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Chapter 5 Troubleshooting
46 CNT-APG002-EN
Tips for specific problems
Table 17 provides tips for troubleshooting specific problems.
Changing the sampling frequency
The major cause of actuator cycling is time lags in the system. If a 10%
change in PID output requires two minutes to affect the measured vari-
able, 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. See “Sampling frequency” on page 12 for more infor-
mation.
Changing the gains
Be careful when changing PID gains. Never change the gains unless the
effects can be measured. Use a doubling/halving technique when increas-
ing or decreasing gains. 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 “Throttling
range” on page 9 for more information.
Ta b le 17 : Tips for specific problems
Problem Tips
Measured variable is
cycling around setpoint
• Slow the sampling frequency
• Decrease PID gains
• Check programming for conflicting actuator
commands
• Actuators may be overridden by minimum-on
and -off times
Overshooting setpoint Reduce gains
Undershooting setpoint Increase gains
Output at maximum Ensure that minimum output is not set to 100%
Output at minimum Ensure that maximum output is not set to 0%