Abstract:
A quad cable construction and a method of manufacturing the same are provided for use in communications for a local area network, while offering significant vapor migration and petroleum immersion resistance characteristics. A cable is provided with inner and outer jackets encompassing a helix configuration of insulated signal conductors. A core filler is provided to substantially fill the core and interstices between the insulated signal conductors. The core filler and inner jacket are formed of vapor proof material and bound with the insulated signal conductors in a manner that substantially fills all grooves and crevices around the insulated signal conductors to substantially prevent vapor migration along the cable length. An outer jacket may be provided that is impervious to gas, thereby permitting the cable to be submerged in petroleum for extended periods of time without affecting operation.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The preferred embodiments of the present invention generally relate to communications and electronics cabling, and in particular to a vapor proof cable, such as for high speed communications and network interconnect cable, and a method of manufacturing the same. 
     2. Background Art 
     Communications and electronics cables are used today in a broad array of applications, many of which require that the cable carry high frequency signals over long distances. The operating frequency range for modem cable is significantly higher than the range needed for past applications, due in part to the evolution of communications and electronics equipment. In addition, today&#39;s applications require that cable operate under environmental conditions that are significantly more demanding than in the past. 
     Communications and electronics applications have been proposed that require cables capable of supporting ethernet protocols, while submerged for extended periods of time in fluid, such as oil, gas, water and the like. In at least one application, networking cables are installed at gasoline service stations to interconnect fuel pump electronics and point of sale (POS) equipment. The point of sale equipment communicates with the fuel pump via an ethernet data transmission protocol, such as established in accordance with the IEEE 802.3 10Base-T standard. Interconnect cable used in service station applications is exposed to petroleum fumes and, in some instances, may be submerged in fuel. Other protocols that cable can be used for include asynchronous transfer mode communication. 
     Heretofore, local area networks, such as used at service stations, typically use category 5 cable as the interconnect cable. Category 5 represents a standard set forth by ANSI, and the TIA/EIA group. Conventional category 5 cable includes twisted groups of insulated conductors. Each twisted group may include two or more conductors forming pairs. Twisted pair cable includes air gaps between an inner surface of the cable jacket and the twisted pair insulated conductors. Twisted pair cable also includes a hollow core between the multiple twisted pair insulated conductors within the cable. The air gaps and hollow core both facilitate the migration of fumes or vapors along the length of the cable. Hence, the potential exists that the cable may transport explosive vapors from the pump to the facility where the clerk is located. 
     In the past, attempts have been made to vapor proof category 5 cable in order to prevent fumes from migrating to the service station and to comply with safety regulations. One method in the past includes stripping away the cable jacket at multiple discrete regions along the length of the cable when the cable is installed to expose the insulated conductors. A potting material is applied to the conductors at each exposed area to form a vapor blocking seal. The potting material is applied at multiple discrete points along the length of the cable to provide a series of discrete or sectional vapor locks. Multiple vapor locks are necessary since the potting material may develop cracks or be improperly applied, thereby permitting vapor to enter the cable and migrate through a vapor lock. Also, the jacket may become damaged between the service station and any given vapor lock, thereby permitting vapor to enter the jacket and migrate toward the service station upstream of a vapor lock. The existing practice of stripping cables and adding potting material is labor intensive, expensive and unreliable and is undesirable. 
     FIG. 1 illustrates a category 5 cable that has been used for ATM and ethernet interconnections heretofore. The cable  10  includes a jacket  12  enclosing four twisted pairs  14 - 17  of conductors arranged in a helix configuration and surrounding a hollow core  18 . The twisted pairs  14 - 17  contact one another and the inner surface  20  of the jacket  12 . The relative positions of the twisted pairs  14 - 17  remain substantially constant with respect to one another. The twisted pairs  14 - 17  are also twisted to form one large helix. The outer boundary of each twisted pair  14 - 17  is denoted by dashed line  28 . Do to the very nature of a helix, the cable  10  includes several peripheral air gaps  24 - 27  located between the inner surface  20  of the jacket  10  and the outer peripheral sections of the twisted pairs  14 - 17 , and air gaps  38  within each twisted pair  14 - 17 . 
     Each twisted pair  14 - 17  comprises two wires  30  and  32  enclosed in insulators  34  and  36 , respectively. A rip cord (not shown) may be provided proximate the inner surface  20  of the jacket  12 . The wires  30  and  32  are copper and the insulators  34  and  36  are formed of a polyolefin or fluoropolymer insulator. The jacket  12  is constructed of riser or plenum rated PVC or fluoropolymer. 
     The cable  10  is arranged in a specific geometry and constructed from materials having a set of desired electrical and physical properties that interact with one another in a particular manner. The overall geometric and material combination affords physical and electrical characteristics that satisfy the requirements of the category 5 standard. Therefore, the cable  10  is approved for use in telecommunications and electronics applications that require category 5 cable. 
     Air is provided in the cable  10  in the core  18  and gaps  24 - 27  and  38 , to achieve specific electrical characteristics. The geometric configuration and dielectric constants for the materials used in the cable  10 , along with the dielectric constant of air in the core  18  and in air gaps  24 - 27  and  38  interact to achieve a desired characteristic impedance and to minimize cross talk between signals transmitted over the twisted pairs  14 - 17 , and interact to minimize attenuation and skew. Therefore, the inclusion of air in the cable  10  is necessary and desirable for category 5 cable. By way of example, the cable  10  exhibits standard electrical characteristics. 
     The cable  10  is able to meet the requirements of the TIA/EIA-568-A standard for the category 5 cable by including air around the insulated conductors  14 - 17 . 
     In certain networking applications, data transmission protocols may be used that differ from the category 5 standard. For instance, in certain ethernet networks, data transmission protocols are used that meet a less strict standard, such as the 10Base-T standard. By way of example, the ethernet network used at service stations, such as in the example explained above, may utilize a data transmission protocol that satisfies the 10Base-T standard. 
     A need remains for an improved network cable that is vapor proof and gas impermeable, while continuing to offer the electrical characteristics needed for high speed data transmissions. It is believed that the preferred embodiments of the present invention, satisfy this need and overcome other disadvantages of conventional cabling which will become more readily apparent from the following discussion. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with at least one preferred embodiment of the present invention, a quad cable is provided including a jacket and at least one quad of insulated signal conductors encased within the jacket. The insulated signal conductors contact one another and are arranged in a helix configuration defining a hollow core. A vapor proof filler substantially fills the hollow core. The jacket and filler fill the gaps and crevices around each insulated conductor to form a hermetic seal along the length of the insulated signal conductors, thereby preventing vapor migration along a length of the cable. In one embodiment, the jacket includes a gas impermeable outer jacket and an inner jacket, while in another embodiment the jacket includes a single unitary jacket. In both embodiments, the single jacket and inner jacket have a dielectric constant higher than a dielectric constant of the insulation on the insulated signal conductors to afford desirable electrical characteristics. The jacket constitutes a pressure extruded compound substantially filling interstices between the insulated signal conductors. The jacket may also include an outer nylon layer substantially impervious to gas. The vapor proof filler represents a pulled core expanded between the insulated signal conductors to substantially fill the hollow core and interstices between the insulated signal conductors. In accordance with one preferred embodiment, the pulled core is formed of cotton, and in an alternative embodiment, the pulled core is formed of an aramid yarn material. 
     According to an alternative embodiment of the present invention, a method of manufacturing a quad cable is provided. The manufacturing method includes the steps of arranging a quad of insulated signal conductors in a helix and in contact with one another. As the insulated signal conductors are arranged in a helix, they define a hollow core therebetween. The manufacturing method further includes introducing a vapor proof filler between the insulated signal conductors to substantially fill the hollow core and crevices between the insulated signal conductors, before the helix is finally formed. As the helix is formed, the insulated conductors are compressed around the core filler to form a hermetic seal with the inner periphery of the conductors. The method further includes applying a pressure extrudable compound around the outer periphery of the insulated signal conductors as a single or inner jacket. The introducing and applying steps form a seal between the insulated signal conductors, filler and jacket substantially void of air gaps to prevent vapor migration along the length of the insulated signal conductors. 
     In at least one alternative embodiment, an inner jacket is pressure extruded over the insulated signal conductors. The inner jacket has a dielectric constant higher than a dielectric constant of the insulation on the insulated signal conductors. The pressure extruding step surrounds the outer perimeter of the signal conductors to substantially fill the interstices between the insulated signal conductors with extruded material. The inner layer may be formed from a polyvinylchloride material. The inner jacket may be encased in a gas impermeable outer layer. The outer layer may be formed of a nylon material. 
     In one alternative embodiment, during the introducing step, the vapor filler is provided between the quad insulated signal conductors before the signal conductors are arranged in a helix and in contact with one another. The vapor proof filler constitutes a soft compressible core. Once the vapor proof filler is properly located between the quad conductors, the quad conductors are compressed and formed into a helix or vice versa. The compression operation causes the vapor proof filler to expand into the grooves between the conductors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing summary, as well as the following detailed description of the preferred embodiments of the present invention, will be better understood when read in conjunction with the appended drawings. For the purpose of illustration, the drawings show embodiments which are presently preferred. It should be understood, however, that the present invention is not limited to the precise arrangements, materials and instrumentality shown in the attached drawings. 
     FIG. 1 illustrates an enlarged cross-sectional view of a conventional multiple differential pair category 5 cable. 
     FIG. 2 illustrates an enlarged cross-sectional view of a quad cable formed in accordance with a preferred embodiment of the present invention. 
     FIG. 3 illustrates an enlarged cross-sectional view of a quad cable formed in accordance with an alternative embodiment of the present invention. 
     FIG. 4 illustrates an enlarged cross-sectional view of a multiple differential pair category 5 cable formed in accordance with an alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates a preferred embodiment of the present invention including a cable  100  having a unitary single jacket  102  that encircles and encases two pair of insulated signal conductors  104 . The insulated signal conductors are formed in a helix configuration and define a hollow core therebetween. The hollow core is substantially filled with a vapor proof material  106 . The vapor proof material  106  extends along a length of the core defined by the conductors  104 . Each conductor  104  includes a conductive center wire  108  surrounded by insulation  110 . The wires  108  carry data transmissions, the characteristics of which are defined in accordance with an ethernet protocol, such as for local area networks complying with the 10Base-T standard, the 100Base-T standard, the ATM standard and the like. The signal conductors  104  carry high frequency transmissions at data rates of 10 Mbits per second, 100 Mbits per second and higher. By way of example only, the cable  100  may carry ethernet data transmissions, such as utilized at a service station for providing an interconnection between fuel pump electronics and service station equipment. The vapor proof material  106  forms a hermetic seal with inner peripheral segments  112 - 115  of the insulated signal conductors  104 . The segments  112 - 115  extend along a length of the insulated signal conductors  104 . The unitary single jacket  102  forms a hermetic seal with the outer peripheral segments  116 - 119  of the insulated signal conductors  104 . The segments  116 - 119  extend along a length of the insulated signal conductors  104 . 
     By way of example only, the cable  100  may be constructed with conductors  104  including two pair of solid tin plated copper having a diameter of approximately 0.0253 inches. The insulation may be 0.0083 inches in thickness and constructed of FEP material. The insulation  110  may have an outer diameter of 0.042 inches. The vapor proof material  106  may be formed of cotton or an aramid yarn type material. The jacket  102  may have an outer diameter of 0.025 inches and may be formed of pressure extruded gasoline resistant Polyurethane. The outer diameter of the cable  100  may be approximately 0.190 inches nominally. A cable  100  having the above-exemplary dimensions and materials satisfies certain standards for supporting data transmission in accordance with an ethernet protocol, such as for a local area network. 
     The dimensions, geometry and materials used in cable  100  are configured in order to achieve desired electrical characteristics, such as for impedance, signal attenuation, skew, capacitance and the like. The insulated signal conductors  104  are formed into a helix or twisted configuration in order to provide uniform transmission characteristics, physical robustness, and protection from electromagnetic interference. The dielectric constants for the vapor proof material  106  and jacket  102  are chosen to be higher than the dielectric constant for the insulation  110  to achieve the desired affective dielectric constant between diametrically opposing conductors that form the differential pair. The outer diameters for the wire  108 , insulation  110  and jacket  102  are controlled to maintain an impedance for the cable  100  within a desired range. In the embodiment of FIG. 2, the cable exhibits an impedance of approximately 100 ohms nominally by TDR or as measured by frequency domain network analysis over the range of 1-100 MHz. By way of example only, the cable  100  exhibits an unbalanced signal pair to ground capacitance of approximately 1,000 pF/1,000 ft. maximum at 1 kHz. By way of example only, the cable  100  experiences near end cross-talk (NEXT) and other electrical characteristics as set forth below in Table 1. 
     
       
         
               
               
             
               
               
             
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Frequency (MHz) 
                 NEXT (dB Nominal) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 5.0 
                 28 
               
               
                 7.5 
                 25 
               
               
                 10.0 
                 23 
               
             
          
           
               
                 Dielectric Withstand: 
                 2500 Vdc For 3 seconds 
               
               
                 Conductor DC Resistance: 
                 28.6 Ohms/1000 ft. Maximum @ 
               
               
                   
                 20° C. 
               
               
                 Conductor DC Resistance Unbalance: 
                 5% Maximum 
               
               
                   
               
             
          
         
       
     
     FIG. 3 illustrates an alternative preferred embodiment for a cable  150  including an outer jacket  152  and an inner jacket  154 . The inner jacket  154  surrounds and hermetically encases a quad configuration of insulated signal conductors  156  that define a hollow core therebetween. A core filler  158  is provided between the insulated signal conductors  156 . The core filler  158  substantially fills the grooves or interstices between the insulated signal conductors  156 . Each insulated signal conductor  156  comprises a wire  160  surrounded by insulation  162 . The core filler  158  is formed of a compressible filament, such as cotton, an aramid yarn and any similar material that exhibits significant vapor blocking characteristics. When the core filler  158  is formed of an aramid yarn material, the core filler  158  also provides added strength to the overall structure of the cable  150 . The inner jacket  154  is pressure extruded around the insulated signal conductors  156 . The inner jacket  154  is formed of a pressure extrudable polyvinylchloride (PVC) material. The outer jacket  152  may be formed of nylon or a similar material that is resistant or impervious to gas and oil (e.g., does not absorb or swell). The core filler  158  forms a hermetic seal with inner peripheral segments  172 - 175  of the insulated signal conductors  156 . The segments  172 - 175  extend along a length of the insulated signal conductors  156 . The inner jacket  154  forms a hermetic seal with the outer peripheral segments  176 - 179  of the insulated signal conductors  156 . The segments  176 - 179  extend along a length of the insulated signal conductors  156 . 
     When the outer jacket  152  is formed of nylon or another material having a dielectric constant higher than that of the insulation  162 , the inner jacket  154  should be constructed with sufficient outer diameter to space the inner diameter  153  of the outer jacket  152  sufficiently far from the insulated signal conductors  156  to prevent the outer jacket  152  from unduly adversely affecting the electrical characteristics of the cable  150 . Nylon typically has a high dielectric constant relative to the dielectric constant of insulation  162 . Also, the dielectric constant of nylon and PVC may change based upon the frequency of transmission signals to which the nylon and PVC are exposed. Thus, when cable  150  is used in ethernet data transmissions carrying high frequency signals, the data signals may influence the dielectric constant of the nylon in the outer jacket  152 , if the outer jacket is located too closely to the insulated signal conductors  156 . Changes in a dielectric constant cause changes in attenuation, impedance, capacitance, etc., which cause reflection losses contributing to signal distortion and increased bit error rates. By way of example only, the inner jacket  154  may have a thickness sufficient to space the inner diameter  153  of the outer jacket a distance d from the insulated signal conductors  156 . 
     The inner jacket  154  is formed of PVC which has a higher dielectric constant than that of the insulated signal conductors  156 . The FEP insulation  162  exhibits a stable dielectric constant that remains constant regardless of the frequency of the transmitted signal. Consequently, the insulation  110  affords impedance matching, low capacitance and other desired electrical characteristics. 
     The cable  150 , as configured with the above described geometry, materials and dimensions, satisfies at least the 10Base-T standard for transmitting ethernet data communications. It is understood that the geometry, materials and dimensions may be varied within a range and still satisfy the 10Base-T standard. The cable  150  is capable of meeting the vapor test defined by UL standard 87, section 36A, paragraph 22.17. The outer jacket  154  is capable of meeting the requirements of the UL standard, subject  758  gas and oil immersion test. 
     By way of example only, the wires  160  may be solid tin plated copper with an inner diameter of approximately 0.0253 inches or 0.024 inches. The insulation  162  may include a thickness of 0.0083 inches and be made of FEP, PFA, polyolefin or other low dielectric materials, thereby forming insulated signal conductors  156  with outer diameters of 0.042 and 0.037 inches, respectively. By way of example only, the inner jacket  154  may include an outer diameter sufficient to maintain a distance d between the insulated signal conductors  156  and the outer jacket  152  of approximately 0.020 inches. The inner jacket  154  may be formed of pressure extruded polyvinylchloride component. The outer jacket  152  may be formed with a thickness of 0.005 inches and may be constructed from nylon material. The foregoing dimensions for the exemplary cable  150  provide an outer diameter of 0.155 inches for a cable including 22 gauge conductors and an outer diameter of 0.140 inches for a cable including 24 gauge conductors. The cable  150  provides the electrical characteristics as set forth below in Table 2. 
     
       
         
               
               
             
               
               
             
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Differential Impedance: 
                 100 Ohms Nominal @ TDR 
               
               
                 Pair-to-Ground Capacitance 
                 1000 pF/1000 ft. Maximum @ 
               
               
                 Unbalance: 
                 1 kHz 
               
             
          
           
               
                 Frequency (MHz) 
                 NEXT (dB Nominal) 
               
               
                   
               
               
                 5.0 
                 28 
               
               
                 7.5 
                 25 
               
               
                 10.0  
                 23 
               
             
          
           
               
                 Dielectric Withstand: 
                 2500 Volts DC For 3 Seconds 
               
               
                 Conductor DC Resistance: 
                 28.6 Ohms/1000 ft Maximum @ 
               
               
                   
                 10° C. 
               
               
                 Conductor DC Resistance Unbalance: 
                 5% Maximum 
               
               
                   
               
             
          
         
       
     
     The cables  100  and  150  in FIGS. 2 and 3 may be manufactured in accordance with an alternative embodiment as set forth hereafter. Initially, the four signal conductors  104 ,  156  and a compressible vapor blocking material  106  or core filler  158  are simultaneously pulled through a quad forming tool. The quad forming tool presses the conductors  104 ,  156  against one another and against the vapor blocking material  106  or core filler  158 , while simultaneously twisting the conductors  104 ,  156  into a helix or quad configuration. As the conductors  104 ,  156  are pressed together, the vapor blocking material  106  or core filler  158  is remolded or shaped to pervade into the crevices and cracks between the conductors  104 ,  156 , and form a hermetic seal with inner and outer peripheral segments  112 - 115 ,  172 - 175 , and  116 - 119 ,  176 - 179 . 
     Next, a plastic compound is pressure extruded around the conductors  104 ,  156  to form the single jacket  102  or inner jacket  154 . The pressure extruding process forces the plastic compound into the interstices between and surrounding the conductors  104 ,  156 . The thickness of the insulation  110 ,  162  and the dimensions of the single jacket  102  or inner jacket  154  are controlled to ensure that the overall combination exhibits the desired electrical characteristics. The vapor proof material  106  or core filler  158  subsequently fills all voids within and along the length of the cable  100 ,  150 . 
     It is understood that the above specific dimensions and particular materials are not required to practice the preferred embodiments of the present invention. Instead, a range of material qualities and dimensions for the various components may be utilized, while still enjoying the advantages and benefits offered by the preferred embodiments of the present invention. By way of example, the following Table 3 sets forth exemplary ranges for the materials used in accordance with the preferred embodiments of FIG.  3 . 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Preferred 
                 Optimal 
                   
               
               
                   
                 Dielectric 
                 Dielectric 
                 Acceptable Dielectric 
               
               
                   
                 Constant Value 
                 Constant Range 
                 Constant Range 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Insulation 
                 2.01 
                 1.8-2.2 
                 1.5-2.9 
               
               
                 Inner Jacket 
                 4.2 
                 3.9-4.5 
                 2.3-6.1 
               
               
                 Outer Jacket 
                 3.50 
                 3.0-4.0 
                 2.0-5.0 
               
               
                   
               
             
          
         
       
     
     The dielectric constant ranges provided in Table 3 are by way of example only and for use with the exemplary materials and dimensions set forth above in connection with FIGS. 2 and 3. It is understood that the ranges for preferable, optimal and acceptable dielectric constants will vary with different materials and dimensions. 
     Optionally, the geometry, materials and dimensions of the cables  100  and  150  may be modified and altered to satisfy other communications and/or electronics standards, provided that such a modification still offers a vapor migration proof cable having desirable electrical characteristics for transmission of high frequency signals. 
     FIG. 4 illustrates an alternative embodiment in accordance with the present invention. A cable  210  is provided for carrying communications transmissions, such as defined by the category 5 standard and the like. The cable  210  includes a jacket  212  enclosing multiple twisted pairs  212 - 217  of conductors arranged in a helix configuration. The insulated conductors  222  and  224  in each twisted pair  212 - 217  are twisted within an outer boundary defined by line  228 . The twisted pairs  212 - 217  are then twisted to form one large helix. Each twisted pair  212 - 217  includes interstitial gaps within boundary  228 . The interstitial gaps within each twisted pair  212 - 217  are filled with an intra-pair gap filler  238 . Outer peripheral air gaps are provided between the boundaries  228  of adjacent twisted pairs  212 - 217  and the inner diameter  220  of the jacket  212 . The peripheral gaps are filled with an inter-pair gap filler  240 . The core is filled with a core filler  218 . 
     The core filler  218 , intra-pair gap filler  238 , and inter-pair gap filler  240  cooperate to hermetically encase the insulated conductors  222  and  224  for each twisted pair  212 - 217 . In the foregoing manner, substantially all air gaps are removed from within the jacket  212  along the length of the cable  210 . 
     By way of example only, the intra-pair gap filler  238  for each twisted pair  212 - 217  may be formed from cotton, an aramid yarn and the like. Similarly, the core filler  218  may be formed of cotton, an aramid yarn and the like. The peripheral inter-pair gap fillers  240  may be formed from pressure extruded plastic compositions, such as PVC and the like. Optionally, a gas impervious jacket  212  may be included. Alternatively, the pressure extruded peripheral inter-pair gap fillers  240  may be expanded to entirely encase the twisted pairs  212 - 217 , such as the inner jacket  156  illustrated in FIG. 3, with or without a thin outer jacket thereabout. 
     According to yet a further alternative embodiment, the number of twisted pairs  212 - 217  may be varied, to as few as one twisted pair or to more than four twisted pairs. 
     The cable  210  illustrated in FIG. 4 may be manufactured in a sequence of steps, whereby the individual twisted pairs  212 - 217  are separately, initially formed with aramid yarn pulled and twisted therewith to form each twisted pair  212 - 217  substantially encased within intra-pair gap fillers  238 . As discussed above in connection with the embodiments of FIGS. 2 and 3, the intra-pair gap filler  238  may be formed of a compressible material, such that, as the insulated conductors  222  and  224  are twisted, the intra-pair gap filler  238  is compressed and molded to substantially fill interstices between the conductors  222  and  224 . 
     Next, the twisted pairs  212 - 217  and encasing intra-pair gap filler  238  are pulled with core filler  218  and twisted to form the larger helix configuration comprised of the core filler  218 , twisted pairs  212 - 217  and intra-pair gap fillers  238 . As the twisted pairs  212 - 217  are twisted into a helix, the core filler  218  is compressed and molded to conform to and substantially fill the interstices between the intra-pair gap fillers  238 . Thereafter, a plastic composition, such as PVC, may be pressure extruded over the twisted pairs  212 - 217  to form peripheral fillers  240  substantially filling the interstices between the outer peripheral portions of the intra-pair gap fillers  238  and the inner surface  220  of the jacket  212 . Finally, the jacket  212  encloses the cable internal structure. 
     While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art, particularly in light of the foregoing teachings. It is therefore contemplated by the appended claims to cover such modifications as incorporate those features which come within the spirit and scope of the invention.