Research On Electronic Differential Control System Of Distributed In-Wheel Motor Electric Vehicle

Globally,one-fifth of energy-related carbon emission comes from road transportation.With the common goal of carbon neutrality around the middle of this century,all countries vigorously develop pure electric vehicles.Among them,the motor and wheel were integrated by electric vehicles with in-wheel motor to directly provide driving torque,which eliminated transmission devices such as the differential of traditional fuel vehicles,and greatly improved transmission efficiency.Because of the omission of the traditional mechanical differential,electronic differential between the driving wheels has become a key factor affecting the steering stability and driving safety during steering.Therefore,it is necessary to research on the electronic differential control system of electric vehicles with in-wheel motor.Only constraining the speed of driving wheels,and having no consider of dynamic performance,the electronic differential control strategies based on driving wheel speed control according to the Ackerman steering model are not suitable for high-speed conditions.In this thesis,an in-wheel motor electric vehicle with front-wheel steering and independent rear-wheel driving was as research object.Based on the torque control of the driving wheels and from the perspective of improving the steering stability,an electronic differential method based on the center of mass slip angle and yaw rate was proposed.The research content of this thesis is as follows.(1)The model of the in-wheel motor and its control system were built and their simulation analysis were performed.By comparing and studying the characteristics of various motors,it was determined that the permanent magnet brushless DC motor which is directly driven was as in-wheel motor of electric vehicle.Based on fuzzy PID control,the speed-current double closed-loop motor control system was designed,and its model was built for simulation analysis.The results show that the motor has fast speed and torque response,strong robustness and good control effect.(2)Vehicle dynamics and related models were built.On the basis of analyzing the vehicle system dynamics,an 8-degree-of-freedom vehicle model including longitudinal motion,lateral motion,yaw motion,roll motion and rotation of four wheels was established.Fully considering the tire characteristics in actual operation,the ‘magic formula’ tire model was established.As a reference,a linear two-degree-of-freedom vehicle model was established to provide the ideal centroid sideslip angle and yaw rate for the electronic differential control system.(3)Some key state parameters of vehicles were estimated.Considering that some key state parameters cannot be directly measured by sensors during vehicle operation,longitudinal velocity estimation based on driven wheel speed,front and rear axle cornering stiffness estimation based on linear 2-degree-of-freedom vehicle model,centroid slip angle estimation based on linear expansion state observer,and road adhesion coefficient estimation based on extended kalman filter algorithm were designed.(4)Two types of electronic differential controller were designed.Taking the actual centroid slip angle and yaw rate to track the ideal value obtained by linear two-degreeof-freedom in real time as the control objectives,based on sliding mode control and fuzzy control,two additional yaw moment solvers were respectively designed.According to the principle that the slip rate of driving wheels cannot be too large,and combined with the total expected driving torque,the additional yaw moment is reasonably distributed to driving wheels on both sides.Taking double line shift and sinusoidal signal as the input of front wheel angle,the vehicle accelerated with low and high initial speed respectively.The control effects of three strategies were verified,which distributed the total driving torque base on average,sliding mode control and fuzzy control.(5)Simulation results of electronic differential control system were analysised.The electronic differential control system based on sliding mode controll has the best effect in general,and maintains excellent differential performance under various working conditions.The electronic differential control system based on fuzzy controll has a certain attenuation in the high-speed driving condition.The electronic differential control system based on the average distribution of the total expected torque is not as good as that of the other two.The two electronic differential control systems designed in this thesis can both track the ideal centroid sideslip angle and yaw rate,improve vehicle handling stability and ensure driving safety while realizing differential driving.