optimal gear shifting strategy for a seven-speed automatic transmission used on a hydraulic hybrid...

7/29/2019 Optimal Gear Shifting Strategy for a Seven-Speed Automatic Transmission Used on a Hydraulic Hybrid Vehicle

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A Thesis

entitled

Optimal Gear Shifting Strategy for a Seven-speed Automatic Transmission

Used on a Hydraulic Hybrid Vehicle

by

Yaoying Wang

Submitted to the Graduate Faculty as partial fulfillment of the requirements for the

Master of Science Degree in Mechanical Engineering

__________________________________________

Dr. Walter W. Olson, Committee Chair

__________________________________________

Dr. Yong Gan, Committee Member

__________________________________________

Dr. Mohammad Elahinia, Committee Member

__________________________________________

Dr. Patricia Komuniecki, Dean

College of Graduate Studies

The University of Toledo

March 2012


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Copyright 2012, Yaoying Wang

This document is copyrighted material. Under copyright law, no parts of this

document may be reproduced without the expressed permission of the author.


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An Abstract of

Optimal Gear Shifting Strategy for a Seven-speed Automatic Transmission

Used on a Hydraulic Hybrid Vehicle

by

Yaoying Wang

Submitted to the Graduate Faculty as partial fulfillment of the requirements for

The Master of Science in Mechanical Engineering

The University of Toledo

May 2011

Hydraulic technology can be used to capture and transfer high levels of

energy extremely quickly and have a longer operating life compared with similarly

sized electric systems. The hydraulic hybrid vehicle (HHV) includes two power

sources that propel the vehicle: a fuel-efficient diesel combustion engine and

hydraulic components. This technology replaces a conventional drive train with a

hydraulic one, which eliminates the need for a mechanical transmission and driveline.

To explore an optimal gear shifting strategy with best fuel economy for a

seven-speed automatic transmission used on a hydraulic hybrid vehicle, a strategy is

designed with a highest possible gear criterion as long as the torque requirement can

be satisfied, except for braking process and torque demanding situations. The

optimization strategy takes several other criteria into consideration, such as high

motor displacement criterion, to improve efficiency and fuel economy. Then the

optimization strategy is developed on the basis of these criteria from two main aspects

of the existing SIMULINK truck model. One approach is based on the hydraulic


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motor working conditions, such as motor displacement, and the other is based on the

drivers intention, which is interpreted as the driver pedal position. This controller is

able to recognize the drivers intention to change the speed and incorporate it into

gear shifting decision making.

Then a SIMULINK controller model is developed based on the optimal gear

shifting strategy and criteria and validated both in fuel economy and power

performance by analyzing the simulation results in the Federal Urban Driving Cycle.


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Acknowledgments

First, I would like to express my sincerest gratitude to my advisor, Dr.

Walter W. Olson, for his guidance on my graduate study and research. It is very

lucky and a great honor to be his student and have the opportunity to work with him

in such an interesting research area of hydraulic power. His academic enthusiasm

and persistence do inspire my motivation for further study and future research work.

Without his tremendous support and patience, I would not be able to complete this

thesis.

Special thanks to my committee members Dr. Yong Gan and Dr.

Mohammad Elahinia, who give constructive advices and comments on my thesis. I

would specifically like to give a thank you to the help and friendship of my partners,

Andrew Sulzer and James Sweetman, who spare their precious time to review and

correct my writing while they are busy working on their theses.

Many thanks to the financial and technical support from Southwest

Research Institute (SwRI), Bapiraju Surampudi, Joe B. Redfield and Glenn R.

Wendel.

Most thankful for my family and friends who always give their

unconditional love and support to me wherever I am and whatever I do. Thank you

all for accompanying me in my heart.


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Table of Contents

Acknowledgments..................................................................................................... v

Table of Contents ..................................................................................................... vi

List of Tables ............................................................................................................. ix

List of Figures ............................................................................................................ x

Nomenclature ........................................................................................................... xii

Chapter 1 Introduction .......................................................................................... 1

1.1 Background of Research ................................................................................. 1

1.2 Problem Statement ........................................................................................... 3

1.3 Work Outline ................................................................................................... 3

Chapter 2 Literature Review ............................................................................... 5

2.1 Hydraulic Hybrid Vehicles ............................................................................ 5

2.1.1. Parallel ..................................................................................................... 7

2.1.2 Series ......................................................................................................... 8


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2.2 Optimal Transmission Strategies ..................................................................... 9

Chapter 3 Control Criteria and Shifting Strategy ........................................ 18

3.1 Control Criteria .............................................................................................. 19

3.2 Shifting Strategy ............................................................................................ 23

3.2.1 Shifting Based on the Driver Pedal ......................................................... 23

3.2.2 Shifting Based on Hydraulic Motor Conditions .......................................... 25

3.3 Control Algorithm ......................................................................................... 25

Chapter 4 Simulation Design ............................................................................ 27

4.1 Transmission Model ...................................................................................... 27

4.2 Controller System Model .............................................................................. 29

4.2.1 Driver Shifting Controller ....................................................................... 30

4.2.2 Speed Computation ................................................................................. 31

4.2.3 Braking Controller .................................................................................. 33

4.2.4 Priority Selection ..................................................................................... 34

4.2.5 Overall Speed Violation of the Hydraulic Motor .................................... 35

4.2.6 Dwell Time Controller ............................................................................ 36

4.2.7 Gear Memory .......................................................................................... 36

4.2.8 Gear Ratio Matching ............................................................................... 37


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Chapter 5 Simulation Results............................................................................ 39

5.1 Fuel Consumption ......................................................................................... 40

5.2 Gear Shifting Schedule .................................................................................. 42

5.3 Tracking Performance ................................................................................... 44

5.4 Hydraulic Motor Speed ................................................................................. 47

5.5 Summary ....................................................................................................... 48

Chapter 6 Summary and Conclusion .............................................................. 50

6.1 Summary ....................................................................................................... 50

6.2 Conclusion ..................................................................................................... 51

6.3 Future Work ................................................................................................... 51

References................................................................................................................. 53


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List of Tables

Table 5.1 Simulation results comparison between controller and original model ....... 49


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List of Figures

Figure 1-1 Configuration block diagram of the hydraulic hybrid vehicle ..................... 2

Figure 2-1 Parallel configuration [5] ............................................................................. 7

Figure 2-2 Series configuration [5] ................................................................................ 8

Figure 3-1 Plots of efficiency vs. motor displacement of each gear ............................ 21

Figure 3-2 Plots of motor displacement vs. acceleration of each gear ........................ 21

Figure 4-1 Original model with default shifting schedule ........................................... 28

Figure 4-2 Block diagrams with the optimal shifting controller in orange .................. 29

Figure 4-3 All block diagrams of the optimal controller top layer .............................. 30

Figure 4-4 Driver shifting controller block diagrams in orange .................................. 31

Figure 4-5 Speed computation block diagrams in grey ............................................... 33

Figure 4-6 Braking controller block diagrams in orange ............................................. 34

Figure 4-7 Hydraulic motor speed limit controller block diagrams in red .................. 35

Figure 4-8 Dwell time controller block diagrams in magenta ..................................... 36

Figure 4-9 Gear memory by using unit delay .............................................................. 37

Figure 4-10 Gear ratio matching controller ................................................................. 38

Figure 5-1 EPA Federal Urban Driving Schedule (FUDS) .......................................... 40

Figure 5-2 Fuel consumption simulation results comparison: fuel consumptionKg

vs. time (seconds)......................................................................................................... 41

Figure 5-3 Controller model simulation results: gear ratio vs. time (seconds) ............ 42

Figure 5-4 Original model simulation results: gear ratio vs. time (seconds) ............... 43


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Figure 5-5 Controller model simulation results: actual and desired vehicle speed (m/s)

vs. time (second) .......................................................................................................... 44

Figure 5-6 Original model simulation results: actual and desired vehicle speed (m/s)

vs. time (second) .......................................................................................................... 45

Figure 5-7 Simulation results comparison between original and controller model:

vehicle speed error (m/s) vs. time (second) ................................................................. 45

Figure 5-8 Controller model simulation results: hydraulic motor speed

(radian/second) vs. time (second) ................................................................................ 47

Figure 5-9 Original model simulation results: hydraulic motor speed (radian/second)

vs. time (second) .......................................................................................................... 48


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Nomenclature

CVT

EPA

FUDS

HHV

IC

SwRI

Continuously variable transmission

Environmental Protection Agency

Federal Urban Driving Schedule

Hydraulic hybrid vehicle

Internal combustion

Final drive ratio

Transmission gear ratio

Minimum transmission gear ratio

Radius of the vehicle tire

Southwest Research Institute

Output torque corresponding to

Output torque corresponding to

Vehicle speed

Motor speed

Torque convertor efficiency

Minimum torque convertor efficiency


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Chapter 1

Introduction

1.1 Background of Research

In many applications especially those where high power densities are

required, hydraulic hybrid systems can offer a more efficient alternative to those

driven by electric motors. Hydraulic technology can be used to capture and transfer

high levels of energy extremely quickly compared with similarly sized electric

systems, which generally require long periods over which batteries have to be

charged. Hydraulic systems are also likely to have a longer operating life than

battery-powered devices.

Similar to an electric hybrid vehicle, which includes a gas or diesel engine

and battery, the hydraulic hybrid vehicle (HHV) includes a diesel engine and a

hydraulic power system that, in laboratory testing, has achieved significant fuel

economy over traditional UPS vehicles. Hydraulic hybrid technology includes two

power sources that propel the vehicle: a fuel-efficient diesel combustion engine and

hydraulic components. This technology replaces a conventional drive train with a


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hydraulic one, which eliminates the need for a mechanical transmission and

driveline.

These vehicles can store energy from the hydraulic system, even after the

vehicle is turned off. This storage allows the vehicle to start with this energy, instead

of relying on the engine to propel the vehicle [1].

To improve fuel economy of a military vehicle or family of vehicles, the

hydraulic hybrid technology is applied and the ways it can benefit ancillary vehicle

functions and enhance the mission usefulness of the vehicle are investigated as the

purpose of this project. Figure 1-1 below is the configuration of the hydraulic hybrid

vehicle developed in this project.

Figure 1-1 Configuration block diagram of the hydraulic hybrid vehicle


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1.2 Problem StatementThe objective of this thesis is to explore an optimal gear shifting strategy for

the best fuel economy with a seven-speed automatic transmission used on a

hydraulic hybrid vehicle. A simulation model of this automatic transmission was

developed in SIMULINK by engineers at Southwest Research Institution that takes

the seven-speed gear ratios as a shifting schedule input to the transmission model

and exports the vehicle speed, vehicle distance, driver pedal position, current gear

ratio, fuel consumption, simulation time, motor pressure, motor displacement, motor

speed and efficiency as outputs to a MATLAB workspace. Though shown as a

seven- speed model, the strategy discussed is for the range from 2nd to 7th gear as a

shift directly from 1st to 2nd gear is not allowed while the vehicle is moving.

In addition, since the transmission is no longer driven by the IC engine, the

instantaneous fuel consumption cannot be computed from outputs of the

transmission model. Part of the work of this thesis is to research and identify better

operational measures by which fuel economy can be optimized.

1.3 Work Outline

Chapter 2 is the literature review which represents the development of

hydraulic hybrid vehicles along with illustrations of two main configurations:

parallel and series. It also introduces some different approaches to transmission

optimization.


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Chapter 3 represents the shifting criteria and analyzes the shifting strategy

for the transmission.

Chapter 4 shows the simulation block diagrams built in

MATLAB/SIMULINK according to the shifting strategy.

Chapter 5 discusses the simulation results and compares the results to the

original shifting model simulation.

Chapter 6 is the conclusion of the optimal shifting strategy simulation of the

transmission and represents the future work.


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Chapter 2

Literature Review

2.1 Hydraulic Hybrid Vehicles

A hybrid vehicle combines two or more sources of power. Currently, hybrid

technology is widely recognized as the most effective measure to solve the energy

problem. Heavy vehicles such as city buses have the characteristics of high

stop-and-go duty cycles and high power flow braking energy, which needs to find an

efficient way to store and reuse the braking energy [2]. The additional power source

can be electrical, chemical, hydraulic, flywheel operated or any other form of power

storage and production [3]. Within the many hybridization options, battery and fuel

cell have the characteristics of high energy density and are well suited for light

vehicles. However, the high internal resistances and handling the wasted battery are

major obstacles for commercialization. Both fuel cell and battery hybrid vehicles can

only marginally recover the braking energy. Moreover, high frequency charging and

discharging will lead to overheating and battery destruction. An ultracapacitor (UC)


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has higher power density than a battery because their operation does not employ a

chemical reaction, but the high cost and relatively lower reliability constrain their

applications [2,5,4].

As an important method of hybrid technology, hydraulic hybrid vehicles

(HHV) now attract the attention of worldwide research institutions and commercial

industrial companies. The first-ever hydraulic-hybrid diesel urban-delivery vehicle

reportedly improves fuel economy by 60 to 70% and reduces carbon-dioxide

emissions more than 40% in initial laboratory testing. EPA estimates that the

technology has the potential to save more than 1,000 gallons/yr for each

urban-delivery vehicle [11].

In a hydraulic hybrid vehicle, the hydraulic power assists the conventional

internal combustion engine by providing additional torque to the driveshaft [6]. The

hydraulic hybrid system with accumulators and hydraulic pump/motors have the

potential for improving fuel economy by operating the engine in the optimum

efficiency range and making use of the regenerative braking during deceleration.

When braking, a hydraulic hybrid can recover and reuse braking energy that is

normally wasted. As the vehicle stops, energy from the wheels pumps fluid from a

low-pressure reservoir into the high-pressure accumulator. When the vehicle

subsequently accelerates, the stored energy propels the vehicle. According to EPA

officials, this process recovers and reuses more than 70% of the energy normally

wasted during braking, and it also reduces brake wear by about 75%, substantially

increasing the savings [11].


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There are two main configurations for hydraulic hybrid vehicles, parallel and

series. Both of them have the main components which consist of a high-pressure

accumulator storing energy by using hydraulic fluid to compress nitrogen gas stored

inside each accumulator, a low-pressure reservoir storing hydraulic fluid after it has

been used by the pump/motor, a pump/motor converting high-pressure hydraulic

fluid into rotating power for the wheels and transmitting braking energy back to the

high-pressure accumulator, and an engine pump transmitting pressurized hydraulic

fluid to the pump/motor, the high-pressure accumulator, or both [1].

2.1.1. Parallel

Shan [7] reports parallel hydraulic hybrid vehicles are easier to implement,

but efficiency gains are limited by the solid link between the wheels and engine.

Figure 2-1 Parallel configuration [5]


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The rear pump/motor acts as the drivetrain, such that in the motor mode, it

drives the axle using high pressure fluid and, as the vehicle brakes, the pump/motor

directs the fluid to the high pressure accumulator by switching into the pump mode.

The high pressure accumulator is usually designed to satisfy internal pressure loads

up to 35MPa (5000 psi), whereas the low pressure accumulator holds the internal

pressure of up to 1.4MPa (200 psi) [8, 9].

2.1.2 Series

Series hydraulic hybrid vehicles allow engine speed to be decoupled from

vehicle speed. This permits a control strategy where the engine and other hydraulic

components operate only near maximum efficiency.

Figure 2-2 Series configuration [5]

In this configuration, the conventional driveline is completely removed; it is


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connected through hydraulic pipes and the drive pump/motors are used to transfer

power and propel the vehicle. The drive pump/motor converts high pressure

hydraulic fluid into rotating power in the motor mode and transmits regenerative

braking energy back to the high pressure accumulator in the pump mode. Since the

direct link between the engine and the driveline components is removed, the engine

is separated from the road and higher efficiencies are anticipated. [7].

2.2 Optimal Transmission Strategies

As it is important part of the vehicle powertrain to convert torque and

rotation, the optimization of the transmission can increase the performance of the

vehicle efficiency and fuel economy significantly. This depends on different working

conditions and rules, and various optimal control methods are applied to different

kinds of transmissions.

A. Haj-Fraj [10] introduces an optimal control approach for gear shift

operations in automatic transmissions as a multistage decision process by making

use of dynamic programming method. Starting from a verified model of a typical

powertrain, Haj-Fraj considers three optimization parameters as a performance

measurement for evaluating the gear shift process: the control data for the clutch

pressure in the gear box, the engine load-reduction and the evaluation of the gear

shift duration. As the passengers comfort is a subjective issue and varies from driver


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to driver, it can generally be stated that the smoother the acceleration change is and

the smaller the peaks of the jerk are, the more comfortable the gear shifting is. To

engage the clutch of the target gear with a very smooth and slow rising pressure in

order to get a very slow transition in the dynamical behavior of the powertrain in

order to get a smooth acceleration but a very long gear shift process. A control law is

derived analytically in an explicit form by minimizing the performance measure over

each process stage and the optimization finally is solved with a sequential quadratic

programming algorithm.

The results have shown that the passengers comfort and the duration of the

gear shift operation represent contrary issues. This means that an improvement in

one criterion leads to a deterioration in the other one. Therefore the optimization

problem represents be leads to find a reasonable solution that can define a

compromise between the two issues.

The shift schedule proposed by Gong Jie [13] for the ground vehicle

automatic transmission by studying the function of the torque converter and

transmission shift schedule can keep the torque converter working in the high

efficiency range under all the working conditions except in the low efficiency range

on the left when the transmission worked at the lowest shift, and in the low

efficiency range on the right when the transmission worked at the highest shift. In

order to evaluate the economic performance of the torque converter, the efficiency of

the torque converter is required no less than an ideal value in use ( =

75% in engineering machinery, = 80% in the normal automobile). The shift


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schedule aims to control and set the common working point of engine and torque

converter so that the torque converter efficiency is no less than the ideal value

so that . In other words, the points satisfying the condition

are used as the shift points.

The gear ratio and corresponding torque are analyzed as the key

factors to determine the shift points. After the experimental test-bed results, the

torque converter efficiency was controlled to be over 75% in the high efficiency

range by gear shift according to the actual load acting on the drivetrain.

This method focuses most on the relationship between the fixed gear ratio

and the continually changing actual gear ratio along with its torque value, which is

much easier to control compared to the dynamic controller stated in Haj-Frajs.

Toshinichi Minowa [15] investigates a powertrain control model for an

automatic transmission providing efficient control for both the engine and the

transmission which leads to better fuel economy and acceleration feeling by

optimizing the shift timing and throttle valve opening. The gear shift timing consists

of the vehicle speed and the throttle valve opening simulating the engine load. To

achieve the optimal fuel consumption, gear shift timing is calculated by comparing

the efficiency of the torque transmitted to the wheels at each gear shift ratio, using

the fuel flow rate characteristic of the engine and torque convertor. The controller is

developed based on the concept that the optimal gear shift timing is selected with the

driven horse-power required for running, and the engine torque is controlled by the

throttle valve opening so as to maintain the driven shaft torque demanded by the


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driver which can be calculated from accelerator pedal angle and vehicle speed. To

avoid the busy shift and fluctuations of the driven shaft torque, hysteresis are

applied to the fuel flow rate calculated every gear shift to prevent gear command

signal changing several times.

The test pattern model used to verify the optimization shift timing results is

very simple and straight forward, so the simulation results are relatively better with

known driving circle.

M. Pachter [17] presents and analyzes differential game model of an

automatic transmission for a road vehicle control which leads to a synthesis of the

optimal feedback strategy for the selection of the transmission ratio of the automatic

gearbox. The vehicle is considered as object, which is governed by two controlling

agents: the setting by the driver of the throttle and the gear-change controller. He

remarks that most road vehicles are equipped with manual or automatic gearboxes

which have a finite number of discrete transmission ratios. But for CVT where the

transmission ratio is a continuous variable, his analysis hinges on the fact that the

dynamic function assumes a discrete set of values. Therefore, in order to analyze the

case of a continuous secondary control variable, one should discretize the

transmission ratio or alternatively, use an adaptation of our theory to a discrete-time

model for the dynamical system.

Quan Zheng [33] introduces a new coordinated engine-transmission control

approach for the neutral idle input clutch application phase aiming at improving fuel

economy during urban driving and reducing engine vibrations transmitted to the


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passenger compartment. It involves ramping up engine speed smoothly and

simultaneously engaging the forward clutch smoothly. Effective coordination can be

realized by controlling the engine and turbine speeds closed loop so that they follow

specified trajectories. Therefore, the controller design objective is to realize optimal

trajectory tracking by synthesizing the control inputs so that the engine and turbine

speeds track the specified desired trajectories closely. Due to the Multi-Input

Multi-Output nature of the control problem, an optimal Linear Quadratic Regulator

with Explicit Model Following is used to allow the system dynamic response to

track two desired trajectories for engine and turbine speeds. The results show that the

proposed control strategy can achieve satisfactory performance.

There are several issues that remain to be explored in this paper. The first

issue is state estimation. In the current formulation, the states are assumed to be

available for feedback implementation. However, state estimation is needed to obtain

clutch pressure and the derivative of clutch pressure, and dynamic estimation of

engine indicated torque is also necessary. The second issue is better inclusion of the

vehicle dynamics in the design. One limitation of the current implementation is that

the state related to vehicle acceleration is ignored in the process of model

simplification. This limits accurate studies of vehicle vibrations under different

control strategies.

In Alarico Macors [34] work, the design of a hydro -mechanical

transmission is defined as an optimization problem in which the objective function is

the average efficiency of transmission, that is to minimize the total loss of the


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transmission, while the design variables are the displacements of the two hydraulic

machines and gear ratios of ordinary and planetary gears. Since the object function

doesn't have an analytical formulation, the optimization problem is solved by a

direct-search algorithm based on the swarm method, which showed a good speed

convergence and the ability to overcome local minima. And the use of evolutionary

algorithms is also able to reduce the importance of the initial research point and the

trapping in local minima far from absolute minimum.

However the advantage of the continuous speed variation of the power-split

drives is counterbalanced by a reduced efficiency, caused by the double energy

conversion taking place in the hydrostatic transmission. Therefore, the design of the

power split drive still need further study. The proposed procedure does not depend

on experience and previous knowledge because no assumption had to be made on the

component's sizing; the optimality of the output is based on the implemented search

algorithm while the quality of the classical designs depends strongly on the

designer's experience.

B.Mashadi [18] designs a gear-shifting strategy of an automated manual

transmission by taking into consideration the effects of these parameters, with the

application of a fuzzy control method. The controller structure is formed in two

layers. In the first layer, two fuzzy inference modules are used to determine the

necessary outputs. In the second layer a fuzzy inference module makes the decision

of shifting by upshift, downshift, or maintain commands. The behavior of the fuzzy

controller is examined by making use of ADVISOR software. It is shown that at


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different driving conditions the controllers make correct decisions for gear shifting

accounting for the dynamic requirements of the vehicle. Both the engine state and

the drivers intention also eliminate unnecessary shifts that are present when the

intention is overlooked. Both strategies for the vehicle to reach the maximum speed

starting from rest allow the gear shift to be made consecutively.

Considerable differences are observed between the two strategies in the

deceleration phase. The engine-state strategy is less sensitive to downshift, taking

even unnecessary upshift decisions. The state intention strategy, however, interprets

the drivers intention correctly for decreasing speed and utilizes engine brake torque

to reduce the vehicle speed in a shorter time.

Magnus Pettersson and Lars Nielsen [21] uses engine control during the

gear shift for a manual transmission without using the clutch during the shift event.

To minimize the total time needed for a gear shift will excite the driveline

resonances which may lead to problems with disengaging the old gear and

synchronizing speeds for engaging the new gear. Internal driveline torque control is a

novel idea introduced in their work for handling resonances and increasing shift

quality. By estimating the transmitted torque and controlling it to zero by engine

control, the gear can systematically be disengaged with minimized driver

disturbances and faster speed synchronization.

Two main advantages of the control system are: fast shifts to neutral gear,

despite disturbances and driveline oscillations at the start of the gear shift; the

control scheme is simple and robust against variations among different gears and


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damping of driveline resonances can be obtained with an observer in combination

with a PID feedback structure.

Different from the manual or automatic transmission with fixed gear ratios,

continuously variable transmission (CVT) with its continuous ratio offers the

potential to substantially improve the part-load fuel efficiency of spark-ignited

engines. The control of CVT has traditionally been designed using static arguments,

like by identifying the best efficiency points in the engine map for each constant

power requirement and by following that curve using some heuristics as much as

possible also in transients.

R. Pfiffner [36] presents the solution of the fuel-optimal control problem for

transient conditions using the numerical optimization package DIRCOL. Based on

this optimal solution a simplified but causal control strategy is proposed which offers

almost the same benefits. The resulting engine operation trajectory is pictured in a

engine torque vs. engine speed figure and the optimal solution therefore consists of

bringing the system to the corresponding best efficiency curve as fast as possible,

and to keep the system on this curve as long as possible. During the periods when

the system moves towards or away from this curve the gear ratio has to be changed

with maximal possible speed while the optimal engine power trajectory is more or

less constant over time.

Michiel Pesgens [35] develops a transmission ratio controller for a

hydraulically actuated metal push-belt continuously variable transmission (CVT),

which consists of an anti-windup PID feedback part with linearizing weighting and a


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set point feedforward which is generated by the hierarchical (coordinated) controller.

Physical constraints on the system, especially with respect to the hydraulic pressures,

are accounted for using a feedforward part to eliminate their undesired effects on the

ratio. The total ratio controller guarantees that at least one of the pressure setpoints is

always minimal with respect to its constraints, while the other is raised above the

minimum level to enable shifting.

This approach has potential for improving the efficiency of the CVT,

compared to non-model based ratio controllers with experimental results showing

that adequate tracking is obtained together with good robustness against actuator

saturation. The largest deviations from the ratio setpoint are caused by actuator

pressure saturation. It is further revealed that all feedforward and compensator terms

in the controller have a beneficial effect on minimizing the tracking error.

Most optimal approaches stated above depend on the working condition of

engine and torque converter. For this case, in this thesis, as the transmission is no

longer driven by the IC engine, the instantaneous fuel consumption cannot be

computed from outputs of the transmission model. Therefore the optimal gear

shifting decision cannot be made based on the best fuel economy of a specific gear

under a certain driving condition. A new method based on the current transmission

model is designed and introduced in the following chapters.


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Chapter 3

Control Criteria and Shifting Strategy

The function of a vehicle transmission is to adapt the traction available from

the drive unit to suit the vehicle, the surface, the driver and the environment. It has a

decisive effect on the reliability, fuel consumption, ease of use, road safety and

transportation performance [37]. The optimization of the transmission can

significantly increase the performance of the vehicle efficiency and fuel economy.

As introduced in chapter 1, a simulation model of a truck was developed in

SIMULINK by engineers at Southwest Research Institution based on the

configuration of the hydraulic hybrid vehicle shown in Figure 1-1. It is a

seven-speed transmission which can provide more efficient fuel performance than a

normal four-speed one. The strategy discussed is for the range between 2nd and 7th

gear as a shift directly from 1st to 2nd gear is not allowed while the vehicle is moving.

In the transmission model, a shifting command that consists of one of the

seven-speed gear ratios is taken as an input, and it outputs the vehicle speed, vehicle


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distance, driver pedal position, current gear ratio, fuel consumption, simulation time,

motor pressure, motor displacement, motor speed and efficiency as outputs to a

MATLAB workspace.

In this chapter, an optimal control strategy for the gear shifting schedule of

the automatic transmission is developed depending on the proper application of the

decision-making algorithm for best fuel economy. Since the transmission is no

longer driven by the IC engine, the instantaneous fuel consumption cannot be

computed from outputs of the transmission model. The control strategy developed in

this chapter is to research and identify better operational measures by which fuel

economy can be optimized.

3.1 Control Criteria

The design of a controller requires certain performance criteria to be

established as objectives. The overall objective of this controller was to provide a

gear shifting schedule that would minimize fuel consumption for a hydraulic hybrid

truck. Unlike conventional vehicles, this trucks drive wheels are not directly

attached to the internal combustion engine. Therefore the torque and wheel speeds

are controlled independently from motor speed. While the internal combustion

engine must provide power to a storage system, the rate at which the power is

consumed does not need to match the rate of power production. Therefore, the

internal combustion engine can continuously operate at the point of its best


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efficiency. When sufficient power has been stored, the internal combustion engine

can be turned off until the stored power has dropped to some replenishment level. At

that time, it would again recharge the storage system. Thus the objective of

minimizing fuel consumption will not depend on matching engine torque and speed

to immediate driving conditions. As it will be explained later, meeting this objective

will depend on promoting higher displacements in a variable displacement hydraulic

motor.

First of all, the analysis of the truck model shows that higher efficiency of

the motor is obtained in a higher gear of the transmission. Therefore, the

transmission should be in the highest possible gear as long as the torque demand can

be met while driving.

The analysis of data produced by the SwRI s-function black box model of

the hydraulic motor in Figure 3-1 shows that the highest efficiency for the motor is

near full displacement. Therefore, the controller should strive to keep the hydraulic

motor working near full displacement. Figure 3-2 shows that high acceleration

requires higher torque, thus requiring a lower gear. However, taking this plot into

account, the transmission should upshift to get higher motor displacement as long as

the torque requirement can be satisfied. So, the highest possible gear criterion is

made for this controller.


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Figure 3-1 Plots of efficiency vs. motor displacement of each gear

Figure 3-2 Plots of motor displacement vs. acceleration of each gear

Another criterion that the controller must meet is to provide the wheel

speed and torque necessary to meet the EPA Federal Urban Driving Cycle. This


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cycle, meant for comparing automobile fuel efficiency consists of 1369 seconds of

velocities simulating large city stop-and-go traffic. For a large truck, this is a very

demanding schedule to meet as the accelerations required are at the torque limits of

the propulsion system. However, the driving cycle cannot be used directly. Initially,

the demands for torque and the conditions for braking must be interpreted from the

actions of the acceleration pedal which will be discussed in the strategy section.

There are also certain safety standards imposed. Based on the limitations

of the truck components, shifting from a lower gear to a higher gear, or shifting from

a higher gear to a lower gear requires that the transmission be shifted into each gear

in between. For example, a shift from 4th gear to 2nd gear, as might be required

driving a braking event, requires that shift sequence is from 4th gear to 3rd gear and

then to 2nd gear. This is required in order to sequence the transmission internal

clutches so the transmission is not destroyed or so that the gear shift can be

successfully made. The vehicle must start from a stop in 2 nd gear. 1st gear is not used

for normal driving conditions.

Another transmission requirement is that each gear during an upshift

requires a dwell time in that gear for conditions to stabilize as the transmission is

under driving force. This requirement is relaxed during downshift so that maximum

use can be made of regenerating braking. Besides that, the gear shifting time is also

taken into consideration as the gear shifting needs certain time to complete.

The hydraulic motor has a rotational speed limitation of 3000 RPM. At

speeds that exceed this, the motors rotating parts produce enough inertia such that


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the motor could explode or otherwise be damaged.

3.2 Shifting Strategy

The optimal shifting strategy is mainly developed from two different

aspects of the truckmodel. One approach is based on the drivers intention which is

given as pedal position output of the transmission model. Based on the concept that

the driving cycle is unknown to the controller, drivers intention is the only input can

be taken into account as a prediction of the driving cycle. The intention of the driver

pressing the pedal indicates the torque or the acceleration is required from driver and

releasing the pedal indicates braking.

In addition to interpreting drivers behavior, another aspect that is taken into

consideration is the hydraulic motor displacement, which can guarantee the

hydraulic motor working within the range where higher efficiency can be achieved.

3.2.1 Shifting Based on the Driver Pedal

The intention of the driver is interpreted from the position of the driver

pedal. In the most basic form, the depression of the pedal is interpreted as the driver

is demanding torque to accelerate while releasing of the pedal is interpreted as the

driver intends to decelerate.

There are two parameters that can be considered here under the drivers


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control, one is the pedal position, and the other is the pedal rate. The pedal position

is represented with a numerical value from 0 to 1 with regard to the full pedal

position as 1. The pedal rate is defined as the rate of increase of pedal position

during one sample time T (T=5 milliseconds). When the driver uses a high rate of

increase in pedal position, it is interpreted as an urgent torque demand. By observing

the relationship between pedal rate and acceleration requirement, the average value

of the pedal rate was found to be 0.01 per sample time from averaging all of the rates

of increase over the FUDS driving cycle. So when driver presses the pedal down

with rate double of the average value per sampling time, a downshift command is

applied to increase torque. In addition, an upshift decision is made when pedal rate is

negative which is interpreted as the driver wants to decelerate by retarding the pedal

based on the highest possible gear criterion.

The release of the pedal indicates that the driver wants to decelerate the

vehicle. The vehicle can decelerate in two ways: coast down and active braking.

However, braking is neither an input nor an output of the truck model. Therefore, a

surrogate had to be developed. If the pedal position is less than 0.05, it is considered

as a potential sign for braking, so a downshift decision is made for braking mode.

Otherwise, in the case of pedal position greater than 5% along with pedal

rate within the range from 0 to 0.005 will keep on the current gear.


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3.2.2 Shifting Based on Hydraulic Motor Conditions

Another aspect that can be considered for minimizing the fuel consumption

is to maximize the hydraulic motor efficiency. According to the relationship between

efficiency and motor displacement, and efficiency and motor speed, the higher motor

displacement and speed, the higher efficiency. So the hydraulic motor needs to work

in the high range of displacement and speed to get high efficiency. As shown in

Figure 3-1, the highest efficiency, 95%, of the motor occurs at a high motor

displacement range from 0.7 to 0.95. A downshift will be applied when motor

displacement value is more than 0.95 to prevent over driving the displacement.

3.3 Control Algorithm

Based on the analysis of the control criteria and strategy, the main algorithm

of the controller can be determined by taking all the input and output parameters of

the transmission model into account.

First of all, the controller will always upshift the transmission to the highest

possible gear provided the driving power requirements can be met based on the

highest possible gear criterion. There are some exceptions for downshift, pedal

position is less than 5%, which indicates braking mode, and pedal rate more than

0.005 per sampling time which indicates more torque requirement from the driver.

Otherwise, when the pedal rate is less than 0.005 per sampling time the controller

will stay in the current gear. Secondly, the maximum motor displacement limitation


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is set to 95% and downshift will be applied if it exceeds that value. When the current

motor speed reaches 3000 RPM (314 radian/second), an upshift is always applied to

reduce the motor speed. In addition, any command derived from the drivers

intention will also be checked with the current vehicle speed to protect the motor

speed from exceeding the maximum value. All the shifting commands must be

checked with the 2nd to 7th gear range controller and then combined with the

2-second dwell time other than when braking. Finally, as the gear shifting needs

certain time to accomplish, the shifting in process (SIP) signal is presented with

either 1 which shows the shifting is still in process or 0 which means the shifting is

already completed. The shifting command only can be applied when the shifting in

process (SIP) signal is 0. In other words, every proposed shifting command must be

checked with SIP signal before it can be delivered to the final output.


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Chapter 4

Simulation Design

SIMULINK provides a platform to simulate the performance of the

optimal shifting controller by running the transmission model in a certain driving

cycle.

4.1 Transmission Model

The transmission model shown in Figure 4-1 is a discrete-time based

SIMULINK model which runs a 1369-second simulation driving cycle with a

0.005-second sampling time. The model simulates the shifting of a 6-speed

transmission under conditions imposed by the EPA Federal Urban Driving Cycle for

a hydraulic hybrid truck. The truck dynamics are contained within a black box

s-function provided by Southwest Research Institute. This s-function uses the gear

ratio as the input, and then provides the vehicle speed, the driver pedal position, the

hydraulic motor speed, the motor displacement and the hydraulic pressure among


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other factors.

Figure 4-1 Original model with default shifting schedule

A simulation model of the controller was developed in SIMULINK based

on the original truck model provided by Southwest Research Institute, presented in

Figure 4-2. The highest level of this controller-truck model consists of three

components: the controller system, the truck s-function and the outputs. The

s-function black box is the same as the original truck model which uses the gear

ratio from the controller system block and outputs 12 parameters including vehicle

speed, vehicle distance, driver pedal position, shifting in process signal (SIP),

simulation time, current gear ratio, fuel consumption, simulation time, motor

In1

Out1

Out2

Out3

Out4

Out5

Out6

Out7

Out8

Pressure (N/m2)

Motor Disp (0-1)

Motor Speed (rad/s)

P*eff

VehicleWithSeriesHydraulicPowerTrainAndFTPDriveCycle+Driver1

VehDistance

VehDistance (m)

tt

Simulation Time (tt) seconds

SIP

Shift In Progress (SIP)

Pressure

Pressure (N/m2 )

PtimesEff

Press*Eff

MotorSpeed

MotorSpeed (rad/sec)

MotorDisp

MotorDisp (0-1)

FuelConsumed

FuelConsumed (Kg)

DriverPedal

DriverPedal (0-1)

DesVehSpd

DesVehSpd (m/s)

CurrentGearRatio

CurrentGearRatio-1

Autom atic S hifti ng

or Des Gear Ratio

ActVeh Spd

ActVehS pd (m/ s)


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pressure, motor displacement, motor speed and efficiency as matrices into MATLAB

workspace.

Figure 4-2 Block diagrams with the optimal shifting controller in orange

4.2 Controller System Model

The controller is a five-input and one-output system which uses vehicle

speed, driver pedal position, shifting in process signal (SIP), motor displacement and

motor speed. The output is the gear ratio for the transmission. The controller

contains the following components which interpret the control algorithm discussed

in chapter 3.


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Figure 4-3 All block diagrams of the optimal controller top layer

4.2.1 Driver Shifting Controller

The shifting decisions made by the controller will be presented in signals of

1, -1 and 0, which represent upshift, downshift, and on action respectively.

The torque requirements are interpreted from the driver pedal position and

the pedal position rate. The pedal position is represented with a numerical value

from 0 to 1 and the pedal rate is defined as the rate of increase of pedal position

during one sample time T (T=0.005 second).

The controller always implements an upshift command unless the torque

requirement cannot be satisfied by the current gear. As mentioned before, a high rate

of increase in pedal position with a value of 0.005 per sampling time is interpreted as

Algorithm ofthe Controller:

1.HighestGear Optimization.

2.Max Motor Speed(314rad/s) firstpriority,upshiftwhenexceed.

3.Max Displacementlimit0.95,downshiftwhenexceed.

4.Always upshiftunless:

1).Pedalposition=0.005/sampletime,thendownshift.

5.Dwelltime5sec, may causeproblems whenbrakingtimetoa stopis soshort

thatthegear cannotshifttothe2ndgear intime,

sonodwelltimeis appliedwhenvehiclespeed< =0.3m/s.

Value is1 if shift is in progress, zero other

used to inhibit shifting

-1,0,1 values

Note:

gear number 1corresponds to2nd Gear

gear number 6corresponsedto7th Gear

Prop GearNumber(1 to 6)

1

GearRat ioz

1

used to

breakalg loop

>= 314

Speed Check

If Prop gearviolatesmotor speed

use current gear(no shift)

Shift Cmd

afterspd ck

> 0

SIP Check

No shifting if SIP

Proposed Gear

MotorSpeed Limit Shift

G e ar R a n ge L i m it e d G ea r

Limit Gearsto Range 2nd -7th

P r op G e a r R a ng e L i mi t ed G e a r

Limit Gearsto Range 1 to 6

(gears2nd to 7th)

Mo to r S p ee d G e ar S h if t C o mma nd : 1 = u p s h if t 0 = n o a c t i o n

If 1 A MotorSpeed Violoation isOccurring!

Mandatesupshift if motorspeed exceedsset value

Highest Priority: Not subject to Dwell Time

VehSpeed

Propgear command without dwelltime

Propgearcommandafterdwel l time

PedalPosition

Propgear

Give priority to Braking VehSpeed 0

Give priority

to an upshift

GearSent

z

1

GearMemory

GearDelay

PropGearafter l imi t

GearCommand Givento Transmission

PropGearafterDwellTime

Dwell Time Controller

DriverPedal

PedalPosition

Displacement

DriverShift Command

DriverCommand Shifting

DriverCommand

Create gearnumber

to go to

1

2

3

4

5

6

*Convert to

GearRation

Current VehVelocity

ProposedGear

ComputedGearMotorSpeed

Computed MotorSpeed

Add

0.78

7thGearRatio1

0.90

6thGearRatio1

1.20

5thGearRatio1

1.69

4thGearRatio1

2.24

3rd GearRatio1

4.18

2nd GearRatio1

5

Displacement

4

MotorSpeed

3

Actual Velocity

2

DriverPedal

1

SIP

Current GearNumber (1to 6)

Current GearNumber (1to 6)


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an urgent torque demand from the driver, a downshift command is applied to

increase torque in this case. In addition, pedal position greater than 0.05 with the

pedal rate staying in the range from 0 to 0.005 indicating the driver is demanding

torque gently and will keep the shifting command in the current gear. This is shown

in Figure 4-4.

Figure 4-4 Driver shifting controller block diagrams in orange

4.2.2 Speed Computation

The maximum hydraulic motor rotational speed is 3000 RPM which can be

l i l :

. l i i . i i i , i .

. l i i i i i i i , .

. l i i i i . l i , i .

. i l li i . , i .

-1 or 0 values

-1 or 0 values1

Driver Shift Command

1

Up

Shift

z

1

Unit Delay

0

No Shift

> 0.95

If HysMotor Disp isgreater

than 0.95: downshift

> 0

If Driver isdepressing pedal:

Torque Demanded : Do not up shift

otherwise allow upshift

2nd priority

>= 0

Give priority of

choice to pedal vel

cmd

>= 0

Give priority of

choice to disp

cmd

>= 0

Final Gear Command

Prioity given to braking

(down shift)

>= 0.05

Driver Pedal Retard:

If pedel depress< 5%

assume braking m odes

i

-1

Down shift

Assume l owest gear

slowing

Difference of

Driver Pedal

position

> 0.005

If pedal depression velocity

isgreated than .005/Ts,

the driver isurgently

demaning torque

1st priority

2

Displacement

1

Pedal Position

0 or 1 values

-1,0,1 values

-1,0,1 values

-1, 0, 1 values

-1 or 0 values-1 or 0 values


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converted into radians per second as:

The proposed gear command made by the driver shifting controller also

had to be considered in the proposed corresponding hydraulic motor rotation speed

based on the proposed gear ratio, , and checked with the maximum hydraulic

motor speed limitation based on the criteria:

The ratio for the final drive is and the radius of the vehicle tire is

This is implemented as shown in Figure 4-5.


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Figure 4-5 Speed computation block diagrams in grey

4.2.3 Braking Controller

In order to get to the lowest gear when the vehicle stops, a downshift

command will be made during the braking process. As there is no braking pedal

signal in the transmission model, the driver pedal position of less than 5% is

predicted braking. A downshift decision is made in the pedal controller to provide

sufficient time to get to the 2nd gear before the vehicle stops. A figure for the

braking controller is shown below.

1

Computed

Gear Motor

Speed

13.4483

Velocity factor (Nf/re)

1

2

3

4

5

6

*Proposed

Gear Ratio

ComputeMotor Speed

from gear ratio

0.78

7thGear Ratio

0.90

6thGear Ratio

1.20

5thGear Ratio

1.69

4thGear Ratio

2.24

3rd Gear Ratio

4.18

2nd Gear Ratio

2

Proposed Gear

1

Current Veh Velocity


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Figure 4-6 Braking controller block diagrams in orange

4.2.4 Priority Selection

In the driver pedal shifting controller, the first priority is given to the

urgent torque demand from the driver with a pedal rate greater than 0.005/T.

Otherwise the algorithm maintains the current gear for torque demand for the

increasing rate of the pedal less than 0.005/T, which indicates moderate torque

demand from the driver. The upshift command with decreasing pedal rate is

secondary to increasing pedal rate. Within the pedal controller, which consists of the

driver pedal shifting controller, braking controller, and hydraulic motor efficiency

controller, the first priority is given to the braking process. The second priority is the

hydraulic efficiency controller, with 95% limitation, which can keep the hydraulic

motor working within the high efficiency range. For the overall controller of the

transmission, first priority is given to the shifting in process (SIP) signal controller as

no new action can be initiated while a previous shift command is being executed.

-1 or 0 values1

Driver Shift Command

0

No Shift

>= 0

Final Gear Command

Prioity given to braking

(down shift)

>= 0.05

Driver Pedal Retard:

If pedel depress < 5%

assume braking modes

downshift

-1

Down shift

Assume l owest gear

slowing

1

Pedal Position

-1,0,1 values


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The hydraulic motor speed controller is the second priority as long as the violation of

the maximum hydraulic motor speed limitation is not violated.

4.2.5 Overall Speed Violation of the Hydraulic Motor

The maximum hydraulic motor rotational speed limitation of 3000 RPM

(134 radians/second), must be strictly obeyed regardless of current hydraulic motor

speed condition or the proposed hydraulic motor speed after calculation. An upshift

is applied when the current hydraulic motor speed exceeds the maximum limitation

to reduce the motor speed. Furthermore, no command can be made by the shift

controller if the corresponding proposed hydraulic motor speed exceeds the

maximum limitation. Figure 4-7 shows the hydraulic motor speed limit controller.

Figure 4-7 Hydraulic motor speed limit controller block diagrams in red

Purpose: to enforce an upshift on motor speed violation

1

Gear Shift Command:

1 = upshift

0 = no action

1

Next Gear Up

>= 314

Motor Speed

Check

0

Hold Gear Constant

1

Motor Speed


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4.2.6 Dwell Time Controller

A two-second dwell time used to stabilize the gear during upshift is applied

to the transmission by using the dwell time controller, except for braking processes.

The controller shown in Figure 4-8 uses a delay loop which lasts for two seconds to

be the timer of the dwell time. It is initiated by a gear change signal to reset the dwell

loop.

Figure 4-8 Dwell time controller block diagrams in magenta

4.2.7 Gear Memory

Every new gear command is made on the basis of the previous gear

Gear delay Loop

-1,0,1 values

1

Prop Gear after

Dwell Tim e

> 0

Switch2

~= 0

Reset for Shift time

When Gear Shift

z

1

Last Shift Time Memory

>= 2

Dwell Tim e

Setting in Seconds

12:34

Digital Clock1

12:34

Digital Clock

Del T

Check Gear Shift

3

Gear Command

Given to Transmission2

Prop Gear after limit

1

Gear Delay


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command. The last gear command of the controller is stored by using a unit delay

block. As shown in Figure 4-9 every proposed shifting command has to be combined

with the last gear command and stored to the next step sampling time. For example,

a downshift command with a value of -1 will be combined with the last gear

command of 4 (5th gear ) and output a gear command as 3.

Figure 4-9 Gear memory by using unit delay

4.2.8 Gear Ratio Matching

Every gear command must be within the 2nd to 7th gear range according to

the criteria in Chapter 3. The gear range controller checks every gear command

before delivering it to the transmission by setting the lowest gear to be 1 (2nd gear)

proposed gear command

1

Gear Ratioz

1

used to

break alg loop

> 0

SIP Check

No shifting if SIP

Proposed Gear1

Gear Sent

z

1

Gear Memory

Create gear number

to go to1

1

2

3

4

5

6

*Convert to

Gear Ration

0.78

7thGear Ratio1

0.90

6thGear Ratio1

1.20

5thGear Ratio1

1.69

4thGear Ratio1

2.24

3rd Gear Ratio1

4.18

2nd Gear Ratio1


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and highest gear to be 6 (7th gear). The the gear ratios of 0.78, 0.9, 1.2, 1.69, 2.24

and 4.18 are matched with the gear numbers from the highest gear 6(7 th gear) to the

lowest gear 1(2nd gear). In doing this, the gear range controller can match each gear

command with its corresponding gear ratio and then output the gear ratio to the truck

model. For instance, the gear range controller takes in a gear command number 4

(5th gear), finds its gearratio 1.2 and then outputs it to the truck model. Figure 4-10

shows the gear ratio matching controller.

Figure 4-10 Gear ratio matching controller

Propsed ge ar

1

Gear Ratioz

1

used to

break alg loop

Gear Sent

1

2

3

4

5

6

*Convert to

Gear Ration

0.78

7thGear Ratio1

0.90

6thGear Ratio1

1.20

5thGear Ratio1

1.69

4thGear Ratio1

2.24

3rd Gear Ratio1

4.18

2nd Gear Ratio1


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Chapter 5

Simulation Results

In this chapter, the simulation results are presented and discussed by

comparing them with the default gear shift schedule. Results include fuel

consumption, tracking performance, and motor speed limit. The simulation runs in a

1369-second velocity-base EPA Federal Urban Driving Schedule (FUDS) with a

discrete 0.005-second sampling time.


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Figure 5-1 EPA Federal Urban Driving Schedule (FUDS)

5.1 Fuel Consumption

Figure 5-2 shows that the total fuel consumption of the controller simulation

is 3.7367 Kg, which is 5.22% less than the fuel consumption of the original default

shifting simulation, 3.9427 Kg. The number of gear changes in the controller

simulation is 315, compared to 197 in the default simulation.

0 200 400 600 800 1000 1200 14000

5

10

15

20

25

30Federal Urban Driving Schedule (FUDS)

Time(sec)

VehicleSpeed(m/s

)


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Figure 5-2 Fuel consumption simulation results comparison: fuel consumption

Kg vs. time (seconds)

As the default shifting schedule of the original model does not include the

2-second dwell time criterion, the fuel consumption of the original model with

2-second dwell time will be a little higher than 3.9427 Kg without dwell time.

0 200 400 600 800 1000 1200 14000

0.5

1

1.5

2

2.5

3

3.5

4Fuel Consumption

Time(sec)

FuelConsumed(Kg

)

Controller Model FuelConsumption

Original Model Fuelconsumption


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5.2 Gear Shifting Schedule

Figure 5-3 Controller model simulation results: gear ratio vs. time (seconds)

The gear shifting schedule of the controller simulation shows no unexpected

oscillations of the gear selection when a 2-second dwell time applied, as illustrated

in Figure 5-3. All the downshifts are either due to the urge demand of torque from

the driver or violations of the hydraulic motor speed and displacement limitations as

expected in the strategy. Shifting with less than 2 seconds dwell time only occurs

when the hydraulic motor speed reaches the maximum limit 314 radian/second

(3000 RPM), or during braking downshift process, as introduced in the previous

chapters.

0 200 400 600 800 1000 1200 14000.5

1

1.5

2

2.5

3

3.5

4

4.5GearRatio o f Controller

Time(sec)

GearRatio


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Figure 5-4 Original model simulation results: gear ratio vs. time (seconds)

The default gear shifting schedule of the original model, shown in Figure 5-4,

presents relatively simple gear changes and unexpected oscillation at approximately

700 seconds, which may be due to the lack of dwell time.

0 200 400 600 800 1000 1200 14000.5

1

1.5

2

2.5

3

3.5

4

4.5GearRatio of Original Shifting

Time(sec)

GearRatio


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5.3 Tracking Performance

Figure 5-5 Controller model simulation results: actual and desired vehicle speed

(m/s) vs. time (second)

0 200 400 600 800 1000 1200 14000

5

10

15

20

25

30VehicleSpeed of Controller

Time(sec)

VehicleSpeed(m/s)

Actual Vehicle Speed

Desired Vehicle Speed


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Figure 5-6 Original model simulation results: actual and desired vehicle speed

(m/s) vs. time (second)

Figure 5-7 Simulation results comparison between original and controller model:

vehicle speed error (m/s) vs. time (second)

0 200 400 600 800 1000 1200 1400

0

5

10

15

20

25

30VehicleSpeed of Original Shifting

Time(sec)

VehicleSpeed(m/s)

Actual V ehicle Speed

Desired Vehicle Speed

0 200 400 600 800 1000 1200 1400-4

-2

0

2

4

6

8

10

VehicleSpeed Error

Time(sec)

VehicleSpeedError(m/s)

Original Model Vehicle Speed Error

Controller Model Vehicle Speed Error


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The combined time-based plots of actual vehicle speed (m/s) and desired

vehicle speed (m/s) are depicted in Figure 5-5 and Figure 5-6. The results of

figure-20 indicate that the truck controlled by the optimal gear shifting controller can

track the desired driving cycle well. One exception occurs between 200 seconds and

300 seconds with a 8m/s peak value difference during the high-acceleration and

high-speed period around 17m/s to 25m/s. This difference can be seen clearly from

Figure 5-7 which shows the error of the actual vehicle speed from both original and

controller model when compared to the desired vehicle speed. The vehicle speed

error stays no more than 3m/s except for the period between 200 seconds and 300

seconds as mentioned before. In fact, the FUDS is used for light-duty vehicle test.

Therefore, the most aggressive acceleration rates might not be realistic for

heavy-duty truck [7], which can account for the peak error of vehicle speed

displayed between 200 seconds and 300 seconds.

The total actual simulation driving distance of the truck is 11428m which is

4.68% less than the desired total distance of FUDS 11989m. The tracking

performance of the original model, as shown in figure-21, has a total driving distance

of 11592m, a 3.31% difference between actual driving distance and desired driving

distance. Though the controller model drives 1.37% less distance than the original

one, the difference is less than the designated maximum difference of 5%. Therefore,

the simulation results of the controller still show good performance on tracking the

desired driving cycle.


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5.4 Hydraulic Motor Speed

Figure 5-8 Controller model simulation results: hydraulic motor speed

(radian/second) vs. time (second)

0 200 400 600 800 1000 1200 14000

50

100

150

200

250

300

350MotorSpeed of Controller Model

Time(sec)

MotorSpeed(rad/s)


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Figure 5-9 Original model simulation results: hydraulic motor speed

(radian/second) vs. time (second)

The hydraulic motor speed is well below the maximum limit of 14

radian/second (3000RPM) , as presented in Figure 5-8. When compared to the

results of original model in Figure 5-9, it is obvious that the hydraulic motor speed

of the controller model is higher, or in other words, keeps in the high-speed range

more often, which eventually results in higher motor efficiency and better fuel

economy.

5.5 Summary

This chapter presents and analyzes the results of the controller model

0 200 400 600 800 1000 1200 14000

50

100

150

200

250

300MotorSpeed of Original Shifting

Time(sec)

MotorSpeed(rad/s)


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simulation and compares them with the results of the original model. Table-1 shows

the comparison between the original model with default shifting schedule and the

model with shifting controller in term of fuel consumption, number of gear changes,

total driving distance, and the difference between actual and desired distance.

Table 5.1 Simulation results comparison between controller and original model

Fuel

consumption

(Kg)

Total

driving

distance (m)

Driving distance

difference between

actual and desired (%)

Shifting

change

times

controller 3.7367 11428 4.68 315

original 3.9427 11592 3.31 197

The controller simulation showed improved fuel consumption and

acceptable tracking performance. However, the original model has slightly better

tracking performance.


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Chapter 6

Summary and Conclusion

6.1 Summary

This paper proposes a fuel-economy optimization gear shifting strategy for

a seven-speed automatic transmission used on a hydraulic hybrid vehicle in order to

maximize fuel economy.

This strategy is designed with a highest possible gear criterion as long as the

torque requirement can be satisfied, except for braking process and torque

demanding situations. The optimization strategy takes several other criteria into

consideration, such as high motor displacement criterion, to improve efficiency and

fuel economy as well. Then the optimization strategy is developed on the basis of

these criteria from two main aspects of the existing SIMULINK truck model. One

approach is based on the hydraulic motor working conditions, such as motor

displacement, and the other is based on the drivers intention, which is interpreted as

the driver pedal position. This controller is able to recognize the drivers intention to


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change the speed and incorporate it into gear shifting decision making.

This paper then develops a SIMULINK controller model based on the

optimal gear shifting strategy and criteria and validates the model both in fuel

economy and power performance by analyzing the simulation results in the Federal

Urban Driving Cycle.

6.2 Conclusion

The simulation results show that the SIMULINK optimal gear shifting

controller model is able to increase the fuel economy by 5.22% with a 3.7367 Kg

fuel consumption compared to the original default shifting schedule.

The controller model is also able to keep the hydraulic motor speed below

the 3000RPM maximum speed limitation when driving. Moreover, by keeping the

hydraulic motor speed higher, the hydraulic motor efficiency can stay in high range

more often to get better fuel economy.

The controller model performs well in the tracking with 4.68% distance

difference between the actual and desired total driving distance which is less than the

5% designed standard.

6.3 Future Work

As there is no braking pedal in the truck model, the braking intention can


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only be predicted from the driver behavior trend on the accelerating pedal. And due

to many influence factors in the truck model, the positive correlation between the

hydraulic motor speed and the displacement can only be concluded approximately.

Therefore, a more efficient schedule for high speed driving period can be developed

based on a braking pedal input and accurate relationship between the hydraulic

motor speed and the displacement, which can track the driving cycle or road

conditions better. Furthermore, with instantaneous fuel consumption output from the

truck model, the optimal shifting strategy can be improved with more efficient

choices. The FUDS only simulates the driving test on flat roads and roads with

slopes are left to be developed in the future.

The optimal gear shifting strategy for a seven-speed automatic transmission

developed in this thesis is part of the research work for a future hydraulic hybrid

truck. The work done by this thesis is presented in simulation, which still needs to be

applied on the hydraulic hybrid truck transmission and adjusted according to the real

conditions of the truck in the future.


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References

[1] R. Hall, UPS and J. J. Kargul, U. S. EPA, Hydraulic Hybrid Promises Big

Savings for UPS, Hydraulics & Pneumatics, Oct 2006, p 42

[2] YJ. Kim, Integrated Modeling and Hardware-In-The-Loop Study for SystematicEvaluation Of Hydraulic Hybrid Propulsion Options. Doctor of Philosophy in the

University of Michigan 2008.

[3] A. Mohaghegh-Motlagh and M. H. Elahinia, Dynamic Force Profile in Hydraulic

Hybrid Vehicles--A Numerical Investigation, Vehicle System Dynamics, Vol. 48, No.

4, April 2010, p 405428

[4] AW. Stienecker, T. Stuart, C. Ashtiani, An Ultracapacitor Circuit for Reducing

Sulfation In Lead Acid Batteries For Mild Hybrid Electric Vehicles. Journal of

Power Sources 2006

[5] A. Mohaghegh-Motlagh, M. Abuhaiba, M. Elahinia, and W. Olson, Hydraulic

Hybrid Vehicles, K. Myer, ed.,Wiley, 2007. ISBN: 978-0-471-79369-4.

[6] EPA, Draft Report on the Environment, 2003.

[7] M. Shan Ph.D. Dissertation Modeling and Control Strategy for Series Hydraulic

Hybrid Vehicle, The University of Toledo, December 2009

[8] A. Mohaghegh-Motlagh and M.H. Elahinia, M. Abuhaiba, and W. Olson (2007)

Application of Smart Materials for Noise And Vibration of Hydraulic Systems,

ASME DETC/CIE, September 47, Las Vegas, NV.

[9] C. Schroeder, A. Mohaghegh-Motlagh, T.M. Nguyen, and M.H. Elahinia,

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[11] Hydraulic Hybrids Boost Fuel Economy, Hydraulics and Pneumatics, Machine

Design, October 2006

[12] B. Kwon, H. Kim, Dynamic Analysis of Shift Quality for Clutch to Clutch

Controlled AT, KSME International Journal, Vol 14, No. 12, (2000), p 1348-1357

[13] J. Gong, D. Zhao, Y. Chen, N. Chen, Study on Shift Schedule Saving Energy of

Automatic Transmission of Ground Vehicles, Journal of Zhejiang University

SCIENCE 2004 5(7), p 878-883

[14] H. Sun, L. Yang, J. Jing, Hydraulic/Electric Synergy System (HESS) Design for

Heavy Hybrid Vehicles, Energy 35 (2010), p 5328-5335

[15] T. Minowa, H. Kimura, N. Ozaki, M. Ibamoto, Improvement of Fuel

Consumption for A Vehicle with An Automatic Transmission Using Driven PowerControl With A Powertrain Model, JSAE Review 17(1996), p375-380

[16] H. Sun, Multi-Objective Optimization for Hydraulic Hybrid Vehicle Based on

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optimal gear shifting strategy for a seven-speed automatic transmission used on a hydraulic hybrid...

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