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Description
3.5 Functional principle
Figure 3-13
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②
③
3.5
Functional principle
3.5.1
Control system
The electropneumatic positioner and an actuator form a control system. The current position
of the actuator is recorded by a servo potentiometer and the actual value x is fed back. The
setpoint and the actual value are simultaneously shown on the digital display.
The control system provides the setpoint digitally to the positioner through the fieldbus. The
bus interface of the fieldbus version of the positioner differs from the previous versions.
However, the basic functions of the positioner as well as the operation and the display are
almost unchanged.
The lifting or rotary movement of the actuator is transferred to a high-quality conductive
plastic potentiometer using suitable attachments, feedback shaft and a backlash-free,
switchable gear drive, and then to the analog input of the microcontroller. The current
position can also be forwarded to the positioner using an external sensor. A Non-Contacting
Position Sensor is used to record the lifting or rotation angle directly on the actuator.
3.5.2
Microcontroller
If required, the microcontroller corrects the angle error of the feedback lever bracket,
compares the potentiometer voltage as an actual value x with the setpoint w that is fed
through terminals 3 and 7, and calculates the controller output increment ±∆y. Depending on
the magnitude and the direction of the control deviation (x-w), the piezo advance controlled
supply or exhaust air valve is opened. The actuator volume integrates the controller
increment for the actuating pressure y which is proportional to the drive rod or the drive
shaft. This controller increment change the actuating pressure until the control deviation
becomes zero.
30
Restrictors
Restrictor Y1
Restrictor Y2, only in the version for double-acting actuators
Hexagon socket-head screw 2.5 mm
SIPART PS2 with PROFIBUS communication
Operating Instructions, 09/2007, A5E00127926-07
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