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Execution Condition ON
The PID operation is executed at the intervals based on the sampling period,
according to the PID parameters that have been set.
Sampling Period and PID Execution Timing
The sampling period is the time interval to retrieve the measurement data for
carrying out a PID operation. PID(––), however, is executed according to the
CPU Unit’s sampling time, so there may be cases where the sampling period is
exceeded. In such cases, the time interval until the next sampling is reduced.
PID Control Method PID control operations are executed by means of PID control with feed-forward
control (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 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 offset (residual deviation) occurs, and
the offset is reduced by making the proportional band smaller. If it is made too
small, however, hunting will occur.
Proportional Operation
(Reverse Operation)
Adjusting the Proportional Band
Operation
amount
SV
Proportional band
SV
Proportional band too narrow (hunting occurring)
Proportional band just right
Proportional band too wide (large offset)
Offset
100%
0%
Integral Operation (I)
Combining integral operation with proportional operation reduces the offset ac-
Special Math Instructions Section 5-21