1
Design and principle of
operation
The pneumatic positioner is used for the cor-
relation between the valve stem position
(controlled variable x) and the input signal
(reference variable w).
In this case, the input signal issued by a con-
troller is compared to the travel (valve stem
position) of the control valve, and a pneu-
matic signal pressure (output variable y) is
delivered.
The positioner consists of a lever with at-
tached shaft and range spring, diaphragm
and the pneumatic control system compris-
ing nozzle, flapper plate and booster.
In addition, a pressure gauge can optionally
be attached for the instrument input signal
and the positioner output signal.
The positioner operates according to the
force-balance principle. In this way, the
stroke of the actuator stem or the plug stem
(controlled variable x) is transmitted to the
lever (1) and the range spring (6) by the
plate (20). This action twists the range
spring and varies the torsional force.
The input signal (p
e
) supplied by the up-
stream controller produces a positioning
force on the diaphragm (8). This force is
compared with the force of the range spring
(6). At the same time, the motion (deflection)
of the diaphragm is transmitted to the flap-
per plate (10.2) by the feeler pin (9.1),
causing pressure to be released from the
nozzle (10.1).
Supply air is supplied to the pneumatic
booster (12) and flows through the X
p
re-
striction (13) and the nozzle (10.1) until it
hits the flapper plate (10.2).
6
EB 8359-1 EN
Design and principle of operation
10.2
13
14
3
1
15
7
8
4
5
6.1 6
Fig. 2 · Positioner with cover removed
