3. Electrohydraulic flow servovalves

3.1 Definition

The electrohydraulic servovalve is a proportional-action device whose input is an electric current and whose output is a hydraulic flow (sometimes pressure). This makes the servovalve the heart and soul of electrohydraulic servo systems.

There is widespread confusion, even in manufacturers' catalogs, between servovalve and solenoid valve. In principle, the term "solenoid valve" should be reserved for an on-off device whose input is an electric current and whose output is a hydraulic flow.

There are currently several hundred types of servo valve on the market, corresponding to dozens of different principle diagrams.

A servovalve includes at least :

• an...