RECOVERY OF IMPACT SIGNATURES IN MACHINE STRUCTURES

Reciprocating machines are difficult to diagnose using traditional fre quency domain techniques because they generate predominantly transient vibrations which have a broad frequency content. We have found that t ransient vibrations, like those generated by valve impacts in a recipr ocating compressor, are best analysed in the time-domain. Unfortunatel y, by the time the vibrations reach the surface of the machine where w e can measure them non-invasively, reverberation and dispersion have d isfigured them so that they look nothing like their originating forces . In order to convert them into a useful form, where they can be chara cterized in terms of timing and strength on a cycle-by-cycle basis, th e vibration signal can be compressed by filtering it with the inverse of the structural transfer function. This is a straightforward process if the exact transfer function is known; however, in practice we expe ct to have simply a 'typical' transfer function from a nominally ident ical machine. Additional transfer function variability results from ch anges in machine operating conditions such as temperature and load. Th e inverse-filtering process can be made robust to transfer function va riability through a combination of cepstral-smoothing and minimum-phas e processing. In addition, if a cepstral comb window is incorporated i nto the signal processing scheme, multiple impact signatures can be re moved. (C) 1995 Academic Press Limited