Motion Control Method For Thermal Protection Of Electric Vehicle In-wheel Motor

In recent years,with the country’s strong support for the research and development of new energy vehicles,all kinds of electric vehicles have gradually appeared in people’s sight.Compared with traditional internal combustion engine vehicles,electric vehicles have the advantages of environmental protection and fast response.Among electric vehicles,distributed electric vehicles have been widely studied due to their high degree of freedom and flexible control.Since its motors are distributed among the four hubs,each tire has individual steering and drive capabilities,resulting in a higher upper limit of control.Due to the small operating space of the in-wheel motor,when the motor works under high load for a long time,a large amount of heat will be generated inside the motor.Due to the limited heat dissipation performance of the in-wheel motor,various components inside the motor are easily accelerated due to high temperature.When the vehicle fails during driving,it will cause the vehicle to become unstable,and may cause a rollover in severe cases,resulting in casualties.Therefore,it is of great significance to study the overheating protection of the drive motor of distributed electric vehicles.In order to ensure that the in-wheel motor always runs at a safe temperature during the driving process of the vehicle,prolong the service life of the motor and ensure the safety of the driver,a research on the motor overheating protection is proposed.The main idea of the current research on vehicle motor thermal protection is:when the internal temperature of the motor exceeds the safe temperature threshold,the motor can be protected by reducing the maximum torque that the motor can output,thereby reducing the temperature.However,the change of the motor torque during the actual driving process of the vehicle will inevitably affect the driving stability of the vehicle.Aiming at the above problems,this paper proposes two motor thermal protection strategies,which are the fault-tolerant control strategy of motor thermal protection control based on sliding mode control and the fault prevention control strategy of motor thermal protection control based on model predictive control（MPC）.Firstly,a fault-tolerant control strategy of motor thermal protection control based on sliding mode control is designed to solve the problem that the existing thermal protection control algorithm does not consider the vehicle stability.The purpose of this method is to ensure that the instability of the vehicle body caused by the torque change is reduced after the thermal protection is triggered.The control system is designed using a layered control strategy.The upper layer obtains the desired vehicle state through a two-degree-of-freedom model,and the resultant force and moment are obtained through the Terminal sliding mode control algorithm according to the obtained state.The middle layer distributes the tire force by designing the optimization function,and considers the motor temperature constraint in the process of solving.The lower layer converts the resulting tire forces into control signals that actually control the vehicle.Finally,the stability problem of the vehicle after triggering the thermal protection is solved.Secondly,in order to prevent the vehicle from triggering thermal protection and at the same time improve the dynamic performance of the vehicle to the greatest extent,a fault prevention control strategy of motor thermal protection control based on MPC is proposed,which also uses a layered control strategy to design the control system.The upper layer obtains the desired driving state of the vehicle through the reference model,and the middle layer uses the MPC algorithm to design the controller.The vehicle dynamics model and the motor thermal model are considered in the design of the controller.The advantages of this design method are:It can predict the driving state of the vehicle and the temperature of the motor in the future time domain.When the temperature of the motor is about to reach the threshold temperature,it can control the temperature of the motor in advance through the optimization solution to keep the temperature at a level that does not rise.Avoid triggering motor thermal protection while minimizing vehicle power loss.The lower layer converts the obtained tire force into a control signal that actually controls the vehicle through the actuator control layer.Finally,the effectiveness of the proposed control method is verified by the cosimulation method of Matlab and Carsim,and the proposed control method is verified in the linear acceleration condition and the acceleration single lane change condition respectively.At the same time,the PID control is compared with the proposed control scheme,and the results show that the proposed control method can significantly improve the stability and dynamics of the vehicle.In addition,the above two methods combine the motor thermal protection control with the vertical and horizontal integrated stability of the whole vehicle,which solves the vehicle stability problem caused during the motor thermal protection control process.