experimental validation of the cad technique for seat comfort design and evaluation

8/11/2019 Experimental Validation of the Cad Technique for Seat Comfort Design and Evaluation

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13B-0103

Experimental Validation of the Computer Aided Design

Technique for Seat Comfort Design and Evaluation

Copyright 2012 SAE International

ABSTRACT

Traditionally, seat comfort analyses are performed on

physically produced seats and, in most cases, human subjects

are asked to sit for a period of time to obtain personal

subjective and objective comfort measurements. When

performed in the seat comfort research laboratory at

Tennessee State University, such procedures required close to900 hours of testing to achieve feasible results. Besides, the

costs of the procedures were noticeably high. One of the

studies performed in the Lab employs computer driven

techniques to avoid physical prototyping for seat comfort

analyses. This technique is sought to eliminate the dependence

on the traditional techniques and reduce resources

consumptions. This study presents the crucial procedures

needed to validate and authenticate the new technique by

comparing its outcomes to the ones obtained by the

traditionally established techniques. The validation process

examines the CAD based system under diverse circumstances

that accentuate the predominant factors of seat comfort. Such

factors include sitters anthropometry, seat dimensions, seatfeatures, seat adjustability and cushions material properties.

The obtained results reflect high correlations between the

outcomes of the CAD based system and the traditional

methods. This study confirms that the proposed system is an

adequate tool that may replace the tedious and expensive

methods to perform seat comfort design and evaluations.

INTRODUCTION

Seat comfort analyses are usually obtained after the designer

completes a design cycle to the point where a physical product

is at hand. Such dependence yields resource exploitation and

complexity of alterations.In the first place, alterations can beretrofitted into the seat, but in most cases they have to be

applied to a new seat production cycle. Another shortcoming

is the dependence on human subjects which is expensive and

indefinite due to the lack of consistency in the humans

subjective inputs [1]. The research team in the Center of

Excellence for Seat Comfort (CoESC) at Tennessee State

University recognizes the need to improve this area of seat

comfort analyses, and therefore, developed an innovative

technique that enhances the design process of seat comfor

with less abuse of resource.

Several researchers have been involved in the study of sea

comfort; most suggest physical prototyping and/or human

involvement. Some researchers advocate that seat comfort

evaluation can be performed through tackling the physica

factors that influence seat comfort such as Biomechanics andPhysiological factors [2],vibration evaluation [3], thermal and

humidity factors [4]. One of the most eminent approaches

used to evaluate seat comfort is the observing of contact

pressure distribution. According to literature surveys

decreasing the contact pressure between the human and the

seat brings about more comfort [1, 3, and 5]. Contact pressure

measurement is usually done using pressure mapping systems

such as TekScan BPMS. CoESC performed a study tha

employed pressure mapping to evaluate the seat comfort for

ejection seats with regards to different rail angles [6]. Part of

this study was geared towards authenticating the outcomes of

the pressure mapping system with the sitters subjective

feedbacks. The results of this study confirm that contacpressure mapping reflects the comfort level of the tested seat.

CoESC suggests employing Computer Aided Design (CAD)

simulation rather than physical prototyping. CAD proved to

enhance the design process with better product

competitiveness, improved quality and superior information

sharing [7]. Researching the area of seat comfort shows

several studies that implemented CAD to perform seat comfor

analyses based on the fit, feel and human ergonomics.

The detection of high contact pressure regions is obtained

using Finite Element Analyses (FEA). FEA can be described

as a technique that demonstrates the reaction of an object in

CAD due to excitations; this may include force loadingscontact pressure, thermal excitations of fluid motion [8]

There are three major stages considered in the finite element

analysis; these are the Pre-processing stage, Analysis stage

and Post-processing stage [9]. In the Pre-processing stage, the

model is constructed with the needed parts to be analyzed then

a meshing stage is accomplished. FEA are implemented in

other researchers studies to detect the areas of high contac

pressures between seat cushion and human buttock tissue
http://www.sae.org/servlets/techpapers/paperHome.do?evtSchedGenNum=206303&evtName=13B-0103&prodGrpCd=PPRES&idTyp=paperhttp://www.sae.org/servlets/techpapers/paperHome.do?evtSchedGenNum=206303&evtName=13B-0103&prodGrpCd=PPRES&idTyp=paper


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Two dimensional models of a human thigh and a cushion were

used by Tang et al. considering the hyperelastic and

viscoelastic properties of the human body and the seat cushion

to simulate its mechanical behaviors. The goals of such studies

were geared toward investigating the effects of vertical

vibration on subcutaneous stress of buttocks using a finite

element modeling approach [10].

The main goal of this paper is to demonstrate the level of

accuracy of the CAD system for seat comfort analysis and its

ability to expedite the design process of new seats endorsed

with high comfort levels. This study is aimed to examine the

proposed systems reliability and repeatability with four

different factors including sitters anthropometrics, variable

seat dimensions, variable seats features and sitting period of

time.

METHODOLOGY

The initial stages of this study consisted of comprehensive

investigations in which the CoESC performed hundreds of

hours of experimentations geared toward examining the

various significant factors pertaining to seat comfort. The

experimentations concurrently explored the objective andsubjective analyses of seat comfort in order to establish a

common ground that relates the measurable factors with the

actual human feelings about the seat comfort. The objective

measurements performed include Oxygen Saturation, Blood

Pressure, Pulse Rate and Pressure Mapping. The subjective

analyses were performed using questionnaires given with a 5-

point scale type for the subject to rate the experienced

comfort. With zero being uncomfortable and five being

comfortable, the outcomes of these investigations reflected

that the pressure mapping tends to reflect high correlation with

the human feeling of comfort.

Several factors were examined and validated by theexperimentations. These factors were established as the ones

that are influential on seat comfort analysis. The factors

include:

Occupants weight distribution on top of the seat surface

Seat components such as cushion, armrests, footrest,

backrest etc.

Seat Measurements and size such as width, height, depth

etc.

Seat adjustability such as seat-pan height adjustment,

backrest angle tilt, etc.

Seat Cushion material properties such as Memory Foam,

Gel cushion, air-filled cushion, etc. Occupants anthropometry

In order to fabricate the human models in the CAD system, the

anthropometrics of the human subjects participating in the

actual experimentations were obtained. As illustrated in Figure

1, these anthropometrics were studied and compared to the Joint Primary Aircraft Training System (JPATS) classified

measurements. The anthropometric variables considered in

this study, shown in Table 1, are selected to agree with the

ones recommended by Reed [11]. The modeling of a human

being was achieved with correct anthropometrics that reflects

the ones of the participants. These models were also simulated

with the proper quasi-linear viscoelastic material properties

that allow its interaction to external loading to behave simila

to human flesh and bones [11].

The seats used in these experiments were the base reference to

the construction of the seat models. The results obtained from

the Tekscan Pressure Mapping System can be presented in

several forms but the most considered form was the threedimensional mapping of the pressure points to be in

accordance with the results obtained by the Finite Element

Analysis. The unified representation of the results is importan

for the researcher to be able to compare and contrast the

outcomes of the proposed system to the ones of the real-life

experimentations.

As signified in the introduction, the most considered metric

used to evaluate seat comfort is by investigating the regions of

high contact pressure between the human subject and the sea

surface. Hence, the experimentations were carried out to

validate the proposed technique by examining the contacpressure observed through traditional seat comfort evaluation

techniques and the ones of the CAD based technique. The

preliminary experiments were geared toward calibrating the

needed tools including the load measuring devices and the

pressure mapping system. Few of the experimentations were

performed to examine the distribution of loads exerted on the

different components of the seat by the occupants weight

Other experimentations aimed to validate the modeling of the

cushion material properties and their simulated reaction when

loaded using the Finite Element Analysis software. However

the foremost validation experimentation was performed using

Tekscan Pressure Mapping System to investigate legitimacy of

the CAD based technique.

Figure 1. Test Subjects Anthropometry Compared to the

JPATS Classes

Table 1.The Recommended Sitting Anthropometric

Measurements [11]


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The Weight Measuring Tool Selection

and Calibration

Human weight distribution on the seat surface was established

as a key factor for seat comfort analyses, therefore, the first

tool considered in achieving the adequate CAD validation is a

load measuring device that can measure the different loads

that are exerted on the seat surface by the occupant. Due to

the special settings of the experiment, the measuring tool

needed for the human-seat interface experimentation must

have low readability, memory feature, continuous reading

capability, flat surface and can operate on top of seat cushion

as well as on a hard surface.

A Taylor scale model number 7544BL was selected and

calibrated using standard weights and laboratory-utilized high

precision scales model S5000. Table 2 represents theoutcomes of this calibration process and the percentage of

error detected.

Table 2.The Outcome of the Load Measuring Device

Calibration Process

Weight*

(lb)

Lab Scale**

(lb)

Taylor Scale

On Hard Surface

(lb)

Error

6.235 6.235 6.20 0.56%

12.47 12.47 12.60 1.04%

18.705 18.705 18.90 1.04%

24.94 24.94 25.10 0.64%

Weight*

(lb)

Lab Scale**

(lb)

Taylor Scale

On Cushion

(lb)

Error

6.235 6.235 6.20 0.56%

12.47 12.470 12.50 0.24%

18.705 18.705 18.80 0.51%

24.94 24.94 24.90 0.16%

* Laboratory Standard weight** Laboratory-utilized high precision scales model S5000.

Contact Pressure Measuring Tool

Selection and Calibration

In order to validate the outcomes of the CAD technique, a

device is needed to obtain the contact pressure between the

human body and the seat surface. Several devices were

considered for this study; however, the CoESC selected the

TekScan Performance-Based Measurement System (PBMS)

Figures 2, 3 and 4 illustrate the implementation and outpu

representations of the selected device. PBMS comply with theneeded features and can provide the outcomes as a map tha

represents the pressure magnitude and location in relation to

the human body.

Figure 2. TekScan Performance-Based Measurement System

(PBMS) [5]

Figure 3. PBMS Digital output [5]

Figure 2. TekScan PBMS Delivers the Contact Pressure in a

Map Representation

iAcromion

Height Sitting

iiButtock-

Popliteal Length

iii Chest Breadth

ivForearm-

Forearm Breadth

vHip Breadth

Sitting

viInterscyeDistance

viiPopliteal Height

Sitting

viiiWaist Breadth

Omphalion

ix Head height


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Calibrating the PBMS is performed on seven (7) different

sitting scenarios where the outcomes of the PBMS are

compared to the ones obtained by the calibrated weight

measuring device. Figure 5 illustrates the outcomes obtained

by the PBMS compared to the ones of the calibrated scale.

Figure 3. Pressure Mat Calibration Graph

The Validation Process of the Computer

Aided Design Technique

Two major stages were considered to perform the validation of

the CAD technique; the first stage is to investigate the actual

distribution of the human loading on top of the seat surface.

The second stage is to examine the contact pressure obtained

from the CAD technique and compare it with the ones

obtained from the actual sitting experimentations (Figure 6).

Figure 4. CAD Validation by Conventional Methods

The loading distribution analyses are important in this study

for proper simulations using the Finite Element Analysis

technique which examines the regions of pressure points

between the seat and the occupant. Several experiments were

performed to examine the distribution of the distribution of

human weight on the seat with different postures and sea

components.

The first stage was performed with 5 different participants

aiming to investigate the load distribution on the seat surface

In the second stage, the pressure measuring device (PBMS) is

used to detect and measure the regions of high pressure points

The degree of repetition was tested for optimality; such testing

shows that fifty consecutive readings from the PBMS were

adequate for precision. The following steps are performed for

five different participants.

Step 1: Testing Seat Height Effects

As illustrated in Figure 7, Seat height is set to 17 inches and

gradually changed to 18 inches and 19 inches. In each trial 50

readings are taken with subject seated up on the pressure mat

with back in straight up position not touching the back support

and the arms placed on top of the thighs.

Figure 5. Testing the Seat Height Effects

Step 2: Testing Armrest Height Effects.

Illustrated in Figure 8, Seat height is set to 18 inches and the

armrest at 7 inches from the seat-pan. The armrest height is

raised to 8 inches then to 9 inches. For each change 50

readings are taken with the subject seated up on the pressure

mat with the back in straight up position not touching the back

support and the arms placed on armrest.

Figure 6. Testing the Armrest Height Effects


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Step 3: Testing Backrest Angle Effects

Seat height is set to 18 inches, the armrest height at 7 inches

and backrest angle at 90. Then the back angle is gradually

increased by 10 as showing in Figure 9. In each trial, 50

readings are taken with subject seated back on the pressure

mat with back leaned on backrest and the arms placed on

armrest.

Figure 7. Testing the Backrest Height Effects

Step 4: Testing Headrest and Footrest Effects

Seat height is set to 18 inches, the armrest height at 7 inches,

backrest angle at 100 and headrest height at 42 inches. Then

50 readings are taken while subject is seated back on the

pressure mat with back leaned on backrest, arms placed on

armrest and head rested on headrest.

The next trial test the effect of the headrest by setting the seat

height is at 18 inches, the armrest height at 7 inches and

backrest angle at 100, headrest height at 40 inches and

footrest at 4 inches height. 50 readings are taken while subject

is seated back on the pressure mat with back leaned on

backrest, arms placed on armrest, head rested on headrest and

feet rested on footrest. See Figure 10.

Figure 8. Testing the Headrest and Footrest Effects

Step 5: Testing Cushion Material Effects

The first trial of this step tests the effects of hard surface seat

In this trial the seat height is set to 18 inches then the hard

surface is placed on seat pan and pressure mat is placed on the

top. 50 readings are taken while the subject is seated up on the

pressure mat with back in straight up position not touching the

back support and the arms placed on top of the thighs. The

second trial is performed on memory foam cushion which isplaced on seat pan and pressure mat is placed on the top. 50

readings are taken while the subject is seated with the same

posture. Subsequently, the gel-filled cushion and the air-filled

cushions were tested.

Step 6: Testing Time Effects

In this step the human subject is asked to sit up for thirty (30)

minutes. This period was compared to 10 minutes, 20 minutes

one hour and two hours. However 30 minutes seem to provide

optimum outcomes. The seat is set on 18 inches high and the

subject sits on the pressure mat with back in straight up

position not touching the back support and the arms placed on

top of the thighs.

Results of CAD Technique Validation

The results of the sitting scenarios were collected and

conditioned in a unified form then tabulated in a manner tha

makes it easier to review. The data shows nine different sets

that represent the trends between the number of seat features

cushion softness and the contact pressure examined for five

different human subjects with diverse anthropometries. The

nine sitting scenarios were simulated with the CAD systemand the regions of contact pressure were analyzed via Finite

Element Analysis. In each simulated case the results were

represented in figures to facilitate the validation process.

The first review was performed to validate the results obtained

by the CAD for different seat features. This includes armrests

backrest, footrest, and headrest. The second set of results was

illustrated to understand the effects of the cushion material on

seat comfort using the criteria of high contact pressure

detection. Five different cushion materials were considered

including hard wood surface, elastic mesh, memory foam, ge

cushion and air-filled cushion. In order to ensure accuracy

calibration testing was performed to compare the reaction ofthe simulated cushions in CAD to the actual loading of the

obtained cushions. This process aims to validate the materia

properties provided by the manufacturer and calibrate the

system by compensating for covers and other materials used

for the support and esthetics. The cushions and human

subjects material properties calibration results are illustrated

by Figures A1-A10 in the Appendix.


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Figure 9. Cushion Material Validation in CAD

Figure 10. Cushion material Validation Results

Error AnalysesAs depicted in the results, the outcomes of the validation

process behave with a trend which agrees that more seat

features seems to promote seat comfort. Furthermore, the

trend detected from the review of the cushion material effects

concurs that changing the softness of the cushion material

seems to reduce the contact pressure level. The average error

manifested in this study is, at maximum, 3.3% which is related

to a pressure that can be produced from having heavier clothes

or due to carrying items like wallets or cellular phones. In any

case, the observed error is insignificant to interfere with the

design decision making or to the evaluation of the sea

comfort level. A case study is presented to demonstrate the

outcomes of the CAD system for seat comfort design and

evaluation. This study involves a new seat design for the

North Americas population and the CAD system is used to

examine the effects of theparameters suggested to satisfy the

comfort level of the given seat. Figure A11 in the Appendix

shows the analyses results of the CAD system with differen

seat height, armrest height, backrest angle, headrest and

footrest existence, and different cushion materials.

CONCLUSION

Several traditional seat comfort evaluation experiments were

performed to test the validity of the system proposed by the

Center of Excellence for Seat Comfort. Human subjects with

diverse anthropometry were employed to sit with varying

scenarios. The testing was properly simulated and analyzed

using the CAD technique. Validation of the system was

obtained by comparing its outcomes to the ones of the actua

sitting. The results obtained from the CAD system

demonstrates the effects of the seat features and seat cushion

softness on seat comfort as established from literatures. When

comparing the outcomes of the CAD system to the traditiona

technique, a great deal of similarity was observed, not only by

the magnitude of the contact pressure, but also by the

distribution of the pressure regions between the human and the

seat surface. An average error of 3.3% was observed which

can be compared to the pressure difference produced by the

occupants clothes and pocket contents. In other words, the

observed error was insignificant and does not interfere with

goals of the study. However, with the resilience of the CAD

system this error can always be reduced. The results of this

study agree strongly with literature survey while more

comprehensive analyses were carried out with this technique.

ACKNOWLEDGMENT

This work was supported by the Boeing Corporation under

contract TBC-TSU-GTA-1. We wish to acknowledge Edward

Winkler and Jarrett Datcher of The Boeing Company for their

support and coordination of this effort.

REFERENCES

1.

De Looze, M. P., Kuijt_Evers, L. F. M. and Dieen, J. V.

Sitting Comfort and Discomfort and the Relationship

with Objective Measures Ergonomics, Vol. 46, No. 102003, pp. 985-997, 2003

2.

Reed, M. P., Saito, M., Kakishima, Y., Lee, N. S., and

Schneider, L. W. An investigation of drive r discomfor

and related seat design factors in extended-duration

driving. SAE Technical Paper 910117. Warrendale, PA

Society of Automotive Engineers, Inc. 19913. Nilsson, L, and Johansson A. Evaluation of Discomfort

Using Real-time Measurements of Whole Body Vibration

and Seat Pressure Distribution While Driving Trucks


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Master of Science Thesis, Lule University of

Technology, Scandinavia. 20064.

Ormu K., and Mufti M., Main Ambient Factors

Influencing Passenger Vehicle Comfort, Proceedings of

2nd International Ergonomics Stubike Toplice, Zagreb,

Croatia October 2004.

5. Milivojevich, A., Blair R., Pageau J. -G, and Russ A.,

Investigating Psychometric and Body Pressure

Distribution Responses to Automotive Seating Comfort,

SAE Technical Paper No. 2000-01-0626, 2000.

6.

Ojetola, O., Onyebueke, L., Winkler, E., Ejection Seat

Cushions Static Evaluation for Three Different rail

angles, SAE International, 2011.

7.

Mamat, R., Wahab, D. A., Abdullah, S., The Integration

of CAD and Life Cycle Requirements in Automotive Seat

Design European Journal of Scientific Research ISSN

1450-216X Vol.31 No.1, pp. 148-156, 2009.

8. Pileicikiene G., Surna A., Barauskas, R., Surna R.,

Basevicius, A., Finite element analysis of stresses in the

maxillary and mandibular dental arches and TMJ articular

discs during clenching into maximum intercuspation,

anterior and unilateral posterior occlusion.

Stomatologija, Baltic Dental and Maxillofacial Journal,

9:121-128, 2007.9.

Roylance, D., Finite Element Analysis Department of

Materials Science and Engineering Massachusetts

Institute of Technology Cambridge, MA 02139. February

28, 2001.

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Tang C, Y., Tsui, C.P, Method of Modeling Muscular

Tissue with Active Finite Elements, U. S. Patent, the

Hong Kong Polytechnic University, Kowloon (HK), 2006

11. Reed, M. P., Schneider, L. W., and Ricci, L. L.,Survey of Auto Seat Design Recommendations forImproved Comfort, Technical Report No. UMRTI-

94-6, University of Michigan. 1994


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APPENDIX

Results obtained for Test Subject 1

Table A1. Test Subject 1 Anthropometric Data

label Gender Age

TS1 Female 24

trial 1 trial 2 trial 3 Average

Gross Weight (lb) 113.0 113.0 113.0 113.0Acromion Height Sitting (in) 18.6 18.5 18.5 18.53

Buttock-Popliteal Length (in) 21.25 21.5 21.5 21.42

Chest Breadth (in) 18 18.75 18.75 18.5

Forearm-Forearm Breadth (in) 19 18.75 18.75 18.83

Hip Breadth Sitting (in) 19.25 19.25 19.25 19.25

Interscye Distance (in) 16.5 16.5 116.5 49.83

Popliteal Height Sitting (in) 17.5 16.5 16.5 16.83

Waist Breadth Omphalion (in) 18.5 19 19 18.83

Table A2. Test Subject 1 Validation Results with Seat Features

S1 Female Contact pressure (PSI)

24 YO, 113 lbs BPMS CAD ERROR

Seat Pan Height

17 in 3.2611 3.3215 0.0604

18 in 3.2229 3.2567 0.0338

19 in 3.4194 3.4335 0.0141

Armrests Height

7 in 2.2998 2.3487 0.0489

8 in 2.7749 2.9011 0.1262

9 in 2.8636 2.9876 0.124

Back Angle

90 1.6395 1.7002 0.0607

100 1.4464 1.4723 0.0259

110 1.1859 1.2009 0.015

Headrest exists 1.0921 1.1232 0.0311

Footrest exists 0.9882 1.0324 0.0442

Figure A1. Test Subject 1 Seat Features Validation Graph


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Table A3. Test Subject 1 Validation Results with Seat Cushion Material

Contact Pressure (PSI)

BPMS CAD ERROR

Cushion Material

Hard surface 4.5817 4.671 0.0893

Elastic mesh 3.2611 3.3215 0.0604

Memory Foam 1.4958 1.5664 0.0706

Gel-filled 1.0129 1.1098 0.0969

Air-filled 0.968 0.9721 0.0041

Figure A2. Test Subject 1Cushion Material Validation Graph


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Results obtained for test subject number 2

Figure 11. Test Subject 2 Seat Feature Validation Graph

Figure A4. Test Subject 2 Materials Effects Validation Graph


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Figure A6. Test Subject 3 Seat Feature Validation Graph

Figure A5. Test Subject 3 Cushion Material Effects Validation Graph


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Case Study for a New Seat Design for the North America Population

Figure A11. Case Study Outcomes forNew Seat Design Using CAD System

experimental validation of the cad technique for seat comfort design and evaluation

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