Jae Y. Lew

Development and Experimental Evaluation of a Slip Angle Estimator for Vehicle Stability Control

Real-time knowledge of the slip angle in a vehicle is useful in many active vehicle safety applications, including yaw stability control, rollover prevention, and lane departure avoidance. Sensors to measure slip angle, including two-antenna GPS systems and optical sensors, are too expensive for ordinary automotive applications. This paper develops a real-time algorithm for estimation of slip angle using inexpensive sensors normally available for yaw stability control applications. The algorithm utilizes a combination of model-based estimation and kinematics-based estimation. Compared with previously published results on slip angle estimation, this present paper compensates for the presence of road bank angle and variations in tire-road characteristics. The developed algorithm is evaluated through experimental tests on a Volvo XC90 sport utility vehicle. Detailed experimental results show that the developed system can reliably estimate slip angle for a variety of test maneuvers.

Algorithms for Real-Time Estimation of Individual Wheel Tire-Road Friction Coefficients

It is well recognized in the automotive research community that knowledge of the real-time tire-road friction coefficient can be extremely valuable for active safety applications, including traction control, yaw stability control and rollover prevention. Previous research results in literature have focused on the estimation of average tire-road friction coefficient for the entire vehicle. This paper explores the development of algorithms for reliable estimation of independent friction coefficients at each individual wheel of the vehicle. Three different observers are developed for the estimation of slip ratios and longitudinal tire forces, based on the types of sensors available. After estimation of slip ratio and tire force, the friction coefficient is identified using a recursive least-squares parameter identification formulation. The observers include one that utilizes engine torque, brake torque, and GPS measurements, one that utilizes torque measurements and an accelerometer and one that utilizes GPS measurements and an accelerometer. The developed algorithms are first evaluated in simulation and then evaluated experimentally on a Volvo XC90 sport utility vehicle. Experimental results demonstrate the feasibility of estimating friction coefficients at the individual wheels reliably and quickly. The sensitivities of the observers to changes in vehicle parameters are evaluated and comparisons of robustness of the observers are provided.

Tire-Road Friction-Coefficient Estimation

Tire-road forces are crucial in vehicle dynamics and control because they are the only forces that a vehicle experiences from the ground. These forces significantly affect the lateral, longitudinal, yaw, and roll behavior of the vehicle. The maximum force that a tire can supply is determined by the maximum value of the tire-road friction coefficient for a given normal vertical load on the tire. For each tire, the normalized traction force p, alternatively called the coefficient of traction, is defined as VfI + F (1) where Fχ, Fψ and Fζ are the longitudinal, lateral, and normal, that is, vertical, forces acting on the tire. The objective of friction-coefficient estimation is to predict the maximum value of the normalized traction force p that each tire can provide. This value, which is called the tire-road friction coefficient μ, depends on the characteristics of the road surface. The value of μ varies between zero and one depending on the type of road surface under consideration, such as icy, snow covered, gravel, and dry asphalt.

Algorithms for real-time estimation of individual wheel tire-road friction coefficients

It is well recognized in the automotive research community that knowledge of the real-time tire-road friction coefficient can be extremely valuable for active safety applications, including traction control, yaw stability control and rollover prevention. Previous research results in literature have focused on the estimation of average tire-road friction coefficient for the entire vehicle. This paper explores the development of algorithms for reliable estimation of independent friction coefficients at each individual wheel of the vehicle. Three different observers are developed for the estimation of slip ratios and longitudinal tire forces, based on the types of sensors available. After estimation of slip ratio and tire force, the friction coefficient is identified using a recursive least-squares parameter identification formulation. The observers include one that utilizes engine torque, brake torque, and GPS measurements, one that utilizes torque measurements and an accelerometer and one that utilizes GPS measurements and an accelerometer. The developed algorithms are first evaluated in simulation and then evaluated experimentally on a Volvo XC90 sport utility vehicle. Experimental results demonstrate the feasibility of estimating friction coefficients at the individual wheels reliably and quickly. The sensitivities of the observers to changes in vehicle parameters are evaluated and comparisons of robustness of the observers are provided.

Development and experimental evaluation of a slip angle estimator for vehicle stability control

Real-time knowledge of the slip angle in a vehicle is useful in many active vehicle safety applications, including yaw stability control, rollover prevention and lane departure avoidance. Sensors to measure slip angle, including two antenna GPS systems and optical sensors are too expensive for ordinary automotive applications. This paper develops a realtime algorithm for estimation of slip angle using inexpensive sensors normally available for yaw stability control applications. Compared to previous results on slip angle estimation that have been published in literature, the algorithm utilizes a combination of model-based estimation and kinematics-based estimation and compensates for the presence of variations in tire-road characteristics. The developed algorithm is evaluated through experimental tests on a Volvo XC90 sport utility vehicle. Detailed experimental results show that the developed system can accurately estimate slip angle for a variety of test maneuvers.

Parameter and State Estimation in Vehicle Roll Dynamics

In active rollover prevention systems, a real-time rollover index, which indicates the likelihood of the vehicle to roll over, is used. This paper focuses on state and parameter estimation for reliable computation of the rollover index. Two key variables that are difficult to measure and play a critical role in the rollover index are found to be the roll angle and the height of the center of gravity of the vehicle. Algorithms are developed for real-time estimation of these variables. The algorithms investigated include a sensor fusion algorithm and a nonlinear dynamic observer. The sensor fusion algorithm requires a low-frequency tilt-angle sensor, whereas the dynamic observer utilizes only a lateral accelerometer and a gyroscope. The stability of the nonlinear observer is shown using Lyapunovs indirect method. The performance of the developed algorithms is investigated using simulations and experimental tests. Experimental data confirm that the developed algorithms perform reliably in a number of different maneuvers that include constant steering, ramp steering, double lane change, and sine with dwell steering tests.

On the Use of Torque-Biasing Systems for Electronic Stability Control: Limitations and Possibilities

This brief paper focuses on the concept of utilizing torque-biasing systems on a four-wheel drive vehicle for improving vehicle stability and handling performance. In contrast to brake-based yaw stability control systems, torque biasing has the potential to provide yaw stability control without slowing down the longitudinal response of the vehicle. An inexpensive system configuration is considered in which the driveline is based on front-wheel drive with on-demand transfer of torque to the rear. The torque-biasing components of the system are an electronically controlled center coupler and a rear electronically controlled limited slip differential. First, modeling of the torque-biasing devices is briefly introduced. Then, a hierarchical control architecture is presented in which an upper controller determines desired yaw moment for achieving yaw rate and slip angle control. The lower controller attempts to achieve the desired yaw moment using torque biasing. Theoretical analysis shows that transfer of longitudinal tire forces can effectively be used to achieve any desired yaw moment for the vehicle. However, the use of torque biasing cannot always achieve the desired transfer of longitudinal tire forces. Simulations show that the proposed control system can always effectively provide understeering yaw moments but can provide oversteering torque moments only during on-throttle maneuvers. Experimental data show that significant stability improvements are obtained using the proposed system for low-friction slalom maneuvers and a T-junction launch maneuver. The results presented in this brief shed important light on the possibilities and limitations of using torque biasing for vehicle yaw stability control

Active driveline torque-management systems

This article reviews various individual wheel torque control technologies for active automotive safety systems. The use of active drive torque-management for vehicle stability control is a relatively new technology that is still being developed by researchers and automotive manufacturers. Current approaches and effects in various types of devices and drivelines were discussed. Control strategies for cornering performance and vehicle yaw and roll stability related to active torque-management systems were investigated. Three major types of ADTM devices, namely, center couplings, ELSDs, and torque-vectoring systems, were described, and the models, control system design, and limitations of each were discussed.

Real-time estimation of roll angle and CG height for active rollover prevention applications

Roll angle and height of the center of gravity are important variables that play a critical role in the calculation of real-time rollover index for a vehicle. The rollover index predicts the real-time propensity for rollover and is used in activation of rollover prevention systems such as differential braking based stability control systems. Sensors to measure roll angle are expensive. Sensors to estimate the c.g. height of a vehicle do not exist. While the height of the center-of-gravity does not change in real-time, it does change with the number of passengers and loading of the vehicle. This paper focuses on algorithms to estimate roll angle and c.g. height. The algorithms investigated include a sensor fusion algorithm that utilizes a low frequency tilt angle sensor and a gyroscope and a dynamic observer that utilizes only a lateral accelerometer and a gyroscope. The performance of the developed algorithms is investigated using simulations and experimental tests. Experimental data confirm that the developed algorithms perform reliably in a number of different maneuvers that include constant steering, ramp steering, double lane change and sine with dwell steering tests.

On the use of torque-biasing devices for vehicle stability control

This paper focuses on the concept of utilizing torque-biasing systems on a four-wheel drive vehicle for improving vehicle stability and handling performance. In contrast to brake-based yaw stability control systems, torque biasing has the potential to provide yaw stability control without slowing down the longitudinal response of the vehicle. An inexpensive system configuration is considered in which the driveline is based on a front-wheel-drive system with on-demand transfer of torque to the rear. The torque biasing components of the system are an electronically controlled center coupler and a rear electronically controlled limited slip differential. A hierarchical control architecture is presented in which an upper controller determines desired yaw moment for achieving yaw rate and slip angle control. The lower controller attempts to achieve the desired yaw moment using torque biasing. Theoretical analysis shows that transfer of longitudinal tire forces can effectively be used to achieve any desired yaw moment for the vehicle. However, the use of torque biasing cannot always achieve the desired transfer of longitudinal tire forces. Simulations show that the proposed control system can always effectively provide under-steering yaw moments but can provide over-steering torque moments only during on-throttle maneuvers. The experimental data show a significant stability improvement for a low-friction slalom maneuver. The results presented in the paper shed important light on the possibilities and limitations of using torque biasing for vehicle yaw stability control