Research On Slip Rate Control Strategy For Electric Vehicles

With the aggravation of environmental and energy problems,the research on new energy vehicles is becoming more and more popular.Electric vehicles not only have high energy utilization efficiency,but also have the advantages of simple structure,high transmission efficiency and distributed control of power,which play an extremely important role in the research on vehicle stability control.In the research process of new energy vehicles,improving the safety performance of electric vehicles in the process of driving has attracted wide attention,especially the control of wheel slip rate in the braking process.The slip rate control first needs to be able to identify the wheel slip rate under different road surfaces,so it is necessary to obtain the slip rate by accurately estimating the speed.Taking four-wheel drive electric vehicle as the research object,this paper first studies the estimation method of vehicle speed,and then discusses the control problem of wheel slip rate.The vehicle speed estimation method studied in this paper is to consider the influence of tire variable parameters and road friction on the accuracy of speed estimation,combine the tire model with the longitudinal dynamics model,establish the nonlinear system state equation,and then use the extended kalman filter algorithm to estimate the longitudinal speed of the vehicle.With the estimated speed,the wheel slip rate can be further obtained.The slippage rate control strategy of electric vehicle is designed to provide a large longitudinal force to achieve good braking performance on the premise of ensuring safety.This paper mainly studies the following aspects:In this paper,a regenerative braking system model related to the energy utilization rate of electric vehicle is built in AMESim,and then the question of whether the motor braking torque can provide the total braking torque needed to ensure the vehicle braking is considered.An electric hydraulic pressing system which can provide the compensation braking torque is added,and the braking system model which can provide the compensation braking torque is built in Simulink.The simulation model of seven degrees of freedom is established,including the body,wheels,LuGre tire model with the parameters reflecting the road conditions,and the model was simplified without affecting the control accuracy,which provided a simulation test platform for the control of braking slip rate.Secondly,due to the estimation of longitudinal velocity of the vehicle is affected by various factors,it is difficult to obtain it directly.This paper considers the friction conditions of tire road,the variable parameters of tire and the noise of sensor,based on the LuGre tire friction model and longitudinal dynamics model,an extended kalman(EKF)estimator is designed to estimate the speed in real time.At last,the validity of the estimation method is proved by the simulation results of Simulink and CarSim.Finally,according to the estimated longitudinal velocity of the vehicle,the slip ratio can be calculated.The longitudinal force of tire can be obtained from LuGre tire friction model,which can effectively overcome the impact of transient friction characteristics in the process of braking.According to the built vehicle model,the controller is designed to control the slip rate near the expected value.In the distribution of braking torque,considering the restriction of the maximum output torque of the motor,the braking torque of the motor is taken as the basic braking torque,the hydraulic braking torque is taken as the compensation braking torque,and the hydraulic braking is added to meet the expected torque,so as to achieve the purpose of braking safety and improve the efficiency of energy recovery.The simulation results show that under different road friction conditions,the designed slip rate controller can make the actual wheel slip rate quickly track the expected value.With the transient friction characteristics of the tire obtained in the braking process,the slip rate also fluctuates to some extent.Therefore,the designed controller is adaptive to the road friction conditions and tire distribution characteristics,effectively ensuring the vehicle handling stability and active safety,and has strong robustness.