4 - 2
MELSEC-Q/QnA
4. FUNCTIONS OF PID CONTROL
4.2 Functions of PID Control
The operation methods for PID control with the PID control instructions are the velocity
type and process value derivative type. The following describes the control executed
for both of these methods:
4.2.1 Operation method
(1) Velocity type operation
The velocity type operation calculates amounts of changes in the MVs
(manipulated values) during PID operation.The actual MV is the accumulated
amount of change of the MV calculated for each sampling cycle.
(2) Process value derivative type operation
The process value derivative type operation executes PID operations by
differentiating the PV (process value).
Because the deviation is not subject to differentiation, sudden changes in the
output due to differentiation of the changes in the deviation generated by
changing the set value can be reduced.
4.2.2 Forward operation and reverse operation
Either forward operation or reverse operation can be selected to designate the
direction of PID control.
(1) In forward operation, the MV (manipulated value) increases as the PV (process
value) increases beyond the SV (set value).
(2) In reverse operation, the MV increases as the PV decreases below the SV.
(3) In forward operation and reverse operation, the MV becomes larger as the
difference between the SV and the PV increases.
(4) The figure below shows the relationships among forward operation and reverse
operation and the MV, the PV, and the SV.
(SV)
(MV)
(PV)
Reverse
operation
Forward
operation
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