Servo Contollers December Cowen

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Operation of a Servovalve in aHydraulic Actuator

December R. Cowen

University of Illinois - UC

March 6, 2002



Outline

General Uses of Hydraulic Actuators

Basic Hydraulic System Operation

Servovalve Location and Components

Servovalve Operation



General Uses of Hydraulic

Actuators

Static, Psuedodynamic, and Dynamic Testing

Load or Position Control



Basic Hydraulic System

Operation



Servovalve Location



Role of Servovalve in System

Servovalve reacts to an input signal from a valve

driver or valve controller by porting hydraulic fluid

in a direction and amount determined.



Servovalve Components

Torque Motor

Two Stages of Hydraulic

Regulation




Torque Motor

Coils

Upper and Lower Pole Pieces

Armature

Two Magnets

Converts input signal from a valve driver or valve controller

into a physical movement for the armature




Second Stage of Regulation

A spool that controls the hydraulic fluid flow to and from

actuator A feedback wire which provides the closed loop control

between the armature and spool




The control signal energizes the coils causing the armature to

rotate.

The armature rotates clockwise or counter clockwisedepending upon the polarity of the signal. The rate at which the

armature rotates is dependent upon the magnitude of the signal.

As the armature/flapper assembly rotates it controls the flow

of hydraulic fluid from the two nozzles.This is an inversely proportional manner; as the flow from one

nozzle increases, the flow from the other decreases.




This resultant change in flow of hydraulic fluid creates a

differential pressure which is used to position the second stage

spool. As the spool moves in response to the differences in pressure,

the feedback wire exerts an opposite torque on the

armature/flapper assembly trying to re-center the flapper.

The spool will continue to move until the feedback wire torqueequals the control signal torque and re-centers the flapper.

Now both nozzle openings are equal.




This causes the pressure at the ends of the spool to equalize

and then the spool will stop.

For any level of input control signal there will be a

corresponding spool position dependent upon the magnitude and

polarity of the control signal.

Therefore, the position of the spool controls the amount, rate,

and duration of flow to the actuator.




When the control signal decreases to zero, the flapper

creates a pressure imbalance to the opposite end of the

spool.

In response to the pressure imbalance, the spool movesuntil the pressure is equalized. The spool moves to a

position that stops the flow of the hydraulic fluid to the

actuator.



Acknowledgements

Greg Banas, Lab Director for NCEL

MTS Training Manual

Servo Contollers December Cowen

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