Optical waveguides – Optical waveguide sensor – Including physical deformation or movement of waveguide
Reexamination Certificate
1999-09-20
2001-08-21
Sikder, Mohammad (Department: 2872)
Optical waveguides
Optical waveguide sensor
Including physical deformation or movement of waveguide
C385S014000, C385S015000
Reexamination Certificate
active
06278811
ABSTRACT:
TECHNICAL FIELD
The present invention relates to fiber optic pressure sensors, and in particular to such sensors that use a separate pressure detecting device.
BACKGROUND OF THE INVENTION
There are many processes and environments in which it is desirable to know the ambient pressure and in which a sensor is used in order to monitor same. One such common process is during exploration and production of hydrocarbons such as oil in which it is necessary to measure the pressure of the hydrocarbons in a reservoir. Another application is the measurement of the fluid pressure associated with pumps or natural drivers for transporting such hydrocarbons from one location to another. Pressure drops across a venturi is one means by which flow of a fluid can be detected, which therefore requires detection of the pressure difference on both sides of such a venturi.
Pressures of such fluids are traditionally measured with a quartz crystal based pressure measuring devices such as that manufactured by Quartzdyne, Inc. of Salt Lake City, Utah as the Quartzdyne™ Series QS High Pressure Laboratory Transducer. Such a pressure measuring device measures the change in mechanical oscillation frequency associated with the elastic deformation of the crystal in response to applied pressure. Quartz is the medium of choice for such applications due to inherent long term stability, as well as its minimal creep and hysteresis properties. The change in frequency with temperature is also very predictable.
Traditionally the change in frequency of the quartz crystal is measured and compared to a reference crystal which is temperature compensated with the resulting data correlated and calibrated to a direct pressure measurement. Although the reliability of such a quartz crystal is extremely high, the electronics required to measure frequency change are subject to failure particularly when the transducer and its associated electronics are subjected to elevated temperatures such as above 125° C.
Certain techniques exist for measuring pressure using a Bragg grating. However, such techniques are either complex, costly, or do not constrain the optical fiber from buckling in the grating region.
For example, a fiber optic grating based sensor is described in U.S. patent application Ser. No. 08/925,598 entitled “High Sensitivity Fiber Optic Pressure Sensor for Use in Harsh Environments” to Robert J. Maron. In that case, an optical fiber is attached to a compressible bellows at one location along the fiber and to a rigid structure at a second location along the fiber with a Bragg grating embedded within the fiber between these two fiber attachment locations and with the grating being in tension. As the bellows is compressed due to an external pressure change, the tension on the fiber grating is reduced, which changes the wavelength of light reflected by the grating. Such a sensor requires a complex bellows structure and does not constrain the fiber from buckling in the grating region.
Another example is described in Xu, M. G., et al, “Fibre grating pressure sensor with enhanced sensitivity using a glass-bubble housing”, Electronics Letters, 1996, Vol. 32, pp. 128-129, where an optical fiber is secured by UV cured cement to a glass bubble at two ends with a grating inside the bubble. However, such a sensor does not constrain the optical fiber against buckling in the region of the grating.
It is also known that a grating-based pressure sensor may be made by placing a polarization maintaining (PM) optical fiber in a capillary tube having rods therein, and measuring changes in grating birefringence caused by changes in the transverse strain on the fiber grating due to transverse pressure forces acting on the capillary tube, as is discussed in U.S. Pat. No. 5,841,131, to Schroeder et al., issued Nov. 24, 1998. However, such a technique may be expensive or complex to implement.
It is therefore desirable to have a fiber optic Bragg grating pressure sensor that can measure the elastic deformation of a pressure detecting device while minimizing non-axial (or transverse) movement of the optical fiber in the region of the Bragg grating.
SUMMARY OF THE INVENTION
Objects of the present invention include provision of a fiber optic Bragg grating pressure sensor that directly senses pressure due to elastic defonnation of a sensing device.
According to the present invention, a fiber optic pressure sensor comprises a pressure detecting device that is elastically deformable as a function of applied pressure; and an optical fiber being wrapped at ;east once around the device and having at least a portion of its length fused to the device such that elastic deformation of the device imparts an axial strain along a longitudinal axis of the fiber due to the applied pressure.
According further to the present invention, the device has a cylindrical geometry, and may be solid or have an axial bore formed therein. According further to the present invention, the fiber has at least one grating disposed therein. In still further accord to the present invention, the grating has a characteristic wavelength that changes as applied pressure changes. According still further to the present invention, the device compreises silica or quartz.
The present invention provides an improvement over the prior art by fusing a fiber and/or grating directly to a sensing device (or element) and/or by fusing the fiber to the device on opposite axial sides of the grating area adjacent to or a predetermined distance from the grating and radially contraining the grating area. Also, one or more gratings, fiber lasers, or a plurality of fibers may be attached to the device. The grating(s) or laser(s) may be attached to the device within, partially within, or to the outer surface of the device. The sensing device is elastically deformable based on applied pressure.
Thus, when the device is elastically deformed due to the ambient pressure of the environment surrounding the device, this elastic deformation is imparted to the grating. This elastic deformation of (or strain on) the device causes a strain along the longitudinal axis of the fiber grating which causes reflection wavelength of the grating to be proportionately changed. The sensing device may be made of a glass material.
Further, the invention may be used as an individual (single point) sensor or as a plurality of distributed multiplexed (multi-point) sensors. Also, the invention may be a feed-through design or a non-feed-through design.
The invention may be used in harsh environments (high temperature and/or pressure), such as in oil and/or gas wells, engines, combustion chambers, etc. In one embodiment, the invention may be an all glass sensor capable of operating at high pressures (>15 kpsi) and high temperatures (>150° C.). The invention will also work equally well in other applications independent of the type of environment.
The fiber grating may be attached to the device by direct fusing or by using a glass solder (e.g., silica solder), or other means which maintain the optical fiber and/or grating fixedly secured to the device. The means of attachment may allows the fiber and/or grating to remain securedly attached at temperatures much higher or lower than ambient, depending on the application.
Also, an additional temperature grating which is at the same temperature as but not attached to the device may be provided to provide for temperature compensation of the pressure grating. The temperature grating can be formed in the same optical fiber as the pressure grating or can be formed in a second optical fiber that is coupled to the first optical fiber.
For any of the embodiments shown herein, the grating may be attached to the device having an initial pre-strain on the grating (compression or tension) or no pre-strain.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.
REFERENCES:
patent: 4636031 (1987-01-01), Schmadel, Jr. et al.
patent: 4915467 (1990-04-01), Berkey
paten
Bailey Timothy J.
Daigle Guy A.
Davis Allen R.
Davis Michael A.
Didden F. Kevin
LandOfFree
Fiber optic bragg grating pressure sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Fiber optic bragg grating pressure sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fiber optic bragg grating pressure sensor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2517211