Chapter 2. SPECIFICATIONS
2 - 9
3) Derivative Action (D Action)
(1)
When a deviation occurs due to alteration of SV or external disturbances, D action restrains the changes of
the deviation by producing MV which is proportioned with the change velocity (a velocity whose deviation
changes at every constant interval) in order to eliminate the deviation.
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D action gives quick response to control action and has an effect to reduce swiftly the deviation by
applying a large control action (in the direction that the deviation will be eliminated) at the earlier time
that the deviation occurs.
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D action can prevent the large changes of control object due to external conditions.
(2)
The period of time from when the deviation has occurred to when the MV of D action become the MV of P
action is called derivative time and represented as Kd.
(3)
The D action when a given deviation occurred is shown as Fig. 2.7.
[Fig. 2.7] Derivative action at a constant deviation
(4) The expression of D action is represented as follows:
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In this expression, an output proportional with the variation rate of deviation is added to P action quantity.
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If the derivative time is increased then P action is strengthened.
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D action is applied when a change of deviation occurs and the deviation at normal state become 0. D
action, therefore, do not reduce offset.
(5) D action is used in either PD action in which P action combines with D action or PID action in which P and I
actions combine with D action.
