 6.2, positive numbers indicate a reduction in stopping distance. However, in Table 6.3 the shorter stopping distances were produced using the maximum pedal application. As before, the statistical significance can be found beneath the difference in mean stopping distance, demonstrating whether the change in stopping distance was significant and to what degree. Significant findings confirm that the BA threshold input did not provide braking performance comparable to the vehicle’s maximum capability. On the other hand, an insignificant finding indicates that the mean stopping distance achieved with a threshold-based input was indistinguishable from the maximum capability of the vehicle. This can be construed as a measure of BA’s effectiveness.

Table 6.3. Displacement Feedback BA Effectiveness Assessment (ft)*

Maximum vs. Threshold

Chrysler 300C

BMW 330i

Cadillac STS

Toyota 4Runner

Volvo XC90

65 mph straight line 16.0 32.2 Insignificant Insignificant 6.5

45 brake in-a-turn 15.4 37.4 16.1 4.9 Insignificant

45 mph straight line 10.8 11.8 Insignificant 7.3 4.0

*All stops performed with brake assist enabled. The magnitudes of the stopping distance differences shown were significant at the p ≤ 0.05 level.

The summary presented in Table 6.3 shows that in 11 of the 15 test conditions, use of maximum pedal displacement provided significantly shorter stopping distances than comparable tests using the BA activation threshold-based applications: 6.5 to 32.2 ft for the straight line tests initiated

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from 65 mph, 4.9 to 37.4 ft brake in-a-turn stops performed from 45 mph, and 4.0 to 11.8 ft for the 45 mph straight line tests.

Practically speaking, the authors believe the results shown in Tables 6.2 and 6.3 are encouraging. Generally speaking, use of displacement feedback-based application thresholds allowed the effect of BA to be successfully evaluated, and demonstrated the technology is capable of producing tremendous reductions in stopping distance for some vehicles—provided the right combination of brake pedal displacement and high application rate are applied. For four of the five vehicles evaluated in this study, the data shown in Table 6.3 indicate that tests using threshold-based applications with BA enabled were able to achieve mean stopping distances within approximately 16 ft of the vehicles’ maximum braking capability (the exception being the 65 mph straight line and 45 mph brake in-a-turn tests performed with the BMW 330i).

6.2.2 Force Feedback-Based Tests

Table 6.4 summarizes the differences in mean stopping distances from the BA enabled versus disabled tests performed with force feedback. In a manner consistent with that of Table 6.2, a positive value indicates the mean stopping distance achieved with BA enabled was shorter than that produced with BA disabled, while a negative value. Conversely, a negative value indicates the mean stopping distance produced with BA enabled was longer than the comparable BA disabled series mean. Also shown in Table 6.4 are significance descriptors to indicate whether the change in stopping distance resulting from BA activation was significant and to what degree.

Table 6.4. Force Feedback BA Mean Stopping Distance Differences (ft)*

Enabled vs. Disabled

Chrysler 300C

BMW 330i

Cadillac STS

Toyota 4Runner

Volvo XC90

65 mph straight line Insignificant Insignificant Insignificant Insignificant 3.4

45 brake in-a-turn -3.1 Insignificant Insignificant Insignificant Insignificant

45 mph straight line Insignificant -1.1 Insignificant Insignificant -3.5

*All brake applications based on inputs believed to reside at the vehicle’s respective brake assist activation threshold. The magnitudes of the stopping distance differences shown were significant at the p ≤ 0.05 level.

This summary clearly shows that use of force feedback does little to differentiate the mean stopping distances produced with BA enabled versus disabled, despite relying on nearly equivalent methodology and analysis techniques successfully used during the displacement feedback-based tests. With force feedback, only 4 of the 15 test conditions shown in Table 6.4 had a statistically significant difference in mean stopping distances, three of which had minor increases attributed to BA (between 1.1 and 3.5 ft longer).

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The lack of significantly different stopping distance means can be attributed to limitations imposed by the force feedback control logic. For each vehicle, there exists a range of command module settings for which stopping distance was not affected by the commanded magnitude. If command module inputs contained within this range are used, there is a high likelihood the maximum, or near maximum braking capability of the vehicle will be realized with or without BA activation. On the other hand, using command module settings below this range will not allow the brake controller to realize high application rates.

Since these low application rates were unable to activate BA for the vehicles evaluated in this study (i.e., those associated with low command module magnitudes), it was necessary for the authors to increase the command module settings. It is believed these settings were responsible for the small (and mostly insignificant) mean stopping distance differences shown in Table 6.4. In other words, the similarity of the force feedback-based BA enabled versus disabled results was not the result of BA intervention. Rather, it was achieved by virtue of the respective command module settings achieving highly effective braking.

Table 6.5 summarizes the mean stopping distances differences observed during force feedback tests performed with maximum and threshold-based applications. Here, BA was enabled for each comparison. In agreement with the convention used in Table 6.3, positive numbers indicate the extent to which the stopping distances achieved with maximum pedal force was shorter than those achieved with threshold-based applications. As before, the statistical significance can be found beneath the difference in mean stopping distance, demonstrating whether the change in stopping distance was significant and to what degree. Significant findings confirm that the BA threshold input did not provide braking performance comparable to the vehicle’s maximum capability. On the other hand, an insignificant finding indicates that the mean stopping distance achieved with a threshold-based input was indistinguishable from the maximum capability of the vehicle.

Table 6.5. Force Feedback BA Effectiveness Assessment (ft)*

Maximum vs. Threshold

Chrysler 300C

BMW 330i

Cadillac STS

Toyota 4Runner

Volvo XC90

65 mph straight line 7.3 6.0 Insignificant 5.4 10.7

45 brake in-a-turn 7.8 7.6 Insignificant 2.4 -3.8

45 mph straight line 11.3 2.9 1.7 Insignificant 5.4

*All stops performed with brake assist enabled. The magnitudes of the stopping distance differences shown were significant at the p ≤ 0.05 level.

The summary presented in Table 6.5 shows that in 11 of the 15 test conditions, use of maximum pedal force provided significantly shorter stopping distances than comparable tests using the applications taken to represent the force feedback-based BA activation threshold: 5.4 to 10.7 ft

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for the straight line tests initiated from 65 mph, 2.4 to 7.8 ft brake in-a-turn stops performed from 45 mph, and 1.7 to11.3 ft for the 45 mph straight line tests.

The results shown in Table 6.5 indicate that while the command module settings taken to represent the force feedback-based BA activation threshold were large enough to make distinctions between BA enabled versus disabled stopping distances insignificant (i.e., the application magnitude dominated any measureable effect of BA activation on stopping distance), these settings were not large enough to consistently realize the maximum braking capability of the respective vehicles. Given the similarity of the enabled and disabled stopping performance shown in Table 6.4, the authors believe this point has little to do with BA. Rather, it is simply believed to pertain to differences in brake application magnitude and foundation brake system gain. In other words, the settings taken to represent the force feedback-based BA activation threshold were large enough to nearly, but not fully, realize maximum braking even without BA activation.

Given its inability to distinguish the stopping performance realized with BA enabled versus disabled, while simultaneously contributing to physically similar stopping distances being realized with maximum and threshold-based brake applications, the authors believe that use of force feedback is unsuitable for evaluating BA performance. Alternatively, use of displacement feedback control logic is recommended.

6.3 Utility of Findings

In this report, the authors have provided information about the brake pedal input magnitudes and rates required to activate the BA systems of five contemporary vehicles. When used in conjunction with knowledge of the human driver’s physical brake application capability, as well as their willingness or reluctance to access it, the data presented in this report may be useful for estimating the extent to which BA activations may occur in real-world driving scenarios. The BA activation thresholds discovered during conduct of the displacement feedback-based tests are considered to be particularly relevant for consideration in such research.

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7.0 REFERENCES

1. Fitch, G.M., Wierwille, W.W., Blanco, M, Hanowski, R.J., “Human Performance Evaluation of Light Vehicle Brake Assist Systems, Task 3: Test Methods and Workplan,” VTTI: DTNH22-050D-01019, Sept.1, 2006.

2. Society of Automotive Engineers, “Stopping Distance Test Procedure, SAE J299 SEP93,” 1996 SAE Handbook, Vol. 2, Parts and Components, 1996.

3. DaimlerChrysler Corporation, “Chrysler 300 Series, 2005 Owners Manual,” Part No. 81-0260543, pg. 112.

4. Bayerische Motoren Werke, “325i, 330i Owner’s Manual for Vehicle,” Part No. 01 41 0 159 259, pg. 78.

5. General Motors Corporation, “2005 Cadillac STS Owners Manual,” Part No. 05STS A First Edition, pg. 4-10.

6. Toyota Motor Company, “2003 Owner’s Manual, 4Runner,” Publication No. OM35804U, Part No. 01999-35804, pg. 250.

7. Volvo Car Corporation, “Owner’s Manual Volvo XC90,” TP 6210, Elanders Graphic Systems AB, Goteborg, 2003, pg. 96.

8. U.S. Department of Transportation, “NHTSA Laboratory Test Procedure for FMVSS 135, Electronic Stability Control Systems,” TP-135-01, December 2, 2005.

9. See Docket Number NHTSA-2001-9663.

10. U.S. Department of Transportation, “NHTSA Laboratory Test Procedure for FMVSS 126, Electronic Stability Control Systems,” TP-126-01, April 10, 2008.

11. Heitzman, E.J., and Heitzman, E.F., “A Programmable Steering Machine for Vehicle Handling Tests,” SAE Paper 971057, SAE SP-1228, February 1997.

12. Heitzman, E.J., and Heitzman, E.F., “The ATI Programmable Steering Machine,” Automotive Testing, Inc. Technical Report, March 1997.

13. Forkenbrock, G.J., Garrott, W.R, Heitz, Mark, O’Harra, Brian C., “A Comprehensive Experimental Examination of Test Maneuvers That May Induce On-Road, Untripped Light Vehicle Rollover – Phase IV of NHTSA’s Light Vehicle Rollover Research Program,” NHTSA Technical Report, DOT HS 809 513, October 2002.

14. Mazzae, E.N., Barickman, F.S., Forkenbrock, G.F., Baldwin, G.H., “NHTSA Light Vehicle Antilock Brake System Research Program Task 5.2/5.3: Test Track Examination of Drivers’ Collision Avoidance Behavior Using Conventional and Antilock Brakes,” DOT HS 809 561, March 2003.

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APPENDIX A1.

Appendix A1 contains brake assist threshold determination plots for the BMW 330i. Figures A1.1 through A1.6 feature outputs from the Displacement Feedback tests. Figures A1.7 through A1.9 present outputs from tests performed with Force Feedback.

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Figure A1.1. 2006 BMW 330i brake assist threshold determination Step 1a (displacement feedback).

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Figure A1.4. 2006 BMW 330i brake assist threshold determination Step 2b (displacement feedback).

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Figure A1.5. 2006 BMW 330i brake assist threshold determination Step 3b (displacement feedback).

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Figure A1.6. 2006 BMW 330i brake assist threshold determination Step 4 (displacement feedback).

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Figure A1.7. 2006 BMW 330i brake assist threshold determination Step 1 (pedal force feedback).

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APPENDIX A2.

Appendix A2 contains brake assist threshold determination plots for the Chrysler 300C. Figures A2.1 through A2.4 feature outputs from the Displacement Feedback tests. Figures A2.5 through A2.7 present outputs from tests performed with Force Feedback.

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Figure A2.1. 2005 Chrysler 300C brake assist threshold determination Step 1 (displacement feedback).

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Figure A2.2. 2005 Chrysler 300C brake assist threshold determination Step 2a (displacement feedback).

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Figure A2.3. 2005 Chrysler 300C brake assist threshold determination Step 2b (displacement feedback).

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Figure A2.4. 2005 Chrysler 300C brake assist threshold determination Step 3 (displacement feedback).

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Figure A2.5. 2005 Chrysler 300C brake assist threshold determination Step 1 (pedal force feedback).

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APPENDIX A3.

Appendix A3 contains brake assist threshold determination plots for the Cadillac STS. Figures A3.1 through A3.6 feature outputs from the Displacement Feedback tests. Figures A3.7 through A3.9 present outputs from tests performed with Force Feedback.

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Figure A3.1. 2004 Cadillac STS brake assist threshold determination Step 1 (displacement feedback).

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Figure A3.2. 2004 Cadillac STS brake assist threshold determination Step 2a, Part 1 (displacement feedback).

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Figure A3.3. 2004 Cadillac STS brake assist threshold determination Step 2a, Part 2 (displacement feedback).

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Figure A3.4. 2004 Cadillac STS brake assist threshold determination Step 2b (displacement feedback).

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Figure A3.7. 2004 Cadillac STS brake assist threshold determination Step 1 (pedal force feedback).

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APPENDIX A4.

Appendix A4 contains brake assist threshold determination plots for the Toyota 4Runner. Figures A4.1 through A4.7 feature outputs from the Displacement Feedback tests. Figures A4.8 through A4.10 present outputs from tests performed with Force Feedback.

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Figure A4.1. 2003 Toyota 4Runner brake assist threshold determination Step 1 (displacement feedback).

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Figure A4.2. 2003 Toyota 4Runner brake assist threshold determination Step 2a (displacement feedback).

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Figure A4.3. 2003 Toyota 4Runner brake assist threshold determination Step 2b (displacement feedback).

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Figure A4.4. 2003 Toyota 4Runner brake assist threshold determination Step 3 (displacement feedback).

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Figure A4.5. 2003 Toyota 4Runner brake assist threshold determination Step 4a (displacement feedback).

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Figure A4.6. 2003 Toyota 4Runner brake assist threshold determination Step 4b (displacement feedback).

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Figure A4.7. 2003 Toyota 4Runner brake assist threshold determination Step 4c (displacement feedback).

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Figure A4.8. 2003 Toyota 4Runner brake assist threshold determination Step 1 (pedal force feedback loop).

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APPENDIX A5.

Appendix A5 contains brake assist threshold determination plots for the Volvo XC90. Figures A5.1 through A5.4 feature outputs from the Displacement Feedback tests. Figures A5.5 through A5.7 present outputs from tests performed with Force Feedback.

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Figure A5.1. 2004 Volvo XC90 brake assist threshold determination Step 1 (displacement feedback loop).

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Figure A5.5. 2004 Volvo XC90 brake assist threshold determination Step 1 (pedal force feedback loop).

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DOT HS 811 371September 2010

a test track evaluation of light vehicle brake assist

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