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Appendix A2
M588-00-880 Issue E
A2.11 How to use PID pressure control
The GXS pump has a built in PID pressure control and auto tune feature that can adjust the GXS dry pump speed when
the pump is on-process so that the pressure reading of a pressure sensor matches the supplied PID set point. Edwards
recommend that the PID is auto tuned after the PID set point is adjusted.
Figure A52 - The simplified system diagram
Classical PID equation
On a GXS pump the Kc, T
i
and T
d
parameters and the time constant can either be set by the user or the pump can
perform an auto tune that calculates these parameters based on the response of the system at the setpoint pressure.
The GXS pump PID pressure control sequence is designed to work with an Edwards strain gauge or Edwards active
Pirani gauge (available as accessories). Due to inaccuracies in the gauges, Edwards do not recommend trying to
control below 20 mbar with a strain gauge. Refer to Appendix A2.11.2 to use a different pressure sensor.
The following menus are used by the PDT to configure and use the PID functionality.
Where:
u is the control signal.
e is the control error.
K
c
is the gain for a proportional controller.
T
i
is the parameter that scales the integral controller.
T
d
is the parameter that scales the derivative controller.
t is the time taken for error measurement.
b is the set point value of the signal, also known as bias or offset.
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