An Electro-Hydraulic Servo Valve or EHSV is an electrically operated
valve that controls how hydraulic fluid is ported to an actuator.
Servo valves and Servo-Proportional Valves operate by transforming
a changing analogue or digital input signal into a smooth set of movement
in a hydraulic cylinder. Servo valves can provide precise control of
position, velocity, pressure and force with good post movement damping characteristics.