Research On Control Strategies Of Handling Stability For Multi-Axle Steering Vehicle

Multi-axle steering is an effective active chassis control technology for improving vehicle handling stability. It can effectively solve the problems of traditional front-wheel-steering vehicle, such as inflexible steering at low speed and poor stability at high speed. Currently, the essence of most control strategies of four wheel steering is active control of rear wheel steering angle. This method can control one state variable of vehicle well, but can't realize the multi-objective control. Besides, it can't guarantee the stability of vehicle in limited driving conditions. As for multi-axle steering technology (the number of steering axles is more than two), proportional control strategy of zero mass-center side-slip angle which is simple is mostly used, and researches on nonlinear dynamic control are very few.Given above current conditions, in this thesis, researches on integrated rear wheel steering angle and yaw moment control strategies of four-wheel-steering vehicle and steering angle control strategies of three-axle vehicle with all-wheel steering were carried out. The specific studies are as follows:(1) In order to solve the problem of excessive understeer of four-wheel-steering vehicle with proportional control of zero mass-center side-slip angle at high speed, a control strategy of adding direct yaw moment control to the proportional control of rear wheel steering angle was proposed. Based on the linear 2DOF model of four-wheel-steering vehicle, an integrated optimal control system was designed based on servo tracking optimal control theory.(2) To enhance the stability of vehicle in limited driving conditions, study on the nonlinear integrated control of four-wheel-steering vehicle was carried out. A nonlinear dynamics model of vehicle was built. Based on the principle of state regulator in the linear quadratic optimal control theory, a nonlinear integrated controller was designed. In order to reflect the dynamic characteristics of vehicle more truly, a virtual prototype model of four-wheel-steering vehicle which is more close to the real vehicle was established by using ADAMS/Car software. Aiming at the nonlinear characteristic of vehicle, an integrated fuzzy control strategy was proposed, and a nonlinear integrated fuzzy control system was designed. Adopting the joint simulation method of ADAMS/Car and MATLAB/Simulink, the performance of the designed control system was tested. (3) Taking the performance of three-axle vehicle at low speed and high speed into comprehensive consideration, study on the optimal control of the vehicle with all-wheel steering was carried out. A linear 2DOF model of three-axle vehicle with all-wheel steering was built. Adopting optimal control algorithm, an optimal control system of all-wheel steering was designed.(4) Taking the nonlinear cornering characteristic of tire into consideration, analysis and control of handling stability for three-axle vehicle in limited driving conditions were carried out. A nonlinear 2DOF dynamics model of three-axle vehicle with all-wheel steering was established. Based on the fuzzy control theory, a nonlinear fuzzy control system of all-wheel steering was designed.Through simulation verification of the control systems, the following conclusions can be drawn:the designed linear control systems can enhance the handling stability of vehicle in normal driving conditions effectively, and the nonlinear control systems can effectively avoid the instability of vehicle during critical steering process.