Research On Autonomous Vehicle Trajectory Following Based On Active Four Wheels Steering

With the growing popularity of automotive in our daily life,how to keep the vehicle body in good gesture and improve the anti-slip ability and minimize the position error in trajectory following under extreme situation are of growing importance.In this paper,active four-wheel steering(4WS)will be applied to automotive trajectory following,and a controller will be designed to enhance the trajectory following ability and vehicle safety under extreme situation.Firstly,a 2DOF vehicle model and steering system model are built to describe vehicle motion and hysteresis characteristic of steering system.By using static steering experiment,the feasibility of 4WS control framework and the accuracy of steering system model are tested.Meanwhile,transform the yaw rate into the second-order system of front wheel angle to describe desired vehicle respond after applying 4WS,and using Bode and Nyquist diagram to figure our the needed parameters of the second-order system and its Stability.Considering the difficulty of getting real-time map information,cubic spline interpolation is used to get continuous trajectory.A MPC controller is designed as well to generate desired vehicle motion.From the simulation result of ISO double change scenario,the MPC controller can improve the trajectory following ability effectively.In order to follow the desired motion given by MPC controller,a LQR controller is designed which goal is to follow the desired yaw rate and vehicle slip angle to decide front and rear wheels steering angle.Considering the non-liner feature of tire lateral force,the weight matrix of the LQR controller will change according to the tire slip angle to assure vehicle maneuverability and safety in extreme situation.From the simulation result of slam and sine wave input,the LQR controller can follow the desired motion accurately.In addition,a look-up table is given to make sure the LQR controller can be employed in hardware in real-time.Meanwhile,fault tolerant strategies are designed to cope with the possible security problems when active rear steering is put into use,and the simulation result proves that these two strategies work well.A simulation model which combines Simulink and Carsim is built to test the control strategy and the simulation result shows that after applying 4WS to automotives,the tire slip angle decreases greatly so that the car becomes more stable.The side wind test shows that the 4WS controller also works well and avoid the unattended steering.Later,the hysteresis characteristic of steering system is put into consideration to find its influence on the accuracy during trajectory following.Lastly,offline vehicle experiment is performed to describe the effort of the trajectory following strategy.