Omron SYSMAC CP Series Instruction & Reference Manual page 302

2 Instructions
Direction of Action
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.
Reverse Action
100%
Output
50%
0%
(MV output designation: 50%)
Adjusting PID Parameters
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 stabilization (settle-
ment time), but it is important not to cause
overshooting, then enlarge the proportional
band.
Control by measured PID
SV
When P is enlarged
• When there is broad hunting, or when oper-
ation is tied up by overshooting and under-
shooting, it is probably because integral
action is too strong. The hunting will be
reduced if the integral time is increased or
the proportional band is enlarged.
Control by measured PID
(when loose hunting occurs)
SV
Enlarge I or P.
2-266
Output
Low SV
High
temperature
temperature
Forward Action
100%
50%
0%
SV
Low
temperature
• When overshooting is not a problem but it is
desirable to quickly stabilize control, then
narrow the proportional band. If the propor-
tional band is narrowed too much, however,
then hunting may occur.
When P is narrowed
SV
Control by measured PID
• If the period is short and hunting occurs, it
may be that the control system response is
quick and the derivative action is too strong.
In that case, set the derivative action lower.
Control by measured PID
(when hunting occurs in a short period)
SV
Lower D.
CP1E CPU Unit Instructions Reference Manual(W483)
High
temperature