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Performance Metrics and Diagnostics

Controller performance may be measured by simply calculating the measurement-setpoint variance. Simple, but of limited use as this measure depends on the underlying disturbances of the process - the more disturbances, the larger the variance, even if the controller model fidelity and tuning remain the same. And it's scale dependent, which makes it difficult to scan large number of controllers for unacceptable values.

Most controller performance software packages get around these limitations by employing a performance index, which compares the current variance to that which would be obtained if an "optimal" controller had been applied to the process over the same time range.

Advantages? The disturbance effect is theoretically removed, as both the actual controller and the optimal controller are subject to the same disturbances. And because a ratio is taken, the number is naturally scaled to be between zero and one.

But what is this "optimal" controller? Is it an adequate representation of what could be applied in practice? Most software packages use a minimum variance controller as the optimal controller (this is the basis for the Harris Index). This may not be a reasonable standard - a minimum variance controller is essentially a PID controller with deadtime compensation. But most controllers in a plant don't have deadtime compensation, and usually don't contain derivative action (as it can be troublesome in practice).

So comparing PI controllers to a minimum variance controller is often not a reasonable comparison. Rather than using a minimum variance controller, the Control Arts ControlMonitor package determines what the variance would be if a well-tuned PI controller had been applied to the process over the same time frame. And because PI control is practically possible, this is a much more realistic, stable, and accurate standard for a controller performance metric.

Other Metrics:

The controller performance index is just one tool available in the ControlMonitor package. Here's a list of the other graphical and numerical metrics available:

  • Surge vessel level controller performance index.
  • Frequency Spectrums
  • Measurement-Output cross-correlation
  • Stiction Index
  • Cycling Index
  • Non-linearity index
  • Service Factor
  • Saturation Factor


Control Arts Inc. 2014