Allen-Bradley 2080-LC20-20QWB User Manual page 172

Appendix D IPID Function Block
164
User Program Scan Time is Important
IMPORTANT
The autotuning method needs to cause the output of the control loop to
oscillate. In order to identify the oscillation period, the IPID must be
called frequently enough to be able to sample the oscillation adequately.
The scan time of the user program must be less than half the oscillation
period. In essence the Shannon, or Nyquist-Shannon, or the sampling
theorem must be adhered to.
In addition, it is important that the function block is executed at a
relatively constant time interval.
Rockwell Automation Publication 2080-UM005B-EN-E - March 2015