Two-level variational chirp component decomposition for capturing intrinsic frequency modulation modes of planetary gearboxes

Due to the advantages of high transmission ratios and large carrying capacity, planetary gear-boxes have been widely used in various industrial machinery. Vibration signals caused by plan-etary gearbox faults usually have intrinsic frequency modulation (FM) modes. Since the intrinsic FM modes have fast time-varying and oscillating characteristics, extraction of the intrinsic FM modes is still a challenging task. In this paper, a two-level variational chirp component decom-position (VCCD) approach is proposed to capture the intrinsic FM modes from vibration signals caused by planetary gearbox faults. Specifically, to avoid complicated sideband analysis, Level 1 of the VCCD is designed to extract gear fault characteristics from an amplitude modulation (AM) signal obtained by the Hilbert transform and a preliminary fault detection conclusion can be drawn to locate gear faults. Level 2 of the VCCD is employed to accurately extract the oscillation FM mode of a FM signal obtained by removing the AM signal from an original vibration signal. Results of Level 2 of the VCCD can not only verify the initial conclusion previously drawn from Level 1 but also explicitly and comprehensively reveal fault features of planetary gearboxes. The potential and effectiveness of the proposed two-level VCCD method are verified by two simulated cases related to planet gear faults under variable speed conditions and two experiments respec-tively related to a broken ring gear under constant speed conditions and a worn sun gear under variable speed conditions.