ABB 800xA Operation page 110

Loop Performance Diagnostics
Set Point Oscillation
The detection of setpoint oscillations is important for the detection of external
excitation in a control loop. The setpoint is usually much less distorted and noisy
than the control error. The setpoint can only oscillate if it is the output of a master
controller in a cascade loop.
External Disturbances
A loop may be oscillating due to a periodic external disturbance. The controller is
not able to remove this oscillation, either because it was not designed to do so or
because it is insufficiently tuned for that task. If the controller is in manual mode, a
significant oscillation is always externally generated. If the controller is in
automatic mode, then a significant external oscillation is identified based on the
following characteristics:
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Data Reliability
Any signal-based analysis can be misleading by using data that is not suitable for
such analysis. Tests for suitability of current data batch form data reliability
diagnosis. Data that has been compressed prior to analysis make results vulnerable
to unreliability. The data is tested on data points that are located far from the rest of
data (outliers that are removed automatically), compression, and quantization.
Harris Index
This measures performance as a ratio of the performance of the current controller to
that of a Minimum Variance Controller (MVC). The MVC is a theoretical controller
that provides "best possible feedback control performance" — it tries to bring the
PV back to SP in the shortest possible time by adapting to the manipulated variable.
The Harris Index works well on fast control loops when there is a good estimate of
deadtime
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The oscillation is somewhat linear
No stiction is detected
The oscillation is almost symmetric
The loop is not saturated
Section 8 Control Loop Asset Monitor (CLAM)
3BUA000150-600