National Instruments Order Analysis Toolset Order Analysis Methods

Chapter 2 Order Analysis

orders overlap, you are unable to derive meaningful information about the

individual orders.

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.

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.