Research On Coordinated Control Of In-wheel Motor Drive Autonomous Vehicles For Trajectory Tracking

Autonomous vehicle is an important part of future intelligent transportation system,and trajectory tracking control is a necessary condition to realize it.Currently,the tracking accuracy and the stability of vehicles in the process of trajectory tracking have received extensive attention.Electric vehicles are the best platform to realize autonomous driving technology,among which in-wheel motor drive electric vehicles can achieve lots of dynamics control,which makes it has the potential to improve the trajectory tracking effects.Aiming at solving the trajectory tracking problem,this paper makes use of the advantages of in-wheel motor drive autonomous vehicles in differential steering and yaw moment control,and conducts coordinated steering and drive trajectory tracking control for autonomous vehicles to improve the control effect.Firstly,the research of trajectory tracking control under autonomous steering with front wheels is carried out.The vehicle dynamics model,the kinematics model and tire model are analyzed.Based on this,the model predictive controller(MPC)is designed,where the objective function and the constraint boundary are set,and the front wheel steering angle used to track the reference trajectory is figured out by quadratic programming.Because it is difficult to guarantee the tracking accuracy and the driving stability of the vehicles under different driving conditions with a fixed preview time,the preview time adaptive control of the MPC is conducted.Considering the curvature of the reference trajectory,prediction horizons are adjusted based on a empirical formula.At the same time,the fuzzy control method is used to adjust the prediction steps based on the trajectory deviation and vehicle speed,thereby achieving preview time adaptive control.A joint simulation platform of AMESim and Simulink is established,which is used to verify the trajectory tracking control strategy.The simulation results show that the MPC controller with adaptive preview time can ensure the trajectory tracking accuracy and stability of autonomous vehicles under different driving conditions.Secondly,based on the autonomous steering,the research of trajectory trackingcontrol under coordinated steering control is carried out.Considering the characteristics of differential steering of in-wheel motor drive autonomous vehicles,the trajectory deviation and heading angle deviation are taken as the control targets,and the torques of the left and right in-wheel motors of the front wheels are taken as the output,and the PID control is used to design the differential steering controller.Also,the joint simulation platform is used to analyze and verify the effectiveness of the differential steering controller under different driving conditions.Then the characteristics of these two steering modes are analyzed,and the weight coefficients which are adaptively adjusted according to the road attachment coefficients,vehicle speeds,and heading angles of the reference trajectory to meet requirements of different working conditions are set to combine them.Comparison simulations of trajectory tracking with autonomous steering and coordinated steering control are performed under different driving conditions,which shows that coordinated steering control can improve the steering flexibility of in-wheel motor drive autonomous vehicles,thereby improving the trajectory tracking accuracy.The lifting effect is more obvious under the conditions of medium and low speed and high attachment roads,and the RMS value of the track deviation is reduced by about 10%.In addition,In order to improve the stability and trajectory tracking accuracy of in-wheel motor drive autonomous vehicles under medium or high speed and low attachment road conditions,based on the autonomous steering,the research of trajectory tracking control under coordinated drive control is carried out.The functions of yaw moment in reducing trajectory deviation and improving vehicle stability are analyzed.Based on this,the deviation compensation model and stability control model are established.The stability of the vehicle is taken into consideration to set weight coefficients,and the yaw moment used for deviation compensation and stability control is figured out through sliding mode control.Finally,the calculated yaw moment is converted into the increments of driving torques of the left and right in-wheel motors,deviation compensation and stability control are achieved through the drive control of in-wheel motors.Simulation analysis of trajectory tracking under autonomous steering,stability control and coordinated drive control under differentdriving conditions are performed.The results show that autonomous vehicles under coordinated drive control can improve the accuracy of trajectory tracking and the stability of the vehicle,the RMS value of trajectory deviation is reduced by about 5%,and the RMS value of the side slip angle is reduced by about 22%,which can obtain the best comprehensive trajectory tracking performance.Finally,in order to verify the effectiveness and reliability of the proposed coordinated steering and drive trajectory tracking control strategy,Hardware-in-the-Loop(HiL)tests are performed.The vehicle model,in-wheel motor model,trajectory tracking control strategies and in-wheel motor control strategy are established.Based on the two sets of VCU and MCU HiL platforms,a in-wheel motor drive autonomous vehicle co-simulation test system is built.HiL test shows that the proposed trajectory tracking control strategies can work in a real controller,where the coordinated steering control can not only improves the steering flexibility of the vehicle,but also improves the accuracy of trajectory tracking;The proposed coordinated drive control can comprehensively improve the trajectory tracking accuracy and the driving stability of the in-wheel motor drive autonomous vehicles.