Optical waveguides – Optical waveguide sensor
Reexamination Certificate
1998-04-02
2001-09-11
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Optical waveguide sensor
C073S801000, C356S480000
Reexamination Certificate
active
06289143
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates in general to sensors, and in particular to a new and useful acoustic emission sensor suitable for use in hostile environments. According to the present invention, a high-frequency fiber optic vibration sensor is adapted to acoustic emission sensing in hostile high-temperature environments, high EMI environments, and applications where explosion hazards exist.
Presently, piezoelectric transducers of the lead zirconate titanate (PZT) type are used as detectors in commercial systems such as acoustic leak locators on boilers. These leak locators can detect and analyze acoustic energy that results when high-pressure steam exits a pin-hole leak in a steam generator tube, and can distinguish between a leaking tube and an operating sootblower. Since the operating temperature range of the PZT acoustic detector is limited to 150° C., an acoustic waveguide must be used to transfer acoustic energy out of the boiler. The waveguide introduces a loss factor of about ten into the detection system, however. See for example, U.S. Pat. Nos. 4,858,462 and 5,351,655.
An optical fiber modulator is disclosed in U.S. Pat. No. 4,810,051. The use of optical fibers for measuring pressure or strain is also disclosed in U.S. Pat. No. 4,770,492. See U.S. Pat. No. 4,678,903 for a self-aligning fiber optic microbend sensor.
A temperature-compensating fiber optic strain sensor is disclosed in U.S. Pat. No. 5,345,519. A pressure system using an optical resonator cavity is disclosed in U.S. Pat. No. 4,933,545. Also see U.S. Pat. No. 4,928,004 for a method and apparatus for measuring strain, utilizing optical fibers. A fiber optic acoustic sensor is disclosed in U.S. Pat. No. 4,162,397. Also see U.S. Pat. No. 4,071,753.
U.S. Pat. No. 4,950,886 to Claus et al. discloses a partially reflecting optical fiber splice for temperature and strain measurement. A particular feature of the '886 patent is that it avoids the need to rely upon microbends and instead uses measurement techniques that correspond to changes in an air gap between the fibers in a particular splice. A system of optical fibers and splices can be located throughout a structure to yield an array of strain and temperature measurements.
U.S. Pat. No. 5,301,001 to Murphy et al. discloses extrinsic fiber optic displacement sensors and displacement sensing systems. The patent relates generally to fiber optic interferometric sensors and, more particularly, to extrinsic Fizeau interferometric fiber optic sensors having a particular application in hostile environments to dynamically monitor strain, temperature or pressure in mechanical structures. As used therein, “strain” is defined to mean strain, temperature, pressure, magnetic fields, and other like phenomena that can be translated into a displacement depending upon the application. Further, the inventors characterized known Fabry-Perot interferometers as having multiple reflections within a cavity, while Fizeau interferometers were said to operate on the principle of a single reflection within the cavity.
U.S. Pat. No. 5,202,939 to Belleville et al. disclosed a Fabry-Perot optical sensing device for measuring a physical parameter such as pressure, temperature, the refractive index, and especially strain in or deformation of a body. A Fabry-Perot interferometer is optically coupled to a wedge-shaped Fizeau interferometer cavity to produce a spatially-spread light signal indicative of the transmittance or reflectance properties of the Fabry-Perot interferometer, the light signal being indicative of the parameter being sensed.
U.S. Pat. No. 5,189,299 discloses an apparatus and method for sensing strain in a waveguide. A comprehensive description of a Fabry Perot Interferometer (EFPI) can be found in an article by Murphy, et al., found in the Proceedings of the 1993 ASME Winter Annual Meeting, New Orleans, La., “Acoustic Wave Response of the Extrinsic Fabry-Perot Interferometer (EFPI) Optical Fiber Sensor”, available in a publication,
Adaptive Structures and Material Systems,
Aerospace Division American Society of Mechanical Engineers, at AD-Vol. 35, pp 395-399, published by ASME, New York, N.Y.
SUMMARY OF THE INVENTION
The use of in-situ fiber optic acoustic emission sensors would allow an improvement in sensitivity in leak detection of about a factor of ten because the lossy waveguide would not be needed. A fiber optic acoustic emission (FOAE) sensor could be located directly on the boiler wall since the FOAE sensor has a potential operating temperature above 400° C. Furthermore, fiber optic acoustic emission sensors are potentially low-cost devices that can be multiplexed to a signal processor using fiber optic multiplexing methods. Thus, multiple FOAE sensors could be located on a boiler wall to obtain good area coverage for low system cost, or they could be located at a plurality of discrete locations all over the boiler or industrial apparatus of interest.
Accordingly, one aspect of the present invention is to provide a fiber optic acoustic emission sensor for vibration sensing. The sensor comprises a pair of optical fibers each having an end face. A hollow core having opposite open ends is provided for receiving the end faces of the optical fibers; means are provided for fixing the optical fibers in the hollow core with the end faces facing each other and spaced by a distance from each other in the core. Finally, a resonant cylinder is provided in which the hollow core is fixed. The resonant cylinder in which the hollow core is fixed comprises a base plate connected to the cylinder having means for receiving the optical fibers and a cover connected to the base plate and over the resonant cylinder. To provide an operable system using the FOAES, signal processing means are provided and connected to the optical fibers for supplying light to, and receiving light from, the optical fibers and for measuring variations in optical phase which result in changes in the light intensity due to vibrations of the hollow core. The light intensity variations are indicative of the sensed vibrations.
Alternative embodiments of the FOAES of the present invention dispense with the hollow tube and instead employ means for fixing the optical fibers directly into a precision hole in the resonant cylinder. The FOAES of the present invention is simple in design, rugged in construction and economical to manufacture.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific benefits attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
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Berthold John W.
Roman Garry W.
Baraona R. C.
Marich Eric
McDermott Technology Inc.
Sanghavi Hemang
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