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10 Control actions
10.1 Fuzzy self-tuning
Fuzzy self-tuning is a function to perform a fine adjustment of PID values automatically.
Stable control can be carried out even if the conditions of the production process are
changed due to various external factors (types and rates of production).
(1) When using the controller for the first time, perform the AT (auto-tuning) or set the proper
PID values by keypad operation.
(2) When the control initiates, the controller performs this function by the PID values
previously adjusted.
(3) When the control result is disordered by disturbance or a change in the process, the
controller checks the convergence status, and performs a fine adjustment of PID values if
required.
(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 to correct overshoot.
(d) When hunting occurs, the controller checks its waveform and performs a fine
adjustment of PID values.
The instrument is constantly in self-tuning status, and when deviation occurs, the tuning
starts. Even in Fuzzy self-tuning status, when very large hunting occurs and the control is
not stabilized, AT automatically starts.
When the AT “Perform” is selected by the keypad, AT initiates, and when the control is
stabilized, the AT is released and the controller returns to self-tuning status.
When lock mode [
] or [ ] is selected, Fuzzy self-tuning or AT does not work.
With a control system in which load fluctuation periodically occurs, the Fuzzy
self-tuning PID control may malfunction.
In such a case, use the controller with the PID control.
10.2 PID
(1) Proportional band (P)
Proportional action is the action which the control output varies in proportion to the
deviation between the SV (desired value) and the processing temperature (PV).
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 SV 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 accelerated. However, the cycle of oscillation is
also accelerated and 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, the restoring value becomes small, and if the derivative
time is extended, an excessive returning phenomenon may occur and the control system
may oscillate.
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