IM-P343-44 CTLS Issue 56
EP500 Standard Electropneumatic Positioner
2.1 Introduction
The EP500 is a 2 wire loop powered positioner requiring a 4-20 mA control signal, and is
designed for use with linear pneumatic valve actuators. The positioner compares the electrical
signal from a controller with the actual valve position and varies a pneumatic output signal to
the actuator accordingly. A mounting kit is supplied to suit all pneumatic actuators compliant
with the NAMUR standard.
2.2 Operating principle
The current signal (1) flows in the coil (2) and generates a magnetic field which attracts the flapper
(3) tapping the nozzle (4). As a consequence of this the pressure inside the actuator increases
and the valve moves. When the valve stem (5) moves it generates through the feedback lever
(6) and the feedback spring (7) a negative feedback which moves the flapper away from the
nozzle and stops the valve movement.
Changes in the current signal will induce changes in the valve positioning.
This mechanism will build a linear correspondence between the current signal and the valve
position expressed in percentage of the valve stroke. The valve opening at 4 mA and at 20 mA
can be adjusted manually as illustrated in Figure 1.
Fig. 1 Functional schematic
2
1
3
4
7
8
10
5
6
9
2. General product information
6 = Feedback lever
7 = Feedback spring
8 = Pneumatic relay
9 = Pneumatic actuator
10 = Span lever
Key:
1 = Current signal
2 = Coil
3 = Flapper
4 = Nozzle
5 = Valve stem
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