Publication 1766-RM001A-EN-P - October 2008
356
Process Control Instruction
PID Rung State
If the PID rung is false, the integral sum (IS) is cleared and CV remains in
its last state.
Feed Forward or Bias
Applications involving transport lags may require that a bias be added to
the CV output in anticipation of a disturbance. This bias can be
accomplished using the processor by writing a value to the Feed Forward
Bias element (word FF). (See page 343.) The value you write is added to
the output, allowing a feed forward action to take place. You may add a
bias by writing a value between -16383 and +16383 to word 6 with your
programming terminal or ladder program.
Application Examples
PID Tuning
PID tuning requires a knowledge of process control. If you are
inexperienced, it will be helpful if you obtain training on the process
control theory and methods used by your company.
There are a number of techniques that can be used to tune a PID loop.
The following PID tuning method is general and limited in terms of
handling load disturbances. When tuning, we recommend that changes be
made in the MANUAL mode, followed by a return to AUTO. Output
limiting is applied in the MANUAL mode.
Procedure
1.
Create your ladder program. Make certain that you have properly
scaled your analog input to the range of the process variable PV and
that you have properly scaled your control variable CV to your
analog output.
TIP
•
This method requires that the PID instruction controls a non-critical
application in terms of personal safety and equipment damage.
•
The PID tuning procedure may not work for all cases. It is strongly
recommended to use a PID Loop tuner package for the best result (i.e.
RSTune, Rockwell Software catalog number 9323-1003D).
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