Servo Hydraulics

Servo Hydraulics are fluid based systems which use electronic controls to manipulate hydraulic pumps and valves and use feedback devices to correct itself. Hydraulic systems are notoriously hard to control for extremely fine manipulations. Using electronic sensors and modern valve control systems they can be utilized to bring the strength of hydraulics to machinery that needs fine control oversight.

These feedback mechanisms are normally some type of encoder or resolver, or pressure sensors in hydraulic systems.

How servo hydraulic amplifiers work

There are three main components of the linear amplifier: the pilot motor (normally a stepper motor however could be a DC motor or AC servo motor); the NC-valve; and the hydraulic cylinder. A flexible coupling is fitted between the pilot motor shaft and the control shaft exiting the NC-valve. At the other end of the control shaft is a thread which enables the movement of the control shaft into/out of the NC Valve. A spool is fitted to the control shaft which is kept in location by a spring inside the spool and around the control shaft. This spring positively locates the spool against a thrust washer at the threaded end of the control shaft.

The rotation of the pilot motor is converted into a linear movement of the spool within the NC-valve using the control thread on the control shaft. As the spool moves either the 'tank' or the 'pressure' ports are opened up. The flow of hydraulic medium is controlled into/out of both the piston area and piston ring area.

The closed-loop within the linear amplifier is achieved mechanically via a feedback spindle located within the piston. Located at the NC-valve end of the feedback spindle is a bush. The feedback spindle (which has an opposite thread to the control shaft) thread rotates in the same direction to the pilot motor input due to the resulting movement of the piston. This rotation moves the spool to it's neutral position. When in the neutral position the piston will once again be stationary.