How does an electro-hydraulic servo-valve work?

Electrical signals are used to control hydraulic fluid flow to and from a hydraulic actuator. The hydraulic actuator itself is used to manipulate a devices such as a steam control valve or a gas turbine inlet guide.

These devices often have a requirement to be positioned anywhere between fully open and fully closed. It is thus the electrical signals controlling the flow of hydraulic fluid supplying actuator that dictates how these devices position themselves. The electrical signals can of course be controlled by other devices such as micro processors or computers.