Cyclostationary and bilinear approaches for gears vibrating signals application to helicopter gear box diagnosis

Our study first deals a comparison between cyclostationarity and bilinearity, and then with different applications of these approaches. Using synthetic and indudtrial signals, we underline that cyclostationarity tools make it possible to determine non-linear links between two frequencies (Quadratic Phase Coupling). Then , we present different applications of cyclostationarity and bilinearity to helicopter noise. These methods enable us to obtain much more interesting results than classical methods such as Fourier, Spectrum, time-frequency studies. First, these techniques allow us to detect and to determine precisely a fault on the system. Moreover, the PSD presents a frequency which is not linked to any physical property of the engine. A cyclostationary study makes it possible to explain precisely the apparition of the frequency. Finally, we underlined a unusual modulation phenomenon. Usually, during a meshing phenomenon, the rotating frequency modulaters the meshing frequency; this is characterised by a spectrum with lateral bands around meshing harmonics. Here we encountered a meshing frequency which modulated another meshing frequency.