Siemens SIPART PS2 6DR50 Series, Manual

Page: 23 / 160

Design and Functional Principle
23
SIPART PS2 Manual
A5E00074631--06
2.4 Method of Operation
The electropneumatic positioner SIPART PS2 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 part-turn actuators and the command variable w is the
actuating current of a controller or a manual control station of 4 to
20 mA.
The stroke or part-turn 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 can
also be preset for the positioner via an external sensor. The detection
of the stroke or rotation andle is performed by a non-contacting posi-
tion sensor ( N on C ontacting Position S ensor) directly at the actuator.
This may correct the angle error of the stroke tap, compares the
potemtiometer voltage as actual value x with the setpoint w fed in at
the terminals 3 and 7 and calculates the manipulated variable
increments

Δ
y. 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. See the block diagram figure 2-10, page 25.
The control algorithm is an adaptive predictive five-point switch
(see figure 2-9, page 24).
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
travel 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.