624
Data Control Instructions Section 3-17
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.
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.
0
0
PI action
P action
Step response
I action
Integral Action
Step response
Ti: Integral time
Manipulated
variable
Deviation
Manipulated
variable
Deviation
PI Action and Integral Time
To Next Page
Loading...
