Rockwell Automation 1441-PEN25-Z Enpac 2500 Data Collector - Manual (Page 74)

74 Rockwell Automation Publication GMSI10-UM002D-EN-E - August 2012
Chapter 2 Setting Up Measurements
Next, find the total collection time for the time waveform measurement using
this formula.
Number and Type of Averages
The number and type of averages are part of the collection specification (Setup >
Collection). You select the collection specification when you set up the
measurement definition.
See Setting Up Collection Specifications on page 69 .
Emonitor and the Enpac 2500 support the following number and types of
averages for data collection.
• Linear - Up to 4096 averages.
• Time Synchronous - Up to 255 averages, and requires a trigger.
• Exponential - 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.
• Peak Hold - Up to 99 averages. Holds the highest measured value in each
bin (line).
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.
EXAMPLE If you had a Maximum frequency value of 7200 CPM and a Number of
lines value of 400, you would:
ATTENTION: 1. Convert CPM to Hz.
ATTENTION: 2. Find the total collection time.
collection time
spectral lines
Fmax (Hz)
------------------------------- =
7200 CPM
60
-------------------------- 120 Hz =
400 lines
120 Hz
--------------------- 3.3 seconds =