10 EB 8395 EN
Design and principle of operation
3 Design and principle of oper-
ation
The Type3724 Positioner is delivered as a
ready-mounted unit on Type3379 Pneumatic
Piston Actuators.
The positioner is used to assign the valve po-
sition (controlled variable x) to the control
signal (reference variable w). The positioner
compares the electric control signal of a con-
trol system to the travel of the control valve
(1) and issues a signal pressure (output vari-
abley) 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)
−Software limit contacts (12)
The travel is measured by an internal pick-up
rod, which is connected to a magnet, as well
as a non-contact magnetoresistive sensor
and the downstream electronics.
The motion of the pick-up rod causes the di-
rection of the magnetic eld to change. This
change is sensed by the sensor (2). The elec-
tronics unit determines the current valve posi-
tion from this information.
The position of the valve is transmitted to the
microcontroller (4) over its A/D converter
(3). The microcontroller contains a modied
PID controller which compares the actual
valve position with the 4 to 20mA control
signal. The resulting output value is passed
on to the D/A converter. In case of a system
deviation, the activation of the i/p module
(6) is changed so that the actuator (1) is
pressurized or vented accordingly over the
downstream air capacity booster (7). The
supply air is supplied to the booster (7) and
the pressure regulator (8).
Unauthorized manual adjustments to the po-
sitioner will damage it!
Do not move the pick-up rod manually.
The output signal pressure supplied by the
booster can be limited to 2.3bar 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 lled
with air or vented as soon as the reference
variable falls below 1% or exceeds 99%
(see set point cutoff in P10 and P11 parame-
ter codes).
NOTICE
!
