Bifurcation in vehicle dynamics and robust front wheel steering control

A control strategy is proposed for designing a steering control for automotive vehicles to protect the vehicle from spin and to realize the improved cornering performance. The vehicle unstabilization is shown to be caused by a saddle-node bifurcation which depends heavily on a rear tire side force saturation. Based on this observation, the saturation characteristics of rear tires are modeled by a linear function with uncertainty terms of a special structure. The nonlinearity of the saturation is included in the part of uncertainty since it changes considerably due to road environments, whose rigorous identification is usually very difficult. Then a result from the linear H-infinity control theory seems to be appropriate to design a front wheel steering controller which compensates the instability against the nonlinear uncertainty, The designed controller is shown to work quite well for nonlinear systems in achieving robust stability and protecting the vehicle from spin. Furthermore, the computer simulations show that the control improves cornering performance in critical motions. The motion realized by the controller resembles the one known as a counter steering which skillful drivers often use.