Rolling Limits

[Lindsey.Wilcox]

This use case came up while working with a customer last week, and I thought it was worth sharing on the forum, since other users may be interested.

The user I was working with was interested in creating limits for a signal.  The limits would be calculated based upon the average and standard deviation of the previous 4 hours of operation;  essentially the user wanted to create a rolling 4 hour boundary.  We worked out the following solution:

1. Create a periodic condition for the time period that you would like to use for this rolling window.  Here I created 4 hour periods:

2. Next, use signal from condition to calculate the average of the signal during each of these 4 hours periods:

3. Use signal from condition again to calculate the standard deviation of the signal during each of these 4 periods.

4. Now use Formula to calculate the upper and lower boundaries based on the average and standard deviation.  Here I’m creating boundaries that are +/- 2 standard deviations.  Note that the ‘.delay(4h)’ is needed to shift the boundary signals so that the PV signal is being compared to the upper and lower boundaries calculated in the previous 4 hours.

5. Once the upper and lower boundaries are identified, use Deviation Search to determine when the signal is outside of these boundaries.