Abstract:
Disclosed is a current sensing system for measuring the current flowing through a current sensing resistance. The system has a calibration resistance coupled to the current sensing resistance and a pair of input resistances each having a first end coupled to one of the ends of the current sensing resistance and a second end coupled to one of the ends of the calibration resistance. A voltage is created across the calibration resistance and the calibration resistance is set at a predetermined level to compensate for the current sensing resistance deviating from a reference value. The voltage is filtered and amplified to produce an output representative of the measured current.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional patent application serial No. 60/154,558 filed Sep. 17, 1999, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a current sensing system calibration method and apparatus, and more particularly, to a current sensing system calibration method and apparatus for a current sensing system that is used for controlling a motor. 
     BACKGROUND OF THE INVENTION 
     Current sensors are known in the art for monitoring current signals. FIG. 1 depicts a conventional current sensing system generally denoted by the numeral  10 . An input current  12  flows through a current sensing resistance  14 , such as a shunt resistance of a motor wiring. A first end  16  of a pair of input resistances  18  is connected to both ends of the current sensing resistance  14 . Input resistances  18  may be used to control the input impedance of the current sensing system  10 . Each of the input resistances  18  has a second end  20  connected to a filter  22 . The current  12  through current sensing resistance  14  establishes a voltage  24  at filter  22 . The filter  22  has a pair of output leads  26  connected to an amplifier  28 . The amplifier  28  generates an output  30  indicative of the current  12 . 
     The current sensing resistance  14  needs to have a reference value so that the voltage  24  input at filter  22  accurately reflects the current  12 . Variations in the current sensing resistance  14  from a reference value will cause variations in the voltage  24  thus producing inaccurate current measurements. Several techniques have been proposed to calibrate the current sensing system  10  to accommodate the value of the current sensing resistance  14 . 
     One calibration technique is to change gain resistors in amplifier  28 . Changing gain resistance in the amplifier, however, may not be practical in certain situations. For example, the gain resistor may be internal to an integrated circuit (IC). The addition of further gain resistors external to the IC would affect gain accuracy and common mode rejection due to variability of the on-chip resistances over build and temperature. 
     Another technique for calibration of the current sensing system  10  involves changing or trimming the current sensing resistance  14  to alter its resistive value. Trimming can be difficult because of the high power involved (80 Amps, 2 milliohms). For example, in a situation wherein a blade shunt is used as the current sensing resistance  14 , trimming would most likely damage the current sensing resistance. Resistors such as thick film shunt resistance are expensive and tend to be unsuitable for trimming. 
     Yet another technique for calibrating the current sensing system  10  is to use software to adjust the signal generated by amplifier  28 . One drawback to the software approach, among others, is that it hurts resolution. 
     SUMMARY OF THE INVENTION 
     Disclosed is a current sensing system comprising a current sensing resistance through which a current to be measured flows. The system further has a calibration resistance coupled to the current sensing resistance and a pair of input resistances each having a first end coupled to one of the ends of the current sensing resistance and a second end coupled to one of the ends of the calibration resistance. A voltage is created across the calibration resistance and the calibration resistance is set at a predetermined level to compensate for any deviation of the current sensing resistance from a reference value. 
     Also disclosed is a motor driving system that uses the current sensing system of the invention to measure a DC bus current. 
     Also disclosed is a method for calibrating the current sensing system. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a prior art current sensing system. 
     FIG. 2 is a current sensing system in one embodiment of the invention. 
     FIG. 3 is an exemplary filter. 
     FIG. 4 is an exemplary amplifier. 
     FIG. 5 a ,  5   b , and  5   c  depict various embodiments for structure of the calibration resistance. 
     FIGS. 6 a  and  6   b  depict one of many applications for the instant invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 2, reference numeral  32  generally designates a current sensing system in an exemplary embodiment of the invention. An input current  12  flows through a current sensing resistance  14 , such as a shunt resistance of a motor wiring. A first end  16  of a pair of input resistances  18  is connected on both ends of current sensing resistance  14 . Each of the input resistances  18  has a second end  20  connected to a calibration resistance  34 , the structure of which is described herein. The calibration resistance  34  is connected in parallel to the input leads of filter  22 . The filter  22  has an input voltage  24  corresponding to the voltage drop across the calibration resistance  34 . The filter  22  has a pair of outputs leads  26  connected to an amplifier  28 . The amplifier  28  generates an output signal on an output lead  30 . As described in further detail herein, the resistive value of the calibration resistance  34  is selected and/or adjusted to compensate for variation in the current sensing resistance  14  from a reference value. 
     Referring to FIG. 3, one embodiment of the filter  22  is shown. The input voltage  24  is applied to a pair of first ends  36 ,  38  of a pair of inductors  40 ,  42 . The pair of inductors  40 ,  42  is magnetically coupled together by a coupling member  44  such as ferromagnetic material. The pair of inductors  40 ,  42  has a pair of second ends  46 ,  48  that are connected to a first end  50  of a first capacitance  52 , and a first end  54  of a second capacitance  56  respectively. The first capacitance  52  and the second capacitance  56  each have a grounded second end. Furthermore, the first ends  50 ,  54  are connected to a first and a second end of a third capacitance  58 . In addition, the first ends  50 ,  54  are connected to the two output leads  26  of the filter  22  as well. 
     Referring to FIG. 4, one embodiment of the amplifier  28  is shown. Two input resistances  62 ,  64  each have a first end connected to the output leads  26  of the filter  22 . An operational amplifier that has an inverting input  68 , a non-inverting input  70 , and an output end that coincides with the output  30 . The inverting input  68  is connected to a second end of the input resistance  64 , as well as a first end of a feedback resistance  72  having its second end connected to the output  30  of the operational amplifier  66 . The non-inverting end  70  is connected to a second end of the input resistance  62 . In addition, the non-inverting end  70  is connected to a midpoint  74  of a voltage divider having a power source  76 , an offset resistance  78 , and a second resistance  80 . The midpoint is connected to a first end of offset resistance  78 , and a second resistance  80  respectively. The offset resistance  78  has a second end coupled to the power source  76 . The second resistance  80  has a second end coupled to ground. 
     Referring to FIGS. 5 a ,  5   b  and  5   c , embodiments of the calibration resistance  34  are shown. In FIG. 5 a , the calibration resistance  34  includes a set of parallel resistances  82  (e.g., four resistances), with each resistance  82  having a switch  84  coupled in series with the resistance  82 . The switches  84  may be activated using a number of techniques. Switches  84  may fusable links opened by passing current through the link or severable links that can be cut using known techniques such as laser, hydro cutting, etc. In this embodiment, the switches  84  are initially closed and then permanently opened. Alternatively, switches  84  may be initially open links which can be closed using known techniques (e.g., jumpers, DIP switches). In this embodiment, the switches  84  are initially open and then closed. In another embodiment, the switches  84  are controllable switches which can assume open or closed states in response to a controller. By closing or opening switches  84 , the resistive value of the calibration resistance  34  can be adjusted in order to compensate for the current sensing resistance  14  deviating from a reference value. 
     In FIG. 5 b , the set of parallel resistances  82  is assigned a sequence of values. In the instant case, the first parallel resistance is assigned a value of R, the second 2R, the third 4R, etc. The calibration resistance  34  includes N resistances, each resistance having a resistive value of 2 N−1 R. The value R is a predetermined resistive value, which is calculated based on a desired value of the calibration resistance  34 . This embodiment allows the calibration resistance  34  to assume a resistive value of infinity (if no switches  84  are closed) to 2 N−1 R/(2 N −1). As can be appreciated, by switchably connecting or disconnecting one or a combination of the switches  84 , the calibration resistance  34  can be modified as necessary. 
     FIG. 5 c  depicts an alternative calibration resistance  34  which includes a set of series resistances  86 . In the exemplary embodiment shown in FIG. 5 c , only three resistances  86  are shown. Each resistance  86  has a switch  88  coupled in parallel with the resistance  86 . The switches  88  can be opened or closed to reach a suitable calibration resistance  34  as described above with reference to FIG. 5 a . The resistive values may be similar to those shown in FIG. 5 b , that is the calibration resistance  34  includes N resistors, each resistor having a resistive value of 2 N−1 R. This embodiment allows the calibration resistance  34  to assume a resistive value of zero (if all switches  88  are closed) to (2 N −1)R. 
     Note that the embodiments depicted in FIGS. 5 a ,  5   b , and  5   c  are far from exhaustive. Various combinations of FIGS. 5 a ,  5   b , and  5   c , may be used for the calibration resistance  34 , even a single resistor if the resistance required can be predetermined in a specific application. 
     Referring to FIGS. 6 a , and  6   b , two of the applications of the invention, among many, are described. In FIG. 6 a , the current sensing system  32  is used for sensing a direct current (DC) bus current  90  of an electric machine, such as an electric motor or generator and the like. A DC source  92  generates the DC bus current  90  that feeds a three phase inverter  94  having phases A, B, and C. The three-phase current is denoted by numerals  96 ,  98 , and  100  and supplies a stator wiring of a PM brushless motor. The current sensing resistance  14  senses the DC bus current  90 . The calibration resistance  34  in combination with resistances  18  calibrates the current signal for the filter/amplifier  22 / 28 . FIG. 6 b  depicts an alternate embodiment in which the current sensing system  32  is used for sensing phase current  100 . 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration only, and such illustrations and embodiments as have been disclosed herein are not to be construed as limiting to the claims.