WIKA CF2S - Manual (Page 31)

Operating Instructions Temperature Indicating Controller CF2S
V1.2 • 08/2004 - 31 -
8. Control actions
8.1 Fuzzy self-tuning
Fuzzy self-tuning is a function to perform a fine adjustment of PID values automatically.
The stable control can be carried out even if the conditions of the process are changed
due to things like change in types and rates of production.
(1) When the control is rising, the controller performs the control by the PID values
turned in advance.
(2) When the control result is disordered by the disturbance or a change in the process,
the controller checks the converging status, and performs a fine adjustment of PID values
if necessary.
(a) If the convergence is performed smoothly, the PID values are not changed.
(b) If the convergent speed is slow, the controller corrects the PID values
to accelerate the convergence.
(c) When overshoot is generated during the convergence, the controller corrects
the PID values so as not to generate the overshoot.
(d) When hunting is generated, the controller checks the waveform and performs
fine adjustment of PID values.
(3) The controller can be used without Fuzzy self-tuning by designating the internal switch.
(4) When using the controller for the first time, perform the PID auto-tuning or set the
proper PID values by key operation.
The instrument is always in self-tuning status itself, and when deviation is created, it starts
the tuning. Even if in Fuzzy self-tuning status, when very large hunting is caused and the
control is not stabilized, it starts auto-tuning automatically.
When the auto-tuning performance is designated by the key operation, tuning is initiated
and when the control is stabilized, the auto-tuning is released and returns to self-tuning status.
When lock mode [ ] or [ ] is designated, Fuzzy self-tuning
or Auto-tuning PID action cannot work.
With the control system in which load fluctuation periodically occurs,
the Fuzzy self-tuning may fall into malfunction.
In such a case, use the controller with the PID auto-tuning mode.
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 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.