Mode Of Operation - Siemens SIPART PS2 Series Operating Instructions Manual

Description

3.4 Mode of operation

● 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.
Depending on the size and direction of the control deviation (x-w), pressurizing or
depressurizing occurs via the pneumatic block. 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 changes 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.
For medium control deviations, the pneumatic block is controlled by pulses modulated by pulse
length. This takes place in the so-called slow step zone.
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SIPART PS2 with 4 to 20 mA/HART
Operating Instructions, 11/2019, A5E00074631-AE