Omron SYSMAC CJ - REFERENCE MANUAL 08-2008 Reference Manual page 806

Data Control Instructions
Proportional Action (Reverse Action)
100%
50%
0%
Proportional band when
MV output designation is
0 (output 0%)
766
Proportional band when
MV output designation is
0 (output 0%)
Proportional band when
MV output designation is
1 (output 50%)
SV
Set point
Proportional band when
MV output designation is
1 (output 50%)
Integral Action (I)
Combining integral action with proportional action reduces the offset accord-
ing to the time that has passed, so that the PV will match the SV. The strength
of the integral action is indicated by the integral time, which is the time
required for the manipulated variable of the integral action to reach the same
level as the manipulated variable of the proportional action with respect to the
step deviation, as shown in the following illustration. The shorter the integral
time, the stronger the correction by the integral action will be. If the integral
time is too short, the correction will be too strong and will cause hunting to
occur.
Step response
0
Deviation
Manipulated
0
variable
Pi Action and Integral Time
Step response
Deviation
Manipulated
variable
Derivative Action (D)
Proportional action and integral action both make corrections with respect to
the control results, so there is inevitably a response delay. Derivative action
compensates for that drawback. In response to a sudden disturbance it deliv-
ers a large manipulated variable and rapidly restores the original status. A
correction is executed with the manipulated variable made proportional to the
incline (derivative coefficient) caused by the deviation.
The strength of the derivative action is indicated by the derivative time, which
is the time required for the manipulated variable of the derivative action to
reach the same level as the manipulated variable of the proportional action
with respect to the step deviation, as shown in the following illustration. The
longer the derivative time, the stronger the correction by the derivative action
will be.
Adjusting the Proportional Band
Proportional band too narrow (hunting occurring)
Proportional band just right
Proportional band too wide (large offset)
Integral Action
PI action
I action
P action
Ti: Integral time
Section 3-18
Offset