Spider DSA User’s Manual
267
order location in an order-normalized spectrum indicates the number of vibration
cycles per shaft revolution. The tracked magnitude (which can be measured using
EU
pk
, EU
rms,
or EU
rms
2
) of an order is the measurement extracted through a
tracking filter with its center frequency located at this order. An Order Power
Spectrum measurement gives a quantitative description of the amplitude, or
power, of the orders in a signal. It provides a good view of all order components of
a signal. This can help you rapidly identify significant forcing mechanisms.
The following figure shows conceptually how angle re-sampling can be used to
analyze vibrations from an engine during start up. Once the signal has been
transformed into its angle domain, the FFT can be applied to analyze the order
spectrum of the vibrations.
Engine Speed in
RPM
Uniformed Sampled Data
Uniformed Sample Rate
Order Spectrum
Synchronously Sampled Data (sampling rate is
determined by both instantaneous tacho speed
and required analysis frequency range)
Frequency Spectrum
Analog signal overplot with
4X per revolution
tachometer signal
Figure 177: Angular data resampling of a chirp signal
An important concept that must be introduced now is called ΔOrder (delta order).
In the FFT based frequency spectrum analysis, the frequency span and frequency
resolution are fixed. The capability of discriminating frequency components is
equal in both low and high frequency. In rotating machine analysis, we need to
have better analysis resolution in the low frequency than that in high frequency.
For example, if the rotating speed is at 60 RPM, we definitely care if the
instrument can tell the difference between 1Hz (order 1) and 2Hz (order 2); in
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