2
T 8394 EN
Principle of operation
The 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
rotational angle of the control valve and issues a signal pres-
sure (output variable y) for the pneumatic actuator.
The positioner mainly consists of the following components
–
Magnetoresistive sensor (2)
–
Analog i/p converter (6) with a downstream booster (7)
–
Electronics unit with microcontroller (4)
The travel or opening angle is measured by the external pick-
up lever, non-contact magnetoresistive sensor and down
-
stream 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
(2). The electronics unit determines the current position of the
actuator stem or opening angle from this information.
The position of the actuator stem or opening angle is transmit-
ted to the microcontroller (3) over the A/D converter (4). The
PD control algorithm in the microprocessor compares this ac-
tual 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 occurs, the activation of the i/p converter is changed so
that the actuator of the control valve (1) is pressurized or vent-
ed accordingly over the downstream booster (7). The supply
air is supplied to the booster and the pressure regulator (8).
Operation
A user-friendly, intuitive concept using three capacitive keys
and a LCD has been developed: Users select parameters by
touching the arrow keys and confirm the settings with the con
-
firmation key. In the menu, all parameters are listed in one
level, meaning there is no need to search through submenus.
All parameter settings can be read and changed on site.
All values are displayed on the LCD. The reading direction of
the LCD can be rotated by 180°.
%
S
mm
%
mm
Q
3
4
11
6
7
8
10
1
w
9
x
y
2
5
1 Control valve
2 Sensor
3 A/D converter
4 Microcontroller
5 D/A converter
6 i/p module
7 Air capacity booster
8 Pressure regulator
9 Fixed restriction
10 Volume restriction
11 Display
Fig. 4:
Functional diagram of Type 3725 Positioner
