CarSim and Matlab-Simulink. Thesis. Master of Science in the Graduate School of the Ohio State University. Tejas Shrikant Kinjawadekar

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1 Model-based Design of Electronic Stability Control System for Passenger Cars Using CarSim and Matlab-Simulink Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of the Ohio State University By Tejas Shrikant Kinjawadekar Graduate Program in Mechanical Engineering The Ohio State University 2009 Thesis Committee: Dr. Dennis A Guenther, Advisor Dr. Gary J Heydinger

2 Copyright by Tejas Shrikant Kinjawadekar 2009

3 ABSTRACT The purpose of this thesis is to develop an Electronic Stability Control (ESC) system model through co-simulation between the softwares CarSim and Matlab- Simulink. The objective is to demonstrate that, given a validated vehicle model, it is possible to construct a simple, functional ESC model which will give comparable performance to actual ESC systems on board the vehicle for certain maneuvers. A vehicle dynamics model of the 2003 Ford Expedition created in CarSim was validated by comparing simulation results with quasi-static and dynamic test data. Bounce and roll tests were used to validate the suspension kinematics and compliance parameters of the vehicle model. The National Highway Traffic Safety Administration s (NHTSA) Sine with Dwell maneuver was used to validate the complete vehicle model. The ESC system model was built in Simulink and was tuned through cosimulation with the CarSim vehicle model. The ESC system was designed to operate in two modes: Yaw Stability Control (YSC) and Roll Stability Control (RSC). YSC used vehicle slip rate (which is the difference between actual and ideal vehicle yaw rates) as the control variable while RSC used lateral acceleration as the control variable. The ESC system used differential braking to control the vehicle s yaw and roll motions. Differential braking selectively applies individual brakes on the vehicle to apply corrective moments/forces. A simple ABS functionality was also incorporated in the model. ii

4 The performance of the system was evaluated using dynamic maneuvers like the Sine with Dwell and Fishhook maneuvers. NHTSA s Sine with Dwell maneuver is part of the Federal Motor Vehicle Safety Standard (FMVSS) 126 and all ESC-equipped vehicles are required to pass this test. This test was used to evaluate the performance of the Yaw Stability Control mode of the ESC system. NHTSA s fixed-time Fishhook maneuver is used to test the rollover propensity of the vehicle and this maneuver was used to evaluate the performance of the Roll Stability Control mode of the ESC system. The thesis report is concluded by discussing the benefits of this model-based approach in the design of a simple, functional ESC system. The limitations of this approach and future work to overcome some of these limitations are also discussed. iii

5 DEDICATION Dedicated to my parents and family, for their love, support and guidance To my friends; for their unconditional support and for all the good times spent together. iv

6 ACKNOWLEDGEMENTS I would like to thank my advisors Dr. Dennis Guenther and Dr. Gary Heydinger for their constant encouragement, support and guidance. They were always very approachable and I thank them for creating a comfortable, friendly work environment in our group. I am very grateful to them for being considerate about my interests and for always giving me the freedom to choose my courses and pursue extra-curricular activities. I also thank Dr. Kamel Salaani for his technical inputs during the course of this project and Don Butler for his help and support. I thank my colleagues Sughosh Rao and Neha Dixit for being such good teammates and for all of their help. I would like to thank my friends and roommates for making this a home-away-from-home for me and my parents for their unconditional support and love. v

7 VITA June 2000.Abhinav Vidyalaya High School, Pune, India June 2006.B.E.M.E. Pune University Sept Aug 2008 University Fellow, The Ohio State University Sept June 2009 Graduate Research Assistant, The Ohio State University PUBLICATIONS Kinjawadekar, T., Dixit, N., Heydinger, G.J., Guenther, D.A., and Salaani, M.K., Vehicle Dynamics Modeling and Validation of the 2003 Ford Expedition with ESC using CarSim, SAE Paper No , April Major Field: Mechanical Engineering Vehicle Dynamics, Automotive Systems FIELDS OF STUDY vi

8 TABLE OF CONTENTS ABSTRACT...ii DEDICATION iv ACKNOWLEDGEMENTS.v VITA...vi LIST OF FIGURES.x LIST OF TABLES...xiii 1. INTRODUCTION Motivation Electronic Stability Control Background ESC: working principle ESC Regulations and tests Role of Simulation in ESC development Thesis Outline Closure 8 2. MODELING AND VALIDATION Parameter Determination and Modeling.9 vii

9 Vehicle Inertia and Center of Gravity Suspension and Steering Systems Tires Powertrain, Aerodynamics and Brake System Model Validation Quasi-static Bounce and Roll Tests Sine With Dwell Test (Conducted with ESC OFF) Brake System Model Tuning Closure ELECTRONIC STABILITY CONTROL (ESC) BACKGROUND AND THEORY History and Background Differential Braking Yaw Stability Control Roll Stability Control ESC Regulations: FMVSS ELECTRONIC STABILITY CONTROL (ESC) MODEL Complexity of Actual ESC Systems CarSim Simulink Interface ESC System Topography Roll Stability Control Yaw Stability Control Summary of tuning parameters.48 viii

10 Thresholds Look-up Tables Closure SIMULATION RESULTS ESC System Performance Evaluation: Yaw Stability Control Mode ESC System Performance Evaluation: Roll Stability Control Mode Closure CONCLUSIONS AND RECOMMENDATIONS Conclusions Recommendations 70 REFERENCES 72 ix

11 LIST OF FIGURES Figure 1 Decrease in yaw moment due to steering as side slip angle increases.3 Figure 2 Vehicle Sprung Mass Datascreen in CarSim..10 Figure 3 Front Suspension Spring Rate.12 Figure 4 Front Suspension Damping Curve...12 Figure 5 Tire Lateral Force versus Slip Angle...14 Figure 6 Front Suspension Bounce Test Comparison 16 Figure 7 Rear Suspension Bounce Test Comparison.17 Figure 8 Front Suspension Roll Test Comparison.18 Figure 9 Rear Suspension Roll Test Comparison..19 Figure 10 Sine with Dwell: Steering Wheel Angle...20 Figure 11 Sine with Dwell: Vehicle Speed Plot Figure 12 Sine with Dwell: Lateral Acceleration..21 Figure 13 Sine with Dwell: Yaw Rate...22 Figure 14 Sine with Dwell: Roll Rate 22 Figure 15 Sine with Dwell: Roll Angle.23 Figure 16 Comparison plots for the tuned brake system model in CarSim..24 Figure 17 Friction Circle...28 Figure 18 Differential Braking..28 x

12 Figure 19 ESC intervention during understeer and oversteer 30 Figure 20 Lateral Forces Acting on a Vehicle during Rollover 31 Figure 21 Steering input and vehicle yaw rate used to assess lateral stability..36 Figure 22 Steering input and lateral displacement used to assess responsiveness 37 Figure 23 CarSim- Simulink interface...41 Figure 24 ESC system topography...42 Figure 25 Roll Stability Control Module..46 Figure 26 Yaw Stability Control Module.47 Figure 27 YSC look-up table...49 Figure 28 Effect of ESC Intervention 52 Figure 29 Sine with Dwell -160: Steering Input, Vehicle Speed, Lateral Acceleration and Yaw Rate 53 Figure 30 Sine with Dwell -160: Roll Rate, Roll Angle and Vehicle Slip Rate 54 Figure 31 Sine with Dwell -160: Wheel Cylinder Pressure Values in MPa..56 Figure 32 Sine with Dwell: 280 Degrees..57 Figure 33 Fishhook (82 kmph): Steering Wheel Angle.60 Figure 34 Fishhook (82 kmph): Vehicle Speed.60 Figure 35 Fishhook (82kmph): Lateral Acceleration.61 Figure 36 Fishhook (82kmph): Yaw Rate.61 Figure 37 Fishhook (82kmph): Roll Angle 62 Figure 38 Fishhook (82kmph): Roll Rate..62 Figure 39 Fishhook (82kmph): Wheel Cylinder Pressures 63 Figure 40 Fishhook (82 kmph): Tire Normal Forces for Baseline Vehicle (no ESC)...63 xi

13 Figure 41 Fishhook (82 kmph): Tire Normal Forces for Vehicle with ESC.64 Figure 42 Fishhook 100 kmph: Steering Wheel Angle, Vehicle Speed, Lateral Acceleration and Raw Rate..65 Figure 43 Fishhook 100 kmph: Roll angle, Roll rate, Tire Normal Forces and Brake Chamber Pressures.66 xii

14 LIST OF TABLES Table 1 Vehicle state values at the time of ESC activation. 44 Table 2 Vehicle Slip Rate Values at the time of ESC activation 44 xiii

15 CHAPTER 1: INTRODUCTION 1.1 Motivation : A healthy transportation system and a nation's economy compliment and support each other. An efficient transportation system of goods and people both public and personal is a very important factor in the economic empowerment of a nation. Conversely, as the economy grows it translates into increased traffic of goods and people. This is reflected in the fact that USA, which has the highest national GDP, has a staggering number of vehicles on its roads. The US Government's Bureau of Transportation Statistics reports that there were about 243,444,343 [1] registered vehicles in the country in With such a large number of vehicles on the roads, the number of crashes and injuries reported annually is also very high. The National Automotive Sampling System (NASS) Crashworthiness Data System (CDS) reports that in 2004, there were 34,314 police-reported passenger vehicle fatal crashes and over 2.5 million serious non-fatal crashes (defined as at least one involved passenger vehicle was towed away). Single vehicle crashes which often involve departure from the roads, accounted for 53% of the fatal crashes. Further, rollovers accounted for 42% of single vehicle fatal crashes [2]. With so many lives at stake, automotive safety has been a critical area for all automotive companies. In the last two decades, there have been extensive developments 1

16 in the field of automotive passive safety such as seatbelts and airbags, improved crumble zones in car bodies, increased use of high strength steel, etc. These developments are useful in mitigating the harm in an accident but they do not help to prevent the accident. Further, these safety features are very beneficial in frontal impact crashes but are less effective in side impact and rollovers [2]. Consequently, active safety systems have received a lot of attention in the last few years. These systems can help in preventing accidents as they attempt to keep the vehicle within its stability envelope. For example, the Antilock Braking System (ABS) prevents wheel lock-up which makes the vehicle retain its steering ability. The next level of vehicle stability control is the Electronic Stability Control (ESC) system. This thesis describes a model-based design approach for the development of a functional ESC system model. The softwares used are CarSim and Matlab-Simulink. 1.2 Electronic Stability Control: Background: In extreme maneuvers when the vehicle is operating at the limits of road traction, the vehicle response to driver inputs is different from normal and hence the driver cannot control the vehicle. For example, in a situation where the vehicle is spinning out, countersteering may help to regain control but a normal driver may not be able to do so leading to loss of control and the vehicle leaving the road. Further, in these extreme situations, the side slip angle increases which decreases the corrective yaw moment that can be applied through steering input i.e. steerability of vehicle decreases (refer Figure 1). Unexpected behavior of a vehicle may induce a panic reaction from the driver which 2

17 could make it even more difficult to regain control. In these situations ESC can help the driver regain control of the vehicle. Figure 1. Decrease in yaw moment due to steering as side slip angle increases (for different roadwheel steering angles- δ). (Source: E.K Liebemann, et al, Robert Bosch GmbH, Safety and Performance enhancement: The Bosch Electronic Stability Control (ESP), SAE Paper No , 2004.) ESC: working principle: An ESC system in present-day automobiles typically consists of two operating modes: Yaw Stability Control (YSC) and Roll Stability Control (RSC). YSC helps the driver maintain the desired heading of the vehicle even when the vehicle is operating at the limits of traction. RSC on the other hand, reduces the possibility of single vehicle, 3

18 un-tripped rollovers. Sports utility vehicles (SUVs) and other vehicles with a high center of gravity have a high rollover propensity and the RSC system is especially beneficial in such vehicles. An ESC system helps to keep the vehicle stable by applying brakes on individual wheels. Systems typically consist of wheel speed sensors, yaw rate sensors, lateral acceleration sensors, steering wheel angle sensor and brake pressure sensor. In the YSC mode, the ESC system calculates the heading of the vehicle using speed and steering angle data from the sensors. The measured lateral acceleration and the vehicle speed are used to calculate the radius of the path along which the vehicle is traveling. The radius and the vehicle speed are then used to calculate the correct yaw rate of a vehicle traveling on that path. The on-board yaw rate sensor measures the actual yaw rate at every instant. The measured and calculated yaw rates are then compared and if the difference exceeds a certain threshold, ESC uses differential braking to produce a correcting yaw moment. In the RSC mode, the lateral acceleration of the vehicle is monitored and compared to a set threshold. If the threshold value of lateral acceleration is reached, there is a possibility of the vehicle rolling over. The RSC system applies individual brakes to keep the lateral acceleration of the vehicle below the threshold. The above section describes the working principle of ESC briefly. Each of the two modes is described in detail in subsequent chapters. Commercially available ESC systems have very complex proprietary algorithms to account for a large range of driver-road interactions. In addition to differential braking, some ESC systems also use engine throttle control to slow down the vehicle during extreme maneuvers which aids the driver in regaining control. Some four wheel drive vehicles with ESC control the torque split 4

19 between the front and rear axles to prevent the vehicle from losing control. During understeer (when the traction on front wheels is less), the torque is transferred to the rear axle. During oversteer (when the traction on the rear wheels is less), the torque is transferred to the front axle. Some vehicles use active steering control for vehicle stability. One or a combination of many of the above technologies is used to ensure that the vehicle operates within its stability envelope. 1.3 ESC Regulations and tests: The benefits of ESC in preventing single car accidents are well accepted. Individual studies by automotive OEM's like Toyota and VW have also established the benefits of ESC [2]. The National Highway Traffic Safety Administration (NHTSA) estimates that ESC installation will help reduce single vehicle crashes in passenger cars by 34% and in SUVs by 59%. NHTSA has introduced the FMVSS 126 standard to speed up the installation rate of ESC in vehicles. This standard requires all vehicles with a Gross Vehicle Weight Rating (GVWR) of lbs (4,536kg) or less to be quipped with an ESC system by model year 2012 [3]. In the FMVSS 126 standard, NHTSA has a defined a dynamic test known as the Sine with Dwell to evaluate the performance of vehicles equipped with ESC [4]. This test is used to evaluate the performance of ESC in correcting oversteer of a vehicle. Previously, all vehicles were given a rollover rating depending on their tendency to rollover which in turn was determined from their Static Stability Factor (SSF). SSF is the ratio of half of vehicle track to the height of vehicle center of gravity (CG). Thus, a vehicle with a low CG would have a high SSF and have a lower tendency to rollover. However, studies showed that the vehicle rollover thresholds 5

20 are much lower than those predicted by SSF due to factors like suspension, tire compliances, and lateral load shift during cornering. To account for these factors, the rollover rating is now dependent on vehicle response in dynamic tests typically the NHTSA Fishhook test. Both, the Sine with Dwell and Fishhook tests will be described in detail in subsequent chapters. These tests have been used to validate the vehicle model and also to tune the ESC model. 1.4 Role of Simulation in ESC development: Commercial ESC systems have very complex proprietary algorithms and the controllers are fine-tuned for a particular vehicle model. Tuning the controller requires a large amount of vehicle performance data under different maneuvers and road conditions. Vehicle instrumentation, field tests, data acquisition and post processing require a lot of time and money. Further, human error in these tests makes the data less repeatable and accurate. Some of these tests involve testing the vehicle at the limits of its performance - which might bring harm to the driver and damage the vehicle. Hence vehicle testing, though necessary, is costly and time consuming. In this scenario, modeling and simulation can have huge benefits. A model will always be based on some approximations, but a representative model can be very useful in generating useful, fairly accurate data. Simulations take much less time than actual field tests. Further they offer flexibility in test conditions and vehicle configurations. For example, testing the vehicle with a different suspension would require removing the present suspension and fitting the new one and re-instrumenting the 6

21 vehicle. All of this could be done by merely changing the spring and damper curves in the vehicle model used in simulation. Simulation results are as good as the model used to make the simulation. As models are based on certain approximations, the results will not be as accurate as those done on a actual vehicle. However, a good model will be predictive of the actual vehicle model. Using simulations to design the controller and fine tuning and verifying the controller operation using actual tests will be a good approach that will help reduce project costs and lead time. 1.5 Thesis Outline: The main objective of this thesis is to develop a simple, functional ESC system model using a validated vehicle model. Chapter 2 briefly describes the parameter determination and vehicle dynamics modeling procedure used to create the 2003 Ford Expedition model in CarSim. The model is then validated by comparing simulation results with data from quasi-static and dynamic tests. Chapter 3 gives the theoretical background of the ESC system and its 2 modes of operation: yaw stability control and roll stability control. This chapter also describes the National Highway Traffic Safety Administration s (NHTSA s) Sine with Dwell and Fishhook maneuvers. These maneuvers will be used to evaluate the ESC system performance. Chapter 4 describes the modeling of the ESC system in Simulink. Various modules of the ESC system and their functions are explained in this section. 7

22 Chapter 5 contains simulation results. The simulations results are compared to actual test data to compare the performance of the ESC system model with the actual ESC system on board the vehicle. The simulation results also contain comparisons of the performances of the baseline vehicle (without ESC) and the vehicle with ESC. This would help to demonstrate the safety benefits of the ESC system. Chapter 6 provides a conclusion for the research and it lists the benefits and utility of the design approach adopted in this study. Improvements to the ESC model and further scope of work are also included in this chapter. 1.6 Closure: The objective of this study is not to build a comprehensive ESC model that would account for all loss-of-control scenarios. Rather, the ESC model developed here is expected to demonstrate that given a validated vehicle model, it is possible to construct a simple, functional ESC model which will give comparable performance to actual ESC systems on board the vehicle for certain maneuvers. The utility of such a model is that it can further be used to test the effect of new components or systems (for example an active suspension system) on an ESC-equipped vehicle. In such studies, it is not prudent to spend time and resources in building a comprehensive ESC model. A model, as described in this study, has considerably less development time and still gives comparable performance to the actual ESC system, in the specific maneuvers. This allows us to allocate resources to the main task of evaluating the effect of the new component or system on the performance of the ESC-equipped vehicle. 8

23 CHAPTER 2: MODELING AND VALIDATION The benefits of using simulations were discussed in Chapter 1. However, these benefits are realized only if the model is representative of the actual system. This chapter discusses the development of a vehicle dynamics model and model validation of the 2003 Ford Expedition in CarSim. The vehicle model will then be validated by comparing simulation results with experimental test data. Bounce and Roll tests in CarSim will be used to validate the suspension and steering kinematics and compliances. Field test data of the Sine with Dwell maneuver(s) will be used for the vehicle model validation. CarSim is a commercially available vehicle dynamics simulation software. It has a large database made up of more than 150 libraries of datasets linked together [5]. These datasets contain vehicle model parameters and simulation settings. Datasets of a generic SUV in CarSim were suitably modified to build the Expedition model. The vehicle modeled is a 2003 Ford Expedition XLT, a 4-door sports utility vehicle with a 5.4L V8 engine, automatic transmission, 4WD and Continental Contitrac SUV, P265/70R17 113S M+S tires. Tire pressures are 35 psi for both front and rear tires. 2.1 Parameter Determination and Modeling: The parameters of the vehicle model created in CarSim are based on various measurements done at SEA, Ltd. These parameters were then used to build the CarSim model. 9

24 2.1.1 Vehicle Inertia and Center of Gravity The Vehicle Inertia Measurement Facility (VIMF) at SEA, Ltd was used to measure vehicle weight, inertia and center of gravity location [6]. This data was based on the SAE axis system but CarSim uses an axis system with Z positive upwards. The data was therefore converted into the CarSim system before being used to define model parameters. Consequently, the Ixz value has a negative sign in CarSim. The sprung mass, wheelbase, CG location and vehicle inertias are entered in the CarSim Sprung Mass datascreen as shown in Figure 2. The sprung mass properties are for the unladen condition. Figure 2. Vehicle Sprung Mass Datascreen in CarSim 10

25 2.1.2 Suspension and Steering Systems: The dimensions, weight, CG location and mass moment of inertia of the following components were measured [7] : front and rear upper and lower control arms, front steering knuckle and rear knuckle, front and rear propeller shafts, rear lateral link, steering link. These components were measured using a trifilar pendulum. Front and Rear Suspension - The 2003 Ford Expedition has a short-arm long-arm coilover-shock independent front suspension with stabilizer bar. The track width, roll center height, axle height, spin inertia for the rotating wheel and rotor, forward movement of wheel center per unit of upward movement, squat/lift ratio, and compliance coefficients are all determined from the measured data [7]. The unsprung mass is made of half of the weight of control arms (as the other half is included in the sprung mass), front wheels and spindles. The front spring rate, as modeled in CarSim, is shown in Figure 3. Note that in CarSim, compression is considered positive while extension is negative. The rear suspension is an independent, multi-link, coil-over-shock suspension with stabilizer bar. The front and rear shock absorbers are different for the 2003 Expedition. Figure 4 shows the front suspension damping curve in CarSim. 11

26 Figure 3. Front Suspension Spring Rate Figure 4. Front Suspension Damping Curve 12

27 Auxiliary roll stiffness - In the roll test, overall roll stiffness was measured, which is the resultant of three stiffness auxiliary roll stiffness, suspension stiffness and tire stiffness. Of these, the auxiliary roll stiffness and the suspension stiffness are in parallel, and the tire stiffness is in series with their composite. Hence, the auxiliary roll stiffness can be found using the following formula: K aux T T T K K K K K K T Tr K K T s r 2 r w T w T s where: K Φ : Overall roll stiffness K w : Suspension stiffness K T : Tire stiffness T r : Track width T s : Suspension spring lateral distance Steering system: Tests showed that the relation between handwheel to road wheel steer angle is approximately linear in the 360 O range. Hence this was modeled as a linear relation in CarSim. The nominal steering ratio was found to be The steering system parameters for kingpin angle, caster angle and lateral offset were based on measurements of the actual vehicle Tires: The CarSim tire look-up table feature was used to model the tire characteristics. Parameters provided in [7] were used to generate the necessary tire characteristic curves for various vertical load conditions. A generic number was used to specify the rolling 13

28 resistance of the tire in CarSim. The plots of tire lateral force vs. slip angle for different values of vertical tire load are shown in Figure 5. The tire was tested to lateral slip angles up to 28 degrees. This model was to be used to simulate vehicle behavior in extreme maneuvers wherein high values of slip angles were expected. Therefore, to represent saturation of the tires, the lateral force values were made constant beyond slip angles greater than 28 degrees. Figure 5. Tire Lateral Force versus Slip Angle Powertrain, Aerodynamics and Brake System: A generic powertrain model from CarSim, similar to the one found in vehicles of this size, was selected. The 2003 Ford Expedition model was created to evaluate the performance of the vehicle in mainly constant speed and coast down tests and hence the 14

29 generic powertrain model from CarSim was sufficient to give accurate results. The powertrain selected was a 5.0 L 236 KW engine with four wheel drive. A generic aerodynamic model (for a vehicle comparable in size to the Ford Expedition) was selected in CarSim. In this, the vehicle frontal area has been modeled as 2.4 m 2. A generic brake system model equipped with ABS was selected in CarSim. The ratios of brake torque to wheel cylinder pressure were then changed to match the braking performance measured during field tests. 2.2 Model Validation : The 2003 Ford Expedition model was validated using two different types of tests: Bounce and Roll tests in CarSim were used to validate the suspension and steering kinematics and compliances, and Sine with Dwell field tests were used to validate the entire vehicle model. The field tests were conducted by the National Highway Traffic Safety Administration s Vehicle Research and Test Center Quasi-static Bounce and Roll Tests: The bounce test is performed by giving vertical motion to the chassis. The camber angle, steer angle, suspension and tire vertical deflections are measured for different vertical loads. Overall, the comparisons for the front and rear bounce kinematics and compliances are found to be very good. The roll test is performed by giving roll motion to the chassis. The response of the CarSim model is compared to the experimental data using parameters like camber angle, 15

30 steer angle, roll moment and roll angle. The measured and simulated front and rear roll characteristics are shown to be in good agreement. Figures 6, 7, 8 and 9 show the comparison of measured data and simulation predictions for the bounce and roll tests, for the front and rear suspensions. Figure 6. Front Suspension Bounce Test Comparison 16

31 Figure 7. Rear Suspension Bounce Test Comparison 17

32 Figure 8. Front Suspension Roll Test Comparison 18

33 Figure 9. Rear Suspension Roll Test Comparison Sine With Dwell Test (Conducted with ESC OFF): The Sine with Dwell maneuver was chosen as it is the maneuver used in FMVSS (Federal Motor Vehicle Safety Standard) 126 regulation for testing the performance of ESC equipped vehicles. This maneuver uses a 0.7 Hz sinusoidal steering input with a dwell of 500 ms after the third quarter cycle. The vehicle is allowed to coast down with throttle off from a speed just above 80 kph and immediately when the speed reaches 80 kph, the steering maneuver is initiated. This maneuver and ESC system performance 19

34 evaluation will be discussed in detail in the next chapter. Figures show the comparison of simulation results and field test data with ESC OFF. In general, the simulation predictions match the measured vehicle response well. However, the simulation predicted roll angles are lower than the experimental data. This could be due to unmodeled compliances in the suspension system and phenomena such as deformation of the tire sidewall which would result in higher roll angles in the field test data. Figure 10. Sine with Dwell: Steering Wheel Angle 20

35 Figure 11. Sine with Dwell: Vehicle Speed Plot Figure 12. Sine with Dwell: Lateral Acceleration 21

36 Figure 13. Sine with Dwell: Yaw Rate Figure 14. Sine with Dwell: Roll Rate 22

37 Figure 15. Sine with Dwell: Roll Angle 2.3 Brake System Model Tuning: The VRTC test data contains runs of the Sine with Dwell maneuver made with the vehicle Electronic Stability Control system ON. In order to get comparable braking performance from the generic brake system chosen in CarSim, the wheel cylinder pressure values from the test data were used as inputs to the CarSim vehicle model. The ratios of brake torque to wheel cylinder pressure in the CarSim brake system model were then changed to approximately match the braking performance in field testing. These ratios were retained when designing the ESC System model. The comparison plots from an ESC run, using the measured brake pressures as inputs to CarSim, are as shown in Figures 16. These results confirm that the modeled brake pressures to brake-torque relationships are sound. 23

38 Figure 16. Comparison plots for the tuned brake system model in CarSim 2.4 Closure: The comparison plots in this chapter showed that the CarSim vehicle dynamics model of the 2003 Ford Expedition is representative of the actual vehicle. This validated model will be used in the subsequent chapters to build and tune a simple, functional ESC model. 24

39 CHAPTER 3: ELECTRONIC STABILITY CONTROL (ESC) BACKGROUND AND THEORY This chapter discusses the theory and background of ESC systems in passenger vehicles. The two main modes of ESC operation: Yaw Stability and Roll Stability are described in detail. The ESC system model built in this project is based on differential braking and this control strategy is explained in detail. This chapter also describes the tests that have been formulated to evaluate the performance of ESC-equipped vehicles. 3.1 History and Background: Since the introduction of automobiles, a lot of effort has gone into making braking systems reliable and easy to use (less driver effort through the use of boost mechanisms, etc). However, intelligent control of braking systems became possible only after digital microcontrollers became commercially available in the late 1970s [8]. Another major technological breakthrough was the development of the Controller Area Network (CAN) bus in the mid 1980s. These became commercially available in The CAN bus enabled different control modules, sensors and actuators in the vehicle to communicate with each other which in turn made it possible to integrate control systems. After this, the first development in the field of braking systems was that of the Antilock Braking System (ABS). ABS was first introduced in 1981 and in the years that have passed, there have been several refinements and improvements that have made this 25

40 technology smarter, cheaper and more reliable. ABS controls wheel slip and prevents the wheels from locking which enables the vehicle to maintain steerability during hard braking. Further, as ABS maintains the wheel slip at an optimum value (approximately 0.2), it maximizes the braking force available. As cars became faster and traffic on roads increased, so did the number of accidents. There was a need to provide additional safety features for occupants and safety features in automobiles became a major selling point for car companies. Companies began to look beyond passive safety and a lot of resources were invested in active safety systems systems which would help to prevent accidents. Most loss-of-control single vehicle accidents occur because the vehicle behaves very differently at limit conditions compared to its normal sub-limit behavior and the driver cannot take the necessary corrective actions. Hence, the goal of an automotive designer should be either to ensure that vehicle behavior at limit conditions does not deviate much from the sub-limit conditions, or to ensure that the vehicle-as far as possible- operates in its sub-limit region. Vehicle stability control systems have been developed to fulfill this goal. In 1995 two BMW models were the first vehicles to be factory fitted with Electronic Stability Control (ESC) Systems developed by Bosch. ESC is a generic name and these systems are known by various commercial names such as DSC-Dynamic Stability Control, VDC- Vehicle Dynamics Control, ESP- Electronic Stability Program, Stabilitrak, etc. An ESC system monitors the vehicle behavior using sensors that measure yaw rate, lateral acceleration, vehicle speed, steering angle, brake and acceleration pedal position, etc. A microcontroller estimates the intended heading and ideal motion of the vehicle and compares that with actual motion. If the difference between these motions 26

41 exceeds a certain threshold value then the controller generates a control signal which is sent to the actuators which in turn exert corrective forces on the vehicle. These corrective forces may be applied through the steering system (active steering control), through differential braking, through torque distribution and engine control (reducing throttle, delayed spark timing, etc). Figure 1 in Chapter 1 showed that steering induced moment decreases at higher slip angles and beyond a certain value (10-12 degrees on dry roads), it is almost zero. Thus at high slip angle values, the corrective forces need to be applied through a mechanism other than steering. This is normally done through differential braking which, in essence, is the individual control of the braking forces on all four wheels. 3.2 Differential Braking: Differential braking involves selectively applying brakes on individual wheels so that the required corrective torque acts on the vehicle. To understand this braking concept, we must first understand the concept of friction circle of a tire. The friction circle is an idealized concept used to explain tire longitudinal and lateral force relationships. The maximum frictional force available at a tire will always be equal to the normal load on the tire multiplied by the coefficient of friction for the tire-road surface combination. This friction force maybe used in the longitudinal (traction or braking) or lateral direction or a combination of the two. In figure 17, Fx is the braking force and Fy is the lateral force. The resultant of these forces is F R which is equal to the product of the normal force on the tire and coefficient of friction as described above. When Fx = 0, Fy = F R and conversely when Fy = 0, Fx = F R. Further, if Fx is increased by applying brakes, 27

42 the direction of the resultant force will change and lesser lateral force will be generated by the tire. Figure 18 explains this concept better. When no brakes have been applied (tire slip (λ) = 0) the lateral force (Fs) is equal to maximum frictional force F R. Figure 17. Friction Circle Figure 18. Differential Braking (Source: E.K Liebemann, et al, Robert Bosch GmbH, Safety and Performance enhancement: The Bosch Electronic Stability Control (ESP), SAE Paper No , 2004) 28

43 When a braking force F B is applied, the resultant force F R changes direction as shown in Figure 18. The moment arm of this force about the yaw axis of the vehicle also changes which in turn changes the yaw moment on the vehicle. Thus the yaw moment acting on the vehicle can be controlled by controlling the braking force on each tire. If the driver applies brakes to retain control of the vehicle, ESC can still modulate brake forces at different wheels to achieve differential braking. The concept of differential braking explained in this section will be used to explain the yaw and roll stability modes of ESC operation in the next section. 3.3 Yaw Stability Control: This mode of ESC is used for lateral (or directional) stability of the vehicle. When the vehicle is operating in sub-limit conditions (slip angles are very low) and has neutral steer, there is an approximately linear relationship between yaw rate and steering wheel angle given by: Yaw rate = (steering wheel angle) X (yaw rate gain). Further for a neutral steer vehicle with negligible slip angle, the yaw rate and lateral acceleration are related as follows: Yaw rate = (Lateral Acceleration) / (vehicle speed). Any of the above two relations can be used to find the ideal yaw rate of a vehicle under given conditions. The actual yaw rate is measured using the yaw rate sensor on board the vehicle. The difference between actual and ideal yaw rates (sometimes called vehicle slip rate ) is used as the control variable for yaw stability. (Vehicle slip rate) = (actual yaw rate) - (ideal yaw rate). If vehicle slip rate has a positive value i.e. the actual yaw rate is higher than ideal, then the vehicle is said to be oversteering. If vehicle slip rate is negative, the vehicle is said to be understeering. 29

44 During oversteer, the vehicle spins out due to excessive yaw while during understeer, the vehicle turns less than desired and is said to plough out. Though excessive understeer and oversteer are both undesirable, oversteer is considered to be more dangerous as regaining stability through steering intervention is very difficult during oversteer. Depending on the sign of vehicle slip rate (i.e. depending on whether the vehicle is understeering or oversteering) and the direction of turn, ESC selectively brakes individual wheels to generate a corrective yaw moment which maintains vehicle stability. (a) (b) Figure 19. (a) ESC intervention during understeer (b) ESC intervention during oversteer (Source : : FMVSS 126 ) Consider Figures 19 (a) and (b) in which a vehicle is shown making a left turn. If the vehicle turns less than intended by the driver (understeer), due to the front wheels losing traction, then ESC produces a correcting moment by braking the rear left wheel. If the 30

45 vehicle turns more than the driver intends (oversteer), then a corrective moment is applied by braking the right front wheel which helps the vehicle to stay on desired course. The magnitude of the braking force depends on the magnitude of the difference between the vehicle slip rate and the threshold value of the vehicle slip rate. 3.4 Roll Stability Control: This control mode is designed to reduce the possibility of a vehicle rollover. Rollover is the motion of a vehicle about its longitudinal axis causing it to tilt to one side or turn into one or more rolls. According to the National Accident Sampling System/Crashworthiness Data System (NASS-CDS) all crashes where a vehicle rolls at least 1/4 th turn are classified as rollover. Rollover crashes are dangerous events and they account for a high percentage of occupant fatalities in vehicle accidents. Figure 20. Lateral Forces Acting on a Vehicle during Rollover 31

46 Figure 20 shows the quasi-static rollover mechanics of a vehicle (i.e. we assume that the vehicle is in a steady turn so that there is no roll acceleration). M is mass of the vehicle; Fyi and Fyo are lateral forces on the inner and outer wheels respectively. Fzi and Fzo are the normal forces on the inner and outer wheels. The lateral acceleration (due to centripetal force) is given by a y. The tire lateral forces Fyi and Fyo balance the force M* a y in the lateral plane. Since the force (M* a y ) acts in a different plane from Fyi and Fyo, this creates a moment on the vehicle which might lead to rollover. From the friction circle theory, if brakes are applied then the lateral forces produced by tires decrease. They can no longer balance the force M* a y and the vehicle slides outward which increases the radius of turn. The braking force also reduces the speed of the vehicle. These two factors reduce the lateral acceleration (as a y = v 2 /r where v= vehicle speed and r = radius of turn) which prevents the vehicle from rolling over. It is desirable that the ESC intervention is minimal so that the driver remains in control most of the time. Braking all wheels would help but it would reduce speed excessively and this is not good from vehicle efficiency standpoint. Hence, roll stability control systems usually brake the outer front wheels to prevent vehicle rollovers. As the vehicle goes into a turn there is a lateral load transfer from the inner wheel to the outer wheel. Consequently lateral forces are higher at the outer wheels. Thus braking the outer wheel is more beneficial than braking inner wheels as a greater reduction in lateral force would be achieved. Further, braking only the front outer wheel ensures that the rear outer wheel produces sufficient lateral forces to prevent the vehicle from deviating from its path excessively. Roll stability systems use lateral acceleration as the control variable. 32

47 A threshold value of lateral acceleration is set and as the vehicle lateral acceleration approaches this value, the ESC system brakes the front outer wheel to prevent rollover. ESC systems on vehicles are integrated with the ABS system. If excessive wheel slip is detected then ABS is given priority over ESC as it is important to prevent wheel lock-up. 3.5 ESC Regulations: FMVSS 126 [3] Recognizing the potential of ESC in reducing single vehicle crashes, the National Highway Traffic Safety Administration has proposed a new Federal Motor Vehicle Safety Standard (FMVSS-126) which aims to reduce the number of accidents by regulating the implementation of ESC in vehicles. FMVSS 126 requires all vehicles with a Gross Vehicle Weight Rating (GVWR) of lbs (4,536kg) or less to be quipped with an ESC system by model year Definition: FMVSS 126 defines ESC as a system which augments vehicle stability by applying brakes on individual wheels to produce correcting yaw moments. (This standard does not require use of engine control for restoring stability.) The ESC system should measure the driver steering steering input, vehicle yaw rate and side slip and use a closed loop computer algorithm to limit understeer or oversteer of the vehicle. Functional requirements: The ESC system defined above must satisfy the following functional requirements : (a) The ESC system must have the means to apply all four brakes individually and a control algorithm that utilizes this capability. 33

48 (b) The ESC must be operational during all phases of driving including acceleration, coasting, and deceleration (including braking). (c) The ESC system must stay operational when the antilock brake system (ABS) or Traction Control is activated. Performance requirements: In addition to the above functional requirements, ESC must satisfy certain performance requirements for lateral stability and responsiveness. NHTSA has defined a dynamic test known as the Sine with Dwell to evaluate the performance of vehicles equipped with ESC [4]. This test is used to evaluate the performance of ESC in correcting oversteer of a vehicle. The test uses a 0.7 Hz single cycle steering input with a 500 ms pause (dwell) between the 3 rd and 4 th quarter of the cycle. The vehicle is accelerated to a speed of 52 mph and is then allowed to coast down to a speed of 50 mph. At this speed, a steering controller is used to give the above steering input. Since the steering input is always initiated at 50 mph, the severity of the test is increased by increasing the amplitude of the steering wheel angles. First, a slowly increasing steer maneuver is used to determine the steering angle (δ 0 ) required to produce a lateral acceleration of 0.3 g. The steering wheel angles used for the Sine with Dwell nominally begin at 1.5* δ 0 and are increased in steps of 0.5* δ 0 till the termination condition is reached or up to 6.5* δ 0 or 270 degrees (whichever is greater). An ESC equipped vehicle that meets the performance requirements at steering angle values of 6.5* δ 0 or 270 degrees (whichever is greater) passes the test. The tests are performed for left-right and right-left steering with ESC enabled. 34

49 Lateral Stability: The performance of the vehicle is assessed for lateral stability and responsiveness. NHTSA has proposed the controlled decay of vehicle yaw rate as the performance criteria to assess lateral stability of the vehicle (Refer Figure 21). The performance targets for a vehicle to pass this test are: yaw rate measured 1 sec after completion of the maneuver steering input first local yaw rate peak produced after 2nd steering reversal 0.35 yaw rate measured 1.75 sec after complet ion of the maneuver steering input first local yaw rate peak produced after 2nd steering reversal 0.20 Responsiveness: An ESC system helps the driver retain directional stability by using braking individual wheels which produces a correcting yaw moment. However, this benefit should not come at the expense of vehicle responsiveness which is critical in obstacle avoidance. For example, consider a situation wherein the driver does an extreme maneuver to avoid an obstacle. If the ESC, on detecting high yaw rates, produces a correcting moment, to keep the vehicle on course, the vehicle will collide with the obstacle. Hence the ESC should give the lateral stability benefits while ensuring a certain level of vehicle responsiveness. The lateral displacement of the vehicle (measured from the line corresponding to its initial heading) is used to assess a vehicle's responsiveness. FMVSS 126 requires that the vehicle must produce a lateral displacement of at least 6 ft (1.83 m) when measured at 1.07 seconds after the initiation of steering maneuver. (Figure 22) 35

50 Figure 21. Steering input and vehicle yaw rate used to assess lateral stability (Source: Forkenbrock G.J, Boyd P.L, NHTSA, Light Vehicle ESC Performance test development, ESV Paper Number , 2007 ) 36

51 Figure 22. Steering input and lateral displacement used to assess vehicle responsiveness (Source: Forkenbrock G.J, Boyd P.L, NHTSA, Light Vehicle ESC Performance test development, ESV Paper Number , 2007 ) 37

52 CHAPTER 4: ELECTRONIC STABILITY CONTROL (ESC) MODEL This chapter describes the construction and working of the Simulink ESC model developed for this thesis project. The first section discusses the complications in actual ESC systems onboard vehicles and the need to build a simplified model. The subsequent sections describe different subsystems and modes of operation of the ESC model. The last section describes the tuning parameters for this model as well as the tuning process. 4.1 Complexity of Actual ESC Systems: Commercial ESC systems on-board vehicles have very complicated, proprietary algorithms. The complexity can be attributed to the following main factors: (1) number of control variables and their accurate estimation, (2) number and type of systems used to generate control forces, and (3) the changes that occur in the plant (vehicle) over time, or due to changing operating conditions. Yaw stability and roll stability are the two main modes of operation of an ESC system. Yaw stability uses vehicle slip rate (difference in actual and ideal yaw rates) as the main control variable while roll stability uses lateral acceleration as the main control variable. However, for yaw stability, using only vehicle slip rate as the control variable might help to limit the difference between the actual and ideal yaw rates, but the vehicle could still heavily slip sideways [9]. 38

53 Hence, vehicle slip angle also needs to be used as a control variable. However, the vehicle slip angle cannot be measured directly and hence estimation algorithms are used to estimate its value. Further, to calculate vehicle slip rate, it is necessary that nominal behavior of the vehicle is estimated correctly. This is accomplished by taking in driver inputs like positions of brake and accelerator pedals, steering wheel angle and steering rate. The nominal vehicle behavior also depends on environmental conditions like tireroad coefficient of friction and the effect of these parameters has to be estimated correctly. This estimation is very important to realize the goal of all active safety systems - to ensure that the driver is in control most of the time and that the system intervention is smart and minimal. Typically Roll stability control systems use lateral acceleration as the main control variable as discussed in [10]. However, this control strategy might fail to interfere when rollover is partially caused due to vertical road inputs [11]. Hence a rollover index based on roll angle and roll rate of the vehicle has been discussed in [11]. Estimation algorithms are also required for calculating the rollover index of the vehicle. Thus, determining the parameters to be included in the main control function, estimating them if they cannot be measured directly, assigning weights to individual parameters and designing controllers for them makes an ESC system very complicated. The second factor listed above was the number and type of systems used to generate control forces. ESC systems may use one or a combination of various systems such as differential braking, engine throttle control, torque distribution control (for allwheel-drive vehicles), control of active suspensions, steering control, etc. to generate the control forces required to stabilize the vehicle. If many of these systems are used 39

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