Electrohydraulic servo valve

An electrohydraulic servo valve (EHSV) is an electrically operated valve that controls how hydraulic fluid is ported to an actuator. Servo valves and servo-proportional valves are operated by transforming a changing analogue or digital input signal into a smooth set of movements in a hydraulic cylinder. Servo valves can provide precise control of position, velocity, pressure and force with good post movement damping characteristics.

Control

A low voltage is used to control the servo valve. The control voltage is passed into an amplifier which provides the power to alter the valve's position. The valve will then deliver a measured amount of fluid power to an actuator. The use of a feedback transducer on the actuator returns an electrical signal to the amplifier to condition the strength of the voltage to the servo valve.^[1][2]

Examples of usage

The twin Moog servo valves are used to deform the shape of the die on this blow molding accessory designed by BMC Limited

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of extruded plastic making up the bottle or container by use of a deformable die.^[3] The mechanical feedback has been replaced by an electric feedback with a position transducer.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. Another example is the regulation of fuel flow 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

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.^[4]

References