Allen-Bradley 2080-LC20-20AWB, User Manual

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Rockwell Automation Publication 2080-UM005E-EN-E - March 2018
185
PID Function Blocks Appendix D
5.
Calculate deviation value with reference to the fluctuation. For example, if
the temperature stabilizes around 22 °C (72 °F) with a fluctuation of
21.7…22.5 °C (71…72.5 °F), the value of ‘ATParams.Deviation’ is:
6.
Set the deviation value, if you have not set it yet.
7.
Change the initialize input to FALSE.
8.
Wait until the ‘AT_Warning’ shows 2. The autotune process is successful.
9.
Get the tuned value from the ‘OutGains’.
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
For °C:
22.5 - 21.7
2
= 0.4 For °F:
72.5 - 71
2
= 0.75