Moving Window Average Alarms

Ignition
1

Hi,

I have some tags being populated from a device. I would like to create various moving window average based alarms - e.g. if the average value in the past n minutes is outside some setpoints, raise an alarm. I’m looking at implementing this for a wide variety of tags, for a wide variety of window sizes.

I can do this with gateway scripts, but I’m not sure if that’s the best way. Are there any other techniques I could use for this?

Thanks

2

I would be using my objectScript expression function (from my Simulation Aids module) in an expression tag, using it’s state variable to hold recent history in a fifo for fast computation. The alarms would then be set on the expression tag as usual.

1 Like
3

Thanks for the suggestion, I’ll try it out and post how I go!

4

Did you manage to implement this ?

I’ve got the exact same request…

5

Try this script:

exprTagUtil.py

Requires my Simulation Aids module, linked above.

6

I was curious how I could do it using base ignition, and what I came up with was:

  1. Create an expression tag referencing the current time (updating at the frequency you want to update data).
  2. Create a change event which logs the data you want to another tag (in this case, it runs a queryTagHistory using the window size to get the average in that period).
  3. If you historise this other tag, you get a history of moving window averages.

I don’t think this would scale well, but my use case was for a few tags in a very small system, but used across multiple different systems.

For any real production system, I would assume the solution proposed by @pturmel is much more performant.