FactoryTalk Historian SE Exception Filter

In FactoryTalk Historian SE Exception and compression are two methods to reduce the number of data points saved in Historian SE while still maintaining a good representation of the actual behavior of the tag being monitored. The data from each tag being monitored still comes into the snapshot of the system. If the data passes Exception test it will be sent to Historian. If the Historian data pass Compression test they will be archived. In a sense both Exception and Compression are Historian filters deciding which data pass through.

Historian SE Exception

When using the Historian SE Exception test, there are four variables to be set:

• ExcMin: Exception minimum time
• ExcMax: Exception maximum time
• ExcDev: Exception Deviation. This determines how much the new value and the old value can deviate from each other before the new value gets set to Historian.
• Engineering Units

A data point can pass an exception test in any of the two possible ways below:

First Set of Conditions

• If the difference between the new value and the last value that passed the exception test is more than the ExcDev. For example if the old value was 110 and the deviation is 5 (engineering units) then any new value between 105 and 115 will be discarded. If the new value is outside those limits then it will be sent to Historian.

AND

• If the difference between the time-stamp of the new value and time-stamp of the old value is more than ExcMin. So the new data has to have a time-stamp of at least a certain period of time after the old data. In many Historian SE applications the ExcMin is set to zero. Therefore all the new data that come pass this test.

Second Set of Conditions

• If the difference between the new value and the last value that passed the exception test is more than the ExcDev. For example if the old value was 110 and the deviation is 5 (engineering units) then any new value between 105 and 115 will be discarded. If the new value is outside those limits then it will be sent to Historian .

AND

• If the difference between the time-stamp of the new value and time-stamp of the old value is more than ExcMax. So if certain period of time has passed (ExcMax) after the old data without any new data being sent to Historian (because the new data didn’t deviate enough) then a new data point will automatically pass the Exception filter. This is so that the application doesn’t go for a long time without any new passed data.

Keep in mind that Exception can NOT be turned off. So if you don’t want to use its functionality simply set all of its parameters to zero. The graph below illustrates how data is passed through an exception filter:

Related posts:

1. FactoryTalk Historian SE Compression Filter
2. New Version of FactoryTalk Historian SE From Rockwell Automation
3. Historain SE Polled Vs Advised Data Collection