Samson 4763 - Design and Principle of Operation

EB 8359-2 EN 3-1
Design and principle of operation
3 Design and principle of oper-
ation
The electropneumatic positioner is used to
assign the valve position (controlled variable)
to the input signal (set point). The positioner
compares the control signal of a control sys-
tem to the travel of the control valve and is-
sues a signal pressure (output variable) for
the pneumatic actuator.
The positioner consists of an electropneumat-
ic converter unit (21) and a pneumatic unit
equipped with a lever (1), shaft (1,1) and
range spring as well as the pneumatic con-
trol system with nozzle, apper plate and
booster. The DC control signal, e.g. 4 to
20mA, issued by the controller, is transmit-
ted to the electropneumatic converter unit
(i/p converter) where it is converted into a
proportional pressure signal p
e
.
Any change of the input current signal caus-
es a proportional change of the air pressure
p
e
fed to the pneumatic control system.
The air pressure p
e
produces a force which
acts on the surface of the measuring dia-
phragm (8) and is compared to the force of
the range spring (6).
The motion of the measuring diaphragm (8)
is transferred to the apper plate (10.2) over
the feeler pin (9.1) and the nozzle (10.1) re-
leases pressure. Any change of either the air
pressure p
e
or the valve stem position causes
the pressure to change in the booster (12)
connected downstream of the nozzle. The
signal pressure p
st
which is released causes
the plug stem to assume a position based on
the set point.
The adjustable volume restriction Q (14) and
Xp (gain) restriction (13) are used to opti-
mize the control loop of the positioner.
The range spring (6), which can be ex-
changed, is assigned to both the rated valve
travel and the span of the set point.
Legend for Fig.3-1 and Fig.3-2
1 Lever for valve travel
1.1 Shaft
2 Pin
2.1 Nut
3 Sleeve
4 Zero point adjustment
5 Fastening screw
6 Range spring
6.1 Bracket
7 Fastening screw
8 Measuring diaphragm
9 Diaphragm plate
9.1 Feeler pin
10 Nozzle block
10.1 Nozzle
10.2 Flapper plate
11 Cover plate
12 Booster
13 Xp restriction
14 Volume restriction Q
15 Hole for fastening screw
20 Plate
21 i/p converter