Experimental Investigation of the Effect of Speckle Noise on Continuous Scan Laser Doppler Vibrometer Measurements

Continuous Scan Laser Doppler Vibrometry (CSLDV) sweeps a laser spot continuously over a structure to measure its mode shapes at hundreds of points simultaneously. It can be used to measure accurate, detailed mode shapes using a hand-held impact hammer, while conventional point-by-point LDV can be impractical and inaccurate in that application because of strike-to-strike variation in the impact location and orientation. Recently presented techniques can be used to transform the CSLDV measurements into a set of responses that can be processed using standard system identification techniques to extract the modes. The resampling method works best when the scan frequency is high relative to the highest natural frequency of interest, and state of the art scanning vibrometers are capable of relatively high scan frequencies, but there is a tradeoff between measurement quality and scan frequency due to laser-speckle noise. This work explores the effect of LDV measurement noise, both the periodic and non-periodic components, on CSLDV measurements. Of particular interests are: the dependence of the signal-to-noise ratio on the scan rate and geometry, the target-to-detector distance of the experimental setup, the Doppler signal quality, and the sampling rate used to acquire the measurement. The results presented sometimes seem contrary to one’s intuition and to the conclusions presented in other works, but they do provide a fairly thorough guide for the experimentalist, enabling the design of CSLDV experiments with the minimum noise level possible. Nomenclature t : Temporal Variable No : # of Samples per Period x : Spatial Variable Naper : # of Non-Periodic Signal Values xm : Maximum Spatial Value Naper,lifted : # of lifted Non-Periodic Values λ : Wavelength Variable TA : Signal Period φi : Phase Variable v : Voltage Signal fsamp : Sampling Frequency y : Response Signal fscan : Scanning Frequency yn : n Pseudo Response Signal h : Surface Imperfection Height yper : Periodic Data of Response Signal d : Target-to-Detector Distance Yaper(ω) : Non-Periodic Frequency Signal θ : Laser Sweep Angle Yaper,lifted(ω) : Lifted Non-Periodic Freq. Signal ηave : Non-Periodic Noise Average ηave,lifted : Lifted Non-Periodic Noise Average ρave : Periodic Noise Average