Siemens 6DR55 Series, Manual

Page: 32 / 178

Design and method of operation
30
SIPART PS2 PA Manual
A5E00127926--05
2.4 Method of operation
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
part--turn actuators and the command variable w is supplied digitally via
the PROFIBUS.
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 of
the positioner can also be set using an external sensor. Determining
the stroke distance or the angle of rotation is done by an external
position detection system (e.g. non--contacting position sensor or a
potentiometer) 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

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.
The control algorithm is an adaptive predictive five--point switch (see
figure 15, page 31). 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
frequency. 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 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.