Electro-Hydraulic Servo Valve Construction, Models and Use

The input to an electro-hydraulic (EH) servovalve is typically a current or a differential current that powers an electromagnetic torque motor. The differential current Δi is typically supplied by an amplifier to avoid excess loading of the interface to the computer or controller. In the simplest (but not typical) form, the torque motor moves a spool valve as shown below. The spool valve allows the hydraulic fluid to pass from the supply to the return across two variable metering orifices with a controlled flow rate QL. If the spool is shifted in the other direction the direction of flow will reverse. Since the clearances between the spool and the valve body is small, the forces required to move a large spool are large. Hence the single stage or direct acting EH valve is limited to low rates of flow (small valves). Figure 7-11 from Merritt shows the pictorial representation of the motion system in this case.