Trajectory Following Control At The Limit Of Handling Of Electric Vehicle With Independent-wheel-drive

In recent years,the research and development of intelligent electric vehicles is a global response to deal with energy problems and improve driving experience,which is the future development direction of the automotive industry and market.Among them,the structure of EV with independent-wheel-drive is simple,the distribution form is more flexible,and the more important thing is that the torque of each wheel is controlled independently.Based on this advantage,it is easier to realize the torque vectoring(TV)function and realize intelligent driving assistance technology.At the same time,the trend of diversified development of vehicles is emerging.At present,in the field of electric vehicles,high-performance electric vehicles and sports driving mode are widely concerned.When the vehicle meets the fierce driving demand,it is very easy to approach the limit of handling,especially when cornering,it is easy to show poor track following and even instability.However,the electric vehicle with independent-wheel-drive can take advantage of torque vectoring,make full use of the ground adhesion ability,actively adjust the yaw characteristics to improve the maneuverability,so as to improve the trajectory following ability under the limit of handling.At the same time,the tire sideslip characteristics will change dramatically under the limit conditions,and its nonlinear characteristics will have a certain impact on the vehicle control under the limit conditions.Therefore,in view of the above existing problems,this paper takes the electric vehicle with independent-wheel-drive which can realize active steering as the research platform,proposes a set of transverse and longitudinal comprehensive control strategy which combines the torque vectoring technology and the sideslip characteristic update,aiming at the actively approaching the limit speed to meet the fierce driving demand and maintaining a good trajectory following ability under the limit conditions.In this paper,the effect of additional yaw moment caused by TV on steady-state steering characteristics is analyzed theoretically by using the 2-DOF linear vehicle model,and the mechanism of improving steering maneuverability is obtained.Then,the 7-DOF vehicle model,motor model and tire model of electric wheel vehicle are built,and the correctness is verified by comparing with Car Sim software.Based on the 7-DOF model,the effect of TV on the improvement of mobility is verified by simulation.Then,through the analysis of typical conditions,the parameters of U-turn condition are determined.At the same time,in order to meet the demand of the shortest cornering time for the fierce driving,the trajectory is planned based on the Clothoid curve,and the map of the limit speed is developed based on the trajectory and the tire adhesion limit circle.A feedforward-feedback longitudinal controller considering the load transfer of the front and rear axles is designed,and the vehicle speed tracking effect is verified through the 7-DOF vehicle model.After that,in order to improve the vehicle’s trajectory following ability at the limit of handling,this paper designs a lateral and longitudinal comprehensive hierarchical control strategy which combines the TV technology and the updating of sideslip characteristic parameters.Specifically,the lateral controller for trajectory tracking is designed based on model predictive control(MPC)algorithm.The predictive model is a 2-DOF vehicle model with TV.The optimal front wheel angle control sequence and torque vectoring gradient control sequence are solved by quadratic programming.In order to improve the adaptability to the strong nonlinear characteristics of the tire under the limit conditions,this paper designs the strategy of identifying the cornering stiffness and adaptively updating the tire sideslip angle constraint,which is used to update the relevant parameters of the prediction model in real time and upgrade the control accuracy.Based on the 7-DOF model,the feasibility verification and parameter adjustment of MPC controller,lateral force identification and cornering stiffness / lateral force intercept estimation modules are completed.In addition,the paper has developed the bottom driving control strategy,which realizes the slip ratio control and between-wheeltorque correction function respectively.The longitudinal control demand is combined with the additional yaw moment demand,and the longitudinal speed tracking and trajectory following effect are guaranteed.Finally,in order to verify the feasibility of the lateral and longitudinal comprehensive control strategy designed in this paper,based on the MATLAB / Simulink and Car Sim cosimulation platform,the test verification is carried out to explore the speed tracking and trajectory following effect and vehicle posture control effect under at the limit of handling.Meanwhile,a compact driving simulator is established to conduct the driver-in-loop trajectory following experiment,and a co-simulation platform is built to complete the MPC based active steering trajectory tracking experiment.Compared with the experimental results of the lateral and longitudinal comprehensive control strategy,the analysis shows that the comprehensive control strategy designed in this paper can optimize the front wheel angle output,accurately complete the updating of sideslip characteristic parameters,significantly improve the trajectory following effect,shorten the cornering time,and give full play to the advantages of independent torque control of electric wheel vehicle,so as to improve the limit handling ability.The trajectory following control method designed in this paper can provide a reference for the development of driverless and ADAS technology of electric vehicles with independentwheel-drive in the fierce driving mode,improve the extreme handling ability,and provide certain technical support for the personalized,diversified and intelligent development of highperformance electric vehicles.