Abstract:
A fiber optic sensor employs at least two flexural discs that are spaced apart from one another along a central axis. A fiber optic coil is affixed to at least one of the flexural discs. A proof mass is disposed between the flexural discs. A first stop member is disposed between the proof mass and one flexural disc. A second stop member is disposed between the proof mass and the other flexural disc. The first and second stop members are sized to provide space between the proof mass and the corresponding flexural disc to allow for normal motion of the flexural discs, while interfering with movement of the flexural discs to prohibit unwanted extreme motion. The fiber optic sensor can be used for OTDR measurements of acceleration for real-time oilfield monitoring applications as well as other fiber-based interferometric measurement applications. A coupling structure preferably couples the outer edges of the flexible disks, the mass being attached to the coupling structure.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates broadly to fiber optic sensors for measuring linear acceleration. More particularly, this invention relates to fiber optic sensors that employ an optical fiber coil affixed to a flexural disc. 
         [0003]    2. Description of Related Art 
         [0004]    The flexure or strain of an optical fiber coil affixed to a flexible disc is a well-known basis for measuring acceleration resulting from momentum forces acting on the disc in a direction normal to the disc. The amount of flexure is determined interferometrically, where interferometric measurements of strain in the optical fiber coil provide high resolution, high data rates, require low power, are immune to electromagnetic interference, and can readily be adapted for remote sensing and/or rugged applications. 
         [0005]    The mass which provides the inertia, and hence the force to cause flexure of the disc, usually consists of the disc itself and the optical fiber coil affixed thereto. This mass is typically small. As a result, the sensitivity of the strain measurements is poor although the frequency response extends to high frequency. Additional mass can be coupled to the disc in order to improve the sensitivity of the strain measurements at the expense of frequency response. 
         [0006]    For example, U.S. Pat. Nos. 6,384,919 and 5,369,485 each describe a flexural disc fiber optic sensor having a center-supported flexural disc with additional mass that is affixed to the outer edge of the disc and disposed outside the outer circumference of the disc. US Patent Application 2005/0115320 describes a flexural disc fiber optic sensor having a center-supported flexural disc with additional mass that is affixed to the outer edge of the disc and disposed above and below the outer portion of the disc. Such additional mass improves the sensitivity of the device, but makes the device more susceptible to damage from high-g shocks. 
         [0007]    U.S. Pat. No. 6,650,418 describes an edge-supported flexural disc that employs viscoelastic shear and compression dampers. The shear damper is in contact with the inner edge portion of the disc. The compression dampers are compressed against the fiber optic coils affixed above and below the disc. The compression dampers act to dampen extreme motions of the disc that would be otherwise caused in the disc, for example, as a result of high-g shocks. In this manner, the compression dampers protect the device against damage from such extreme motion. Disadvantageously, the shear and compression dampers of the &#39;418 patent are difficult to manufacture at small tolerances, and thus are impractical in applications requiring significant proof mass for high sensitivity measurements. 
         [0008]    Thus, there remains a need in the art for flexural disc fiber optic sensors that are suitable for applications requiring high sensitivity measurements while affording protection from high-g shocks that may be experienced by the sensors. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The invention provides a flexural disc fiber optic sensor that is suitable for applications requiring high sensitivity measurements while affording protection from high-g shocks that may be experienced by the sensors. 
         [0010]    The invention also provides a flexural disc fiber optic sensor that can be manufactured at small tolerances, and thus is suitable for applications requiring high sensitivity measurements. 
         [0011]    The invention further provides a flexural disc fiber optic sensor that has a compact design suitable for installation in a borehole that traverses an oilfield. 
         [0012]    Accordingly, as discussed in detail below, the fiber optic sensor of the present invention employs at least two flexural discs that are spaced apart from one another along a central axis. A fiber optic coil is affixed to one of the flexural discs. A proof mass is disposed between the flexural discs. A first stop member is disposed between the proof mass and one flexural disc. A second stop member is disposed between the proof mass and the other flexural disc. The first and second stop members are sized to provide space between the proof mass and the corresponding flexural disc to allow for normal motion of the flexural discs, while interfering with movement of the flexural discs to prohibit unwanted extreme motion. 
         [0013]    In the preferred embodiment, the stop members are made of a thermoplastic material and formed by a self-aligned process that provides for accurate sizing of the stop members at small dimensions (e.g., dimensions that provide for operating gaps less than 25 μm). 
         [0014]    In the preferred embodiment, radially inner portions of the flexural discs are rigidly connected to a central support structure and radially outer edge portions of the flexural discs are rigidly connected together and to the proof mass. 
         [0015]    The fiber optic sensor can be used for Optical Time Domain Reflectometry (OTDR) measurements of acceleration over spaced-apart locations in a fiber optic waveguide, which can be installed in a borehole that traverses an oilfield for real-time oilfield monitoring applications. Such OTDR measurement can also be used in fiber-based interferometric measurement applications. 
         [0016]    Additional advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0017]      FIG. 1  is a cross-section schematic view of a fiber optic sensor in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    Turning now to  FIG. 1 , a fiber optic sensor  10  according to the present invention includes a top flexural disc  11 A and a bottom flexural disc  11 B that are rigidly attached to a central support structure (e.g., the center post  12  and corresponding central support members  13 A,  13 B). In the preferred embodiment, the radially inner portion  15 A of the top flexural disc  11 A is permanently affixed between the central support member  13 A and a backing disc  17 A by welding, adhesive material, or other suitable means (for example, by welding along the interface  41  through the radially inner portion  15 A of the top flexural disc  11 A to the central support member  13 A). The backing disc  17 A interfaces to an annular flange portion  19 A of the central support member  13 A. The central support member  13 A is rigidly attached to the center post  12  by welding, adhesive material, or other suitable means (for example, by welding along an interface  43  that is exposed by a cutout  45  in the top wall of the central support member  13 A). 
         [0019]    Similarly, the radially inner portion  15 B of the bottom flexural disc  11 B is permanently affixed between the central support member  13 B and a backing disc  17 B by welding, adhesive material, or other suitable means (for example, by welding along the interface  47  through the radially inner portion  15 B of the bottom flexural disc  11 B to the central support member  13 B). The backing disc  17 B interfaces to an annular flange portion  19 B of the central support member  13 B. The central support member  13 B is rigidly attached to the center post  12  by welding, adhesive material, or other suitable means (for example, by welding along an interface  49  that is exposed by a cutout  51  in the bottom wall of the central support member  13 B). In this configuration, the top and bottom flexural discs  11 A,  11 B are centrally supported by rigid attachment to the central support structure (members  13 A,  13 B and the center post  12 ) such that the top and bottom flexural discs  11 A,  11 B are axially-aligned to one another. 
         [0020]    The top flexural disc  11 A has a top surface  21 A opposite a bottom surface  21 B. Similarly, the bottom flexural disc  11 B has a top surface  23 A opposite a bottom surface  23 B. A fiber optic coil  25  is affixed to the top surface  21 A of the top flexural disc  11 A by adhesive material or other suitable means. For simplicity of illustration, the fiber optic coil  25  is indicated as a solid component. However, it should be understood that the fiber optic coil  25  is a multi-layer spiral-wound coil that may be formed in accordance with well-known techniques for forming such coil. 
         [0021]    An outer edge coupler  27  extends between the radially outer edge portions  29 A,  29 B of the flexural discs  11 A,  11 B and is rigidly attached thereto by welding, adhesive material, or other suitable means (for example, welding at interfaces  53 ,  55 ) such that the radially outer edge portions  29 A,  29 B of the top and bottom flexural discs  11 A,  11 B are rigidly connected together. A proof mass  31 , which is preferably made of tungsten, is rigidly attached to the outer edge coupler  27  and is disposed in the space between bottom surface  21 B of the top flexural disc  11 A and the top surface  23 A of the bottom flexural disc  11 B. Preferably, the outer edge coupler  27  includes a flange  33  that extends radially inward between the two flexural discs  11 A,  11 B. The proof mass  31  is supported by the flange  33  in the space between bottom surface  21 B of the top flexural disc  11 A and the top surface  23 A of the bottom flexural disc  11 B. The proof mass  31  is rigidly attached to the flange  33  by adhesive material, welding, or other suitable means (for example, by adhesive material at the interfaces  57 ,  59 ). In this manner, the proof mass  31  is rigidly connected by the outer edge coupler  27  to the radially outer edge portions  29 A,  29 B of the flexural discs  11 A,  11 B. The additional mass provided by the outer-edge-coupled proof mass  31  improves the sensitivity of the device in response to axial acceleration forces and the strain measurements based thereon. 
         [0022]    The fiber optic coil  25  of the fiber optic sensor  10  is optically coupled (preferably by a splice or other suitable means) to a fiber optic waveguide for interferometric measurements of strain and acceleration based thereon. 
         [0023]    During operation, acceleration forces along the central axis CA cause the radially outer edge portions  29 A,  29 B of the two flexural discs  11 A,  11 B together with the proof mass  31  to move together in a direction parallel to the central axis (denoted by arrow  36 ) relative to radially inner portions  15 A,  15 B of the two flexural discs  11 A,  11 B and the center support structure (central support members  13 A,  13 B and center post  12 ). 
         [0024]    High-g force loading can potentially induce extreme motion in the two flexural discs  11 A,  11 B and thus damage the fiber optic sensor. In order to prevent such extreme motion, the fiber optic sensor  10  employs a top end-stop ring  33 A and a bottom end-stop ring  33 B. The top end-stop ring  33 A is disposed between the bottom surface  21 B of top flexural disc  11 A and proof mass  31 . Similarly, the bottom end-stop ring  33 B is disposed between the top surface  23 A of the bottom flexural disc  11 B and proof mass  31 . The top and bottom end-stop rings  33 A,  33 B are made of a thermoplastic material and sized by a self-aligned process that ensures sufficient and accurate gaps are provided adjacent the end-stop rings  33 A,  33 B. Such gaps allow for unhindered flexing movement of the flexural discs  11 A,  11 B during normal operation. However, under extreme g-force loading, the end-stop rings  33 A,  33 B interfere with flexing movement of the top and bottom flexural discs to preclude unwanted extreme motion of the top and bottom flexural discs. 
         [0025]    In the self-aligned process, the end-stop rings  33 A,  33 B are initially assembled in a compressed state and then heated to a temperature above their heat-deflection temperature, which is the temperature at which permanent deformation is taking place at the given compressed state. The end-stop rings  33 A,  33 B are then allowed to cool. Such cooling causes shrinkage of the dimensions of the respective end-stop rings  33 A,  33 B, and thus, with appropriate time and temperature, the appropriate gaps adjacent thereto form. 
         [0026]    In the preferred embodiment, the top end-stop ring  33 A is disposed between the backing disc  17 A and a top annular cut out surface  35 A of the proof mass  31 , and the bottom end-stop ring  33 B is disposed between the backing disc  17 B and a bottom annular cut out surface  35 B of the proof mass  31 . The self-aligned process forms gaps  37 A,  37 B adjacent to the corresponding end-stop rings  33 A,  33 B. The end-stop rings  33 A,  33 B can be attached to the corresponding backing discs  17 A,  17 B by use of an adhesive, mechanical fixing, or other suitable means. In this configuration, the gaps will be disposed between the corresponding end-stop rings and the proof mass (not shown). Alternatively, the end-stop rings  33 A,  33 B can be attached to the proof mass  31  by use of an adhesive, mechanical fixing, or other suitable means. In this configuration, the gaps will be disposed between the corresponding backing discs and the end-stop rings as shown. In yet another alternative, the end-stop rings  33 A,  33 B can be allowed to float in the space between the backing discs  17 A,  17 B and the proof mass  31 . In this configuration, the gaps can be on either the top or bottom sides of the end-stop rings  33 A,  33 B. 
         [0027]    In the preferred embodiment, the two end-stop rings  33 A,  33 B are made of a low friction, high temperature thermoplastic material. The thicknesses of these rings are arranged such that they are slightly oversized relative to the space available between the backing discs  17 A,  17 B (which are rigidly connected to the center post  12 ) and the proof mass  31 , after accounting for the machining tolerances of these parts (typically on the order of ±0.05 mm). This represents a tight but easily achievable machining tolerance with modern machinery. 
         [0028]    The apparatus is assembled. In this initial assembly, the end-stop rings  33 A,  33 B are compressed due to their oversized thickness dimensions. The assembly is then heated to a temperature above the heat deflection temperature for the end-stop rings  33 A,  33 B, which causes the thermoplastic material of the end-stop rings  33 A,  33 B to soften markedly. Consider an example where the end stop rings are made of PTFE and sized to induce compression forces of 1.8 MPa upon initial assembly. The heat-deflection temperature for PTFE at 1.8 MPa is approximately 55° C. Thus, by heating the initial assembly to a temperature in the range between 150° C. and 200° C., the PTFE end-stop rings  33 A,  33 B will soften and deform to a point where the end stop rings  33 A,  33 B completely fill the spaces between the backing discs  17 A,  17 B and the proof mass  31 . In this configuration, motion of the proof mass  31  and flexural discs  11 A,  11 B relative to the central support structure is inhibited. The assembly is then allowed to cool. Such cooling causes shrinkage of the dimensions of the respective end-stop rings  33 A,  33 B, and thus forms the appropriate gaps adjacent thereto. 
         [0029]    Such shrinkage can be controlled by analysis of the thermal expansion coefficients of the materials of the parts of the assembly, the dimensions of such parts, and the heating and cooling temperatures used in the self-aligned processing. For example, consider the following example:
       the center post  12  is made of a material with a thermal expansion coefficient of approximately 13 ppm/° C. and has a length of 10 mm;   the end-stop rings  33 A,  33 B are made of PTFE having a thermal expansion coefficient of approximately 100 ppm/° C. and each have a thickness of 1 mm;   the proof mass is made of tungsten having a thermal expansion coefficient of 4.5 ppm/° C. and has a thickness between surfaces  35 A and  35 B of 8 mm; and   the initial assembly is heated to a temperature of 200° C. and cooled to a temperature of 25° C.       
 
         [0034]    In this example, the following calculations can be made. The shrinkage of the center post  12  can be calculated as 10 mm*13 ppm/° C.*175° C.=22.8 μm. The shrinkage of the proof mass  31  can be calculated as 8 mm*4.5 ppm/° C.*175° C.=6.3 μm. The total shrinkage of the end-stop rings  33 A,  33 B is calculated as 2*1 mm*100 ppm/° C.*175° C.=35 μm. Hence, the net total shrinkage is calculated as the shrinkage of the proof mass  31  added to the shrinkage of the end-stop rings  33 A,  33 B less the shrinkage of the center post  12 , which is (6.3+35−22.8) μm=18.5 μm. This net shrinkage is divided by 2 to give the net shrinkage per end-stop ring of 9.3 μm, assuming zero-stress state at 200° C. In this manner, a gap on the order of 10 μm can be realized by the self-aligned process described herein. Smaller gaps (e.g., in the range between 5 to 10 μm per end-stop ring) can be formed by similar materials and methods. Advantageously, the self-aligned process described herein forms a simple, reliable method to form a small gap for the end-stop rings of the sensor. Such gaps would otherwise prove extremely difficult to manufacture. 
         [0035]    The thermoplastic material used for the end-stop rings  33 A,  33 B must have an appropriate expansion coefficient to develop the required gap, and must also survive repeated high-g shocks at a defined upper temperature. Although PTFE is described above, there are a number of thermoplastic materials that can be used, including using glass-fiber/carbon-filled PTFE, PFA, FEP, PEEK, or various other high temperature plastics. 
         [0036]    The flexural discs  11 A,  11 B are preferably formed of a structural material such as alloys of aluminum, nickel, iron, or copper. The fiber optic sensor  10  is typically mounted inside a protective housing (not shown) that is suitable for the desired application. The housing may be formed of any suitable material, such as plastics or metal, that will allow sufficient structural stiffness to ensure that structural resonance frequencies are far from the frequency range of interest. The housing may be manufactured by any suitable means such as machining or casting. 
         [0037]    Advantageously, the flexural disc fiber optic sensor of the present invention utilizes a proof mass that is edge-coupled to two flexural discs and thus affords improved sensitivity. The end stops protect the device from high-g shocks that may be experienced by the sensor due to its increased mass. Moreover, the self-aligned process for sizing the end stops provides a cost-effective solution that allows for the small tolerances required for high sensitivity measurements. Finally, the flexural disc fiber optic sensor has a compact design suitable for installation in a borehole that traverses an oilfield, as well as for other optical fiber-based interferometric measurement applications. 
         [0038]    There have been described and illustrated herein embodiments of a flexural disc fiber optic sensor. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. Thus, while a particular self-aligned process has been disclosed for sizing the end stops of the sensor, it will be appreciated that other methodologies can be used as well. In addition, while a particular sensor design has been disclosed, it will be understood that other designs can be used. Also, while the fiber optic sensor is described as part of particular OTDR methodologies and systems, it will be recognized that it can readily be used in other OTDR methodologies and systems and well as in other optical fiber-based interferometric methods and systems. Moreover, while particular materials and thermal processing parameters have been disclosed in reference to the self-aligned process for forming the end stops, it will be appreciated that other material and thermal processing parameters could be used as well. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its scope as claimed.