Control/structure interaction in hydraulic power steering systems

A hydraulic power steering system uses feedback control to assist the driver in applying steering input commands to the vehicle. A small vibration, known as shudder, can occur in the system during low speed steering manoeuvres. It is shown that power steering shudder is a closed-loop limit cycle caused by the interaction of the hydraulic and structural dynamics of the vehicle. The model equations are derived, the theoretical background behind the approach is presented and some specific analytical results that reveal the control/structure interaction are examined. This understanding can be used to synthesize modifications which reduce the likelihood of shudder. Some examples of the effect of various hydraulic component modifications are shown.