Abstract:
A nonmetallic electrical contact or wiper composed of material such as carbon fiber, in which an electrical signal is transmitted along a length of the carbon fibers. The carbon fibers can be fused or conductively bonded together. The carbon fibers can be affixed to a carrier using various bonding and fastening techniques. The carrier can be electrically conductive or not depending on the application and can be affixed to the carbon fibers by bonding, fusing, or mechanical fastening. Alternatively, the electrical contact can be formed entirely from the carbon fiber material, with any carrier being also formed of carbon fiber material.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to an electrical contact or an electrical contact assembly typically used in an electromechanical device and, more particularly, to a contact or contact assembly that utilizes carbon fibers in various forms as the element that makes electrical contact with another element of the electromechanical device. 
     2. Description of Background 
     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 variable resistive devices are used in a dynamic state they can not be fixed or restricted in their movement and must have the freedom to be positioned along any length of their respective resistive or conductive paths. 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 and not produce polymers or debris which acts as an insulator and distorts the output signal. 
     Presently the contact or wiper materials used for these variable resistive devices are composed of various 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. 
     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. 
     Now that reduction in size, improved accuracy, 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. 
     Accordingly, the need exists for improvements in electrical contacts and contact assemblies and, particularly, for improvements in the materials and assemblies employed therefor. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a contact or contact assembly for use in electromechanical applications that can effectively eliminate the above-noted defects inherent in previously proposed systems. 
     It is another object of this invention to eliminate the above-described negative conditions and characteristics of previously known systems and to improve considerably the useful life of the system by providing a contact or wiper formed of nonmetallic material, such as one composed of carbon fibers. This carbon fiber material, through special processing, not only overcomes the negative conditions caused by metal composition contacts or wipers, but considerably improves total performance in all other aspects. 
     It is a further object of the present invention to provide a wiper contact or contact assembly for use in electro-mechanical components or applications that is more compatible with present state of the art fabrication techniques and materials used for resistive and conductive track substrates and that appreciably reduces or eliminates the negative aspects inherent in presently used or previously proposed designs or materials. 
     In accordance with one aspect of the present invention an existing contact carrier is employed and in place of the previously used metal contacts, carbon fibers are employed that are specially attached to a carrier. 
     According to one aspect of the present invention, a nonmetallic electrical contact, such as one made of carbon fiber material, is processed and formed in such a manner as to allow the multiple strands of carbon fiber when properly positioned to be electrically conductive for transmitting umimpeded electrical signals along its longitudinal length. Such carbon fiber strands may be fused or conductively bonded by any of various techniques to provide essentially uniform conductivity and redundant transmission of the electrical signal. The carbon fiber material can be affixed to a carrier or the fibers may be utilized without a carrier. Such a carrier, if used, may be metallic or non-metallic and may be affixed to the carbon fiber bundles 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 carbon fiber material as that used for the actual contact. Forming of the carbon fiber contact can involve cross-layering of the material in nonparallel orientations to provide additional structural integrity, as well as to assist in the postforming operation. 
     The inventive 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. 
     A further aspect of the present invention is that 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. 
    
    
     The above and other objects, features, and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof to be read in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1D are side elevations showing respective embodiments of electrical contacts according to the present invention; 
     FIGS. 2A-2C are front elevations and respective enlargements showing embodiments of electrical contacts corresponding to FIGS. 1A-1C, respectively; 
     FIG. 3 shows two views of a carbon fiber contact formed as a matrix of layers of carbon fibers; 
     FIG. 4 shows two views of a carbon fiber contact formed as a matrix of layers of carbon fibers; 
     FIG. 5 shows two views of an electrical contact formed solely of carbon fibers according to an embodiment of the present invention; 
     FIG. 6 shows two views of an electrical contact formed solely of carbon fibers according to another embodiment of the present invention; 
     FIG. 7 shows two views of a carbon fiber electrical contact affixed to an electrically conductive beam according to an embodiment of the present invention; 
     FIG. 8 shows two views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused accordingly to an embodiment of the present invention; 
     FIG. 9 shows two views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused according to an embodiment of the present invention; 
     FIG. 10 shows two views of an electrical contact in which the carbon fibers are mechanically captured and chemically fused accordingly to an embodiment of the present invention; 
     FIG. 11 shows two views of an electrical contact employing multiple layers on a carrier according to an embodiment of the present invention; and 
     FIG. 12 shows two views of an electrical contact formed as a single carbon fiber element. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention provides a contact or wiper element for transmitting electrical signals, either in a low voltage mode (under 15 volts) or a low current mode (under 500 ma), between a resistive and/or a conductive track and some external circuit termination. In one embodiment-the 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. 
     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 . 
     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. 
     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 bundles forming the specialized end constructions  12 ,  16 , and  20 , respectively. That is, the enlargement of FIG. 2A shows carbon fiber bundles  26  arranged in one layer forming the rake end  12 . Similarly, bundles  28  and  30  respectively form knuckle end  16  and pointed end  20  in FIGS. 2B and 2C, respectively. 
     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 inadjacent layers are essentially nonparallel and may be perpendicular to each other. 
     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 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. 
     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 very low resistance synthetic resin compound to restrict movement, add structural stability, and provide multidirectional electrical continuity. 
     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. 
     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 . 
     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 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. 
     In the embodiment of the present 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 the 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. It is a feature of the present invention that those ends  12 ,  16 ,  20  can be fabricated free of any other material, such as the low-resistance, synthetic resin compound or the like, for a length less than {fraction (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. 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. 
     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 collars  102 ,  104 ,  106 , in a single plane and/or chemically bonded with a low-resistance, electrically 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. 
     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. 
     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 an electrically conductive synthetic resin compound shown generally at  130 . 
     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, all other embodiments herein shown and described can be formed of multiple layers. So too, the various embodiments of the present invention can be used with a carrier that can be electrically conductive or not, depending upon the desired application. 
     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. 
     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 defined by the appended claims.