(s2)Improved cepstral techniques for operational modal analysis

摘要

All vibration-based machine condition monitoring systems use response signals, which are a mixture of forcing function and transfer function effects, and often an assumption is made as to whether a change is due to one or the other. Operational modal analysis (OMA) is one way of determiningthe dynamic properties of a structure or machine from the response signals only, while in operation, and thus provides the potential to separate them from the forcing functions. It then becomes a powerful machine diagnostic tool. Forcing functions and transfer functions are additive in thecepstrum, at least for single-input, multiple-output (SIMO) situations, and, moreover, they are often located in disparate regions in the cepstrum, allowing them to be separated. In recent years, new cepstral analysis methods have been developed to assist OMA in two ways: as a preprocessingtool to enhance the modal properties, remove other effects, such as forcing functions (often at discrete frequencies), and simplify the application of standard OMA techniques; and to perform OMA by fitting a pole/zero model of the structural dynamic properties to the enhanced response data.The current paper shows the success of both of these methods in the case of simulated signals from a variable speed gearbox, by vastly reducing the effects of the very complicated forcing function. An extension from earlier proposed approaches is the determination of zeros (normally maskedby noise in response signals) using transmissibilities, measured between pairs of responses and the noise being reduced by averaging. These new results confirm that limited, and only partially successful, earlier results were contaminated by non-linearities in the support of the test object.