When a rotating machine (or its part) vibrates, it generates acoustic signals related to rotational speed that are caused by mechanical flaw such as misalignment, imbalance, bearing defect, etc.
Order (harmonic component) analysis has been one of the most popular analysis methods for detecting such flaws in various fields of industry: automotive; aerospace, industrial equipment, household appliances, etc. Reliability engineers mostly rely on order analysis for examining rotating machinery. Behavioral analysis for rotating or reciprocating machinery is essentially related with the rotational speed and its harmonic components.
Some of those components are observable in acoustic signal itself in the time domain. However the pattern appears fairly complex. One way to reduce this complexity is to apply a power spectrum. It is more distinguishable in the frequency domain by depicting machine-related signal component. In the power-spectrum, signals that are periodic in the time domain appear as peaks in the frequency domain. For example, if a vibration peak occurs twice every revolution at the same shaft position, a peak appears at the second order in the order spectrum. However, the power spectrum is not applicable when the speed of rotating machine changes.
Order tracking is a method to evaluate orders (harmonic component) regardless of the rotation speed of machine. This can be done by synchronizing the sampling of input signals to the instantaneous angular position of the machine shaft using a resampling technique. As illustrated in Figure 1, rather than a constant number of samples per time, this results in a constant number of samples per revolution and transforms the analysis to the order domain rather than the frequency domain.