The behavior of a semi-active hydraulic damper is derived based upon fluid mechanics theories. A 1-DOF suspension system with the damper is analyzed theoretically and experimentally. The damper is controlled by an on-off action of relief valves and the criterion for the action of the relief valves is the sign of the multiplication of the relative displacement and relative velocity. It is shown that the maximum acceleration at the system can be effectively reduced by the on-off characteristics of the damper compared with the case of passive damper. This study also examines the sensitivity of the system acceleration to various parameters of the damper.
