Rockwell Automation 1747-PT1, D1747NP002 Hand-Held Terminal - Manual (Page 368)

Chapter 26
PID Instruction
26–24
8. Adjust the gain while observing the relationship of the output to the
setpoint over time.
Note that gain adjustments disrupt the process when you change values.
To avoid this disruption, switch to the MANUAL mode prior to making
your gain change, then switch back to the AUTO mode.
9. When you notice that the process is oscillating above and below the
setpoint in an even manner, record the time of 1 cycle. That is, obtain the
natural period of the process. Record the gain value. Return to the
MANUAL mode (stop the process if necessary).
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.
If the cycle time is 20 seconds for example, 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 will be 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 will be 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
all “integral buildup” and “gain error” is removed at the time each
adjustment is made. This allows you to see the effects of each adjustment
immediately.