ABSTRACT
Hydraulically-operated systems have been used in a wide variety of applica tion ranging from precision control systems, such as robotics and machining, to high power heavy industrial systems, such as, machine tools forging processes, rolling mill and webbing as depicted in Figure 1. In the these processes, hydraulically-operated systems keep playing an important role due to its high power capability, high accuracy, high stiffness and compactness. Even if many engineers have tried to substitute hydraulic systems with electrical power systems due to relative easiness of control, the trial often has been proven to be fail in some cases [1-3].
