National Instruments Order Analysis Toolset - Manual (Page 24)

Chapter 2 Order Analysis
© National Instruments Corporation 2-9 LabVIEW Order Analysis Toolset User Manual
orders overlap, you are unable to derive meaningful information about the
individual orders.
Harmonic Analysis
Harmonic analysis is suitable for the analysis of rotating machinery only
when the rotational speed remains constant. In classical harmonic analysis,
the fundamental frequency does not change over time. Although the phases
and amplitudes of the individual harmonics can vary over time, the center
frequencies of all the harmonics remain constant.
When using the Fourier transform, you obtain the best results with the
machine running at a constant rotational speed while taking measurements.
If you want to take measurements at a different rotational speed, you have
to run the machine to that rotational speed and take another measurement.
Testing with discrete rotational speed increments is time consuming.
Testing with discrete rotational speed increments also can be inaccurate or
impossible if you cannot control the rotational speed of the system well or
the system is not allowed to run at the critical rotational speed for a
sufficient length of time.
Order Analysis
An important goal of order analysis is to uncover information about the
orders that might become buried in the power spectrum due to a change
in rotational speed. While the orders are hidden in the overall power
spectrum in Figure 2-6, the orders have distinguishable features in the
frequency-time spectral map. The observation that orders have
distinguishable features in the frequency-time spectral map serves as the
starting point for uncovering information about orders that are difficult to
see in the power spectrum.
Order Analysis Methods
The basic technique of order analysis involves obtaining the instantaneous
speed of the rotating shaft of a machine from a tachometer or encoder
signal. The speed is then correlated to the noise or vibration signal
produced by the machine to obtain information about the order
components, such as waveforms, magnitudes, and phases. In order
analysis, revolutions, rather than time, serve as the basis for signal analysis.
Thus, in the spectrum domain, the focus is on orders instead of frequency
components.