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CNT-APG002-EN 45
®
Chapter 5
Troubleshooting
This chapter offers a general troubleshooting procedure and tips for spe-
cific problems.
Troubleshooting procedure
When following this troubleshooting procedure, change only one thing at
a time, then wait to see the effect the change has on the system.
Follow these steps to troubleshoot a PID loop:
1. Make sure that the system is not in override.
2. Graph the measured variable, setpoint, and valve position over time
to determine how the system performs.
Look at the big picture. Can the system actually accomplish what it
needs to? What is happening to the measured variable? Is it oscillat-
ing or failing to reach setpoint?
3. Check the programming logic for:
• DDC statement in an *IFT or *IFF clause
• Failure conditions that are always true
• Output connected to the wrong part of a Switch block in TGP
• Conflicting programming demands, such as bad logic interlocks
between program modules
4. Check PID property settings for:
• Output minimum incorrectly set to 100%
• Output maximum incorrectly set to 0%
• Sampling frequency that is too fast
5. Check the system for disturbances from:
• Outside air intake
• Bad actuator linkages
• Faulty sensors
6. Change PID gains.
• Reduce gains when experiencing system overshoot, output at
minimum or maximum, or cycling of output around setpoint
• Increase gains when experiencing system undershoot