7-18
Section
Data Control Instructions
430
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 com-
pensates for that drawback. In response to a sudden disturbance it delivers a
large manipulated variable and rapidly restores the original status. A correction
is executed with the manipulated variable made proportional to the incline (de-
rivative 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 re-
spect 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.
Derivative Action
PD Action and Derivative Time
Step response
PD action
P action
Td: Derivative time
D action
Ramp response
Deviation
Manipulated
variable
Deviation
Manipulated
variable
PID Action
PID action combines proportional action (P), integral action (I), and derivative
action (D). It produces superior control results even for control objects with dead
time. It employs proportional action to provide smooth control without hunting,
integral action to automatically correct any offset, and derivative action to speed
up the response to disturbances.
Step Response of PID Control Action Output
Ramp Response of PID Control Action Output
PID action
I action
P action
D action
Step response
Deviation
Manipulated
variable
PID action
I action
P action
D action
Ramp response
Deviation
Manipulated
variable
