Laser speckle studies for vibration and torque measurement
Laser speckle studies for vibration and torque measurement
This thesis presents fundamental research in the areas of Laser Doppler Velocimetry (LDV) and Laser Speckle Metrology for application to engineering measurements of vibration and torque.
The laser speckle phenomenon results from interference of the component wavelets of a coherent laser beam scattered from an optically rough surface. The particular speckle pattern formed is unique to the rough surface creating it and, importantly, movement of the surface produces sympathetic movement of the speckle pattern viewed. Further to this, periodic target motions, particularly rotation, cause a periodic repeat in the speckle pattern formed.
In Laser Vibrometry - the application of LDV to vibration measurement - speckle pattern motion is shown to cause phase and amplitude modulation of the frequency modulated Doppler signal. Phase modulation is most important, creating noise in the instrument output that is indistinguishable from genuine vibration and is given the name `pseudo-vibration'. Both phase and amplitude modulation can be reduced by collection of an increased number of speckles on the photodetector used and a minimum of 5 in the ratio of detector length (in the direction of speckle motion) to speckle size is recommended. It is also established that a small target laser beam for tilting and rotating targets or a larger target beam for targets moving in-plane is beneficial in limiting speckle-induced Doppler signal modulation.
The suitability of Laser Vibrometry for measurements on rotating targets is studied further in an in-depth analysis of this application. It is shown that spurious measurement dependencies on in-plane vibration and speed fluctuations can mask the intended, out-of-plane vibration measurement entirely compounding the difficulties in data interpretation introduced by pseudo-vibrations. A new technique is presented whereby two simultaneous measurements are used to resolve the genuine vibration components at non-synchronous frequencies.
University of Southampton
1994
Rothberg, Steven Joseph
(1994)
Laser speckle studies for vibration and torque measurement.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
This thesis presents fundamental research in the areas of Laser Doppler Velocimetry (LDV) and Laser Speckle Metrology for application to engineering measurements of vibration and torque.
The laser speckle phenomenon results from interference of the component wavelets of a coherent laser beam scattered from an optically rough surface. The particular speckle pattern formed is unique to the rough surface creating it and, importantly, movement of the surface produces sympathetic movement of the speckle pattern viewed. Further to this, periodic target motions, particularly rotation, cause a periodic repeat in the speckle pattern formed.
In Laser Vibrometry - the application of LDV to vibration measurement - speckle pattern motion is shown to cause phase and amplitude modulation of the frequency modulated Doppler signal. Phase modulation is most important, creating noise in the instrument output that is indistinguishable from genuine vibration and is given the name `pseudo-vibration'. Both phase and amplitude modulation can be reduced by collection of an increased number of speckles on the photodetector used and a minimum of 5 in the ratio of detector length (in the direction of speckle motion) to speckle size is recommended. It is also established that a small target laser beam for tilting and rotating targets or a larger target beam for targets moving in-plane is beneficial in limiting speckle-induced Doppler signal modulation.
The suitability of Laser Vibrometry for measurements on rotating targets is studied further in an in-depth analysis of this application. It is shown that spurious measurement dependencies on in-plane vibration and speed fluctuations can mask the intended, out-of-plane vibration measurement entirely compounding the difficulties in data interpretation introduced by pseudo-vibrations. A new technique is presented whereby two simultaneous measurements are used to resolve the genuine vibration components at non-synchronous frequencies.
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Published date: 1994
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Local EPrints ID: 462852
URI: http://eprints.soton.ac.uk/id/eprint/462852
PURE UUID: 53f5ceca-5f10-4d6e-b16b-e5835c7bb400
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Date deposited: 04 Jul 2022 20:16
Last modified: 04 Jul 2022 20:16
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Author:
Steven Joseph Rothberg
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