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[FISHER)
Configuration
5-49
5.11.1
Principle of Operation
Gain scheduling
means that the controller
switches
between
three
different
sets of PI0 parameters,
depending
on the value of a chosen
reference
signal (GSref).
The reference
signal may be an internal
signal (the output signal or the process value) or an external
signaf
received
on either Al3 or Al4.
The controller
stores as many as three different
sets of PID
parameters
to use in three user-defined
ranges
(percent
of reference
signal).
When the reference
signal passes from one range to the next,
the controller
uses the set of parameters
for that range.
The user may manually
select and store the parameters
using
FCODE
36 thru 44, or use the autotuner
to derive and store them.
Ensure the process
and reference
signal levels
are in the appropriate
range when using the
Autotuner
to derive the parameters.
Either drive
the process
manually
to the required
level or
initiate
autotuning
when the process
is at the
appropriate
level.
When the Autotuner
is used to automatically
set the gain scheduling
table, the gain schedule
reference
(GSref) signal must be within the
preset limits (FCODE
34 and 35) before tuning
procedures
begin.
The tuning sequence
shows how the parameter
table is completed.
Note that the reference
signal level changes
between
each tuning.
The labels 1 thru 4 at the bottom
of the diagram,
indicate
the first
Tuning
1
2
3
4
thru fourth tuning cycles.
Each box represents
all three PID parame-
DPR900~c48 ters. The T beside the boxes shows that tuning
is complete
for that
range.
The values
derived
from each tuning
are placed
in
the ranges that have not yet been tuned.
The
percentage
ranges
given in the example
are the
default
ranges.
In the range O-IO seconds,
the
increments
are 0.1 seconds.
May 1991
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