Abstract:
Methods and circuits for reduction of errors in a current shunt are disclosed, for example sensing lines for Kelvin sensing in which the sensing lines are of identical material to the high-resistance portions of the shunt, and welded thereto. This allows application of a current shunt with lower output voltage and thus lower power losses than the contemporary art implementations, while maintaining high accuracy with regard to temperature changes.

Description:
BACKGROUND  
       [0001]    It is not easy to measure currents accurately and over large dynamic ranges. 
         [0002]    The most common method of current sensing is to pass the current through a resistor (a current shunt) and to measure the resulting voltage drop, which develops according to Ohm&#39;s law. A well-known current sensor circuit based on this principle is illustrated in  FIGS. 1   a,    1   b,  and  1   c.    
         [0003]    Input terminals  3 / 3   a  and  4 / 4   a  allow connection of the current shunt  1  into the circuit where current has to be measured.  FIG. 1   a,  as an example, shows that the actual connection points on the input terminals are simple holes  3   a  and  4   a  to which properly terminated power cable can be attached, both electrically and mechanically, by means of bolts and nuts or other suitable fasteners. This configuration is typical of current sensors capable of measuring several tens to several hundreds of Amperes. 
         [0004]    An electronic circuit (omitted for clarity in  FIG. 1   a ) measures voltage across the current shunt  1 , via the sense lines  9  and  10 ; the actual current value is derived by the Ohm&#39;s law with the knowledge of the shunt&#39;s resistance. 
         [0005]    Pick-up points  7  and  8  on the current shunt  1  follow the principle of “Kelvin sensing” that reduces errors associated with resistance of the sense connections and wires, considering the fact that there is almost no current in these sensing connections. The point made by “Kelvin sensing” is that because the current in lines  9  and  10  is extremely small, the voltage drop along lines  9  and  10  is likewise extremely small, and thus the voltage drop along lines  9  and  10  does not introduce very much error in the overall current measurement process. It will be appreciated that the pick-up points  7  and  8  are separate from and are specifically located apart from the main terminals  3   a / 4   a  of the shunt. 
         [0006]    Typically, the shunt is created by joining three conducting sections with varying conductive properties. Sections  3  and  4  are made from highly conductive material (typically copper), and central section  2  is made from a material that has higher resistance as compared with that of copper, a material whose resistance has little or no dependence upon the magnitude of the current passing through the material, and the material having a resistance that has little or no dependence upon the temperature of the material. Some investigators choose this material to be “manganin”, an alloy of typically 86% copper, 12% manganese, and 2% nickel. The reasons for such a construction, among a few, include the desire to equalize the current density in the resistive material  2 , and to minimize errors arising out of resistance variations due to magnitude of current or due to changes in temperature. The choice to have a central section differing in its material from end sections, and the choice of particular material for that central section, are outside of the scope of the present discussion, and as will later be appreciated, the teachings of the invention offer their benefits in ways that are not dependent upon such choices. 
         [0007]    The thoughtful reader will appreciate that in the particular case where a shunt is selected to have such a central section  2  of non-identical material from the end sections, the shunt amounts to a thermocouple circuit, with the junctions of the thermocouples created by the joining of dissimilar materials in areas  5  and  6 , and schematically depicted in the electrical model in  FIG. 1(   b ). Temperature difference between areas  5  and  6  will create thermoelectric voltage on the sense lines  9  and  10 ; unfortunately, this voltage creates an error in the measured voltage across the shunt, and thus creates an error in the ultimate derived value for the measured current. 
         [0008]    Furthermore, the attachment method of the sense lines  9  and  10  at points  7  and  8  may have a large effect on the total thermoelectric errors, as illustrated in  FIGS. 1   b  and  1   c.  Typical sense lines  9  and  10  are made from copper and may, in fact, be simply traces on a printed circuit board (PCB). Connections to the current shunt at points  7  and  8  are typically made by soldering the sense lines to the shunt; solder material  7   b  in  FIG. 1  c is located between copper section  3  and sense line  9 ; at areas  7   a  and  7   c  this creates another pair of thermocouples. Any temperature difference between areas  7   a  and  7   c  will produce additional voltage errors. 
         [0009]    The same considerations apply for the thermocouples created at junction  8  and shown schematically in  FIGS. 1   b  as  8   a,    8   b,  and  8   c.    
         [0010]    Stating the situation in a different way and looking at  FIG. 1   b  it is clear that thermoelectric voltages described above are connected in series with the voltage to be measured in element  2 , thus giving rise to errors in the measured voltage and in the derived current value. 
         [0011]    The designer of a shunt as shown in  FIG. 1   a  must, of course, select locations  7  and  8  for the connection points for lines  9  and  10 . Locations  7  and  8  will usually be selected to lie along a center line. Likewise nut-and-bolt connection points  3   a  and  4   a  will usually be selected to lie along the center line just mentioned. Finally, the designer of the shunt will usually be seen to design the shunt generally to have more or less reflective symmetry about the center line (to the left and to the right in  FIG. 1   a ); usually this reflective symmetry is due not so much to any conscious choice as merely to simplicity and ease of manufacture. 
         [0012]    The designer of the shunt as shown in  FIG. 1  a must also select locations  7  and  8  (vertically in  FIG. 1   a ) along the center line. The usual design choice is to maximize the voltage output at sense lines  9  and  10  so as to maximize the signal-to-noise ratio for the sensed voltage. To maximize the voltage, the designer selects locations  7  and  8  to be farther apart from each other rather than closer together. The locations  7  and  8  are selected to be less far apart than nut-and-bolt connections  3   a  and  4   a,  but still quite far apart. It is not desired that location  7  be particularly nearby to point  3   a  and it is not desired that location  8  be particularly nearby to point  4   a.    
         [0013]    It would be very desirable if apparatus could be devised which would reduce or eliminate errors in the derived current value that arise because of such thermoelectric voltages. 
       SUMMARY OF THE INVENTION 
       [0014]    Structures are described in which identical or substantially identical materials are used for sense lines as for the shunt materials to which the sense lines are electrically connected, and in which many if not all thermocouple-induced errors are eliminated when compared with prior-art structures. Methods and circuits for reduction of errors in a current shunt are disclosed, for example sensing lines for Kelvin sensing in which the sensing lines are of identical material to the high-resistance portions of the shunt, and welded thereto. This allows application of a current shunt with lower output voltage and thus lower power losses than the contemporary art implementations, while maintaining high accuracy in the face of temperature changes. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0015]    This invention will be described with respect to a drawing in several figures, of which: 
           [0016]      FIGS. 1   a,    1   b,  and  1   c  depict prior-art circuits; 
           [0017]      FIGS. 2   a ,  2   b ,  2   c  and  2   d  disclose one method of the invention; 
           [0018]      FIGS. 3   a  and  3   b  show a current shunt with expanded capabilities according to the invention; and 
           [0019]      FIGS. 4   a  and  4   b  display an alternate simplified embodiment that could be used under specific limited conditions. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    One purpose of the invention is to reduce the errors associated with thermoelectric voltages in the current shunt. In the simplest form (referring to  FIGS. 2   a  and  2   b ), the method according to this invention requires simultaneous fulfillment of several requirements, namely:
       a.) Connecting points  15 / 16  for the sense lines  19 / 20  must be located on the element  2  of the shunt  11  (and not on the elements  3 / 4  as in the previous art).   b.) Material of the sense lines  19 / 20  must be the same as the material of the resistive element  2 .   c.) The attachment technique between the element  2  and sense lines  19 / 20  must not introduce any other materials into the joint. A preferred method of attachment will soften and liquefy small adjacent areas of both element  2  and sense lines  19 / 20 ; after cooling down element  2  and sense lines  19 / 20  will remain attached to each other and will maintain low resistance of the attachment points, as illustrated in  FIG. 2   c . Suitable industrial processes may include welding by application of heat or localized electrical current, a so-called technique of Electrical Resistive Welding (ERW), also known as “Spot Welding”, “Electrical Discharge Welding”, “Flash Welding”; methods that apply high-frequency RF, plasma, high-intensity electron beams, and X-ray energy may be utilized as well; an underlying requirement is that the selected process applies the energy in a small and well-defined area, and causes localized softening, liquefaction, and diffusion of the materials of the element  2  and sense lines  19 / 20 , without significant effect on the rest of the element  2  and sense lines  19 / 20 , as these may have been previously processed by thermal treatment in order to obtain the required properties for utilization in the current shunt.   d.) At the input ports  24  of electronic measuring apparatus (detail omitted from  FIG. 2   a  for clarity), both sense lines  19 / 20  must terminate in the area  23  that assures equal temperature of both junctions  21 / 22  between sense lines  19 / 20  and input lines  24  of the measuring apparatus; this arrangement may be termed an “isothermal block”.   e.) Sense lines  19 / 20  should extend well past the surface and/or the edge of the Current Shunt  11 , in order to provide a measure of thermal isolation between the Current Shunt body and the isothermal block  23 ; for the same reason, it is advantageous if the material of the sense lines  19 / 20  has relatively high thermal resistance; in other words, it is preferred that the material of elements  19 / 20  should not easily conduct heat. On one hand, as the heat in metals and metal alloys is typically transferred by the same transport mechanism as the electricity, by the electrons, if the material of sense lines  19 / 20  has lower conductivity than sections  3 / 4  then it will have corresponding lower ability to conduct heat; on the other hand, if cross-section of sense lines  19 / 20  is small, it will also impede heat conduction. An optimal configuration would depend on the particular construction of the current shunt and the intended use environment. Furthermore, additional means of thermal isolation could be applied between the shunt  11  and isothermal block  23 ; in particular, if block  23  is located on the PCB that is also attached to the shunt  11 , then a strategically located slot in the PCB substrate will increase the thermal isolation of block  23  from shunt  11 ; referring to  FIG. 2   d , utilization of an advantageously shaped slot  26  in the PCB  27  will help in creating a low thermal differential between points  21  and  22 , thereby improving functions of the isothermal block  23 .       
 
         [0026]    When all the above requirements a.) through e.) are fulfilled, the measurements from the resulting current shunt  11  should be substantially free from thermoelectric errors. 
         [0027]    An alert reader will notice that voltage-sensing points  15 / 16  according to the invention are located contrary to the previous-art method of maximizing the voltage output from the current shunt. Saying this another way, in  FIG. 1   a  the points  7  and  8  were described as being as far apart from each other as could be arranged while still keeping point  7  away from point  3   a  and while still keeping point  8  away from point  4   a.  In contrast in  FIG. 2   a  the points  15  and  16  are closer together (in a topological sense) than points  7  and  8  in  FIG. 1  a. In  FIG. 1  a, the points  7  and  8  have junctions  5  and  6  between them, while in  FIG. 2   a , the junctions  5  and  6  have points  15  and  16  between them. Depending on the materials choices in  FIG. 1   a  and in  FIG. 2   a , and depending on choices of dimensions of the shunt in  FIG. 1   a  and in  FIG. 2   a , it may well work out that the sense voltage developed from a given amount of current to be measured is smaller in  FIG. 2   a  than in  FIG. 1   a.  It might be thought that this would make the sensing arrangement of  FIG. 2   a  less accurate than the arrangement of  FIG. 1   a  (due to a poorer signal-to-noise ratio) but such is not the case. 
         [0028]    Indeed the output voltage of the shunt according to the invention may be somewhat smaller compared to the output voltage of the shunt according to the prior art, while still providing improved accuracy as compared with the prior-art arrangement, even if all other parameters of the shunt and the passing current through the shunt are the same. 
         [0029]    One of the points being made here is that the improved shunt offers an output signal that is effectively free from thermoelectric errors, and that some reduction in the output signal (as compared with the prior art) can be tolerated and readily compensated for by the electronic measuring apparatus; for example, such measuring systems are described in U.S. Pat. No. 8,264,216 entitled “High-accuracy low-power current sensor with large dynamic range” and in published international patent application WO12/117275 entitled “Current sensor”. 
         [0030]    In addition, a signal that is not contaminated with thermoelectric errors can be much smaller for the same signal-to-error ratio than a large signal that has substantial error component; this allows significant reduction of energy loss due to heating in the shunt. Saying this another way, the improved signal-to-noise ratio offered by the teachings of the invention permit the shunt designer to design it so as to have a smaller resistance. The smaller resistance means less I 2 R loss in the shunt and thus less heating. Less heating means less energy loss due to the heating, and means smaller temperature-related errors introduced into the measurement process. Compared with prior art, this invention allows one or more orders of magnitude of improvement for the losses in the shunt. 
         [0031]      FIG. 3   a  discloses a current shunt  12  that can supply information about the actual operating temperature of the shunt element  2 , in addition to reduced thermoelectric errors. 
         [0032]    Two extra sensing points are created at locations  15   a  and  16   a,  with material of the leads  19   a / 20   a  being copper or other suitable material, but not the same material as the shunt element  2  or leads  19 / 20 . The attachment method should be the same as outlined in requirement c.) above. 
         [0033]    The alert reader that appreciated the existence of thermocouples at  5  and  6  in  FIG. 1  will immediately recognize that points  15   a  and  16   a  in  FIG. 3   a  create thermocouples as well. Such thermocouples have outputs which can be sensed via leads  19   a / 20   a  (and, of course, utilizing leads  19 / 20 ). This allows for the sensing of the temperature at  15   a  and  16   a;  in turn, knowledge of the temperature of the shunt element  2  permits compensation for the changes of resistance of element  2 , and possibly eradication of residual thermoelectric errors due to slightly different material properties between leads  19 / 20  and element  2 . 
         [0034]      FIG. 3   b  presents an electrical schematic model of the shunt  12  in  FIG. 3(   a ). 
         [0035]    While voltage sensing points  15 / 16  are preferably located on the center line of the shunt  12  (in accord to the best current measurement functionality), the points  15   a  and  16   a  are located on the lines emanating from points  15 / 16  and perpendicular to the center line of the shunt; the intent here is that point  15   a  is located at such a position on the section  2  that its temperature is substantially the same as the temperature of point  15 ; likewise for points  16   a  and  16  (the reader is reminded that the temperatures of points  15  and  16  are not necessarily the same).  FIG. 3   a  shows the center line by dashed and dotted lines passing through holes  3   a  and  4   a.    
         [0036]    At the same time any voltage differential from point  15   a  to point  15 , and between  16   a  and  16 , generated due to the current passing through the shunt, is nearly zero. 
         [0037]    Stated differently, pair  15   a / 15  is located on what are hoped to be isopotential and isothermal lines (as present on element  2  due to temperature gradients and the current passing through the shunt); likewise for pair  16   a / 16 . 
         [0038]    A further improvement of the shunt in  FIG. 3  is to introduce another thermocouple sensing point exactly in the middle of element  2  between points  15  and  16 . Such a sensing point is omitted for clarity in  FIG. 3   a . If such a thermocouple is present, then better knowledge of the temperature distribution across the volume of shunt element  2  can be gained, with correspondingly better ability to compensate for changes due to temperature in the resistance of element  2 . 
         [0039]      FIG. 4  shows another configuration of the shunt according to the invention. While this arrangement is not capable of complete reduction of thermoelectric errors under all possible conditions, it could still achieve significant reduction of thermoelectric errors, and may be desirable due to simpler construction. 
         [0040]    In  FIG. 4   a  the sense points  17 / 18  are located respectively on the portions  3  and  4  of the shunt. The sense leads  19 ,  20  are made from the same material as the shunt element  2 . Such an arrangement creates a thermocouple at point  17 , and a thermocouple at point  18  in addition to a thermocouple between element  2  and sections  3  at area  5 , and a thermocouple between element  2  and section  4  in the area  6 . This is illustrated in electrical schematic model form in  FIG. 4   b.    
         [0041]    Isothermal block  23  keeps the temperature of the leads  19 / 20  the same at joints  21 / 22  to the input lines  24  of the electrical measuring apparatus (omitted for clarity in  FIG. 4   a ). 
         [0042]    The output voltage of the shunt consists of the sum of the voltage across the shunt due to the passing current, together with the voltages developed across the thermocouple pairs  17 / 5  and  6 / 18 . As the distance between area  5  and point  17  (likewise, area  6  to point  18 ) is much smaller than the distance between area  5  and  6 , the magnitude of the temperature difference is much smaller between  5  and  17  (also,  6  and  18 ) as compared with the likely temperature differential from  6  to  5 . Correspondingly, the magnitude of generated thermoelectric voltage in pairs  5 / 17  and  6 / 18  is much smaller than the generated voltage in pair  5 / 6 . Therefore, the sum of the thermoelectric error voltages from pairs  5 / 17  and  6 / 18  will be smaller than the error from pair  5 / 6  in the original prior-art arrangement in  FIGS. 1   a,    1   b,  and  1   c.    
         [0043]    Saying this differently, the designer of shunt  13  in  FIG. 4   a  will likely select a location for point  17  that is fairly close to junction  5 , and will likely select a location for point  18  that is fairly close to junction  6 . Again as discussed above in connection with  FIG. 2   a , the designer will likely select locations for points  17  and  18  (and points  3   a  and  4   a ) that are on the center line of shunt  13 . 
         [0044]    Even better performance is expected when the temperature gradient imposed across the shunt is generated in the element  2  itself, as is typically the case when current is passing through the shunt and creates heat in element  2  and, to lower degree, in sections  3  and  4 . (The reader is reminded that the resistance of element  2 , due to material selection by the designer, is typically much higher than the resistance of both sections  3  and  4 ). 
         [0045]    Under such conditions, the thermoelectric voltages generated in pairs  5 / 17  and  6 / 18  are roughly the same in magnitude but opposite in sign; the voltages from pairs  5 / 17  and  6 / 18  will cancel each other, and the desired output of the shunt at  24  will be substantially free from errors. 
         [0046]    Stating a set of requirement for the arrangement in  FIG. 4   a:  
       f.) Connecting points  17 / 18  for the sense lines  19 / 20  are located on elements  3 / 4  of the shunt  13 , as close as possible to respective areas  5  and  6 .   g.) Material of the sense lines  19 / 20  should be the same as the material of the resistive element  2 .   h.) The attachment technique between elements  3 / 4  and sense lines  19 / 20  must not introduce any other materials into the joint. A preferred method of attachment was discussed in c.) above.   i.) At the input ports of electronic measuring apparatus, both sense lines  19 / 20  must terminate in “isothermal block” area  23  that assures equal temperature of both junctions  21 / 22  between sense lines  19 / 20  and input lines  24  of the measuring apparatus.   j.) Thermal isolation is incorporated between the Current Shunt body and the isothermal block  23 .       
 
         [0052]    It will be appreciated that the alert and thoughtful reader may readily devise myriad obvious variations and improvements upon the invention, without departing from the invention at all. Any and all variations and improvements are intended to be encompassed within the claims which follow.