Design And Principle Of Operation - Samson 3767 Mounting And Operating Instructions

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 (flapper 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
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
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).
6
.
e
at the measuring dia-
e
The controlled supply air flows through the X
restriction (8) and the nozzle (2.1, 2.2) and
hits the diaphragm lever (flapper 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 ) flows through the volume restriction
st
(11) to the pneumatic actuator, causing the
plug stem to move to a position correspond-
ing to the reference variable.
The adjustable X restriction (8) and volume
p
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
shut off and the actuator is vented. The actu-
ator springs move the valve to its fail-safe po-
sition.
p
is
st
EB 8355-2 EN