Mechanical-hydraulic 4-edge servo valvein which the valve control spool is connected to a drive motor (e.g. a stepper motor), while the working element (e.g. a hydraulic motor) is linked to the valve housing (Figure R 35).
If the valve control spool is now deflected, the hydraulic motor then receives pressure oil and runs behind the valve control spool. If the control shaft is still, the motor will continue to turn until the control openings are closed by the turning housing.
Figure R 35: Rotation servo valve