Abstract:
A robust transmission element for transmitting information between downhole tools, such as sections of drill pipe, in the presence of hostile environmental conditions, such as heat, dirt, rocks, mud, fluids, lubricants, and the like. The transmission element maintains reliable connectivity between transmission elements, thereby providing an uninterrupted flow of information between drill string components. A transmission element is mounted within a recess proximate a mating surface of a downhole drilling component, such as a section of drill pipe. The transmission element may include an annular housing forming a trough, an electrical conductor disposed within the trough, and an MCEI material disposed between the annular housing and the electrical conductor.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. The Field of the Invention  
           [0002]    This invention relates to oil and gas drilling, and more particularly to apparatus and methods for reliably transmitting information to the surface from downhole drilling components.  
           [0003]    2. The Relevant Art  
           [0004]    For several decades, engineers have worked to develop apparatus and methods to effectively transmit information from components located downhole on oil and gas drilling strings to the ground&#39;s surface. Part of the difficulty lies in the development of reliable apparatus and methods for transmitting information from one drill string component to another, such as between sections of drill pipe. The goal is to provide reliable information transmission between downhole components stretching thousands of feet beneath the earth&#39;s surface, while withstanding hostile wear and tear of subterranean conditions.  
           [0005]    In an effort to provide solutions to this problem, engineers have developed a technology known as mud pulse telemetry. Rather than using electrical connections, mud pulse telemetry transmits information in the form of pressure pulses through fluids circulating through a well bore. However, data rates of mud pulse telemetry are very slow compared to data bandwidths needed to provide real-time data from downhole components.  
           [0006]    For example, mud pulse telemetry systems often operate at data rates less than 10 bits per second. At this rate, data resolution is so poor that a driller is unable to make crucial decisions in real time. Since drilling equipment is often rented and very expensive, even slight mistakes incur substantial expense. Part of the expense can be attributed to time-consuming operations that are required to retrieve downhole data or to verify low-resolution data transmitted to the surface by mud pulse telemetry. Often, drilling or other procedures are halted while crucial data is gathered.  
           [0007]    In an effort to overcome limitations imposed by mud pulse telemetry systems, reliable connections are needed to transmit information between components in a drill string. For example, since direct electrical connections between drill string components may be impractical and unreliable, other methods are needed to bridge the gap between drill string components.  
           [0008]    Various factors or problems may make data transmission unreliable. For example, dirt, rocks, mud, fluids, or other substances present when drilling may interfere with signals transmitted between components in a drill string. In other instances, gaps present between mating surfaces of drill string components may adversely affect the transmission of data therebetween.  
           [0009]    Moreover, the harsh working environment of drill string components may cause damage to data transmission elements. Furthermore, since many drill string components are located beneath the surface of the ground, replacing or servicing data transmission components may be costly, impractical, or impossible. Thus, robust and environmentally-hardened data transmission components are needed to transmit information between drill string components.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of the foregoing, it is a primary object of the present invention to provide robust transmission elements for transmitting information between downhole tools, such as sections of drill pipe, in the presence of hostile environmental conditions, such as heat, dirt, rocks, mud, fluids, lubricants, and the like. It is a further object of the invention to maintain reliable connectivity between transmission elements to provide an uninterrupted flow of information between drill string components.  
           [0011]    Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, an apparatus for transmitting data between downhole tools is disclosed in one embodiment of the present invention as including an annular housing having a circumference. The annular housing is shaped to include a trough around the circumference thereof. An electrical conductor is disposed within the trough. A magnetically-conducting, electrically-insulating material (hereinafter “MCEI material”) may be located within the trough of the annular housing to contain and channel a magnetic field emanated from the electrical conductor, and to prevent direct physical contact between the electrical conductor and the housing.  
           [0012]    In selected embodiments, the MCEI material conforms to the trough in the annular housing. A trough may also be formed in the MCEI material to accommodate the electrical conductor. In certain embodiments, the MCEI material may be provided in the form of multiple segments positioned around the circumference of the trough of the annular housing. The annular housing may be formed to retain the MCEI segments in substantially fixed positions within the housing. In certain embodiments, the MCEI material may be a ferrite, a composition containing a ferrite, or a material having similar magnetic and electrical properties to a ferrite.  
           [0013]    In selected embodiments, a trough formed in the annular housing may include one or several retaining shoulders. Likewise, the MCEI material may be formed to include one or several corresponding shoulder to mechanically engage the retaining shoulder, thereby effectively positioning the MCEI material with respect to the annular housing and preventing the MCEI material from exiting the trough of the annular housing. In selected embodiments, the electrical conductor is coated with an insulating material. In other embodiments, the electrical conductor may simply be a single coil within the annular housing or may comprise a plurality of conductive strands coiled around the circumference of the annular housing.  
           [0014]    The annular housing may be configured to reside in an annular recess milled, formed, or otherwise provided in a substrate, such as in the mating surfaces of the pin end or box end of a drill pipe or other downhole component. Correspondingly, the exterior surface of the annular housing may be formed to include one or more locking shoulders. The annular recess may also include one or more corresponding locking shoulders to engage locking shoulders of the annular housing, thereby preventing separation of the annular housing from the substrate.  
           [0015]    In selected embodiments, the annular housing is dimensioned to reside substantially flush with the surface of the substrate when in the annular recess. Likewise, the MCEI segments may also be dimensioned or designed to reside in the trough of the annular housing such that they are substantially flush with the annular housing, the substrate, or both. In selected embodiments, the apparatus may comprise a biasing member, such as a spring or elastomeric material. This biasing member may be located between the annular recess and the annular housing, or may be located between the annular housing and the MCEI material, for example.  
           [0016]    In another aspect of the present invention, an apparatus for transmitting data between downhole tools may include an annular housing having a circumference. The annular housing may have a substantially U-shaped cross-section around the circumference thereof. An MCEI material may be placed or located within the annular housing. The MCEI material may have a substantially U-shaped cross-section substantially conforming to the inside of the annular housing, although this is not necessary.  
           [0017]    An electrical conductor may be disposed within the U-shape cross-section of the MCEI material. In certain embodiments, the MCEI material may be comprised of a plurality of MCEI segments positioned around the circumference of the annular housing. The annular housing may be formed to retain the MCEI segments in substantially fixed positions. In selected embodiments, the MCEI material may comprise a ferrite, compositions including a ferrite, or materials have ferrite-like magnetic and electrical properties.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    The foregoing and other features of the present invention will become more fully apparent from the following description, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments in accordance with the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which:  
         [0019]    [0019]FIG. 1 is a perspective view illustrating one embodiment of transmission elements installed into the box and pin ends of a downhole-drilling pipe to transmit and receive information along a drill string;  
         [0020]    [0020]FIG. 2 is a perspective view illustrating one embodiment of the interconnection and interaction between transmission elements;  
         [0021]    [0021]FIG. 3 is a perspective cross-sectional view illustrating various features of one embodiment of an improved transmission element in accordance with the invention;  
         [0022]    [0022]FIG. 4 is a perspective cross-sectional view illustrating one embodiment of a multi-coil or multi-strand conductor within a transmission element, and various locking shoulders used to retain the MCEI segments within the annular housing;  
         [0023]    [0023]FIG. 5 is a perspective cross-sectional view illustrating one embodiment of a single conductor or coil used within the transmission element;  
         [0024]    [0024]FIG. 6 is a perspective cross-sectional view illustrating one embodiment of a single conductor or coil surrounded by an electrically insulating material used within the transmission element;  
         [0025]    [0025]FIG. 7 is a perspective cross-sectional view illustrating another embodiment of a transmission element having a flat or planar area formed on the conductor in accordance with the invention;  
         [0026]    [0026]FIG. 8 is a perspective cross-sectional view illustrating one embodiment of a transmission element having various biasing members to urge components of the transmission element into desired positions;  
         [0027]    [0027]FIG. 9 is a perspective cross-sectional view illustrating one embodiment of a transmission element having a shelf or ledge formed in the annular housing to accurately position the transmission element with respect to a substrate;  
         [0028]    [0028]FIG. 10 is a perspective cross-sectional view illustrating one embodiment of a transmission element having an elastomeric or elastomeric-like material to urge the components of the transmission element into desired positions; and  
         [0029]    [0029]FIG. 11 is a perspective cross-sectional view illustrating on embodiment of an annular housing capable of retaining MCEI segments in substantially fixed positions within the annular housing.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of embodiments of apparatus and methods of the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of various selected embodiments of the invention.  
         [0031]    The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. Those of ordinary skill in the art will, of course, appreciate that various modifications to the apparatus and methods described herein may easily be made without departing from the essential characteristics of the invention, as described in connection with the Figures. Thus, the following description of the Figures is intended only by way of example, and simply illustrates certain selected embodiments consistent with the invention as claimed herein.  
         [0032]    In an effort to overcome limitations imposed by mud pulse telemetry systems, reliable connections are needed to transmit information between components in a drill string. For example, since direct electrical connections between drill string components may be impractical and unreliable due to dirt, mud, rocks, air gaps, and the like between components, converting electrical signals to magnetic fields for later conversion back to electrical signals is suggested for transmitting information between drill string components. Like a transformer, current traveling through a first conductive coil, located on a first drill string component, may be converted to a magnetic field. The magnetic field may then be detected by a second conductive coil located on a second drill string component where it may be converted back into an electrical signal mirroring the first electrical signal. A core material, such as a ferrite, may be used to channel magnetic fields in a desired direction to prevent power loss. However, past attempts to use this “transformer” approach have been largely unsuccessful due to a number of reasons.  
         [0033]    For example, power loss may be a significant problem. Due to the nature of the problem, signals must be transmitted from one pipe section, or downhole tool, to another. Thus, air or other gaps are present between the core material of transmission elements. This may incur significant energy loss, since the permeability of ferrite, and other similar materials, may be far greater than air, lubricants, pipe sealants, or other materials. Thus, apparatus and methods are needed to minimize power loss in order to effectively transmit and receive data.  
         [0034]    Referring to FIG. 1, drill pipes  10   a ,  10   b , or other downhole tools  10   a ,  10   b , may include a pin end  12  and a box end  14  to connect drill pipes  10   a ,  10   b  or other components  10   a ,  10   b  together. In certain embodiments, a pin end  12  may include an external threaded portion to engage an internal threaded portion of the box end  14 . When threading a pin end into a corresponding box end  14 , various shoulders may engage one another to provide structural support to components connected in a drill string.  
         [0035]    For example, a pin end  12  may include a primary shoulder  16  and a secondary shoulder  18 . Likewise, the box end  14  may include a corresponding primary shoulder  20  and secondary shoulder  22 . A primary shoulder  16 ,  20  may be labeled as such to indicate that a primary shoulder  16 ,  20  provides the majority of the structural support to a drill pipe  10  or downhole component  10 . Nevertheless, a secondary shoulder  18  may also engage a corresponding secondary shoulder  22  in the box end  14 , providing additional support or strength to drill pipes  10  or components  10  connected in series.  
         [0036]    As was previously discussed, apparatus and methods are needed to transmit information along a string of connected drill pipes  10  or other components  10 . As such, one major issue is the transmission of information across joints where a pin end  12  connects to a box end  14 . In selected embodiments, a transmission element  24   a  may be mounted proximate a mating surface  18  or shoulder  18  on a pin end  12  to communicate information to another transmission element  24   b  located on a mating surface  22  or shoulder  22  of the box end  14 . Cables  26   a ,  26   b , or other transmission media  26 , may be operably connected to the transmission elements  24   a ,  24   b  to transmit information therefrom along components  10   a ,  10   b.    
         [0037]    In certain embodiments, an annular recess may be provided in the secondary shoulder  18  of the pin end  12  and in the secondary shoulder  22  of the box end  14  to house each of the transmission elements  24   a ,  24   b . The transmission elements  24   a ,  24   b  may have an annular shape and be mounted around the radius of the drill pipe  10 . Since a secondary shoulder  18  may contact or come very close to a secondary shoulder  22  of a box end  14 , a transmission element  24   a  may sit substantially flush with a secondary shoulder  18  on a pin end  12 . Likewise, a transmission element  24   b  may sit substantially flush with a surface of a secondary shoulder  22  of a box end  14 .  
         [0038]    In selected embodiments, a transmission element  24   a  may be coupled to a corresponding transmission element  24   b  by having direct electrical contact therewith. In other embodiments, the transmission element  24   a  may convert an electrical signal to a magnetic field or magnetic current. A corresponding transmission element  24   b , located proximate the transmission element  24   a , may detect the magnetic field or current. The magnetic field may induce an electrical current into the transmission element  24   b . This electrical current may then be transmitted from the transmission element  24   b  by way of an electrical cable  26   b  along the drill pipe  10  or downhole component  10 .  
         [0039]    As was previously stated, a downhole drilling environment may adversely affect communication between transmission elements  24   a ,  24   b  located on successive drill string components  10 . Materials such as dirt, mud, rocks, lubricants, or other fluids, may inadvertently interfere with the contact or coupling between transmission elements  24   a ,  24   b . In other embodiments, gaps present between a secondary shoulder  18  on a pin end  12  and a secondary shoulder  22  on a box end  14 , due to variations in component tolerances, may interfere with communication between transmission elements  24   a ,  24   b . Thus, apparatus and methods are needed to reliably overcome these as well as other obstacles.  
         [0040]    Referring to FIG. 2, in selected embodiments, a transmission element assembly  33  may include a first transmission element  24   a  mounted in the pin end  12  of a drill pipe  10  or other tool  10 , and a second transmission element  24   b  mounted in the box end  14  of a drill pipe  10  or other tool  10 . Each of these transmission elements  24   a ,  24   b  may be operably connected by a cable  26   a , such as electrical wires, coaxial cable, optical fiber, or like transmission media. Each of the transmission elements  24  may include an exterior annular housing  28 . The annular housing  28  may function to protect and retain components or elements within the transmission element  24 . The annular housing  28  may have an exterior surface shaped to conform to a recess milled, formed, or otherwise provided in the pin  12  or box end  14  of a drill pipe  10 , or other downhole component  10 .  
         [0041]    In selected embodiments, the annular housing  28  may be surfaced to reduce or eliminate rotation of the transmission elements  24  within their respective recesses. For example, anti-rotation mechanisms, such as barbs or other surface features formed on the exterior of the annular housing  28  may serve to reduce or eliminate rotation.  
         [0042]    As is illustrated in FIG. 2, a transmission element  24   b  located on a first downhole tool  10  may communicate with a transmission element  24   c  located on a second downhole tool  10 . Electrical current transmitted through a coil  32  in a first transmission element  24   b  may create a magnetic field circulating around the conductor  32 . A second transmission element  24   c  may be positioned proximate the first transmission element  24   b  such that the magnetic field is detected by a coil  32  in the transmission element  24   c.    
         [0043]    In accordance with the laws of electromagnetics, a magnetic field circulated through an electrically conductive loop induces an electrical current in the loop. Thus, an electrical signal transmitted to a first transmission element  24   b  may be replicated by a second transmission element  24   c . Nevertheless, a certain amount of signal loss occurs at the coupling of the transmission element  24   b ,  24   c . For example, signal loss may be caused by air or other gaps present between the transmission elements  24   b ,  24   c , or by the reluctance of selected magnetic materials. Thus, apparatus and methods are needed to reduce, as much as possible, signal loss that occurs between transmission elements  24   b ,  24   c.    
         [0044]    Referring to FIG. 3, a perspective cross-sectional view of one embodiment of a transmission element  24  is illustrated. In selected embodiments, a transmission element  24  may include an annular housing  28 , an electrical conductor  32 , and a magnetically-conducting, electrically-insulating material  34  separating the conductor  32  from the housing  28 .  
         [0045]    The MCEI material  34  may prevent electrical shorting between the electrical conductor  32  and the housing  28 . In addition, the MCEI material  34  contains and channels magnetic flux emanating from the electrical conductor  32  in a desired direction. In order to prevent signal or power loss, magnetic flux contained by the MCEI material  34  may be directed or channeled to a corresponding transmission element  24  located on a connected downhole tool  10 .  
         [0046]    The MCEI material  34  may be constructed of any material having suitable magnetically-conductive and electrically-insulating properties. For example, in selected embodiments, certain types of metallic oxide materials such as ferrites, may provide desired characteristics. Ferrites may include many of the characteristics of ceramic materials. Ferrite materials may be mixed, pre-fired, crushed or milled, and shaped or pressed into a hard, typically brittle state. Selected types of ferrite may be more preferable for use in the present invention, since various types operate better at higher frequencies.  
         [0047]    Since ferrites or other magnetic materials may be quite brittle, using an MCEI material  34  that is a single piece may be impractical, unreliable, or susceptible to cracking or breaking. Thus, in selected embodiments, the MCEI material  34  may be provided in various segments  34   a - c . Using a segmented MCEI material  34   a - c  may relieve tension that might otherwise exist in a single piece of ferrite. If the segments  34  are positioned sufficiently close to one another within the annular housing  28 , signal or power loss between joints or gaps present between the segments  34   a - c  may be minimized.  
         [0048]    The annular housing  28 , MCEI material  34 , and conductor  32  may be shaped and aligned to provide a relatively flat face  35  for interfacing with another transmission element  24 . Nevertheless, a totally flat face  35  is not required. In selected embodiments, a filler material  38  or insulator  38  may be used to fill gaps or volume present between the conductor  32  and the MCEI material  34 . In addition, the filler material  38  may be used to retain the MCEI segments  34   a - c , the conductor  32 , or other components within the annular housing  28 .  
         [0049]    In selected embodiments, the filler material  38  may be any suitable polymer material such as Halar, or materials such as silicone, epoxies, and the like. The filler material  38  may have desired electrical and magnetic characteristics, and be able to withstand the temperature, stress, and abrasive characteristic of a downhole environment. In selected embodiments, the filler material  38  may be surfaced to form to a substantially planer surface  35  of the transmission element  24 .  
         [0050]    In selected embodiments, the annular housing  28  may include various ridges  40  or other surface characteristics to enable the annular housing  28  to be press fit and retained within an annular recess. These surface characteristics  40  may be produced by stamping, forging, or the like, the surface of the housing  28 . In selected embodiments, the annular housing  28  may be formed to retain the MCEI material  34 , the conductor  32 , any filler material  38 , and the like. For example, one or several locking shoulders  36  may be provided or formed in the walls of the annular housing  28 . The locking shoulders  36  may allow insertion of the MCEI material  34  into the annular housing  28 , while preventing the release therefrom.  
         [0051]    Referring to FIG. 4, in selected embodiments, the electrical conductor  32  may include multiple strands  32   a - c , or multiple coils  32   a - c , coiled around the circumference of the annular housing  28 . In selected embodiments, multiple coils  32   a - c  may enable or improve the conversion of electrical current to a magnetic field. The coils  32   a - c , or loops  32   a - c , may be insulated separately or may be encased together by an insulation  38  or filling material  38 .  
         [0052]    Referring to FIG. 5, in another embodiment, the transmission element  24  may include a single coil  32 , or loop  32 . The single loop  32  may occupy substantially the entire volume within the MCEI material  34 . An insulated conductor  32  may simply provide a rounded surface for interface with another transmission element  24 .  
         [0053]    Referring to FIG. 6, in another embodiment, the conductor  32  may be much smaller and may or may not be surrounded by a filler material  38 . The filler material  38  may be leveled off to provide a planar or substantially flat surface  44  for interfacing with another transmission element  24 . In certain cases, a larger electrical conductor  32  may provide better performance with respect to the conversion of electrical energy to magnetic energy, and the conversion of magnetic energy back to electrical energy.  
         [0054]    Referring to FIG. 7, in selected embodiments, a transmission element  24  may have a rounded shape. The annular housing  28 , the MCEI material  34 , and the conductor  32  may be configured to interlock with one another. For example, the annular housing  28  may be formed to include one or more shoulders  48   a ,  48   b  that may interlock with and retain the MCEI material  34 .  
         [0055]    In certain embodiments, a biasing member  50  such as a spring  50  or other spring-like element  50  may function to keep the MCEI material  34  loaded and pressed against the shoulders  48   a ,  48   b  of the annular housing  28 . The shoulders  48   a ,  48   b  may be dimensioned to enable the MCEI material  34  to be inserted into the annular housing  28 , while preventing the release thereof. In a similar manner, the conductor  32  may be configured to engage shoulders  49   a ,  49   b  formed into the MCEI material  34 . In the illustrated embodiment, the conductor  32  has a substantially flat or planar surface  44 . This may improve the coupling, or power transfer to another transmission element  24 .  
         [0056]    Referring to FIG. 8, in another embodiment, locking or retaining shoulders  52   a ,  52   b  may be milled, formed, or otherwise provided in a substrate material  54 , such as in the primary or secondary shoulders  16 ,  18 ,  20 ,  22  of drill pipes  10  or downhole tools  10 . Likewise, corresponding shoulders may be formed in the annular housing  28  to engage the shoulders  52   a ,  52   b.    
         [0057]    A biasing member, such as a spring  50   a , or spring-like member  50   a , may be inserted between the annular housing  28  and the MCEI material  34 . The biasing members  50   a ,  50   b  may enable the transmission element  24  to be inserted a select distance into the annular recess of the substrate  54 . Once inserted, the biasing members  50   a ,  50   b  may serve to keep the annular housing  28  and the MCEI material  34  pressed against the shoulders  48   a ,  48   b ,  52   a ,  52   b.    
         [0058]    In addition, shoulders  48   a ,  48   b ,  52   a ,  52   b  may provide precise alignment of the annular housing  28 , MCEI material  34 , and conductor  32  with respect to the surface of the substrate  54 . Precise alignment may be desirable to provide consistent separation between transmission elements  24  communicating with one another. Consistent separation between transmission elements  24  may reduce reflections and corresponding power loss when signals are transmitted from one transmission element  24  to another  24 .  
         [0059]    Referring to FIG. 9, in selected embodiments, a transmission element  24  may include an alignment surface  58  machined, cast, or otherwise provided in the exterior surface of the annular housing  28 . The alignment surface  58  may engage a similar surface milled or formed into an annular recess of a substrate  54 . This may enable precise alignment of the annular housing  28  and other components  32 ,  34  with the surface of a substrate  54 .  
         [0060]    In certain embodiments, the conductor  32  may be provided with grooves  54   a ,  54   b  or shoulders  54   a ,  54   b  that may engage corresponding shoulders milled or formed into the MCEI material  34 . This may enable a surface  44  of the conductor  32  to be level or flush with the surface of the MCEI material  34  and the annular housing  28 . In some cases, such a configuration may enable direct physical contact of conductors  32  in the transmission elements  24  when they are coupled together. This may enhance the coupling effect of the transmission elements  24  and enable more efficient transfer of energy therebetween. As is illustrated in FIG. 9, lower shoulders  56   a ,  56   b  formed into the annular housing  28  and the MCEI material  34  may provide a substantially fixed relationship between the annular housing  28  and the MCEI material  34 .  
         [0061]    Referring to FIG. 10, in selected embodiments, a biasing member  50  composed of an elastomeric or elastomeric-like material may be inserted between components such as the annular housing  28  and the MCEI material  34 . As was previously described with respect to FIG. 7, the biasing member  50  may keep the MCEI material  34  pressed up against shoulders  48   a ,  48   b  of the annular housing  28  to provide precise alignment of the MCEI material  34  with the annular housing  28 .  
         [0062]    Referring to FIG. 11, in selected embodiments, the annular housing  28  may be formed, stamped, milled, or the like, as needed, to maintain alignment or positioning of various components within the annular housing  28 . For example, various retention areas  60  may be formed into the annular housing  28  to provide consistent spacing of MCEI segments  34   a - c . The retention areas  60  may simply be stamped or hollowed areas within the annular housing  28 , or they may be cutout completely from the surface thereof.  
         [0063]    Likewise, one or multiple ridges  62  or other surface features  62  may be provided to retain the annular housing  28  in an annular recess when the annular housing  28  is press-fit or inserted into the recess. The annular housing  28  may also include various shoulders  64   a ,  64   b  that may engage corresponding shoulders milled or formed into the annular recess to provide precise alignment therewith and to provide a consistent relationship between the surfaces of the transmission element  24  and the substrate  54 .  
         [0064]    The present invention may be embodied in other specific forms without departing from its essence or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes within the meaning and range of equivalency of the claims are to be embraced within their scope.