P
AGE
35
3.4.2 ZIEGLER NICHOLS TUNING METHOD
This procedure has been determined empirically to yield 1/4 amptitude decay tuning parame-
ters that are determined by watching the system in a sustained oscillation (curve C, page 36,
the ultimate proportional band and ultimate time period) and then using these values from this
sustained oscillation to calculate ideal parameters.
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 similair to the sustained oscilaltion (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:
Proporational 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 overdamped response is desired, multiply the proportional band by two.
