Publication 1762-RM001C-EN-P
19-24 Process Control Instruction
10. Set the loop update time (and STI time interval if applicable) to a
value of 5 to 10 times faster than the natural period.
For example, if the cycle time is 20 seconds, and you choose to set the
loop update time to 10 times faster than the natural rate, set the loop
update time to 200, which would result in a 2-second rate.
11. Set the gain K
c
value to 1/2 the gain needed to obtain the natural
period of the process. For example, if the gain value recorded in step
9 was 80, set the gain to 40.
12. Set the reset term T
i
to approximate the natural period. If the natural
period is 20 seconds, as in our example, you would set the reset term
to 3 (0.3 minutes per repeat approximates 20 seconds).
13. Now set the rate T
d
equal to a value 1/8 that of the reset term. For our
example, the value 4 is used to provide a rate term of 0.04 minutes
per repeat.
14. Place the process in the AUTO mode. If you have an ideal process,
the PID tuning is complete.
15. To make adjustments from this point, place the PID instruction in the
MANUAL mode, enter the adjustment, then place the PID instruction
back in the AUTO mode.
This technique of going to MANUAL, then back to AUTO, ensures that
most of the “gain error” is removed at the time each adjustment is
made. This allows you to see the effects of each adjustment
immediately. Toggling the PID rung allows the PID instruction to
restart itself, eliminating all of the integral buildup. You may want to
toggle the PID rung false while tuning to eliminate the effects of
previous tuning adjustments.
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