YOKOGAWA UP35A, User Manual

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6-19
IM 05P02D41-01EN
Monitoring and Control of Regular Operations
6
Description and Tuning of Derivative Time
If the control object has a large time constant or dead time, the corrective action will
be too slow with proportional action or proportional-plus-integral action alone, causing
overshoot. However, even just sensing whether the deviation is on an increasing or a
decreasing trend and adding some early corrective action can improve the controllability.
Thus the derivative action (D action) is action that changes the output in proportion to the
deviation derivative value (rate-of-change).
The derivative time is defined as the time required with PD action to develop, when a
constant-slope change in deviation is imposed, an output change due to derivative action
that is exactly equal to the change due to proportional action.
Output = e + T
D
e
100
P
d
dt
P = 100%
Output change due to P action
Output change due to D action
Time Derivative
time
Small
derivative time
Large
derivative time
e : Deviation
T
D
: Derivative time
Deviation
Output (%)
(On-time ratio)
To manually tune the derivative time
• Adjust from shorter time to longer time.
• If you see a short-period oscillation, the time is too long.
The longer the derivative time set, the stronger the corrective action, and the more likely
the output will become oscillatory. Oscillations due to derivative action are characterized
by a short period.
D = OFF should always be used when controlling fast-responding inputs such as
pressure and flow rate, or inputs characterized by rapid fluctuation, such as optical
sensors.
Time
SP
• If D is too large, short-period oscillation will appear in
the measured temperature.
Temperature
6.3 Adjusting PID Manually