Cashco Siemens PS2 I/P - Manual (Page 22)

Design and Functional Principle
22
SIPART PS2 Manual
A5E00074631-01
2.3 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 rotary 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 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.
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 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 action
versions. Only one pressure chamber is aerated or deaerated in the
single action version. The resulting pressure operates against a spring.
In the double action 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 2-9, page 24.
The control algorithm is an adaptive predicative five--point switch (see
figure 2-8, page 23).
The valves are controlled with continuous contact at large control errors
(fast action zone). At medium control errors the valve is controlled by
pulse length modulated pulses (slow action 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 floating 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 hour 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.
Figure 2-9, page 24 shows the block diagrams for single-- and double--
acting actuators with the linear actuator as an example.