MTS Series 252 Servovalves - Manual (Page 14)

Series 252 Servovalve Product Information 14
About the Servovalve
Introduction
About the Servovalve
The heart of a servohydraulic system is the servovalve. It is the final control
element in most MTS closed-loop systems. The servovalve responds to
command signals generated by the software and processed by the controller and
output through the valve driver module. The servovalve regulates the direction
and flow of the hydraulic fluid entering the actuator from the hydraulic pressure
ports. The direction that the spools move determines the direction of fluid flow to
the actuator. A pressure difference is what causes the fluid to move.
Typical Closed-Loop System
Servovalve Function
Control signal
In a closed-loop hydraulic system, the servovalve uses the control signal from an
electronic control device (controller) to operate a valve that regulates the
movement of a hydraulic actuator.
The control signal is created by comparing the program command signal (the
desired actuator position) and the feedback signal from a transducer (the actual
actuator position). Any difference between the two is called DC error, which is
the command to the servovalve to supply hydraulic fluid to the actuator until the
desired actuator position is achieved.
Servovalve at rest
The servovalve’s controlling element is the torque motor, which receives an
electrical input from the controller. A flapper is attached to the armature of the
torque motor. The flapper moves from side to side as the armature moves in
response to control signals from the controller. The flapper assembly is
mechanically attached to the armature. There are two nozzles, one on each side of
the flapper.
Because the nozzle-flapper valve is the first control point of hydraulic fluid, it is
called the first stage. As long as there is no command for actuator motion, the
flapper is centered between the two nozzles.
At the same time, pressurized hydraulic fluid entering the valve is applied
equally to both sides of the spool, which does not move. This is the second stage.
Test
Command
Control
Signal
Inner
Loop
Outer
Loop
Manifold
Hydraulic
Power
Unit
Valve Driver
Feedback
Controller
Control
Mode