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Chapter 7 Usage of Various Functions
7-21
(b) Integral operation (I operation)
With integral operation, the manipulate value (MV) is increased or decreased continuously in accordance time in
order to eliminate the deviation between the SV and PV. When the deviation is very small, the proportional
operation can not produce a proper manipulate value and an offset remains between PV and SV. The integral
operation can eliminate the offset value even the deviation is very small.
The period of the time from when the deviation has occurred in I action to when the MV of I action become that of
P action is called Integration time and represented as Ti.
Integral action when a constant deviation has occurred is shown as the following Fig. 7.3.
Fig. 7.3 The integral action with constant deviation
The expression of I action is as following;
As shown in the expression, Integral action can be made stronger or weaker by adjusting integration time (Ti) in
I action. That is, the more the integration time (the longer the integration time) as shown in Fig. 7.4, the lesser the
quantity added to or subtracted from the MV and the longer the time needed for the PV to reach the SV.
As shown in Fig. 7.5, when the integration time given is short the PV will approach the SV in short time since the
quantity added or subtracted become increased. But, If the integration time is too short then oscillations occur,
therefore, the proper P and I value is requested.
Integral action is used in either PI action in which P action combines with I action or PID action in which P and D
actions combine with I action.
∫
= Edt
Ti
Kp
MV