Samson 3725 series, Mounting And Operating Instructions

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8 EB 8394 EN
Design and principle of operation
3 Design and principle of oper-
ation
The Type 3725 Electropneumatic Positioner is
mounted on pneumatic control valves and is
used to assign the valve position (controlled
variable x) to the control signal (reference
variable w). The positioner 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 pneumatic actuator.
The positioner mainly consists of the follow
-
ing components (see Fig. 1):
−
Magnetoresistive sensor (2)
−
Analog i/p converter (6) with a down -
stream air capacity booster (7)
−
Electronics unit with microcontroller (4)
The travel or opening angle is measured by
the pick-up lever, non-contact magnetoresis -
tive sensor and downstream electronics.
The pick-up lever is connected to a magnet
inside the device. The motion of the pick-up
lever causes the direction of the magnetic
field to change. This change is sensed by the
sensor. The electronics unit determines the
current position of the actuator stem or open -
ing angle from this information.
The position of the actuator stem or opening
angle is transmitted to the microcontroller (3)
over the A/D converter (4). The PD control
algorithm in the microprocessor compares
this actual position to the 4 to 20 mA control
signal after it has been converted by the
A/D converter (3). In case of a system devi
-
ation, the activation of the i/p module (6) is
changed so that the actuator of the valve (1)
is pressurized or vented accordingly over the
downstream booster (7). The supply air is
supplied to the booster and the pressure reg-
ulator (8).
The output signal pressure supplied by the
booster can be limited to 2.3 bar by soft -
ware.
The volume restriction Q (10) is used to opti -
mize the positioner by adapting it to the ac -
tuator.
Tight-closing function
The pneumatic actuator is completely filled
with air or vented as soon as the reference
variable falls below 1 % or exceeds 99 %
(see end positions set in P10 and P11 pa
-
rameter codes).