Operational modal analysis of outdoor slanted rotational blades under natural excitation using a long-range tracking continuously scanning laser Doppler vibrometer

In this study, we present the measurements of slanted rotational blades (SRBs) under natural excitation, which accurately represent the operational conditions of wind-driven generators and turbine blades. To track and scan the SRB, we develop a novel long-range tracking continuously scanning laser Doppler vibrometer (LR-TCSLDV) system. By sweeping a laser spot across the blade surface to capture real-time rotation, we apply an operational modal analysis (OMA) method, including an improved natural excitation technique (NExT), to determine the natural frequencies of SRB. The extraction of mode shapes (MSs) from SRBs employs a demodulation method for responses from broadband excitation. The estimation method of natural frequency is an improved NExT, whose derivation begins at the derivation of the measured vibration considering the tilt angle of the laser beam and the pitch angle of the SRB and ends at the cross-correlation function of the corresponding output responses. The tracking of the laser spot scanning the SRB is realized through visual identity, a LabVIEW module that utilizes grayscale recognition to define the position of the SRB. Outdoor experiments need additional precautions to minimize the influence of natural elements on visual identity, where the motion of the laser spot is controlled by a programmed path. An experimental investigation of the OMA method using the LR-TCSLDV system is conducted. Modal parameter estimation of the SRB using the OMA method is performed at different constant rotational speeds and validation is carried out by comparing the results from a stationary slanted blade.