Controllability and Stability Aspects of Actively Controlled 4WS Vehicles

Four wheel steering (4WS) of passenger cars has become a topic of interest in recent vehicle dynamics literature. In the present work, a linear two-degree of freedom model (L2DF) has been used to examine controllability and stability aspects of various 4WS algorithms. Yaw rate r and lateral velocity v were used as model degrees of freedom, and as state feedback variables for the implementation of 4WS controllers of various types. With controllers developed using the L2DF model, investigations were performed into the performance of such controllers when implemented using a nonlinear three-degree of freedom model (N3DF) which included roll and the possibility of tire saturation. Desirable steady-state properties for v and r can be obtained using the robust controllers developed through the use of the L2DF model. Finally, the stability of the system is shown to depend upon tire cornering stiffness, and is examined both qualitatively and quantitatively.