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Derivative Time Constant
Initially set the derivative to between 1/4th and 1/10th
of the Integral time value. Increase the derivative time
if the process overshoots/undershoots. Increase it a
little at a time, but if the process becomes unstable,
decrease it until the oscillation stops. Induce a load
disturbance or make a setpoint change to verify that
the process stabilizes. If not decrease the value some
more and re-test.
Derivative Time
Too Long: Oscillates and over corrects
when process disturbed
Too Short: Slow warm up and disturbance
response under-corrects
Process Disturbance
Process Disturbance
Note:
When controlling a modulating valve, it is usually
recommended that derivative is set to OFF to avoid
excessive valve activity. Derivative can cause process
instability in these processes.
Anti Wind-up
If after fully optimising the tuning, there is an overshoot
of the setpoint at start-up or in response to large set-
point changes, the reset wind-up inhibit point can be
reduced to suspend integral action until the process is
closer to setpoint. If set too low control deviation can
occur (the process settles, but is offset above or below
the setpoint). It this is observed, increase the value until
the deviation error is removed.
Anti Wind
-
up
Too Smal
l
: Overshoots setpoint before
settling
Too Short: Slow to setpoint or offset
above/below setpoint
Manual Reset
For proportional only control, after making all other
adjustments, if a positive control deviation error exists
(process is offset above the setpoint) reduce the man-
ual reset until the error is eliminated. If there is a nega-
tive error (process is offset below the setpoint) increase
manual reset until the error is eliminated.
For PID or PI control, typically set manual reset to ap-
proximately 80% of power needed to maintain set-
point, but lower values can be used to inhibit start-up
overshoot if required.
Manual Reset
Too High: Overshoots setpoint at start-up
Too Low: Slow to setpoint
Содержание Chromalox 4081
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