| The transmission system of wheel motor driven vehicle has simple structure,and the driving torque of each driving wheel is independent and controllable.By controlling the left and right wheels of the vehicle to produce different torques,the wheels will deflect around their respective kingpins,so as to realize the flexibility control of steering.This steering mode is also known as differential steering.At present,the research on differential steering vehicles often focuses on a single longitudinal or transverse motion,and there is a lack of relevant strategies to coordinate the two.Therefore,in order to improve the handling stability of front wheel differential steering vehicle driven by hub motor under different working conditions,a coordinated controller is designed to control the longitudinal and transverse force of the vehicle.The main work and conclusions of this paper are as follows:(1)The vehicle model is selected by using the vehicle dynamics simulation software Carsim,and the corresponding vehicle parameters are extracted from it.In order to verify the correctness of the extracted vehicle parameters,a linear two degree of freedom vehicle model is established and compared with it.After that,the hub motor model is established in Simulink and connected to Carsim vehicle model through S-function interface,and the dynamic model of electric vehicle driven by hub motor is built.(2)The dynamics models of front wheel drive,electric wheel drive and differential steering system are established.The nonlinear tire model is established to obtain the cornering stiffness of front and rear tires under different adhesion coefficients.A control method of front wheel differential steering vehicle driven by hub motor based on sliding mode variable structure control theory is proposed to realize its basic steering function through differential steering,which can be used for subsequent comparative simulation test with longitudinal and transverse force coordinated control.(3)For the front wheel differential steering vehicle driven by hub motor,a hierarchical longitudinal and transverse force coordinated control is proposed.The upper controller obtains the resultant force and resultant moment required by the tracking reference model of the front wheel differential steering vehicle through sliding mode control.The lower controller uses the minimization of tire load rate as the objective function to solve the resultant force and resultant torque calculated by the upper controller.At the same time,considering the limiting conditions such as tire attachment circle and maximum torque of motor,the resultant force and resultant torque are decomposed into longitudinal force and transverse force required to be provided by four wheels.Through the inverse model of tire cornering characteristics and the superposition scheme of longitudinal force,the input parameters required for the vehicle model of front wheel differential steering driven by hub motor are transformed.(4)The high and low adhesion coefficient roads are set in Simulink to carry out the comparative simulation test between the hub motor-driven front wheel differential steering vehicle with or without coordinated control and the reference model.The results show that compared with the uncoordinated control,the designed coordinated controller can effectively coordinate the tire force of the front wheel differential steering vehicle driven by the hub motor,and ensure that there is no tire force saturation.Then the simulation test is carried out by combining Car Sim and Simulink.The results show that the proposed coordinated control can well control the front wheel differential steering vehicle to steer stably on different attached roads and keep the vehicle speed constant within the reasonable range of tire load rate. |
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