| With the aggravation of environmental protection and energy problems,electric vehicles(EV)are facing significant development opportunities.Compared with the traditional centralized electric vehicle,the wheel torque of electric vehicle driven by in-wheel motor can be controlled independently,which has the advantage of distributed drive.Differential drive assist steering(DDAS)is a new technology of power steering based on the unique advantage of electric vehicle driven by in-wheel motor.It uses the difference of torque between the left and right front wheels to realize power steering,eliminating the power output components of the traditional power steering system.At the same time,the controller can be integrated into the vehicle controller,which has compact structure,small space occupation,low cost.So it has broad research prospects.But the actuator of DDAS is a left and right front wheel motor.It interferes with each other as part of the actuator of the Vehicle Stability Control(VSC)system.At the same time,it will produce an additional yaw moment for the vehicle,which will undoubtedly affect the stability of the vehicle.Aiming at the existing problems,this paper designs a global hierarchical coordinated control system based on phase plane theory between DDAS and VSC.This paper can be divided into four parts as follows:1.Firstly,based on the MATLAB/Simulink,this paper builds the dynamic model of electric vehicle driven by in-wheel motor,including four-DOF mechanical steering system model,seven-DOF vehicle model,tire model,motor model and driver model,and compares it with CarSim software to verify its correctness,which lays the foundation for the follow-up research and simulation.2.Based on the deficiencies of the existing DDAS control strategies,this paper introduces the Active Disturbances Rejection Control(ADRC)technology to design the controller of the DDAS,and designs the differential active return controller and the switching rules of the return mode.The simulation results show that the ADRC controller has better control effect than the PID controller.Then,the influence of DDAS on vehicle stability is systematically analyzed.3.It is found that the influence of DDAS on vehicle stability varies with different road adhesion conditions.Considering that the phase plane can accurately judge the vehicle stability,the phase plane theory is introduced to coordinate control.Firstly,a non-linear two-DOF vehicle model is established,and theb-bandb-w_r phase planes under various working conditions are drawn.Compared with other factors,the stability boundary of theb-bphase plane is less affected by other factors.Therefore,theb-bphase plane is chosen as the basis of coordinated control,and the boundary parameter table of the stability region about the road adhesion coefficient is worked out.Then a hierarchical coordinated control system of DDAS and VSC is designed based onb-bphase plane.The system is divided into four layers:the parameter estimation layer,the control region division layer,the coordination decision layer and the control allocation layer.The parameter estimation layer estimates the side slip angle and the road adhesion coefficient;the control region division layer divides the corresponding control area based on theb-bphase plane;the coordination decision layer includes the DDAS controller and the VSC controller as well as their coordination controller;the control allocation layer consists of the torque allocation controller and the slip rate controller.Firstly,in order to ensure vehicle stability,a VSC controller based on feedforward and sliding mode feedback is designed.Then,based on the characteristics of the phase plane and the analysis of the influence of DDAS on vehicle stability,theb-bphase plane is divided into three control areas:stable area,coordinated control area and unstable area.At the same time,considering the different road adhesion conditions,DDAS has different effects on vehicle stability.Fuzzy control and offline optimization by simulated annealing algorithm are used to solve the boundary parameters of control area in high adhesion and low adhesion respectively.Then,in summary,the coordinated control strategies of sub-regions under high and low attachment were formulated respectively.The control of the other two areas is the same as that of the high adhesion area.Then,based on the different control regions,the control strategy of sub-region torque allocation is designed.Finally,a slip ratio control controller based on the optimal slip ratio identification is designed to control the slip instability of the bottom wheels.4.In order to better verify the coordinated control strategy proposed in this paper,based on CarSim and MATLAB/Simulink,a joint simulation platform of electric vehicle driven by in-wheel motor,and the coordinated control strategy designed in this paper is simulated and verified under several typical working conditions.At the same time,a small driving simulator is built to complete driver in-the-loop test verification.The simulation and test results show that the coordinated control strategy designed in this paper can give full play to the advantages of DDAS in improving steering portability and turning maneuverability,and extend the working range of DDAS on the premise of ensuring the stability of the vehicle,thus ensuring the stable and reliable driving of the electric vehicle equipped with DDAS. |
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