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PID Control Method
PID control actions are executed by means of PID control with feed-forward con-
trol (two degrees of freedom).
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
Target response
Disturbance response
(1)
(2)
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 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 Action (Reverse Action)
Adjusting the Proportional Band
SV
Proportional band too narrow (hunting occurring)
Proportional band just right
Proportional band too wide (large offset)
Offset
Manipulated
variable
Proportional
band
100%
0%
SV
PID and Related Instructions
Section 5-24
Содержание CVM1D
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