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Flaws in machinery often let themselves be known as a series of periodic impacts, which in the frequency domain often translate into a family of harmonics spaced according to the speed of the machine.In this example we look at the acceleration response at a housing, where a flaw is invoked twice with different speeds. We have no tachometer signal here, but the machine is run at constant speed in the two segments, so that the orders are easily seen in the spectrograms. Rotate PLUS can show your plots as waterfalls or as spectrograms.
Spectrogram with harmonics
Frequency cut at 63 seconds- the same as above, but with the proportions of the display modified to emphasize the cursor cut. (Another handy feature of Rotate)
Frequency cut at 200 seconds.
Sideband cursors at 200 seconds, period 8.594 Hz. From the raw data displayed below, it is difficult to determine families of orders other than by using sideband cursors. Sideband cursors, another new useful feature of Rotate PLUS, are limited, though when data is noisy, or there are a number of high amplitude orders obscuring the harmonic families as depicted below.
Sideband cursors at 63 seconds, period 11.25 Hz.
Millstrum: Families of harmonics with identified modulation.The millstrum, which is essentially the fourier transform of the logarithm of the absolute value of the frequency spectrum identifies periodicity in the frequency domain. The normal formulation, called the real cepstrum, has as its abscissa the period of the harmonics, but by inverting period, we are back in the frequency domain, and have the graphical presentation shown here. This presentation was suggested by Mr. Robert Miller of the Modal Shop, hence the nomenclature millstrum. In the presence of a tachometer signal, we can also go to the order domain for easier identification of the periodicities.
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