Samson 3767 - Design and Principle of Operation

6 EB8355-2EN
Designandprincipleofoperation
2 Designandprincipleofoper-
ation
The positioner consists of an electropneumat-
ic converter and a pneumatic unit equipped
withaleverfortravelpick-up,ameasuring
diaphragm, and the pneumatic control system
withnozzle,diaphragmlever(apperplate),
andbooster.
The positioner is designed either for direct
attachmenttoSAMSONType3277Actuators
or for attachment to actuators according to
NAMUR(IEC60534-6)usinganadapter
housing.
Thepositionercanbeadditionallyequipped
with either inductive limit contacts and/or a
solenoid valve or position transmitter.
Thecontrolsignal,e.g.4to20mA,issuedby
the controller is transmitted to the electropneu
-
maticconverter(13)whereitisconvertedinto
a proportional pressure signal p
e
.
The positioner operates according to the
force-balanceprinciple.Thevalvetravel,i.e.
thevalveposition,istransmittedtothepick-
uplever(1)overthepin(1.1)anddetermines
theforceoftherangespring(4).Thisforceis
compared to the positioning force generat
-
edbythepressurep
e
at the measuring dia-
phragm(5).
If either the control signal or the valve posi
-
tionchanges,thediaphragmlever(3)moves,
altering the distance to the nozzle (2.1 or
2.2),dependingontheadjusteddirectionof
action of the positioner.
Thesupplyairissuppliedtothebooster(10)
andthepressureregulator(9).
ThecontrolledsupplyairowsthroughtheX
p
restriction(8)andthenozzle(2.1,2.2)and
hitsthediaphragmlever(apperplate).
Anychangeinthereferencevariableorthe
valve position causes the pressure to change
upstreamordownstreamofthebooster.
Theaircontrolledbythebooster(signalpres
-
sure p
st
)owsthroughthevolumerestriction
(11)tothepneumaticactuator,causingthe
plug stem to move to a position correspond
-
ingtothereferencevariable.
TheadjustableX
p
restriction(8)andvolume
restriction(11)areusedtooptimizetheposi
-
tioner control loop.
Thepick-uplever(1)andtherangespring
(4)mustbeselectedtomatchtheratedvalve
travel and the nominal span of the reference
variable.
Positionerwithinductivelimitcontacts
In this version, the rotary shaft of the position-
ercarriestwoadjustabletagswhichactuate
thebuilt-inproximityswitches.
Positionerwithsolenoidvalve
When the positioner is equipped with a so-
lenoidvalve,thevalvecanbemovedtothe
fail-safeposition,regardlessoftheposition
-
er's output signal. If a control signal corre-
spondingtothebinarysignal'0'(OFF)isap-
plied to the input, the signal pressure p
st
is
shut off and the actuator is vented. The actu
-
atorspringsmovethevalvetoitsfail-safepo-
sition.