Continuous Scan Laser Doppler Vibrometry (CSLDV)
Encyclopedia
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.
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.
Laser Doppler Vibrometer
A laser Doppler vibrometer is a scientific instrument that is used to make non-contact vibration measurements of a surface. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam...
(LDV) in which the laser
Laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
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 & Disadvantages
CSLDV can allow one to capture the mode shapesNormal mode
A normal mode of an oscillating system is a pattern of motion in which all parts of the system move sinusoidally with the same frequency and with a fixed phase relation. The frequencies of the normal modes of a system are known as its natural frequencies or resonant frequencies...
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
Structural Dynamics
Structural dynamics is a subset of structural analysis which covers the behaviour of structures subjected to dynamic loading. Dynamic loads include people, wind, waves, traffic, earthquakes, and blasts. Any structure can be subject to dynamic loading. Dynamic analysis can be used to find 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
Speckle noise
Speckle noise is a granular noise that inherently exists in and degrades the quality of the active radar and synthetic aperture radar images....
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.