Robust control with decoupling performance for steering and traction of 4WS vehicles under velocity-varying motion

This paper mainly studies robust steering and traction of four-wheel steering (4WS) vehicles with varying velocity, mass, moment of inertia, and road-tire interaction. To this end, a nonlinear vehicle model is developed which both takes the acceleration and braking effects on the system dynamics into account and avoids using the complicated side force relation. Based on this model, a nonlinear input-output decoupling controller is first designed, by which the vehicle system can be decoupled into three two-order subsystems, i.e., the velocity subsystem controlled by the longitudinal acceleration/braking force, the lateral motion subsystem by the front steering angle, and the yaw motion subsystem by the rear steering angle, so that the load of the driver can be relieved. Especially, a new decoupling condition is derived. It is proved that a vehicle with the front wheel braking or rear wheel drive can always be decoupled provided it does not accelerate so fast or brake so hard that its front or rear wheels are lifted from the ground. Furthermore, in order to reduce the effects of the vehicle parameter variations on steering performance, a robust control scheme is proposed. The corresponding controller and observer gains can be obtained by solving two new Riccati algebraic equations. Meanwhile, by properly choosing the form of the observer, the robust controller does not destroy the decoupling structure of the longitudinal and yaw motions. The numerical simulation shows that the robust control with decoupling performance can improve safety and comfort of the vehicle driving.