I. Introduction
Active suspension systems have been widely studied over the last three decades. The basic idea in active control of suspension is to use an active element (the actuator) to apply a desired force between the vehicle body and wheel axle. This desired force is computed by the vehicle control unit to achieve certain performance objectives under external disturbances, such as passenger's comfort under road imperfections [1]. The main advantage of an active suspension is its adaptation potential where the suspension characteristics can be adjusted according to the profile of the road being traversed.