Abstract:
A method for resistance projecting welding the components of a current sensor is disclosed. The method comprises the steps of providing a first blade having a projection extending outward and a first conductor, supporting the first blade and the first conductor, positioning the first blade adjacent to the first conductor so that the projection of the first blade contacts an opposed surface on the first conductor defining a contact area, supplying an electrical current of a predetermined magnitude and predetermined duration to the first blade and the first conductor to create a weld at the contact area and continually applying a compressing force of a predetermined magnitude to the first blade throughout the supplying step to resistance projection weld the first blade and the first conductor at the contact area. The present method overcomes the drawbacks of the prior art by providing an alternative method to fabricate current sensors. Finally, the method reduces part production costs and increases part reliability while obtaining desired production rates.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is based upon, and claims the benefit of, U.S. Provisional Patent Application No. 60/189,530 filed on Mar. 15, 2000, which is herein incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the field of electric meters and, more particularly, to sensors utilized in metering devices. 
     Many electrical devices, such as electronic electric meters and induction or electronic type watt-hour meters for measuring electric power and usage, require current sensors for sensing the line current and producing an output signal related to the line current regardless of temperature. 
     Present sensor fabrication methods include electron beam welding of copper blades to nickel-copper resistors. While the prior art has been effective for many years, electron beam welding is not an economical means of fabricating sensors. Further, consistency of electron beam welding between different suppliers is difficult to maintain. 
     Previously, projection welding, and more specifically resistance projection welding, was not typically utilized when it was desired to bond a copper part to a copper nickel part. Copper traditionally does not withstand high pressures as is typically found with a projection welding. Further, when applying a projection welding technique to copper and copper alloy parts, the parts are heated to the liquid state. When the current ceases to flow through the parts, the parts can collapse while in the liquid state. Also, current sensors require consistent metallurgical bonding, as well as impedance consistency on each side of the conductors utilized within the current sensor. These characteristics are not attributable to projection welding due to the inconsistencies in the weld penetration. 
     SUMMARY OF THE INVENTION 
     The above discussed and other drawbacks and deficiencies are overcome or alleviated by the present invention. 
     In an exemplary embodiment of the present invention, a method for resistance projection welding the components of a current sensor is disclosed. The method comprises the steps of providing a first blade having a projection extending outward and a first conductor, supporting the first blade and the first conductor, positioning the first blade adjacent to the first conductor so that the projection of the first blade contacts an opposed surface on the second member defining a contact area, supplying an electrical current of a predetermined magnitude and predetermined duration to the first blade and the first conductor to create a weld at the contact area and continually applying a compressing force of a predetermined magnitude to the first blade throughout the supplying step to resistance projection weld the first blade and the first conductor at the contact area. 
     The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following drawings and detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the following FIGURES, in which: 
     FIG. 1 is an isometric view of a meter assembly and current sensors manufactured according to the present invention; 
     FIG. 2 is an isometric view of the current sensor of FIG. 1; 
     FIG. 3 is a front plan view of a first blade of the current sensor of FIG.  2  and projections; 
     FIG. 4 is an enlarged cross section view of the current sensor of FIG. 2 showing the conductors and a current comparator; 
     FIG. 5 is a side view of the current sensor of FIG. 2 showing welds; and 
     FIG. 6 is a side view of the resistance welding components employed for carrying out the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, an electronic meter  10  for measuring electricity according to the present invention is shown. As would be readily understood by those skilled in the art, the meter  10  can measure watt-hours, var-hours, or other quantities. The meter  10  includes a cover  14 , a base  16 , a nameplate  12 , a bezel  28 , and circuit boards  26 . The cover  14  attaches to the base  16  forming a housing to enclose the nameplate  12 , bezel  28  and circuit boards, as is generally known in the art. The housing may have other shapes, such as generally rectangular as would also be readily understood by those skilled in the art. 
     The meter  10  also includes two current sensors  18 ,  20  positioned in side-by-side relation within the cover  14 . The present invention however is not limited to a two current sensors but may be applied to other configurations, such as a three current sensors. Current sensors  18 ,  20  comprise an electrically conductive plate  30  having first and second blades  38 ,  40 . First and second blades  38 ,  40  are preferably made of copper. First blade  38  functions as a line terminal and second blade  40  functions as a load terminal. 
     The meter  10  is connected to a panel  22  typically mounted on the exterior of a building. An incoming power cable  24  enters the panel  22 . Panel  22  electrically connects the meter  10  to the incoming power cable  24  in order to measure the amount of electricity consumed. The meter  10  is electrically connected to the power cable  24  by insertion of the first and second blades  38 ,  40  through corresponding openings (not shown) within the base  16  and then into corresponding sockets  58  within the panel  22  as would be understood by those skilled in the art. 
     Referring to FIGS. 2 and 3, the current sensor  18  located on the left side of the circular base  16  (FIG. 1) will now be described. However, it is understood by those skilled in the art that current sensor  20  includes similar elements. 
     First blade  38  includes a main body portion  142  having an outer edge  100  and an inner edge  144  with a flat surface  102  therebetween. A similar flat surface (not shown) opposes flat surface  102 . Depending from the inner edge  144  of the main body portion  142  of the first blade  38  is a first arm  74  and a second arm  76 . First arm  74  is substantially parallel to second arm  76 . First and second arms  74 ,  76  each include a fixed end  148  continuous with the main body portion  142  and an unattached free end  152 . The unattached free ends  152  of first and second arms  74 ,  76  of first blade  38  include edges  92 ,  94 , respectively. 
     Second blade  40  includes a main body portion  156  having an outer edge  104  and an inner edge  146  with a flat surface  106  therebetween. A similar flat surface (not shown) opposes flat surface  106 . Depending from the inner edge  146  of the main body portion  156  of the second blade  40  is a first arm  78  and a second arm  80 . First arm  78  is substantially parallel to second arm  80 . First and second arms  78 ,  80  each include a fixed end  150  continuous with the main body portion  156  and an unattached free end  154 . The unattached free ends  154  of first and second arms  78 ,  80  of second blade  40  include edges  96 ,  98 , respectively. 
     First arm  78  of second blade  40  includes a projection  136  extending along edge  96  at the unattached free end  154 . Second arm  80  of second blade  40  includes a projection  108  extending along edge  98  of the unattached free end  154 . Preferably, projections  108 ,  136  are ninety degree angled projections. Projections  108 ,  136  are preferably produced using a stamping process. 
     In like manner but not shown, edges  92 ,  94  of first and second arms  74 ,  76  of the first blade  38  also include projections  108 ,  136 , respectively. 
     Current sensor  18  also includes a first conductor  46  located between edge  94  of arm  76  and edge  98  of arm  80  and a second conductor  48  located between edge  92  of arm  74  and edge  96  of arm  78 . First and second conductors  46 ,  48  are generally cylindrical shaped with an outer surface  128 . First and second blades  38 ,  40  and first and second conductors  46 ,  48  define plate  30 . First and second conductors  46 ,  48  are preferably made of a nickel and copper. Plate  30  includes an opening  60  in a medial portion thereof defining a first current branch  34  at an upper end  52  of plate  30  and a second current branch  36  at a lower end  42  of plate  30 . First and second current branches  34 ,  36  conduct first and second portions of an input electrical current and are positioned adjacent opposing sides of opening  60 . Preferably, first conductor  46  is a resistor having a predetermined resistance and second conductor  48  is a resistor having a predetermined resistance. 
     Referring to FIG. 4, current sensor  18  includes a current comparator. Current comparator includes a coil  50  disposed around a core  54 , for measuring electricity. An electrical conductor  44 , preferably a resistor having a predetermined resistance, extends between the first and second current branches  32 ,  34  through a central opening  56  of the core  54 . When current is passing through the conductor  44 , the conductor  44  develops magnetic flux in the core  54 , which, in turn, develops an electric output signal in the coil  50  that is proportional to the current flowing through the plate  30 . 
     Referring to FIG. 5, a cross-section view of the current sensor  18  is shown depicting welds  82 ,  84 ,  86 ,  88  on both sides of first and second conductors  46 ,  48 , respectively. More specifically, the welds  82 ,  84 ,  86 ,  88  are located between edges  94 ,  98 ,  92 ,  96 , respectively, and the adjacent sides of the first and second conductors  46 ,  48 , respectively. Welds  82 ,  84  locate the first conductor  46  in the first current branch  34 . Welds  86 ,  88  locate the second conductor  48  in the second current branch  36 . In this way, first conductor  46  is integrated into the first current branch  34  and second conductor  48  is integrated into the second current branch  36 . 
     Referring to FIGS. 2,  5  and  6 , a method for welding the first and second conductors  46 ,  46  will be detailed. First and second blades  38 ,  40  are welded to the first and second conductors  46 ,  48 , respectively by use of a resistance projection welding method. FIG. 6 shows the resistance projection welding components employed in the following method. The first and second blades  38 ,  40  and first and second conductors  46 ,  48  are not shown. 
     First and second blades  38 ,  40  are placed in holding members  110 , preferably gripping devices suitable to secure the flat surfaces  102 ,  106  of the first and second blades  38 ,  40 , respectively. The holding members  110  are supported by a main body (not shown) of a welding apparatus, shown generally at  140 , and maintain first and second blade  38 ,  40  alignment and restraint during the welding method. In particular, arms  74 ,  76 ,  78 ,  80  are restrained such that equal force can be applied to the projections along edges  92 ,  94 ,  96 ,  98 , respectively. Holding members  110  also conduct the input electrical current to the first and second blades  38 ,  40  during the welding method. 
     The first and second conductors  46 ,  48  are then placed within respective die enclosures  112  that are located directly under the first and second blades  46 ,  48 . Prior to welding, edges  94 ,  98  and corresponding projections  108  located along edges  94 ,  98  are positioned in opposing relation to each other and engage surface  128  of first conductor  46 . Similarly, edges  92 ,  96  and corresponding projections  136  located along edges  92 ,  96  are positioned in opposing relation to each other and engage surface  128  of second conductor  48 . The first and second conductors  46 ,  48  are fixedly positioned within the die enclosures  112  during the welding method. 
     First and second conductors  46 ,  48  are simultaneously welded to the first and second blades  38 ,  40  during one operation as described below. 
     An upper cylinder  114 , preferably pneumatic, is movably connected to a rod  124  that is connected to a holding device  126 . Upper cylinder  114  is lowered onto the flat surfaces  102 ,  106  of first and second blades  38 ,  40  and positioned adjacent to a first contact area  64 , a second contact area  130 , a third contact area  132  and a fourth contact area  134  as defined by the respective edges  92 ,  94 ,  96 ,  98  that engage the outer surfaces  128  of the respective first and second conductors  46 ,  48 . The upper cylinder  114  applies a downward clamping force on the flat surfaces  102 ,  106  of the first and second blades  38 ,  40  of at least about 2000 pounds during the weld cycle. Two side cylinders  116 , preferably pneumatic, are moved horizontally to engage outer edge  100  of first blade  38  and outer edge  104  of second blade  40 . The side cylinders  116  provide a compressive force to the outer edges  100 ,  104  of the first and second blades  38 ,  40 , respectively, of at least about 2000 pounds. Preferably, the compressive force applied by the side cylinders  116  is approximately about 3200 pounds to 3700 pounds for the duration of the weld cycle. The upper cylinder  114  ensures against buckling of the first and second blades  38 ,  40  under the application of the compressive forces applied by the side cylinders  116 . The high compressive forces are required to ensure adequate conductivity at the weld interface to support the high current required to heat the first and second blades  38 ,  40  and thereby weld the first and second conductors  46 ,  48  between the respective edges  92 ,  94 ,  96 ,  98  of arms  74 ,  76 ,  78 ,  80 . 
     The welding cycle is initiated by a high current that flows through the first and second iblades  38 ,  40  causing local heating at the interface with the first and second conductors  46 ,  48 . The electrical current is applied to the first and second blades  38 ,  40  for only an extremely short duration, which duration is only about one-half to about one of an alternating current wave cycle. For a one cycle weld time, a high voltage of approximately about 20 volts alternating current (vac) corresponding to a 63 kilo ampere (kA) input current level for approximately about one sixtieth of a second (weld cycle) is initiated by a transformer  118 . For a one-half cycle weld time, a high voltage of approximately about 20 vac corresponding to an 80 kA input current level for approximately about one twentieth of a second (weld cycle) is initiated by the transformer  118 . Transformer  118 , as shown in FIG. 6, is electrically connected to a fixture (not shown) that is actuated by side cylinders  116 . The power supply to the transformer  118 , and hence the welding current and its duration supplied thereto to the holding members  110 , is controlled by a control unit  120 . 
     Since the high level of input current is supplied in very short time duration, it is necessary to collapse the projections  108 ,  136  (FIG. 3) before the first and second blades  38 ,  40  and first and second conductors  46 ,  48  collapses due to prolonged exposure to high temperatures created by the current flow. This is achieved by the use of spring assemblies  122  that are mounted to holding device  126  and act to rapidly remove the compressive forces from the first and second blades  38 ,  40  once the predetermined weld cycle is complete. Spring assemblies  122  are continuously biased for rapid follow-up movement once the projections  108 ,  136  collapse. The spring assemblies  122  significantly lower the moving mass of the holding device  126  during projection collapse from about 300 pounds to less than about 10 pounds. This permits the weld cycle to be reduced to one-half cycle while maintaining the necessary forces exerted on the first and second blades  38 ,  40  and consequently, on the first and second conductors  46 ,  48  during the short duration weld cycle. 
     The welds  82 ,  84 ,  86 ,  88  are completed by the application of high compressive forces exerted on the first and second blades  38 ,  40  by the side cylinders  116 . The application of the compressive forces coupled with the heating of the projections  108 ,  136  (FIG. 3) as well as the first and second blades  38 ,  40  in the immediate vicinity of the projection, effect the collapse of the heated projections  108  (FIG. 3) to thereby create welds  82 ,  84 ,  86 ,  88 . The method produces welds  82 ,  84 ,  86 ,  88  that have a weld (joint) strength to be able to withstand handling, meter assembly and testing. The welds  82 ,  84 ,  86 ,  88  have tensile strengths equivalent to that of copper nickel. The welds  82 ,  84 ,  86 ,  88  also do not exhibit severe expulsion as this can cause unwanted debris during meter assembly. The resistance projection welding method employs the necessary high forces to support the high current that is required to effect the welding of the first and second conductors  46 ,  48  to the first and second blades  38 ,  40 . Further, the application of high pressure from the upper cylinder  114  and the side cylinders  116  in a short duration ensures minimum weld penetration as the weld heat effected zone does not penetrate through the first and second blades  38 ,  40 . Homogenous properties within the first and second conductors  38 ,  40  are achieved. 
     At the conclusion of the welding method as described hereinabove, the current sensor  18  is held in place until welds  82 ,  84 ,  86 ,  88  are cooled. The upper cylinder  114  retracts, the side cylinders  116  retract and the current sensor  18  is removed from the holding members  110  and the die enclosures  12 . 
     The method disclosed herein advantageously employs a resistance projection welding method to bond first and second conductors  46 ,  48  to first and second blades  36 ,  38 ,  40  of current sensor  18 . Resistance projection welding for current sensor  18  provides an inexpensive and reliable method as compared electron beam welding, to simultaneously weld the first and second conductors  46 ,  48  into the first and second current branches  36 ,  38 , respectively. To effect the bond using a resistance projection welding method, a high voltage source is required to yield a high current sufficient to effect the welding at the interfaces of the first and second conductors  46 ,  48  to the first and second blades  38 ,  40 . High conductivity is required at the interfaces to maintain contact during the welding method. To support the high current flowing through the first and second current branches  34 ,  36 , the high clamping force is applied from the upper cylinder  114  proximate to the first and second conductors  46 ,  48 . Potential buckling of the first and second blades  38 ,  40  under the high compressive forces exerted by the two side cylinders  116  is thus averted while ensuring weld integrity. 
     The restraint of the first and second conductors  46 ,  48 , as well as the first and second blades  38 ,  40 , during the welding method ensures that the mechanisms behind the projection collapse are controlled properly and the local forging characteristics of the cooper at the bond lines are not compromised. In this way, small variances between the welds  82 ,  84 ,  86 ,  88  at the bondline have large effects on the repeatability of current sensor  18  thermal response. 
     The method described herein above welds the first and second conductors  46 ,  48 , made of nickel and copper, to first and second blades  36 , that are made of copper, respectively. The resistance for the first and second current branches  34 ,  36  are equal thus ensuring impedance consistency and proper sensor performance. This is accomplished by producing welds  82 ,  84 ,  86 ,  88  using a high pressure, short cycle resistance projection welding method. The resistance projection welding method produces welds  82 ,  84 ,  86 ,  88  at a lower cost than traditional electron beam welding and has the necessary quality and reliability comparable to welds produced with traditional welding methods. By developing a resistance projection welding method for use in manufacturing a current sensor  18 , desired production rates are achieved, part fabrication costs are substantially reduced and sensor performance is consistent and reliable during production. Finally, joint strength is great enough to withstand handling, meter assembly and testing. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.