A Lifting Algorithm for Output-only Continuous Scan Laser Doppler Vibrometry
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چکیده
Continuous Scan Laser Doppler Vibrometry (CSLDV) can greatly accelerate modal testing by continuously sweeping the measuring laser over the structure, effectively capturing the response of the structure at tens or even hundreds of points simultaneously. The authors recently extended this technique to the case where the input forces are unmeasured and random using harmonic power spectrum. This paper presents a variant on the proposed method that combines lifting, a resampling approach, with the output only algorithm. Lifting causes all of the peaks in the harmonic power spectrum to collapse onto a single peak for each mode, greatly simplifying modal parameter estimation. The proposed approach works by estimating and then lifting the harmonic correlation function, which is analogous to the impulse response of the system. The proposed algorithm is evaluated on a simulated beam and compared with the previous output only methods, indicating that the new approach gives comparable results to those of the previous methods but the data reduction is far simpler. The algorithm is then used to identify the first several modes of a parked wind turbine under wind excitation, capturing the deformation shape along one blade in detail. A new long range Remote Sensing Vibrometer (RSV) from PolyTec was employed for these measurements. This new vibrometer allows the first several modes of the turbine to be captured, from a standoff distance of 77 meters without the retro-reflective tape applied to the turbine. The speckle noise in the measurements is found to be remarkably small, allowing a 36 Hz scan frequency to be employed, which corresponds to a surface velocity of the laser spot of more than 500 m/s.
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تاریخ انتشار 2012