| Due to the oil shock and environmental contamination,pure electric vehicles have turn into the emphasis of national development due to their clean characteristic and energy conservation,and took the opportunity to quickly overtake in this respect,becoming the world’s first-class and even the standard setter of the electric vehicle industry.In recent years,with the improvement of the intelligent level of automobiles,the four-wheel independent drive/brake chassis-by-wire has become the core executive assembly technology of intelligent electric vehicles.There is no mechanical connection between the drive/brake pedal and the drive/brake actuator,and the fourwheel drive/brake torque is independently controllable.By optimizing the distribution of the four-wheel driving/braking torques,not only the driving stability of the vehicle can be improved,but also the energy-saving effect of the excavated vehicle can be maximized,and the driving economy of the vehicle can be improved.However,only four-wheel friction braking force distribution or only single-axis electro-mechanical braking force distribution is considered currently.Moreover,there are few control strategies for the apportionment of the four-wheel braking force and electro-mechanical braking force together.Therefore,this paper adopts fourwheel independent drive/brake electric vehicle as the object of study,takes stability and energy efficiency as the control objectives,and studies the optimal control strategy of regenerative braking energy recovery under straight and turning conditions.By coordinating the distribution of regenerative braking and friction braking,the braking efficiency and energy recovery efficiency can be boosted on the precondition of guaranteeing the vehicle’s driving stability.First,the simulation model of four-wheel independent drive/brake EV is built,including the vehicle dynamics model with seven degrees of freedom,hub motor model,power battery model and electro-mechanical brake model,and EMB was modeled by bond graph,which is important to the establishment of the simulation platform.Secondly,the optimal control strategy of regenerative braking energy recovery under straight braking condition is proposed.Under the constraints of various actuators and ECE braking regulations,taking braking stability and minimum braking energy loss as control objectives,the genetic algorithm is used to optimize the torque distribution coefficient and electro-mechanical distribution coefficient of front and rear axles offline.Then,threedimensional MAP of three coefficients is obtained to allocate the electro-mechanical braking torque of four wheels in real time.The clamping force control strategy based on cascade PI control and clearance control of EMB is proposed.The controller and model are built in MATLAB/Simulink,and the response and follow-up are verified.Thirdly,the optimal control strategy of regenerative braking energy recovery under turning braking condition is proposed.According to the linear two degrees of freedom vehicle model,the sliding mode controller is studied to obtain the additional yaw moment in order to track the target yaw rate,and the ultimate longitudinal force provided by the ground during turning is calculated.Based on the above,the strategy selection module and two turning torque distribution control strategies are proposed,including the braking torque distribution strategy based on online optimization and braking torque distribution strategy under braking condition of extreme turning.In normal turning braking,the minimum brake energy loss and tire adhesion utilization rate are taken as optimization objectives,comprehensively consider the target total braking force and yaw moment command tracking error and the capacity constraints of each electromechanical actuator,and realize real-time torque distribution optimization Control;In extreme turning braking,in order to give priority to the stability of the driving direction,target total braking torque and yaw moment and tire adhesion utilization rate are taken as control objectives to optimize the torque distribution.In order to meet the requirements of different driving conditions,a fuzzy controller is designed to dynamically adjust the weight value of tire adhesion utilization rate in real time.Finally,the co-simulation platform of four-wheel independent drive/brake electric vehicle is built based on MATLAB/Simulink and Car Sim,the stability and economic effect of the optimal control strategy of regenerative braking energy recovery under straight and turning condition are verified by simulation.The straight result shows that,in addition to the low average braking stability rate,compared with I-curve and rule-based distribution,the energy recovery rate of GA offline optimization distribution can effectively improve 0～4.2% and 1.32%～17.9% under three typical braking intensities of high,medium,and low;respectively.Under NEDC and LA92 cycle condition,compared with rule-based distribution,the energy recovery rate of GA offline optimization distribution can improve 1.1% and 5.4%,respectively.The turning result shows that,compared with fixed proportional distribution,the braking energy recovery rate of the proposed electromechanical braking force distribution strategy using the SQP algorithm has the obvious improvement of more than 10% under different braking intensities,different initial vehicle speeds and different radius,it also has better driving stability under extreme cornering braking conditions. |
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