Steering movement is the most basic movement of the vehicle, in the car driving process, the driver through the steering wheel has always been to control the direction of the car, in order to achieve their own driving intention. Four Wheel Steering (4WS) is an advanced vehicle control technique which can markedly improve vehicle steering characteristics. Compared with traditional front wheel steering vehicles, 4WS vehicles can steer the front wheels and the rear wheels individually for cornering, according to the vehicle motion states such as the information of vehicle speed, yaw velocity and lateral acceleration. Therefore, 4WS can enhance the handling stability and improve the active safety for vehicles.
Based on the theory of Vehicle Dynamics and Sliding Mode Control, this paper investigates the following issues, Firstly, a 2DOF 2WS vehicle model is built up by using the state-space equations, which will be used to compare the 4WS vehicle model containing vehicle lateral and yaw; Secondly, based on the 4WS vehicle model with nonlinear tire lateral force characteristics, the control algorithm is designed to use feed-forward plus feed-back control framework by following the reference model. And the simulation is processed in MATLAB/Simulink and CarSim to verify the control algorithm. By comparison and analysis of the simulation results, By following the reference model, the performances of the yaw velocity and lateral acceleration responses are largely different. When set the speed at 30 km/h, 50 km/h and 80 km/h in simulations, the traditional steering stability of 2WS vehicle is not more stable than the four-wheel steering vehicle at different speeds. Consequently, the use of sliding mode control can effectively improve the steering performance of the vehicle in the steering, a good way to track the target path, and 4WS car to improve the vehicle’s handling stability.