National-instruments Order Analysis Toolset Manuel d'utilisateur télécharger pdf (Page 20)

Chapter 2 Order Analysis

Figure 2-3. PC Fan Running at Constant Speed

The bottom plot in Figure 2-3 depicts the tachometer pulses and the signal

from an accelerometer mounted on the PC fan. The plot on the left in

Figure 2-3 illustrates a conventional power spectrum based on the fast

Fourier transform (FFT). The upper-right plot in Figure 2-3 shows the

frequency-time spectral map computed from the short-time Fourier

transform (STFT) with a 1,024-point Hanning window. Because of the

constant rotational speed of the fan during data acquisition, you can

identify several peaks in both the power spectrum and the frequency-time

spectral map. The peaks indicate different orders and appear in the

frequency-time spectral map as white lines.

STFTSpectrum

570

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100

Frequency (Hz)

31.9 31.9 31.9

1 2 ... 15 16 17 18 19 20 21 22 23 24 25 ... 62 63

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National-instruments Order Analysis Toolset Manuel d'utilisateur Page 20