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Rockwell Automation Publication 2080-UM002N-EN-E - November 2022 329
Appendix E PID Function Blocks
How Autotune Works
The auto tune process begins when the ‘Initialize’ is set to FALSE (Step 7.) At this moment, the
control output increases by the amount of ‘Step’ and the process waits for the process value to
reach or exceeds ‘first peak’.
First peak is defined as:
For Direct Operation: First peak = PV1 - (12 x Deviation)
For Reverse Operation: First peak = PV1 + (12 x Deviation)
Where PV1 is the process value when Initialize is set to FALSE.
Once the process value reaches first peak, the control output reduces by the amount of Step
and waits for the process value to drop to the second peak.
Second peak is defined as:
For Direct Operation: Second peak = PV1 - (3 x Deviation)
For Reverse Operation: Second peak = PV1 + (3 x Deviation)
Once the process value reaches or falls below second peak, calculations commence and a set
of gain will be generated to parameter OutGains.
Troubleshooting an
Autotune Process
You can tell what is going on behind the autotune process from the sequences of control
output. Here are some known sequences of control output and what it means if autotune fails.
For the ease of illustrating the sequence of control output, we define:
• Load: 50
• Step: 20
Output Sequence 1: 50
✟
70
✟
30
Sequence Condition Autotune Result Action for Autotune Fail
Process value reached 'first peak' and
'second' peak in time
Likely successful NA
Output Sequence 2: 50
✟
70
✟
50
Sequence Condition Autotune Result Action for Autotune Fail
Process value not able to reach 'first peak' Likely unsuccessful Reduce Deviation or Increase Step
Output Sequence 3: 50
✟
70
✟
30
✟
50
Sequence Condition Autotune Result Action for Autotune Fail
Process value not able to reach second peak Likely unsuccessful Increase Deviation or increase Step
Output Sequence 4: 50
✟
70
Sequence Condition Autotune Result Action for Autotune Fail
Process value not able to reach First peak in
time
Likely unsuccessful Increase ATDynamSet