Two degree of freedom/H infinity controller synthesis for active four wheel steering vehicles

This paper proposes an active front and rear wheel steering control system that simultaneously achieves both lateral acceleration and yaw rate responses always desirable regardless of changes in vehicle dynamics. First, this paper describes a method to accurately estimate physical parameters in the four wheel steering vehicle model, including the dynamics of a steering actuator, by applying the maximum likelihood estimation method. Next, the structure of the proposed front and rear steering control system is described. This control system has for its purpose to coincide both the lateral acceleration and yaw rate responses of the vehicle with the responses of the respective desirable reference models, as well as to make it a robust system against changes in vehicle dynamics and external disturbances. To achieve both objectives, a two-degree-of-freedom control system theory is employed. Then the paper shows the viability of the proposed steering control system through computer simulations. Finally, the results of proving ground tests carried out with a test vehicle equipped with a front and rear wheel steering mechanism driven by the digital computer and electric powered actuator are shown. From these results, it was confirmed that the proposed active front and rear wheel steering control system would contribute greatly to the improvement of vehicle handling qualities and preventive safety.