Research On Torque Vector Control And Stability Of Four-wheel Drive Electric Vehicles

With the support of electric vehicle in various countries,electric vehicles have ushered in unprecedented opportunities for development.Due to the diversification of electric vehicle power system,more and more drive technologies are being applied to electric vehicles.Among them,the drive torque directional distribution technology is a hot spot in the current drive technology research.The torque vectoring control can realize the directional distribution of the driving torque,which enables each driving wheel to make full use of the road adhesion and reduce the occurrence of wheel slip;at the same time,the body posture of the vehicle can be adjusted through the differential torque between the wheels,to improve vehicle stability of vehicle.Moreover,the torque distribution technology avoids the disadvantage of low power density of the distributed drive,and has the advantage that the distributed drive can realize the directional distribution of the driving torque,which is an important direction of the four-wheel drive technology electric vehicles.Therefore,it is great significance to carry out theoretical and engineering practice research on the directional distribution of torque in four-wheel drive vehicles.Based on the drive system configuration proposed in this paper,the torque distribution characteristics of the electronically controlled limited-slip differential are studied,and then the influence of torque distribution on vehicle stability is analyzed through vehicle dynamics.a torque distribution control strategy covering straight driving and steering driving is established.Finally,the Matlab/Simulink and Carsim co-simulation is performed.The correctness and effectiveness of the torque distribution control strategy proposed in this paper are verified.The main contents of this paper are as follows:（1）Analyze the current electric vehicle power system and determine the fourwheel drive system in this paper,study the torque distribution characteristics of the electronically controlled limited-slip differential,and analyze the wet-type electronically controlled limited-slip differential.The torque transmission characteristics of the multi-plate clutch and its influencing factors are determined through the bench test,and the mathematical model of the electronically controlled limited-slip differential is finally established.（2）In this paper,the calculation formulas of parameters such as dynamic wheel load,ideal yaw rate,and slip rate are determined based on the two-degree-of-freedom model and principles of vehicle dynamics,which provides a basis for the subsequent construction of the vehicle and control model.At the same time,according to the tire attachment ellipse theory and the curve of slip rate and adhesion rate,the principle that the vector torque can change the longitudinal force and lateral force of each driving wheel through the distribution of driving torque is analyzed,and the effect of torque distribution on the vehicle is expounded.It can improve the stability of the vehicle in the state of straight driving,make full use of the adhesion coefficient of each wheel on the road surface,and change the gisture of the vehicle when turning,increase the stability margin of the vehicle,and improve the cornering ability.（3）Based on the dynamic analysis,this paper proposes a torque distribution control strategy for the vector four-wheel drive system,which is mainly composed of two parts: the straight driving condition and the steering driving condition;the control strategy in the straight driving condition includes the feedback control based on the slip rate difference between the front and rear axles And the control based on the target slip rate of the left and right wheels of the rear axle;and the driving torque distribution control based on the direct yaw moment in the steering driving condition.（4）In this paper,the control strategy model built in Matlab/Simulink software will be co-simulated with Carsim software,offline simulation will be carried out with various working conditions and the simulation results will be analyzed.The improvement of stability and the control of the wheel slip rate in the straight driving condition;in the steering driving condition,the improvement of the steering state and the improvement of the handling stability of the vector four-wheel drive system were verified when the vehicle was steered.