For a hydraulic fluid to do work, it must flow to the actuator and or motors, then return to a reservoir. The fluid is then filtered and re-pumped. The path taken by hydraulic fluid is called a hydraulic circuit of which there are several types.

Closed center circuits supply full pressure to the control valves, whether any valves are actuated or not. The pumps vary their flow rate, pumping very little hydraulic fluid until the operator actuates a valve. The valve's spool therefore doesn't need an open center return path to tank. Multiple valves can be connected in a parallel arrangement and system pressure is equal for all valves.

hydraulic circuit

Fig. 1: Hydraulic Circuit Directional Control

Computer controlled hydraulic valves usually employ electromagnetic actuators to set the pushrod’s displacement over the entire retract/extend range. Apriori knowledge of the hydraulic reactive forces inside the control valve is crucial for the design of any magnetic actuator requiring CFD analysis of all fluid flow patterns over the entire range of operation.

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Fig. 2: Hydraulic forces developed inside a Linear Control Valve (ANSYS CFX)

The ANSYS CFX CFD simulator is able to automatically derive hydraulic reactive forces by inserting control surfaces at proper locations. Therefore, the reactive hydraulic force/displacement characteristic for the valve’s spool becomes available as part of the system report.

ForceDisplacementCharacteristic

Fig. 3: Force-/Displacement Characteristic for Valve Spool (Courtesy AlphaFluid)

There are ways to outfit hydraulic actuators with inexpensive internal displacement sensors that allow for an exact position control while in operation. Combined with computer controlled hydraulic valves, this technique allows for displacement accuracies of < 0.1 percent over the full range of operating conditions.