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Rockwell Automation Publication 2080-UM002L-EN-E - November 2021
Appendix F PID Function Blocks
•
PID_OutputRegulator
This user-defined function block regulates the output of
IPIDCONTROLLER within a safe range to ensure that there is no
damage to the hardware used in the process.
IF RMIN
≤
RIN
≤
RMAX, then ROUT = RIN,
IF RIN < RMIN, then ROUT = RMIN,
IF RIN > RMAX, then ROUT = RMAX.
•
PID_Feedback
This user defined function block acts as a multiplexer.
IF "FB_RST" is false, FB_OUT=FB_IN;
If "FB_RST" is true, then FB_OUT=FB_PREVAL.
•
PID_PWM
This user defined function block provides a PWM function, converting a
real value to a time-related ON/OFF output.
•
SIM_WATERLVL
This user defined function block simulates the process depicted in the
application example shown before.
IMPORTANT
User Program Scan Time is 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. One can typically achieve this using STI interrupt.