Omron CP - PROGRAMMING MANUAL 05-2007 - Page 656

623
Data Control Instructions Section 3-17
For the third cycle, the surplus 20 ms is added to 60 ms. Because the sum
of 80 ms is less than 100 ms, PID(190) will not be executed. For the fourth
cycle, the 80 ms is added to 60 ms. Because the sum of 140 ms is greater
than 100 ms, PID(190) will be executed and the surplus of 40 ms (i.e.,
120 ms – 100 ms = 20 ms) will be carried forward. This procedure is re-
peated for subsequent cycles.
Control Actions 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 vari-
able (MV) is made proportional to the deviation. An example for reverse oper-
ation 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 at the minimum value of 0% (or 50%, depending on the
setting of the manipulated variable output designation parameter). 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 stronger the corrective action will be. With proportional action an offset
(residual deviation) generally occurs, but the offset can be reduced by making
the proportional band smaller. If it is made too small, however, hunting will
occur.
1 cycle
1 cycle 1 cycle 1 cycle
1 cycle
Processing
Initial processing
(60 ms) (60 ms + 60 ms = 120 ms)
(20 ms + 60 ms = 80 ms)
(80 ms + 60 ms = 140 ms)
(PID processing
with initial values)
Reading of
measurement
time
Less than 100 ms, so
PID is not executed.
Greater than 100 ms,
so PID is executed
and 20 ms is carried
forward.
Less than 100 ms, so
PID is not executed.
Not executed. Executed
Not executed.
Executed
Greater than 100 ms,
so PID is executed and
40 ms is carried
forward.
SV
SV
100%
50%
0%
Adjusting the Proportional Band
Proport
i
onal Act
i
on
(
Reverse Act
i
on
)
Manipulated variable
Output: 0%
Output: 50%
Proportional band too narrow (hunting occurring)
Offset
Proportional band just right
Proportional band too wide (large offset)
Proportional band
when output is 0%
Proportional band
when output is 50%