42 CNT-SVX08F-EN
PID Control
Changing the Sampling Frequency
The major cause of actuator cycling is time lags in the system.
Note: If a 10% change in PID output requires two minutes to affect the process variable, it does no
good to have the
sampling frequency set to two seconds.
The integral contribution will build up before any significant change in error can be measured. A
sampling
frequency of 30 to 60 seconds would work much better in this situation. In other words,
to fix a cycling system, slow down the loop! Refer to the section, “Sampling Frequency,” p. 34.
Changing the Gains
Be careful when changing PID gains.
Note: Never change
gains unless the effects can be measured.
Use a doubling/halving technique when increasing or decreasing gain
s. If the PID gains are set to
4, 1, and 0 respectively, and you are going to reduce them, try 2, 0.5, and 0. If the system now
undershoots, try gains of 3, 0.75, and 0 respectively. Refer to the section, “Throttling Range and
Gain,” p. 32.
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