Appendix I B-3
PID Parameters
STEPS:
Set Default Parameters
1 Turn off all derivative control by setting Er and EH to “0”.
2 Minimize integral control by setting nt and nH to “800”.
3 Set proportional control of bn and PBH to an initial value of approximately
10% of setpoint. For example, if your setpoint is 150°F {66°C}, set these
parameters to “15”.
Run a Test - Proportional
4 Start the Thermolator and observe it attempting to reach setpoint.
5 Decrease the values of bn and PBH until the system beings to oscillate around
the setpoint. You may have to cool-down your system and repeat the experiment
several times so you can accurately observe the process temperature approaching
setpoint.
6 Multiply the value determined by step 5 by “2” and enter it as parameters bn
and PBH.
Continue the Test – Proportional + Integral
7 Decrease the integral setting nt and nH by a factor of two and run the system
through a thermal cycle. For example, change it from “800” to “400”, then “200”,
then “100, etc. Repeat as necessary. You should observe the steady-state error
disappear and the system reach setpoint. If the system begins to oscillate around the
setpoint, you have gone too far.
8 Cool-down your system and repeat the experiment so you can truly observe the
process temperature approaching setpoint. You will probably observe the
temperature overshooting the setpoint. If overshoot is acceptable for your process,
there is no reason to continue tuning. If you would like to eliminate overshoot in
exchange for slightly longer times until setpoint it reached, read on, as overshoot can
be eliminated using the next step.
Continue the Test – Proportional + Integral + Derivative
9 Set the derivative parameters Er and EH to “1”. Run the system through a
thermal cycle and observe the overshoot the first time it reaches setpoint.
10 Double the derivative parameters Er and EH and run the machine through
another thermal cycle. If you have a floating valve, you may wish to observe its
position, since derivative control will help the system properly anticipate the slow
operating time for such a valve.
11 Repeat the doubling process of parameters Er and EH until overshoot is
satisfactorily eliminated. If the system stutters or temporarily reverses temperature
direction as approaching setpoint, your derivative response is too aggressive and you
need to decrease this parameter.
(Continued)
(Continued)
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