Operational modal analysis of a rotating structure using image-based tracking continuously scanning laser Doppler vibrometry via a novel edge detection method

Abstract

A novel edge detection method is developed for an image-based tracking continuously scanning laser Doppler vibrometer (CSLDV) system to track a rotating structure without attaching any encoder or mark to it. The edge detection method can determine real-time positions of points on edges of the rotating structure by processing images captured by a camera in the tracking CSLDV system. Once a point on an edge of a rotating structure is determined, the position of the rotating structure is determined. The tracking CSLDV system can generate a scan path on the rotating structure and control its laser spot to sweep along the scan path. A newly developed improved demodulation method is used to process measured data of response of the rotating structure under random excitation and estimate its modal parameters including damped natural frequencies and undamped mode shapes. Damped natural frequencies of the rotating structure are estimated from fast Fourier transforms of measured data. Undamped mode shapes are estimated by multiplying measured data by sinusoidal signals whose frequencies are estimated damped natural frequencies and applying low pass filters to measured data multiplied by the sinusoidal signals. Experimental investigation of the edge detection method is conducted by using the tracking CSLDV system to track and scan a rotating fan blade. Modal parameters of the rotating fan blade under random excitation with different constant speeds and its instantaneous undamped mode shapes with a non-constant speed are estimated.