Degree of cyclic target protrusion defined on squared envelope spectrum for rotating machinery fault diagnosis

Rotating machinery fault diagnosis is critical for safe operation of modern mechanical equipment so as to avoid serious damage and economic loss caused by malfunctions. Envelope analysis is an effective method in the field of rotating machinery fault diagnosis, in which the selection of optimal demodulated band containing the most obvious fault features by fault index is a critical step. However, the existing fault indexes are often defined on the basis of global ratio in their domains of definitions, which may result in incorrect band selection. To address the problem, a novel index named degree of cyclic target protrusion (DCTP) for fault diagnosis is proposed in this paper. DCTP is defined on squared envelope spectrum to take adequately fault characteristic frequency (FCF) into consideration. Specifically, Otsu method is utilized to calculate local threshold for determining cyclic target amplitude and cyclic background in two elaborate bands. In this aspect, DCTP is defined as a weighted local ratio of cyclic target amplitude to cyclic background. Because it focuses on FCF, DCTP can avoid interference by extraneous components. DCTP is used to replace the kurtosis index of fast kurtogram (FTC) to establish the DCTPgram which is compared with FTC and ratio of cyclic content based method in roller bearing fault diagnosis and planetary gearbox fault diagnosis. Experimental results and comparison studies demonstrate that the DCTPgram has a significant advantage for rotating machinery fault diagnosis.