| With the development of motor technology,distributed drive electric vehicles(DDEVs)have attracted wide attention due to their characteristics of fast phase response,high reliability,and flexible drive,at the same time,it promotes the development of lateral stability of vehicles.However,the vehicle is easy to be unstable because of the nonlinear dynamic characteristics of the tire under extreme conditions,which will threaten the safety of the driver and passengers.Therefore,it is very necessary to study the stability analysis and stability control of DDEVs under extreme conditions.With the development of autonomous driving technology,the study of stability as a single control objective no longer meets the actual needs.As another difficult problem under extreme conditions,path tracking and stability coordination control have gradually become the focus of research.Therefore,this paper takes DDEVs as the object,aiming at the problems of vehicle stability and path tracking accuracy under extreme conditions,the research on stability analysis and coordinated control of vehicles under extreme conditions is carried out.The main work includes:(1)Dynamics modeling of distributed drive electric vehicle.Based on the traditional vehicle model in Car Sim,we modified the vehicle and built the DDEVs wheel motor model in Matlab/Simulink,and jointly built the whole vehicle dynamics model through Car Sim and Matlab/Simulink,under high speed-high adhesion and high speed-low adhesion conditions,the validity of the built model is verified,which shows that it can provide a reliable platform for subsequent simulation verification.(2)Analysis and boundary design of vehicle stability region based on the phase plane.To analyze the evolution law of the vehicle stability state and design a suitable stability domain boundary,the phase plane theory was used to analyze the influence of vehicle speed,road adhesion coefficient,front-wheel steering angle on the vehicle stability region,and based on the traditional double-line method,further consider the yaw rate constraint and propose a multi-constrained the sideslip angle and yaw rate(β-ω)phase plane stability analysis method.Meanwhile,the relationship functions between the vehicle state parameters and the stability region boundary are established to achieve the adaptive adjustment of the vehicle stability region boundary according to the actual vehicle state.(3)Research on vehicle adaptive coordinated stability control based on stability margin.An adaptive coordinated stability control method based on phase plane stability margin was proposed to solve the stability control problem of DDEVs under extreme condition and to avoid the influence of subsystems on vehicle longitudinal velocity.According to the phase plane,the vehicle stability coordination region was divided and a stability margin index was defined.The lateral stability of the vehicle was further improved by the divisional coordination control of active front-wheel steering(AFS)and direct yaw moment control(DYC).An adaptive vehicle stability controller based on model prediction was designed,and the weight of the control variables of the MPC controller is adjusted online according to the stability margin to change the output of AFS and DYC.The co-simulation result of Car Sim and Matlab/Simulink shows that the proposed method can effectively guarantee the lateral stability of the vehicle in extreme condition,compared with the fixed weight MPC,the proposed method improves the sideslip angle by 3.82% and reduces the influence on the longitudinal velocity of the vehicle by 3.32%.(4)Path following and stability coordinated control based on stability margin.Aiming at the problem of vehicle path tracking and stability coordination control under extreme condition,a layered coordinated control method based on stability margin was proposed.The upper coordination controller established the vehicle path-following and stability coordination weight functions according to the vehicle stability margin and coordination region and determined the control objectives and weights at different stability levels.The middle-level model prediction controller adjusted the control output weight of the MPC controller online according to the stability margin to obtain the front wheel angle and yaw moment that meet the control requirements,and the lower-level moment distribution controller performs longitudinal force distribution based on the axle load ratio.The joint simulation analysis by Car Sim and Matlab/Simulink shows that,compared with the control method that only considers stability,the proposed coordinated control method can effectively improve the sideslip angle by 19.81%,the lateral displacement error is reduced by 52.13%.Therefore,the proposed path tracking and stability coordination control method based on stability margin can ensure the stability and path tracking accuracy of the vehicle under extreme condition. |
