Investigation of Noise Sources in Laser Doppler Vibrometer Measurements
The laser Doppler vibrometer (LDV) is a tool which can detect a Doppler shift induced in a laser beam incident on a vibrating target. This Doppler shift is processed to yield information about the velocity at which the target is vibrating. LDV systems have evolved from custom built laboratory instruments in the late 1960s to widely used, commercially available tools. A particular aspect of LDV systems that has not been thoroughly investigated in a unified manner is the way in which measurement conditions impart noise into the velocity signal. This work investigates several noise sources including target surface roughness, standoff distance, beam focus, angle of incidence, and amplitude-dependent noise. There is little literature dedicated to addressing the consequences of these fundamental noise sources, all of which confront LDV systems in every measurement. The aim of this dissertation is to study these noise sources and contribute meaningful analysis that will be of use to those in the LDV community who are concerned with maximizing measurement quality. A novel three-dimensional contour scanning LDV measurement system has been developed which allows measurement along vectors relative to the surface normal of the target. This unique system raised issues which motivated the investigation of noise sources in LDV. A new method of looking at the noise field, which considers standoff distance and beam focus, is presented, illustrating the complicated nature of LDV noise. These noise maps provide a starting point to a more detailed investigation of surface roughness effects. Methods for characterizing measurement noise arising from focus error and angle of incidence when the target has a rough surface are proposed and results from experiments performed on surfaces machined to a precise roughness average are used to draw conclusions about the effects of surface roughness.With LDV, the speckle phenomenon causes increased noise in measurements when the vibration amplitude increases. An experiment to characterize this behavior is presented, the results of which illustrate when this amplitude dependent noise will have potentially adverse effects on the measurement.
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