32
Phenomenon
Countermeasure
Probable cause
Hunting does not stop.
The control period is too long for the
characteristics of the controlled
device, which causes hunting in
synchronization with the control
period.
Shorten the control period.
The temperature is influenced by
continuous external disturbance (i.e.,
the temperature is affected by an
external disturbance before a previous
disturbance has be stabilized. and it
looks as if hunting did not stop at all).
In this case, however, 100%
improvement will not be possible
because the temperature is influenced
by a continuous external disturbance
instead of hunting. Change the stable
range to a wider setting than the
range of the temperature change so
that HT will not be ON.
The response fluctuates, becoming
good and bad.
There is a heater or cooling device not
controlled by the control output of the
Temperature Controller (e.g., forced
heating or cooling is executed using
alarm outputs).
In this case fuzzy self-tuning may not
work. Set the control mode to the
advanced PID and adjust the PID
constants manually.
The response fails to reach
operational requirements.
There are continuous characteristics
changes.
Do either one of the following.
•
Adjust the stable range so that
fuzzy self-tuning will not be turned
ON.
•
Set the control mode to the
advanced PID and adjust the PID
constants manually.
The response speed of the controlled
device is so fast that it cannot be
retrieved by a sampling period of 500
ms.
The Thermac E5
J Temperature
Controller cannot support a sampling
period of less than 500 ms. Use an
ES100-series Digital controller, which
has a shorter sampling period.
The temperature is affected by
devices near the system.
Fuzzy self-tuning does not support
any countermeasure against
interference. Set the control mode to
the advanced PID and adjust the PID
constants manually.
4-3
Terminology
4-3-1 Stable Value, Stable Range, and Stability Judgement Time
If the measured value continuously coincides with the set point, it can be said
that the measured value is stable. The measured value will not continuously
coincide perfectly with the set point if there is noise inference. It is, however, pos-
sible to make the measured value stay within a permissible range called the
stable range. Even if the temperature is within the stable range, the Temperature
Controller may respond to external disturbance or hunting. Therefore, it cannot
be said that the temperature is stable unless the temperature stays within the
stage range continuously for a certain time. This time is called stability judge-
ment time. Like PID constants, stability judgement time is adjusted with fuzzy
self-tuning according to the characteristics of the object to be controlled. Fuzzy
Terminology
Section 4-3
