Method Of Operation - Siemens PROFIBUS PA SIPART PS2 Manual

2.3

Method of Operation

SIPART PS2 PA Manual
A5E00127926-03
The electropneumatic positioner forms a control circuit with the
pneumatic actuator in which the actual value x is the position of the
actuator bar in linear actuators or the position of the actuator shaft in
rotary actuators and the command variable w is supplied digitally via
the PROFIBUS.
The stroke or rotary movement of the actuator is transferred by the
appropriate mounting accessories, the feedback shaft and a play-free
switchable gearwheel to a high quality conductive plastic potentiometer
and to the analog input of the microcontroller. The current position of
the positioner can also be set using an external sensor. For this the
stroke and the angle of rotation are sensed by a Non-Contacting
Position Sensor directly on the actuator.
The positioner may correct the angle error of the stroke tap, compares
the actual value x with the setpoint w and calculates the manipulated
variable increments Δy. Depending on the size and direction of the
control error (x-w) the piezo-controlled supply air or exhaust air valve is
opened. The volume of the actuator integrates the positioning
increments to actuating pressure y open which moves the actuator bar
or actuator shaft approximately proportionally. These positioning
increments change the actuating pressure until the control error
becomes zero.
The pneumatic actuators are available in single and double-acting
versions. Only one pressure chamber is aerated or deaerated in the
single-acting version. The resulting pressure operates against a spring.
In the double-acting version, two pressure chambers are counteractive.
In this case the one volume is deaerated when the other volume is
aerated.
The control algorithm is an adaptive predictive five-point switch (see
figure 2-8, page 30). The valves are controlled with continuous contact
at large control errors (fast step zone). At medium control errors the
valve is controlled by pulse length modulated pulses (short step zone).
No actuating pulses are output in the small control error zone (adaptive
dead zone). The dead zone adaptation and the continuous adaptation
of the minimum pulse lengths in automatic operation cause the best
possible control accuracy to be achieved at the lowest switching fre-
quency. The start parameters are determined during the initialization
phase and stored in a non-volatile memory. These are basically the real
actuating path with the mechanical limit stops, the travel times, the size
of the dead zone etc.
In addition the number of fault messages, changes in direction and the
number of strokes are determined and stored every 15 minutes during
operation. These parameters can be read out and documented by the
communication programs such as PDM and AMS. Conclusions as to
the wear on the fitting can be drawn (diagnostic function) especially by
comparing the old value with the currently determined values.
Design and Functional Principle
29