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Symptom: People who
tune controllers spend unnecessary time in the tuning process. The end
result is usually not the desired performance.
Probable cause:
Without clear knowledge of the process, controller tuning becomes a trial
and error procedure. Most people who attempt to tune controllers have never
been exposed to a technique to understand and define the process. These
people usually have a fear of bumping the process.
Corrective action:
An open loop response test can give the process definition to greatly help
in controller tuning.
The plot shown is an example
of an open loop response test of a self regulating process. It is performed
by placing the controller in manual and changing the controller output
(labeled here as CO) a small amount so as not to cause a process or
operational problem.
From the plot, the process
parameters can be determined:
Deadtime - The
time from the CO bump until the PV starts to move.
Time constant
- The time required for the PV to change 63% of its complete change.
Process gain - The
process gain is calculated by Gain =
DPV/DCO.
The CO is then bumped back
to its original starting value. A test at a different controller output may
result in different process parameters. |
