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Symptom:
Do
you observe that your liquid level applications are difficult to control?
Are they somewhat unstable or change performance reactions at different
times?
Probable cause:
Liquid
levels are different from most other processes in that they normally
integrate rather than self regulate. (See
Loop Tip #3). Therefore, the controller tuning should be compatible for
an integrating process.
Corrective action:
An open loop response
test will define the integrating process so that controller tuning can
result in the desired performance.
The open loop response
is the same procedure as in any process. (See Loop Tip #3). With the
controller in manual, bump the controller output (CO) a small amount, but
with enough of a change to cause the PV to move. Once the PV has moved
enough to give a good trend, bump the CO back to the original value.
The process output is
the slope of the PV change in % of span per time period. The process input
is the change of CO. Therefore the process gain is:
This integrating
process gain becomes a time function of a value per time. This example is
for a tank with vertical and parallel sides. A different tank shape will
result in different process gains at different levels |
