Toward safe and human friendly hydraulics: The passive valve

Hydraulic systems, as power sources and transmissions, 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 humans. If hydraulic systems are passive, they would be both safer to interact with, and easier for humans to control. In this paper, it is shown that a critical hydraulic component, the directional control valve, is not passive. However, the directional valve, as a one-port or a two-port device can become passive if appropriate spool valve dynamics are imposed. 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 passive method that relies on hardware modification, and an active feedback method, are proposed.