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Marathon Monitors Inc.
AACC 2000 Carbon Nov. 1, 1997
55
Typical automatic tuning cycle
Calculation of the cutback values
Low cutback and High cutback are values that restrict the amount of overshoot, or
undershoot, that occurs during large step changes in process variable (for example, under
start-up conditions).
If either low cutback, or high cutback, is set to ‘Auto’ the values are fixed at three times
the proportional band, and are not changed during automatic tuning.
Adaptive tune
Adaptive tuning is a background algorithm, which continuously monitors the error from
setpoint and analyses the control response during process disturbances. If the algorithm
recognises an oscillatory, or under-damped, response it recalculates the Pb, ti and td
values.
Adaptive tune is triggered whenever the error from setpoint exceeds a trigger level. This
trigger level is set in the parameter ‘drA.t’, which is found in the Autotune list. The
value is in display units. It is automatically set by the controller, but can also be
manually
re-adjusted.
Adaptive tune should be used with:
1. Processes whose characteristics change as a result of changes in the load, or setpoint.
2. Processes that cannot tolerate the oscillation induced by a One-shot tune.
Adaptive tune should not be used:
1. Where the process is subjected to regular external disturbances that could mislead the
adaptive tuner.
2. On highly interactive multiloop applications. However, moderately interactive loops,
such as multi-zone extruders, should not give a problem.
Time
Setpoint
Process Variable