2 Replies Latest reply on Jan 5, 2018 12:25 PM by Roger Palmen

    Problems with PI Interface - Compression and Deviation Processing


      Hi all,


      I have a problem/doubt with the PI Interface and the processes of exception and deviation for filter this data coming from OPC.


      The following picture shows a samples of humidity.



      There is an alarm behaviour saying that an alarm is triggered ON after that 5 minutes are elapsed out of thresholds (red line). The time counter of 5 minutes is decreased when the condition is ON, and it is stopped and resetted when air condition value go back inside thresholds plus a hysteresis (yellow line).


      As you can see in the picture, you go out from lower threshold (RED line) between 08:16:09 and 08:16:40. If we consider 08:16:10 as first time that you are out of threshold, the alarm should be triggered ON at 08:21:10.


      Here is the problem, in the report the alarm appears at 08:20:46, 24 seconds before the theoritical time.



      If I go to the PI DA and take a look in the values, I see this:



      As you can see the picture shows the 10 samples showed in the first trend. Considering that the alarm is triggered ON at 08:20:46 must be a value 5min before so at 08:15:46.

      However the first time that the value is under 40 is at 08:16:40 so the alarm would be at 08:21:40. I don't know if the samples that trigger ON the alarm if filtered.



      • The PLC samples every 2msec.
      • Scan class of the Interface is 00:00:01.
      • Humidity sensor has a Resolution of 0,1
      • This is the configuration of my tag (loc4=1)


      And don't know if the problem is coming from the machine or if it is problem from the PI System.


      Thanks in advanced.


      Best regards,

      Julio Flores

        • Re: Problems with PI Interface - Compression and Deviation Processing

          Hi Julio,


          Looking at your tag configuration here, the compression deviation and exception deviation are both set to 0, which means no values should be compressed/excepted out, so I don't think that's what is at play here.


          To get a bit more information, how is this alarm triggered? Is the alarm tag coming from the PI Analysis Service and based on the data you're receiving from the PLC? If so, it's possible that the evaluation time of the Analysis is set to periodic (rather than event based) and thus the calculation actually occurs in between the two events (08:16:09 and 08:16:40) giving you the behavior you're seeing.




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          • Re: Problems with PI Interface - Compression and Deviation Processing

            Simply looking at the archived value in the archive editor is not a good enough indication of when the value changed. The data could have been coming in a little bit later. I would suggest setting up some additional debugging for that tag so you can see when the data is being read and when it passed to the PI Data Archive.

            /db=4096 for the tag of interest; this will show you what is read from the OPC Server

            pibufss -trace; This will show you what is sent from pibufss to the snapshot


            Additionally, since your tag seems to be using Advise polling, then make sure you are running version or later since a lot of ExcMax not being respected with advise tags issues were resolved.



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