Research On Torque Distribution And Yaw Stability Control For Four-wheel Drive Electric Vehicle

The four-wheel drive electric vehicle(FWEV)with in-wheel motors has been widely recognized and developed in recent years.It has shown great advantages in short drive chain,compact structure and fast generated torque.Meanwhile,the control for longitudinal and lateral dynamics of FWEV is faced with technological changes and new challenges.This paper focuses on the key problems of torque planning for longitudinal dynamic control and yaw stability for lateral dynamic control in application.Firstly,a distribution method for torque of FWEV is introduced to take into accounts both driving performance and energy saving optimization.Secondly,a robust invariant set control method is proposed to prevent over-steer and improve yaw stability performance.Finally,a model-based design method for desired yaw rate is employed to avoid reduction of yaw stability control performance resulting from an improper desired yaw rate.The work of this paper is based on requirements of application,and the proposed control method has practical value.In this paper,the torque planning and distribution are studied firstly for the longitudinal motion control,which form the basis of drive control of FWEV.The task of torque planning is to compute the total driving torque of motors based on the commands of driver(i.e.braking,accelerating,etc.),vehicle status(i.e.motor speed,driving torque,etc.)and the status of power system(i.e.battery temperature,state of charge(SOC),motor temperature,etc.).In order to make the structure of torque planning clear,hierarchical planning method is adopted in this paper.The input information of torque planning is decomposed into three parts: driver's commands,vehicle's status and constraints.Based on that,the torque is planned to satisfy both requirements of driver's commands and vehicle's constraints.For the non-uniqueness problem of torque distribution,a distribution method based on optimal adhesion rate is presented to provide optimal driving performance for FWEV.However,the distribution method may reduce energy efficiency.Therefore,a regulation method based on optimal driving efficiency is further addressed to save energy.The current research on yaw stability control of FWEV concentrates on approach design of yaw rate tracking control.Therefore,there is a lack of research on under-steer and over-steer during yaw stability control.This paper presents a single track model of FWEV with uncertainties and analyzes the mechanism of over-steer according to the lateral dynamics model and the nonlinear saturation characteristic of tire force.An over-steer identification method based on the tire slip angle of rear axle is proposed.Thus,the problem of preventing over-steer is transformed into yaw stability control with state constraint.In order to improve the dynamic and steadystate performance of yaw rate tracking control and avoid conservation resulting from state constraint control,the yaw stability control is further decomposed into unconstrained and constrained control problems in this paper.When the state constraint does not satisfy,the unconstrained yaw rate tracking control is applied to enhance the yaw response performance.Once the constraint satisfies,a robust invariant set yaw rate control is proposed to keep the state in a robust invariant set to prevent over-steer.A novel yaw stability control strategy to prevent over-steer for FWEV is introduced in this paper.The simulation results show the effectiveness of the proposed method for enhancing yaw rate response performance and preventing over-steer.The design of desired yaw rate is a key technical problem faced by yaw stability control of FWEV.Currently,the steady-state value of the single track vehicle model is generally taken as desired yaw rate.In addition,a bounded yaw rate is calculated directly according to road adhesion coefficient and vehicle speed.The design method mentioned above is difficult to apply in practice because of its poor applicability.This paper analyzes the reason for the poor applicability and puts forward an approach via calculating steady-state of the single track vehicle model modified by the Dugoff tire model.And the design of desired yaw rate is transformed into a nonlinear optimization problem.Genetic algorithm is adopted to solve nonlinear optimization problem in this paper.To solve the problem of online calculation in practical application,this paper presents a method of off-line optimization and MAP online calculation.The simulation results show the proposed method has a wider scope of application,and the method in literature is a special case of this method.In this paper,several key technologies for yaw stability control of FWEV are studied.This research is motivated by application requirements and focuses on challenging problems faced by yaw stability control of FWEV.A torque distribution method considering both driving performance and energy efficiency is presented to form the basis of drive control.A yaw stability control approach for preventing over-steer and a design method for desired yaw rate are proposed from a new and applicable point of view.The method of this paper is to supplement and expand existing methods,and it is meaningful for the application of yaw stability control of FWEV.