244
When
overshooting is prevented
with simple PID control, stabilization of distur
-
bances
is slowed (1). If stabilization of disturbances is speeded up, on the other
hand,
overshooting occurs and response toward the target value is slowed (2).
With feed-forward PID control, there is no overshooting, and response toward
the target value and stabilization of disturbances can both be speeded up (3).
Simple PID Control Feed-forward PID control
As the target response is slowed,
the disturbance response worsens.
As the disturbance response is
slowed, the target response worsens.
Overshoot
T
arget response
Disturbance response
(1)
(2)
Control Operations Proportional Operation (P)
Proportional operation is an operation in which a proportional band is estab-
lished with respect to the set value (SV), and within that band the operation
amount
(the control output amount) is made proportional to the
deviation. If the
present value (PV) is smaller than the proportional band, the
operation
amount
will be 100%. If within the proportional band the operation amount
is made pro
-
portional to the deviation and gradually decreased until the SV and PV match
(i.e.,
until the deviation is 0), the operation
amount will return to the previous val
-
ue (forward operation).
The
proportional band is expressed as a percentage with
respect to the total in
-
put
range. With proportional operation an
of
fset (residual deviation) occurs, and
the
of
fset is reduced by making the proportional band smaller
. If it is made
too
small, however, hunting will occur.
Proportional Operation
(Forward Operation)
Adjusting the Proportional Band
Operation
amount
SV
Proportional band
Proportional band too narrow (hunting occurring)
Proportional band just right
Proportional band too wide (large of
fset)
Offset
100%
0%
Integral Operation (I)
Combining
integral operation with proportional
operation reduces the of
fset ac
-
cording
to the time that has passed. The strength of the integral
operation is indi
-
cated by the integral time, which is the time required for the integral operation
amount to reach the same level as the proportional operation amount with re-
spect
to the step deviation, as shown in the following illustration. The shorter the
integral
time, the stronger the correction by the integral operation will be. If the
Special Math Instructions Section 5-21
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