Research On Dynamic Analysis And Control Method Of Steer-by-wire System For Vehicle

Steering by wire system (SBW), which has no mechanical linkage between the steering wheel and the front wheels, is one of the indispensable components in the future intelligent automaobile which has the fuction of driving without driver. The transmission ratio of SBW system can be randomly designed, so its steering characteristics can remain the same while the speed changes. Furthermore, it can add an additional steering angle to the driver's input, which can optimize the vehicle's response on the driver's input or improve the stability in case of emergency. So recently it becomes the research focal.Firstly, the control aims and demands of the front wheel steering module were presented in this paper. And the control system based on hierarchical structure, which included the control system of front steering angle(the upper control) system and the control system of steering actuator motor(the lower control) system, was proposed. For the lower control system, the dynamics equations of the front wheel steering module were established and a new PIλDμcontrol method based on fractional calculus was designed. The simulation results showed that the normal steering function and the aligning function could be realized by the PIλDμcontroller and the robust of the steering-by-wire system could be improved. For the upper control system, a front-wheel steering angle control strategy based on fuzzy control was designed to overcome the pitfalls of the traditional steering system. And the simulation results showed that the ideal transmission ratio based on fuzzy control could meet the demands. The control system based on hierarchical structure is simulated for the model and the control algorithm with typical operating conditions. And the simulation results showed that the hierarchical control svstem was effective.Then a coordinated control strategy for active front steering (AFS) and direct yaw moment (DYC) was proposed to avoid serious over-steering and under-steering. And AFS controller and DYC controller based on variable structure control were designed. A coordinated controller was also designed to adjust the controllers—AFS and DYC according to the control logic. Simulation results show that the vehicle with coordinated control of AFS and DYC achieve better the stability when steering.Improper time-delay which exists in the control systems, will has unfavorable effect on the characteristics of the control system. So control time-delay was introduced to the study of vehicle atability which had been discussed in the paper. Some researches on the stability of linear time-delay system, the effects of time-delay on vehicle dynamic characteristics and the time-delay compensation of the controlled system were carried out. The simulation and analysis of the time-delay effects and compensation were performed. The simulation results show the controlled system with time-delay compensation can achieve better stability.At last, this paper developed SBW bench test devices from two aspects of hardware and software. With this test device, the control system based on hierarchical structure of the front wheel steering module was verified. The results of test and simulation were basically agreed.The innovation of this paper is as the following. Firstly, the fractional order control theory has been studied, the five parameters of fractional PIλDμcontroller were achieved through the optimization method. To solve the issue that fractional order system can not be directly in the MATLAB simulation environment, simulation model of fractional PIλDμcontroller was constructed. And the fractional order control method was successfully appied to the normal steering and the aligning control of the front wheel steering module. Secondly, the fractional PIλDμcontroller and the fuzzy controller were combined organically to achieve the hierarchical control. Vehicle stability control strategy based on AFS and DYC coordinated control was proposed to overcome the defects when AFS and DYC control worked alone. The time-delay effects and compensation were performed for the vehicle with the steering by wire system. This study provides new ideas and methods for the research and development of the steering by wire system.