Passivity properties and passification of electrohydraulic valves

Hydraulic systems, as power source and transmission, offer many advantages over electromechanical or purely mechanical counterparts in terms of power density, flexibility and portability. Many hydraulic systems require touching and contacting the physical environments; and many of these systems are directly controlled by human. If hydraulic systems are passive, they would be both safer to interact with, and easier for human to control. In this paper, it is shown that a critical hydraulic component, the directional control valve is not passive. However, the directional valve is a one-port or a two-port device passified by imposing appropriate spool dynamics. Methods to passify the valve for both first order and second order spool dynamics are considered. In the case of second order spool dynamics, a method that relies on hardware modification, and an active feedback method are proposed.