Measuring and testing – Vibration – Sensing apparatus
Patent
1979-11-09
1982-07-20
Kreitman, Stephen A.
Measuring and testing
Vibration
Sensing apparatus
128665, 128746, G01B 902
Patent
active
043399541
DESCRIPTION:
BRIEF SUMMARY
This invention concerns the measurement of small movements and more particularly the measurement of movements, typically vibrations, having magnitudes less than the wavelengths of light.
Interferometry is a well-established technique for making such measurements and a more recently developed technique of this kind is that known as laser speckle interferometry. This last technique derives from the finding that the scattering and reflexion of coherent light from an irregular surface produces a field which can be imaged as a speckled pattern of relatively light and dark areas, and that two such fields from respectively different surfaces can interfere to produce a pattern which is modulated in phase upon movement of one of the surfaces in the direction of the incident light.
It is to be noted that a speckle pattern itself will be subject to variation together with movement of the surface from which it is derived, but this variation has previously been considered too random and/or fine grained to be of direct use. Indeed, early opinions of speckle pattern phenomena regarded the same as undesirable noise effects associated with laser illumination.
In any event, laser speckle interferometry as so far practised has entailed discrete recording in various ways of an imaged interference pattern created by one relative disposition of the two surfaces for comparison therewith of the directly corresponding pattern created by a changed disposition of the two surfaces in order to obtain a measure of the movement leading from one disposition to the other. The recording step of this procedure necessarily involves a complexity of equipment and/or processing compared to an intrinsically instantaneous measurement technique.
Also, laser speckle interferometry as so far practised has entailed the provision of separate beams of coherent light, often derived from a common laser source, to respectively illuminate the two surfaces. This involves a complexity of optical equipment and, possible more important, can render difficult or impracticable the application of the technique to surfaces to which access is difficult.
In contrast to the situation just described the present invention provides laser speckle interferometry techniques, and related techniques, which require no discrete recording of interference patterns and which can be operated with a single coherent light beam. The presently proposed techniques in fact have two aspects respectively associated with the advantages just mentioned and these two aspects are preferably, but not necessarily, deployed together in application of the invention.
According to one of these aspects of the invention there is provided a method of measuring the movement of an oscillating irregular surface, which comprises illuminating that surface with coherent light, arranging a photodetector for direct response to scattering and reflexions of said light from said surface, and employing from the output of said photodetector variations in the component thereof at the frequency of said movement to represent such movement.
This aspect of the invention derives from the finding that the photodetector has an amplitude-modulated component which corresponds to the surface movement. This finding arises when the oscillating surface is employed alone or in association with a similarly illuminated stationary surface, the photodetector being located in corresponding fields of both surfaces in the latter case, and also when the photodetector has a near or far field location relative to the surface or surfaces.
While a detailed analysis of this phenomenon has yet to be finalised, it is at present considered that the relevant modulated output component results from mixing at the photodetector of the scattered and reflected fields as these are converted to electrical signal form. Certainly, in the case when two surfaces are involved, the presence of an interference effect has been confirmed by employing a piezoelectric crystal as one surface and vibrating the same at known frequency and amplitude, to find that the r
REFERENCES:
patent: 4009707 (1977-03-01), Ward
patent: 4180328 (1979-12-01), Drain
K. J. Ebeling, "Measurement of In-Plane Mechanical Vibrations in the Sub-Angstrom Range by Use of Speckle Imaging", Optics Communications, vol. 24, No. 1, pp. 125-128, Jan. 1978.
U. Kopf, "Use of Television Apparatus in Optico-Coherent Measurement of Mechanical Vibrations in the .mu.m. Range", Messtechnik, vol. 80, No. 4, pp. 105-108, Apr. 1972.
F. P. Chiang et al., "Dynamic Laser Speckle Interferometry Applied to Transient Flexure Problem", Applied Optics, vol. 16, No. 12, pp. 3085-3086, Dec. 1977.
G. J. Jako et al., "Use of a New Photoelectric Device (Fotonic Sensor) for Vibration Measurements in the Ear", Journal Acoustical Soc. Am., vol. 40, No. 5, p. 1263, Nov. 1966.
"Dynamic Statistical Properties of Vibrating Laser Speckle in the Diffraction Field", by N. Takai, et al., Applied Optics, vol. 17, No. 23, Dec. 1, 1978.
Anson Michael
Chung Shin-Ho
Pettigrew Alan G.
Kreitman Stephen A.
National Research Development Corporation
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