Study On Differential Drive Coordinated Active Steering System Of Electric Vehicle With Independent-Wheel-Drive

In recent years,governments and auto giants have paid more and more attention to the development of electric vehicles(EV).Especially,the electric vehicles with independent-wheeldrive have broad prospects due to the unique advantage that each drive wheel torque is independently controllable.Differential drive assisted steering(DDAS)technology is a new type of power-assisted steering technology developed based on the characteristics of independently controllable drive torque of each driving wheel.The DDAS system applies different driving torques to the wheels on both sides of front axle to generate assisting torque to the steering system,which can not only improve steering portability,but also simplify the hardware structure of the steering system and improve system integration level.At the same time,active front steering(AFS)technology that can effectively improve low-speed steering flexibility and high-speed steering stability has also been more and more widely recognized.Therefore,this paper proposes a new type of differential drive coordinated active steering system of electric vehicle with independent-wheel-drive,aims to solve the problem of mutual interference between these two subsystems after adopting DDAS technology.This system can improve the comprehensive performance of the vehicle steering system,and provide an effective solution for the development of steering technology for electric vehicles with intelligent driving assistance systems.Firstly,this paper makes a theoretical analysis of the causes of mutual interference when the two subsystems work based on the formulation of the control strategy of the AFS system and the DDAS system.First of all,AFS system works by actively adjusting the steering angle of front wheels,and the change of steering angle will cause steering resistance torque to change.And the change of steering resistance torque will reflecte on the steering wheel through the mechanical steering system,leading to fluctuations of steering wheel torque.In addition,since the DDAS system provides steering assistance indirectly,and the control strategy of DDAS system is direct steering torque control strategy,and the intervention of AFS system does not cause the steering wheel angle to change.Therefore,when AFS system intervenes,the steering wheel torque will be the same as before,but the actual aligning torque has changed.This inconsistency easily leads to the driver's wrong perception of the driving state of the vehicle.More importantly,the working principle of DDAS system determines that it will generate a yaw moment when providing steering assistance.This yaw moment will necessarily affect the yaw movement of vehicle,and affect the change characteristics of ideal yaw angular velocity gain.At the same time,this yaw moment will also make the steering sensitivity of vehicle too large,especially when the speed is high,which will make the driver too nervous.These problems severely limit full use of the advantages of these two subsystems.After that,this paper develops the coordination control strategies to solve the above problems.Firstly,the steering wheel torque transient fluctuation suppression module is designed based on the steering resistance torque estimation and torque differential control,and the reference steering wheel torque correction module is designed based on the overlay angle of AFS system,thereby decreasing the AFS system intervention on the DDAS system.Then,based on the reference yaw rate calculated by the linear two-degree-of-freedom vehicle model,a yaw rate feedback control module was designed to reduce the influence of the DDAS system on the ideal steering characteristics of vehicle.Furthermore,considering that DDAS system will interfere the stability of vehicle when vehicle is driving on a low-adhesion road,but AFS system can only achieve a good stability control effect when the tire cornering characteristic in linear region.In non-linear region,the DYC system based on differential driving or braking can achieve a more ideal control effect.But DYC system has conflicts with DDAS system because it shares actuators with DDAS system.Therefore,this paper perfects the coordinated control strategy of DDAS and AFS,developed a more comprehensive layered control strategy considering the vehicle stability.According to the vehicle state parameters such as the sideslip angle,the work area of subsystems is divided into three parts: the stable area,the transition area,and the unstable area.The output of each subsystem is adjusted through the coordination controller.Specifically,when vehicle in stable area,AFS system and DDAS system work normally,and DYC system is enabled;when vehicle in unstable area,AFS system and DDAS system are enabled,and DYC system is started;To avoid sudden intervention or exit of subsystems,a transition area is set.In this area,the weight coefficient of each subsystem is calculated in real time by a linear function according to the vehicle's sideslip angle.When the vehicle state is closer to stable zone,the weight coefficients of AFS system and DDAS system are made close to 1,the weight coefficients of DYC system are made close to 0;otherwise,the weight coefficients of AFS system and DDAS system are made close to 0,and the control coefficient of DYC system is made close to 1.Finally,in order to verify the effectiveness of the control strategy designed in this paper more accurately,the simulation verification is carried out based on the MATLAB/Simulink and CarSim co-simulation platform in many typical driving conditions,such as double lane change maneuver.Simulation results show that when the vehicle is in a stable state,the control strategy can effectively weaken the mutual interference between AFS system and DDAS system,make the two subsystems work together effectively;when the vehicle is unstable,the control strategy can coordinate the output of each subsystem well according to the characteristics of each subsystem to ensure the stability of the vehicle first.