An electrohydraulic servo valve 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.
History of electrohydraulic servo valves
The electrohydraulic servo valve first appeared in World War II. The EHSVs in use during the 1940s were 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 motors as the first stage. 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 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
Manufacturing
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.
