ABSTRACT
The task of steering a car may be subdivided into
Path planning by preview
Keeping the car - considered as a mass point - on the planned path
Stabilization of yaw motions induced by path tracking
Control of yaw motions generated by external forces.
In this paper it is shown how an automatic control system with yaw rate feedback can help the driver in the above tasks. It decouples the lateral motion at the front axle from the yaw mode. The remaining steering transfer function from steering wheel to front lateral acceleration is thereby reduced to a first order low pass. The yaw mode is stable in forward driving. It may be desirable however to increase the damping of the yaw mode. This can be accomplished by additional rear wheel steering with yaw rate feedback. Finally the steering characteristics of the car can be shaped by the two feedforward paths from the steering wheel to the reference inputs of the front and rear wheel steering feedback loops. The control system automates the subtasks 3) and 4) and drastically simplifies subtask 2) for the driver.
The robustness aspect of the problem is that uncertain operating conditions like speed, mass, road surface condition enter into the model. For the half car model (with an assumption on the longitudinal mass distribution) it is shown that the decoupling result holds generically, i.e. for all vehicle parameters and all operating conditions. Pole placement by rear wheel steering remains as a robustness problem. Also robustness with respect to the simplifying model assumptions generates interesting robustness issues. It is my hope that this paper will stimulate new application-motivated research in robust control theory.
