Research Of Hierarchical Control Method For Four-wheel-steering-by-wire Of Vehicle

With the growing of traffic system and the rising of people’s living standard, automobile has become indispensable in people’s life as an important tool. Automobile steering system is the key part of automobile control, for it has the vital role to decide whether the car can run safely and steadily or not. Therefore, the study of steering system has major research and practical value. Steering-by-wire cancels the mechanical connections between steering wheel and steering cartwheels, replaced by the electronic signal transmission, which can fundamentally solve problem of the big steering difference between low and high speed by traditional mechanical steering method. Four-wheel steering can significantly improve the front wheel steering in coordination with the front and rear wheels. So, the research of control method for the four-wheel-steering-by-wire selected as the research object in this paper has important practical significance.Firstly, the basic structure, the working principle and the basic requirements of the four-wheel-steering-by-wire system are introduced given in this paper. In addition, the hierarchical control method is put forward which includes the upper Angle control and the lower execution module control. Then, Models of the steering wheel module, the execution module and the two degrees of freedom vehicle are built using the knowledge of vehicle dynamics.Finally, different control methods are designed in this paper according to the different characteristics of the upper and lower control. For the upper control, the ideal transmission ratio control method based on fuzzy control is used to control the front wheel angles, for it can simultaneously consider the two factors of speed and steering wheel. The rear wheel angles are controlled by a comprehensive control method based on the front wheel Angle feed-forward and yawing angular velocity feed-back. For the lower control, the fractional order PIλDμ controller based on fractional order differential and integral calculus theory is adopted to control the steering execution module. Matlab-Simulink, a commonly used simulation tool is also employed to test the control method designed in this paper. The simulation results show that the control method applied here is reasonable and effective.There are plenty of bright spots in this paper:the idea of employing hierarchical control method in four-wheel-steering-by-wire control can make the best of the advantages of hierarchical control and improve the control performance. In addition, the ideal transmission control method based on fuzzy control, can not only consider the influence factor of speed, but also the rotation of the steering wheel at the same time. It is much simpler and more efficient than traditional ideal transmission ratio control method. The new fractional order PIλDμ is applied in this paper and its control effect is much better than traditional PID controller, too.