Residual angular speed analysis based on laser Doppler vibrometer and its application in planetary gearbox diagnosis

Due to the complex transmission paths, diagnosis for planetary gearbox faces many challenges. The Laser Doppler vibrometer (LDV) has been considered a promising tool for non-contact vibration analysis. However, the inherent speckle noise induced by the curved and rotating surface can interfere the identification of the concerned fault impacts. This article presents a residual angular speed analysis approach for the off-plane LDV, featuring a two-step speckle noise reduction process. First, an Improved Signal-to-Noise Ratio (ISNR) metric is developed to guide offset adjustments. Second, a size-adaptive median filter method is designed to mitigate the effects of inevitable spikes. The proposed process can effectively smooth the speckle noise of LDV with a demodulation bandwidth at 100 kHz, to reduce the risk of misdiagnosis. The proposed method is validated by two planet gear fault cases with varying spallation sizes, achieving a similar performance compared to the high-resolution rotary encoder.