Study On Modeling And Steering Stability Of Four-Wheel Steering Vehicle

Four-wheel steering(4WS)technology,as one of the effective control methods to improve vehicle handling stability and driving safety,has attracted more and more attention and research.A large number of studies and vehicle tests show that 4WS technology has significant advantages in improving the vehicle's low-speed steering flexibility and improving the safety of high-speed driving.Based on it,this thesis takes four-wheel steering vehicle as the research object,aims to study the factors affecting the stability of four-wheel steering vehicle,and explore better control strategy and control method of four-wheel steering.The main research contents are as follows:(1)According to Newton's second law and Newton mechanics vector system,the mathematical models of two-degree-of-freedom and three-degree-of-freedom 4WS vehicle are established,simulation comparison analysis was carried out on the two models respectively.And the results show that the three-degree-of-freedom model fits more closely with the real steering characteristics.Secondly,yaw rate gain curves of front-wheel steering and 4WS were obtained by Matlab programming based on the yaw rate gain formula of two-degree-of-freedom vehicles.Simulation results show that four-wheel steering has better steering characteristics than two-wheel steering vehicles.(2)Taking two-degree-of-freedom 4WS mathematical model as an example,two common control methods of front and rear wheel proportional control and proportional and yaw feedback combined control are established,analyze the influence of the front wheel angle step excitation for 4WS vehicle under the two control methods,the simulation results show that these two kinds of control method each have advantages and disadvantages in low speed and high speed.Compared to the front-wheel steering,these two methods improve the maneuverability of the vehicle at low speed and the steering stability at high speed.(3)Research on the rear wheel control method for 4WS.According to the basic principle of fuzzy control,the fuzzy PID controller is constituted by the control strategy of proportional and yaw velocity feedback.The input of the controller is the deviation and deviation rate between yaw velocity and ideal yaw rate,and the output is the change of three parameters.At the same time,the basic principle of particle swarm optimization fuzzy PID parameters is introduced.The realization process of the fuzzy PID controller based on particle swarm optimization and the optimal design of the controller are analyzed,and the simulation model of the fuzzy controller based on particle swarm optimization is established.(4)Setting up subsystems including vehicle tires,suspension system,steering system,braking system,power system and body in Adams/View,set the rear-wheel rotation angle as system state variable,then complete the four-wheel steering vehicle multi-body dynamics model assembly under the Car module.The Adams four-wheel steering model was export from the Car/Control interface,input and output variables were set,and the joint simulation with Matlab was conducted.The front wheel angle step test and ISO line shift test were conducted respectively to verify the effectiveness of the four-wheel steering control method studied above.(5)Study on the influence of time-delay on the operating stability of 4WS vehicles with electric power steering.By analyzing the structure and working principle of the electric power steering system,the mechanism of time-delay and how to express the magnitude of time-delay qualitatively are deduced.The ideal model of driver is established and combined with the Adams four-wheel steering model derived from Car/Control interface to form the human-vehicle closed-loop control system.Different output response curves are obtained by changing the size of time delay through joint simulation.The simulation results show that the time-delay has a great influence on the vehicle's side-slip angle and yaw rate,which will lead to the vehicle's instability.