OPTIMAL CONTROL STRATEGY FOR LOW SPEED AND HIGH SPEED FOUR-WHEEL-ACTIVE STEERING VEHICLE

Abstract

In this work, based on the optimal control theory approach, a four-wheel-active steering (4WAS) system is proposed for low speed and high speed applications. A model following the control structure is adopted consisting of a feed-forward and feedback compensation strategy that serves as correction inputs to enhance the vehicle's dynamic behavior. The velocity dependent feed-forward control inputs are based on the driver's steering intention while the feedback control inputs are based on the vehicle's state feedback errors, being the sideslip and yaw rate of the vehicle. Numerical simulations are conducted using the Matlab/Simulink platform to evaluate the control system's performance. The performance of the 4WAS controller is tested in two designated open loop tests, being the constant steer and the lane change maneuver, to evaluate its effectiveness. A comparison with conventional passive front-wheel-steering (FWS) and conventional four-wheel-steering (4WS) systems shows the preeminent result performance of the proposed control strategy in terms of the response tracking capability and versatility of the controller to adapt to the system's speed environment. In high speed maneuvers, the improvement in terms of yaw rate tracking error in rms is evaluated and the proposed active steering system considerably beat the other two structures with 0.2% normalized error compared to the desired yaw rate response. Meanwhile, in low speed, turning radius reductions of 25% and 50% with respect to the capability of normal or typical FWS vehicles are successfully achieved.