Actuator rate limits in robust car steering control

Robust unilateral decoupling of car steering makes the yaw rate unobservable from the lateral acceleration at the front axle and thereby attenuates the effect of yaw disturbance torques on steering. The control system showed remarkable safety benefits in an experimental car. During the experiments, however, there occurred undesirable limit cycles caused by steering actuator rate limitations. In this paper limit cycles are analyzed by a describing function approach in combination with the representation of limit-cycle-free regions in a parameter plane of velocity and road-tire friction coefficient. The results are formulated in terms of required actuator bandwidth that achieves robustness in the entire operating range. It turns out that the use of a fading integrator reduces the required actuator bandwidth.