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Chapter 7 Usage of Various Functions
7-20
2) Specification
(1) Control operation
(a) Proportional operation (P operation)
P action means a control action that obtain a manipulate value which is proportional to the deviation
(E : the difference between SV and PV)
The deviation (E) is obtained by difference between SV and PV and the formula of deviation is as following;
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PVSVKpMV −×=
where, Kp : the proportional constant (gain), SV: set value, PV: present value
When E happens, MV by P operation is like Fig.7.1
Fig 7.1 MV by P operation
If the Kp is too large, the PV reaches to the SV swiftly, but it may causes a bad effect like oscillations.
If the Kp is too small, oscillation will not occur. However, the PV reaches to the SV slowly and an offset may
appear between PV and SV shown in the Fig. 7.2.
The manipulation value (MV) varies from 0 to 4,000. User can define the maximum value of MV (MV_MAX) and
minimum value (MV_MIN) within the range 0 ~ 4,000.
When an offset remains after the system is stabilized, the PV can be reached to the SV by adding a certain value.
This value is called as bias value, and user can define the bias value
Fig. 7.2 The relation between Proportional constant (Kp) and prosent value (PV)
Time
: Deviation
: Manipulating value
Deviation(E)
Manipulate value (MV)
Oscillation
Offset
: Kp is too large
: Kp is too small