Development of a Tractor-Semitrailer Roll Stability Control Model

DEVELOPMENT OF A TRACTOR - SEMITRAILER

ROLL STABILITY CONTROL MODEL

A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

By Santhosh Chandrasekharan, B.E. * * * * *

The Ohio State University 2007

Master’s Examination Committee: Approved by Dr. Dennis Guenther, Adviser

Dr. Gary J. Heydinger ______Adviser

Graduate Program in Mechanical Engineering

ABSTRACT

The National Highway Traffic Safety Administration (NHTSA), which is a division of the United States Department of Transportation, aims at preventing injuries and loss of lives caused by accidents on U.S. roads. It achieves its goals by creating public awareness, conducting research on different aspects of crash avoidance and implementing safety standards. Many of the accidents on U.S. roads involve heavy trucks and hence they constitute a major part of the research work carried out by NHTSA.

The current focus on heavy trucks includes the prevention of rollovers which is a major contributor to fatalities occurring every year. Many truck manufacturers have started offering Electronic Stability Control (ESC) systems as an OEM option. A typical

ESC system is designed to prevent the occurrence of rollovers as well as other loss of control events. NHTSA is focusing on studying the effectiveness of these commercial

ESC systems in preventing rollovers in heavy trucks, and is looking at the feasibility of mandating ESC systems on heavy trucks in the future. This project is a part of NHTSA’s study of ESC systems and its results would further aid the understanding of the working of commercial ESC systems.

Comprehensive testing of the ESC systems on heavy trucks is prohibitive due to

inherent reasons such as safety factor and the incurred costs. Hence it becomes essential

to have a model of an ESC system that would simulate the working of a commercial

system, so that maneuvers that are too risky to performed in reality can be executed in simulation and the responses of the truck studied. Tests were performed on a tractor- semitrailer combination truck fitted with ESC systems. Data collected from these tests were used to model the ESC system.

The tractor and the semitrailer used in testing were modeled using a commercial

vehicle dynamics package. Models of heavy truck brake chambers and Ant-lock Braking

Systems (ABS) were previously developed at VRTC. These existing models were integrated with the newly developed truck model that included the ESC model. The

collected test data was studied to understand the behavior of the ESC system and the way

it responds. This information was used to create a model of the ESC system that has Roll

Stability Control (RSC) feature in it. The completed ESC system model was integrated with the ABS and brake models and with the truck model, thus creating one complete model of the heavy truck.

Simulations were performed with the ESC systems switched OFF and ON to

study the working of the developed ESC system model and its effectiveness in preventing

wheel lift and rollovers. Maneuvers simulated include J-Turn, slowly increasing steer

(SIS) and fish hook. The ESC system model was found to be effective in quickly detecting potential rollover threats and immediately act by dropping the throttle and

applying brakes thereby preventing rollovers by slowing down the vehicle.

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DEDICATION

To my parents for their unconditional love and blessings

ACKNOWLEDGMENTS

I wish to express my sincere gratitude to Dr. Dennis Guenther, who was a constant source of support for me throughout this work and whose guidance and encouragement kept me focused on the project and the courses for my Masters degree. I would also like to thank Dr. Paul Grygier and Dr. Riley Garrott for permitting me to conduct my research at the National Highway Traffic Safety Administration’s Vehicle

Research & Test Center (VRTC) and for providing me with all the necessary facilities required for the project.

I would like to express my appreciation to Dr. Gary Heydinger and Dr. Kamel

Salaani, for enlightening me on the concepts of heavy truck mechanics and for patiently answering my questions on the subject. I would also like to thank Dr. Ashley Dunn and

Scott Zagorski, whose help was invaluable to me especially during the initial stages of the project. I would also like to thank all the engineers at VRTC, especially Frank

Barickman and Devin Elsasser, for providing me the required test data and information.

I would like to thank my parents for their wishes and prayers, whose constant support mitigated my separation from them for the last two years. I largely owe the successful completion of this project as well as my future career to them. Lastly, I would like to thank all my friends who put up with my temperamental nature and stood by me for the last two years.

VITA

May 4, 1984………………….....Born – Chennai, India

May, 2005………………………B.E., Anna University, Chennai, India

Sept. 2005 to Aug. 2006……..…University Fellow, The Ohio State University

Sept. 2006 to present…………....Graduate Research Assistant, The Ohio State University

FIELDS OF STUDY

Major Field: Mechanical Engineering Vehicle Dynamics

TABLE OF CONTENTS

ABSTRACT…………...... II DEDICATION.……………………………………………………………...... IV ACKNOWLEDGMENTS ...... V VITA………………...... VI LIST OF TABLES...... X LIST OF FIGURES………… ...... XI CHAPTER 1 INTRODUCTION………...... 1 1.1 Motivation...... 1

1.2 Simulation...... 3

1.3 Previous Studies...... 3

1.4 Objective...... 7

1.5 Thesis Overview ...... 7

CHAPTER 2 VEHICLE TESTING & DATA ANALYSIS...... 9 2.1 Vehicle Description ...... 9

2.1.1 Truck Braking ...... 10 2.1.2 Anti-lock Braking System (ABS) ...... 10 2.2 Vehicle Testing...... 12

2.2.1 Load Configurations ...... 12 2.2.2 ESC Systems ON/OFF...... 15 2.2.3 Vehicle Instrumentation...... 16 2.2.4 Test Maneuvers...... 16 2.3 Data Analysis...... 17

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2.3.1 Data Processing...... 17 2.3.2 ESC Activation Signals...... 18 2.3.3 Steering Controller Data Sets...... 19 2.3.4 Test Data ...... 20 2.3.5 RSC Activation ...... 27 2.3.6 Steady State Lateral Acceleration Gain ...... 28 CHAPTER 3 TRUCK MODEL……………………………………………………. 31 3.1 Overview...... 31

3.2 TruckSim Model...... 31

3.2.1 Tractor Model ...... 32 3.2.2 Trailer Model ...... 36 3.2.3 TruckSim Model Summary...... 37 3.3 Simulink Model ...... 38

3.3.1 Randomly Varied Friction Generator ...... 39 3.3.2 S-Function...... 40 3.3.3 Brake Torque and Pneumatic Subsystems...... 40 3.3.4 ABS Controllers...... 41 3.3.5 External Engine and Torque Converter Models ...... 43 CHAPTER 4 ELECTRONIC STABILITY CONTROL SYSTEM MODEL...... 47 4.1 Overview...... 47

4.2 ESC Working Principle ...... 48

4.3 Development of ESC System Model...... 49

4.3.1 Threshold Selector ...... 50 4.3.2 ESC Activation Module...... 53 4.3.3 ESC Throttle Control ...... 58 4.3.4 Slip Ratio Calculator...... 59 4.3.5 Steer Axle Braking...... 60 4.3.6 Differential Drive Axle Braking ...... 62 4.3.7 Trailer Axle Braking...... 67 4.3.8 Brake Demand Sensor...... 69 4.4 Functioning ...... 70

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CHAPTER 5 SIMULATION RESULTS………………………………………….. 73 5.1 Overview...... 73

5.2 GVWR with High C.G. Height...... 74

5.2.1 ESC OFF...... 75 5.2.2 ESC ON ...... 82 5.3 GVWR with Low C.G. Height ...... 93

5.3.1 Simulation with ESC OFF ...... 94 5.3.2 ESC ON ...... 96 5.4 LLVW...... 107

5.4.1 ESC OFF...... 107 5.4.2 ESC ON ...... 112 5.5 NHTSA Fish hook Test ...... 122

5.5.1 GVWR with High C.G. Height...... 123 5.5.2 GVWR with Low C.G. Height ...... 131 5.5.3 LLVW...... 140 CHAPTER 6 CONCLUSIONS AND RECOMMENDATIONS...... 149 6.1 Conclusions...... 149

6.2 Recommendations...... 151

REFERENCES………………………………………………………………………...153 APPENDIX A – VARIABLES RECORDED IN TEST…………………………….155 APPENDIX B – SIMULINK ENVIRONMENT…………………………………… 158

LIST OF TABLES

Table 2.1 - Vehicle Information…………………………………………………….…....10 Table 2.2 - Truck Loading …………………………………………………...……….....13 Table 2.3 - ESC ON/OFF combinations………………………………………………....16 Table 2.4 - RSC Activation Time and Lateral Acceleration Values……………..………28 Table 3.1 - Tractor Track Width and Wheelbase……………………………………...... 33 Table 3.2 - Tractor Unsprung Mass C.G. and Spin Axis Height……………………...... 33 Table 3.3 - Tractor Unsprung Mass Properties………………………………….……….34 Table 3.4 - Tractor Sprung Mass Properties………………………………….………….34 Table 3.5 - C.G. Locations of Custom Payloads for Trailer Model in TruckSim…….....37

LIST OF FIGURES

Figure 1.1 - Top Half of Tractor – Trailer base Simulink Model………………………..5 Figure 1.2 - Bottom half of Tractor-Trailer base Simulink Model………………………7

Figure 2.1 - 4s/4m ABS layout ………………………………………………………..11 Figure 2.2 - Semitrailer in LLVW Condition ………………………..………………...14 Figure 2.3 - Semitrailer in GVWR Condition with Low C.G. Height …………………14 Figure 2.4 - Semitrailer in GVWR Condition with High C.G. Height …...……………15 Figure 2.5 - Path of Vehicle Motion …………………………………...………………21 Figure 2.6 - Steering wheel Angle and Truck Speed as Functions of Time ...…………22 Figure 2.7 - RSC Throttle and RSC Brake Activation Signals Overlaid on the Lateral Acceleration Plot …………………………………………………….….23 Figure 2.8 - Tractor Brake Chamber Pressures with RSC Throttle & RSC Brake Signals …………………………………………………………...... 24 Figure 2.9 - Tractor Wheel Speeds …………………………………………………….25 Figure 2.10 - Trailer Brake Chamber Pressures with RSC Throttle & RSC Brake Signals …………………..………………………………………………………..26 Figure 2.11 - Trailer Wheel Speeds ……………………..………………………………26 Figure 2.12 - Lateral Acceleration vs. Road Wheel Angle …..………………………….29 Figure 3.1 - Reduction in Truck Speed from 55 mph at different Retarder Levels ……45 Figure 4.1 - Schematic of the ESC Model …………………………………………..…49 Figure 4.2 - Relationship between Truck Weight and Rollover Threshold …………....52 Figure 4.3 - Threshold Selector Module ……………………………………………….53 Figure 4.4 - Steady State Lateral Acceleration Gain vs. Truck Speed …………...……55

Figure 4.5 - ESC Activation Module…………………………………………………...57 Figure 4.6 - ESC Throttle Control …………………………………...………………...59 Figure 4.7 - Brake Severity Number as a function of the difference between Lateral Acceleration and Critical Threshold……………………………...………61 Figure 4.8 - Differential Drive Axle Braking ………………………...………………..65 Figure 4.9 - Trailer Axle Braking……………………………………………………....67 Figure 4.10 - Trailer ABS Simulator ……………………………………………...... 68 Figure 4.11 - Roll Stability Control System Model ……………………………………..71 Figure 5.1 - Steering Angle, Truck Speed and Tractor Lateral Acceleration as Functions of Time from Test 397………………………………….………………..77 Figure 5.2 - Roll Angle, Truck Articulation Angle and Yaw Rates as Functions of Time from Test 397……………………………………………………...……..78 Figure 5.3 - Steering Angle and Truck Speed from Simulation – GVWR High C.G. (ESC OFF)………………………………………………………….…....80 Figure 5.4 - Tractor Lateral Acceleration from Simulation - GVWR High C.G. (ESC OFF) ……………………………………………………………………..81 Figure 5.5 - Roll Angle, Truck Articulation Angle and Yaw Rates as Functions of Time from Simulation - GVWR High C.G. (ESC OFF)……………………….82 Figure 5.6 - Steering Angle, Vehicle Speed and Tractor Lateral from Test Data – GVWR High C.G. (ESC ON)……………………………………….…...83 Figure 5.7 - Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - GVWR High C.G. (ESC ON)………………………………..…...85 Figure 5.8 - Brake Chamber Pressures of Trailer Axles from Test Data – GVWR High C.G. (ESC ON)………………………………………………………...... 86 Figure 5.9 - Roll Angles, Truck Articulation Angles and Yaw Rates from Test Data – GVWR High C.G. (ESC ON)……………………………………………87 Figure 5.10 - Steering Angle and Truck Speed from Simulation – GVWR High C.G. (ESC ON)……………………………………………88 Figure 5.11 - Tractor Lateral Acceleration from Simulation - GVWR High C.G. (ESC ON)………………………………………………………...... 89

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Figure 5.12 - ESC System Model Initiated Braking from Simulation - GVWR High C.G. (ESC ON)……………………………………………………...... 90 Figure 5.13 - Tractor Brake Chamber Pressures from Simulation - GVWR High C.G. (ESC ON)………………………………………………………………. .91 Figure 5.14 - Trailer Brake Chamber Pressures from Simulation – GVWR High C.G. (ESC ON)……………………………………………92 Figure 5.15 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR High C.G. (ESC ON)……………………………………………93 Figure 5.16 - Steering Angle and Truck Speed from Simulation –GVWR Low C.G. (ESCOFF)………………………………………………………………..95 Figure 5.17 - Tractor Lateral Acceleration from Simulation – GVWR Low C.G. (ESC OFF)………………………………………………………………...……95 Figure 5.18 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR Low C.G. (ESC OFF)…………………………………………...96 Figure 5.19 - Steering Angle, Truck Speed and Tractor Lateral Acceleration from Test Data - GVWR Low C.G. (ESC ON) ...... 97 Figure 5.20 - Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - GVWR Low C.G.(ESCON)………………………………………98 Figure 5.21 - Brake Chamber Pressures of Trailer Axles from Test Data - GVWR Low C.G. (ESC ON)…………………………………………………………..99 Figure 5.22 - Roll Angles, Truck Articulation Angle and Yaw Rates from Test Data - GVWR Low C.G. (ESC ON)……………………………………….….100 Figure 5.23 - Steering Angle and Truck Speed from Simulation – GVWR Low C.G. (ESC ON)……………………………………………………………….101 Figure 5.24 - Tractor Lateral Acceleration from Simulation - GVWR Low C.G. (ESC ON)……...... 102 Figure 5.25 - RSC system initiated braking for Tractor and Trailer from Simulation - GVWR Low C.G. (ESC ON)…………………………………………103 Figure 5.26 - Tractor Brake Chamber Pressures from Simulation - GVWR Low C.G. (ESC ON)…………………………… …………………………………104

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Figure 5.27 - Trailer Brake Chamber Pressures from Simulation - GVWR Low C.G. (ESC ON)……………………………………………………………….105 Figure 5.28: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR Low C.G. (ESC ON)…………………………………………...106 Figure 5.29 - Steering Angle, Truck Speed and Tractor Lateral Acceleration from Test Data – LLVW (ESC OFF)………………………………………...108 Figure 5.30 - Yaw Rates from Test Data - LLVW (ESC OFF)……………………...... 109 Figure 5.31 - Steering Angle and Truck Speed from Simulation - LLVW (ESC OFF)..110 Figure 5.32 - Tractor Lateral Acceleration from Simulation - LLVW (ESC OFF) …....111 Figure 5.33 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - LLVW (ESC OFF)……………………………………………..……….112 Figure 5.34 - Steering Angle, Vehicle Speed and Tractor Lateral acceleration from Test Data - LLVW (ESC ON)……………………………………………….113 Figure 5.35 - Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - LLVW (ESC ON)…………………………………………….…114 Figure 5.36 - Brake Chamber Pressures of Trailer Axles from Test Data - LLVW (ESC ON)……………………………………………………………………..115 Figure 5.37 - Yaw Rates from Test Data - LLVW (ESC ON)..…………………….….116 Figure 5.38 - Steering Angle and Truck Speed from Simulation - LLVW (ESC ON).117 Figure 5.39 - Tractor Lateral Acceleration from Simulation - LLVW (ESC ON)……..118 Figure 5.40 - RSC system initiated braking for Tractor and Trailer from Simulation - LLVW (ESC ON)……………………………………………………....119 Figure 5.41 - Tractor Brake Chamber Pressures from Simulation - LLVW (ESC ON)…………………………………………………………………..…120 Figure 5.42 - Trailer Brake Chamber Pressures from Simulation - LLVW (ESC ON).121 Figure 5.43 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - LLVW (ESC ON)………………………………………………………122 Figure 5.44 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation – GVWR with High C.G. (ESC OFF) ……………………….…………..124 Figure 5.45 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC OFF)……………………… …………...124

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Figure 5.46 - Roll Angles, Truck Articulation Angles and Yaw Rates from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC OFF)…………..125 Figure 5.47 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON)………………………………..……126 Figure 5.48 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON)……………………………………..127 Figure 5.49 - RSC system model Initiated Braking from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON) ……………………….…………..128 Figure 5.50 - Tractor Brake Chamber Pressures from Simulation - GVWR with High C.G. (ESC ON)…………………………………………………………129 Figure 5.51 - Trailer Brake Chamber Pressures from Simulation - GVWR with High C.G. (ESC ON)……………………………………………………………….130 Figure 5.52 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR with High C.G. (ESC ON)……………………………………..131 Figure 5.53 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC OFF) …………………………………....132 Figure 5.54 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC OFF)…...…………………………..……133 Figure 5.55 - Roll Angles, Truck Articulation Angles and Yaw Rates from Test Data - GVWR with Low C.G. (ESC OFF)…………………………………….134 Figure 5.56 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON) ……………………………………..135 Figure 5.57 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON) ……………………….…………….136 Figure 5.58 - RSC system model Initiated Braking from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON)……………………………………137 Figure 5.59 - Tractor Brake Chamber Pressures from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON ……………………………………...138 Figure 5.60 - Trailer Brake Chamber Pressures from Simulation - GVWR with Low C.G. (ESC ON)………………………… …………………………………....139

Figure 5.61 - Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR with Low C.G. (ESC ON)……………………………………...140 Figure 5.62 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - LLVW (ESC OFF)………………………………….……………..……141 Figure 5.63 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - LLVW (ESC OFF)……………….……………………………………..142 Figure 5.64 - Roll Angles, Truck Articulation Angle and Yaw Rates from Fish Hook Maneuver Simulation - LLVW (ESC OFF)…………………………….143 Figure 5.65 - Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - LLVW (ESC ON)………………… ……………………….…………..144 Figure 5.66 - Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - LLVW (ESC ON)………………………………………………………144 Figure 5.67 - RSC system model Initiated Braking from Fish Hook Maneuver Simulation - LLVW (ESC ON)…………………………………………..…………145 Figure 5.68 - Tractor Brake Chamber Pressures from Fish Hook Maneuver Simulation - LLVW (ESC ON)………………. …………………………………...... 146 Figure 5.69 - Trailer Brake Chamber Pressures from Simulation ………………. ……147 Figure 5.70 - Roll Angles, Truck Articulation and Yaw Rates from Fish Hook Maneuver Simulation - LLVW (ESC ON)………………………………………...148 Figure B.1 - Upper Half of Simulink Environment with S-Function, RSC System Model and Engine Model…………….………………………………………...159 Figure B.2 - Lower Half of Simulink Environment with ABS Controllers and Braking System Models..…………….……………………………………….....160

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CHAPTER 1

INTRODUCTION

1.1 Motivation

In the year 2005, about 8.5 million trucks were registered in the US and together

they traveled over 200 billion miles. The high amount of miles traveled resulted in many

accidents involving trucks. According to the Large Truck Crash Facts 2005 [1] released by the Federal Motor Carrier Safety Administration (FMCSA) in February 2007, there were 4,533 large trucks involved in fatal crashes in the year 2005. Of the 8.5 million registered trucks, about 2.1 million were combination trucks which included truck tractors pulling trailers. These combination trucks accounted for 3,378 of the total fatal crashes involving large trucks. Events such as rollover and jackknife proved most fatal for such combination trucks. In 2005, approximately 349 rollovers occurred that were fatal.

The National Highway Traffic Safety Administration (NHTSA) has recently

established a Federal Motor Vehicle Safety Standard (FMVSS) No. 126 [2] that requires

vehicles with a gross weight rating of 4,536 Kg (10,000 pounds) or less to have

Electronic Stability Control (ESC) systems by 2012. NHTSA estimates that this move

would save 5,300 to 9,600 lives and prevent 156,000 to 238,000 injuries annually [2].

ESC, a generic term defined by the Society of Automotive Engineers (SAE), may

have only a Roll Stability Control (RSC) feature in it or may include a combination of

Roll Stability Control and Yaw Stability Control (YSC) features. They are designed to prevent the occurrence of rollover, jackknife or other loss of control events. The RSC system senses the lateral acceleration and if it approaches critical values, then the RSC system applies braking on the wheels and reduces the lateral acceleration. The YSC system, in addition to lateral acceleration, also senses the yaw rate and steering wheel angle to determine the intended path. Based on the inputs it receives, the YSC system applies the brakes on individual wheels to prevent any excessive understeer or oversteer

[3].

The high volume of fatal accidents in heavy trucks has led NHTSA to investigate the feasibility of mandating ESC systems on heavy trucks. Tractor-semitrailers were involved in 3,077 fatal accidents in the year 2005, which constituted about 62.4% of the total large truck fatal accidents [1]. 6x4 tractors constitute a major part of the air-braked vehicles being manufactured every year. All manufacturers of 6x4 tractors sell ESC systems either as standard or optional equipment. Hence, NHTSA has begun research on

Tractor-semitrailers with a 6x4 tractor. This study is part of the research conducted at

NHTSA’s Vehicle Research and Test Center (VRTC) to study ESC systems and the effectiveness of Roll Stability Control.

1.2 Simulation

TruckSim, MATLAB and Simulink were the software programs used in this

study. The simulation environment consists of a file created in Simulink which includes

models of the braking system, ABS controllers and the ESC system. A multi-body

dynamics model of the tractor-semitrailer combination was created in TruckSim and it

was run in conjunction with the Simulink environment. The inputs to the ESC model

were obtained from TruckSim, while TruckSim obtained its inputs of friction coefficient

at wheels, torque converter output and brake torques from the Simulink environment.

Simulation was performed for different vehicle loads and maneuvers to replicate the

actual on-track tests conducted with the truck and also to study the effectiveness of the

developed ESC system model.

1.3 Previous Studies

Several research programs have been carried out at VRTC to aid the understanding of heavy truck braking. A braking system model and an Anti-lock Braking

System (ABS) model were first created by Dunn [4, 5] in his study of jackknife stability

of articulated vehicles. Zagorski [6, 7] developed a model of a Sterling tractor in his

study of the effects of disc and drum brakes on jackknife stability on Class VIII vehicles

with multiple trailers. The braking system models and the ABS controllers were further

improved by Zaugg [8, 9]. A similar ABS controller was developed for a semitrailer by

Shurtz [10, 11] in his study of the effects of ABS controller parameters on heavy truck braking.

The base Simulink environment developed by Shurtz [10] for a tractor-semitrailer has been adopted for the development of the ESC model. Figure 1.1 shows the top half of the Simulink environment by Shurtz. It has the S-Function which serves as the link between Simulink and TruckSim. The inputs to TruckSim are the brake torques and friction coefficients at the wheels of the truck which is estimated by the Random Friction

Generator. Figure 1.2 shows the bottom half of the Simulink environment. It has the ABS controllers, the brake torque and pneumatic subsystems for the tractor and the trailer. The inputs for the ABS controllers are longitudinal wheel speeds, vehicle speed and acceleration values which are obtained as outputs from TruckSim. The ABS controller computes the slip ratios at each wheel while braking and gives a command of build, hold or dump to each brake chamber. Complete explanations for the working of the ABS controller models can be found in Zaugg [8] and Shurtz [10].

wheel_long_speed_tractor

Selector4

Equiv alent (tangential) WHEEL Speeds (kph) (10) Equiv lant (tangential) Wheel Speeds (mph) (10) 1/1.6093 wheel_long_speed

kph to mph

wheel_long_speed_trailer

Selector5

Tire-Wheel Assy . accel/decel (rad/s^2) (10) wheel_radial_accel

tractor_speed kph to mph

Volv o Speed (mph) 1/1.6093 tractor_speed

g to m/s^2 Volv o Long. Accel. (m/s^2) 9.80665 tractor_accel trailer_speed Brake Torque Outputs (lb-f t) (10) torque_vehicle 1.356 Brake Volvo Motion (4) Demux Brake Torque Outputs kph to mph to TSIM (N-m) (10) lb-ft to N-m 1/1.6093 trailer_speed Volv o Speed (mph)

Demux g to m/s^2 INPUTS Volv o Long. Accel. (m/s^2) f rom TruckSim 9.80665 trailer_accel

tractor_speed varia v ariable MU lev els (40) TruckSim 5-axle tractor / semi-trailer model

Treadle Pressure (MPa) Treadle Pressure (psi) 145.0377 5 Randomly varied Mux Brake Torques

Friction Generator & MU Levels MPa to psi

Vehicle Dy namics (29) vehicle_dynamics activation Prim_treadle_output

Constant

REFERENCE Ptreadle P_treadle_output

Rear wheel locations (4) Treadle Pressure Dynamics Subsystem wheel_locations ECBS or Pneumatic controlled

activation Sec_treadle_output wheel_forces Constant1 Wheel forces (20)

REFERENCE Ptreadle P_treadle_output

Demux1 Treadle Pressure Dynamics Subsystem1 ECBS or Pneumatic controlled1

treadle_pressure_reference

Figure 1.1 – Top Half of Tractor – Trailer base Simulink Model 6

Figure 1.2 – Bottom half of Tractor-Trailer base Simulink Model 1.4 Objective

The purpose of this study was to understand the functioning of commercial ESC systems and to develop an ESC model that simulates the functionality of those systems.

The effectiveness of ESC systems cannot be completely studied by testing the vehicle on a test track, as the testing procedure is restricted by safety concerns. High costs involved in the procurement of trucks and ESC systems coupled with factors such as time and resources also prohibit extensive testing. This necessitates the need of an ESC model that can be used in conjunction with a truck model to simulate the behavior of a truck in reality.

Trucks fitted with ESC systems were tested by VRTC at the Transportation

Research Center Inc., Ohio (TRC) to evaluate their roll stability. The test data from these tests were studied to understand the functioning of the ESC systems in preventing rollovers. This information was used to develop a Simulink model of the ESC system with a Roll Stability Control. A model of the truck used for testing was created in

TruckSim. The ESC model was integrated with truck model. Simulations were performed to further aid the understanding of the ESC systems.

1.5 Thesis Overview

Chapter 2 deals with the collection of data and its analysis. This chapter gives the details of the vehicle testing carried out at TRC. The various maneuvers adopted for

testing the truck, the different load configurations used while testing and the status of the

ESC systems of both the tractor and the trailer are described. Details of data analysis and processing are included in this chapter. The collected data is processed and then studied using Matlab to understand the response of the ESC system.

Chapter 3 outlines the development of the truck model. First, it describes the modeling of the truck in TruckSim. Then it deals with the modifications done to the ABS and braking system models to cater to the configurations of the truck under study.

Chapter 4 describes the modeling of the ESC system for the tractor. The chapter explains the various modules in the ESC system model and their functions.

The results of the simulation are included in Chapter 5. Information such as the different maneuvers and load configurations simulated are detailed in this chapter.

Chapter 6 discusses the findings from the simulations. It also suggests improvements to the model and the scope for further studies.

CHAPTER 2

VEHICLE TESTING AND DATA ANALYSIS

2.1 Vehicle Description

To understand the functioning of a commercial ESC system, a truck with a

Volvo tractor and Fruehauf van semitrailer was tested on the Vehicle Dynamics Area

(VDA) of the Transportation Research Center Inc., Ohio. The tractor had a factory installed ESC system while the trailer was retrofitted with a trailer based Roll Stability

Control (RSC) System.

The 2006 Volvo tractor used for testing has a 6x4 configuration. This denotes that

the vehicle has three axles: a steer axle, a leading drive axle and a trailing drive axle.

Thus of its 6 wheel locations, 4 are driven by the powertrain.

The following table gives information about the tractor and the semitrailer used during testing.

Tractor : 2006 Volvo Trailer : 2001 Fruehauf VNL64T630 Configuration 6 X 4 53 ft (16,154mm) Dry Van Wheelbase 211 in (5,271mm) 464 in (11,785mm) ABS System Bendix ABS 4S/4M Meritor Wabco Trailer ABS Brake Type Meritor S-Cam– 15X4 F Meritor S-Cam– 16.5X7 16.5x7 R ESC Bendix Electronic Stability Meritor Wabco Roll Stability Support Program (ESP)

Table 2.1: Vehicle Information

2.1.1 Truck Braking

The Volvo tractor has a pneumatic braking system in it. The braking system has two air reservoirs: a primary and a secondary. When the driver applies the brake, the action opens the treadle valve which distributes the pressurized air to the modulators present in the wheels of the tractor and the trailer. The modulator is controlled by the

ABS. The modulated air pressure is given to the brake chambers which brake the vehicle.

2.1.2 Anti-lock Braking System (ABS)

The main purpose of the ABS is to prevent the wheels from locking up during braking as locked wheels cause the vehicle to lose directional stability. The ABS continuously monitors the wheel speeds and if the slip ratios exceed specific thresholds, the ABS commands the modulators to dump the pressure in the brake chambers of the wheels that lock. The reduction in the brake pressure decreases wheel slip, thus enabling

the vehicle to regain stability. The Volvo tractor has a 4s/4m ABS which means that the system has 4 wheel speed sensors and 4 modulators. The following figure shows a typical

4s/4m ABS system for a 6x4 tractor.

Figure 2.1 – 4s/4m ABS layout [16]

2.2 Vehicle Testing

The truck was tested with different load conditions and with combinations of the tractor and semitrailer ESC systems either turned ON or OFF.

2.2.1 Load Configurations

During testing, the load configuration of the truck was varied to study the responses of the truck as well as the ESC system to different loads and C.G. heights of the truck. A total of three load configurations were used for testing:

1. Lightly Loaded Vehicle Weight (LLVW)

2. Gross Vehicle Weight Rating (GVWR) with low C.G. height

3. Gross Vehicle Weight Rating (GVWR) with high C.G. height

The LLVW configuration denotes the empty truck with outriggers fitted on either side to the chassis of the semitrailer. Figure 2.2 shows the semitrailer in LLVW condition with its C.G. location marked.

GVWR represents the maximum legally permitted truck weight of 80,000 pounds

(36,287 kg). Increase in the truck weight to attain GVWR conditions were achieved by adding concrete ballast blocks each weighing approximately 4,400 lb (2000 kg) to the semitrailer. While loading the truck, the blocks were arranged into two stacks; one to the front of the semitrailer and the other towards its rear. Figure 2.3 shows the semitrailer with the concrete ballast blocks arranged to get GVWR condition with a low C.G. height.

The rollover stability of vehicles decreases with increase in their C.G. heights and so a series of tests were carried out with GVWR load having a high C.G. height. The increase in C.G. height was achieved by placing the ballast blocks on top of spacer blocks each approximately weighing 495 lb (225 kg). The blocks were secured onto the floor of the semitrailer using load rails. Figure 2.4 shows the semitrailer in GVWR condition with high C.G. height. The C.G. height shown is that of the entire semitrailer with the load blocks. The blocks with the thick outlines are the spacer blocks which were used to increase the C.G. height of the load stacks.

The following table shows the weight of individual stacks on both low and high

C.G. GVWR conditions along with the combined ballast C.G. location.

Weight of Ballast Stacks Combined Ballast CG location (mm) Front stack (kg) Rear stack Height from trailer floor Distance from (kg) front of trailer Low CG 9,993 8,950 297 6,744 High CG 10,464 8,650 1,250 6,950

Table 2.2: Truck Loading

Figure 2.2 – Semitrailer in LLVW Condition (All Dimensions in mm)

Figure 2.3 – Semitrailer in GVWR Condition with Low C.G. Height (All Dimensions in mm)

Figure 2.4 – Semitrailer in GVWR Condition with High C.G. Height (All Dimensions in mm)

The trailer was fitted with outriggers to prevent the tractor and trailer from rolling over while testing. The outriggers were fitted to the trailers approximately at their geometric center and added 3,000 lb (1,360 kg) to the trailer. The total roll inertia of the trailer was changed due to this addition.

2.2.2 ESC Systems ON/OFF

Both the tractor and the semitrailer have their own ESC systems. The functioning of each system is different as they operate with their unique logic to maintain the stability of the units they control. To understand the working of both systems, it is necessary to study the independent functioning of the systems. Tests were done with different

combinations of the tractor and semitrailer ESC systems being either ON or OFF and the

following four combinations were used to test the vehicle:

Tractor ESC Trailer ESC Combination 1 OFF OFF Combination 2 ON OFF Combination 3 OFF ON Combination 4 ON ON

Table 2.3: ESC ON/OFF combinations

2.2.3 Vehicle Instrumentation

The truck was fitted with many sensors to measure variables while testing. The truck had a Controller Area Network (CAN) bus which is defined by SAE J1939 standard

and Global Positioning Systems (GPS). The measured variables include hand wheel

angle, lateral, longitudinal and vertical acceleration, yaw rate, roll rate, pitch rate, vehicle

speed and wheel speeds. The full list of the collected data is shown in Appendix A. From

the CAN bus, the ESC status signal was measured which indicated the activation and

deactivation of the ESC system.

2.2.4 Test Maneuvers

The truck was tested by performing several maneuvers such as J-Turn, slowly increasing steer (SIS), lane change, double lane change, and constant radius turn. The SIS and a set of J-Turn maneuvers were performed using a steering controller. Tests were

performed at different speeds. Speeds were increased by 5 mph in successive tests until

either wheel lift occurred or directional stability was lost. Tests were carried out on Dry

Asphalt and Wet Jennite surfaces.

2.3 Data Analysis

As the purpose of this research is to create an ESC system model with Roll

Stability Control for the tractor it is necessary to look at how the tractor ESC alone responds to the different maneuvers without the intervention of the trailer ESC system.

For this purpose, only the data from tests in which the tractor ESC was ON and the trailer

ESC was OFF were considered. Data from the tests done on the dry asphalt surface were used for this research. The collected data were imported into MATLAB for analysis.

2.3.1 Data Processing

The collected data had to be processed and filtered before it could be analyzed. It

had noise content which was removed by filtering the data channels using Butterworth

filters in MATLAB with a cut-off frequency of 6 Hz.

As the sensors mounted on the truck for measuring variables were offset from the

C.G. location of the truck, the measured data had to be corrected for this offset. In

addition to this, during a turn the vehicle experiences roll which slightly increases the

measured lateral acceleration. Hence, the collected data had to be corrected for roll too.

Engineers at VRTC had developed codes in MATLAB to correct data channels for C.G.

offset and vehicle roll. These codes were modified to suit the truck under study. During

testing, the vehicle was instrumented with laser height sensors on the trailing drive axle,

trailer chassis and the second axle of the trailer to measure the vertical displacement at

those points. The data from these sensors were used to calculate the roll angles. Based on

the calculated roll angle, necessary corrections to the measured lateral acceleration values were made.

2.3.2 ESC Activation Signals

The ESC system installed in the tractor had both RSC and YSC features. These

two features control the brakes as well as the engine. So a total of four channels of data

were recorded for the ESC systems: RSC Brake, RSC Throttle, YSC Brake and YSC

Engine. Each channel is an activation signal for that particular control. When a feature

activates, the value of the corresponding data channel rises from zero to one. When it

deactivates it falls back to zero. Thus for each test there are four channels of ESC

activation data that indicate the individual status of RSC and YSC features.

These activation signals from different tests were overlaid on the tractor lateral

acceleration plots of the corresponding tests. As the activation data channels show the

time at which the ESC system is activated and deactivated, its intersection with the lateral acceleration curves gives the values of the variable at the time of ESC activation and deactivation. The RSC part of the ESC system was observed to activate once the lateral acceleration exceeded certain threshold values. It was observed that on activation, the

RSC first cuts the engine throttle and then applies braking. This was observed by

studying the sequence of activation of the data channel. First, the value of the RSC

Throttle data channel increased to one followed shortly by the RSC Brake data channel.

During the tests, the YSC was found to remain inactive. This is primarily due to

the fact that the test data considered were from a dry surface where the friction coefficients between the wheels and the road surface are high enough to maintain traction and hence the chances of loss of directional control are less. But high CG coupled with high lateral acceleration is enough to initiate a rollover even at high friction surfaces which causes the RSC part of the ESC system to activate.

2.3.3 Steering Controller Data Sets

Tests were performed both with and without a steering controller. To understand

the functioning of the ESC system, it was necessary to get a pattern of its response to a

repeated set of tests with the same inputs. Tests in which the drivers manually steered the

truck differed from one another in terms of the input steering angle values. These tests

were performed by having the drivers follow a path marked by a set of cones. Each of

these tests proved to be different from the others as the driver gave different steering

inputs in order to maintain the path. This was primarily observed in the driver controlled

J-Turn tests.

The tests performed with steering controller were much more repeatable. The

steering controller was used to control the steering profile as well as the steering rates. A constant steering rate of 13.5 deg/s was maintained for the Slowly Increasing Steer maneuvers and a steering rate of 48.57 deg/s was maintained for the J-Turn tests. When

the data from these tests were studied, the response of the ESC system was observed to

be more uniform. Hence, the data from these tests were primarily used for the modeling

of the ESC system.

2.3.4 Test Data

The data from a test performed with the truck in GVWR condition with Low C.G.

height is discussed in this section. The executed maneuver is Slowly Increasing Steer and the targeted speed of the vehicle while entering the maneuver was 35 mph (56 kph).

Figure 2.5 shows the path of the vehicle during the test. The GPS data was used to plot

the vehicle path. Eastings and Northings are the measured values of the truck’s X and Y

positions from the GPS.

4 x 10 Maneuver plot

7.184 Maximum Acceleration

7.182 End of Maneuver

7.18

7.178

7.176 Northings GPS (m) (m) GPS Northings Start of Maneuver

7.174

7.172

5.1114 5.1116 5.1118 5.112 5.1122 5.1124 5.1126 5.1128 5.113 5.1132 Eastings GPS (m) 5 x 10

Figure 2.5 – Path of Vehicle Motion

The SIS maneuver was executed according to SAE J266 standards. Once the vehicle attained a constant speed of 35mph (56kph), the maneuver was begun by triggering the steering controller. A constant steering rate of 13.5 deg/s was maintained by the controller for a period of 10 seconds. Figure 2.6 shows the steering angle and vehicle speed as functions of time during the maneuver.

Steering Angle vs Time 0 Steering Angle RSC Throttle RSC Brake -50

-100 Steering Angle (deg)

-150

6 8 10 12 14 16 18 20 22 24 Time(sec)

Tractor Velocity vs Time

Velocity (kph) Velocity 20

0 6 8 10 12 14 16 18 20 22 24 Time(sec)

Figure 2.6 - Steering Wheel Angle and Truck Speed as Functions of Time

The lateral acceleration of the tractor as a function of time is shown in Figure 2.7.

The ESC activation signals (RSC Throttle and RSC Brake) were superimposed on the lateral acceleration plot to show the values of lateral acceleration at which the RSC system activated. From the plot it can be observed that the RSC Throttle control is activated first, followed by the RSC Brake control. This indicates the sequence of events as they occurred.

Tractor Lateral acceleration vs Time 0 A y RSC Throttle RSC Brake -0.05

-0.1

-0.15

-0.2 Lateral Acceleration(g)

-0.25

-0.3

6 8 10 12 14 16 18 20 22 24 Time(sec)

Figure 2.7 – RSC Throttle and RSC Brake Activation Signals Overlaid on the Lateral Acceleration Plot

The response of the ESC system was also studied by observing the brake chamber pressures of the individual wheels of the tractor as shown in Figure 2.8. This gives an idea of how the ESC system selectively applies braking on the wheels.

Steer Axle of Tractor 150 Inner Wheel Outer Wheel 100 RSC Throttle RSC Brake 50 Pressure, kPa Pressure, Brake chamber 0 13 14 15 16 17 18 19 20 21 Time(sec) 1st Drive Axle of Tractor 200

100 Pressure, kPa Pressure, Brake chamber 0 13 14 15 16 17 18 19 20 21 Time(sec) 2nd Drive Axle of Tractor 200

100 Pressure, kPa kPa Pressure, Brake chamber 0 13 14 15 16 17 18 19 20 21 Time(sec)

Figure 2.8 – Tractor Brake Chamber Pressures with RSC Throttle & RSC Brake Signals

Figure 2.9 shows the wheel speeds of the tractor. The drop in the wheel speeds is due to braking by the ESC system. The inner wheels have a slightly lesser speed than the outer wheels, because of the weight transfer to the outer side of the vehicle during the maneuver. This causes the inner wheels to have less traction. And so when the ESC applies braking, the inner wheels tend to slow down a bit more than the outer wheels.

Steer Axle of Tractor :Wheel speeds vs Time 60 Inner Wheel Outer Wheel 50

40 Wheel Speed(kph) 6 8 10 12 14 16 18 20 22 24 Time (sec) 1st Drive Axle of Tractor: Wheel speeds vs Time 60

40 Wheel Speed (kph) 6 8 10 12 14 16 18 20 22 24 Time (sec) 2nd Drive Axle of Tractor: Wheel speeds vs Time 60

40 Wheel Speed (kph) 6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 2.9 – Tractor Wheel Speeds

The tractor ESC was also observed to brake the trailer as shown in Figure

2.10. Since the tractor is the leading unit, its lateral acceleration is used by the tractor

ESC to brake the trailer as a preemptive measure and also to slow down the entire vehicle. Figure 2.11 shows the speeds of the trailer wheels.

Trailer Axle Braking 300 Inner & Outer Wheels RSC Throttle RSC Brake 250

200

150

100 Brake chamber Pressure, kPa

0 13 14 15 16 17 18 19 20 21 Time(sec)

Figure 2.10 – Trailer Brake Chamber Pressures with RSC Throttle &RSC Brake Signals

First Axle of Trailer :Wheel speeds vs Time 60 Inner Wheel 55 Outer Wheel

Wheel Speed (kph) 40

35 6 8 10 12 14 16 18 20 22 24 Time (sec)

Second Axle of Trailer: Wheel speeds vs Time 60

Wheel Speed (kph) 40

35 6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 2.11 – Trailer Wheel Speeds

2.3.5 RSC Activation

The data collected from all the tests were saved as different files with the

corresponding test number being the filename. A set of codes were written in MATLAB

to automate the process of finding the values of lateral acceleration, vehicle speed, and

steering angle at the time of ESC activation. The set included an initiation file. When

executed, the code prompts the user to enter the test number of the run to be analyzed.

Upon entering the test number, the code automatically executes other files which analyze the data from that test. The codes output the values of different variables at the times of

ESC activation and deactivation.

This procedure was repeated on data from different tests. The responses of the

ESC system in all these tests were compared to find out the value of lateral acceleration

at which the ESC system activates in every test. Table 2.4 shows a compilation of the

data collected from analyzing the different data sets.

From Table 2.4, it can be inferred that ESC activation occurs at 0.62 g for LLVW

condition, 0.30 g for GVWR with Low C.G. and 0.36 g for GVWR with high C.G.

Test No Load Maneuver Speed ESC Lateral Acceleration (kph) Activation (in g) at ESC duration Activation (s) 983 LL SIS 48 5.65 0.61 989 LL SIS 56 8.69 -0.63 292 Low J-Turn 40 2.92 -0.30 927 Low SIS 40 3.13 0.29 936 Low SIS 56 3.43 -0.29 373 High J-Turn 43 2.23 -0.35 374 High J-Turn 52 2.43 -0.36 375 High J-Turn 56 3.33 -0.37 LL – Lightly loaded Low – GVWR with Low C.G. High – GVWR with High C.G. SIS - Slowly Increasing Steer

Table 2.4: RSC Activation Time and Lateral Acceleration Values

2.3.6 Steady State Lateral Acceleration Gain

The steady state lateral acceleration gain of the vehicle was estimated, as it gives an insight into the relation between the lateral acceleration and the steering angle. This value can be used to calculate the Understeer coefficient of the vehicle, which is useful to determine whether the vehicle is understeering or oversteering.

The steering ratio of the vehicle was measured at VRTC and found to be 20.5 deg/deg for the tractor. The road wheel angle was found by dividing the steering wheel angle by the steering ratio. The lateral acceleration values were then plotted against the road wheel angle. The slope of this curve gives the steady state lateral acceleration gain.

Lateral acceleration vs Road wheel Angle 0.18 30mph 0.16 32mph G = 0.0436 K =10.8662 us 35mph 0.14 G = 0.0406, K =10.0215 us 0.12

0.1

0.08

0.06 G=0.0376, K =10.2303

Lateral acceleration, in g in acceleration, Lateral us 0.04

0.02

0 2 3 4 5 6 Road wheel angle, in deg

Figure 2.12 – Lateral Acceleration vs. Road Wheel Angle

The above figure shows the plot of lateral acceleration as a function of road wheel angle for three different tests conducted at 30, 32 and 35 mph.

The understeer coefficient for a run with constant speed with slowly increasing steer was calculated from the formula:

L1 180 K ( 9 )81. ××−= us G u2 π (2.1)

Where Kus = Understeer coefficient in deg/g

G = Steady-state lateral acceleration gain in g/deg

L – Wheel base in m

u – Truck velocity in m/s

The understeer coefficient for the tractor was computed to be 10.37 deg/g. The positive value of the coefficient shows that the vehicle is understeering.

CHAPTER 3

TRUCK MODEL

3.1 Overview

The complete truck model developed for the purpose of studying the effectiveness of the ESC system includes a vehicle dynamics model of the tractor-semitrailer in

TruckSim, ABS and ESC systems models in Simulink. The Simulink environment also includes the braking system of the entire truck, which includes the brake treadles and the individual brake chamber models. All these systems function together to simulate the behavior of the actual truck. The modeling of the entire system is elaborated in the following sections.

3.2 TruckSim Model

TruckSim was used as the vehicle dynamics software to model the tractor and the trailer. Previous projects at VRTC have involved the modeling of trucks in TruckSim.

Models created in TruckSim can be integrated to run with models in Simulink. In addition, TruckSim provides animations for the runs and also provides the user with capability of simulating different load conditions and maneuvers. Hence, TruckSim was chosen as the software for creating the dynamic models of the tractor-semitrailer.

TruckSim version 6.01 was used for developing the models.

3.2.1 Tractor Model

Modeling of the tractor in TruckSim requires information about different vehicle parameters like dimensions, sprung masses, unsprung masses, suspension properties, and tire data. Some of the parameters were measured directly from the truck and others which could not be measured were estimated by scaling previous truck model parameters.

VRTC had a TruckSim model of a NHTSA’s 1991 Volvo tractor. This model was scaled to that of the 2006 Volvo tractor. The 2006 Volvo tractor was measured for its mass, wheel base, and track width. These values were entered into the TruckSim interface. The two drive axles were modeled with dual wheels which was the case with the actual Volvo tractor. A typical 5th wheel was selected for the trailer hitch. The coordinates of the 5th wheel in the tractor were measured and these values were entered into TruckSim.

3.2.1.1 Track Width and Wheelbase

The track width of the tractor was measured between the centerlines of the tires for the steer axle. For the drive axles, it was measured between the centers of the dual wheels. The origin for the tractor in TruckSim is at the ground just below the centerline

of the front axle (steer axle). The wheelbase for the drive axles were measured from this origin. The measurements were corroborated by comparing with the dimensions provided by the manufacturer and are shown in the table below.

Track Width [in] (mm) Wheelbase [in] (mm) Steer Axle 83 (2,108) 0 (0) Leading Drive 73 (1,854) 187 (4,750) Trailing Drive 73 (1,854) 239 (6,071)

Table 3.1: Tractor Track Width and Wheelbase

3.2.1.2 Sprung and Unsprung Mass Properties

The locations of the center of gravity heights of the steer axle and drive axles were estimated. Table 3.2 shows the C.G. heights of each axle along with their spin axis heights. The spin axis is the axle’s centerline of rotation.

C.G. height [in] (mm) Spin Axis height [in] (mm) Steer Axle 19.7 (500) 18.9 (480) 19.7 (500) Leading Drive 20.5 (520) 19.7 (500) Trailing Drive 20.5 (520)

Table 3.2: Tractor Unsprung Mass C.G. and Spin Axis Height

Table 3.3 shows the estimated mass and the roll/yaw inertias of the unsprung masses.

Unsprung Mass [lb] (kg) Axle Roll and Yaw Inertia [lb-ft2] (kg-m2) Steer Axle 1,162 (527) 15,780 (665) 2,214 (1004) Leading 13,930 (587) 2,145 (973) Trailing 14,048 (592)

Table 3.3: Tractor Unsprung Mass Properties

The sprung mass roll, pitch and yaw inertias were obtained by scaling. The sprung mass was estimated to weigh 13794 lb (6256 kg). The estimated Roll, Pitch and Yaw inertias of the sprung mass are given in Table 3.4.

Inertia [lb-ft2] (kg-m2)

Roll Inertia 206,709 (8,710.9)

Pitch Inertia 652,380 (2,7491.8)

Yaw Inertia 590,913 (2,4901.5)

Table 3.4: Tractor Sprung Mass Properties

3.2.1.3 Tractor Suspension and Tires

The suspension and the tire models developed by VRTC for the 1991 Volvo truck were retained for the 2006 Volvo tractor model. The springs and the shock absorber parameters were defined in the model.

The steer and drive axle tire models for the tractor were also taken from the 1991

Volvo model.

3.2.1.4 Tractor Steering and Braking

A typical steering system available by default in TruckSim was used for the tractor steer axle. The steering gear ratio of the system was changed to the measured value of 20.5 deg/deg. Default values were retained for the steering system compliances.

As the ABS and braking system models were available in Simulink, the brakes options for all three axles were selected to be No Pressure/No Torque Gains/Without

ABS.

3.2.1.5 Powertrain

As the Volvo tractor has four driven wheels, a powertrain with four wheel drive was selected. The selected powertrain has a gear ratio of 3.2 for the front and rear differentials.

TruckSim has an example engine and torque converter model in Simulink called the External Engine and Torque Converter Model. This external model was integrated

within the Simulink model and used during the simulations. Since the engine and torque converter models are defined in Simulink, the No Engine and No Torque Converter options were selected for the options of engine and torque converter in TruckSim. An external engine model was selected because it gives the option of controlling the throttle input to the engine in Simulink. This is useful while modeling the RSC system, which needs to have engine throttle control. This concept is explained in detail in Section 4.3.3.

3.2.2 Trailer Model

TruckSim has a default 48 ft semitrailer and this was used to model the 53 ft

Fruehauf semitrailer used in testing. The inertias of the sprung mass were calculated taking into account the outriggers fitted to the sides of the trailer. The trailer was modeled with two axles. Like the tractor, the braking system and the ABS models for the trailer are in Simulink file. The brakes option for the trailer was set to No Pressure/No Torque

Gains/without ABS in TruckSim. Tire models developed by VRTC were used in the

Fruehauf trailer model.

TruckSim provides the user with the option of adding loads to the vehicle. Up to four loads can be added to the trailer. Three different trailer models were created, simulating the three different loading conditions used while testing. The empty trailer represents the LLVW condition. The two GVWR conditions were modeled by adding custom payloads to the empty trailer. Two loads were added for each of the GVWR conditions, simulating the front and rear stacks used in testing.

The total weight of each stack used during testing was calculated. Then the C.G. location and the Roll, Pitch and Yaw inertias of the load stacks were computed. These values were then used to create front and rear loads for both GVWR with Low C.G. and

GVWR with High C.G.

The following table shows the C.G. locations of each custom payload added to create the two GVWR conditions. The hitch is the point of reference (origin) for measuring longitudinal (X-axis) and lateral (Y-axis) distances within the semitrailer in

TruckSim, while the ground is the reference for vertical (Z-axis) distance.

Individual Stack CG locations Height from Distance from trailer hitch ground (mm) (mm)

Low CG Front Stack 1,487 1,146 Rear Stack 1,501 11,521 High CG Front Stack 2,296 822 Rear Stack 2,617 11,833

Table 3.5: C.G. Locations of Custom Payloads for Trailer Model in TruckSim

3.2.3 TruckSim Model Summary

TruckSim allows using individual models in different data sets, which permits the same tractor to be used in different simulations without reconstructing the entire model

for each simulation run. Three different data sets were created to represent the truck in the three loading conditions. This was achieved by using the Volvo tractor model and one of the three modeled semitrailers for each of the data sets. Thus a completely modeled truck in three different loading configurations of LLVW, GVWR with Low C.G., and

GVWR with High C.G. were available for simulation.

The complete vehicle model for the 6x4 tractor with a semitrailer was mathematically represented by 143 ordinary differential equations. These equations represented the kinematic and dynamic behavior of the truck. It consisted of 32 bodies having 29 multibody degrees of freedom. A total of 49 multibody coordinates and 64 auxiliary coordinates were used in the model. The model had 164 active forces and 117 active moments.

3.3 Simulink Model

The Simulink file, which is run in parallel with TruckSim, has the models of the braking system as well as the ABS for the tractor and the trailer. The Simulink environment developed by Shurtz [10] for his Sterling tractor-semitrailer model was used as the base model for the Volvo tractor-Fruehauf semitrailer combination. The design and controls of the braking system and ABS were modified to suit the configurations of the truck under study.

Brake chambers and ABS were first modeled by Dunn [6, 7] for his study of jackknife stability in tractor-semitrailers. This was further improved by Zaugg [8, 9] and adapted for the Sterling tractor-semitrailer model by Shurtz [10, 11]. The modules in the

Simulink environment are explained in the following sections. Complete descriptions of each module can be found in [8] and [10].

3.3.1 Randomly Varied Friction Generator

In reality, the friction coefficients encountered by each wheel of the vehicle are different due to the irregularities of the road surface. The Randomly Varied Friction

Generator simulates this reality. This module was first modeled by Dunn [6].

The user inputs a nominal coefficient of friction when initiating the run. This module adds a band limited white noise to the nominal value. The resultant is given as the friction coefficient value to the wheels of the steer axle.

The module then calculates the time that would elapse for the wheels of the drive axles and the trailer axles to cross the point which is the current location of the steer axle wheels. For this, it computes the delay time based on the vehicle speed and the distance of the drive axles and the trailer axles from the steer axle. Then the module inputs the friction coefficient values to the wheels of the drive axles with the calculated delay.

The friction coefficient values determined by this module are multiplexed (the process of combining a number of input signals into one input vector) together and given as an input to the TruckSim model.

3.3.2 S-Function

The link between the Simulink file and the TruckSim model is the s-function.

Every truck in TruckSim has a vehicle code assigned to it based on its axles’ configurations. The truck that needs to be integrated with Simulink environment is defined in the s-function.

The vehicle code for the Volvo tractor-Fruehauf semitrailer is s_ss + ss. The ‘s’ represents solid axles. Axles in the same vehicle unit that do not have any load equalization linkages are separated by the underscore. A ‘+’ indicates the presence of a hitch (fifth wheel) in a combination vehicle [12].

The s-function module has an input and the output. The input variables are obtained from the Simulink environment. These input variables are exported to the

TruckSim model when the model is run. The output variables from the TruckSim model are exported into the Simulink environment. Both the input and the output variables are predefined in the TruckSim model. The individual variables that constitute the inputs given to s-function and the outputs obtained from it are arranged in the same order in which they are defined inside TruckSim.

3.3.3 Brake Torque and Pneumatic Subsystems

The truck has a pneumatic braking system which uses air as the controlling medium. The compressible nature of the control medium results in dynamics in the braking system. Pressurized air is made to travel the full length of the tractor-semitrailer

combination, resulting in considerable time delays. These characteristics are modeled by the brake torque and pneumatic subsystems.

The pneumatic modules simulate the pressures at the brake chambers while the brake torque modules simulate the brake torques at the wheels. Each axle has its own subsystem. Thus there are a total of 5 subsystems; 3 for the tractor and 2 for the semitrailer.

A first order dynamic system has been developed previously [6] to model the working of the actual brake chamber. The treadle pressure from TruckSim is given as an input to this system. Delay times are incorporated in the system to account for the time delays of the pressure signal from the treadle to the brake chamber in each axle. Based on the treadle pressure, the pneumatic system computes the brake chamber pressures for the wheels. The brake torque system computes the brake torque at each wheel, depending on the brake chamber pressures. Comprehensive information about the initial development and further modifications of the Brake Torque and pneumatic subsystems can be found in

[6], [8], and [10].

The working of the pneumatic system is controlled by the ABS controllers. The

ABS controllers send command inputs to the pneumatic system, thereby modulating the brake chamber pressures and thus ultimately the brake torques.

3.3.4 ABS Controllers

The ABS system uses a set of sensors and modulators. The sensors measure the speed of the wheels on which they are mounted. The modulators modulate the pressure of

the pneumatic medium in the brake chambers. Different configurations of ABS are obtained by using different number of sensors and modulators. Some of the common configurations are 2s/2m, 4s/2m, 4s/3m and 4s/4m. A three axle tractor can have a 6s/4m or a 6s/6m ABS too.

The tractor and the trailer have their own ABS controllers that separately control the braking of their respective units. Shurtz [10] used a 4s/4m ABS controller for his

Sterling tractor model. The Volvo tractor too had the same ABS configuration. But the

Sterling tractor has four wheel locations while the Volvo tractor being modeled has six wheel locations. Hence, this change in the configuration was taken care of by making suitable modifications to the ABS controller. The controller was integrated with the tractor braking system such that the axles sensed by the controllers were the steer axle and the trailing drive axle, which is the case with the actual vehicle. It was reasoned that while braking, there tends to be a weight shift towards the front. This would make the trailing drive axle wheels be more prone to wheel lift and eventually locking on braking.

Hence the wheels of the trailing drive axle were sensed instead of those in the leading drive axle.

The wheels of the steer axle had one modulator each to modulate the pressure in their brake chambers. The remaining two modulators were each designed to control one side of the drive axles. Thus, the left wheels of the leading and trailing drive axles were controlled by one modulator and the right wheels were controlled by the fourth modulator

(See Figure 2.1).

The ABS controller, while in operation, continuously monitors the wheel speeds using the wheel speed sensors. During braking the controller is activated. It compares the individual wheel speeds with the actual vehicle speed. If the slip ratio in one or more wheels exceeds certain threshold values, then it automatically modulates the pressure in the brake chamber of those wheels. The ABS prevents the wheels from locking by making them decelerate at the same rate as the vehicle, before applying brakes again.

The Fruehauf trailer has a 4s/4m ABS. The ABS controller used by Shurtz [10] for his trailer model was a 4s/4m system too. So this model was directly used for the

Fruehauf trailer model. The trailer ABS has an option to select the configuration of the

ABS. When initiating the simulation, the initiation file for the model is programmed to ask the user to input the desired configuration for the trailer ABS. Based on the user input, the ABS configuration is fixed for the trailer for that particular run.

3.3.5 External Engine and Torque Converter Models

TruckSim has an example external engine and torque converter model in

Simulink. The external engine model receives the throttle as input from TruckSim and computes the engine torque. The engine torque is given as input to the torque converter model which calculates the torque converter output shaft torque. This torque converter output shaft toque is given as input to TruckSim.

The throttle from TruckSim is given to the engine model through the ESC system model. This means that ESC system can intervene and control the throttle input given to the engine. This feature is explained in more detail in Section 4.3.3.

3.3.5.1 Engine Retarder

The Volvo tractor has an engine retarder which brakes the vehicle when the throttle input in cut. There are three types of engine retarders in use: Compression brake,

Hydraulic retarder and Jacobs engine brake. When the ESC system activates, it first cuts the engine throttle input before braking the wheels. This cause the engine retarder to brake the vehicle. The engine retarder generates a retardation torque which slows down the vehicle.

The vehicle was tested to study the performance of the engine retarder at VRTC.

The results were used to create a simple model of the retarder to be used by the ESC system model. The engine retarder in the truck has the option to be set at three levels:

OFF, Low and High. The tests were conducted at 55 mph. The vehicle was made to coast once it reached 55 mph with no retarder and with the retarder set at low and high modes.

The deceleration effects caused by the retarder were studied.

Figure 3.1 shows the deceleration in the speeds caused by engine braking during the tests. Data from all the three tests conducted at different levels of retarder setting are shown in the plot.

55 No Retarder

Speed (mph) 40

Retarder: LO

Retarder: HI 30

25 5 10 15 20 25 30 Time (sec)

Figure 3.1: Reduction in Truck Speed from 55 mph at different Retarder Levels [17]

The engine retarder was modeled in VRTC [17] as an equation given below:

TaaRe=−1ω − 0 (3.1)

Where:

TR : Retarder torque

ωe : Engine speed

aa10, : Retarder parameters (positive values)

The coefficients for this equation were obtained by means of trial and error. The coefficients for the various retarder levels were determined to be the following:

No retarder: a1 = 0, a0 = 0

Low Retarder setting: a1 = 1.1, a0 = 500

High retarder setting: a1 = 2.1, a0 = 1150

The retarder torque is added to the engine output torque which is used to calculate the torque converter output shaft torque.

CHAPTER 4

ELECTRONIC STABILITY CONTROL SYSTEM MODEL

4.1 Overview

The 2006 Volvo tractor has an ESC system by Bendix Commercial Vehicle

Systems LLC called the Bendix Electronic Stability Program (ESP). It includes both RSC and YSC features. As the focus of this project is to develop a model of an ESC system with RSC feature only, comprehensive study and analysis of data were carried out only for those tests that had just the RSC activation. The model was developed on a Simulink platform making it suitable for integration with any YSC model that could be developed in the future.

The ESC system model was developed in the same Simulink file that has the ABS and the braking systems model and it runs in conjunction with them. An ‘m-file’ was developed in MATLAB during previous research works [6, 8, 10] for initializing the

ABS system. This initiation file was suitably modified such that it initiates the ESC system too. The initiation scripts were designed to give the user the choice to run the

model either with the ESC system switched ON or OFF for the tractor, thus providing the user with the capability to simulate runs with both ESC ON and OFF.

4.2 ESC Working Principle

When a vehicle is turning, the lateral acceleration of the vehicle tends to create a force at its center of gravity which can cause the vehicle to roll over. This tendency to roll over is limited by the friction between the tires and the road surface. During high speed turning or sharp maneuvers, the lateral acceleration will create high forces at the vehicle

C.G. When these lateral forces overcome the balancing effect of the tire-road friction forces, the wheels on the inside of the vehicle will tend to lift off the ground, which might eventually lead to a rollover [3].

The commercial RSC system has sensors that measure variables such as lateral acceleration, steering angle, brake pressures and the wheel speeds. When the RSC system senses that the lateral acceleration value has a reached a critical limit that could potentially cause a rollover, it gets activated. Then it cuts the engine throttle and appropriately applies braking on the wheels of the vehicle to slow it down, thereby preventing rollover.

The roll stability of a vehicle is influenced primarily by the vehicle load and its

C.G. height. For the same weight, an increase in the C.G. height would result in the decrease of the vehicle’s roll stability.

4.3 Development of ESC System Model

The ESC system was developed based on functional properties of the actual system rather than complex mathematical models. A schematic of the ESC model is shown below. The commercially available ESC systems have their own set of sensors which monitor the vehicle and then act based on the measurements. This principle of operation is used in the ESC model too. The ESC model receives inputs from TruckSim which include the tractor lateral acceleration, wheel speeds, brake treadle pressure, and steering angle. Based on the inputs, it exercises control over the vehicle brakes as well as the engine throttle control as shown in Figure 4.1.

Lateral acceleration, Wheel Speeds

Steering Angle, Brake Pressures,

Input Throttle TruckSim ESC Model model (RSC)

External Engine & Torque TC Output ESC Controlled Shaft Converter model throttle Input Torque

Braking System & ABS model

Figure 4.1 – Schematic of the ESC Model

The developed RSC system has different modules. The development and the functioning of these modules are elaborated in the following sections.

4.3.1 Threshold Selector

Every vehicle has a rollover threshold which is a critical value of its lateral acceleration. If the instantaneous lateral acceleration value equals the rollover threshold, the vehicle becomes highly prone to rollover. The commercial ESC systems monitor the lateral acceleration of the vehicle and once it reaches the critical threshold value, it cuts the engine throttle and depending on the severity of the maneuver, initiates braking to slow down the vehicle [3]. The rollover threshold is highly influenced by the gross vehicle weight as well as the C.G. height of the vehicle. Thus different configurations have different rollover threshold values.

The data from the test track showed that the ESC activates at different lateral acceleration values for different truck weights. For the LLVW condition (17,364 kg) ESC activation was approximately at 0.62 g and for the GVWR condition (36,287 kg) the ESC activation was at around 0.30 g. This indicates the fact that the commercial system estimates the rollover threshold for the vehicle based on its weight. The Bendix ESP has a set of load sensors. Though a complete working detail of the sensors are not known, it is guessed that the system measures the pressure in the air bags of the truck’s air suspension system. Based on the air bag pressures, it computes the vehicle weight and then estimates its rollover threshold. The threshold selector module simulates this method for computing the critical threshold for the vehicle.

The loading configuration and the total weight of the truck can be varied for every test. The load may have a high or low C.G. height. This necessitates the ESC system model to compute a threshold value for the current run so that it can appropriately intervene in the case of a potential rollover incident. The ESC system model estimates the critical threshold purely based on the truck weight. This estimation of the critical threshold value for the truck is achieved by the threshold selector module. The module receives the vertical tire forces at all 10 wheel locations from TruckSim from which the total truck weight is calculated. Based on the calculated truck weight, the module estimates a critical threshold value for the truck.

To estimate the critical threshold based on the truck weight, a relationship between the truck weight and its rollover threshold was developed. Literature studies were carried out to understand the correlation between truck weight and its effect on the rollover threshold. UMTRI had estimated the influence of the geometric properties of the truck loading on its rollover threshold [13]. Baker et al had theoretically fixed the rollover thresholds for a truck for a range of weights [14]. The results from these two reports along with the observed ESC activation from the test data were used to plot a graph between the truck weight and its rollover threshold (Figure 4.2). The points were fitted onto a quadratic curve in Matlab and an equation for this curve was obtained:

3.4 − 210 −5 xexey +×−×= 3.13.4 (4.1)

y – Rollover threshold, in g

x – Truck weight, in kg

Truck Weight vs Rollover Threshold 0.9 Data Quadratic Fit 0.8

0.7

0.6

0.5

y = 6.1e-010*x2 - 5.3e-005*x + 1.4 Rollover Threshold(g's)

0.4

0.3

0.2 1.5 2 2.5 3 3.5 4 Truck Weight (kg) 4 x 10

Figure 4.2 – Relationship between Truck Weight and Rollover Threshold

The above relationship is valid only for truck weights ranging from 17,364 kg to

36,500 kg. Simulink has a function block that calculates the output as a function of the input. The function is an equation that can be defined within the block. The quadratic equation that gives the relation between truck weight and the critical threshold was defined within this function block. Thus the input to the block is the estimated truck weight and the output from it is the estimated critical threshold for the truck.

When the maneuver begins, the initial numerical dynamics of the truck simulation causes errors in the estimated truck weight. To avoid this, the simulation is begun at -3.6 seconds and steering input is maintained at zero degrees until one second into the

simulation. Thus a total of 4.6 seconds is given for the truck to attain stability. Once one second elapses into the simulation, the module is designed to freeze the estimated critical threshold value. The value which is the output at one second into the simulation would be taken as the critical threshold of the truck for that particular run. The threshold selector module is shown in Figure 4.3.

Threshold Selector selects the critical Lateral Acceleration threshold values

== -3.6 Clock Compare Load Condition Truck_load To Constant Wheel f orces Truck Load: 1 if LL, 0 if loaded Truck Weight wheel_forces Tire force sensor f(u) roll_thr

Critical Threshold Critical Threshold Equation to Workspace

roll_thr Critical Threshold 1 Critical threshold

Figure 4.3 – Threshold Selector Module

4.3.2 ESC Activation Module

This module activates the ESC system if the instantaneous tractor lateral acceleration equals or exceeds the estimated critical threshold value. In addition to comparing the actual lateral acceleration values with the threshold value, this module also predicts the lateral acceleration of the tractor in advance and compares it with the threshold value. By predicting the tractor’s lateral acceleration in advance, this module gives the ESC system the capability to intervene before the truck reaches a critical

condition such as wheel lift during the maneuver. This methodology is used by the commercial system [3] and hence is employed in the ESC system model too.

4.3.2.1 Predicting Tractor Lateral Acceleration

The Bendix ESP system predicts the lateral acceleration in advance which allows the system to respond earlier to a critical situation [3]. The system measures the steering angle of the tractor and uses it to estimate the lateral acceleration that would result as a result of the maneuver, even before the tractor attains those values. This enables the ESC system to react sooner than it would otherwise by just using the values from the lateral acceleration sensors.

For this purpose, the steady state lateral acceleration gain for the tractor was calculated from the test data (Section 2.3.6). As the tests were carried out at different speeds, the process of calculating the gain was repeated for all those different tests. The steady state lateral acceleration gain values were then plotted against the speed of the respective tests. The points were fitted to a line as shown in Figure 4.4 and the slope of this line was computed.

The real time steering angle and truck speed values are given as inputs to this module. The road wheel angle (δ ) is obtained by dividing the steering angle by the steering ratio. The tractor lateral acceleration is then predicted by multiplying this slope value with the instantaneous speed of the tractor and the road wheel angle as shown in equation 4.2.

y = SlopeA × Speed ×δ (4.2)

Figure 4.4 – Steady State Lateral Acceleration Gain vs. Truck Speed

4.3.2.2 ESC Model Activation

The module compares the predicted and the actual lateral accelerations with the critical threshold value. If either the predicted or the actual lateral accelerations equals or exceeds the critical threshold, the ESC activation module generates an activation signal of value 1; else it outputs a value of 0.

Figure 4.5 shows the ESC activation module. The module was designed in such a way that it provides the user with the option of running the simulation with either the

ESC system turned ON or OFF. When the simulation is initiated, the initiation scripts were designed to query the user to input ‘1’ if the ESC system is to be turned on or ‘0’ if

the run is to be made with the ESC turned off. This value of 1 or 0 which the user inputs is stored as a constant called ESCONOFF in the Matlab working directory. This

ESCONOFF constant controls the final output of the module. Even if the ESC activation module generates an activation value of 1 as the lateral acceleration crosses the threshold, the value of 1 is given as a final output only if the constant ESCONOFF is equal to 1

(which implies that the user has opted to have the ESC system switched on for the run). If the user decides to switch off the ESC during the initiation, the value of the constant

ESCONOFF would be zero, and this would cause the ESC activation module to output a final activation signal of value zero, irrespective of whether the threshold value is exceeded or not.

ESC Activation subsystem provides the activation signals for the ESC system based on the estimated values and thresholds

Prediction of Lateral Acceleration from Steering angle

2 Steering angle Predicted Ay |u| >= Steering Angle Abs Lateral Acceleration Predictor Relational -C- Operator1 3 ESC ON/OFF Critical Threshold 0 Switch zero

Comparison of predicted and actual Lateral Acceleration values

OR 1 ESC Activation Logical Operator2

1 |u| 57 >=

Lateral Acc Abs1 Relational -C- Operator3 ESC ON/OFF1

Switch1 0

Zero

Figure 4.5 – ESC Activation Module 4.3.3 ESC Throttle Control

During the data analysis in Sec. 2.3.4, it was observed that the ESC system cut the engine throttle upon its activation. This ensures that even if the driver is pressing the accelerator pedal, the power is not transmitted to the wheels during ESC induced braking.

To include this feature in the ESC system model, an external engine and torque converter model was added to the Simulink interface.

In reality, the torque demands are measured by the amount the accelerator pedal is depressed by the driver and the engine supplies the necessary torque based on the driver demand. The engine model is integrated with the truck model to simulate this feature.

The engine model calculates the output engine torque based on the input throttle it receives from TruckSim. TruckSim generates the throttle input based on the target speed.

The TruckSim generated throttle value ranging from 0 to 1, with the extreme values representing zero throttle and full throttle respectively. The engine torque generated by engine is used by the torque converter model to calculate the torque converter’s output shaft torque. This calculated torque value is given as input to TruckSim.

The ESC model was designed to have control over the input throttle given to the engine model. The ESC throttle control module receives the activation signal from the

ESC activation module. If the activation signal equals 1, then the ESC throttle control module is designed to cut the engine throttle and output a value of zero for the engine throttle. This value of zero is maintained as long as the ESC system remains activated.

Once the ESC system gets deactivated and the activation signal falls to zero, the ESC throttle control module outputs the TruckSim throttle value directly, thus enabling

TruckSim to have direct control over the engine torque generation. The following figure shows the ESC throttle control module.

This subsystem cuts the throttle on ESC Activation

0 Zero Throttle

ESC_activation Convert 1 mod_thrt To prevent Paramater Precision loss 1 thrt Switch

Figure 4.6 – ESC Throttle Control

4.3.4 Slip Ratio Calculator

This module computes the slip ratios of the wheels of the steer axle and the trailing drive axle. The ABS in the truck has sensors that measure the speeds of the wheels of the steer and the trailing drive axles as well as the longitudinal vehicle velocity.

These values are used to calculate the slip ratios of the wheels. The slip ratio calculator simulates this functioning. The following equation is used by the module to measure the slip ratio:

_ − vspdWh .RS = (4.3) v

Where S.R. = Slip ratio

Wh_spd = Wheel speeds, in kph

v = Truck velocity, in kph

4.3.5 Steer Axle Braking

Braking the steer axles provide the ESC system with additional capability for slowing down the vehicle. During braking there tends to be a shift in the vehicle weight toward its front [15]. By braking the steer axle, additional braking power can be obtained compared to just braking the drive axles [3]. Hence, a module was developed specifically to proportion brake pressure to the brake chambers of the steer axle. During a maneuver in which the vehicle is turning, it is possible that wheels on the inner side of the drive axles may lift off the ground. Under such conditions if braking is initiated, only the wheels that are not lifted off the ground provide the braking for the tractor, thereby reducing the total braking power. In such cases, steer axle braking would prove to be advantageous.

4.3.5.1 Brake Severity Estimator

The braking modules for both the steer and the drive axles involve the calculation of a brake severity number (Brs). This brake severity number is calculated based on the instantaneous lateral acceleration value and is then used to calculate a brake pressure value for the brake chambers of the steer and drive axles. An equation was developed to calculate the brake severity number from the lateral acceleration value.

The test data was studied to understand the braking of the wheels by the ESC system. As the maneuver becomes tighter, the lateral acceleration value increases and consequently the ESC system increases the brake pressures. The relation between the between the braking pressure and lateral acceleration was studied. The information

collected information was used to develop a quadratic equation which gives the brake severity number from the measured lateral acceleration.

Brs =Δ+Δ+372 5.5 0.069 (4.4)

Where Δ = Measured lateral acceleration (Ay) – Critical threshold value

1 Data Quadratic Fit 0.9

0.8

0.7

0.6

0.5

0.4 Brake Severity Number(Brs) 0.3

0.2

0.1

0 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 Difference between Lateral Acceleration and Critical threshold (g)

Figure 4.7 – Brake Severity Number as a function of the difference between Lateral Acceleration and Critical Threshold

The brake severity estimator finds out the difference between the instantaneous lateral acceleration value and the critical threshold value. This difference is fed as an input to the developed equation which outputs the brake severity number (Brs). The brake severity number ranges from 0 to 10. The brake pressure value is obtained by multiplying

Br with 11. If Br equals 10, which is its maximum possible value, then the output pressure is 110 psi, which is the maximum possible pressure in the brake chambers.

Likewise, for Br equal to 0, the output pressure is 0 psi. The brake severity number is rounded off to the nearest integer. Thus for every increase in Br by 1, the pressure increases by 11 psi.

4.3.5.2 Slip Ratio Sensor and Pressure Reducer

This subsystem monitors the slip ratio of the inner wheel of the steer axle. If the slip ratio exceeds a fixed value, then it drops the pressure supply to the steer axle brake chambers by 10 psi.

4.3.6 Differential Drive Axle Braking

Both the leading and the trailing drive axles are braked by this module. This module can brake the two sides of the drive axles differentially. If the wheels on one side of the tractor lift off during a turn, then the module would increase the braking pressure on the outer wheels which are still in contact with the ground and reduce the pressure in

the inner wheels which have lifted off. This ensures that the braking power is properly distributed between the wheels of the drive axles, so as to utilize the available braking power to the maximum.

As the ESC system is built on the ABS platform, it uses the same set of modulators for controlling the brake pressures in the wheels of the drive axles. This implies that the system can have two controls for the four wheels of the drive axle; one for the left wheels and another for the right wheels of the drive axles.

4.3.6.1 Brake Severity Estimator

This module also has a subsystem for estimating the brake severity number based on the lateral acceleration. This brake severity number is then used by the module to compute the required brake pressure for the wheels.

4.3.6.2 Pressure Modulator

The module has a subsystem called the pressure modulator which appropriately assigns brake pressures to the inner and outer wheels depending upon the instantaneous slip ratios of the inner wheels. Based on the steering angle, the module determines the direction of turn of the vehicle. By finding out the direction of turn, the inner and the outer wheels are determined for the maneuver in progress. The module then monitors the slip ratio of the inner wheel of the trailing drive axle, the values for which are obtained from the slip ratio calculator.

During a sharp turn, there will be a lateral weight shift towards the outer wheels.

This results in reduced contact between the inner wheels and the ground. If the slip ratios of the inner wheels of the drive axles reach a significant value, then on applying high brake pressure, the wheels will tend to lock and there is no further use in braking the inner wheels as they will not provide braking for the truck. To prevent the wheels from locking and fully utilize the available braking power, the module monitors the slip ratio of the inner wheel of the trailing drive axle. When the value reaches a prefixed limit, it dumps the pressure being given to the inner wheels to a value of 103 kPa (15 psi).

Every brake chamber has its own pop-out pressure (or push-out pressure), which is the minimum pressure required to move the brake pad against the spring forces. This value of 103kPa is slightly greater than the pop-out pressure for the brake chamber, which has been estimated by Dunn [6] to be 48 kPa (7 psi). At pressure levels below 48 kPa, the brake chamber movement is prevented by static friction (stiction). When the inner wheels regain sufficient contact with the ground, the ESC system would increase the pressure in the inner wheels to extract braking effort from the inner wheels too. Under such circumstances, the pressure in the brake chambers of the inner wheels would already be at a value oh 103 kPa, which would eliminate the time required by the brake chamber to overcome the static friction and to begin movement. Thus, maintaining a pressure value above the pop-out pressure results in quicker response of the inner wheels to braking requirements.

4.3.6.3 Inner/Outer Wheels Braking Distributor

This module assigns the appropriate braking pressure calculated by the previous module to the left and the right wheels of the tractor, depending on which wheels constitute the inner and outer ones. The brake pressure calculated by the previous module is given as input to this module in the form of a vector which has two channels; one each for the inner and the outer wheels. Based on the vehicle’s direction of turn, the module establishes the inner and the outer wheels and then appropriately supplies the braking pressures to their corresponding wheels.

Brake Pow er Calculator calculates the required brake pressure on each w heels depending on the load of the vehicle

lateral_acc_tractor |u| g's abov e CT DA Pr DA Braking roll_thr Abs2 DA wheel Press Add DA wheel Pr DA Brake Pressure 1 SR of inner wheel Drive Axle Braking Brake Severity Estimator Inner Wheels' Pressure Modulator In/Out Wheels Braking Distributor1

1 SR L3 SR L3 SR of inner wheel of Trailing D/A

66 2 SR R3

SR R3 Drive Axle Inner Wheel SR

Figure 4.8 – Differential Drive Axle Braking

4.3.7 Trailer Axle Braking

The braking signal for the semitrailer is also provided by the tractor. Glad-hand is a coupling that connects the tractor with the semitrailer. The braking signals generated by the tractor are sent to the semitrailer through this glad-hand connector.

The test data was studied to understand the braking of the trailer axle brake chambers. It was observed that the ESC system does not maintain the pressure continuously in the trailer brake chambers as it does for the tractor. At periodic intervals, it dumps the pressure in the chambers to almost zero and then starts building the pressure.

The reason for this is that the tractor ESC system has no knowledge of the presence or the absence of a trailer ABS. It is designed to assume that the trailer has no ABS and hence simulates the functioning of an ABS by automatically dumping the pressure periodically, so as to prevent wheel lock-up from occurring in the trailer wheels.

The pressure estimated for the drive axles are the reference value used for braking the trailer axles. Only the pressure of the outer wheels is used for braking the trailer axles, as the inner wheels pressure may be dumped by the ESC system based on wheel lift. The module has a trailer ABS simulator. This subsystem pulses the reference signal given to the trailer’s brake chambers. The pressure is allowed to build for a period of 0.9 seconds and then dumped to zero for the next 0.6 seconds. This cycle is repeated by the ABS simulator as long as the ESC system remains activated. The duration of the pulses were arrived at after careful study of the trailer brake chamber pressures from the test data. The module determines the time at which the ESC system activates. It subtracts this value from the current simulation time to create a separate reference time value which is used

to generate a pulse. This pulse signal begins from the moment of ESC activation and has a value of 1 for 0.9 seconds and 0 for the next 0.6 seconds. This cycle is repeated by the pulse generator. The generated pulse is used to modulate the pressure given to the brake chambers of the trailer axle. The trailer braking module and the ABS simulator are shown in Figures 4.8 and 4.9 respectively.

This module pulses the brake pressure signal to the trailer axle to simulate the w orking of a trailer ABS

trl_brk_press Brake Pr f rom DA

Trailer Axle Braking

ESC ACT TIME

Trailer ABS Simulator 1 1 ESC Activation Trailer Axle Braking

treadle_pressure_reference

ESC Pressure/Treadle Pressure swi t ch

ESC_act_time

ESC Actv Time 1 z ESC_act_time Add Unit Delay Out1 Time at which ESC Activates Enabled Subsystem This unit finds out the time at which ESC activates

Figure 4.9 – Trailer Axle Braking

The Brake Pressure signal is given to trailer axle treadle according to the defined pulse. The pulse is begun from the time ESC activates

Enable

1 Brake Pr from DA

t 1 Clock Trailer Axle Braking 2 Add Pulse ESC ACT TIME Generator 0

Zero ESC Pressure/Treadle Pressure swi tch

Figure 4.10 – Trailer ABS Simulator

4.3.8 Brake Demand Sensor

When the ESC system is active and is braking the vehicle, it takes over the brake control from the driver. Under such conditions if the driver suddenly senses an obstacle and applies braking to slow down the vehicle faster than the ESC system is decelerating it, the ESC system has to sense the increased braking demand from the driver and provide the necessary braking power. The commercial ESC system utilizes a brake demand sensor which compares the driver’s braking demand and the ESC system’s braking and appropriately supplements the driver’s demand [3].

This feature was incorporated in the ESC system model. When the ESC system gets activated, the brake demand sensor module continuously compares the driver’s braking demand and the ESC system braking. If the module senses that the driver’s demand is higher than the ESC system braking, then it allows for overriding the ESC system and supplies the driver’s brake demand to the brake chambers.

4.4 Functioning

The inputs to the TruckSim model are the friction coefficients at the wheels which are estimated by the randomly varied friction generator, brake torque values for the brake chambers of all wheel, and the torque converter output shaft torque. The maneuver was defined for each run by specifying the steering angle as a function of time in the

TruckSim control panel. TruckSim provides the user with the option of either defining an initial speed or a constant target speed for the run.

Many variables were obtained as output from TruckSim during the run. Some of these variables are given as inputs to the ABS and the ESC system. The ESC system has been designed such that the ABS has priority over the ESC system. If the ESC system is applying brakes on the truck wheels and it leads to a wheel lock-up, then the ABS can interfere and dump the pressure in the brake chambers of wheels that lock.

On initiation, the Matlab scripts prompt the user to first define a nominal coefficient of friction, the ABS configuration and tractor ESC status. The nominal friction coefficient is used by the randomly varied friction generator to generate varying frictions at the wheels of the truck. The next prompt gives the user the choice to select the default ABS configuration of the 2006 Volvo tractor or to configure it. Once the ABS configurations are specified, the user is asked to enter whether the tractor ESC should be turned ON or OFF for the run. Then the initiation file defines all the parameters required for the braking system and the ABS models.

After initiation, the ESC system estimates the truck weight and then computes the rollover threshold. The ESC system continuously compares the predicted lateral acceleration with the threshold value. If the predicted value equals or exceeds the threshold then the ESC system activates. On activation, the ESC system immediately cuts the engine throttle. This is done to ensure that the engine is not accelerating the vehicle at the same time the ESC system is trying to decelerate it. Then depending upon the severity of the maneuver, the ESC applies braking on the wheels of the tractor and the trailer. The complete RSC system model is shown in Figure 4.10. The Simulink environment with the RSC system is shown in Appendix B.

Figure 4.11: Roll Stability Control System Model

CHAPTER 5

SIMULATION RESULTS

5.1 Overview

The behavior of the Volvo truck was simulated by running the model using different maneuvers. Simulations were performed with the tractor ESC model switched

ON as well as OFF so as to study the effectiveness of the ESC system model in preventing rollovers. The simulation runs were made on a surface with a nominal coefficient friction of 0.9 which is approximately the surface friction of the track on which the truck was tested.

The TruckSim control panel provides the user with the options to directly define the inputs for the model such as the steering angle, vehicle speed and braking. The simulation maneuvers were specified by defining the steering angle as a function of time.

This was achieved by selecting the open loop control for the steering input. The steering angle used for each simulation was designed such that the truck model attains the same

levels of lateral acceleration as the actual truck during testing. There are no driver initiated braking for any of the simulations and so the brake control was not specified.

Simulations were performed either at a constant speed or by dropping the throttle at the commencement of the maneuver. These two conditions represent actual test conditions. Tests involving the SIS maneuvers were conducted at constant speeds. In these tests, the driver tried to maintain the vehicle speed at a constant target speed throughout the maneuver. The J-Turn maneuvers were performed with a dropped throttle

(clutch-in). Once the vehicle attained a target speed, the driver stops giving throttle input

(by releasing the accelerator pedal) thereby allowing the vehicle to coast through the rest of the maneuver.

The tests were repeated for the truck under all three loading conditions (LLVW,

GVWR with low C.G. height and GVWR with high C.G. height) at different speeds.

5.2 GVWR with High C.G. Height

This section describes the simulation of the truck in GVWR configuration with high C.G. height. From the test data it was apparent that the vehicle in this load configuration was more prone to wheel lifts compared to the other two configurations and hence most susceptible to rollovers. During severe maneuvers, wheel lifts were observed in both the tractor and the trailer. In a few extreme cases the outriggers came into contact with the ground.

To perform the simulations, the maneuver as well as the truck speed had to be defined. The maneuver executed by the model in simulation was defined such that it caused the tractor to attain the same lateral acceleration as the tractor in the tests. The truck was also run at the same speed as the vehicle in the tests from which data for the maneuver are taken. By simulating the truck such that the same lateral acceleration levels are attained, the responses of the truck model and the RSC system model in simulations could be compared with those of the actual truck and its ESC system.

Tests conducted using the J-Turn maneuver were considered for the simulations in this section. Some of these tests had the steering angle input given manually by the driver, while others received it from the steering controller. The data from J-turn tests conducted with steering controller has been used in this section to perform simulations.

5.2.1 ESC OFF

5.2.1.1 Test Data

The data for the maneuver and truck speed were obtained from Test 397. The test was conducted with the same load configuration and with the tractor and trailer ESC systems OFF. The steering input and the targeted speed are the two required information from the test needed to simulate the run. The data from the test run and the simulation results are plotted in this section to explain the maneuver and the truck’s response.

The targeted speed of the vehicle in the test was 32 mph (52 kph). Once the target speed was attained, the driver took his foot off the accelerator pedal to cut the throttle

input to the engine and initiated the maneuver by triggering the steering controller. The steering controllers gave a steering input at the rate of 48.57 deg/s for a period of 3.5 seconds. The instantaneous value of the steering input at the end of 3.5 seconds was -194 degrees and this value was maintained for about 9 seconds after which the input was reduced to zero. A constant steer angle of -28 degrees was given before the commencement of the maneuver.

Figure 5.1 shows the steering angle, vehicle speed and tractor lateral acceleration as functions of time. During the course of the maneuver, the speed drops by 12 kph

(7.5 mph) while the tractor lateral acceleration attains a peak value of -0.47 g after which it begins to fall due to the continuous drop in the speed of the vehicle. When the tractor lateral acceleration attained a value of -0.45 g, the drive axles’ wheels lifted off the ground and the outrigger hit the ground at when the lateral acceleration reached -0.47 g.

The outriggers, which were fitted to the trailer, prevented a potential rollover incident.

Steering Angle 0

-50

-100

-150 Steering Angle (deg) -200 15 20 25 30 Time (sec) Vehicle Speed 55

35 Vehicle Speed (kph) 30 15 20 25 30 Time (sec) Tractor Lateral Acceleration 0

-0.1

-0.2

-0.3

-0.4 Lateral Acceleration (g) -0.5 15 20 25 30 Time (sec)

Figure 5.1: Steering Angle, Truck Speed and Tractor Lateral Acceleration as Functions of Time from Test 397

Roll Angle 10 Trailer Roll Angle

5 Angle (deg)

0 6 8 10 12 14 16 18 20 22 24 Time (sec) Truck Articulation Angle 0 Articulation Angle -5

-10

Angle (deg) -15

-20 6 8 10 12 14 16 18 20 22 24 Time (sec) Trailer Yaw Rate 0 Tractor Yaw Rate -5 Trailer Yaw Rate

-10

-15 Yaw Rate (deg/s) Rate Yaw -20 6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 5.2: Roll Angle, Truck Articulation Angle and Yaw Rates as Functions of Time from Test 397

The above figure shows the plots of the roll angle, truck articulation angle and the yaw rates from the test data. The roll angles for the tractor were not available and so only the trailer roll angles are shown.

5.2.1.2 Simulation

The test maneuver was simulated by defining the steering input in the TruckSim control panel such that the tractor attained the same lateral acceleration levels as in the test. From 0 to 1 second of the simulation, the steering was maintained at -28 degrees.

After one second, the steering angle was increased to 235 degrees in a period of 3.5 seconds and was then maintained at that value. Negative steering angle indicates that the direction of turn is towards the right side of the vehicle.

To simulate the dropping of throttle by the driver at the commencement of the maneuver, the vehicle target speed was defined as a function of time in the TruckSim control panel. A constant value of 52 kph was maintained from the beginning of the simulation until one second into the simulation. At the end of one second, the target speed was dropped to zero. This causes the TruckSim model to give zero throttle input to the engine model exactly at the commencement of the maneuver, thereby simulating the actual sequence of events.

Steering Angle 0

-50

-100

-150

Steering Angle (deg) -200

-250 0 1 2 3 4 5 6 7 8 9 Time (sec)

Tractor Speed 55

40 Speed (kph)

30 0 1 2 3 4 5 6 7 8 9 Time (sec)

Figure 5.3: Steering Angle and Truck Speed from Simulation – GVWR High C.G. (ESC OFF)

The above figure shows the input steering and the truck speed as functions of time. The truck speed dropped by about 12.4 mph (20 kph) in the simulation. The tractor lateral acceleration is shown as a function of time in Figure 5.4. The tractor attained a peak lateral acceleration value of -0.44 g at 4.9 seconds during the simulation and the trailer axles’ wheels lifted off at this moment. At 5.7 seconds, the drive axles’ wheels lifted off leading to a rollover of the truck. The value of the tractor lateral acceleration at which drive axles’ wheels lift off was observed to be almost the same as the value from the test data.

0 Simulation A y A Predicted from Hand wheel angle y -0.05 RSC Brake RSC Throttle -0.1 Estimated Critical Threshold Level

-0.15

-0.2

-0.25

-0.3 Lateral acceleration (g)

-0.35

-0.4

-0.45 Drive Axle wheels lift off leading to truck rollover -0.5 0 1 2 3 4 5 6 7 8 9 Time (sec)

Figure 5.4: Tractor Lateral Acceleration from Simulation - GVWR High C.G. (ESC OFF)

The roll angles, truck articulation angle and the yaw rate are shown in

Figure 5.5. The TruckSim model becomes invalid when the vehicle rolls over and it results in incorrect responses. Hence, the lateral accelerations of the tractor shown in

Figure 5.4 are not valid after about 6 seconds of the simulation.

Roll angle vs Time

0 Tractor Roll Angle Trailer Roll Angle

-5

-10 Roll angle (deg)

-15 0 1 2 3 4 5 6 7 Time (sec) Articulation angle vs Time 20

Angle (deg) 5

0 0 1 2 3 4 5 6 7 Time (sec) Yaw rate vs Time 0 Tractor Yaw Rate -5 Tractor Yaw Rate

-10

-15 Yaw rate (deg/s)

-20 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.5: Roll Angle, Truck Articulation Angle and Yaw Rates as Functions of Time from Simulation - GVWR High C.G. (ESC OFF)

5.2.2 ESC ON

5.2.2.1 Test Data

Simulation was next performed with the tractor ESC system ON. Test 400, which was conducted with the tractor ESC system ON and the trailer ESC system OFF, was used as the reference for this simulation. Figure 5.6 shows the steering angle, vehicle

speed and the corresponding tractor lateral acceleration from the test data. The maximum steering input was almost equal to the maximum steering input of Test 397. The RSC feature of the ESC system got activated during the test. The plots show only RSC brake signal. The reason for the absence of the RSC throttle control signal is due to the fact that the test was done with a dropped throttle (clutch-in) and hence there is no engine braking provided by the ESC system.

Steering Angle 0 Steer angle RSC Brake RSC Throttle -100

Steering Angle (deg) -200 10 12 14 16 18 20 22 Time (sec) Vehicle Speed

Vehicle Speed (kph) 30 10 12 14 16 18 20 22 Time (sec) Tractor Lateral Acceleration 0

-0.2

-0.4

Lateral Acceleration (g) Acceleration Lateral 10 12 14 16 18 20 22 Time (sec)

Figure 5.6: Steering Angle, Vehicle Speed and Tractor Lateral acceleration from Test Data – GVWR High C.G. (ESC ON)

An initial steering angle of -9 degrees was maintained and the target speed for the maneuver was 32 mph (52 kph). The steering input had a maximum value of -178 degrees. The ESC system activated during the maneuver and applied braking on the wheels. As a result of this, the vehicle speed dropped by about 9.6 mph (15.5 kph) to 22.8 mph (36.6 kph). The tractor attained a maximum lateral acceleration value of -0.41 g during the maneuver.

The ESC system applied braking on the steer axle and the drive axles as shown in Figure 5.7. The steer axle received a maximum braking pressure of 55 psi (376 kPa). The drive axle wheels were braked differentially with the outer wheels receiving a higher braking pressure than the inner wheels. The maximum pressure in the brake chambers of the outer and inner wheels were 75 psi (514 kPa) and 15 psi (103 kPa) respectively.

Steer Axle of Tractor 400 Inner Wheel 300 Outer Wheel RSC Brake 200 RSC Throttle 100 Pressure, kPa kPa Pressure, Brake chamber 0 11 12 13 14 15 16 17 18 19 20 21 Time (sec) Leading Drive Axle of Tractor

400

200 Pressure, kPa Pressure, Brake chamber 0 11 12 13 14 15 16 17 18 19 20 21 Time (sec) Trailing Drive Axle of Tractor

400

200 Pressure, kPa Pressure, Brake chamber 0 11 12 13 14 15 16 17 18 19 20 21 Time (sec)

Figure 5.7: Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - GVWR High C.G. (ESC ON)

The ESC system also applied braking on the trailer axle wheels. The maximum pressure in the trailer axle wheels’ brake chambers were 52 psi (360 kPa). The brake chamber pressures for the trailer axle are shown in Figure 5.8.

Leading Axle of Trailer 400 Inner Wheel Outer Wheel 300 RSC Brake RSC Throttle

200 Pressure, kPa kPa Pressure, Brake chamber 100

0 11 12 13 14 15 16 17 18 19 20 21 Time (sec)

Trailing Axle of Trailer 400

300

200 Pressure, kPa Brake chamber 100

0 11 12 13 14 15 16 17 18 19 20 21 Time (sec)

Figure 5.8: Brake Chamber Pressures of Trailer Axles from Test Data – GVWR High C.G. (ESC ON)

Roll Angle

6 Trailer Roll Angle RSC Throttle RSC Brake 4

Angle (deg) 2

0 6 8 10 12 14 16 18 20 22 24 Time (sec) Truck Articulation Angle 0 Articulation Angle RSC Throttle -5 RSC Brake

-10 Angle (deg) -15

6 8 10 12 14 16 18 20 22 24 Time (sec) Trailer Yaw Rate 0 Tractor Yaw Rate Trailer Yaw Rate -5 RSC Throttle RSC Brake -10 Yaw Rate (deg/s) -15

6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 5.9: Roll Angles, Truck Articulation Angles and Yaw Rates from Test Data – GVWR High C.G. (ESC ON)

The above figure shows the plots of roll angles, truck articulation angles and the yaw rates from the test data.

5.2.2.2 Simulation

The maneuver was simulated by using the same steering input as the simulation in the previous section. As the target speed is the same as in the previous run, speed profile created for the previous simulation was retained. Figure 5.10 shows the simulation

steering angle and vehicle speed as functions of time. The vehicle speed drops by 10.6 mph (17 kph) during the maneuver due to ESC initiated braking. Figure 5.11 shows the tractor lateral acceleration from simulation as a function of time. The tractor attained a peak lateral acceleration of -0.37 g during the maneuver.

Steering Angle 0 Steering Angle RSC Brake -50 RSC Throttle

-100

-150 Steering Angle (deg) -200

-250 0 2 4 6 8 10 12 14 Time

Tractor Speed 55

40 Speed(kph)

30 0 2 4 6 8 10 12 14 Time(sec)

Figure 5.10: Steering Angle and Truck Speed from Simulation – GVWR High C.G. (ESC ON)

0 Simulation A y A Predicted from Hand wheel angle y -0.05 RSC Brake RSC Throttle Estimated Critical Threshold Levels -0.1

-0.15

-0.2

-0.25 Lateral acceleration (g)

-0.3

-0.35

-0.4

0 2 4 6 8 10 12 14 Time (sec)

Figure 5.11: Tractor Lateral Acceleration from Simulation - GVWR High C.G. (ESC ON)

ESC Initiated braking for steer axle 800 Outer Wheels 600 Inner Wheels

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for drive axles 800

600

400

200 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for trailer axles 800

600

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.12: ESC System Model Initiated Braking from Simulation - GVWR High C.G. (ESC ON)

Figure 5.12 shows the braking initiated by the tractor ESC system model during the simulation. All the pressure signals shown were given to their corresponding brake chambers.

Steer Axle Brake Chamber Pressures 500 Outer Wheel 400 Inner Wheel 300 RSC Brake 200 RSC Throttle 100 Pressure (kPa) 0 0 2 4 6 8 10 12 14 Time (sec) Leading Drive Axle Brake Chamber Pressures 500 400 300 200 100 Pressure (kPa) 0 0 2 4 6 8 10 12 14 Time (sec) Trailing Drive Axle Brake Chamber Pressures 500 400 300 200 100 Pressure (kPa) Pressure 0 0 2 4 6 8 10 12 14 Time (sec)

Figure 5.13: Tractor Brake Chamber Pressures from Simulation - GVWR High C.G. (ESC ON)

The pressures in the brake chambers of the tractor are shown in Figure 5.13. The maximum brake pressure in the tractor steer axle was 62 psi (428 kPa). The drive axle wheels were braked differentially. The inner wheels received a maximum pressure of

15 psi (103 kPa) while the outer wheels received a maximum brake pressure of 61 psi

(417 kPa).

The trailer axles’ brake chamber pressures are shown in Figure 5.14. The tractor

ESC system does not brake the trailer wheels continuously like the tractor wheels. Instead it periodically dumps the pressure in the wheels to simulate the functioning of trailer

ABS. The outer wheels attained two peak values of 29 psi (198 kPa) and 43 psi (295 kPa)

respectively while the peaks in the brake chambers of the inner wheels were 29 psi

(198 kPa) and 26 psi (176 kPa).

Leading Trailer Axle Brake Chamber Pressures 300 Outer Wheel 250 Inner Wheel RSC Brake 200 RSC Throttle

150

100 Pressure (kPa)

0 0 2 4 6 8 10 12 14 Time (sec)

Trailing Trailer Axle Brake Chamber Pressures 300 Outer Wheel 250 Inner Wheel RSC Brake 200 RSC Engine

150

100 Pressure (kPa)

0 0 2 4 6 8 10 12 14 Time (sec)

Figure 5.14: Trailer Brake Chamber Pressures from Simulation - GVWR High C.G. (ESC ON)

The roll angles, truck articulation angles and the yaw rates of the truck are plotted in Figure 5.15.

Roll angle vs Time 2 Tractor Roll Angle 0 Trailer Roll Angle RSC Brake -2 RSC Throttle

Roll angle (deg) -4

0 2 4 6 8 10 12 14 Time (sec) Articulation angle vs Time

30 Articulation Angle RSC Brake 20 RSC Throttle

10 Angle (deg)

0 0 2 4 6 8 10 12 14 Time (sec) Yaw rate vs Time 0 Tractor Yaw Rate Trailer Yaw Rate -5 RSC Brake RSC Throttle -10 Yaw rate (deg/s) -15 0 2 4 6 8 10 12 14 Time (sec)

Figure 5.15: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR High C.G. (ESC ON)

5.3 GVWR with Low C.G. Height

This section describes the simulation of the truck in GVWR condition with a low

C.G. height. Though tests were conducted using both J-Turn and SIS maneuvers, only the

SIS maneuvers were performed using the steering controller. The simulations in this section were performed with SIS maneuvers which were modeled based on the test runs.

Test data was not available for the SIS maneuvers with the ESC system OFF. So the data

from tests conducted with the ESC system ON were used to build a maneuver for simulating runs with both ESC OFF and ON conditions.

5.3.1 Simulation with ESC OFF

Data from Test 936 which was conducted with tractor ESC system ON was used as the basis to model the maneuver for the simulation. The steering controller was set to give a steering input at the rate of 13.5 deg/s on being triggered.

The test was conducted at a constant speed with the driver trying to maintain a target speed throughout the maneuver. The target speed for the maneuver during this particular test was 35 mph (56.3 kph). Once the target speed was achieved, the driver triggered the steering controller which initiated the maneuver.

Once the maneuver was constructed in TruckSim control panel using the steering data, the simulation was performed. Figure 5.16 shows the steering angle and truck speed from the simulation. An initial steering angle of 7 degrees was given for a period of one second after which the steering input was increased at the rate of 13.5 deg/s. This steering rate was maintained for a period of about 11 seconds until the steering input reached 150 degrees after which it was decreased to zero. The tractor lateral acceleration, shown in

Figure 5.17, attained a peak value of -0.34 g.

The roll angles, truck articulation angles and the yaw rates of the tractor and the trailer are shown in Figure 5.18.

Steering Angle 0

-50

-100 Steering Angle (deg)

-150

0 2 4 6 8 10 12 14 Time

Tractor Speed 60

Speed (kph) 40

30 0 2 4 6 8 10 12 14 Time(sec)

Figure 5.16: Steering Angle and Truck Speed from Simulation - GVWR Low C.G. (ESC OFF)

0 Simulation A y A Predicted from Hand wheel angle y -0.05 Estimated Critical Threshold

-0.1

-0.15

-0.2 Lateral acceleration (g)

-0.25

-0.3

0 2 4 6 8 10 12 14 Time (sec)

Figure 5.17: Tractor Lateral Acceleration from Simulation – GVWR Low C.G. (ESC OFF)

Roll angle vs Time 0 Tractor Roll Angle -0.5 Trailer Roll Angle

-1

-1.5 Roll angle (deg) -2 0 2 4 6 8 10 12 14 Time (sec) Articulation angle vs Time 10

5 Angle (deg)

0 0 2 4 6 8 10 12 14 Time (sec) Yaw rate vs Time 0 Tractor Yaw Rate Tractor Yaw Rate -5

-10 Yaw rate (deg/s)

0 2 4 6 8 10 12 14 Time (sec)

Figure 5.18: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR Low C.G. (ESC OFF)

5.3.2 ESC ON

5.3.2.1 Test Data

The steering angle, truck speed and lateral acceleration obtained from the test data are shown in Figure 5.19. The RSC part of the ESC system got activated during the maneuver and applied brakes which caused the vehicle speed and hence the tractor lateral acceleration to decrease. This caused the truck speed to drop by 7.2 mph (11.6 kph). The tractor attained a maximum lateral acceleration of -0.32 g.

Steering Angle 0 Steer angle -50 RSC Brake RSC Throttle -100

-150

Steering Angle (deg) 8 10 12 14 16 18 20 22 Time (sec) Vehicle Speed 60

Vehicle Speed (kph) 0 8 10 12 14 16 18 20 22 Time (sec) Tractor Lateral Acceleration 0

-0.1

-0.2

-0.3

Lateral Acceleration (g) 8 10 12 14 16 18 20 22 Time (sec)

Figure 5.19: Steering Angle, Truck Speed and Tractor Lateral Acceleration from Test Data - GVWR Low C.G. (ESC ON)

The tractor brake chamber pressures are shown in Figure 5.20. The steer axle received a maximum pressure of 15 psi (103 kPa). The drive axles were braked differentially. The outer wheels received a maximum brake pressure of 27 psi (186 kPa) while the inner wheels received a maximum pressure of 15 psi (103 kPa). All the brake chambers of the trailer axles received a pressure of 27 psi and they are shown in Figure

5.21.

Tractor Steer Axle Brake Chamber Pressures 150 Inner Wheel Outer Wheel 100 RSC Brake RSC Throttle 50 Pressure, kPa Pressure, 0 10 12 14 16 18 20 22 Time (sec)

Tractor Leading Drive Axle Brake Chamber Pressures 200

150

100

50 Pressure, kPa Pressure, 0 10 12 14 16 18 20 22 Time (sec)

Tractor Trailing Drive Axle Brake Chamber Pressures 200

150

100

50 Pressure, kPa Pressure, 0 10 12 14 16 18 20 22 Time (sec)

Figure 5.20: Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - GVWR Low C.G. (ESC ON)

Brake Chamber Pressures in Trailer Axles 200 Inner Wheels Outer Wheels 180 RSC Brake RSC Throttle

160

140

120

100 Pressure, kPa Pressure, 80

0 8 10 12 14 16 18 20 22 Time (sec)

Figure 5.21: Brake Chamber Pressures of Trailer Axles from Test Data - GVWR Low C.G. (ESC ON)

The roll angles, truck articulation angles and the yaw rates are shown in Figure

5.22.

Roll Angle

Tractor Roll Angle 8 Trailer Roll Angle 6 RSC Throttle 4 RSC Brake

Angle (deg) 2

0 6 8 10 12 14 16 18 20 22 24 Time (sec) Truck Articulation Angle 0 Articulation Angle RSC Throttle -5 RSC Brake

-10 Angle (deg)

6 8 10 12 14 16 18 20 22 24 Time (sec) Trailer Yaw Rate 0 Tractor Yaw Rate Trailer Yaw Rate -5 RSC Throttle RSC Brake -10 Yaw Rate (deg/s)

6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 5.22: Roll Angles, Truck Articulation Angle and Yaw Rates from Test Data - GVWR Low C.G. (ESC ON)

5.3.2.2 Simulation

Simulation was performed with the tractor ESC system switched ON using the same maneuver and target speed. The RSC system got activated during the maneuver and brakes all wheels of the truck. The truck speed drops by 4.1 mph (6.6 kph) due to the braking as shown in Figure 5.23.

100

Steering Angle 0 Steering Angle RSC Brake RSC Throttle -50

-100 Steering Angle(deg)

-150

0 2 4 6 8 10 12 14 Time (sec)

Tractor Speed 60

Speed(kph) 40

30 0 2 4 6 8 10 12 14 Time (sec)

Figure 5.23: Steering Angle and Truck Speed from Simulation – GVWR Low C.G. (ESC ON)

101

0 Actual A y A Predicted from Hand wheel angle y -0.05 RSC Brake RSC Throttle Estimated Critical Threshold

-0.1

-0.15

-0.2 Lateral acceleration (g)

-0.25

-0.3

0 2 4 6 8 10 12 14 Time (sec)

Figure 5.24: Tractor Lateral Acceleration from Simulation - GVWR Low C.G. (ESC ON)

The tractor lateral acceleration attains a maximum value of -0.31 g. Figure 5.25 shows the RSC system initiated braking for all the wheels of the tractor and the trailer.

This is the actual braking command generated by the RSC system for each brake chamber. The maximum RSC system initiated pressures for the steer axle wheels’ brake chambers was 22 psi (150 kPa). The drive axles were braked differentially. The maximum initiated pressure for the outer and inner wheels were 22 psi (152 kPa) and 15 psi (103.4 kPa) respectively. The trailer axle wheels’ brake chambers received a maximum pressure value of 22 psi (152 kPa).

102

ESC Initiated braking for steer axle Outer Wheels 150 Inner Wheels 100

50 Pressure (kPa) Pressure 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec) ESC Initiated braking for drive axles

150

100

50 Pressure (kPa) 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec) ESC Initiated braking for trailer axles

150

100

50 Pressure (kPa) 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec)

Figure 5.25: RSC system initiated braking for Tractor and Trailer from Simulation - GVWR Low C.G. (ESC ON)

103

Steer Axle Brake Chamber Pressures 150 Outer Wheel Inner Wheel 100 RSC Brake RSC Throttle 50 Pressure (kPa) 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec) Leading Drive Axle Brake Chamber Pressures 150

100

50 Pressure (kPa) 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec) Trailing Drive Axle Brake Chamber Pressures 150

100

50 Pressure (kPa) 0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec)

Figure 5.26: Tractor Brake Chamber Pressures from Simulation - GVWR Low C.G. (ESC ON)

The tractor and trailer brake chamber pressures are shown in Figure 5.26 and 5.27 respectively. These are the actual pressures inside the brake chambers as computed by the models used for the simulation. The steer axle brake chamber attained a maximum pressure value of 20.4 psi (140 kPa). The outer and inner wheels of the drive axle wheels’ brake chambers attained maximum pressure values of about 20 psi (138 kPa) and 14.2 psi

(98 kPa). This difference between the control pressure signal created by the RSC system and the actual pressure in the brake chambers is due to the dynamics of the pneumatic medium as well as that of the brake chamber.

104

Leading Trailer Axle Brake Chamber Pressures 150 Outer Wheel Inner Wheel RSC Brake 100 RSC Throttle

50 Pressure (kPa) Pressure

0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec)

Trailing Trailer Axle Brake Chamber Pressures 150 Outer Wheel Inner Wheel RSC Brake 100 RSC Engine

50 Pressure (kPa) Pressure

0 9 9.5 10 10.5 11 11.5 12 12.5 13 13.5 14 Time (sec)

Figure 5.27: Trailer Brake Chamber Pressures from Simulation - GVWR Low C.G. (ESC ON)

The trailer axle brake chambers attained maximum pressures of 9.5 psi (65 kPa) and 19.3 psi (132.7 kPa) as shown in the figure above.

105

Roll angle vs Time 0

-0.5 Tractor Roll Angle -1 Trailer Roll Angle -1.5 RSC Throttle

Roll angle (deg) RSC Brake -2 0 2 4 6 8 10 12 14 Time (sec) Articulation angle vs Time 20 Articulation Angle 15 RSC Throttle RSC Brake 10

Angle (deg) 5

0 0 2 4 6 8 10 12 14 Time (sec) Yaw rate vs Time 0

-5 Tractor Yaw Rate Tractor Yaw Rate RSC Throttle -10

Yaw rate (deg/s) rate Yaw RSC Brake

0 2 4 6 8 10 12 14 Time (sec)

Figure 5.28: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR Low C.G. (ESC ON)

106

5.4 LLVW

This section describes the simulation of the truck in LLVW condition. The truck was tested in LLVW condition using both J-Turn and SIS maneuvers.

5.4.1 ESC OFF

5.4.1.1 Test Data

The data from Test 209 which was performed using a J-Turn maneuver is described in this section. The steering input was given manually by the driver.

Figure 5.29 shows the steering angle, vehicle speed and the tractor lateral acceleration from the test data. The vehicle speed dropped by about 13.7 mph (22 kph) during the test. The tractor attained a maximum lateral acceleration of 0.75 g. no wheel lifts were observed during the test. The roll angles and articulation angles were not available for this test. The yaw rates of the tractor and the trailer are shown in Figure

5.30.

107

Steering Angle 300

200

100

Steering Angle (deg) -100 8 9 10 11 12 13 14 15 16 Time (sec) Vehicle Speed 70

55 Speed (kph)

50 8 9 10 11 12 13 14 15 16 Time (sec) Lateral acceleration vs Time 0.8

0.6

0.4

0.2

0 Lateral Acceleration (g) 8 9 10 11 12 13 14 15 16 Time (sec)

Figure 5.29: Steering Angle, Vehicle Speed and Tractor Lateral Acceleration from Test Data - LLVW (ESC OFF)

108

Yaw Rates of Tractor and Trailer 25 Tractor Trailer

Yaw Rate (deg/s) 10

0 8 9 10 11 12 13 14 15 16 17 Time (sec)

Figure 5.30: Yaw Rates from Test Data - LLVW (ESC OFF)

5.4.1.2 Simulation

The simulation was performed with the truck in LLVW condition such that the tractor attained the approximately the same lateral acceleration levels as the test.

109

Steering Angle 300

250

200

150

100 Steering Angle (deg) 50

0 0 1 2 3 4 5 6 7 8 Time (sec)

Tractor Speed 70

Speed (kph) 50

40 0 1 2 3 4 5 6 7 8 Time(sec)

Figure 5.31: Steering Angle and Truck Speed from Simulation - LLVW (ESC OFF)

Figure 5.31 shows the steering angle and the tractor speed. The speed drops by about 11.2 mph (18 kph). A maximum steering angle of 300 degrees was given during the simulation.

110

1 Simulation A y A Predicted from Hand wheel angle 0.9 y Estimated Critical Threshold Levels 0.8

0.7

0.6

0.5

0.4 Lateral acceleration (g)

0.3

0.2

0.1

0 0 1 2 3 4 5 6 7 8 Time (sec)

Figure 5.32: Tractor Lateral Acceleration from Simulation - LLVW (ESC OFF)

The tractor lateral acceleration is shown in Figure 5.32. The tractor attains a maximum lateral acceleration of 0.73 g. The roll angles, truck articulation angles and yaw rate are shown in Figure 5.33.

111

Roll angle vs Time 4

2 Tractor Roll Angle Trailer Roll Angle 1 RSC Throttle Roll angle(deg) RSC Brake 0 0 1 2 3 4 5 6 7 Time (sec) Articulation angle vs Time 0 Articulation Angle -5 RSC Throttle RSC Brake -10

Angle (deg) -15

0 1 2 3 4 5 6 7 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate 10 Tractor Yaw Rate RSC Throttle

Yaw rate (deg/s) RSC Brake 0 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.33: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - LLVW (ESC OFF)

5.4.2 ESC ON

5.4.2.1 Test Data

The data from Test 217 which was performed using a J-Turn maneuver is described in this section. The steering input was given by the driver in this test.

112

The steering angle, vehicle speed and tractor lateral acceleration are shown in

Figure 5.34.

Steering Angle 250 Steer angle 200 ROP Brake 150 ROP Engine 100 50

Steering Angle (deg) 0 8 9 10 11 12 13 14 15 16 17 Time (sec) Vehicle Speed

50 Speed (kph)

40 8 9 10 11 12 13 14 15 16 17 Time (sec) Lateral acceleration vs Time

0.8

0.6

0.4

0.2

Lateral Acceleration(g) 0 8 9 10 11 12 13 14 15 16 17 Time (sec)

Figure 5.34: Steering Angle, Vehicle Speed and Tractor Lateral acceleration from Test Data - LLVW (ESC ON)

113

Steer Axle of Tractor 300 Inner Wheel Outer Wheel 200

100 Pressure (kPa) Pressure 0 8 9 10 11 12 13 14 15 16 17 Time(sec) Leading Drive Axle Brake Chamber Pressures 80

20 Pressure (kPa) Pressure 0 8 9 10 11 12 13 14 15 16 17 Time(sec) Trailing Drive Axle Brake Chamber Pressures 80

20 Pressure (kPa) Pressure 0 8 9 10 11 12 13 14 15 16 17 Time(sec)

Figure 5.35: Brake Chamber Pressures of Tractor Steer Axle and Drive Axles from Test Data - LLVW (ESC ON)

The steer axle brake chamber’s received a maximum pressure of 38 psi (263 kPa).

The drive axles’ outer and inner wheels had maximum pressures of 6.8 psi (46.4 kPa) and

5.8 psi (40 kPa) as shown in Figure 5.34. The trailer axles’ brake chamber pressures had peak values of 31.9 psi (219 kPa), 46 psi (318 kPa) and 15.7 psi (108 kPa).

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Leading Trailer Axle Brake Chamber Pressures

300 Inner Wheel Outer Wheel 250

200

150

Pressure (kPa) 100

0 8 9 10 11 12 13 14 15 16 17 Time(sec)

Trailing Trailer Axle Brake Chamber Pressures

300

250

200

150

Pressure (kPa) Pressure 100

0 8 9 10 11 12 13 14 15 16 17 Time(sec)

Figure 5.36: Brake Chamber Pressures of Trailer Axles from Test Data - LLVW (ESC ON)

Roll angles and truck articulation angles were not available from the test data. The yaw rates of the tractor and the trailer are shown in Figure 5.36.

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Yaw Rate 25 Tractor Yaw Rate Trailer Yaw Rate RSC Throttle RSC Brake

Yaw Rate (deg/s) 10

0 6 8 10 12 14 16 18 20 22 24 Time (sec)

Figure 5.37: Yaw Rates from Test Data - LLVW (ESC ON)

5.4.2.2 Simulation

The maneuver for the simulation was designed so as to cause the tractor to attain the same lateral accelerations as tractor in the test. The steering angle and the tractor speed during the simulation are shown in Figure 5.37.

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Steering Angle 300 Steering Angle 250 RSC Brake RSC Throttle 200

150

100 Steering Angle(deg) 50

0 0 1 2 3 4 5 6 7 8 Time (sec)

Tractor Speed 70

Speed (kph) 50

40 0 1 2 3 4 5 6 7 8 Time (sec)

Figure 5.38: Steering Angle and Truck Speed from Simulation - LLVW (ESC ON)

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1 Simulation A y A Predicted from Hand wheel angle 0.9 y RSC Throttle RSC Brake 0.8 Estimated Critical Threshold Levels

0.7

0.6

0.5

0.4 Lateral acceleration (g)

0.3

0.2

0.1

0 0 1 2 3 4 5 6 7 8 Time (sec)

Figure 5.39: Tractor Lateral Acceleration from Simulation - LLVW (ESC ON)

The tractor lateral acceleration is shown in the figure 5.32. The tractor attains a maximum lateral acceleration value of 0.68 g during the simulation.

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ESC Initiated braking for steer axle 250

) 200 Inner Wheels Outer Wheels 150 100 50 Pressure (kPa Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for drive axles 250 200 150 100 50 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for trailer axles 250 200 150 100 50 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.40: RSC system initiated braking for Tractor and Trailer from Simulation - LLVW (ESC ON)

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Steer Axle Brake Chamber Pressures 250 200 Outer Wheel Inner Wheel 150 RSC Brake 100 RSC Throttle 50 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 7 8 Time (sec) Leading Drive Axle Brake Chamber Pressures 200

150

100

50 Pressure (kPa) 0 0 1 2 3 4 5 6 7 8 Time (sec) Trailing Drive Axle Brake Chamber Pressures 200

150

100

50 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 7 8 Time (sec)

Figure 5.41: Tractor Brake Chamber Pressures from Simulation - LLVW (ESC ON)

Figure 5.40 shows the braking pressures in the axles of the tractor. The steer axle receives a maximum braking pressure of 26 psi (177 kPa). The drive axles’ outer and inner wheels received maximum pressure values of 24 psi (167 kPa) and 12 psi (84 kPa) respectively.

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Leading Trailer Axle Brake Chamber Pressures 200 Inner Wheel Outer Wheel 150 RSC Brake RSC Throttle 100

Pressure (kPa) Pressure 50

0 0 1 2 3 4 5 6 7 8 Time (sec)

Trailing Trailer Axle Brake Chamber Pressures 200

150

100

Pressure (kPa) 50

0 0 1 2 3 4 5 6 7 8 Time (sec)

Figure 5.42: Trailer Brake Chamber Pressures from Simulation - LLVW (ESC ON)

The above figure shows the brake chamber pressures in the trailer axles. The inner wheels experienced considerable slip causing the ABS to dump the pressure in the brake chambers of the inner wheels.

The roll angles, truck articulation angle and the yaw rates are shown in Figure

5.42.

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Roll angle vs Time 4 Tractor Roll Angle 3 Trailer Roll Angle 2 RSC Throttle RSC Brake 1 Roll angle (deg) 0 0 1 2 3 4 5 6 7 Time (sec) Articulation angle vs Time 0 Articulation Angle -5 RSC Throttle -10 RSC Brake

Angle (deg) -15

-20 0 1 2 3 4 5 6 7 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate Tractor Yaw Rate 10 RSC Throttle RSC Brake Yaw rate (deg/s) rate Yaw 0 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.43: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - LLVW (ESC ON)

5.5 NHTSA Fish hook Test

To study the effectiveness of the ESC system model in preventing the truck model from rolling over, severe maneuvers were adopted which lead to the rollover of the truck when simulated with the ESC system switched OFF. Then the truck was simulated with

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ESC system model switched ON to study the functioning of the ESC system model in preventing rollovers.

The fish hook test is one of the most severe maneuvers developed by the NHTSA.

The maneuver simulates the steering that might be given by a driver in panic in case of a sudden lane departure. It represents a typical loss of control situation to which a driver would driver react by giving rapid steer inputs that could possibly lead to the truck rolling over. The simulations were run with dropped throttle (clutch-in) after the vehicle reached a target speed of 40 mph (64 kph).

5.5.1 GVWR with High C.G. Height

5.5.1.1 ESC OFF

Simulation was first performed with the fully loaded truck having a high C.G. height. The tractor ESC system model was switched OFF for the simulation run. In a span of 2.5 seconds, the steering angle is increased to 294 degrees and then dropped to

-294 degrees as shown in Figure 5.43. During this maneuver the tractor attains peak lateral acceleration values of 0.54 g and -0.79 g after which it rolls over. The point at which the tractor’s drive axle wheels lift off is marked in Figure 5.44.

The roll angles, truck articulation angles and the yaw rates are shown in Figure

5.45.

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Steering Angle 300 Steering Angle 200 RSC Brake RSC Engine 100

-100 Steering Angle (deg) -200

-300 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time

Tractor Speed 70

50 Speed(kph) 40

30 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time(sec)

Figure 5.44: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation – GVWR with High C.G. (ESC OFF)

0.8 Simulation A y A Predicted from Hand wheel angle y 0.6 RSC Brake RSC Throttle Estimated Critical Threshold Levels 0.4 data6

0.2

-0.2

Lateral acceleration (g)

-0.4

-0.6

Tractor Drive Axle Wheels Lift-off -0.8 leading to Rollover

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time (sec) Figure 5.45: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC OFF)

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Roll angle vs Time 10 Tractor Roll Angle 0 Trailer Roll Angle RSC Throttle -10 RSC Brake

-20 Roll angle (deg) -30 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time (sec) Articulation angle vs Time 20

0 Articulation Angle

Angle (deg) RSC Throttle -10 RSC Brake

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate Tractor Yaw Rate RSC Throttle 0 RSC Brake

-20 Yaw rate (deg/s)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Time (sec)

Figure 5.46: Roll Angles, Truck Articulation Angles and Yaw Rates from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC OFF)

5.5.1.2 ESC ON

Simulation was next performed with the tractor ESC switched ON. The steering angle and the target speed were retained. During the maneuver, the tractor ESC system model got activated and applied braking on the wheels of the tractor and trailer wheels to bring the lateral acceleration down below the critical threshold. This prevented the truck from rolling over compared to the previous simulation, when the truck went on to roll over. The steering angle and the vehicle speed are shown in Figure 5.46.

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Steering Angle 300 Steering Angle 200 RSC Brake RSC Throttle 100

-100

Steering Angle (deg) -200

-300 0 1 2 3 4 5 6 Time

Tractor Speed 70 60 50

Speed(kph) 20 10

0 0 1 2 3 4 5 6 Time(sec)

Figure 5.47: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON)

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0.8 Simulation A y A Predicted from Hand wheel angle y RSC Throttle 0.6 RSC Brake Estimated Critical Threshold Levels

0.4

0.2

0 Lateral acceleration (g)

-0.2

-0.4

-0.6

0 1 2 3 4 5 6 Time (sec)

Figure 5.48: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON)

The ESC system model estimated a critical threshold value of 0.28 g for the truck.

The maximum lateral acceleration values are restricted by the ESC braking to 0.45 g and

-0.43 g as shown in Figure 5.47. The braking stops once the actual lateral acceleration value drops below the critical threshold value.

The ESC initiated braking is shown in Figure 5.48. The brake chamber pressures in the tractor and trailer axles are shown in Figures 5.49 and 5.50 respectively.

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ESC Initiated braking for steer axle 800 Left Wheel 600 Right Wheel 400 200

Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for drive axles 800 600 400 200

Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for trailer axles 800 600 400 200

Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.49: RSC system model Initiated Braking from Fish Hook Maneuver Simulation - GVWR with High C.G. (ESC ON)

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Steer Axle Brake Chamber Pressures 800 Left Wheel 600 Right Wheel RSC Brake 400 RSC Throttle 200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec) Leading Drive Axle Brake Chamber Pressures 800

600

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec) Trailing Drive Axle Brake Chamber Pressures 800

600

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.50: Tractor Brake Chamber Pressures from Simulation - GVWR with High C.G. (ESC ON)

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Leading Trailer Axle Brake Chamber Pressures 800 Left Wheel Right Wheel 600 RSC Brake RSC Throttle

400

Pressure (kPa) Pressure 200

0 0 1 2 3 4 5 6 Time (sec)

Trailing Trailer Axle Brake Chamber Pressures 800

600

400

Pressure (kPa) 200

0 0 1 2 3 4 5 6 Time (sec)

Figure 5.51: Trailer Brake Chamber Pressures from Simulation - GVWR with High C.G. (ESC ON)

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Roll angle vs Time

Tractor Roll Angle 5 Trailer Roll Angle RSC Brake 0 RSC Throttle

Roll angle (deg) -5

0 1 2 3 4 5 6 Time (sec) Articulation angle vs Time

20 Articulation Angle RSC Brake 10 RSC Throttle

0 Angle (deg)

-10

0 1 2 3 4 5 6 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate Tractor Yaw Rate 10 RSC Brake 0 RSC Throttle -10

Yaw rate (deg/s) rate Yaw -20

0 1 2 3 4 5 6 Time (sec)

Figure 5.52: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR with High C.G. (ESC ON)

5.5.2 GVWR with Low C.G. Height

5.5.2.1 ESC OFF

The simulation was performed with the truck in GVWR with low C.G. height condition using the fish hook maneuver at target speed of 64 kph. The steering angle and vehicle speed are shown in Figure 5.52. The tractor attains peak lateral acceleration values of 0.55g and -0.73 g as shown in Figure 5.53 which causes the truck to rollover.

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Steering Angle

Steering Angle 200 RSC Brake RSC Throttle 0

-200 Steering Angle (deg)

0 1 2 3 4 5 6 Time Tractor Speed

Speed(kph) 20

0 0 1 2 3 4 5 6 Time(sec)

Figure 5.53: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC OFF)

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0.8 Actual A y A Predicted from Hand wheel angle y 0.6 RSC Brake RSC Throttle Estimated Critical Threshold Levels 0.4

0.2

-0.2 Lateral acceleration (g)

-0.4

-0.6

Tractor Drive Axles' Wheels Lift-off -0.8 leading to Rollover

0 1 2 3 4 5 6 Time (sec)

Figure 5.54: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC OFF)

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Roll angle vs Time

Tractor Roll Angle 0 Trailer Roll Angle

-10

-20 Roll angle (deg) -30 0 1 2 3 4 5 6 Time (sec) Articulation angle vs Time

Angle (deg) -10

-20 0 1 2 3 4 5 6 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate Tractor Yaw Rate 0

-20 Yaw rate (deg/s) rate Yaw

0 1 2 3 4 5 6 Time (sec)

Figure 5.55: Roll Angles, Truck Articulation Angles and Yaw Rates from Test Data - GVWR with Low C.G. (ESC OFF)

5.5.2.2 ESC ON

Simulation was then performed with the tractor ESC system ON. The steering angle and the vehicle speed are shown in Figure 5.55. During the maneuver, the tractor attains peak lateral acceleration values of 0.48 g and -0.5 g as shown in Figure 5.56. But the ESC system model applies brakes on the wheels of the truck, thereby quickly bringing down the lateral acceleration of the truck thus preventing rollover. The ESC

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initiated brake pressures, the tractor axle and trailer axle wheels’ brake chamber pressures are shown in Figures 5.57, 5.58 and 5.59 respectively.

Steering Angle 300

200 Steering Angle RSC Brake 100 RSC Throttle

-100 Steering Angle(deg) -200

-300 0 1 2 3 4 5 6 Time (sec)

Tractor Speed

40 Speed (kph)

20 0 1 2 3 4 5 6 Time (sec)

Figure 5.56: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON)

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0.8 Simulation A y A Predicted from Hand wheel angle y RSC Throttle 0.6 RSC Brake Estimated Critical Threshold Levels

0.4

0.2

0 Lateral acceleration (g)

-0.2

-0.4

0 1 2 3 4 5 6 Time (sec)

Figure 5.57: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON)

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ESC Initiated braking for steer axle 800 Left Wheel 600 Right Wheel 400

200 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for drive axles 800

600

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for trailer axles 800

600

400

200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.58: RSC system model Initiated Braking from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON)

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Steer Axle Brake Chamber Pressures 800 Left Wheel 600 Outer Wheel RSC Brake 400 RSC Throttle 200 Pressure (kPa) 0 0 1 2 3 4 5 6 Time (sec) Leading Drive Axle Brake Chamber Pressures 800

600

400

200 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec) Trailing Drive Axle Brake Chamber Pressures 800

600

400

200 Pressure (kPa) Pressure 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.59: Tractor Brake Chamber Pressures from Fish Hook Maneuver Simulation - GVWR with Low C.G. (ESC ON)

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Leading Trailer Axle Brake Chamber Pressures 800 Left Wheel 600 Right Wheel RSC Brake 400 RSC Throttle

200 Pressure (kPa)

0 0 1 2 3 4 5 6 Time (sec) Trailing Trailer Axle Brake Chamber Pressures 800

600

400

200 Pressure (kPa) Pressure

0 0 1 2 3 4 5 6 Time (sec)

Figure 5.60: Trailer Brake Chamber Pressures from Simulation - GVWR with Low C.G. (ESC ON)

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Roll angle vs Time

Tractor Roll Angle 2 Trailer Roll Angle RSC Throttle 0 RSC Brake

Roll angle (deg) -2

0 1 2 3 4 5 6 Time (sec) Articulation angle vs Time

20 Articulation Angle RSC Throttle 10 RSC Brake

0 Angle (deg) -10

0 1 2 3 4 5 6 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate 10 Tractor Yaw Rate RSC Throttle 0 RSC Brake -10

Yaw rate (deg/s) rate Yaw -20

0 1 2 3 4 5 6 Time (sec)

Figure 5.61: Roll Angles, Truck Articulation Angles and Yaw Rates from Simulation - GVWR with Low C.G. (ESC ON)

5.5.3 LLVW

5.5.3.1 ESC OFF

This section deals with the simulation of the lightly loaded truck with the tractor

ESC system switched OFF. The steering angle and the vehicle speed are shown in Figure

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5.61. The tractor attains peak lateral acceleration values of 0.58 g and -0.76 g. At the lateral acceleration value of -0.76 g the tractor drive axle wheels lift-off. Though this does not lead to a rollover, the occurrence of drive axle wheel lift indicates the fact that the truck almost reached the stage of rolling over.

Steering Angle

) Steering Angle

eg 200 RSC Brake (d e

l RSC Throttle

ng 0 A ng i

eer -200 St

0 1 2 3 4 5 6 Time Tractor Speed

Speed(kph) 20

0 0 1 2 3 4 5 6 Time(sec)

Figure 5.62: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - LLVW (ESC OFF)

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0.8 Simulation A y A Predicted from Hand wheel angle 0.6 y RSC Brake RSC Throttle 0.4 Estimated Critical Threshold Levels

0.2

-0.2 Lateral acceleration (g)

-0.4

-0.6

Tractor Drive Axles' Wheels Lift-off -0.8 0 1 2 3 4 5 6 Time (sec)

Figure 5.63: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - LLVW (ESC OFF)

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Roll angle vs Time 4 Tractor Roll Angle 2 Trailer Roll Angle 0 -2 -4 Roll angle (deg) 0 1 2 3 4 5 6 Time (sec) Articulation angle vs Time 20 10 0

Angle (deg) -10

0 1 2 3 4 5 6 Time (sec) Yaw rate vs Time

Tractor Yaw Rate 20 Tractor Yaw Rate 0

-20

Yaw rate (deg/s) 0 1 2 3 4 5 6 Time (sec)

Figure 5.64: Roll Angles, Truck Articulation Angle and Yaw Rates from Fish Hook Maneuver Simulation - LLVW (ESC OFF)

5.5.3.2 ESC ON

The truck was then simulated with the tractor ESC system switched ON. The steering angle and the vehicle speed are shown in Figure 5.64. The ESC system got activated during the maneuver as the ESC estimated critical threshold of 0.66 g is exceeded by the tractor. The ESC initiated braking restricts the peak lateral acceleration to 0.59 g and -0.74 g which then dropped down further. Wheel lift was not observed in either the tractor or the trailer.

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Steering Angle

Steering Angle 200 RSC Brake RSC Throttle 0

-200 Steering Angle (deg)

0 1 2 3 4 5 6 7 Time (sec) Tractor Speed

Speed(kph) 20

0 0 1 2 3 4 5 6 7 Time(sec)

Figure 5.65: Steering Angle and Truck Speed from Fish Hook Maneuver Simulation - LLVW (ESC ON)

0.8 Actual A y A Predicted from Hand wheel angle 0.6 y RSC Brake 0.4 RSC Throttle Estimated Critical Threshold Levels

0.2

-0.2

Lateral acceleration (g) -0.4

-0.6

-0.8 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.66: Tractor Lateral Acceleration from Fish Hook Maneuver Simulation - LLVW (ESC ON)

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ESC Initiated braking for steer axle

400 Left Wheel 300 Right Wheel

200 100 Pressure (psi) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for drive axles 800

600

400

200 Pressure (psi) Pressure 0 0 1 2 3 4 5 6 Time (sec) ESC Initiated braking for trailer axles 800

600

400

200 Pressure (psi) Pressure 0 0 1 2 3 4 5 6 Time (sec)

Figure 5.67: RSC system model Initiated Braking from Fish Hook Maneuver Simulation - LLVW (ESC ON)

The above figure shows the ESC system initiated braking signal for the tractor and the trailer axles. Figure 5.67 and 5.68 show the pressures in the brake chambers of the tractor and trailer axle wheels.

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Steer Axle Brake Chamber Pressures 200 Left Wheel 150 Right Wheel 100 RSC Brake RSC Throttle ssure Pa) 50 Pre (k 0 0 1 2 3 4 5 6 7 Time (sec) Leading Drive Axle Brake Chamber Pressures 250 200 150 100 50 Pressure (kPa) 0 0 1 2 3 4 5 6 7 Time (sec) Trailing Drive Axle Brake Chamber Pressures 250 200 150 100 50 Pressure (kPa) 0 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.68: Tractor Brake Chamber Pressures from Fish Hook Maneuver Simulation - LLVW (ESC ON)

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Leading Trailer Axle Brake Chamber Pressures 250 Left Wheel Right Wheel 200 RSC Brake RSC Throttle 150

100 Pressure (kPa) Pressure 50

0 0 1 2 3 4 5 6 7 Time (sec)

Trailing Trailer Axle Brake Chamber Pressures 250

200

150

100 Pressure (kPa) 50

0 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.69: Trailer Brake Chamber Pressures from Fish Hook Maneuver Simulation - LLVW (ESC ON)

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Roll angle vs Time

2 Tractor Roll Angle Trailer Roll Angle 0 RSC Throttle RSC Brake -2 Roll angle (deg) 0 1 2 3 4 5 6 7 Time (sec) Articulation angle vs Time

20 Articulation Angle 10 RSC Throttle 0 RSC Brake

Angle (deg) -10

0 1 2 3 4 5 6 7 Time (sec) Yaw rate vs Time

20 Tractor Yaw Rate Tractor Yaw Rate 0 RSC Throttle RSC Brake -20

Yaw rate (deg/s) rate Yaw 0 1 2 3 4 5 6 7 Time (sec)

Figure 5.70: Roll Angles, Truck Articulation and Yaw Rates from Fish Hook Maneuver Simulation - LLVW (ESC ON)

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CHAPTER 6

CONCLUSIONS AND RECOMMENDATIONS

6.1 Conclusions

A Roll Stability Control system model was developed in Simulink for a 6x4 tractor of a combination truck model. The RSC system model simulates the functioning of RSC features of a commercial ESC system. Strategies were developed to estimate the critical threshold for the truck used in the simulation. A brake severity estimator was developed which computes the brake pressures to be applied at the brake chambers of the tractor and the semitrailer. A differential braking module was implemented for the brake chambers of the tractor drive axles. This module modulates the pressures signal supplied to the brake chambers of the inner and outer wheels based on the severity of the maneuver. The tractor ESC system model was also designed to brake the trailer axles similar to the commercial ESC system.

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The TruckSim model together with the ABS and tractor ESC system models in

Simulink was able to simulate the behavior of the actual truck to a reasonable extent.

Simulations conducted with the tractor ESC system model switched OFF and ON clearly show that the developed ESC system model was capable of preventing the truck from rolling over when subjected to severe maneuvers at high speeds. The model has all the capabilities of the actual system in terms of total braking power. It can cut the engine throttle and brake the wheels of the tractor and the trailer.

The simulations showed that the ESC system model prevented wheel lift from occurring during most of the maneuvers. The worst case scenario simulated was with the fully loaded truck with high C.G. height using the fish hook maneuver at a speed of 64 kph. Such a maneuver provides a multitude of safety concerns if it is to be performed with the actual vehicle. During this simulation with the ESC system model switched

OFF, the vehicle rolled over on attaining high levels of lateral acceleration. When the simulation was repeated with the ESC system model switched ON, it was observed that the trailer wheels lifted off momentarily before being brought back into contact with the ground, while the drive axle wheels never lifted off. The ESC system applied braking appropriately on the wheels of the vehicle ensuring its stability.

It should be noted at this point that the ESC system model was developed predominantly based on the test data collected from the truck having Volvo tractor fitted with Bendix ESP. Hence much of the response of the ESC system was modeled based on the response of the Volvo tractor as well as the functioning of the Bendix ESP. Other commercially available ESC systems in the market are likely to have different pattern of

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responses. Difference could include the levels of lateral acceleration at which the ESC system would activate for different loads, total braking power and the number of axles braked. The overall truck response would also change during braking if the ABS configuration is changed. Hence the simulation results cannot be compared with the performance of all the commercial systems and expected to be similar.

6.2 Recommendations

A more accurate truck model for the Volvo could be developed by using the suspension parameters of the actual Volvo tractor. Simulations that were performed using maneuvers executed with the actual vehicle showed that the maximum lateral accelerations attained by the model were slightly less than the actual vehicle. This could arise due to inaccurate suspension compliances and models of the damping system, steering system and the tires. Hence the modeling of the Volvo tractor as well as the

Fruehauf trailer suspension system would result in a model whose responses would be more accurate.

When severe maneuvers are executed at low friction surfaces, it is possible for the vehicle to lose directional stability due to high braking caused by the Roll Stability

Control feature of the ESC system. Hence it is suggested that a complete ESC system, one that has both RSC and YSC features, be developed. The existing RSC model can be easily integrated with any YSC model that could be developed in the future.

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Further testing of the vehicle is recommended in order to achieve a complete understanding of the ESC system behavior. Tests need to be conducted using the SIS maneuver with the tractor ESC system switched ON and trailer ESC switched OFF for all three load configurations. In addition to this, tests should also be conducted with ESC systems switched OFF in both tractor and the trailer. This would give a more clear idea of the ESC activation sequence, threshold levels for different loads and braking patterns.

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REFERENCES

1. Large Truck Crash Facts 2005, Analysis Division, Federal Motor Carrier Safety Administration, U.S. Department of Transportation. February 2007. http://ai.volpe.dot.gov/CarrierResearchResults/HTML/2005Crashfacts/2005LargeTru ckCrashFacts 2. Federal Motor Vehicle Safety Standard No. 126, Final Regulation, National Highway and Traffic Safety Administration April 5, 2007 http://www.safercar.gov/esc/Rule.pdf 3. Bendix ABS 6 Advanced with ESP Stability system, Frequently Asked Questions to help you make an Intelligent Investment in Stability. http://www.bendix.com/bendix/abs6/downloads/StabilityFAQ.pdf 4. Zagorski, Scott B. Compatibility of ABS Disc/Drum Brakes on Class VIII Vehicles with Multiple Trailers and their Effect on Jackknife Stability. MS Thesis. The Ohio State University, 2003 5. Zagorski, S. B., Guenther, D. A., and Heydinger, G. J., “A Study of Jackknife Stability of Class VIII Vehicles with Multiple Trailers with ABS Disc/Drum Brakes,” SAE Paper 2004-01-1741, 2004. 6. Dunn, Ashley L. “Jackknife Stability of Articulated Tractor Semi-trailer Vehicles with High-Output Brakes and Jackknife Detection on Low Coefficient Surfaces”. PhD. Dissertation. The Ohio State University, 2003 7. Dunn, A. L., Heydinger, G. J., and Guenther, D. A., “In-Depth Analysis of the Influence of High Torque Brakes on the Jackknife Stability of Heavy Trucks,” SAE Paper 2003-01-3398, 2003. 8. Zaugg, Brian C. “Development of Heavy Truck ABS and Limit Maneuvers Model”. MS Thesis. The Ohio State University, 2004. 9. Zaugg, B. C., Heydinger, G. J., Guenther, D. A., Dunn, A. L., Zagorski, S.B., and Grygier, P.A., “The Development of a Heavy Truck Heavy Truck ABS Model,” SAE Paper 2005-01-0413, 2005. 10. Shurtz, Matthew L. “Effects of ABS Controller Parameters on Heavy Truck Model Braking Performance”. MS Thesis. The Ohio State University, 2006. 11. Shurtz, Matthew L., Guenther, D. A., Heydinger, G. J., and Zagorski, S. B., “Refinements of a Heavy Truck ABS Model”. SAE Paper 2007-01-839, 2007

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12. TruckSim Version 6.0 Reference Manual ,Ann Arbor, MI: Mechanical Simulation Corporation, 2005 13. “Impact of Geometric Features on Truck Operations and Safety at Interchanges”, Final Technical Report, The University of Michigan Transportation Research Institute, August 1985 14. Baker, D., Bushman, R., and Berthelot, C., “The Effectiveness of Truck Rollover Warning System”, Paper # 01-2646, August 2000. http://www.irdinc.com/library 15. Gillespie, Thomas. D., Fundamental of Vehicle Dynamics. SAE, Warrendale, PA, 1992 16. Meritor WABCO. Anti-Lock Braking System (ABS) for Trucks, Tractors, and Buses. Maintenance Manual MM-0112. pp. 2 http://www.meritorhvs.com/MeritorHVS_Documents/mm0112.pdf 17. Salaani, M.K., Grygier, P.A., Heydinger, G.J., Engine Retarder Brake, VRTC, August 2007

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APPENDIX A

VARIABLES RECORDED IN TESTS

The variables recorded during the vehicle testing are listed in this Appendix.

Variables that are measured specifically for the tractor and the trailer are listed under the respective units.

A total of three Global Positioning Systems (GPS) were used to measure vehicle positions and speed. These variables are listed under the GPS section. Some variables were measured taken from the vehicle Controller Area Network (CAN) bus and they are listed under the section named J1939.

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Tractor Data Trailer Data Longitudinal Acceleration Longitudinal Acceleration Lateral Acceleration Lateral Acceleration Vertical Acceleration Vertical Acceleration Roll Rate Roll Rate Yaw Rate Yaw Rate Pitch Rate Pitch Rate Throttle Position Wheel Speeds (4) Brake Pedal Switch Brake Temperatures (8) Hand Wheel Angle Brake Chamber Pressures (4) Longitudinal Velocity Suspension Air Bag Pressure (4) Wheel Speeds (6) Individual Brake Temperatures (12) Brake Chamber Pressures (6) Suspension Air Bag Pressure (6) Ambient Temperature Reservoir Pressures (2)

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GPS Data J1939 Data Eastings GPS1 J1939 Handwheel Northings GPS1 J1939 Throttle Elevation GPS1 J1939 Speed Speed Eastings GPS1 J1939 Individual Wheel Speeds Speed Northings GPS1 J1939 ESC Status Speed Up GPS1 J1939 VDC 1 Eastings GPS2 J1939 VDC 2 Northings GPS2 Elevation GPS2 Speed Eastings GPS2 Speed Northings GPS2 Speed Up GPS2 Eastings GPS3 Northings GPS3 Elevation GPS3 Speed Eastings GPS3 Speed Northings GPS3 Speed Up GPS3

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APPENDIX B

SIMULINK ENVIRONMENT

The complete Simulink model that has all the modules is shown in this Appendix.

Figure B.1 shows the top half of the Simulink environment that has the Randomly Varied

Friction generator, S-Function, RSC model, External Engine and Torque Converter model. Figure B.2 shows the bottom half of the Simulink environment that has the tractor and trailer ABS controllers and the brake torque & pneumatic subsystems for all the axles.

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wheel_long_speed_trailer wheel_long_speed trailer_speed tractor_speed tractor_speed trailer_speed Selector5 wheel_long_speed_tractor Equiv (mph) lant (tangential) Speeds Wheel (10) Selector4 trailer_accel tractor_accel Volv (mph) o Speed treadle_pressure_reference Volv Long. o Accel. (m/s^2) Volv Long. o Accel. (m/s^2) Volv (mph) Speed o Equiv lantWheel (10) (mph) Speeds (tangential) g to m/s^2 kph to m ph g to m/s^2 kph to m ph 9.80665 9.80665 1/1.6093 1/1.6093 kph to m ph 1/1.6093 wheel_radial_accel x wheel_forces Tractor_roll Treadle Pressure (psi) wheel_locations emu vehicle_dynamics D inp_engine_model MPa psi to 145.0377 145.0377 Treadle Pressure (MPa) Treadle Volv (4) Motion o Vehicle DyVehicle namics (29) (30) forces Wheel Rear wheel locations (4) locations wheel Rear x Equiv alent (tangential) WHEEL Speeds (kph)(10) Tire-Wheel Assy Tire-Wheel . accel/decel (10) (rad/s^2) emu D Demux2 INPUTS from TruckSim from TruckSim 5-axle 5-axle TruckSim tractor / semi-trailermodel nk Environment with RSC System Mo del, S-Function and Engine Model a O ke Bra vari TC & MU Levels& MU Mux Brake Torques Torques Mux Brake Tractor ESC Brake Torque Outputs (N-m)(10) toBrake Torque TSIM lb-ft to N-m lb-ft 1.356 v MU lev ariable els (40) TC OutputTC Shaf t torque Throttle Input Randomly varied varied Randomly Friction Generator Brake Torque Outputs (lb-fBrake Torque t) (10) EngineModel TorqueConverter & torque_vehicle tractor_speed Figure B.1 – Upper Half of Simuli

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ent with ABS Controllers and Braking System with ABS Controllers and Braking ent Figure B.2 – Lower Half of Simulink Environm

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