An improved multiple discrete time system for operational modal analysis of a slanted rotational blade under natural excitation via a long-range tracking continuously scanning laser doppler vibrometer system

Abstract

A long-range tracking continuously scanning laser Doppler vibrometer (LTCSLDV) system is able to continuously track the speed and simultaneously sweep the surface of a distanced rotational blade with predicted paths. During the long-range continuous scanning, laser beams including raw responses with modal parameter information are transmitted from the measured distanced rotational blade to the laser Doppler vibrometer (LDV). However, classical signal processing methods for raw responses obtained from a LTCSLDV measurement, which are the polynomial and demodulation method, cannot implement standard modal estimation. Its reason is that continuous scanning responses includes density virtual measuring points and one of its results is that some significant modal parameter, especially damping ratio, cannot be estimated directly. This work provides an improved multiple discrete time system (MDTS) for LTCSLDV measurements that is capable of extracting the responses of multiple virtual measuring points on the virtual scanning path with equally intervals as if these responses are measured at fixed pseudo-measurement points. Therefore, extensively complex signal processing can be implemented on the processed responses through program or commercial software only if the amount of the data of them is sufficient for modal analysis. The measurements were performed on a slanted rotational blade (SRB) in real operational condition with the considerations of the pitch angle of the SRB and the tilt angle of the direction of laser beam emitted from a LDV. The predicted tracking and scanning path of the laser spot in long-range measurements is realized through visual identification with a high-speed color camera.