Novel Image-based Tracking Continuously Scanning Laser Doppler Vibrometer Systems for Vibration Measurement of Rotating Wind Turbine Blades

Abstract

The objective of the proposed research is to develop efficient vibration monitoring and structural health monitoring (SHM) methods for a horizontal-axis rotating wind turbine blade using a novel image-based tracking continuous scanning laser Doppler vibrometer (CSLDV) and new signal processing methods associated with tracking continuous scanning laser vibrometry. The image-based tracking CSLDV consists of a camera, a scanner with a set of orthogonal mirrors, and a single-point laser head. Currently, there are no suitable non-contact in-situ vibration monitoring and SHM methods for rotating wind turbine blades, and vibration monitoring and SHM of wind turbine blades are mainly accomplished by visual inspection of stationary blades or by using a limited number of embedded sensors. The image-based tracking CSLDV is capable of rapidly obtaining spatially dense vibration shapes (VSs), such as mode shapes and operating deflection shapes (ODSs) of a rotating structure under random excitation, by continuously sweeping its laser spot over the structure surface. VSs measured by the image-based tracking CSLDV can have more measurement points than those by a commercial scanning laser Doppler vibrometer in a point-by-point manner, and the former needs much less measurement time. Different image processing methods are developed for the image-based tracking CSLDV to extract the rotating structure from its background in captured images of the camera. Estimated real-time positions of the rotating structure can be used for estimating its rotation speed. Once the position of the rotating structure is determined in captured images, a scan path can be generated on it and rotation angles of mirrors of the scanner can be controlled so that the laser spot of the image-based tracking CSLDV can be swept along the path. New operational modal analysis methods are developed based on rigorous rotating beam and plate theories, which can estimate modal parameters, such as damped natural frequencies, modal damping ratios, undamped mode shapes, and ODSs, of the rotating structure under random excitation. A novel demodulation method with a reference signal is developed to identify positions of damages in a beam without its base-line information. Prototypes of an image-based short-range tracking CSLDV and an image-based long-range tracking CSLDV are developed for experimental validation of the proposed methods. Rotation speeds and modal parameters of rotating blades with different speeds that are excited by air flow that is considered as random excitation are estimated. Both undamped mode shapes of rotating blades on a straight scan path and their full-field undamped mode shapes are estimated and compared with each other. The proposed methods can address major challenges to monitor the vibration of a rotating horizontal-axis wind turbine blade and detect its potential damage in operational conditions.