West Control Solutions MRC 7000 Controller Manual - Manual (Page 43)

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4.3.2 ZIEGLER NICHOLS TUNING METHOD
This procedure has been determined empirically to yield ideal 1/4 amptitude decay tuning
parameters that are determined by watching the system in a sustained oscillation (curve C,
page 44, the ultimate proportional band and ultimate time period) and then using these values
from this sustained oscillation to calculate ideal parameters.
To aid in determining the process oscillation, the instrument configuration parameters can be
adjusted. By reducing the chart upper value Cru and increasing the chart lower value CrL,
the resolution of the oscillation can e better observed. Also the chart rotation time Crt can be
reduced to improve the observation of the oscillation cycle time.
Determining Ultimate Proportional Band and Ultimate Time Period
1. Set Manual Reset rSet to 0.0, set ArSt to 0.0 and set rAtE to 0.0.
2. Enter the Control mode of operation, observe the process reaction.
3. Set the Proportional Band (PB) at 100 and upset the process and observe the response.
One easy method for imposing the upset is to move the setpoint for a few seconds and
then return it to its original value.
4. Achieve a response curve similar to the sustained oscillation (curve C), this is the Ultimate
Proportional Band (UPB) and Ultimate Time Period (UTP).
a) If the response curve from step 3 does not damp out, as in curve A from the
drawing, the PB is too low. The PB should be increased and step 3 repeated.
b) If the response in step 3 damps out, the PB is too high. The PB should be
decreased and step 3 repeated.
These values obtained for Ultimate Proportional Band (UPB) and Ultimate Time Period (UTP)
are used to calculate ideal P, PI, PD, PID tuning parameters using the following Ziegler-
Nichols equations:
Proportional only control (P) P (Pb) = 2 x UPB (degrees or units)
Proportional plus automatic reset (PI) P (Pb) = 2.2 x UPB (degrees or units)
I (ArSt) = 1.2 / UTP (repeats per minute)
Proportional-plus-derivative (or rate) (PD) P (Pb) = 1.7 x UPB (degrees or units)
D (rAtE) = UTP / 8 (minutes)
Proportional-plus automatic reset-plus
derivative (PID)
P (Pb) = 1.7 x UPB (degrees or units)
I (ArSt) = 2 / UTP (repeats per minute)
D (rAtE) = UTP / 8 (minutes)
If an over damped response is desired, multiply the proportional band by two.
(Referenced illustration on next page)