The lateral motion of vehicle is a phenomenon in which the tire side-slip angles of front and rear wheels are generated by driver’s steer input, and each lateral force is produced proportional to the tire side-slip angle. If the lateral force is saturated and started to decrease in certain conditions, the vehicle might get into an unstable motion called spin, which is dangerous. The vehicle stability control system is an idea that can enhance the vehicle stability and handling in emergency situations through the control of the braking forces at the individual wheels.
In this study, we propose a systematic controller design scheme of the vehicle stability control system. Main objective of this research is to develop a control algorithm that can be used in real applications. We propose the hierarchically structured controller consisted of the direct yaw moment controller for the lateral motion of vehicle, the braking force distribution scheme, and the wheel slip controller for the generation of the braking forces at each wheel based on multiple sliding mode control theory. The proposed direct yaw controller generates the desired yaw moment, the braking force distribution scheme decides the braking wheel and the desired slip at each wheel, and the wheel slip controller manages the hydraulic braking system to generate the desired braking force at each wheel.
The proposed direct yaw moment controller for the lateral motion brings the vehicle sideslip angle and the yaw rate close to the desired ones so that the vehicle dynamics becomes stable and the vehicle traces the desired course even in limit cornering. The braking force distribution shares the braking forces at each wheel based on the vehicle dynamics. A controller for the slip in each wheel follows the desired slip using the pulse width modulation so that the vehicle stability controller can intervene braking force at each wheel. To design the vehicle stability control system, we will analyze the multiple sliding mode controller, and propose the moving multiple sliding mode control methodology.
This control system requires the estimation of yaw rate, side slip angle, and road friction in order to control braking force at the individual wheels. The proposed observer consisted of the state observer for vehicle motion identification and the road condition estimator for the identification of the road friction coefficient. The state observer uses 2 degrees-of-freedom bicycle model with the Dugoff tire and estimates the system variables based on the Kalman filter. The road condition estimator uses the same vehicle model and identifies the tire-road friction based on the recursive least square method.
We show the validity and usefulness of the proposed controller scheme of both controller and observer under various road conditions through computer simulations of a fifteen degrees-of-freedom nonlinear vehicle model.