Rockwell Automation 1756-HIST2G FactoryTalk Historian ME 2.2 - Manual (Page 118)

F ACTORY T ALK H ISTORIAN ME M ODULE U SER ’ S G UIDE
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This simple example illustrates how FactoryTalk Historian ME applies data
compression. In practice, the curves are more complex than straight lines, and the
compression specifications for each tag must be tuned properly to achieve a balance
between storage efficiency and accuracy.
The same principle applies to compressing real-world data. FactoryTalk Historian
ME uses a compression algorithm to determine which events it needs to keep in order
to provide an accurate data history. The CompDev, CompMin, and CompMax
attributes allow you to control the granularity of the compression algorithm.
When a new Snapshot arrives, the previous one is evaluated according to the
compression specifications to see if it is a significant event. If so, it is sent to the
Event Queue. If not, it is discarded. This process is called compression.
There are three instances where an event will bypass the compression process and be
put in the Event Queue:
„
If the Compressing attribute for the point is set to OFF.
„
If the timestamp is older than the timestamp of the current Snapshot. Such an
event is considered out of order.
„
If the Status attribute of the point has changed.
The compression method used by FactoryTalk Historian ME allows it to keep more
data online than conventional scanned systems. The data are also much more detailed
than in an archiving system based on averages or periodic samples.
S WINGING D OOR C OMPRESSION
The compression method is called swinging door compression. Swinging door
compression discards values that fall on a line connecting values that are recorded in
the archive. When a new value is received by the Snapshot subsystem, the previous
value is recorded only if any of the values since the last recorded value do not fall
within the compression deviation blanket. The deviation blanket is a parallelogram
extending between the last recorded value and the new value with a width equal to
twice the compression deviation specification.
Each point has three attributes that comprise the compression specifications:
„
Compression Deviation (CompDev)
CompDev is the half-width of the deviation blanket (as shown in the illustration).
CompDevPercent is similar to CompDev, but it specifies the compression
deviation in percent of Span rather than in engineering units.
„
Compression Minimum time (CompMin)