KR100443386B1 - Fiber Optic Seismic Sensor - Google Patents

Abstract

The fiber optic seismic sensor 10 includes a pair of central support members 30 and 32 formed of a metal such as aluminum. The support plate 42 has an inner portion 39 retained between the central support members 30, 32 and an outer portion 41 extending out of the central support members 30, 32. The pair of hollow cylindrical substrates 14, 18 are opposite sides of the outer portion 41 of the support plate 42 with the central support members 30, 32 extending through the center of the substrates 14, 18. Mounted in an axial alignment on the top. The central support member 30 has a cavity 36 at one end, and the central support member 32 extends therefrom with a protrusion 38 arranged to be received within the cavity 36 in the first central support member 30. Have The support plate 42 includes a passage 44 arranged to receive the protrusion 38 so that the inner portion 39 of the support plate 42 is between the first and second central support members 30, 32. Is retained. The first spirally wound optical fiber coil 12 is formed on the outer end 22 of the first substrate 14, and the second spirally wound optical fiber coil 16 is formed of the second substrate 18. It is formed on the outer end 26. The interferometer 50 includes a first spirally wound optical fiber coil 14 of a first leg and a second spirally wound optical fiber coil 16 of a second leg. Acceleration along an axis perpendicular to the plane of the optical fiber coils 12, 16 causes bending of the support plate 42, resulting in a change in the length of the optical fiber coils 12, 16.

Description

Fiber Optic Seismic Sensor

Measurement of the deflection or deformation of an elastic disk in response to acceleration or pressure involves the principle of operation of a number of different acceleration and pressure sensors. The amount of deformation or displacement is determined interferometrically, mechanically, piezoelectrically or by a change in capacitance or inductance between the elements. However, all these systems have drawbacks such as limited sensitivity, high cost, limited maximum deviation and operating environment sensitivity. Some errors due to operating conditions are fundamental, such as limited physical bending capacitance in response to acceleration, which makes the desired output indistinguishable from the signal elements associated with the noise source. Other operating condition errors are due to changes in physical dimensions, modulus of elasticity, refractive index, etc., caused by changes in temperature and pressure.

Interferometric strain measurements show good accuracy and resolution. When carried out by optical fibers, interferometric systems include simple and robust sensor devices with low power requirements, immunity to electromagnetic interference, remote sensing and fast adaptation to high data rates. Interferometric measurements of acceleration and pressure using optical fiber media are performed via remote signal transmission of multiple sensors within a single fiber using multiple time divisions. The fibers themselves are relatively insensitive to unit length and do not produce errors due to tension from ambient pressure, acceleration, and the like.

Many accelerometers are preferred push-pulls of US Pat. No. 4,959,539 to Hoffler et al. On Sep. 25, 1990, which discloses a hydrophone with a disk supported around it for bending caused by sonic vibration. It has been developed to use disk mounted helical coils of optical fiber to provide a push-pull effect. The flat helix of the optical fiber is fixed to each side of the disk and arranged as two legs of the interferometer. The deflection of the disk is made long on the opposite surface while shortening the length of the optical path of the spiral on one surface. A pair of disks may be mounted on the body so that the measured sonic pressure gradient is present across the disk with the spirals connected for push-pull operation as two legs of the optical fiber interferometer.

U. S. Patent No. 5,317, 929, issued June 7, 1994 to Brown et al., Discloses a fiber optic accelerometer based on the dual disk structure described above. The centrally located mass is clamped between opposing flexible disks. The flat helical coil of the optical fiber is fixed to the surface of the flexible disk and arranged to be included in the two legs of the interferometer.

U. S. Patent No. 5,883, 308, issued March 16, 1999 to one of the inventors of the present invention, discloses a fiber optic accelerometer comprising a ring that twists in response to an acceleration perpendicular to the plane of the ring. The ring is mounted to the wall of the casing by the peripheral flange. The casing has two concentric walls that divide the interior of the casing into a disc-shaped center and an annular periphery. The ring is arranged to have a moment of inertia that twists the ring in response to linear acceleration along the input axis. The flat helical fiber optic coils are arranged to be included in the legs of the interferometer for generating optical signals that are secured to the top and bottom of the ring and that can be processed to determine acceleration. The central part of the casing is hollow so that the device is neutrally buoyant, making it suitable for use in sonar sensing applications.

The present invention generally relates to a device for sensing linear acceleration. In particular, the present invention relates to an earthquake sensor comprising a fiber optic interferometer arranged to provide an optical signal in response to seismic vibrations.

The fiber optic seismic sensor according to the invention preferably comprises a central support assembly formed of a metal such as aluminum. The support plate has an inner portion retained within the central support assembly and an outer portion extending beyond the central support assembly. The first substrate has an inner end mounted on the first side of the outer portion of the support plate. The first substrate is formed of a generally hollow cylinder having an inner radius and an outer radius, an inner wall and an outer wall, the inner radius being dimensioned to receive the central support assembly in the first substrate such that the central support assembly and the inner wall of the first substrate are spaced apart from each other. Has The second substrate has an inner end mounted on the second side of the outer portion of the support plate. The second substrate is generally formed of a hollow cylinder having an inner radius and an outer radius, an inner wall and an outer wall. The inner radius is dimensioned to receive the central support assembly in the second substrate such that the inner support wall and the inner wall of the second substrate are spaced apart from each other. The first spirally wound optical fiber coil is formed on the outer end of the first substrate, and the second spirally wound optical fiber coil is formed on the outer end of the second substrate. The first and second fiber optic coils are arranged to be generally flat and spaced concentrically. The interferometer is formed to include a first spirally wound optical fiber coil of the first leg and a second spirally wound optical fiber coil of the second leg. The interferometer is arranged such that the acceleration along the axis perpendicular to the plane of the optical fiber coil causes the bending of the support plate to increase the length of one of the optical fiber coils and cause a corresponding decrease in the passage length of the other optical fiber coils.

The central support assembly preferably comprises a first central support member having a cavity in the first end and a second central support member having an extended protrusion and is arranged to be received in the cavity in the first central support member. The support plate includes a passageway arranged to receive the protrusion, such that the support plate is held between the first and second central support members.

The understanding of the object of the present invention and its structure and method of operation can be more fully understood by studying the following description of the preferred embodiments and with reference to the accompanying drawings. <Brief Description of Drawings>

1 is a cross-sectional view of a fiber optic seismic sensor according to the present invention.

FIG. 2 is a partial cross-sectional perspective view of the fiber optical seismic sensor of FIG. 1. FIG.

3 is a top plan view of a spirally wound optical fiber coil that may be included in the fiber optical seismic sensors of FIGS. 1 and 2.

4 is a diagram schematically illustrating an interferometer formed to include a pair of spirally wound optical fiber coils.

5 illustrates a fiber optic seismic sensor system with cable links between successive fiber optic seismic sensors.

1 and 2 show a fiber optic seismic sensor 10 according to the present invention. As shown in FIGS. 1-3, the fiber optic seismic sensor 10 includes a first spirally wound optical fiber coil 12 mounted on a substrate 14. A second spirally wound optical fiber coil 16 is mounted on the substrate 18. The substrates 14 and 18 are preferably formed of a material such as polycarbonate.

The first spirally wound optical fiber coil 12 is preferably mounted in an annular recess 20 formed in the first side 22 of the substrate 14. Thus, the recess 20 has side edges that suppress the inner and outer diameters of the coil 12 to help form the coil on the substrate 14. The coil 12 is preferably secured to the substrate 14 with a suitable adhesive. The fiber coil 16 is similarly mounted in an annular recess 24 formed in the first side 26 of the substrate 18.

The coils 12, 16 preferably have an inner diameter of about 18 mm and an outer diameter of about 22 mm. The coils 12, 16 preferably have a plurality of suitable layers such that each coil has a length of about 6.0 to 6.6 m.

The fiber optic seismic sensor 10 further includes a first central support member 30 and a second central support member 32. The central support members 30 and 32 are each preferably formed to have a generally cylindrical shape and are formed of a metal such as aluminum. The first end 34 of the central support member 30 has a cylindrical recess 36 therein, and the second central support member 32 has a corresponding cylindrical protrusion 38 extending from the end 40. 1 and 2, when the first and second central support members 30, 32 are axially aligned with the ends 34, 40 facing each other, the recess 36 and the projections ( 38 is arranged such that the protrusion 38 is hermetically fitted in the recess 36.

The fiber optic seismic sensor 10 preferably further comprises a support plate 42 having a generally circular outer periphery. The passage 44 is formed in the center of the support plate 42. The recess 36, the protrusion 38 and the passage 44 are conveniently formed to have a cylindrical shape, although other shapes may be used. The passage 44 is formed to be slightly larger than the diameter of the protrusion 38.

The assembly of the fiber optic seismic sensor 10 positions the support plate 42 on the second central support member 32 having a protrusion 38 extending through the passage 44. Thereafter, the first center support member 30 is mounted on the second center support member 32 having a protrusion 38 extending into the recess 36. The central support members 30, 32 have an outer diameter smaller than the diameter of the support plate 42. Thus, the portions 39 of the support plate 42 are clamped between the inner ends 34, 40 of the central support members 30, 32, respectively. The portion 41 of the support plate 42 vibrates freely in response to seismic vibrations along the longitudinal axis of the central support members 30, 32.

The substrate 14 is preferably formed in a generally cylindrical shape with a central cylindrical passage 46 therein. The substrate 18 is preferably the same as the substrate 14 and has a central cylindrical passage 48. Substrates 14 and 18 have outer surfaces 43 and 45 respectively.

The passages 46 and 48 have a diameter larger than the diameter of the central support members 30 and 32. The substrates 14 and 18 and the support plate 42 all preferably have approximately the same diameter. The assembly of the fiber optic seismic sensor 10 comprises arranging the substrates 14, 18, so that the respective ends 34, 40, with the substrates facing opposite sides 42A, 42B of the support plate 42, respectively. ) And axially. In this arrangement, the central support member 30 extends through the central passage 46 of the substrate 14. The central support member 32 extends through the central passage 48 with the portion of the protrusion 38 extending into the passage 46. Suitable adhesives, such as epoxy resins, are used to adhere the substrates 14, 18 to the support plate 42.

In some configurations, it may be desirable to add mass 47 to the outer edge 40 of support plate 42. The mass 47 may be a ring mounted to the outer edge of the support plate 42 and the sides 43, 45 of the substrates 14, 18, respectively. The added mass 47 may optionally be formed integrally with the support plate 42. The added mass 47 increases the scale factor of the fiber optic seismic sensor 10.

The fiber optic seismic sensor 10 may include a housing 70 that may be formed of metal or artificial material. The housing 70 may conveniently be formed from an open end hollow cylinder 72 of polycarbonate and a pair of polycarbonate discs 74, 76. The lengths of the assembled central support members 30 and 32 and the support plate 42 should be equal to the height of the cylinder 72. The end 78 of the first central support member 30 is joined to the center of one side of the disk 74. The end of the cylinder 72 is joined with the disk 74. The disk 76 is joined to the other end of the cylinder 72 with the ends 78, 80 of the central support members 30, 32 in contact with the inner surfaces of the disks 74, 76, respectively. The lengths of the assembled substrates 14, 18 and the support plate 42 are smaller than the height of the cylinder 72. Thus, the central support members 30, 32 have a space between the first side surfaces 22, 26 of the respective substrates 14, 18 and the disks 74, 76 of the housing 70, while the disks 74, 76) firmly maintained. Thus, the seismic vibration along the longitudinal axis of the central support members 30, 32 deflects the support plate 42 and changes the length of the optical fiber coils 12, 16.

As schematically shown in FIG. 4, the fiber coils 12, 16 are included as two legs of the interferometer 50. 4 shows the shape of a well-known Michelson interferometer, but the invention is implemented using a Mach-Zehnder interferometer. The laser 52 provides an optical signal to the optical fiber 54, guiding the optical signal to the optical coupler 56. Coupler 56 may be any suitable coupler structure well known in the art. The coupler output is delivered to each fiber coil 12, 16.

The housing 70 also has a suitable passage 82 to allow optical fibers to be sent between the interior and exterior of the element of the housing 70. In some embodiments of the invention, it may be convenient to mount the coupler 56 in the opening, as shown in US Pat. No. 5,883,308.

The seismic vibration causes the support plate 42 to warp, thereby changing the length of the optical fiber coils 12 and 16. Since the coils 12, 16 are on opposite sides of the support plate 42, the bending of the edges of the support plate increases the length of one coil and decreases the length of the other coil. The change in length causes a phase difference in the optical signals propagating in the coils 12, 16. Reflectors 58 and 60 are formed at the ends of the fiber coils 12 and 16 to couple the optical signals back to coupler 56 to form an interference pattern. Optical fiber 62 guides the output of the interferometer to photodetector 64 where the interference pattern is converted into an electrical signal. The electrical signal can then be processed to determine the magnitude of the seismic vibration that causes a phase difference between the optical signals in the two legs of the interferometer.

The dimensions of the various elements of the fiber optic seismic sensor 10 may vary. General dimensions are only given herein as examples illustrating how the fiber optic seismic sensor 10 can be constructed. Substrates 14 and 18 may have dimensions of about 26 mm and a height of about 5.4 mm. The recesses 20 and 24 may have a width of about 4 mm and a depth of about 0.85 mm. The outer edges of the recesses 20, 24 are preferably about 2 mm from the outer edges 43, 45 of the substrates 14, 18, respectively. The dimensions of the passages 46 and 48 of the substrates 14 and 18 are about 10.0 mm. The central support members 30, 32 have a diameter of about 8.0 mm and a connected length of about 13.0 mm. The protrusion 38 has a diameter of about 4.0 mm and a length of about 1.6 mm. There is a small gap 35 of about 0.4 mm between the end of the protrusion 38 and the inner edge of the recess 36. The support plate 42 is preferably made of brass and has an outer diameter of about 26 mm and a thickness of about 0.2 mm. The disks 74 and 76 preferably have a thickness of about 3.0 mm. The cylinder 72 preferably has a height of about 13.0 mm, a diameter of about 32 mm and a wall thickness of about 2.0 mm.

The fiber optical seismic sensor 10 is configured to have the ability to detect seismic vibrations in the frequency range of 3 Hz to 1000 Hz in accordance with the above description.

The seismic vibration sensing system may comprise one or more fiber optic seismic sensors 10 according to the present invention. The sensors can be arranged separately or connected to one another. As shown in FIG. 5, the array 90 may be formed to include a fiber optic telemetry cable 91 that is connected between successive fiber optic seismic sensors 10A, 10B, and the like. Fiber optical couplers 92A, 92B and the like are connected between the sensor and the telemetry cable. Where junctions (not shown) are needed, sensors 10A, 10B, etc., are used to connect the telemetry cables and couplers.

The structures and methods disclosed herein describe the principles of the present invention. The invention may be embodied in other specific forms without departing from its spirit or main features. The described embodiments are to be considered in all respects as illustrative and not restrictive. Accordingly, the appended claims rather than the foregoing description are intended to limit the scope of the invention. All modifications to the embodiments described herein within the meaning and range equivalent to the claims are to be embraced within the scope of the invention.

Claims (4)

A pair of central support members 30, 32, A support plate 42 having an inner portion 39 retained between the central support members 30, 32 and an outer portion 41 extending out of the central support members 30, 32; A first substrate 14 having an inner end 23 mounted on the first side 42A of the outer portion 41 of the support plate 42, A second substrate 18 having an inner end 27 mounted on a second side 42B of the outer portion 41 of the support plate 42, A first spirally wound optical fiber coil 12 formed on the outer end 22 of the first substrate 14, A second spirally wound optical fiber coil 16 formed on the outer end of the second substrate 18, An interferometer 50 formed to include a first spirally wound optical fiber coil 12 of a first leg and a second spirally wound optical fiber coil 16 of a second leg, The first substrate 14 is generally formed by a hollow cylinder having an inner radius and an outer radius, the inner wall 27 and the outer wall 43, and the inner radius is the central support members 30 and 32 and the first substrate 14. The inner wall 37 is dimensioned to receive the central support members 30, 32 in the first substrate 14 so as to be spaced apart from each other, The second substrate 18 is generally formed by a hollow cylinder having an inner radius and an outer radius, an inner wall 37 and an outer wall 45, and the inner radius is the inner wall of the central support members 30 and 32 and the second substrate. Has dimensions that accommodate the central support members 30, 32 in the second substrate 18 so that 37 is spaced apart from each other, The first and second fiber optic coils 12, 16 are arranged generally flat, concentric and spaced apart from each other, The interferometer 50 increases the length of one of the fiber coils 12, 16 along the axis perpendicular to the plane of the fiber coils 12, 16 and increases the length of the other fiber coils 12, 16. Fiber optic earthquake sensor, characterized in that it is arranged to produce a deflection of the support plate 42, correspondingly reducing. The method of claim 1, further comprising a housing 70, The first central support member 30 is mounted to the housing 70 and has a cavity 36 therein, The second central support member 32 has a protrusion 38 mounted in the housing 70 and extending therefrom and arranged to be received in the cavity 36 in the first central support member 30, The support plate 42 includes a passage 44 arranged to receive the protrusion 38 so that the inner portion 39 of the support plate 42 is between the first and second central support members 30, 32. Fiber optical seismic sensor, characterized in that it is retained. The fiber optical earthquake sensor according to claim 2, wherein there is a gap (35) between the protrusion (38) and the first central support member (30). The fiber optical seismic sensor according to claim 1, further comprising an inertial ring (47) mounted around the outer circumference of the support plate (42).