Emerson Process Management El-O-Matic E and P Series E25-E950 - Handbuch (Seite 33)

6 DOC,F20,EDN Rev.: -
1.2 Operating Principles
The positioner provides the means for a pneumatic actuator to
be accurately positioned to any point between full open and full
closed position. The actuator’s movement is controlled in
proportion to a 4-20 mA incoming signal.
The actuator’s movement is monitored by the integral feedback
potentiometer (7), the signal from this and the incoming signal
are fed to the comparative electronics on the main circuit board
(1).
Providing these two signals are equal the resulting signal sent
to the I/P converter (2) is “Neutral” and the spool of the pilot
valve (3) is held in the mid or blocked position.
In this state the actuator remains locked in the last set position,
that is at it’s “Set point”.
If, either the input mA. signal or the valve position changes then
the difference is sensed and the signal to the I/P is either
increased or decreased. This causes a corresponding move-
ment of the pilot spool (3) which in turn starts the actuator’s
movement (4) towards the new “Set point”. On reaching this the
two signals are again equal and movement stops at this new
“Set point”.
For single acting (spring return) actuators only a single air line
is used, the other port at the spool valve (3) is plugged off.
1.0 Introduction / Einführung / Introductie
1 2 3
4
5
6 7
4-20 mA
Signal input
4-20 mA Signal input =
4-20 mA Eingangssignal =
4-20 mA Stuursignaal
1.1