188 Rockwell Automation Publication 2080-UM005E-EN-E - March 2018
Appendix D PID Function Blocks
a pre-defined function block, IPIDCONTROLLER, and four user-defined
function blocks. These four are:
• 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.
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.
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