WIKA CF1H - Manual (Page 53)

Operating Instructions Temperature Indicating Controller CF1H
V 1.2
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01/2007 - 53 -
8.2 Explanations of PID
(1) Proportional band (P)
Proportional action is the action which the control output varies in proportion to the deviation between
the setting value and the processing temperature.
If the proportional band is narrowed, even if the output changes by a slight variation of the processing
temperature, better control results can be obtained as the offset decreases.
However, if when the proportional band is narrowed too much, even slight disturbances may cause
variation in the processing temperature, and control action changes to ON/OFF action and the so
called hunting phenomenon occurs.
Therefore, when the processing temperature comes to the balanced position near the setting value
and a constant temperature is maintained, the most suitable value is selected by gradually narrowing
the proportional band while observing the control results.
(2) Integral time (I)
Integral action is used to eliminate offset. When the integral time is shortened, the returning speed to
the setting point is quickened. However, the cycle of oscillation is also quickened and the control
becomes unstable.
(3) Derivative time (D)
Derivative action is used to restore the change in the processing temperature according to the rate of
change. It reduces the amplitude of overshoot and undershoot width.
If the derivative time is shortened, restoring value becomes small, and if the derivative time is made
longer, an excessive returning phenomenon may occur and the control system may be oscillated.