Omron CS1G/H-CPUxxH Instructions Manual page 704

Data Control Instructions
1 cycle
Processing
Initial processing
(PID processing
with initial values)
Reading of Less than 100 ms, so
measurement PID is not executed.
time
Control Actions
Proportional Action (Reverse Action)
Manipulated
variable
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.
1 cycle 1 cycle
(60 ms) (60 ms + 60 ms = 120 ms)
Not executed. Executed
Greater than 100 ms,
so PID is executed
and 20 ms is carried
forward.
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.
Output: 0%
Output: 50%
SV
Proportional band
Integral Action (I)
Combining integral action with proportional action reduces the offset accord-
ing to the time that has passed, so that the PV will match the SV. The strength
of the integral action is indicated by the integral time, which is the time
required for the manipulated variable of the integral action to reach the same
level as the manipulated variable of the proportional action with respect to the
step deviation, as shown in the following illustration. The shorter the integral
1 cycle
(20 ms + 60 ms = 80 ms)
Not executed.
Less than 100 ms, so
PID is not executed.
Adjusting the Proportional Band
Proportional band too narrow (hunting occurring)
SV
Proportional band just right
Proportional band too wide (large offset)
Section 3-18
1 cycle
(80 ms + 60 ms = 140 ms)
Executed
Greater than 100 ms,
so PID is executed
and 40 ms is carried
forward.
Offset
683