Study On Stability Control Methods Of Four-wheel-steer/drive Vehicles

Four-wheel steer/drive electric vehicle has a significant advantage over centrally driven vehicles in vehicle stability control because of its independently controllable corner torque.This thesis takes four-wheel-steer/drive electric vehicles as the research object,and studies the wheel steering-torque control method based on lateral and yaw stability.The main research contents are as follows:(1)Dynamic simulation model of Four-wheel steer/drive electric vehicle including the vehicle body,wheels,tires is built.The validity of the model is verified by simulation,which laid the foundation for the subsequent research on wheel steering-torque control method;(2)In order to fully combine the advantages of four-wheel steering and four-wheel torque control in vehicle stability control,and consider the nonlinear characteristics of the tire at the same time,a linear time-varying(LTV)control model is established.Based on LTV model,rear steering angle and additional yaw moment are decoupled based on the LQR algorithm.The acquisition of the optimal weight matrix in LQR control is discussed using genetic algorithms and fuzzy logic.The simulation results show that the linear time-varying optimal control(LTVLQR)method has better performance when the tire is in the nonlinear region,and under a given step input of the front steering angle,compared with the control method based on linearized tire model,the maximum lateral speed is decreased by 45.8%,and tracking effect of vehicle yaw rate is increased by 6.47%.(3)LTV-LQR control method is affected by the weight matrix and the real-time performance,a control method composited of feedforward additional rear wheel steering angle aimed to eliminate the vehicle slip angle and direct yaw moment integrated(ARS+DYC)is proposed.Considering the tire is in a strong non-linear region under instability conditions,a method for estimating the axle cornering stiffness is proposed,which can well represent the nonlinearity of the tire and achieve decoupling from the slip angle.The simulation results show that,under the given increasing-sine angle?step front wheel steering angle and single line changing conditions,the ARS+DYC method can make the vehicle slip angle and yaw rate back to normal quickly under the extreme conditions which can effectively avoid sideslip and severe spin.(4)In order to verify the control effect of the two kinds of control methods in the real-car simulation experiment,Carsim-Simulink co-simulation environment is established,and the control methods are analyzed under the conditions of high-speed sharp turn,bisectional road,and high-speed double line changing.The simulation test results show that: LTV-LQR and ARS+DYC can achieve the optimization of vehicle dynamics.The output of LTV-LQR control variable is less than ASR+DYC,which has less interference to the driver,and ARS+DYC directly targets the vehicle slip angle and the yaw rate,the control effect is more excellent under extreme conditions.The verification of the effects of the two methods laid the foundation for further design of the electronic stability controller for Four-wheel steer/drive electric vehicles under full operating conditions.