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Chapter 3
Setting Up Measurements
80
If your Maximum frequency is specified in CPM, convert CPM to Hertz using
the following formula.
Next, find the total collection time for the time waveform measurement using
this formula.
EXAMPLE
Number and Type of Averages
The number and type of averages are part of the collection specification (Setup >
Collection). See
Collection Specification Dialog Box on page
Select the collection specification when you set up the measurement definition.
The Emonitor software and the Dynamix 2500 data collector support the
following number and types of averages for data collection.
Averages
Linear
Time Synchronous
Exponential
Peak Hold
Averaging is useful for reducing random errors. Random errors include
background vibration due to some source other than the machine being
measured. If you collect more averages, you have fewer random errors. However,
collecting more averages requires more time.
Overlap processing (Percent overlap) speeds up the averaging process. The
overlap amount determines how much of the time samples are overlapped for
successive averages. The greater the overlap, the faster the sample can be collected.
The disadvantage is that the greater the overlap, the less new data there is, and the
greater the influence of random errors.
Rockwell Automation Publication 1441-UM001B-EN-P - September 2012
Fmax (CPM)
-------------------------------
Fmax (Hz)
=
spectral lines
------------------------------ -
collection time
=
Fmax (Hz)
If you had a Maximum frequency value of 7200 CPM and a Number of
lines value of 400, you would:
1. Convert CPM to Hz.
7200 CPM
------------------------- -
=
120 Hz
60
2. Find the total collection time.
400 lines
---------------------
=
3.3 seconds
120 Hz
Description
Up to 4096 averages.
Up to 255 averages, and requires a trigger.
Up to 99 averages. Uses exponential weighting. Use for measurements on a
continuous signal that may be slowly varying. Also use to obtain a uniform
statistical error over all frequencies.
Up to 99 averages. Holds the highest measured value in each bin (line).
60
75.
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