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INP-TN512878-E
5.2 Variable gain control
To change the gain of the controller in correspondence to the status change of the process is called
variable gain. The arithmetic operation expression indicated below is basically used for gain
arithmetic operation of PID.
{
(
)
}
1
-
K
2
-
K
K
D
K
1
1
-
K
K
PV
2
-
PV
+
PV
DT
T
+
DV
T
DT
+
DV
-
DV
P
100
=
V
M
~
∆
Proportional
term
Integral
term
Differential term
Figure 5-4
As is understood from the arithmetic operation expression indicated above, it is necessary to multiply
gain to the output ( ) of the PID wafer.
It is possible to change the gain by changing the proportional term (DV
K
-DV
K-1
) in the above
expression, but the effect that is same as that of change in process (change in DV) arises, P action is
activated as a result and the gain rapidly changes.
It is necessary to pay attention, as the effect is different between the case where proportional band P is
changed and the case where the proportional term is changed. The method to change the output
( ) of the PID wafer is better for so-called variable gain.
To change the integration gain, multiply the gain to the integral term (X
2
) of the integration wafer.
Integration (I) cut action occurs when the input is changed to 0%. No shock will occur to the
integral term even when the gain is changed. Change to the weight of the differential term can also
be performed in the same manner. But care should be exercised because it is more sensitive to
changes in the gain, as a difference of two times is used. There are many methods for wafer
connection for variable gain, and it is necessary to select a method depending on the actual process
status.
V
M
~
∆
X1 : Differential term
X2 : Integral term
X3 : Proportional term
DV : Deviation
TI : Integration time
TD : Differential time
P : Proportional band
K : Present value
V
M
~
∆
Summary of Contents for CC-M
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