Noncontact speckle-based velocity sensor [laser Doppler vibrometer]

Laser light illuminating a rough surface generates speckle phenomena in the space around the surface. In this paper, the authors propose a noncontact laser Doppler vibrometer (LDV) sensor, using the speckle phenomena, for the assessment of target velocity and displacement at low vibration frequencies. The sensor is realized using few optical components in a very simple optical configuration. The methodology for the experimental and analytical determination of the sensor sensitivity is reported, and two speckle-processing algorithms have been used to appraise the best cross-correlation function for target velocity measurement. The algorithms are based on binary correlation and fast Fourier transform analysis. Experimental results coming from the comparison of both the proposed techniques are reported. Moreover, results obtained with the proposed speckle sensor have been compared with reference measurement systems. Results show that this noncontact sensor is characterized by a reduced optical complexity and low cost. It appears to be particularly suitable for use in the analysis of low velocity rough surfaces.