Abstract:
An electrical contact device, configured for electrical signals to be transmitted therethrough and for movable contact with an electrically conductive track, includes a composite carbon fiber material including plural carbon fiber elements aligned in substantially the same direction. At least a portion of the plural carbon fiber elements is bonded together in a semi-conductive (low-resistance) synthetic resin compound.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of application Ser. No. 09/899,776 filed Jul. 5, 2001, which in turn is a continuation-in-part of application Ser. No. 09/498,872 filed Feb. 7, 2000 (now U.S. Pat. No. 6,444,102). 
     
    
     TECHNICAL FIELD 
       [0002]    This disclosure relates generally to an electrical contact or an electrical contact assembly (such as used in an electromechanical device), and more particularly to a contact or contact assembly, which is formed of a composite material using plural carbon fiber elements bonded together and firmly fixed in a semi-conductive (low-resistance) synthetic resin compound, to collectively make electrical contact with another element of the electromechanical device. 
       BACKGROUND 
       [0003]    Variable resistive devices utilize elements that vary a voltage or current in order to provide an electrical signal that indicates a relationship to a physical position of a contact or wiper on a resistive or conductive element. Because these contacts or wipers are used in a dynamic state they cannot be fixed or restricted in their movement and must have the freedom to slide or move along any length of their respective resistive or conductive paths. These elements or tracks are custom formulated by each manufacturer and will vary in composition and properties. Because the contact and element have the potential for creating constant friction, the contact or wiper must therefore be produced of a material that is electrically, physically, and environmentally compatible with the resistive and/or conductive track when in the presence of an electrically active and physically dynamic system. The contact or wiper must also provide a long useful life, while maintaining uniform positive engagement with the resistive or conductive element, at a specified applied force, and should not encourage or stimulate the growth of polymers or debris, which act as an insulator and which distort the output signal. 
         [0004]    Presently the contact or wiper materials used for these variable resistive devices are composed of various solid precious metals, clad or coated metals, or precious metal alloys. These precious metal containing contacts, in a dynamic state and in the presence of electrical activity, act as catalysts to generate polymers and debris which degrade the resistive track output signals. This results in the early termination of accurate performance and useful life. 
         [0005]    Initially metal contacts or wipers were used with wirewound resistive or metallic conductive elements, because wirewound elements were the most precise devices. As time evolved great improvements were made in the non-wirewound product area, and they supplanted the wirewound resistive element, but the contact or wiper has always created problems relative to the resistive element because in the presence of an electrical current and dynamic performance, the precious metal components of the metallic contact provide the catalyst to generate polymers and debris, which interfere with the accuracy of the output signal. 
         [0006]    Now that reduction in size, improved accuracy, lower voltages, reduced currents, and a reduction in electrical contact resistance are required in modern servo feedback positioning systems, non-metallic contact materials must be considered to obtain the necessary and sorely needed improvements in these performance characteristics and elimination of the polymers and debris. 
         [0007]    Also, the primary metal currently used in the precious metal alloy is Palladium. This metal has seen a 1,800% price increase since its introduction for use in this application. The price increase has been largely due to an uncertain supply of this metal. 
         [0008]    Also, new environmental laws are being introduced world-wide mandating that automotive components, which are the largest industry using the device described above, be 100% recyclable. The precious metal currently being used cannot be recycled, so that there will be a conflict with this mandate. 
         [0009]    Accordingly, the need exists for improvements in electrical contacts and contact assemblies and, particularly, for-improvements in the materials and assemblies employed therefor. 
       BRIEF SUMMARY 
       [0010]    In an aspect of this disclosure, there is provided a contact or contact assembly for use in electromechanical applications that can improve considerably the useful life of the system by providing a contact or wiper formed of nonmetallic material, more specifically, one formed of a composite carbon fiber material including plural carbon fiber elements bonded together and firmly fixed in a semi-conductive (low-resistance) synthetic resin compound for structural stability and electrical continuity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other aspects, features and advantages can be more clearly understood from the following detailed description with reference to the accompanying drawings wherein: 
           [0012]      FIGS. 1A-1D  are side elevations showing respective embodiments of electrical contacts according to various exemplary embodiments; 
           [0013]      FIGS. 2A-2C  are front elevations and respective enlargements of the embodiments illustrated in  FIGS. 1A-1C , respectively; 
           [0014]      FIGS. 3(   a ) and  3 ( b ) show respective views of a carbon fiber contact formed as a matrix of layers of carbon fibers, according to another exemplary embodiment; 
           [0015]      FIGS. 4(   a ) and  4 ( b ) show respective views of a carbon fiber contact formed as a matrix of layers of carbon fibers, according to another exemplary embodiment; 
           [0016]      FIGS. 5(   a ) and  5 ( b ) show respective views of an electrical contact formed solely of carbon fibers, according to another exemplary embodiment; 
           [0017]      FIGS. 6(   a ) and  6 ( b ) show respective views of an electrical contact formed solely of carbon fibers, according to another exemplary embodiment; 
           [0018]      FIGS. 7(   a ) and  7 ( b ) show respective views of a carbon fiber electrical contact affixed to an electrically conductive beam, according to another exemplary embodiment; 
           [0019]      FIGS. 8(   a ) and  8 ( b ) show respective views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused, according to another exemplary embodiment; 
           [0020]      FIGS. 9(   a ) and  9 ( b ) show respective views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused, according to another exemplary embodiment; 
           [0021]      FIGS. 10(   a ) and  10 ( b ) show respective views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused, according to another exemplary embodiment; 
           [0022]      FIGS. 11(   a ) and  11 ( b ) show respective views of an electrical contact employing multiple layers on a carrier, according to another exemplary embodiment; 
           [0023]      FIGS. 12(   a ) and  12 ( b ) show respective views of an electrical contact formed as a single carbon fiber element, according to another exemplary embodiment; 
           [0024]      FIG. 13  is an exploded view showing the carbon fibers in juxtaposition with two carbon fiber nonwoven mats, according to another exemplary embodiment; and 
           [0025]      FIG. 14  is an end view showing the several layers making up a composite carbon fiber material, according to another exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    This disclosure provides guidance to obtain a contact or wiper element for transmitting electrical signals, either in a low voltage mode (under 45 volts) or a low current mode (under 1000 ma), between a resistive and/or a conductive track and some external circuit termination. 
         [0027]    In an aspect, there is provided a contact or wiper element comprises one or more thin, single layers of carbon fiber elements, all aligned in one direction bonded together and firmly fixed in a very low-resistance, synthetic resin compound for structural stability and electrical continuity and which form part of a composite carbon fiber material (various embodiments of which are described below). Such composite carbon fiber material, not only overcomes the negative conditions caused by metal composition contacts or wipers, but considerably improves total performance in many other aspects. The material is designed to facilitate a virtual drop-in replacement contact or wiper. Such wiper contact or contact assembly for use in electromechanical components or applications is more compatible with present state of the art fabrication techniques and materials used for resistive and conductive track substrates. 
         [0028]    In accordance with another aspect, a nonmetallic electrical contact, one made of composite carbon fiber material, is processed and formed in such a manner as to allow the multiple carbon fiber elements at the center layer of the composite material when properly positioned to be electrically conductive for transmitting unimpeded electrical signals along their longitudinal length. Such carbon fiber elements are fused or conductively bonded by any of various techniques to provide essentially uniform conductivity and redundant transmission of the electrical signal. Additional, off-axis electrical conductivity is provided by nonwoven carbon fiber mats placed on the sides of the multiple strands of carbon fiber. The composite carbon fiber material can be affixed to a carrier or the material may be utilized without a carrier. Such a carrier, if used, may be metallic or non-metallic and may be affixed to the composite carbon fiber material by any of various bonding, fusing, and fastening techniques. The carrier can also be electrically nonconductive, depending upon the application. Alternatively, the carrier can be formed of the same homogenous composite carbon fiber material as that used for the actual contact. Forming of the carbon fiber contact layer of the composite material can involve cross-layering of the material in nonparallel orientations to provide additional structural integrity, as well as to assist in the post-forming operation. 
         [0029]    The aforementioned wiper contact is rigid enough to sustain and maintain a consistent position relative to its parallel alignment to the resistive or conductive track of the substrate element and yet is flexible enough in a perpendicular position to the track to allow some variation in movement to sustain uniform contact position, spring rate and pressure. Thus, the electrical output signal maintains its integrity. 
         [0030]    In another aspect, the contact surface of the wiper contact that is adjacent to the resistive or conductive track is composed of multiple points of contact, rather than either a small number of metal fibers or just one broad band of a rigid beam contact. This ensures a more redundant positive footprint with the resistive or conductive track, which reduces contact resistance and variable electrical noise. 
         [0031]    Further, the use of carbon and thermoplastics ensures the supply of such a product well into the future. Each of these materials is 100% recyclable and readily available at a substantially reduced cost compared to the currently used precious metal. The resulting unit price will also prove to be less expensive than current products. 
         [0032]    As shown in  FIGS. 1A-1C , the ends of the contact or wiper may be specially formed to give the engagement portion of the contact or wiper added strength and permit better mating of the carbon fiber element to the track of the device. In  FIG. 1A , the contact  10  has a rake end  12 . In  FIG. 1B , the contact  14  has a knuckle end  16 . In  FIG. 1C , the contact  18  has a pointed end  20 . 
         [0033]    The contact or wiper  22 , as shown in Fig. ID, may also engage a mechanical strip  24  for support or for attachment purposes. The mechanical strip  24  may be electrically conductive or not, depending upon the desired application. 
         [0034]      FIGS. 2A ,  2 B, and  2 C correspond, respectively, to  FIGS. 1A ,  1 B, and  1 C and show the arrangement of the carbon fiber packages that are part of the composite material forming the specialized end constructions  12 ,  16 , and  20 , respectively. That is, the enlargement of  FIG. 2A  shows carbon fiber packages  26  arranged in one layer forming the rake end  12 . Similarly, packages  28  and  30  respectively form knuckle end  16  and pointed end  20  in  FIGS. 2B and 2C , respectively. The other layers of the composite material are not shown because the structures of the carbon fiber packages would be obscured. 
         [0035]    In the embodiment shown in  FIG. 3 , the contact or wiper element  40  is formed of a carbon fiber matrix, whose adjacent three carbon fiber layers  42 ,  44 ,  46  are essentially perpendicular to each other. The carbon fibers forming layers  42 ,  44 ,  46  are not bundled but are discretely placed-in a cross-hatching matrix, wherein the fibers in alternate layers may be parallel to each other, but those in adjacent layers are essentially nonparallel and may be perpendicular to each other. 
         [0036]      FIG. 4  shows a similarly constructed contact  50  in which the carbon fibers of only one layer  52  perform the actual contacting and an inner layer  54  and second outer layer provide structural support. The additional layers of the composite material are shown in  FIG. 14 . 
         [0037]    The matrix composition shown in the embodiments of  FIGS. 3 and 4  reinforces and strengthens the minuscule carbon fiber strands to provide support for retaining stable contact position. The carbon fiber strands may be continuous or discontinuous and the matrix need not necessarily be homogeneous. 
         [0038]    Corresponding to the structure shown in  FIG. 1D , the matrix compositions of  FIGS. 3 and 4  can use an additional mechanical support strip, which can be electrically conductive depending upon the desired application. The carbon fibers of the matrix composition shown in  FIGS. 3 and 4  are firmly fixed in a semi-conductive (very low resistance) synthetic resin compound to restrict movement, add structural stability, and provide multidirectional electrical continuity. Such synthetic resin compound preferably has carbon fibers added therein (such as by addition of milled or chopped carbon fiber pellets, 250 microns or less in diameter, to the mixture during the fabrication process) in order to improve the cross conductivity of the compound. 
         [0039]    As shown in  FIG. 5 , the planar form of a carbon fiber contact element  60  can consist of a single layer, not a matrix of carbon fiber strands, arranged in a horseshoe shape or upside-down U to provide a continuous, unbroken path from one end  62  of the carbon fiber element strands, one of which is shown typically at  64 , to the other end  66 , even though the carbon fiber strands may change direction by more than 90 degrees. In this embodiment each carbon fiber strand  64  will be both perpendicular and parallel to the resistive or conductive track, not shown, and each opposing end  62 , 66  of the continuous carbon fiber strands  64  will essentially contact different parallel resistive or conductive tracks, not shown. The horseshoe-shaped contact  60  can employ a carrier, not shown, which can be electrically conductive or not, depending on the desired application. 
         [0040]    A similar construction is shown in  FIG. 6 , wherein the contact  70  has a right-angle transition portion  72  in the path from one end  74  to the other end  76 . 
         [0041]    In the embodiment shown in  FIG. 7 , a contact assembly  80  has a carbon fiber element formed as a very short strip  82  firmly and conductively attached at  84  by a conductive (or semi-conductive) adhesive to a parallel portion  84  of a thin beam  86  composed of electrically conductive material. This beam construction provides a means for the current or voltage signal to flow unimpeded from the resistive or conductive track to the end terminus, thereby incorporating the compatible and desirable characteristics of the carbon fiber contact material with beam members formed of materials other than carbon fiber. When this embodiment is in use, the carbon fiber element  82  will be essentially perpendicular to the plane of the resistive or conductive track at all times. 
         [0042]    In the exemplary embodiment shown in  FIGS. 2A ,  2 B, and  2 C, the planar form of the carbon fiber element consists of one or more parallel layers of carbon fiber strip arranged so that free ends  12 ,  16 ,  20  of the carbon fiber elements  10 ,  14 ,  18 , respectively, are designated as the ends that will contact the tracks of the resistive element or conductive element. 
         [0043]    Such ends  12 ,  16 ,  20  are preferably free of any other material, such as the low-resistance, synthetic resin compound or the like, for a length less than 3/16″ to permit only the actual carbon fiber material to contact the respective tracks, thereby providing improved mating between the ends  12 ,  16 ,  20  of the contacts  10 ,  14 ,  18  and the tracks, not shown, of the respective conductive elements. 
         [0044]    On the other hand, the portions of the carbon fiber elements which are free of the low-resistance synthetic resin compound may be, according to requirements of the particular application, more extended such that the free ends are more like fingers or rake ends, such as in the exemplary embodiment shown in  FIG. 12 . For example, width, thickness and length ratios of multiple independent fingers or rake ends may be select to obtain more optimal mechanical damping effects, such as in order to operate in high frequency modes that may include vibration and mechanical shock. 
         [0045]    While at least a portion of the carbon fiber elements may be encapsulated by (and bonded together in) the semi-conductive synthetic resin compound, the free ends may be obtained by protecting or shielding said free ends in the encapsulation process, or by trimming, or otherwise removing, any portion of encapsulating envelope that covers said ends as a result of the encapsulation process. Further, the free ends are preferably groomed to remove (that is, so as to be substantially free of) non-carbon fiber particles. 
         [0046]    The free end of the contact may remain parallel in the same plane or, as shown in  FIGS. 2A ,  2 B, and  2 C, the free end may be bent or formed to an angle perpendicular to the primary length of the strip or formed into a knuckle shape depending upon the application. 
         [0047]    In the embodiments shown in  FIGS. 8 ,  9 , and  10 , each contact or wiper element  90 ,  92 ,  94 , respectively, is fabricated in narrow strips of carbon fiber element, one of which is shown at  96 ,  98 ,  100 , respectively, wherein each strip is less than 0.015 of an inch in width and is composed or one or more parallel strands of carbon fibers. A number of these strips are arranged in a single flat plane, with each strip being essentially parallel to, but not fused or chemically bonded to, each other. The multiple independent parallel strips are mechanically captured by respective fastening parts (such as collars)  102 ,  104 ,  106 , in a single plane and/or chemically bonded with a low-resistance, semi-conductive synthetic resin compound at one end of the assembled strips, so that the independent multiple strip sections will be electrically uniform in their output signal and also be receptive to further assembly operations. 
         [0048]    As shown in  FIGS. 8 ,  9 , and  10 , the free ends  108 ,  110 ,  112  of the respective multiple strip sections  90 ,  92 ,  94  that are to function as the intimate contact points with the track of the resistive or conductive element can remain coplanar to the strip or be formed as a rake as shown in  FIG. 8 , a knuckle as shown in  FIG. 9 , or other compatible contact geometry, such as the point as shown in  FIG. 10 . This feature permits the assembly to contain multiple contact strips, such as  96 ,  98 ,  100 , each with relatively independent mechanical movement in a direction perpendicular to the resistive or conductive track of the substrate element. 
         [0049]      FIG. 11  is an embodiment similar to that of  FIG. 7  wherein multiple layers  120 ,  122 ,  124 , of carbon fiber elements are attached to a shorter leg  126  of an L-shaped carrier  128 . The carbon fibers in each layer  120 ,  122 ,  124  are substantially aligned to be parallel and the layers may be attached to the carrier by a semi-conductive synthetic resin compound shown generally at  130 . 
         [0050]    As shown in the embodiments of  FIGS. 3 ,  4 , and  11 , the electrical contact devices are formed of multiple layers of carbon fibers in various alignments. Similarly, other exemplary embodiments herein shown and described can be formed of multiple layers. So too, the various embodiments can be used with a carrier that can be electrically conductive or not, depending upon the desired application. 
         [0051]    Conversely, as shown in  FIG. 12 , an electrical contact or wiper  140  can be formed of only a single carbon fiber element  142  that can be around 0.010 to 0.015 inches in thickness. Although a rake end  144  is provided in this embodiment, any of the other end treatments described above are also appropriate. 
         [0052]    As noted hereinabove, all of the embodiments described so far can be formed from a composite carbon fiber material that has as its core a carbon fiber structure that has carbon fiber collections arranged in one layer, as in  FIGS. 2A-22C , or in multiple layers, as in  FIG. 3 . 
         [0053]    As shown in  FIG. 13 , a layer of the carbon fiber collections  150  has mats  152 ,  154  formed of nonwoven carbon fibers arranged on each flat side. Alternatively, only a single nonwoven carbon fiber mat could be employed. Although not shown in  FIG. 13 , following the placement of the mats  152 ,  154  on the carbon fiber collection structure  150 , a thermoplastic resin (or resin compound) is applied to the exterior surfaces of the mats  152 ,  154 . This thermoplastic resin, or polymer or resin compound, completes the structure and bonds the mats  152 ,  154  to the carbon fiber structure  150 , thereby forming a stable composite material with all of the carbon fiber material encapsulated in an elastomeric matrix, with only the carbon fiber tips being exposed. The nonwoven carbon-fiber mat  152  or  154  is substantially isotropic and the fibers are so randomly arranged as to provide little or no directionality in the plane of the mat. 
         [0054]    The nonwoven carbon fiber mat provides a primary electrical current carrying capacity and also provides improved mechanical strength to the overall construction. More specifically, the nonwoven carbon fiber provides off-axis mechanical stability and increase the spring rate characteristics of the structure, as well as off-axis current carrying capability, where the off-axis term relates to a longitudinal direction of the finally manufactured electrical contact. 
         [0055]    The nonwoven carbon fiber mat is available commercially from Hollingsworth &amp; Vose Company, East Walpole, Mass. and ranges in thickness from 0.08 mm to 0.79 mm. 
         [0056]      FIG. 14  is an end view of the assembled composite material  160  described above in which the nonwoven carbon fiber mats  152 ,  154  are arranged on the carbon fiber structure  150  and in which resin layer  162  is applied over the nonwoven carbon fiber layer  152  and a resin layer  164  is applied over the nonwoven carbon fiber mat  154  so that all of the carbon fiber materials are encapsulated in an elastomeric matrix, with only the working ends of the carbon fibers being exposed. This results in a stable composite material that can be formed to any desired shape, as described and shown in regard to the several embodiments shown herein. 
         [0057]    It is understood, of course, that the foregoing description is presented by way of example only and is not intended to limit the spirit or scope of the present invention, which is to be delimited by the appended claims.