Abstract:
A repeater is disclosed in one embodiment of the present invention as including a cylindrical housing, characterized by a proximal end and a distal end, and having a substantially cylindrical wall, the cylindrical wall defining a central bore passing therethrough. The cylindrical housing is formed to define at least one recess in the cylindrical wall, into which a repeater is inserted. The cylindrical housing also includes an annular recess formed into at least one of the proximal end and the distal end. An annular transmission element, operably connected to the repeater, is located in the annular recess. In selected embodiments, the annular transmission element inductively converts electrical energy to magnetic energy. In other embodiments, the annular transmission element includes an electrical contact to transmit electrical energy directly to another contact.

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 between downhole drilling components.  
         [0003]     2. The Relevant Art  
         [0004]     The need for signal repeaters to counteract signal loss encountered when transmitting data from downhole components to the earth&#39;s surface is known or has been suggested. Nevertheless, in downhole telemetry systems transmitting data on wires or cables integrated directly into the drill string, few if any useable implementations are known for repeating and amplifying data signals. The following references teach repeaters that are used in wireless electromagnetic or acoustic wave transmission systems, and are not applicable to wired solutions. Furthermore, none of the references address all of the challenges, such as cable routing from the repeater up and down the drill string, that are inherent in wired solutions.  
         [0005]     U.S. Pat. No. 6,218,959 issued Apr. 17, 2001 to Smith describes a system and method of fail-safe communication of information transmitted in the form of electromagnetic wave fronts that propagate through the earth between surface equipment and downhole components. The system comprises two or more repeaters disposed within a well bore such that the two repeaters receive each signal carrying the telemetered information. The repeater that is farther from the source includes a memory device that stores information carried in the signal. A timer device, in the repeater that is farther from the source, triggers the retransmission of the information after a predetermined time period, unless the repeater that is farther from the source has detected a signal carrying the information, generated by the repeater, that is closer to the source.  
         [0006]     U.S. Pat. No. 6,177,882 issued Jan. 23, 2001 to Ringgenberg et. al teaches downhole repeaters that utilize electromagnetic and acoustic waves to retransmit signals carrying information and methods for use of the same. The repeaters and methods provide for real-time communication between downhole equipment and the surface, and for the telemetering of information and commands from the surface to downhole tools disposed in a well using both electromagnetic and acoustic waves to carry information. The repeaters and methods detect and amplify signals carrying information at various depths in the well bore, thereby alleviating signal attenuation.  
         [0007]     U.S. Pat. No. 6,160,492 issued Dec. 12, 2000 to Herman teaches an electromagnetic telemetry system for changing the operational state of a downhole device. The system comprises an electromagnetic transmitter disposed in a first well bore that transmits a command signal. An electromagnetic repeater disposed in a second well bore receives the command signal and retransmits the command signal to an electromagnetic receiver disposed in a third well bore that is remote from the first well bore. The electromagnetic receiver is operably connected to the downhole device such that the command signal received from the electromagnetic repeater is used to prompt the downhole device to change operational states.  
         [0008]     U.S. Pat. No. 6,144,316 issued Nov. 7, 2000 to Skinner teaches an electromagnetic and acoustic signal repeater for communicating information between surface equipment and downhole equipment. The repeater comprises an electromagnetic receiver and an acoustic receiver for respectively receiving and transforming electromagnetic input signals and acoustic input signals into electrical signals that are processed and amplified by an electronics package. The electronics package generates an electrical output signal that is forwarded to an electromagnetic transmitter and an acoustic transmitter for generating an electromagnetic output signal that is radiated into the earth and an acoustic output signal that is acoustically transmitted.  
         [0009]     U.S. Pat. No. 6,075,461 issued Jun. 13, 2000 to Smith teaches an apparatus, method and system for communicating information between downhole equipment and surface equipment. An electromagnetic signal repeater apparatus comprises a housing that is securably mountable to the exterior of a pipe string disposed in a well bore. The housing includes first and second housing subassemblies. The first housing subassembly is electrically isolated from the second housing subassembly by a gap subassembly having a length that is at least two times the diameter of the housing. The first housing subassembly is electrically isolated from the pipe string and is secured thereto with a nonconductive strap. The second housing subassembly is electrically coupled with the pipe string and is secured thereto with a conductive strap. An electronics package and a battery are disposed within the housing. The electronics package receives, processes, and retransmits the information being communicated between the downhole equipment and the surface equipment via electromagnetic waves.  
         [0010]     In view of the foregoing, what are needed are apparatus and methods providing signal amplification in high-speed downhole telemetry systems that transmit data using cables or wires directly integrated into the drill string.  
         [0011]     What are further needed are apparatus and methods to seal electronics of the repeater from the surrounding environment, while providing routing of cables to and from the repeater traveling uphole and downhole.  
         [0012]     It would be a further advance to provide apparatus and methods that not only repeat or amplify a signal, but could also gather data from various sensors such as inclinometers, pressure transducers, thermocouplers, accelerometers, imaging devices, seismic devices, and the like, as well as provide control signals to various of these device to control them remotely.  
       SUMMARY OF THE INVENTION  
       [0013]     In view of the foregoing, it is a primary object of the present invention to provide a robust repeater for amplifying signals in high-speed downhole telemetry systems that transmit data using cables or wires directly integrated into the drill string. It is a further object to provide adequate isolation of electronics of the repeater from the surrounding environment, while providing means of routing cables to and from the repeater traveling uphole and downhole. It is a further object to not only boost or amplify a signal, but to also gather data from various sensors such as inclinometers, pressure transducers, thermocouplers, accelerometers, imaging devices, seismic devices, and the like, as well as provide control signals to various of these device to control them remotely.  
         [0014]     Consistent with the foregoing objects, and in accordance with the invention as embodied and broadly described herein, a repeater is disclosed in one embodiment of the present invention as including a cylindrical housing, characterized by a proximal end and a distal end, and having a substantially cylindrical wall, the cylindrical wall defining a central bore passing therethrough. The cylindrical housing is formed to define at least one recess in the cylindrical wall, into which a repeater is inserted. The cylindrical housing also includes an annular recess formed into at least one of the proximal end and the distal end. An annular transmission element, operably connected to the repeater, is located in the annular recess.  
         [0015]     One or several channels may be formed within the cylindrical housing that extend from the recess to the proximal end, the distal end, or both. In selected embodiments, the annular transmission element inductively converts electrical energy to magnetic energy. In other embodiments, the annular transmission element includes an electrical contact to transmit electrical energy directly to another contact. In certain embodiments, at least one battery is located in another recess provided in the cylindrical housing.  
         [0016]     In selected embodiments, the cylindrical housing is inserted into the bore of a host downhole tool. The host downhole tool may include a pin end and a box end, the pin end having an external threaded portion and the box end having an internal threaded portion. In certain embodiments, the box end lacks an integrated secondary shoulder. In this case, a secondary shoulder insert, independent from the box end, may be inserted into the box end, and may be capable of absorbing stresses normally incident on an integrated secondary shoulder.  
         [0017]     In selected embodiments, stresses normally incident on a secondary shoulder are not imposed on the cylindrical housing. Surface characteristics of the secondary shoulder insert may engage corresponding surface characteristics of the inside diameter of the host tool to transfer a load, incident on the secondary shoulder insert, to the host tool.  
         [0018]     In selected embodiments, the repeater circuit further comprises a data acquisition circuit to acquire data from at least one sensor. The sensor may be a pressure transducer, an inclinometer, a thermocoupler, an accelerometer, an imaging device, a seismic device, or the like. The repeater circuit may also include added functionality including signal filtering circuitry, signal error checking circuitry, device control circuitry, a modem, a digital signal processor, a microcontroller, and the like.  
         [0019]     In another aspect of the invention, a downhole link module includes a cylindrical housing, characterized by a proximal end and a distal end, having a substantially cylindrical wall, the cylindrical wall defining a central bore passing therethrough. The cylindrical housing is formed to define at least one recess in the cylindrical wall to accommodate a repeater circuit. A data acquisition circuit, located within the recess, is connected to the repeater circuit to acquire data from at least one sensor.  
         [0020]     In yet another aspect of the invention, a downhole repeater may include a cylindrical housing, characterized by a proximal end and a distal end, having a substantially cylindrical wall, the cylindrical wall defining a central bore passing therethrough. The cylindrical housing has at least one recess formed into the outer rounded surface of the cylindrical wall, accommodating a signal repeater.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     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:  
         [0022]      FIG. 1  is a profile view of a drill rig illustrating a context in which an apparatus and method in accordance with the invention may be used;  
         [0023]      FIG. 2  is a perspective view illustrating one embodiment of a link module configured for insertion into a host downhole tool;  
         [0024]      FIG. 3  is a perspective cross-sectional view illustrating one embodiment of the internal makeup of a link module in accordance with the present invention;  
         [0025]      FIG. 4  is an inverted perspective view illustrating one embodiment of various electronic components that may be included within a link module in accordance with the present invention;  
         [0026]      FIG. 5  is a schematic block diagram illustrating one embodiment of various components that may be included within a link module circuit in accordance with the invention;  
         [0027]      FIG. 6  is a perspective cross-sectional view illustrating one embodiment of a host downhole tool that may be used to house or enclose a link module in accordance with the present invention;  
         [0028]      FIG. 7  is an exploded, perspective, cross-sectional view illustrating certain selected embodiments of components used in conjunction with a link module and a host downhole tool in accordance with the present invention; and  
         [0029]      FIG. 8  is an enlarged cross-sectional view illustrating more detail of various component components illustrated in  FIGS. 6 and 7 .  
     
    
     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]     Referring to  FIG. 1 , a drill rig  10  may include a derrick  12  used to operate a drill string  14 . The drill string  14  may be comprised of multiple sections of drill pipe  16  and other downhole tools  16 . A drill bit  20  may be connected to the end of the drill string  14 . In certain instances, a drill string  14  may extend into the ground 20,000 feet or more. Thus, when information is transmitted up or down the drill string  14 , ample opportunity exists for signal loss.  
         [0033]     Signal loss may occur as a data signal is transmitted from one downhole tool to another. In certain instances, an electrical signal may be converted to a magnetic field or vice versa when encountering tool joints, losing energy each time it is converted. Signal loss may occur because of signal attenuation in cables or wires due to the sheer length of the drill string. Thus, apparatus and methods are needed to ensure that data received from a drill bit  20  or other downhole tool  16  is safely transmitted to the surface. In selected embodiments, one or several repeaters  18  or signal boosters  18  may be inserted at desired intervals along the drill string  14 , such as every 1000 to 5000 feet. In selected embodiments, a repeater  18  may be integrated into an existing drill pipe  16  or downhole tool  16 , or the repeater  18  may be a downhole tool  18  dedicated exclusively to that function.  
         [0034]     Referring to  FIG. 2 , a link module  30 , or a repeater  30 , may include a cylindrical housing  34  defining a central bore  32 . The cylindrical housing  34  may be substantially circular, or in other embodiments, may be polygonal. The central bore  32  may have a diameter that is slightly smaller than the inner bore diameter of a typical section of drill pipe  16  to accommodate and provide space to components of the link module  30 , or repeater  30 .  
         [0035]     Nevertheless, in selected embodiments, as batteries and electronic components become more compact, it is feasible that the central bore  32  of the link module  30  could be substantially equal to that normally encountered in sections of drill pipe  16  or other downhole tools  16 . The link module  30  may be configured for insertion into a host downhole tool. Thus, the link module  30  may be removed or inserted as needed to access or service components located therein.  
         [0036]     In selected embodiments, the link module  30  may include one or several grooves  36  or seal contact surfaces  36  to seal the link module  30  within a host downhole tool. The host downhole tool will be described in more detail in the description of  FIG. 6 . Seals inserted into the seal contact surfaces  36  or grooves  36  may prevent fluids such as drilling mud, lubricants, oil, water, and the like from contaminating circuitry or components inside the link module  30 . Moreover, the entry of other substances such as dirt, rocks, gasses, and the like, may also be prevented.  
         [0037]     In selected embodiments, the link module  30  may include one or several recesses  38   a - c  to house various components contained by the link module  30 , orrepeater  30 . Selected recesses  38  may contain circuitry while others  38  may be used for batteries or other components. One or several channels  41  may be milled or formed into the cylindrical housing  34  to provide for the routing of wires between recesses  38 . In selected embodiments, a connector  40  may be used to connect link module circuity to a cable, wire, or other link, traveling up or down the drill string  14 . An aperture  42  may likewise be provided for routing cable, wire, or other transmission means up or down the drill string  14 .  
         [0038]     Referring to  FIG. 3 , an inverted cross-sectional view of the drawing of  FIG. 2  is illustrated. As illustrated, the link module  30  may be characterized by a general wall thickness  48 . Likewise, in regions proximate recesses  38  or other channels  41 , a thinner wall thickness  50  may be present. Nevertheless, a critical wall thickness  48  should be maintained to provide structural reliability to the link module  30  to support stresses encountered in a downhole environment. The cylindrical housing  34  may be constructed of any suitable material including steel, aluminum, plastics, and the like, capable of withstanding the pressures, stresses, temperatures, and abrasive nature of a downhole environment.  
         [0039]     As illustrated, one or several transmission paths  42   a ,  42   b  may be milled or formed into the wall of the link module  30  to provide an outlet for cables, wires, or other transmission media exiting the recess  38 . In selected embodiments, connector  40  may be provided to simply link up with or connect to repeater circuitry, or in other embodiments, a channel  42   a  may enable the routing of cables, wires, and the like from a repeater circuit, within the recess  38 , to a transmission element (not shown). For example, a transmission element may be provided in an annular recess  44  milled or otherwise formed into the end of the cylindrical housing  34 .  
         [0040]     Referring to  FIG. 4 , a link module  30 , or repeater  30 , is illustrated equipped with components or circuitry needed to provide functionality to the link module  30 . For example, batteries  54  connected in series or parallel may be inserted into selected recesses  38  of the link module  30 . Wires  56  may be routed through channels  41  interconnecting the recesses  38  to connect the batteries  54  together, or to connect the batteries to the link module circuit  58 .  
         [0041]     Likewise, the link module circuit  58 , or components  58 , may be located within other recesses  38 . As was previously stated, a conductor  60 , cable  60 , or other transmission media  60 , may travel from the link module circuit  58  to a transmission element  52 . The transmission element  52  may transmit energy to another transmission element  52  in contact therewith. The transmission element  52  may have an annular shape and may transmit energy by direct electrical contact, or may convert an electrical current to a magnetic field. The magnetic field may then be detected by another transmission element  52  in close proximity thereto located on a subsequent downhole tool  16 .  
         [0042]     Referring to  FIG. 5 , in selected embodiments, a link module circuit  58  within the link module  30  may include various components to provide desired functionality. For example switches  64 , multiplexers  64 , or a combination thereof may be used to receive, switch, and multiplex signals, received from uphole  66   b  and downhole  66   a  sources, into and out of the link module circuit  58 . The switches/multiplexers  64  may direct traffic such as data packets or other signals into and out of the link module circuit  58 , and may ensure that the packets or signals are transmitted at proper time intervals, frequencies, or a combination thereof.  
         [0043]     In certain embodiments, the multiplexer  64  may transmit several signals simultaneously on different carrier frequencies. In other embodiments, the multiplexer  64  may coordinate the time-division multiplexing of several signals. Signals or packets or received by the switch/multiplexer  64  may be amplified  68  and filtered  70 , such as to remove noise. In certain embodiments received signals may simply be amplified. In other embodiments, the signals may be received, data may be demodulated therefrom and stored, and the data may be remodulated and retransmitted on a selected carrier frequency having greater signal strength. A modem  74  may be used to demodulate analog signals received from the switch/multiplexer into digital data  64  and modulate digital data into analog signals for transfer to the switches/multiplexer where they may be transmitted uphole or downhole  
         [0044]     The modem  74  may also perform various tasks such as error-checking  76 . This is typically performed when the data is digital. The modem  74  may also communicate with a microcontroller  78 . The microcontroller  78  may execute any of numerous applications  86 . For example, the microcontroller  78  may run applications  86  whose primary function is acquire data from one or a plurality of sensors  82   a - c . For example, the microcontroller  78  may interface to sensors  82  such as inclinometers, thermocouplers, accelerometers, imaging devices, seismic data gathering devices, or other sensors. Thus, the link module circuit  58  may include circuitry functioning as a data acquisition tool.  
         [0045]     In other embodiments, the microcontroller  78  may run applications  86  that may control various devices  84  located downhole. That is, not only may the link module circuit  58  be used as a repeater, and as a data gathering device, but may also be used to provide control signals to selected devices as needed. The link module circuit  58  may include a memory device  80  such as a FIFO  80  that may be used to store data needed by or transferred between the modem  74  and the microcontroller  78 .  
         [0046]     Other components of the link module circuit  58  may include non-volatile memory  90 , which may be used to store data, such as configuration settings, node addresses, system settings, and the like. One or several clocks  88  may be provided to provide clock signals to the modem  74 , the microcontroller  78 , or any other device. A power supply  72  may receive power from an external power source such as the batteries  54  illustrated in  FIG. 4 . The power supply  72  may provide power to any or all of the components located within the link module circuit  58 . Likewise, an RS232 port  92  maybe used to provide a serial connection to the link module circuit.  
         [0047]     Thus, the link module circuit  58  described in  FIG. 5  may have many more functions than those supplied by a simple signal repeater. The link module circuit  58  may be though of as a node  30  connected to a downhole network, and may provide many of the advantages of an addressable node on a network. The addressable node may amplify signals received from uphole  66   b  or downhole  66   a  sources, be used as a point of data acquisition, and be used to provide control signals to desired devices  84 . These represent only a few examples of the versatility of the link module  30 . Thus, the link module circuit  58 , although useful and functional as a repeater  30 , may have a greatly expanded capability.  
         [0048]     Referring to  FIG. 6 , a host downhole tool  94  may be used to house the link module  30 . For example, a host downhole tool  94  may include a first portion  96   b  threadable into a second portion  96   a . The first portion  96   a  may include a pin end  95  connectable to another downhole tool  16 . Likewise, a second portion  96   b  may include a box end (not shown) connectable to the pin end of another downhole tool  16 .  
         [0049]     The first and second portions  96   a ,  96   b  may have a standard bore size  98  typical of various downhole tools  16 . An oversize bore  100  may be provided to accommodate the link module  30 , which may have a narrowed bore  102  smaller than the standard bore  98 , but sufficient to accommodate the flow of mud or other drilling fluids flowing therethrough. Nevertheless, as was previously stated, as electronic circuitry, batteries, and the like become smaller and more compact, the diameter of the narrow bore  102  will more closely approximate the diameter of the standard bore  98 .  
         [0050]     Drill pipe  16  suitable for use with the present invention typically includes a pin end that threads into a corresponding box end of another downhole tool. Normally, a primary shoulder on a pin end mates to a corresponding primary shoulder on the box end. Likewise, a secondary shoulder on the pin end mates to a corresponding secondary shoulder on the box end.  
         [0051]     Although a primary shoulder may absorb the majority of the joint stress between two interconnected downhole tools, stress absorbed by the secondary shoulder is significant to the strength of the joint. Thus, when threading a first portion  96   b  of a host downhole tool  94  into a second portion  96   a , the structure  96   a ,  96   b  should provide at least as much strength as is provided by a normal pin end and box end connection.  
         [0052]     As is illustrated, the portion  96   a  lacks a secondary shoulder to enable insertion of link module  30  into the oversize bore  100 . Thus, in selected embodiments a secondary shoulder insert  104  may be inserted into the portion  96   a  to absorb stress normally incident on a secondary shoulder. In addition, since the insert  104  absorbs stress normally incident on a secondary shoulder, pressure may be relieved from the link module  30 . More details with respect to the secondary shoulder insert  104  are provided in the description of  FIG. 8 .  
         [0053]     In addition, a transmission interface  106  may be provided that couples to the link module  30  to permit routing of a transmission path from the link module  30  into the portion  96   b  of the host downhole tool  94 . More details with respect to the transmission interface  106  are provided in the description of  FIG. 8 .  
         [0054]     Referring to  FIG. 7 , an exploded perspective view of the host downhole tool  94 , containing the link module  30 , is illustrated. As illustrated, a first portion  96   a  may include a threaded pin end  95 . An annular transmission element  52 , which may operate by inductive coupling or direct electrical contact, may reside within an annular recess formed or milled into the pin end  95 . A conductor  60  or other cable  60  may be connected to the transmission element  52  and be transmitted along the section  96   a.    
         [0055]     As was previously mentioned, an oversized bore  100 , larger than the standard bore  98 , may be provided to accommodate the link module  30 . Likewise, within the inside diameter of the pipe section  96   a , insert grooves  112  or other surface characteristics  112  may be provided to engage corresponding grooves or surface characteristics of the secondary shoulder insert  104 . The pipe section  96   a  may also include internal threads  110  that may couple to external threads  108  of the other section  96   b.    
         [0056]     Also illustrated are the secondary shoulder insert  104 , insert grooves  105  or surface characteristics  105  that may engage corresponding grooves  112  in the pipe section  96   a , a transmission interface  106  that may slide into the secondary shoulder insert  104  to couple to the link module  30 . Also illustrated are several springs that may be used to keep the transmission interface  106  pressed firmly against the link module  30  to ensure that signal coupling successfully occurs between each component  30 ,  106 .  
         [0057]     The springs  114  may include a separator  115  used to isolate the springs  114  and improve the range of bias. Lastly, an annular buttress  116  may sit within the pipe section  96   b  and provide a fixed surface for the springs  114  to press against. Added details with respect to the annular buttress  116 , springs  114 , spacer  115 , transmission interface  106 , and the secondary shoulder insert  104  are provided in an enlarged cross-sectional view in  FIG. 8 .  
         [0058]     Referring to  FIG. 8 , an enlarged cross-sectional view of the joint between pipe sections  96   a ,  96   b  shown in  FIG. 6  is illustrated. For example, external threads of the pipe section  96   b  may thread into internal threads  110  of the other pipe section  96   a . As was previously explained, due to the lack of a natural secondary shoulder, a secondary shoulder insert  104  may include grooves  105  or threads  105  that may engage corresponding grooves  112  formed in the internal diameter of the section  96   a . Thus, the secondary shoulder insert  104  may provide a quasi-secondary shoulder, but also be removed to allow insertion and removal of the link module  30  from the pipe section  96   a.    
         [0059]     As was also previously described, a transmission interface  106  may fit within the inside diameter of the secondary shoulder insert  104  and be pressed firmly against the link module  30  to provide effective signal coupling therefrom. For example, the link module  30  may include an annular transmission element  52 . The transmission interface  106  may also include an annular transmission element  52   b  in close proximity to the transmission element  52   a  to provide efficient signal coupling therebetween.  
         [0060]     The transmission interface  106  may include a link transition area  120  where the cable may transition from the transmission interface  106  into a bore within the pipe section  96   b . In order to keep the transmission interface  106  pressed firmly against the link module  30 , several annular springs  114  may be provided to provide a biasing force.  
         [0061]     In selected embodiments, the annular springs  114  may be separated by a separator ring  115  to provide addition range of motion to the bias. Likewise, an annular buttress  116  may sit against a shoulder  122  formed in the pipe section  96   b  to provide a firm push-point for the springs  114 . As was previously mentioned in the description of  FIG. 2 , various seals  118  in grooves or recesses of the link module  30  may seal against the inside diameter of the pipe section  96   a  thereby keeping out unwanted contaminants.  
         [0062]     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.