Omron CPM1 - PROGRAMING MANUAL 02-2001 Programming Manual page 447

Data Control Instructions
Control Actions
Proportional Action (Reverse Action)
100%
Manipulated
variable
0%
Proportional Action (P)
Proportional action is an operation in which a proportional band is established
with respect to the set value (SV), and within that band the manipulated variable
(MV) is made proportional to the deviation. An example for reverse operation is
shown in the following illustration
If the proportional action is used and the present value (PV) becomes smaller
than the proportional band, the manipulated variable (MV) is 100% (i.e., the
maximum value). Within the proportional band, the MV is made proportional to
the deviation (the difference between from SV and PV) and gradually decreased
until the SV and PV match (i.e., until the deviation is 0), at which time the MV will
be 0% (i.e., the minimum value). The MV will also be 0% when the PV is larger
than the SV.
The proportional band is expressed as a percentage of the total input range. The
smaller the proportional band, the larger the proportional constant and the stron-
ger the corrective action will be. With proportional action an offset (residual devi-
ation) generally occurs, but the offset can be reduced by making the proportional
band smaller. If it is made too small, however, hunting will occur.
Proportional
band
SV
SV
Integral Action (I)
Combining integral action with proportional action reduces the offset according
to the time that has passed. The strength of the integral action is indicated by the
integral time, which is the time required for the manipulated variable of the inte-
gral action to reach the same level as the manipulated variable of the proportion-
al action with respect to the step deviation, as shown in the following illustration.
The shorter the integral time, the stronger the correction by the integral action
will be. If the integral time is too short, the correction will be too strong and will
cause hunting to occur.
Step response
Deviation
Manipulated
variable
Pi Action and Integral Time
Step response
Deviation
Manipulated
variable
Adjusting the Proportional Band
Proportional band too narrow (hunting occurring)
Proportional band just right
Proportional band too wide (large offset)
Integral Action
PI action
I action
P action
Ti: Integral time
Section 7-18
Offset
429