Research On The Dynamics And Control Strategy Of In-wheel-motor Driving Vehicle’s Steering

The four wheels of electric vehicle with in-wheel motors are directly driven by the four in-wheel motors respectively,of which the motion states are mutually independent.Therefore,it’s one of the most promising driving types for its high efficiency,and attracts wide concern of major countries,companies and institutes.Given that,this thesis is focused on the steering control of four in-wheel-motor driving vehicle,and the kinematics and dynamics of the designed vehicle during the steering are analyzed.The steering control strategy of coordination with speed and torque is proposed,and its effectiveness is validated through simulation test in Simulink platform.Firstly,the configuration of the four in-wheel-motor driving vehicle is determined by comparison of different driving types.The electric power steering based on rack and pinion is applied in the front steering operating mechanism,and the auxiliary differential system with coordination of speed and driving torque is adopted for the rear axle.Based on the designed vehicle,its kinematics and dynamics analysis are carried out.The constraint relations of the four wheels and in-wheel motors in kinematics with steady-state side-slip angle of the vehicle are derived,as well as the vehicle model with three degrees of freedom which includes yaw,roll and lateral vehicle motion.Given the fact that tire is the only mechanism that interact with the ground,the analysis of the four tires’ forces during the vehicle’s motion are carried out,and the non-linear mechanical model of tire is established with the Uni-Tire theory.Next,the permanent magnet synchrony motor is preferable motor type for the inwheel motor.Its mathematical model and control principle are described in order to establish the models of in-wheel motor and its Direct Torque Control method.As for the control algorithm of rotational speed and torque,PI controller is adopted to regulate the motor’s speed and hysteresis comparator is adopted to regulate the motor’s torque.Thus,the promptness and accuracy of speed’s and torque’s regulation can be assured.The influence of the motor’s dynamic characteristics may have on the vehicle during its steering can be simulated with certain accuracy.Then,based on comprehensive comparison of the existing steering control methods,a steering control strategy of coordination with speed and torque for four inwheel-motor driving vehicle is proposed.As for the motor and wheel’s rotational speed control,artificial neural network algorithm is adopted to obtain the target speed values of four wheels.As for the torque control,sliding mode control algorithm is adopted to regulate each motor’s output torques based on the vehicle’s target steady-state yaw velocity and wheel’s target slip ratio,between which the former one is enforced to realize the additional yaw-moment to regulate vehicle’s motion state,i.e.stability control,and the latter one is enforced to eliminate the fluctuation of the wheels’ target rotational speed,i.e.torque distribution control based on wheel’s slip ratio.When the stability control intervenes,the torque distribution control will quit to avoid the conflict.The target values of speeds and torques will then be input into the motors to respond and interact with the ground to realize the steering control of the vehicle.Finally,the Simulink models of four in-wheel-motor driving vehicle and its steering control strategy are established based on aforementioned theoretical analysis.The effectiveness of the control strategy is tested in three different driving conditions,and the results show that the rotational speeds and torques of in-wheel motors can be regulated quickly by the steering control systems to ensure the lateral stability of vehicle,which is beneficial to the handling stability.Therefore,the effectiveness of the presented steering control system is validated,which provides a feasible method for inwheel-motor driving vehicle’s steering control and will help promote the commercializing development of in-wheel-motor vehicles and the improvement of new energy automotive industry in China.