Removal of shaft speed related components from the response signals of a machine with varying speed prior to Operational Modal Analysis

摘要

It is an advantage to remove discrete frequency components from response vibration signals before carrying out operational modal analysis (OMA), because they tend to disrupt the operation of normal OMA algorithms. The removal of such components can be achieved by carrying out order tracking, to remove the effects of speed variations by resampling with a fixed number of samples per period of the frequencies to be removed, so that they are in discrete lines in the order spectrum, and can then in principle be removed by techniques designed for constant speed machines. If the speed variations are greater than 1% or so, the spectrum of the order tracked signals is no longer a frequency spectrum, so modal properties, which are related to frequency, can no longer be obtained by OMA. This paper introduces a method where signals are transformed into the order domain, and after the removal of shaft speed related components are transformed back to the time domain to allow normal OMA to be applied. The method is demonstrated using signals from a gearbox casing excited (via a shaker) with a varying frequency signal containing 22 harmonics corresponding to speed variations of ±15%.