Omron CP - PROGRAMMING MANUAL 05-2007 Programming Manual page 659

Data Control Instructions
Direction of Action
Reverse Action
Output
Adjusting PID Parameters
626
When using PID control, select either of the following two control directions. In
either direction, the MV increases as the difference between the SV and the
PV increases.
• Forward action: MV is increased when the PV is larger than the SV.
• Reverse action: MV is increased when the PV is smaller than the SV.
Low
High
SV
temperature
temperature
The general relationship between PID parameters and control status is shown
below.
• When it is not a problem if a certain amount of time is required for stabili-
zation (settlement time), but it is important not to cause overshooting,
then enlarge the proportional band.
Control by measured PID
SV
When P is enlarged
• When overshooting is not a problem but it is desirable to quickly stabilize
control, then narrow the proportional band. If the proportional band is nar-
rowed too much, however, then hunting may occur.
When P is narrowed
SV
Control by measured PID
• When there is broad hunting, or when operation is tied up by overshooting
and undershooting, it is probably because integral action is too strong.
The hunting will be reduced if the integral time is increased or the propor-
tional band is enlarged.
Control by measured PID
(when loose hunting occurs)
SV
Enlarge I or P
Forward Action
Output
Low
temperature
Section 3-17
High
SV
temperature