Multi-mode degradation assessment and vibro-acoustic monitoring of harmonic drive for aerospace applications

Harmonic drives (HDs) in aerospace manipulators operate under severe tribo-mechanical stresses, leading to multiple interacting degradation modes that complicate reliability assessment and predictive maintenance. To address these challenges, a run-to-failure testing procedure was developed for aerospace HDs under representative operational conditions, enabling quantitative damage assessment through post-test examination. Post-test disassembly identified three dominant failure modes: flexspline and wave generator interface wear, circular spline and flexspline tooth wear, and wave generator flexible thin-walled bearing fatigue. In conjunction with the quantitative characterization of each failure mode, multi-channel vibration data were processed using cyclostationary analysis to construct degradation-sensitive health indicators (HIs) tailored for monitoring wear-type and fatigue-type faults. Experimental results validated that these indicators exhibited superior monotonicity, trendability, and robustness under limited samples and run-to-failure test conditions. Furthermore, integrating the acoustic emission technique with vibration sensing improved early fault detection capability and reduced false alarm rates. By combining comprehensive failure analysis with fault-specific HI design, this study presents an integrated methodology supporting both failure mechanism analysis and reliability-based prognostics of aerospace HD degradation.