Marathon Computer AACC 2000 Typical automatic tuning cycle, Calculation of the cutback values

Marathon Monitors Inc.

Typical automatic tuning cycle

Process Variable

Setpoint

Time

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.

AACC 2000 Carbon 55 Nov. 1, 1997