Output-Only Modal Analysis Using Continuous-Scan Laser Doppler Vibrometry and Application to a 20kW Wind Turbine

Continuous-scan laser Doppler vibrometry (CSLDV) is a method whereby one continuously sweeps the laser measurement point over a structure while measuring, in contrast to the conventional scanning LDV approach where the laser spot remains stationary while the response is collected at each point. The continuous-scan approach can greatly accelerate measurements, allowing one to capture spatially detailed mode shapes along a scan path in the same amount of time that is typically required to measure the response at a single point. The method is especially beneficial when testing large structures, such as wind turbines, whose natural frequencies are very low and hence require very long time records. Several CSLDV methods have been presented that employ harmonic excitation or impulse excitation, but no prior work has performed CSLDV with an unmeasured, broadband random input. This work extends CSLDV to that class of input, developing an output-only CSLDV method (OMA-CSLDV). This is accomplished by adapting a recently developed algorithm for linear time-periodic systems to the CSLDV measurements, which makes use of harmonic power spectra and the harmonic transfer function concept developed by Wereley. The proposed method is validated on a randomly excited free-free beam, where one-dimensional mode shapes are captured by scanning the laser along the length of the beam. The natural frequencies and mode shapes are extracted from the harmonic power spectrum of the vibrometer signal and show good agreement with the first seven analytically-derived modes of the beam. The method is then applied to identify the shapes of several modes of a 20kW wind turbine using a ground based laser and with only a light breeze providing excitation.

Single source three dimensional capture of full field plate vibrations

Measurement of the vibrations of plates can offer significant challenges to the experimentalist, particularly when the plates are lightweight, exhibit large amplitude deflections, nonlinear responses or are initially curved. The use of accelerometers adds masses which can change the dynamics of lightweight plates. Large amplitude oscillations and initial curvatures cause complications when using a laser vibrometer, as they make it difficult to get consistent reflections back to the receiver. Furthermore, large or nonlinear oscillations challenge inherent assumptions on which the vibrometer’s algorithms depend. A high speed video camera avoids these issues, but makes it hard to extract numerical data. This paper describes a method that extends the capabilities of a high speed video camera by using a mirror, allowing post-processing software to stereoscopically resolve an array of points on the plate surface to 3D coordinates, capturing the complete shape and position of the plate throughout vibration. This method avoids all the problems mentioned above and gives very clear insight into plate vibration. Some example results of this method are presented, using thermally bistable carbon laminate plates filmed at a 1000 frames per second. These plates pose the challenges described, and also exhibit an unusual oscillatory motion where the plates ‘snap’ between two statically stable states. The method is shown to provide clear insight into the rich dynamics of these plates