Research On Control Strategy Of Four-wheel Steering Vehicle

With the development of automobile industry,improving the stability and safety of automobile operation has become one of the important hot spots in the development of automobile technology,and the stability and safety requirements are higher under the transition to working conditions.The traditional front wheel steering vehicle at low speed when the turning radius is large,poor mobility,highspeed prone to rollover,poor safety,as one of the active safety of the vehicle fourwheel steering(4WS)gradually into the view of everyone.4WS system that is,when the car is steering,the rear wheels will produce a certain coefficient of the speed and the angle of the front wheel to participate in the steering.Because the vehicle steering system is a more complex nonlinear system,the traditional mechanical control and hydraulic control have problems such as low accuracy and long response time,so the design of excellent control strategy has become the focus of research.Based on the improvement of the traditional two-degree-offreedom vehicle model to improve stability and safety,the control algorithm of the 4WS system is studied in depth and the simulation test is verified.The traditional two-degree-of-freedom linear model optimizes the distortion of steering characteristics when expressing the vehicle’s motion state,such as distortion of steering characteristics at high speed or at large corners.On the basis of the 2-DOF model,linear vehicle models of 3-DOF was constructed with the factors of roll-over in mind.Considering the nonlinear characteristics of tires,the tire model was constructed by using magic formula.Then the vehicle model and tire model were combined to build 3-DOF 4WS nonlinear vehicle models.The 2-DOF linear model and the 3-DOF nonlinear model were simulated and compared under step input to verify the correctness of the model.On this basis,the steering characteristics analysis and manipulation stability analysis of 4WS vehicles are obtained,and the critical speed and the ideal cross-swing angle speed are obtained,which lays a foundation for the research of 4WS control strategy.The traditional fuzzy PID controller was improved with zero angle on the center of mass as the control target,and the simulation model of the whole vehicle was built on the basis of which the simulation analysis was carried out.The analysis indicates that the controller can improve the stability of the vehicle,but there are problems of large centrique side angle and slow response speed.For the problem,the score order PID controller was proposed,the mathematical model of fractional order PID controller was designed,and the parameters of the fractional order PID controller were adjusted by the pole order search method and the approximate solution method,in order to conforms to the requirements of the control system,and then the simulation model of the fractional order PID control vehicle was set up.In order to verify the validity of the verification controller,the improved fuzzy PID controller,fractional order PID control and front wheel steering were compared with the simulation test under angle step input,sine signal input and pulse signal input,and the results were obtained.The advantages of fractional PID controllers were further verified.