Control Technology Of Applying On-demand 4WD System Based On RWD Vehicle

In recent years,on-demand 4WD vehicles have been gradually welcomed by consumers due to their superior power,steering stability and economy,and are popular with the SUV market.Based on the dangerous phenomena of rear-wheel-drive vehicles such as easy slippage of wheels on low-adhesion roads and easy side-slipping of vehicles,as well as the advantages of good performance and beautiful appearance under normal road conditions,we have developed a set of control technology based on the application of on-demand 4WD system in rear-wheel drive vehicles.This paper relies on the national key research and development plan project "new energy vehicle key special project" : high-performance in-wheel motor and modular assembly integration key technologies and applications（No.: SQ2021YFB2500007）.Now a domestic rear-wheel-drive vehicle is used as a prototype,and a set of electronically controlled on-demand 4WD system is applied to it to achieve the improvement on vehicle performance,mainly by analyzing the dynamic characteristics of the vehicle including the tire model,studying the motor-type inter-axle torque distribution device including the control motor,force transmission mechanism,clutch and other components,and designing a suitable and reliable on-demand 4WD system control technology,Finally the model is established and simulated to verify the effectiveness of its control.The main contents of this paper include:（1）The influence of rear axle drive torque on vehicle dynamics and stability is analyzed.Based on the vehicle dynamics model,the influence of the rear axle driving torque on the longitudinal dynamics and steering stability of the vehicle is obtained by analyzing the vehicle dynamics characteristics,tire mechanical characteristics and tire coupling characteristics,which lays a solid theoretical foundation for the follow-up control strategy research.（2）The model of the inter-shaft torque distribution device based on the motor-driven multi-plate friction clutch is established.Firstly,the working principle and power transmission route of worm gear mechanism,ball cam mechanism and multi-plate clutch are analyzed;Then the clutch torque characteristics under different working conditions are studied,and the relevant influencing factors are get;After that,the control mechanism of the motor-driven clutch components in the inter-shaft torque distribution device is analyzed and obtained;Finally,the model of the torque distribution device between shafts by Simulink is established,and the friction torque characteristics transmitted by the clutch in the slipping and locking state are obtained through simulation.The simulation results show that the transmitted torque has the characteristics of fast response,high accuracy and strong reliability.（3）The control strategy of the on-demand 4WD system is designed.First of all,based on the difference in torque distribution requirements of the four-wheel drive system when driving straight and when turning,it is proposed that: in straight driving conditions,the "feedforward control based on driver’s acceleration intention recognition + PID feedback control of BP neural network based on the relative difference between the front and rear axle slip rates" is adopted;under turning conditions,the "sliding mode adaptive feedback control based on vehicle yaw rate deviation and center around mass slip angle deviation" is adopted.Then,a coordinated control strategy is proposed: coordination between the four-wheel drive system and the braking system;coordination between the four-wheel drive system and key command signals.In order to identify the clutch overheating flag,a temperature estimation strategy is proposed,and the reliability of the strategy is verified by Simulink simulation and bench test.（4）It is verified that the on-demand 4WD control strategy can improve vehicle performance by simulation.Carsim and Simulink are used to create a co-simulation environment of the on-demand 4WD control system,and several typical straight-runnings and turning conditions are designed.The simulation results are analyzed and the effectiveness of the designed control strategy is verified: Compared with the rear-wheel drive control mode,the on-demand 4WD control strategy designed in this paper can make the vehicle driving on the road with low adhesion coefficient has a higher wheel adhesion rate,effectively suppress the slippage of the driving wheel,and greatly improve the longitudinal dynamic performance of the whole vehicle.It can also improve the steering stability of the turning vehicle to a certain extent,reduce the oversteering phenomenon of the vehicle,effectively restrain the vehicle from sliding,and control the vehicle not to deviate from the track.