
14 EB 8384-1 EN
Design and principle of operation
3 Design and principle of oper-
ation
ÎRefertoFig.1
The electropneumatic positioner is mounted
on pneumatic control valves and is used to
assign the valve position (controlled variable
x) to the control signal (set point w). The po-
sitioner compares the electric control signal
of a control system to the travel or opening
angle of the control valve and issues a signal
pressure (output variable y) for the pneumat-
ic actuator.
The positioner is designed depending on
which accessories are selected either for di-
rectattachmenttoSAMSONType3277Ac-
tuators or for attachment to actuators ac-
cordingtoNAMUR(IEC60534-6).
Additionally, a coupling wheel included in
the accessories is required to transfer the ro-
tary motion for rotary actuators according to
VDI/VDE3845.
Springless rotary actuators require a revers-
ingamplierincludedintheaccessoriesto
permit the powered operation in either direc-
tion.
The positioner consists of a travel sensor sys-
tem proportional to resistance, an analog
i/p converter with a downstream air capaci-
ty booster and the electronics with microcon-
troller.Thepositioneristtedwithtwoad-
justable software limit contacts as standard
to indicate the valve's end positions.
The position of the valve stem is transmitted
as a either an angle of rotation or travel over
the pick-up lever to the travel sensor (2) and
supplied to an analog PD controller (3). An
%
S
mm
%
mm
w
x
Q
PD
A2
A1
45
3
11
12
6
7
8
10
1
w
9
x
y
2
G
13
1 Control valve
2 Travel sensor
3 PD controller
4 A/D converter
5 Microcontroller
6 i/p converter
7 Air capacity booster
8 Pressure regulator
9 Flow regulator
10 Volume restriction
11 Limit contacts
12 Display
13 Inductive limit contact
(optional)
Fig.1: Functional diagram
A/D converter (4) transmits the position of
the valve to the microcontroller (5).
The PD controller (3) compares this actual
positiontothe4to20mADCcontrolsignal
(reference variable) after it has been convert-
ed by the A/D converter (4).
In case of a system deviation, the activation
of the i/p module (6) is changed so that the
actuator of the control valve (1) is pressur-
ized or vented accordingly over the down-
stream booster (7). This causes the valve
plug to move to the position determined by
the set point.
The supply air is supplied to the booster and
the pressure regulator (8). An intermediate
owregulator(9)withxedsettingsisused
to purge the positioner and, at the same
time, guarantees trouble-free operation of
the booster.
The output signal pressure supplied by the
boostercanbelimitedto2.4barbyactivat-
ing the P9 parameter.
The volume restriction (10) is used to opti-
mize the positioner by adapting it to the ac-
tuator size.
Tight-closing function:
Thepneumaticactuatoriscompletelylled
with air or vented as soon as the set point
fallsbelow1%orexceeds99%(seeset
point cutoff function in P10 and P11 param-
eters).