FactoryTalk Historian SE Compression 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 Compression
Unlike Exception test, Compression test can be turned ON or OFF. When using Historian SE compression there are four parameters to be set:
- CompMin: Compression minimum time
- CompMax: Compression maximum time
- CompDev: Compression Deviation. This determines how much the new value and the old value can deviate from each other before the new value gets archived.
- Engineering Units
Compression method is called the swinging door compression. It acts similar to the Exception test except that the deviation blanket in Compression is a parallelogram drawn from last recorded value and the new value with a width of twice the the deviation specification.
With the swinging door algorithm, a value is stored if a straight line drawn between the last stored value and the next value does not come within the compression deviation specification of all the intermediate points. Two slopes are required to carry out this test (taken from Swinging Door Compression article).
If Compression test is set to ON, a data point can pass an exception test in any of the two possible instances below:First Set of Conditions
First Set of Conditions
- If the difference between the new value and the last value that passed the exception test is more than the CompDev. 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 CompMin. 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 CompDev. 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 CompMax. 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.
The following illustration shows how a set of data points can pass through a compression test.
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