| With the increasingly serious problems of energy security,environmental pollution and climate change,the power system is accelerating its transformation to a clean and low-carbon direction with pure electric drive as the main line.With the advantages of four-wheel independent steering,high power transmission efficiency and high potential to improve the economy,stability and safety of the whole vehicle,four-wheel hub drive has become an important research direction of pure electric vehicle power system.As the core research topic of four-wheel hub drive electric vehicle,torque optimal distribution method is very important to improve the stability and economy of the whole vehicle,so it has attracted the attention of all walks of life.In the process of engineering realization of torque optimal distribution method,there are some key control technologies to be solved urgently,such as longitudinal drive force distribution considering economy and stability,four-wheel hub drive force distribution based on torque vector control,and four-wheel hub drive anti-skid control under complex working conditions,Therefore,it is significant to carry out the theoretical research and engineering implementation of torque optimal distribution method of four-wheel hub drive electric vehicle with production vehicle application ability.This research relies on the"Research on distributed drive control technology of pure electric vehicles"of China FAW science and technology research project,aiming at exploring advanced control algorithms with the application ability of mass production models,aiming at many power system control technical problems encountered in the engineering implementation of torque optimal distribution method,A hierarchical structure of torque optimal distribution method for four-wheel hub drive electric vehicle is established:the upper control realizes the optimization algorithm and switching logic of longitudinal driving force distribution considering economy and stability,so as to ensure the smooth switching of longitudinal driving force in the transition process;The real-time optimization of additional yaw moment is realized by model predictive control with multi constraints and multi objectives;The torque vector distribution of wheel hub driving torque is realized based on the optimal tire adhesion ratio;The lower control ensures that the wheels are in the best sliding state through the four-wheel hub drive anti-skid control based on complex sliding conditions;The evaluation method of torque optimal distribution control is established.Combined with simulation and real vehicle verification,the effectiveness of the torque optimization method proposed in this paper is proved.In the research,a set of design scheme of four-wheel hub drive electric vehicle with satisfactory functions and up to standard performance is formulated,and a real vehicle verification platform of four-wheel hub drive electric vehicle is established.According to the real vehicle data,the dynamic simulation model of four-wheel hub drive electric vehicle is built and verified.The main research contents and conclusions are as follows:1.The hierarchical structure of torque optimal distribution method for four-wheel hub drive electric vehicle is established and the integrated control of torque distribution is realized.Realize the longitudinal driving force distribution control algorithm in the upper control:use the mass production on-board sensing information to complete the development of algorithms such as longitudinal speed estimation integrating wheel slip rate control,road adhesion coefficient estimation based on fuzzy logic and road slope estimation,so as to provide accurate input for longitudinal driving force distribution.From the point of view of improving the economy of the whole vehicle,the objective function and constraints of the efficiency optimization of the drive system are established;On this basis,by establishing the optimization process of longitudinal driving force distribution coefficient covering all working conditions,the optimal efficiency distribution coefficient of ideal driving system is calculated;By introducing tolerance constraints,the fluctuation of ideal optimal distribution coefficient is solved,the longitudinal driving force distribution based on economy is realized,and the average power consumption of the whole vehicle is reduced.From the perspective of improving the stability of the whole vehicle,based on the optimal utilization of longitudinal adhesion coefficient,an optimal control algorithm for the utilization of longitudinal adhesion coefficient is established.From the perspective of economy and stability,the switching logic of the two allocation algorithms is studied,and the distribution weight of longitudinal driving force is designed by using fuzzy logic to realize the smooth switching of longitudinal driving force in the transition process.NEDC simulation results show that the longitudinal driving force distribution algorithm effectively reduces the average power consumption of the whole vehicle,and the average power consumption is reduced by 2.64%compared with the two-wheel drive control mode(front axle follow-up and rear axle drive).The simulation verification of medium and low additional speed shows that the control algorithm effectively delays the time when the vehicle enters the excessive slip state by 0.45s,and improves the acceleration value before the intervention of driving anti-skid function by 26%,so the straight-line driving stability of the whole vehicle is improved..2.The four-wheel hub driving force distribution control algorithm based on torque vector control is realized:the real-time estimation of centroid sideslip angle is realized by using the sensor signal configured by mass production vehicle and extended Kalman filter.In the process of target setting of torque vector control:the phase plane analysis method is used to determine the stability boundary of centroid sideslip angle;In the process of setting the target yaw rate,a critical yaw rate compensation control method considering vehicle steering hysteresis and road adhesion constraints is proposed,and a real-time and reliable yaw rate stability boundary is established.The real-time optimization of additional yaw moment is realized by model predictive control with multi constraints and multi objectives.On the basis of minimizing the value of additional yaw moment,the actual yaw rate and centroid deflection angle of the vehicle are controlled to follow the target value;By solving the equivalent quadratic programming problem with constraints,the optimal additional yaw moment is obtained.Based on the optimal tire adhesion rate,the objective function of the optimal distribution problem of four-wheel hub driving force is established.Combined with the constraints such as longitudinal driving force constraint,additional yaw moment constraint and tire friction circle constraint,the optimal distribution problem of four-wheel hub driving force is transformed into a quadratic programming problem;In view of the situation that there may be no feasible solution under special working conditions such as lateral force saturation and driving anti-skid function intervention,the relevant constraints are relaxed to realize the driving force distribution of four-wheel hub covering all working conditions.The application of the algorithm shortens the response time of vehicle yaw rate under angular step condition by 0.027s;The maximum lateral acceleration under steady-state rotation condition is increased by 0.35 m/s~2,and the understeer characteristic is still low in the range of large lateral acceleration;The passing speed under high attached hunting condition is increased by 8.98%,which effectively reduces the insufficient and excessive steering degree of the vehicle and improves the handling stability of the vehicle.3.In the lower level control,the anti-skid control algorithm of four-wheel hub drive based on complex working conditions is realized:firstly,a calculation method of anti-skid control target of four-wheel hub drive is proposed:in case of non all wheel excessive slip,taking the optimization of tire longitudinal driving force as the target,the basic target slip rate is used to improve the low attached traction capacity;In case of excessive wheel slip,the target slip rate compensation strategy focusing on front wheel traction control and rear wheel stability control is formulated,and the rear wheel gradually transitions to the target slip rate meeting the Lyapunov stability conditions with the increase of vehicle speed.Secondly,the design,parameter setting and false trigger mechanism of slip rate controller are completed.Finally,the coordination strategy of target slip rate compensation and four-wheel hub driving force suitable for complex slip conditions is formulated.The simulation results show that under the uniform low additional speed condition,the algorithm realizes the rapid and stable convergence of four-wheel slip rate,improves the starting acceleration ability of the vehicle,and the vehicle has good lateral stability and straight-line driving ability;Under the condition of opposite acceleration,the wheel slip rate on the skidding side can converge quickly and stably,and the vehicle has good acceleration ability;In the low attachment and high attachment docking acceleration condition,the algorithm can quickly identify the road attachment jump and control the vehicle to quickly return to high attachment acceleration level.4.Combined with industry standards and enterprise product development experience,the evaluation method of torque optimal distribution control is established.The dry tile pavement shows that the longitudinal driving force distribution algorithm proposed in this paper can effectively effectively improves the linear driving stability,increases the acceleration of the vehicle by 1m/s~2,and delays the intervention time of the driving anti-skid function by 0.35 s.The real vehicle acceleration tests on uniform low attachment Road,split road and butt road show that the four-wheel hub drive anti-skid control algorithm proposed in this paper effectively inhibits the excessive wheel slip on the low attachment Road,the slip rate converges quickly and stably,and improves the traction and straight-line driving stability of the whole vehicle;Compared with the drive anti-skid control function in an electronic stability system,the control algorithm has a certain improvement in slip rate,stability control time,starting acceleration ability and lateral stability.The real vehicle test shows that the four-wheel hub driving force distribution control function proposed in this paper improves the Steering transient response performance,shortens the vehicle yaw rate response time of 0.025s under the steering wheel angle step condition,and reduces the yaw rate overshoot.The steady-state response of steering is improved:the maximum lateral acceleration under steady-state rotation condition is increased by 0.2 m/s~2,and the difference between insufficient steering degree and front and rear axle side deflection angle under large lateral acceleration condition is effectively reduced.Improved excessive steering stability:effectively avoid the rear axle side slip and tail flick under sinusoidal stagnation conditions.The stability of line shifting is improved:the passing speed under the condition of high attachment and double line shifting is increased by 6.7%;In the 55km/h test,the steering wheel angle operation of the driver is reduced by 45%and the speed loss is reduced by 7.2%.The continuous cornering stability is improved:the passing speed under serpentine condition is increased by 15%;In the 60km/h test,the driver's steering wheel operation is reduced by 30.3%.;Compared with a certain type of electronic stability system,the four-wheel hub driving force distribution control algorithm proposed in this paper has a certain improvement in reducing the loss of vehicle speed,reducing the amount of driver steering wheel angle operation and improving the understeer/oversteer of the vehicle;In the snake condition,the control algorithm proposed in this paper can not only realize the anti instability control function,but also improve the stable driving boundary of the vehicle. |
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