Setting Up and Collecting Off Route Orders Measurements
dataPAC User’s Guide
123
Example 2 - A uniform sampling rate for a rotor running at variable speeds. When the
machine speed increases, the period of the time waveform becomes shorter. But, the data
samples were taken at a fixed sampling rate. Due to the speed increase, the number of
samples in each cycle decreases. As shown in the figure above, 15 samples were taken in the
first cycle, 10 in the second one, and 7 in the third one. Although the spacing between the
data samples seems to become larger and larger in the time waveform, the samples actually
have exactly the same time interval. The frequency spectrum was widely “smeared” due to
the frequency shift. When the smearing problem occurs, the rotating frequency and related
components appear in multiple bins or lines in a frequency spectrum.
Example 3 - Variable sampling rate synchronized with the rotational speed of a variable
speed machine. A tachometer is used to sense the shaft speed and provide a pulse or an
integral number of pulses for each revolution. In this particular case, 12 pulses were
generated for a complete revolution and each pulse indicated that the shaft had rotated 30
degrees. For the illustration purpose, the time waveform was sampled at the same rate as the
tachometer pulses in this example, i.e., one sample per every 30 degrees. As shown in the
figure above, 12 samples were taken for every cycle regardless how fast or slow the shaft
was rotating.
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