2 Instruction Descriptions
2-402
NJ-series Instructions Reference Manual (W502)
z Derivative Action (D)
If the proportional and integral actions are used together, the offset will reach 0 and the process
value will reach the set point. However, if disturbance causes the process value to change quickly,
time is required to restore the original state. The derivative action functions to quickly return the pro-
cess value to the set point when there is a disturbance. The derivative action differentiates the devi-
ation on the time axis and then increases the absolute value of the manipulated variable in
proportion to the result. In other words, the larger the change in the process value is, the larger the
absolute value of the manipulated variable for the derivative action is. The changes in the manipu-
lated variable for the derivative action when a deviation occurs in stepwise fashion are shown below.
The changes in the manipulated variable when the derivative and proportional actions are combined
are also shown.
One of the parameters for the derivative action is the derivative time. This is the time for the manipu-
lated variable from the derivative action to equal the manipulated variable from the proportional
action when a ramp deviation occurs. The longer the derivative time is, the stronger the derivative
action is. A long derivative time provides a rapid response to disturbances, but it can also cause
hunting.
The total of the manipulated variables for the proportional, integral, and derivative actions is the
manipulated variable for PID control. The changes in the manipulated variable for PID control for a
stepwise and ramp deviations are shown below.
Manipulated Variable for Derivative Action
0
Stepwise response
Time
Deviation
Manipulated Variable for Derivative and
Proportional Actions Together
0
Stepwise response
Time
Deviation
00
Time
Manipulated
variable
Proportional and
derivative actions
Proportional action only
Time
Derivative action only
Manipulated
variable
Ramp response
0
Time
Deviation
Proportional and derivative actions
Proportional action
Derivative action
0
Time
Derivative time
Manipulated
variable
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