Research On Path Tracking Algorithm For Four-Wheel-Independent-Drive Autonomous Electric Vehicle

As the direction of the development of automobile industry,electric drive and intelli-gent has become the research hot topic of scholars,institutes and enterprises at home and abroad.Electric vehicles can not only reduce human consumption of non-renewable resources and improve the environment,but also bring better NVH quality which is difficult for traditional fuel vehicles to achieve.Four-wheel-independent-drive is a unique driving form of electric vehicle.For the power system is directly integrated into the wheel,the drive torque and speed of each wheel can be controlled accurately and independently.This structure lays a foundation for the realization of advanced control algorithm.Autonomous vehicle is the advanced stage of vehicle intelligence and the key technology to realize "zero death" in traffic accident.Trajectory tracking is the basic requirement of realizing autonomous driving.In this paper,the FWID-EV is taken as an object to study the trajectory tracking control strategy of autonomous vehicles,which not only meets the requirements of tracking desired trajectory accurately,but also meets the requirements of improving stability at high speed and low adhesion coefficient road.Firstly,the simulation platform of trajectory tracking control for FWID autonomous vehicle is built by Matlab/Simulink and Car Sim.According to the structure characteristics of in-wheel motor,the power system and chassis system of Car Sim model are modified.A simple speed control strategy and torque allocation method are designed,what’s more the reliability of the model is verified by the open-loop rotation angle simulation experiment.Secondly,in order to meet the requirements of stability at high speed and low adhesion coefficient road,a sliding mode control(SMC)algorithm based on tracking desired yaw rate and a longitudinal speed control strategy based on fuzzy adaptive PI are designed.The equivalent control term is designed based on the SMC and 3-DOF vehicle model.In order to reduce buffeting,the hyperbolic tangent function is used instead of the discontinuous symbol function when designed the switching robust control.In order to consider the influence of velocity change on trajectory tracking accuracy and improve the stability and robustness of longitudinal velocity control,the velocity error and velocity error change rate are taken as the input of the fuzzy controller,and the parameters of PI controller are adjusted on-line by fuzzy Inference,then the stability longitudinal speed is guaranteed.The torque allocation algorithm based on pseudo-inverse method is designed with tire utilization ratio as the optimization Function.Thirdly,the MPC optimization algorithm for active front wheel steering is established.In order to simplify the vehicle model and reduce the difficulty of control,a 2-DOF vehicle model which joint vehicle kinematics and lateral dynamics is established based on small angle hypothesis and linear tire model.When designing the optimization function,the tracking accuracy is regarded as the most basic objective.Then,the control increment constraint is added to the optimization problem for improving the ride comfort.In order to make the yaw velocity represent the stability of the vehicle,the constraint of the vehicle slip angle deflection angle is added to the optimization solution.The optimal problem is transformed into quadratic programming,and the control quantity of front wheel angle is obtained by solving quadratic programming with effective set.Then,a trajectory tracking hierarchical strategy is designed.The upper controller is used to solve the front wheel angle,the middle layer is used to track the yaw rate,and the lower layer is used to control the longitudinal velocity and solve the torque assignment problem.Finally,the simulation verification of multi-track,multi-speed and multi-road adhesion coefficient is carried out based on the simulation platform established in this paper.The results show that the strategy established in this paper can achieve the stable tracking ability of the target trajectory at high speed and low adhesion coefficient road.