Laser Doppler vibrometry(LDV) is a precise and non-contact optical interferometry used to measure vibrations of
structures and machine components. LDV can only provide a point-wise measurement, or a scanning measurement via
moving the laser beam rapidly onto the vibrating object which is assumed to be invariant in the scanning course.
Consequently, LDV is usually impractical to do measurement on transient events. In this paper, a new self-synchronized
multipoint LDV is proposed. The multiple laser beams are separated from one laser source, and different frequency shifts
are introduced into these beams by a combination of acousto-optic modulators. The laser beams are projected on
different points, and the reflected beams interfere with a common reference beam. The interference light intensity signal
is recorded by a single photodetector. This multipoint LDV has the flexibility to measure the vibration of different points
on various surfaces. In this study, two applications in experimental mechanics area are presented. Firstly, the proposed
system is used to measure the resonant frequencies of structure in a shock test. Secondly, The proposed multi-point LDV
is also used to measure the mode shape of a beam with an artificial crack. Compared with the original vibration mode
shape, the crack location can be identified easily.
Optical coherent detection is a precise and non-contact method for measurement of tiny deformation or movement of an
object. In the last century, it can only be used on the static or quasi-static measurement of deformation between two
statuses. Recently it has been applied on dynamic measurement with the help of high-speed camera. The advantage of
this technique is that it can offer a full-field measurement. However, due to the limited capturing rate of high-speed
camera, its capability in temporal domain cannot meet the requirements of many applications. In this study, several
issues in high-speed-camera-based optical interferometry are discussed. For example, introduction of carrier in temporal
and spatial domain, signal processing in temporal-frequency domain, and the introduction of dual-wavelength
interferometry in dynamic measurement. The discussion leads to a clue to select suitable technique to fulfill whole-field
dynamic measurement at different ranges.
Shearography is a whole-field, noncontact optical technique that allows the direct measurement of first-order derivatives of deflection on spatial coordinates, depending on the measurement setup. In many cases, the curvatures and twists of an object provide more interesting parameters, as they are directly related to the induced stresses when an object is subjected to external loads. We describe the use of digital shearography for the measurement of these stress-related parameters through phase retrieval when an object is undergoing continuous deformation. A sequence of shearograms is captured by a high-speed camera during the deformation. To avoid the problem of phase ambiguity, either a spatial or temporal carrier is introduced. A comparison of spatial and temporal carrier is also presented. The obtained three-dimensional matrix is then analyzed by Fourier and windowed-Fourier transform in a spatial and temporal domain and a high-quality spatial distribution of the deflection derivative, curvature and twist are extracted at any instant.
This article presents a novel fiber-based multi-beam laser Doppler vibrometer (LDV). In this design, a single
wavelength laser source at 1550 nm combined with several acousto-optic modulators (AOM) form the transmitter head
of the LDV. At the receiver side, one single high-speed photo-detector is employed, instead of multiple detectors
according to other reported multi-beam laser Doppler vibrometer.1, 2 Utilization of spatial encoding technique allows us
to produce transmitted laser beams with different frequency shifts. In this work, a laser source passes through a sequence
of totally four AOMs at different regimes, producing a 4×5 laser beam matrix which is then sent onto different points of
vibrating targets for measurement. The backscattered light signals are collected back into a single mode fiber by a fiber
collimator and combined with a common reference beam. This mixture of optical signals passes through an Erbium
Doped Fiber Amplifier (EDFA) before it is detected by a high-speed fiber-based InGaAs photo-detector. With a digital
demodulation algorithm implemented in Labview, the phase variations and thus the vibrations of different testing points
can be extracted separately from their corresponding frequency bands. The experimental results show it is possible to do
a precise vibration measurement on twenty testing points simultaneously using this novel multi-beam LDV.
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