An electrohydraulic servo valve (EHSV) is an electrically-operated

valve A valve is a device or natural object that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically fitting ...

that controls how hydraulic fluid is sent to an actuator.https://www.moog.com/literature/ICD/Valves-Introduction.pdf

Servo Servo may refer to: Mechanisms * Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback ** AI servo, an autofocus mode ** Electrohydraulic servo valve, an electrically operated valve that c ...

valves are often used to control powerful

hydraulic cylinder A hydraulic cylinder (also called a linear hydraulic motor) is a mechanical actuator that is used to give a unidirectional force through a unidirectional stroke. It has many applications, notably in construction equipment ( engineering vehicles ...

s with a very small

electrical signal In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...

. Servo valves can provide precise control of position, velocity, pressure, and force with good post-movement damping characteristics.

History of electrohydraulic servo valves

The electrohydraulic servo valve first appeared in

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...

. The EHSVs in use during the 1940s was characterized by poor accuracy and slow response times due to the inability to rapidly convert electrical signals into hydraulic flows. The first two-stage servo valve used a solenoid to actuate a first stage spool which in turn drove a rotating main stage. The servo valves of the World War II-era were similar to this — using a solenoid to drive a spool valve. Advancement of EHSVs took off in the 1950s, largely due to the adoption of permanent magnet

torque motor In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of the ...

s as the first stage (as opposed to solenoids). This resulted in greatly improved response times and a reduction in power used to control the valves.

Description

Types

Electrohydraulic servo valves may consist of one or more stages. A single-stage servo valve uses a torque motor to directly position a spool valve. Single-stage servo valves suffer from limitations in flow capability and stability due to torque motor power requirements. Two-stage servo valves may use flapper, jet pipe, or deflector jet valves as hydraulic amplifier first stages to position a second-stage spool valve. This design results in significant increases in servo valve flow capability, stability, and force output. Similarly, three-stage servo valves may use an intermediate stage spool valve to position a larger third stage spool valve. Three-stage servo valves are limited to very high power applications, where significant flows are required. Furthermore, two-stage servo valves may be classified by the type of feedback used for the second stage; which may be spool position, load pressure, or load flow feedback. Most commonly, two-stage servo valves use position feedback; which may further be classified by direct feedback, force feedback, or spring centering.

Control

A servo valve receives pressurized hydraulic fluid from a source, typically a hydraulic pump. It then transfers the fluid to a hydraulic cylinder in a closely controlled manner. Typically, the valve will move the spool proportionnaly to an electrical signal that it receives, indirectly controlling flow rate. Simple hydraulic control valves are binary, they are either on or off. Servo valves are different in that they can continuously vary the flow they supply from zero up to their rated maximum flow, or until the output pressure reaches the supplied pressure. More complex servo valves can control other parameters. For instance, some have internal feedback so that the input signal effectively control flow or output pressure, rather than spool position. Servo valves are often used in a

feedback control Feedback occurs when outputs of a system are routed back as inputs as part of a chain of cause-and-effect that forms a circuit or loop. The system can then be said to ''feed back'' into itself. The notion of cause-and-effect has to be handled c ...

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

Manufacturing

One example of servo valve use is in

blow molding Blow molding (or moulding) is a manufacturing process for forming hollow plastic parts. It is also used for forming glass bottles or other hollow shapes. In general, there are three main types of blow molding: extrusion blow molding, injectio ...

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.

Aircraft

Servo valves are used to regulate the flow of fuel into a

turbofan engine The turbofan or fanjet is a type of airbreathing jet engine that is widely used in aircraft propulsion. The word "turbofan" is a portmanteau of "turbine" and "fan": the ''turbo'' portion refers to a gas turbine engine which achieves mechanic ...

governed by FADEC. In

fly-by-wire Fly-by-wire (FBW) is a system that replaces the conventional manual flight controls of an aircraft with an electronic interface. The movements of flight controls are converted to electronic signals transmitted by wires, and flight control ...

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.

References

{{DEFAULTSORT:Electrohydraulic Servo Valve Control devices Valves Hydraulics