Research On Four-wheel Differential Steering Control Of In-wheel Motor-driven Electric Vehicles

In-wheel motor-driven electric vehicles have the advantages of high energy transmission efficiency,fast and accurate acquisition of driving information,and simple transmission structure,which provide a new platform for dynamic control of automobile systems.Since the torque of inwheel motor can be controlled independently,it is convenient to realize differential steering by controlling the driving torques of wheels on the both sides of the same axis.Therefore,the emergence of in-wheel motor-driven electric vehicle provides the possibility for four-wheel differential steering mechanism.At present,the differential steering system can achieve three functions,that is,sliding steering that does not need side-turning of wheels,differential assisted steering to reduce the driver’s load,and differential steering as a fault-tolerant mechanism to deal with the failure of steer-by-wire system.In this paper,the in-wheel motor-driven electric vehicle is taken as the research object,and the differential steering system is taken as the only steering source,the feasibility of four-wheel differential steering was studied.The main work contents and conclusions of this paper are as follows:(1)A simulation model of in-wheel motor-driven electric vehicle is established by combining Carsim and Matlab/Simulink.Firstly,the vehicle model is selected in Carsim,and the basic parameters such as vehicle mass,wheelbase and centroid height are extracted;Then,according to the research needs,the in-wheel motor is selected and checked,the in-wheel motor model and the driver speed control model are established in Matlab/Simulink,and these two models are connected to the Carsim vehicle model through the S-Function interface,so as to complete the establishment of the in-wheel motor-driven electric vehicle model.Finally,a reference model is established,and the validity of the built vehicle model is verified by simulation.The results show that the response of the simulation model is consistent with that of the reference model,which indicates that the established vehicle model is effective and the extracted vehicle parameters are reasonable,laying a foundation for the subsequent co-simulation of four-wheel differential steering vehicles.(2)A dynamic model of a four-wheel differential steering vehicle is established.Firstly,the dynamic model of the differential steering system is established according to the principle of differential steering.Then,the semi-empirical model of magic tire is used as the tire model,and the nonlinear tire model was established.Finally,the dynamic model of four-wheel differential steering vehicle is established.In addition,the dynamic model of four-wheel steering vehicle is established to prepare for the control research of four-wheel differential steering.(3)The control of four-wheel steering vehicle is studied to obtain the ideal front and rear wheel angles.Firstly,the dynamic coupling relationship of vehicle is introduced.Then,decouple the sideslip angle and yaw rate of the four-wheel steering vehicle to solve the coupling problem of these two variables.Then,an asymptotic tracking controller is designed for the decoupled fourwheel steering vehicle model,so that the side slip angle and yaw rate of the center of mass could track the ideal values of the reference model respectively.Finally,the steering wheel angle step input and double-line shifting conditions are simulated in Matlab to verify the effectiveness of the controller.The research results show that the designed controller can ensure that the four-wheel steering vehicle has the same steering characteristics as the reference model.At the same time,the ideal front and rear wheel angles of the four-wheel steering vehicle are obtained,which lays the foundation for the distribution of the differential torque between the axles of the four-wheel differential steering vehicle.(4)Design control strategy to study the feasibility of four-wheel differential steering.Firstly,a sliding mode controller with yaw rate as the control objective is designed to calculate the total differential torque required when the four-wheel differential steering vehicle turns.Then,according to the ideal front and rear wheel angle relationship of the four-wheel steering vehicle,the torque divider is designed to distribute the total differential torque to the front and rear axles.Finally,Matlab is used to carry out steering wheel angle step simulation and double line shifting simulation to verify the effectiveness of the control strategy.The research results show that the designed control strategy can ensure that the four-wheel differential steering vehicle to realize the steering function.Considering that the vehicle itself is a nonlinear system,the four-wheel differential steering control simulation of the in-wheel motor-driven electric vehicle is carried out by combining Carsim and Matlab.and the simulation results are compared with those of the in-wheel motor-driven electric vehicle with only front wheel differential steering,the results show that fourwheel differential steering can not only effectively reduce the vehicle’s center of mass slip angle,but also has superiority in path tracking.