Electrohydraulic servo valve
An electrohydraulic servo valve (EHSV) is an electrically operated valve that controls how hydraulic fluid is sent to an actuator. Servo valves are often used to control powerful hydraulic cylinders with a very small electrical signal. Servo valves can provide precise control of position, velocity, pressure, and force with good post movement damping characteristics.
A servo valve receives pressurized hydraulic fluid from a source, typically a hydraulic pump. It then transfers the fluid to a hydraulic cylinder at a pressure that is proportional to an electrical signal that it receives. Most hydraulic control valves are binary, they are either on or off. Servo valves are different in that they can continuously vary the pressure they supply from zero up to the pressure that they receive from their source.
Servo valves are often used in a feedback control where the position or force on a hydraulic cylinder is measured, and fed back into a controller that varies the signal sent to the servo valve. This allows very precise control of the cylinder. 
Examples of usage
One example of servo valve use is in blow molding where the servo valve controls the wall thickness of extruded plastic making up the bottle or container by use of a deformable die. The mechanical feedback has been replaced by an electric feedback with a position transducer. Integrated electronics close the position loop for the spool. These valves are suitable for electrohydraulic position, velocity, pressure or force control systems with extremely high dynamic response requirements.
Servo valves are used to regulate the flow of fuel into a turbofan engine governed by FADEC. One such example is Honeywell's servo valve which is part of the fuel control mechanism for the CFM International CFM56 engine powering the Boeing 737NG and Airbus A320 passenger aircraft
In fly-by-wire aircraft the control surfaces are often moved by servo valves connected to hydraulic cylinders. The signals to the servo valves are controlled by a flight control computer that receives commands from the pilot and monitors the flight of the aircraft. Some airliners that use these systems are the Airbus A320, A330, A340, A350, A380, Boeing 787, and Embraer E-Jet E2 family.
Principle of operation
An electric command signal (flow rate set point) is applied to the integrated position controller which drives the pilot stage. The thereby deflected nozzle flapper system produces a pressure difference across the drive areas of the spool and affects its movement. The position transducer (LVDT) which is excited via an oscillator measures the position of the spool (actual value, position voltage). This signal is then demodulated and fed back to the controller where it is compared with the command signal. The controller drives the pilot stage until the error between command signal and feedback signal will be zero. Thus the position of the spool is proportional to the electric command signal.
- Description of a Servo valve
- Servovalve Introduction
- Deformable die fitted with Moog servo valves
- [http://icas.org/ICAS_ARCHIVE/ICAS2006/PAPERS/048.PDF ICAS Archive THE A380 FLIGHT CONTROL ELECTROHYDROSTATIC ACTUATORS, ACHIEVEMENTS AND LESSONS LEARNT]