Mode Of Operation - Siemens SIPART PS2 Operating Instructions Manual

Description

3.4 Mode of operation

● Display
● Buttons
● Terminal strips to connect the option module to the basic electronics
3.4 Mode of operation
Control loop
The electropneumatic positioner forms a control loop with the pneumatic actuator:
● The actual value x represents the position of the actuator spindle for linear actuators or
the position of the actuator shaft for part-turn actuators.
● The higher-level control loop provides the setpoint w.
The stroke or rotary movement of the actuator is transferred to a potentiometer using
suitable attachments, positioner 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 (NCS) is used to record the stroke or rotary angle directly
on the actuator.
The microcontroller:
● Corrects the angle error of the shaft pick-up if necessary.
● Compares the potentiometer voltage as actual value x with setpoint w.
● Calculates the manipulated variable increments ±∆y.
The piezo-controlled inlet or exhaust air valve is opened depending on the magnitude and
direction of the control deviation (x-w). 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.
Pneumatic actuators are available in single and double-acting versions. In a single-acting
version, only one pressure chamber is ventilated and depressurized. The pressure
developed works against a spring. In a double-acting version, two pressure chambers work
against each other. Ventilating the volume of one chamber simultaneously depressurizes the
volume of the other.
Control algorithm
The control algorithm is an adaptive, predictive five-point controller.
In case of large control deviations, the valves are controlled using permanent contact. This
takes place in the so-called fast step zone.
In case of medium control deviations, valves are controlled using pulse-length modulated
pulses. This takes place in the so-called slow step zone.
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SIPART PS2 with and without HART
Operating Instructions, 02/2016, A5E00074631-AB