Patent Publication Number: US-8979592-B2

Title: Electrical connector for high-speed data transmission

Description:
TECHNICAL FIELD 
     The field of this disclosure relates to electrical connectors and, in particular, to a cable-terminating electrical connector system having enhanced shielding to reduce interference and crosstalk amongst different wires of the cable and different conductors of the connector system. 
     BACKGROUND 
     Increasingly, electronic devices transmit and receive high-frequency electrical signals representing digital data. High-speed data transmission, such as so-called Ultra High-Speed (UHS) data transmission involves the transmission of data between electronic devices at rates of 1 to 10 gigabits per second using signal frequencies of 100 MHz to 500 MHz. There is a desire for future high-speed data transmission at even faster rates and at even higher frequencies. For example, UHS data transmission may be achieved over 1000BASE-T Ethernet networks using category 5, 5E, 6 or 6A cables. Such high-speed digital data networks are not confined to terrestrial applications, especially as high-speed electronics are developed for aerospace and other suitable applications. 
     High-speed digital data transmission is facilitated by a data transmission system with a relatively high signal to noise ratio. For example, one system includes a 1000BASE-T Ethernet network that includes category 5, 5E, 6 or 6A cables. Cables in such a system are designed to propagate data signals without generating or introducing appreciable noise, and are terminated by electrical connectors at either end to either connect cables together, or to connect cables to electronic devices. Electrical connectors commonly used for terrestrial applications, such as the RJ-45 style connector, have proved to be less than suitable for aerospace and other applications. In aerospace and other applications, electrical connectors are subjected to a variety of harsh environmental conditions, such as the presence of moisture, vibrations and mechanical shock, relatively high amounts of external electrical and magnetic interference, and pressure changes, all of which can detrimentally affect an electrical connector&#39;s performance, that is, its ability to transmit data signals while maintaining a relatively high signal to noise ratio. Common electrical connectors for aerospace and other suitable applications, such as the Quadrax-style connector, may work for data transfer rates less than 1 gigabit per second, but tend to exhibit, induce, generate or introduce excessive noise during high-speed data transmission at rates faster than 1 gigabit per second. 
     Because degraded performance of an electrical connector adversely affects the ability of a system to transfer data at high rates, the present inventor has recognized a need for a robust electrical connector capable of facilitating high-speed data transfer in aerospace and other suitable applications, for example, in aircraft electronic systems having performance criteria meeting gigabit data transfer standards such as 1000BASE-T. In addition, the present inventor has recognized a need for an improved connector with a streamlined design and is easily assembled without sacrificing performance. Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an electrical connector assembly including mating socket and plug connectors. 
         FIGS. 2 and 3  are perspective views of the plug connector of  FIG. 1 . 
         FIG. 4  is an exploded view of the plug connector of  FIG. 2 . 
         FIG. 5  is a cross-section view of the plug connector of  FIG. 2   
         FIG. 6  is a front view of a mating end of the plug connector of  FIG. 2 . 
         FIG. 7  is a side view of the socket connector of  FIG. 1 . 
         FIG. 8  is an exploded view of the socket connector of  FIG. 7 . 
         FIG. 9  is a front view of a mating end of the socket connector of  FIG. 7 . 
         FIG. 10  is a perspective view of an insert for receiving the electrical connector assembly of  FIG. 1 . 
         FIG. 11  is a cross-section view of the electrical connector assembly mated with the insert of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the drawings, this section describes particular embodiments and their detailed construction and operation. Throughout the specification, reference to “one embodiment,” “an embodiment,” or “some embodiments” means that a particular described feature, structure, or characteristic may be included in at least one embodiment. Thus appearances of the phrases “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the described features, structures, and characteristics may be combined in any suitable manner in one or more embodiments. In view of the disclosure herein, those skilled in the art will recognize that the various embodiments can be practiced without one or more of the specific details or with other methods, components, materials, or the like. In some instances, well-known structures, materials, or operations are not shown or not described in detail to avoid obscuring aspects of the embodiments. 
     An embodiment of an electrical connector system  10  is described with reference to  FIGS. 1-11 . The following briefly describes an example arrangement of the components of electrical connector system  10 , which includes a plug connector  15  and a socket connector  20 . Electrical connector system  10  may be used to connect two cable segments together for high-speed data transfer, for example, data transferred at rates of 1 gigabit per second and faster by signals generated at frequencies ranging from approximately 100 MHz to approximately 600 MHz and faster. 
     With reference to  FIG. 1 , an electrical connector system  10  includes a plug connector  15  that mates and interfaces with a socket connector  20  to create an electrical connection between two cables or other wiring (omitted from the figures for clarity). With particular reference to  FIGS. 4 and 5 , plug connector  15  includes a rear shell  22  and a front shell  24 . Rear and front shells  22 ,  24  house an electrically insulating sheath  64  (or another non-conductive enclosure) having multiple pin contacts  66 . Front shell  24  includes a cantilever structure  52  extending forwardly in an axial direction  12  (e.g., parallel to axis  12 ) from a rear face  48 . The free end of cantilever structure  52  includes a catch  56  that mates with a pair of retention slots  44  on rear shell  22  to latch together and retain rear and front shells  22 ,  24 . 
     Socket connector  20  includes many similar components that may be arranged in a similar fashion as described with respect to plug connector  15 . For instance, with reference to  FIGS. 8 and 9 , socket connector  20  includes rear and front shells  22 ′,  24 ′ and an insulating sheath  64 ′ housing multiple socket contacts  94 . Socket connector  20  further includes a cantilevered structure  52 ′ that mates with retention slots  44 ′ to latch together rear and front shells  22 ′,  24 ′. 
     One difference between plug and socket connectors  15 ,  20  is the configuration of their respective mating ends  96 ,  98  ( FIGS. 6 and 9 ). In one embodiment, front shell  24  of plug connector  15  includes a pair of tangs  100  on mating end  96 , while front shell  24 ′ of socket connector  20  includes a tongue  104  shaped to mate with mating end  96  of plug connector  15 . In some embodiments, tongue  104  has a smaller circumference in relation to front shell  24 , and in particular, to mating end  96  of front shell  24 , and is dimensioned to provide an interference fit with mating end  96 . 
     In one example assembly process of electrical connector system  10 , mating end  98  of socket connector  20  may be moved along axial direction  12  to connect into mating end  96  of plug connector  15 . As plug connector  15  and socket connector  20  are slidably moved together and mated, pin contacts  66  are inserted into and received by socket contacts  94 . As described above, the interference fit between tongue  104  and tangs  100  provide a mechanical engagement where tangs  100  surround and bear against tongue  104  of socket connector  20 . Tangs  100  help retain the connectors  15 , in a mated configuration even when subjected to mechanical vibrations and stresses, such as mechanical and thermal stresses. 
     The following describes further detailed aspects of this and other embodiments of the electrical connector system  10 . It should be understood that certain embodiments may be illustrated or described herein in the context of particular electrical connectors, such as socket and plug connector assemblies, or other similar connectors. However, as will become apparent from the following disclosure, the embodiments described herein may be implemented with different kinds of connectors and coupling devices. 
     As briefly mentioned above, plug connector  15  and socket connector  20  may include a number of identical or substantially similar components. Accordingly, the following description may group and describe like components or may refer to like components with prime numbers to avoid repetition. In addition, to provide an easy frame of reference, certain complementary components are illustrated and described as being carried by one of the electrical connectors  15 ,  20 . It should be understood that although components may be illustrated and described with respect to one connector and not the other, the location of such components may be interchangeable between the electrical connectors  15 ,  20  without departing from the principles of the disclosed subject matter. 
       FIG. 1  illustrates an electrical connector system  10  according to one embodiment. Electrical connector system  10  includes a plug connector  15  that mates and interfaces with a socket connector  20  to create an electrical connection between cables or other wiring (not illustrated for clarity). With reference to  FIGS. 2-6 , plug connector  15  includes a rear shell  22  and a front shell  24 . Each of rear and front shells  22 ,  24  are preferably made from an electrically conductive material that provides EMI shielding (i.e., to inhibit electromagnetic interference). For instance, rear and front shells  22 ,  24  may be made from aluminum alloys, steel, copper or other suitable electrically conductive material. In other embodiments, rear and front shells  22 ,  24  may be made from an insulating material, such as polyetherimide or other suitable plastic, and coated or plated with an electrically conductive material, such as silver, gold, or nickel. 
     With particular reference to  FIGS. 4 and 5 , rear shell  22  includes a rear face  26 , an opposite front face  28 , and a cavity  30  extending between rear and front faces  26 ,  28 . To establish a frame of reference, rear shell  22  may be divided into three roughly equal regions, including a leading portion  32  (adjacent rear face  26 ), a trailing portion  34  (adjacent front face  28 ), and a central portion  36  spanning between leading and trailing portions  32 ,  34 . In one embodiment, leading portion  32  may have a smaller circumference than both trailing portion  34  and central portion  36 . In such a configuration, rear shell  22  may taper smoothly from leading portion  32  to central portion  36 . In other embodiments, rear shell  22  may have a uniform circumference throughout. 
     One or both of leading and trailing portions  32 ,  34  may include one or more grooves  38  formed on an exterior surface  40  of rear shell  22 . If desired, central portion  36  may also include grooves  38 , but in some embodiments, central portion  36  is free of grooves  38 . Grooves  38  are preferably circumferential, that is, each groove  38  defines a continuous loop around exterior surface  40  at leading and trailing portions  32 ,  34 . Grooves  38  provide a suitable surface for receiving and securing a heat-shrink tubing or other sealing material to form a moisture ingress resistant seal after rear and front shells  22 ,  24  have been mated (as further described below). In an example assembly process, grooves  38  may be filled with epoxy or other adhesive substance and a heat-shrink tube placed thereon. When heat is applied, the heat-shrink tube softens and flows into grooves  38 . After the assembly cools, the tube has a solid mechanical grip with the front shell  24 . In some embodiments, each groove  38  may include a rounded edge  42  instead of sharp corners that could damage or rupture the heat-shrink tubing and thereby degrade the moisture seal. 
     Rear shell  22  further includes a pair of retention slots  44  that may be located on central portion  36  and arranged on opposite sides of rear shell  22 . In some embodiments, slots  44  may be bores that form a passageway extending from an interior surface  46  to an exterior surface  40  of rear shell  22 . In such a configuration, slots  44  penetrate through rear shell  22  and into cavity  30  ( FIG. 5 ). Retention slots  44  are sized to engage a snap-lock catch  56  of cantilever structure  52  as described in more detail below with reference to front shell  24 . In other embodiments, retention slots  44  may not penetrate through rear shell  22 , but may instead be entirely contained and formed on interior surface  46  at a sufficient depth to engage snap-lock catch  56 . 
     With reference  FIGS. 4 and 5 , the following description relates to features of front shell  24 . Front shell  24  includes a cantilever structure  52  extending outwardly from a rear face  48  along or parallel to the axis  12 . Cantilever structure  52  may include one or more cantilever beams  54 , each having a snap-lock catch  56  on a free end thereof. Snap-lock catch  56  includes a radiused or curved surface  58  and a neck  60  that engages an edge  88  of retention slot  44  as further described in detail below. In some embodiments, cantilever beams  54  may further include a number of grooves  50  formed on an interior surface  51  (i.e., a surface facing axis  12 ). Grooves  50  may facilitate gripping a cable or wiring (not shown) and function as a strain relief or overall shield braids of a wiring pair. 
     As illustrated in  FIG. 4 , in one configuration having two cantilever beams  54 , each beam  54  extends generally parallel to axis  12 , with interior surface  51  facing one another and catch  56  facing in opposite directions. In this parallel configuration, catch  56  is arranged to correspond with the position of slots  44  of rear shell  22 . In other embodiments, cantilever structure  52  may include more than two beams  54  that may be arranged in any variety of configurations as desired. 
     Preferably, cantilever structure  52 , including beams  54  and catch  56 , is integrally formed as a part of the front shell  24 . For instance, front shell  24  and cantilever structure  52  may be formed as a monolithic structure, such as by a molding, casting, or injection molding process. Alternatively, front shell  24  and cantilever structure  52  may be machined from a single block of metal. In still other embodiments, front shell  24  and cantilever structure  52  may be formed as separate components and cantilever structure  52  may be fastened, adhered, welded, or otherwise mounted using any suitable techniques. 
     As mentioned previously, rear and front shells  22 ,  24  house an insulating sheath  64  therein. Insulating sheath  64  is preferably a non-conductive enclosure that may be molded or machined from a polymeric material, such as, fiber reinforced or unreinforced thermoplastic polyetherimide resin. Insulating sheath  64  holds pin contacts  66  extending outwardly from a front end  68  in alignment with the axial direction  12 . In some embodiments, insulating sheath  64  may include an integrated cantilever top  70  with a button  72  positioned on a top surface  74  of cantilever top  70 . When sheath  64  is inserted between rear and front shells  22 ,  24 , button  72  may contact an inner wall  76  of front shell  24  to press cantilever top  70  downward toward pin contacts  66  to help pinch and retain pin contacts  66  in position. Additional details and other example embodiments of insulating sheaths are described in detail in U.S. patent application Ser. No. 13/314,174, filed Dec. 7, 2011 and published as U.S. Publication No. 2012/0171884, the disclosure of which is incorporated by reference by reference herein. 
     The following section provides additional details of interior components of rear and front shells  22 ,  24  and describes an example assembly method for mating rear and front shells  22 ,  24  to form plug connector  15 . It should be understood that the plug connector  15  may be assembled in a variety of ways and that the steps described below are not intended to establish a particular sequence of assembly. 
     With particular reference to  FIG. 5 , insulating sheath  64  is inserted into front shell  24  and pushed forward until front end  68  of sheath  64  contacts a collar  78  formed on inner wall  76  of front shell  24 . Preferably, no tools are needed to insert sheath  64  into front shell  24 . Sheath  64  slides into front shell  24  until front end  68  of sheath  64  is flush (or substantially flush) against a face  80  of collar  78 . In some embodiments, inner wall  76  and sheath  64  may have corresponding dimensions to provide an interference fit for securely retaining the sheath  64  therein. In other embodiments, the sheath  64  and the inner wall  76  may each include corresponding keying features (not shown) to securely lock the sheath  64  in position. Front shell  24  includes an opening or bore  82  formed through or between collar  78  to accommodate pin contacts  66  when sheath  64  is inserted. 
     After sheath  64  is secured within front shell  24 , rear shell  22  may be moved along the axial direction  12  toward front shell  24  so that cantilever beams  54  enter cavity  30 . In some instances, cantilever beams  54  may need to be pinched slightly toward each other (e.g., toward the axis  12 ) to ensure that cantilever beams  54  properly enter cavity  30 . As rear shell  22  is moved along the axial direction  12  toward front shell  24 , curved surface  58  of catch  56  rides against interior surface  46  of rear shell  22 . When catch  56  reaches slots  44 , catch  56  snaps into slots  44  and latches together rear and front shells  22 ,  24 . In this configuration, neck  60  engages an edge  88  of rear shell  22  to retain cantilevered beams  54  in position and resist pulling apart the rear and front shells  22 ,  24 . In some embodiments, interior surface  46  may taper or narrow inwardly from front face  28  toward retention slots  44  so as to urge cantilever beams  54  inwardly toward one another. In such embodiments, when catch  56  engages slots  44 , catch  56  may be driven outwardly to form a solid mechanical engagement with slots  44 . 
     To retain sheath  64  in position, rear shell  22  further includes an internal stop  90  formed as part of interior surface  46  within cavity  30 . In an assembled configuration, a rear end  92  of sheath  64  rests against stop  90  to securely retain sheath  64  within plug connector  15 . Preferably, slots  44  and stop  90  are each positioned a distance inward from front face  28  of rear shell  22 , and sheath  64  and cantilever beams  54  are dimensioned so that rear end  92  of sheath  64  contacts stop  90  simultaneous with catch  56  engaging retention slot  44 . In such configuration, sheath  64  is tightly secured within plug connector  15  to limit or eliminate any sliding movement of sheath  64  within plug connector  15 . 
     After rear and front shells  22 ,  24  have been latched together, a heat-shrink tubing or other material may be applied to seal plug connector  15  as described previously. In some embodiments, front shell  24  may include one or more grooves  62  with similar structure and function as grooves  38  on rear shell  22 . In such embodiments, the heat-shrink tubing may cover all of rear shell  22  and up to or beyond grooves  62  of front shell  24  to help maintain the moisture seal at the junction of the rear and front shells  22 ,  24 . 
     As mentioned previously, socket connector  20  may include several components that are identical to or similar as components forming plug connector  15 . It should be understood that it may not be necessary for plug and socket connectors  15 , to use identical components, and that such components may include some differences. One advantage of using identical components is to reduce the number of unique components necessary to create an electrical connector, such as electrical connector system  10 . 
     To provide a brief summary of its components, socket connector  20  is described generally with reference to  FIGS. 7-8 . Socket connector  20  includes rear and front shells  22 ′,  24 ′ and an insulating sheath  64 ′ housed therebetween. Insulating sheath  64 ′ houses multiple socket contacts  94  that mate with pin contacts  66  of plug connector  15 . Sheath  64 ′ may be inserted into front shell  24 ′ in similar steps as described with respect to sheath  64  of plug connector  15 . In some embodiments, sheath  64 ′ may be longer than sheath  64  to house the entire length of socket contacts  94  to avoid exposing any portion of socket contacts  94 . Socket connector  20  further includes cantilever structure  52 ′ on front shell  24 ′ that mates with retention slots  44 ′ of rear shell  22 ′ to latch together rear and front shells  22 ′,  24 ′ and securely retain sheath  64 ′ therein. 
     One difference between plug and socket connectors  15 ,  20  is their respective mating ends  96 ,  98 . Mating ends  96 ,  98  include features configured to mate with one another to form electrical connector system  10 . Such keying features are further described below with respect to an example embodiment illustrated in the figures. It should be understood that these mating features may be interchangeable between plug and socket connectors  15 ,  20  without departing from the principles of the disclosure. 
     With reference to  FIGS. 4-6 , mating end  96  of plug connector  15  includes cantilevered tangs  100 , which may be formed as an integral part of front shell  24 . In some embodiments, tangs  100  may be formed by creating longitudinal slits  102  on front shell  24 . Preferably, slits  102  are formed on mating end  96  to create a pair of opposing tangs  100 . In other embodiments, mating end  96  may include more tangs  100  that may be created by making additional slits  102 . 
     With reference to  FIGS. 7-9 , front shell  24 ′ of socket connector  20  includes a tongue  104  on mating end  98 . Tongue  104  may be dimensioned to have a slightly smaller circumference relative to mating end  96  of front shell  24  to provide an interference fit between tangs  100  and tongue  104  when plug and socket connectors  15 ,  20  are mated. Further details of a mated configuration are described below. 
       FIG. 6  illustrates a view of mating end  96  of plug connector  15  according to one embodiment and  FIG. 9  illustrates a view of mating end  98  of socket connector  20  according to another embodiment. With particular reference to  FIGS. 6 and 9 , the following describes an example assembly of mating plug and socket connectors  15 ,  20  to form electrical connector system  10 . 
     As illustrated in  FIGS. 6 and 9 , respectively, mating end  96  includes exposed pin contacts  66  aligned along axis  12  and mating end  98  includes socket contacts  94  aligned along axis  12 . Preferably, pin contacts  66  do not extend beyond mating end  96  to protect pin contacts  66  from damage. In some embodiments, collar  78  of front shell  24  ( FIG. 5 ) may include an internal pocket  84  for receiving a facial seal  86  that functions to form an environmental seal and hinder moisture, dust, or other contaminants from entering plug connector  15 . Facial seal  86  is made from a resilient material and sits in pocket  84  without being glued or otherwise adhered in place. In some embodiments, facial seal  86  may be a standard O-ring. Additional details relating to facial seal  86  are discussed below with relation to mating plug connector  15  and socket connector  20 . 
     In one assembly of electrical connector system  10 , plug connector  15  is moved in the axial direction  12  toward socket connector  20 . As plug and socket connectors  15 ,  20  are slidably moved together and mated, pin contacts  66  are inserted into socket contacts  94 . Plug and socket connectors  15 ,  20  may be pushed toward one another until a front end  106  of tongue  104  contacts facial seal  86  of plug connector  15 . Front end  106  may compress facial seal  86  into pocket  84  as the plug and socket connectors  15 ,  20  are mated. When fully mated, front end  106  of tongue  104  contacts and rests against front face  79  of collar  78 . 
     Preferably, plug and socket connectors  15 ,  20  are not twisted or rotated when they are jointed, but are instead linearly joined along axial direction  12  so that pure compression forces are imparted to facial seal  86 . Such linear compression without substantial torsion provides controlled, predictable compression and expansion of facial seal  86  as well as helps prevent tearing or otherwise breaking down the material of facial seal  86 . 
     In a mated configuration, tangs  100  of plug connector  15  surround tongue  104  of socket connector  20 . In this configuration, tangs  100  bear against tongue  104  and provide a solid mechanical connection between plug and socket connectors  15 ,  20 . Tangs  100  help preserve a solid mechanical connection between plug and socket connectors  15 ,  20  to maintain shielding at the mating junction against external electromagnetic interference that may otherwise interfere with the cables terminated by plug and socket connector  15 ,  20 . 
     In some embodiments, electrical connector system  10  may be part of a larger assembly of similar connectors. For instance, electrical connector system  10  may be inserted into a larger connector housing (not shown), such as a housing for a MIL-DTL-38999 connector.  FIGS. 10 and 11  illustrate an example embodiment of a housing insert  112  that may be used to house plug and socket connectors  15 ,  20  within a larger connector housing. 
     With reference to  FIGS. 10 and 11 , housing insert  112  includes a front face  114 , an opposite back face  116 , and a bore  118  extending between the faces  114 ,  116 . Bore  118  includes a recessed channel  120  that may extend from front face  114  to a shoulder  134  formed a distance inward of back face  116 . Bore  118  is dimensioned to slidably receive plug and socket connectors  15 ,  20  and may have a general oval shape corresponding to an oval shape of plug and socket connectors  15 ,  20 . It should be understood that in other embodiments, bore  118  may be another shape, such as a circular shape, to correspond to the shape of the plug and socket connectors  15 ,  20 . Housing insert  112  further includes a slot  122  formed on a top surface  124 . In some embodiments, slot  122  penetrates through housing insert  112  from top surface  124  into recessed channel  120 . The bore  118 , channel  120 , and slot  122  are sized to receive and engage a retention latch  108  of plug and socket connectors  15 ,  20  (see  FIG. 1 ). Additional details of retention latch  108  and an example mating arrangement are described below. 
     With particular reference to  FIGS. 1 and 3 , plug and socket connectors  15 ,  20  each include a retention latch  108 . Retention latch  108  is preferably formed as an integral part of front shell  24  and includes a cantilevered arm  110  and a catch  126 . In some embodiments, retention latch  108  may be recessed inwardly into rear shells  24 ,  24 ′ to help minimize exposure of retention latch  108  and protect against potential damage to cantilevered arm  110 . 
       FIG. 11  illustrates a mated configuration of plug connector  15  with housing insert  112 . Although not illustrated or specifically described, the same or similar assembly process may be used to insert socket connector  20  into housing insert  112 . With reference to  FIG. 11 , plug connector  15  slides into bore  118  through back face  116  of housing insert  112 . As plug connector  15  slides through bore  118 , cantilevered arm  110  of retention latch  108  is pushed downward toward front shell  24  by an interior edge  128  of shoulder  134 . Plug connector  15  slides through bore  118  until retention latch  108  snaps into channel  120  and through slot  122 . In this locked configuration, plug connector  15  sits securely within housing insert  112 , with a leading edge  130  of retention latch  108  being flush against a front surface  132  of channel  120  and catch  126  being flush against a shoulder  134  of channel  120 . Housing insert  112  may then be seated in a larger connector housing, such as a MIL-DTL-38999 connector or other connectors. 
     In some embodiments, housing insert  112  may include any number of bores  118  (e.g., four bores  118  are shown in  FIG. 11 ) to retain a desired number of connectors  15 ,  20 . In addition, in other embodiments, channel  120  and slot  122  may be at different positions relative to the positions shown in  FIG. 11  to allow for various configurations of connectors  15 ,  20 . For instance, channel  120  and slot  122  may be at ±90 degrees relative to the position illustrated in  FIG. 11  and the plug connector  15  would be similarly rotated. Accordingly, it should be understood that a number of configurations may be achieved by altering the position of channel  120  and slot  122  as desired. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.