Chapter 3: Configuration 3.6 Setting the Configuration
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• Ringing in the process (disturbances with the same time constant as the
process) can cause problems with adaptive controlling. If these disturbances
cannot be eliminated the controller must be set for adaptive feed forward.
Critical Gain and Critical Period
In a very few cases (like very noisy processes), the AutoTuner may not be able to
determine the control loop characteristics (CriticalGain and CriticalPeriod)
properly. The adaptive function does not work properly without values for these
parameters. In this case you must set them manually.
Ziegler-Nichols developed a method called the Calculated Sensitivity Method.
The CriticalGain and CriticalPeriod parameters can be determined using this
method.
1 Put the unit in Manual mode.
2 Adjust the controller’s parameters so that a pure P-controller is configured.
3 Set the minimum value for the Gain parameter (K = 0.01).
4 Adjust the controller Output signal manually until PV = SP.
5 Read the current Output signal.
6 Set the controller’s Offset parameter the value of the output signal.
7 Put the unit in Auto mode.
8 Change SP by a few percent. (As you change SP, try to keep the value
relatively constant by alternating increases with decreases.)
9 Observe PV and the Output signal to see if the system oscillates.
10 If the system oscillates with a decreasing amplitude, increase Gain, and go
to step 8.
11 If the system oscillates with a increasing amplitude, decrease Gain, and go
to step 8.
12 If the system oscillates with a constant amplitude, you are done!
Read the controller’s Gain setting. This is the setting of the CriticalGain
parameter. Read the oscillation period time. This is the setting of the
CriticalPeriod parameter.
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