Continuous scan laser Doppler vibrometry
Continuous scan laser Doppler vibrometry (CSLDV) is a method of using a laser Doppler vibrometer (LDV) in which the laser beam is swept across the surface of a test subject to capture the motion of a surface at many points simultaneously. This is different from scanning laser vibrometry (SLDV) in which the laser beam is kept at a fixed point during each measurement and quickly moved to a new position before acquiring the next measurement.
Advantages and disadvantages
CSLDV can allow one to capture the mode shapes of a structure with high resolution much more quickly than would be possible with SLDV. Allen & Sracic  show results where measurements were acquired with CSLDV in a hundredth of the time that would be required for LDV. Allen & Aguilar  postulated that the additional detail available from CSLDV might provide important information when validating structural dynamic models. CSLDV also makes testing with an instrumented hammer practical with LDV, and some have speculated that CSLDV might be useful in cases where it is impossible to recreate the input forces, such as explosive loadings.
The primary disadvantage of CSLDV is the additional laser speckle noise that occurs if the laser spot scans the structure too quickly. Speckle noise is caused by micro-scale irregularities in the surface that change the intensity pattern of the laser light received by the LDV as it scans the surface. Similar problems arise when applying LDV to rotating shafts, crankshafts for example. Speckle noise is difficult to predict, depending on the properties of the surface, the geometry of the structure and position of the LDV, so further research is needed to establish the limits of CSLDV.
Sriram et al. seem to have been the first to publish regarding CSLDV, although they studied it for only a few years and discontinued research. Most of the subsequent advances in this area have arisen from the group at Imperial College in London by Stanbridge, Ewins, Martarelli and Di Maio, who coined the term "CSLDV".
The video in the link below illustrates the process and some of the scan patterns that are possible.
- M. S. Allen and M. W. Sracic, "A New Method for Processing Impact Excited Continuous-Scan Laser Doppler Vibrometer Measurements," Mechanical Systems and Signal Processing, vol. 24, pp. 721–735, 2010.
- M. S. Allen and D. M. Aguilar, "Model Validation of a Bolted Beam Using Spatially Detailed Mode Shapes Measured by Continuous-Scan Laser Doppler Vibrometry," in 50th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference Palm Springs, California, 2009.
- S. Rothberg, "Numerical simulation of speckle noise in laser vibrometry," Applied Optics, vol. 45, pp. 4523–33, 2006.
- P. Sriram, J. I. Craig, and S. Hanagud, "Scanning laser Doppler vibrometer for modal testing," International Journal of Analytical and Experimental Modal Analysis, vol. 5, pp. 155–167, 1990.
- A. B. Stanbridge and D. J. Ewins, "Modal testing using a scanning laser Doppler vibrometer," Mechanical Systems and Signal Processing, vol. 13, pp. 255–70, 1999.
- R. Ribichini, D. Di Maio, A. B. Stanbridge, and D. J. Ewins, "Impact Testing With a Continuously-Scanning LDV," in 26th International Modal Analysis Conference (IMAC XXVI) Orlando, Florida, 2008.
- M. Martarelli, "Exploiting the Laser Scanning Facility for Vibration Measurements," in Imperial College of Science, Technology & Medicine. vol. Ph.D. London: Imperial College, 2001.
- M. Martarelli and D. J. Ewins, "Continuous scanning laser Doppler vibrometry and speckle noise occurrence," Mechanical Systems and Signal Processing, vol. 20, pp. 2277–89, 2006.