Omron SYSMAC CJ - REFERENCE MANUAL 01-2008 Reference Manual page 805

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
766
• If the sampling period is greater than or equal to the cycle time, PID con-
trol is not executed with each cycle, but PID(190) is executed when the
cumulative value of the cycle time (the time between PID instructions) is
greater than or equal to the sampling period. The surplus portion of the
cumulative value (i.e., the cycle time's cumulative value minus the sam-
pling period) is carried forward to the next cumulative value.
For example, suppose that the sampling period is 100 ms and that the cy-
cle time is consistently 60 ms. For the first cycle after the initial execution,
PID(190) will not be executed because 60 ms is less than 100 ms. For the
second cycle, 60 ms + 60 ms is greater than 100 ms, so PID(190) will be
executed. The surplus of 20 ms (i.e., 120 ms – 100 ms = 20 ms) will be
carried forward.
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.
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.
1 cycle
1 cycle
(20 ms + 60 ms = 80 ms)
Not executed.
Executed
Less than 100 ms, so Greater than 100 ms,
PID is not executed. so PID is executed and
40 ms is carried
forward.
Section 3-18
(80 ms + 60 ms = 140 ms)