Allen-Bradley MicroLogix 1200 Reference Manual page 268

19-26 Process Control Instruction
Publication 1762-RM001C-EN-P
Determining the Initial Loop Update Time
To determine the approximate loop update time that should be used for
your process, perform the following:
1. Place the normal application values in MinS and MaxS.
2. Type: 50 in CO%.
3. Type: 60 in CO% and immediately start your stopwatch.
4. Watch the PV. When the PV starts to change, stop your stopwatch.
Record this value. It is the deadtime.
5. Multiply the deadtime by 4. This value approximates the natural
period. For example, if deadtime = 3 seconds,
then 4 x 3 = 12 seconds ( ≅ natural period)
6. Divide the value obtained in step 5 by 10. Use this value as the loop
updated time. For example, if:
natural period = 12 seconds, then 12/10 = 1.2 seconds.
Therefore, the value 120 would be entered as the loop update time.
(120 x 10 ms = 1.2 seconds)
7. Enter the following values: the initial setpoint SP value, a reset T
a rate T of 0, a gain K
d
step 17.
Set the PID mode to STI or Timed, per your ladder diagram. If STI is
selected, ensure that the loop update time equals the STI time interval.
Enter the optional settings that apply (output limiting, output alarm,
MaxS - MinS scaling, feed forward).
8. Return to page 19-23 and complete the tuning procedure starting with
step 4.
of 1, and the loop update time determined in
c
of 0,
i