Research On Torque Coordination And Optimal Control Of Four Wheel Drive Electric Vehicle

New energy vehicles with pure motor-driven,as an important kind of transportation to approach a much more further achievement for society,has been widely concerned by the community.Four-wheel drive electric vehicle-a new type of electric vehicles,equipped with in-wheel motors on the four wheels,makes power output more stable and efficient,what‘s more,it also obtains the characteristics of compact structure,highly efficient transmission,easy to control and so on.Driving and braking independently provides more management space for the safety and energy saving of vehicles,which considering not only the safety and comfort,but also the energy efficiency and environmental protection,and obtaining a better performance in vehicle handling dynamics than the traditional drive mode.Hence,four-wheel drive electric vehicle has been considered as one of the main means of transportation for the future wisdom city of low-carbon society.However,the control technology development and application promotion of four-wheel drive electric vehicle are still facing great challenges.Due to the independent drive,brake,active steering and other actuators,four-wheel drive electric vehicle with redundant control degrees of freedom is a complex over-actuated control system.At the same time,the interaction of multi-degree of freedom distributed actuators forms a vertical influence on lateral and longitudinal complex coupling nonlinear dynamics vehicle system,which increasing the difficulty of the control system designing,optimization and implementation.In addition,the factors influencing the energy consumption in four-wheel drive electric vehicles are complicated,which brings more challenges to energy efficiency optimization and management.Therefore,how to design the multi-actuator coordination control strategy of distributed chassis system becomes the key point to meet the safety and economic requirements.The main purpose of this paper is to study the torque coordination control method of four wheel drive electric vehicle,aiming to solve the mutual coupling and constraint problem between drive,steering and other actuators in the over-actuated system of the distributed wire-controlled electric chassis,and to realize the multi-objective demands of”safety,power and energy saving” for the four-wheel drive electric vehicle.We present a torque coordination control strategy based on the predictive control method,which can effectively solve the optimization problem with multiple objectives and variables and efficiently handling the system constraints.To simulate the real vehicle dynamics characteristics,an electric vehicle model based on a commercial simulation software AMESim is developed,and the results verify the effectiveness of the proposed torque coordination control system under various different test conditions.And then a preliminary realtime implementation of the optimization control method is carried out through real-time hardware-in-the-loop experiment.In order to solve the safety problem of the vehicle drifting out of control in critical driving condition,a coordination control method of distributed electric drive system is proposed taking advantage of the redundant motor actuators.Firstly,the vehicle dynamics model with nonlinear tyre characteristics is established to describe the vehicle steering motion performance using the mechanism modeling method.Secondly,an integrated control strategy combining vehicle yaw stability and motor torque distribution is adopted to meet the dynamics coupling between vehicle and actuators.The MPC-based holistic optimization method of integrated system is proposed,meanwhile the vehicle state tracking and longitudinal slip ratio constraint penalty items are designed as the objective functions.Then,to deal with the unmeasurable problem of vehicle sideslip angle in a steady state,an extended Kalman filter state estimation method using nonlinear tyre force characteristics is presented.To facilitate the rapid realization of the vehicle control system,a hierarchical yaw stability control strategy is further designed.In the upper layer,the triple-step nonlinear yaw stability control method is proposed with the nonlinearity of the tyre lateral force.In the lower part,a torque dynamic allocation control method is presented to reduce the optimization dimension,which considers the dynamic characteristics of tyre longitudinal slip ratio and actuators' constraints.Finally,the offline simulation and real-time test are carried out to verify that the proposed control system can coordinate multiple actuators to improve the vehicle stability.On the basis of ensuring the safety of the vehicle,a torque optimization control method for energy management is designed to improve the economy of electric vehicle.Firstly,the energy dynamics model of electric vehicle is established by using the data and mechanism methods,which includes the energy efficiency of the motor and the slip loss characteristics of the tyre.Secondly,aiming at meeting the power demand and actuator saturation constraint,an energy-efficient optimization method based on model predictive control is proposed for the energy management,which considers the differences of slip and energy efficiency between the distributed motors.Then,to resolve the fast implementation problem of receding horizon optimization algorithm with multi-constraint and multi-variable in vehicle real-time system,a solution method combining analytical structure and numerical optimization is presented,using the linear equations as the conversion mode.This method can improve the computational efficiency of online optimization by reducing the iterative optimization calculations.To resolve the unmeasurable problem of the tyre slip ratio,a nonlinear observer considering the measured motor torque as the correction term is proposed.Finally,the offline simulation and real-time bench test are carried out to verify that the proposed control method can reduce the driving energy consumption,and improve the vehicle fuel economy.At the bottom executive control,in order to meet the frequent driving demand and economy saving demand of the vehicle control system,a motor torque optimization control method,which takes inverter switch as the core,is proposed based on the finite set predictive control for PMSM motor of vehicle bottom drive unit.Firstly,we analyze the characteristics of PMSM and inverter system with discrete switching input and continuous state and develop a hybrid system model to describe the dynamic characteristics.Secondly,considering the existing stator current safety constraint and the switch input finite selection constraint,the inverter switch is used as the optimization variable,an MPC-based controller is designed to achieve the accurate and fast torque response and enhance the efficiency of motor work area,and the objective functions consist of tracking the desired current change and reducing the driving energy consumption.To further solve the rapid solution problem of the mixed integer programming with long time domain,a solution method based on dynamic programming and the pruning function of the Lyapunov function stability criterion and current constraint is proposed to reduce the optimization burden and improve the computational efficiency of the driving torque optimization control.Finally,the validity of the proposed motor control method is verified by simulation,which lays the foundation for the further research of vehicle control system.This paper puts forward a clear demonstration of the proposed electric vehicle torque coordination control scheme,and a detailed derivation process of the control method.In order to verify the validity of the controller,the simulation and real-time test are carried out based on the AMESim simulation vehicle model and the d SPACE real-time hardware-in-the-loop experiment platform,then goes the analysis of relevant results.The results show that the method of torque coordination and optimization of the four-wheel drive electric vehicle proposed in this paper has achieved a definitely satisfactory effect.Further research of this paper includes the following aspects: Research on vehicle torque control method with good anti-vertical perturbation robust performance;Research on energy optimization strategy and braking energy recovery strategy considering real-time road and traffic information;Further research on the rapid optimization algorithm of the receding horizon optimization and coordination control to meet the real-time rapid control requirements of vehicle system.The experiment validation of the proposed controller needs to be further studied in the simulator and real four-wheel drive electric vehicle.