MTS Series 256 Servovalve - Manual (Page 16)

Series 256 Servovalve Product Information 16 Introduction
Moving the spool
A command from the controller causes the armature to rotate clockwise or
counterclockwise (depending on the polarity of the command). The command
causes the flapper to block one of the nozzles which diverts hydraulic flow to that
end of the spool. The spool moves and opens hydraulic pressure to one control
port and the return line to the other control port. The control ports are connected
to each end of the actuator.
Stopping the spool
The feedback wire works like a spring. The spool moves until the feedback wire
torque equals the torque from the magnetic forces. This causes the flapper to
move back toward the centered position. The spool stops at a position where the
feedback wire spring torque equals the torque input current of the command. The
spool position is proportional to the input command current.
Although the pressures are equal on both sides of the spool (so the spool is no
longer moving), control flow from the servovalve keeps the actuator moving.
Stopping actuator
movement
When the actuator has moved the desired amount, the valve drive command
decreases to zero. Hydraulic fluid flow to the actuator stops, and so does the
actuator.
Series 256 Servovalve—Multiple Stages
The Series 256 Servovalve contains three stages. The first and second stages are
formed by a Series 252 Servovalve. The first stage is a torque motor armature
that controls a flapper valve in the second stage. This valve, in turn, controls the
flow of hydraulic fluid (and therefore the movement of the spool) in the second
stage. The position of the spool in the second stage, in turn, controls the flow of
hydraulic fluid to the third stage. The third stage of the Series 256 Servovalve is
the main stage, which contains a larger four-way spool. The position of the third
stage spool is measured by a linear variable differential transformer (LVDT).