Sliding Mode Control for All-Wheel Steering of Four-Axle Vehicle

The four-wheel steering (4WS) has been studied for a long time. And many control algorithms, such as proportional control, optimal control, sliding-mode control and H2/H∞ control, have been applied to it. However, few works expand those control algorithms to four-axle vehicle. The traditional four-axle vehicle can be steerable in the first and the second axle, which is called double-front-axle-steering (DFAS) vehicle. By adding Electro-hydraulic Power Steering System to the third and the forth axles, the DFAS vehicle becomes an all-wheel steering (AWS) vehicle. Some control algorithms could apply to it to control the steering angles of the third and the forth axle like the application to 4WS vehicle to control the rear wheel steer. In fact, due to the large size and high center of mass, reducing steer radius and enhancing stability through all-wheel steering is more important than the application in two-axle vehicle. In the paper, a sliding mode controller (SMC) to control the steering angles of the third axle and forth axle is proposed for a four-axle vehicle to improve handling and stability. In order to design the SMC, a linear all-wheel steering model of four-axle vehicle is established firstly, which considers the tire cornering stiffness perturbation and the crosswind disturbance. The yaw rate and sideslip angle are considered as two important state variables for this model. Then a reference model which contains an ideal yaw rate and zero-sideslip angle is developed. Finally, the switching function of the SMC is chosen based on the error of state variable between the all-wheel steering model and the reference model. The SMC aims to make the all-wheel steering model track the reference model through controlling the steering angles of the third and the fourth axle. Unfortunately, in this control strategy, adverse-phase steering appears between the third and the fourth axle, causing serious tire wear problem. So a modified sliding mode controller is given, which is a compromise between controller performance and tire wear. In order to investigate the effect of the modified controller, two typical tests, double lane change test and crosswind disturbance test, are carried out through numerical simulation. The controller built in Matlab/Simulink, and a high precision four-axle vehicle mode developed in TruckSim make two tests easily to accomplish co-simulation of driver-controller-vehicle close-loop system. The simulation results show that the modified SMC has robustness to vehicle parameter perturbation and insensitivity to crosswind disturbance. Moreover, all-wheel steering four-axle vehicle has better handling performance and stability compared with traditional DFAS vehicle.