Samson 3730-0 Mounting and operating instructions - Design And Principle Of
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Design and principle of
operation
The electropneumatic positioner is mounted
to pneumatic control valves and is used to
assign the valve position (controlled vari-
able x) to the control signal (reference vari-
able w). The DC control signal received from
a control unit is compared to the travel of
the control valve and issues a signal pres-
sure (output variable y).
A supply pressure between 1.4 to 6 bar is
required. The electric input signal is a refer-
ence variable between 4 to 20 mA.
The positioner is designed depending on the
corresponding accessories for direct attach-
ment to Type 3277 Actuators or for attach-
ment to actuators according to IEC 60534-6
(NAMUR).
The positioner basically consists of a travel
sensor system which is proportional to resis-
tance, an analog i/p converter with down-
stream air capacity booster, and analog
controller electronics.
The position of the valve is transmitted as
linear travel motion via pick-up lever and
travel sensor (2) to an analog PD controller
(3). The PD controller compares this actual
position to the 4 to 20 mA DC control signal
received from a control unit. In case of a
system deviation, the operation of the i/p
converter (6) is changed so that the actuator
(1) is filled or vented via the downstream air
capacity booster (7). This causes the valve
plug to move to the position determined by
the reference variable.
The pneumatic air capacity booster (7) and
the pressure regulator (8) are provided with
supply air. An intermediate flow regulator
(9) with fixed settings is used to purge the
positioner and also guarantees trouble-free
operation of the pneumatic booster.
The output signal pressure supplied by the
booster can be limited over the DIP switch
S5.
The volume restriction Q (10) and the switch
S6 are used to optimize the positioner by
adapting it to the actuator size and chang-
ing the gain factor.
Tight-closing function:
After activating the tight-closing function (re-
fer to section 7.12), the pneumatic actuator
is completely filled with air or vented as
soon as the reference variable falls below or
exceeds the corresponding deactivating
point of 4.5 mA or 19.5 mA.
In the case of three-way valves, the function
allows the plug move to the end position
with full actuator force.
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EB 8384-0 EN
Design and principle of operation