Research On Variable Gear Ratio And Stability Control Of Steer-by-wire Four-wheel Steering Electric Vehicle

In recent years,smart vehicles technology has ushered in the development opportunity,and electric vehicles have also attracted more and more attention,which makes people’s travel more intelligent,efficient and humane.Among them,steer-by-wire four-wheel steering electric vehicles have gradually become the direction of future driverless vehicles development due to their mobility at low speed and handling stability at high speed.This thesis researches the steer-by-wire four-wheel steering electric vehicle,and improves the maneuverability at low speed and stability at high speed of the vehicle by designing a reasonable variable angle transmission ratio algorithm to control the rotation angle of front and rear wheels.Firstly,according to the working principle of the steer-by-wire system,the steer-by-wire system model is built,and the tire model and driver model are built in the vehicle model,so as to constitute a nonlinear 7 degrees of freedom vehicle model,and the accuracy and rationality of the built model are verified through model comparison and analysis,which lays a foundation for the subsequent algorithm research of steer-by-wire four-wheel steering vehicle.Secondly,according to the steering angle transmission ratio of steer-by-wire front wheels,a particle swarm fuzzy control scheme is proposed.The fuzzy controller can achieve precise control of the steering without considering the complexity of the model,but it is too dependent on the designer’s experience and cumbersome debugging,so the particle swarm intelligence algorithm is used to optimize the fuzzy rules and membership function.In order to avoid particles falling into local optimum,variable inertia weights and variable learning factors are introduced to ensure the global optimization ability of particles,so as to improve the control effect of the controller.Through the steady state rotation test,double line shift and angular step simulation conditions simulation results,it can be seen that the proposed control strategy meets the steering characteristics of steering light at low speed and steering stability at high speed,improves the handling stability of automobiles,and verifies the feasibility of the scheme.At the same time,in the research of the online control active rear wheel steering control algorithm,a fuzzy sliding mode control scheme including variable angle transmission ratio is proposed.After analyzing the stability of the vehicle,the center of mass side declination angle is zero and the ideal yaw angle speed of tracking is selected as the control target,and the variable angle transmission ratio is introduced into the steer-by-wire four-wheel steering vehicle,and the sliding mode controller is designed,which has strong ability to resist external interference,but is prone to jitter,so the exponential approach rate and fuzzy control theory are introduced into it to improve the control performance of the controller.Through the simulation results of open-loop and closed-loop tests,it can be seen that compared with the traditional four-wheel steering vehicle and front-wheel steering vehicle,the fuzzy sliding mode controller has better maneuverability at low speed,better stability at high speed,and better resistance to side wind,which verifies the effectiveness of the scheme.Finally,aiming at the handling stability of the steer-by-wire four-wheel steering vehicle on the low adhesion road surface,the optimal control scheme is adopted,and the ideal vehicle model is used as the tracking object,and the front and rear wheel corners of the vehicle are controlled at the same time,so as to improve the stability of the vehicle.Through the simulation results of open-loop and closed-loop tests,it can be seen that the stability of the full steer-by-wire four-wheel steering vehicle is improved compared with the steer-by-wire active rear-wheel steering vehicle,which verifies the effectiveness of the control algorithm.