Envelope pre-processing techniques for rolling element bearing diagnosis in variable speed conditions

Abstract

In rotating machine diagnosis, the squared envelope spectrum (SES) is one of the most efficient indicators of the presence of cyclostationarity, which is a typical symptom of damage in rolling element bearings. Similarly to other cyclostationary tools, the SES should be applied on the pure random part of the signal, otherwise spurious peaks may appear and hide the cyclostationary content. Therefore, conventional deterministic/random separation (DRS) techniques are usually used as a pre-processing step to suppress the deterministic (periodic) component. In speed-varying regimes, the SES is combined with computed order tracking to obtain an order-domain representation rather than a Hertzian one. Through being order tracked, the deterministic components undergo speed-dependent variations in their magnitude and phase, thus losing their periodicity. This necessarily invalidates the working assumption of conventional DRS techniques and, consequently, jeopardises the efficiency of the SES. Recently, some sophisticated pre-processing techniques have been proposed to deal with this issue, the most important of which are the improved synchronous average (ISA), the cepstrum pre-whitening (CPW) and the generalised synchronous average (GSA). This paper aims to provide a comparative study of these techniques by highlighting their pros and cons. The comparison is performed on real-world signals.