Path Tracking Control of Four-wheel Independent Steering Electric Vehicles Based on Optimal Control

The four-wheel independent steering electric vehicle (4WIS EV) with steer-by-wire (SBW) system features advanced low-speed maneuverability and high-speed stability, which is considered as an ideal autonomous vehicle. Path tracking is an essentail part of the intelligent transportation applications. For 4WIS EVs, the optimal control method is used in this pape to investigate the trajectory tracking issue. Firstly, a 2 degrees of freedom (DOF) vehicle dynamic model is established. The relation of the four wheel steering angles is derived based on the Ackermann geometry. The steering control rule is designed by minimizing the side slip angle. Based on the optimal control theory, linear quadratic regulator (LQR) is adopted to realize the optimal control of vehicle trajectory tracking. Using Carsim and Simulink, the co-simulation platform is built and simulation is conducted under different vehicle speeds and design parameters, which verifies that the designed LQR controller has good path tracking performance. To reduce the influence of the disturbance quantities on the control system, an improvement is applied to the controller and a state feedback-feedforward structure is formed. The simulation shows that the feedforward adjustment further reduces the tracking error. At last, the optimal controller is embedded in the vehicle control unit. Different sets of experiments with the double-lane-change and cosine reference trajectories are conducted. Experimental results show that the designed optimal control method can track the expected trajectories well and verify the stability performance of the algorithm.