However, these same functions can be accomplished with digital valves. The illustration shows four normally closed (NC) valves grouped into a hydraulic integrated circuit to provide the same functionality as a 3-position spool valve. With all valves in the neutral condition, as shown, fluid flow to and from the pump, reservoir, and actuator is blocked.
Energizing solenoid A routes pressurized fluid to the cap end of the cylinder, causing the rod to extend. Simultaneously energizing solenoid D routes fluid from the cylinder’s rod end to tank. In similar manner, energizing only solenoids B and C causes the rod to retract and routes fluid from the cylinder’s cap end to tank.
The valves in the figure at right are arranged to function as a 3-position closed-center spool valve. An open-center valve could be achieved simply by making all the digital valves normally open (NO) instead of normally closed. Likewise, tandem- and float-center configurations can be accomplished by using various combinations and placement of NO and NC digital valves.
An advantage of digital valves over spool valves is that digital valves use a poppet or plunger, so no cross-port leakage occurs. A limitation is that they cannot provide the proportional control of spool valves. However, acceleration and deceleration ramps can be achieved by using soft-shift valves.
