Improvement of high speed 4-WS vehicle handling performance by sliding mode control

This paper presents a new sliding mode control of four wheel steering of an automobile for handling improvement. The sliding mode four wheel steering (SM4WS) not only improves the directional stability and controllability but also provides good disturbance rejection for unexpected side wind. The SM4WS makes vehicle response faster than conventional 2WS without suspension and tire modification. The linear three-degree-of freedom vehicle handling model is used to investigate the vehicle handling performance. In simulation of the J-turn, the yaw rate overshoot reduction of a typical mid-size car improved by 40% compared to a 2WS case. Although the lateral deviation of SM4WS is almost the same as that of 2WS the yaw rate reduction was approximately 20% to that of 2WS system. The simulation of the J-turn maneuver shows that the proposed scheme gives faster yaw rate response and smaller side slip angle than the 2WS case. When the rear tire pressure is lower than normal condition, the car becomes less understeer and is worse in directional controllability. But, SM4WS is insensitive to such parameter variations because of the robustness properties of the sliding mode control.