Laser Doppler vibrometry (LDV) is a well-known interferometric technique to measure the motions, vibrations and mode shapes of machine components and structures. The drawback of commercial LDV is that it can only offer a pointwise measurement. In order to build up a vibrometric image, a scanning device is normally adopted to scan the laser point in two spatial axes. These scanning laser Doppler vibrometers (SLDV) assume that the measurement conditions remain invariant while multiple and identical, sequential measurements are performed. This assumption makes SLDVs impractical to do measurement on transient events. In this paper, we introduce a new multiple-point laser coherent detection system based on spatial-encoding technology and fiber configuration. A simultaneous vibration measurement on multiple points is realized using a single photodetector. A prototype16-point laser coherent detection system is built and it is applied to measure the vibration of various objects, such as body of a car or a motorcycle when engine is on and under shock tests. The results show the prospect of multi-point laser coherent detection system in the area of nondestructive test and precise dynamic measurement.
Generally there are two categories of noncontact laser interferometric methods commonly used in dynamic measurement, camera-based full-field interferometry and photo-sensor-based laser Doppler interferometry. The two methods have different advantages and disadvantages thus are suitable for different applications. The camera-based interferometry enjoys the valuable merit of full-field observation and measurement. In this paper, one typical full-field interferometry, digital holography, is employed to monitor the growth process of aqueous sodium chlorate crystals. The phase proportional to the solution concentration is retrieved from the holograms captured by CCD camera in real time. There exist no phase ambiguity problem in holography compared with other optical interferometric methods. On the other hand, laser Doppler interferometry is usually a point-wise measurement but with a very high temporal sampling rate. A multipoint laser Doppler interferometer is proposed for modal parameter measurement in this paper. The multiple transient vibration signals of spatially separated points on a beam structure subjected to a shock excitation are recorded synchronously. The natural frequencies and mode shapes are extracted in the signal processing stage. This paper shows that laser interferometry is able to contribute more to the practical applications in dynamic measurement related fields.
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.