Abstract:
A magnetic field sensor is provided with a device for producing an output signal that is a function of a magnetic field to be sensed. A calibration coil is arranged so as to change the magnetic field when energized, and a switch for selectively energizing the calibration coil is included. Calibration of the magnetic field sensor is accomplished by operating the switch to cause a known current to flow through the calibration coil, measuring the resultant change in the magnetic field, and calculating a sensitivity for the magnetic field sensor from the measured change in the magnetic field and the known current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to magnetic field sensors and more particularly to auto-calibrated magnetic field sensors used as current sensors. 
     2. Background Art 
     Magnetic field sensors of all kinds are used in many applications. One type of magnetic field sensor is the current sensor, which is widely used in circuit breaker and metering applications. For instance, many circuit breakers use at least one current transformer to sense the current level in the electrical distribution circuit being protected. The current transformer includes an annular core that encircles the line conductor of the distribution circuit and has a multi-turn winding wound thereon. The current flowing through the line conductor generates a magnetic field that produces flux in the transformer&#39;s core, resulting in an output from the multi-turn winding that is indicative of the current level. If the current exceeds a predetermined level, then the circuit breaker mechanism is tripped. The sensor must be calibrated properly so that the output from the multi-turn winding accurately represents the current in the line conductor. 
     All such magnetic field sensors are affected by temperature and other environmental factors that can adversely impact the performance of the sensor. Over time, these adverse effects that will affect the calibration of the sensor. The loss of calibration of the sensor will reduce its accuracy. 
     Accordingly, it would be desirable to have a magnetic field sensor that can calibrate itself and remove the affects of temperature and other environmental factors on its sensitivity. 
     BRIEF SUMMARY OF THE INVENTION 
     The above-mentioned need is met by the present invention which provides a magnetic field sensor having a means for producing an output signal that is a function of a magnetic field to be sensed. A calibration coil is arranged so as to change the magnetic field when energized, and a means for selectively energizing the calibration coil is included. Calibration of the magnetic field sensor is accomplished by causing a known current to flow through the calibration coil, measuring the resultant change in the magnetic field, and calculating a sensitivity for the magnetic field sensor from the measured change in the magnetic field and the known current. 
     The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the concluding part of the specification. The invention, however, may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
     FIG. 1 is a schematic view of a current sensor set for normal operation. 
     FIG. 2 is a schematic view of the current sensor of FIG. 1 set for auto-calibration. 
     FIG. 3 is a graph plotting voltage against current in the calibration of a current sensor. 
     FIG. 4 is a schematic view of a second embodiment of a current sensor set for normal operation. 
     FIG. 5 is a schematic view of the current sensor of FIG. 4 set for auto-calibration. 
     FIG. 6 is an exploded view of an integrated circuit that can be used in a current sensor. 
     FIG. 7 is a sectional view of one embodiment of an integrated circuit that can be used in a current sensor. 
     FIG. 8 is a sectional view of another embodiment of an integrated circuit that can be used in a current sensor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIGS. 1 and 2 schematically show a current sensor  10  for sensing the current flowing through a conductor  12 . The current sensor  10  includes a generally C-shaped core or flux concentrator  14  having a relatively small gap  16  formed therein and defining a central opening through which the conductor  12  passes. Although the conductor  12  is shown as having a single pass through the core  14 , it could be configured as a having multiple turns on the core  14 . The core  14  is preferably manufactured from a high permeability magnetic material such as ferrite or iron. Thus, a current flowing through the conductor  12  will generate a magnetic field in the core  14 . 
     A Hall-effect device  18  is disposed in the gap  16 . The Hall-effect device  18  comprises a base  19  that is made of a semiconductor material and is arranged in the gap  16  so as to be perpendicular to the magnetic field created by the core  14 . The base  19  has first and second input terminals  20  and  22  and first and second output terminals  24  and  26 . The first input terminal  20  is connected to a bias circuit  28 , which is in turn connected to a supply  30  such as a constant voltage source. The bias circuit  28  is conventional circuitry that generates a bias current that flows through the base  19  from the first input terminal  20  to the second input terminal  22  to energize the Hall-effect device  18 . When a magnetic field is applied to the Hall-effect device  18 , a voltage is developed across the output terminals  24  and  26  that is proportional to the magnetic field. Specifically, the output voltage varies as a function of the strength of the magnetic flux density, and hence the magnetic field created by the conductor  12 . 
     The output terminals  24  and  26  of the Hall-effect device  18  are connected to output circuitry in the form of a conventional amplifier circuit  32 . The amplifier circuit  32  amplifies the output voltage of the Hall-effect device  18 . The amplified output voltage is then supplied to a processor  34 . 
     The current sensor  10  further includes an electronically controlled switch  36  and a calibration or auto-compensation coil  38  wound on the core  14  so as to change the magnetic flux density generated in the core  14  when energized. The switch  36  is a single pole, double throw switch having a pivoting pole  40  and first and second contacts  42  and  44 . The pole  40  is connected to the second input terminal  22  of the Hall-effect device  18 , and the second switch contact  44  is connected to ground. The calibration coil  38  is connected between the first switch contact  42  and ground and comprises a relatively small number of turns, preferably in the range of 50-100 turns. (By comparison, a typical current transformer has on the order of 1000 turns or more.) The small number of turns for the calibration coil  38  is selected so that the magnetic flux density generated in the core  14  will increase by approximately 5-15% when the calibration coil  38  is energized. The switching of the pole  40  is controlled by the processor  34 , as will be described in more detail below. 
     For normal operation of the current sensor  10 , the switch  36  is switched to its first position (FIG. 1) wherein the pole  40  engages the second switch contact  44 . In this switch position, the bias current flows through the Hall-effect device  18  and to ground. Accordingly, the output voltage of the Hall-effect device  18  is a function only of magnetic flux density generated in the core  14  due to the current flowing through the conductor  12 , i.e., the current to be sensed. Thus, the output voltage of the Hall-effect device  18  is representative of the current to be sensed. For auto-calibration, the switch  36  is switched to its second position (FIG. 2) wherein the pole  40  engages the first switch contact  42  so that the bias current flows through the Hall-effect device  18  and then through the calibration coil  38  and then to ground. In this switch position, the coil  38  is energized and the output voltage of the Hall-effect device  18  is a function of the magnetic flux density generated in the core  14  due to the current flowing through the conductor  12  and the bias current flowing through the calibration coil  38 . 
     This is shown graphically in FIG. 3, which plots voltage against current. Specifically, V H1  represents the output voltage of the Hall-effect device  18  inputted to the processor  34  when the switch  36  is in its first or normal operation position and V H2  represents the output voltage of the Hall-effect device  18  inputted to the processor  34  when the switch  36  is in its second or auto-calibration position. For the purposes of performing a calibration, it is preferred that V H1  be detected very close in time to when V H2  is measured. This will avoid drift in the calibration process, and will enable the calibration process even when the current in the conductor  12  has a high bandwidth. As described above, V H1  corresponds to the magnetic flux density generated in the core  14  due to only the current flowing through the conductor  12 , and V H2  corresponds to the magnetic flux density generated in the core  14  due to both the current flowing through the conductor  12  and the bias current flowing through the calibration coil  38 . Thus, point A as shown in the graph of FIG. 3 corresponds to V H1  and I 1 , where I 1  is equal to the current flowing through the conductor  12  (i.e., the current to be sensed, I s ) and point B corresponds to V H2  and I 2  where I 2  is equal to the current to be sensed plus the bias current (I s +I b ). Points A and B define a curve  46 , the slope of which is equal to the sensitivity of the current sensor  10 . The curve  46  thus represents how the current is a function of the measured voltage. The processor  34  determines the sensitivity, S, using the following equation:              S   =           V   H2     -     V   H1           (       I   s     +     I   b       )     -     I   s         =       Δ                   V   H         I   b                 (   1   )                                
     Thus, a value of the sensitivity is obtained from the known bias current and the measured change in the output voltage. This value is obtained in the presence of, and independently of, all external and internal interference and conditions such as the ambient temperature, the temperature of the core  14 , any effects of the high permeability material used in the flux concentrator  14 , and the like. 
     Once the sensitivity has been determined, the current sensor  10  is properly calibrated and normal operation thereof can resume. For normal operation, the switch  36  is switched to its first position (as shown in FIG. 1) wherein the pole  40  engages the second switch contact  44 . In this switch position, the bias current flows through the Hall-effect device  18  and to ground. The resulting output voltage from the Hall-effect device  18  that is fed to the processor  34  represents the magnetic flux density generated in the core  14  due to the current flowing through the conductor  12 . The processor  34  then uses the sensitivity S obtained in the most recent auto-calibration phase to determine the current I s  flowing through the conductor  12  from the measured output voltage V H  with the equation:                I   s     =       V   H     S             (   2   )                                
     The processor  34  controls the switching of the switch  36  between its first and second positions. The processor  34  causes the switch  36  to be switched to its second position (as shown in FIG. 2) and thereby initiate a calibration in response to one or more of a variety of triggers. For instance, the processor  34  could be programmed to automatically initiate a calibration of the current sensor  10  on a periodic basis, such as once a minute or once a day. The period between calibrations would depend on factors such as the nature of the sensor  10 , its application and its environment. The processor  34  could be programmed to initiate a calibration in response to a significant change in temperature of the current sensor  10 . That is, if the temperature of the current sensor or the ambient temperature increased or decreased more than a predetermined amount, then the processor  34  would cause the switch  36  to be switched to its second position so as to obtain a calibration of the sensor  10 . To this end, a temperature sensor  48  is provided for sensing the temperature of the current sensor  10  and producing a signal corresponding to the sensed temperature. The temperature signal is fed to the processor  34 . The temperature sensor  48 , shown schematically in FIGS. 1 and 2, could be a separate sensor located so as to sense the temperature of the current sensor  10 . Alternatively, the temperature sensor  48  could be integrated on the base  19  of the Hall effect device  18 . The processor  34  could also be provided with a manual input  50 , such as a toggle switch, that a human operator could use to manually cause the processor to initiate a calibration. The processor  34  can be provided with all of these triggers or any subset thereof. 
     Alternatively, the current sensor  10  could be set up to operate such that the switch  36  is in its second position for normal operation and switched by the processor  34  to its first position for auto-calibration. In this mode, the bias current flows through both the Hall-effect device  18  and the calibration coil  38  and then to ground during normal operation, and the bias current flows through just the Hall-effect device  18  and then to ground during auto-calibration. Accordingly, the output voltage V H1  of the Hall-effect device  18  during normal operation is a function of the magnetic flux density generated in the core  14  due to the current I s  flowing through the conductor  12  and the bias current I b  flowing through the calibration coil  38 . The output voltage V H2  of the Hall-effect device  18  during auto-calibration is a function only of magnetic flux density generated in the core  14  due to the current I s  flowing through the conductor  12 . The processor  34  is thus still able to determine the sensitivity from the measured output voltages and the known bias current I b  using equation (1) above. Then, during normal operation, the processor  34  uses the sensitivity S obtained in the most recent auto-calibration phase to determine the current I s  flowing through the conductor  12  from the current measured output voltage V H  with the equation:                I   s     =         V   H     S     -     I   b               (   3   )                                
     An advantage of using the second switch position shown in FIG. 2 during normal operation is that the addition of the bias current is more likely to place the measured output voltage in the linear region of the curve  46  shown in FIG.  3 . This will make calibration and the current sensing function easier because the performance in the linear region is defined by a single parameter (i.e., the slope). 
     Although a Hall-effect device is described herein, it should be noted that the current sensor  10  can be implemented using any magnetic field sensitive device, such as magneto-resistors, giant magneto-resistors, MOSFET magnetic field sensors, magneto-transistors and the like. Either orthogonal magnetic field sensors (sensors sensitive only to magnetic fields perpendicular to their faces) or lateral magnetic field sensors (sensors sensitive only to magnetic fields parallel to their faces) can be used. When using a magnetic sensor, such as a giant magneto-resistor, that does not require a flux concentrator, the core  14  is not used. In this case, the calibration coil  38  could be formed on an air-core or any other arrangement that will cause the calibration coil  38  to create a magnetic field substantially in the same direction as the magnetic field to be sensed. 
     The above-discussed embodiment is referred to herein as the “current mode” embodiment in that it operates with a constant current flowing through the Hall-effect device. FIGS. 4 and 5 schematically show a second embodiment that is referred to as the “voltage mode” embodiment in which a constant voltage is supplied to the Hall effect device. In the embodiment, a current sensor  110  for sensing the current flowing through a conductor  112  includes a generally C-shaped core or flux concentrator  114  having a relatively small gap  116  formed therein and defining a central opening through which the conductor  112  passes. Although the conductor  112  is shown as having a single pass through the core  114 , it could be configured as a having multiple turns on the core  114 . The core  114  is preferably manufactured from a high permeability magnetic material such as ferrite or iron. Thus, a current flowing through the conductor  112  will generate a magnetic field in the core  114 . 
     A Hall-effect device  118  is disposed in the gap  116 . The Hall-effect device  118  comprises a base  119  that is made of a semiconductor material and is arranged in the gap  116  so as to be perpendicular to the magnetic field created by the core  114 . The base  119  has first and second input terminals  120  and  122  and first and second output terminals  124  and  126 . The first input terminal  120  is connected to a supply  130  such as a constant voltage source, and the second input terminal  122  is connected to ground. Thus, a constant voltage is applied across the input terminals  120  and  122  such that a current flows through the base  119  of the Hall-effect device  118 , thereby energizing the Hall-effect device  118 . When a magnetic field is applied to the Hall-effect device  118 , a voltage is developed across the output terminals  124  and  126  that is proportional to the magnetic field. Specifically, the output voltage varies as a function of the strength of the magnetic flux density, and hence the magnetic field created by the conductor  12 . 
     The output terminals  124  and  126  of the Hall-effect device  118  are connected to output circuitry in the form of a conventional amplifier circuit  132 . The amplifier circuit  132  amplifies the output voltage of the Hall-effect device  118 . The amplified output voltage is then supplied to a processor  134 . 
     The current sensor  110  further includes an electronically controlled switch  136  and a calibration or auto-compensation coil  138  wound on the core  114  so as to increase the magnetic flux density generated in the core  114  when energized. The switch  136  has a first switch contact  142  and a second switch contact  144 . The first switch contact  142  is connected to a bias circuit  128 , which is in turn connected to a second voltage supply  131 . The calibration coil  138  is connected between the second switch contact  144  and ground. The bias circuit  128  is conventional circuitry that generates a bias current that flows through the calibration coil  138  when the switch  136  is closed. The calibration coil  138 , like that of the first embodiment, comprises a relatively small number of turns, preferably in the range of 50-100 turns. Switching of the switch  136  is controlled by the processor  134  in the same manner as that described above in connection with the current mode embodiment. 
     For normal operation of the current sensor  110 , the switch  136  is switched to its open position (FIG.  4 ). With the switch  136  open, current from the constant voltage source  130  flows through the Hall-effect device  118  and to ground, but no current flows through the calibration coil  138 . Accordingly, the output voltage of the Hall-effect device  118  is a function only of magnetic flux density generated in the core  114  due to the current flowing through the conductor  112 , i.e., the current to be sensed. Thus, the output voltage of the Hall-effect device  18  is representative of the current to be sensed. For auto-calibration, the switch  36  is switched to its closed position (FIG. 5) so that the bias current flows through the calibration coil  138 . With the switch  136  closed, the coil  138  is energized and the output voltage of the Hall-effect device  118  is a function of the magnetic flux density generated in the core  114  due to the current flowing through the conductor  112  and the bias current flowing through the calibration coil  138 . 
     As with the current mode embodiment, the output voltage V H1  of the Hall-effect device  118  when the switch  136  is open and the output voltage of the Hall-effect device  118  V H2  when the switch  136  is closed are both inputted to the processor  134 . As before, V H1  corresponds to the magnetic flux density generated in the core  114  due to only the current I s  flowing through the conductor  112 , and V H2  corresponds to the magnetic flux density generated in the core  114  due to both the current I s  flowing through the conductor  112  and the bias current I b  flowing through the calibration coil  138 . Thus, the processor  134  again determines the sensitivity S from the known bias current and the measured change in the output voltage according to equation (1) above. 
     Once the sensitivity has been determined, the current sensor  110  is properly calibrated and normal operation thereof can commence. For normal operation, the switch  36  is opened (as shown in FIG.  4 ). In this switch position, the resulting output voltage from the Hall-effect device  118  that is fed to the processor  134  represents the magnetic flux density generated in the core  114  due to the current flowing through the conductor  112 . The processor  134  then uses the sensitivity obtained in the most recent auto-calibration phase to determine the current flowing through the conductor  112  from the measured output voltage from equation (2) above. 
     The processor  134  controls the switching of the switch  136  between its first and second positions. As is the first embodiment, the processor  134  can be programmed to automatically initiate a calibration of the current sensor  110  on a periodic basis, in response to a significant change in temperature of the current sensor  110  as detected by a temperature sensor  148 , or in response to a manual input  150 . The processor  134  can be provided with all of these triggers or any subset thereof. 
     As described above in connection with the first embodiment, the current sensor  110  could be set up to operate such that the switch  136  is in its second position for normal operation and switched by the processor  134  to its first position for auto-calibration. In this case, the processor  134  would still determine the sensitivity from the known bias current and the measured change in the output voltage according to equation (1) above, and then, during normal operation, the processor  134  would use the sensitivity obtained in the most recent auto-calibration phase to determine the current flowing through the conductor  112  from the measured output voltage V H  with equation (3) above. The current sensor  110  could also be implemented without a flux-concentrating core. 
     The current mode and voltage mode embodiments as described above use discrete components. However, both embodiments can also be implemented using integrated circuit technology such as standard CMOS (complementary metal-oxide semiconductor) technology. Specifically, FIG. 6 shows an exploded view of an integrated circuit  52  in which a calibration or auto-compensation coil  54  and a Hall-effect device  56  are both fabricated on a single substrate. The Hall-effect device  56  includes first and second input terminals  58  and  60  and first and second output terminals  62  and  64 . The Hall effect device can be fabricated using an N-diffusion for the current mode embodiment (as shown in FIG. 7) or an n-well for the voltage mode embodiment (as shown in FIG.  8 ). The calibration coil  54  is a planar coil formed in metals layers such as Metal 1  and Metal 2  in a standard CMOS process directly on top of the Hall-effect device  56 . The coil  54  has a cross-over leg  66  that is formed so as to cross over the other loops of the coil  54  without contacting. In this implementation, the integrated circuit  52  could include on-chip electronics to obtain the bias currents, temperature sensing, switching, logic control, and signal amplification and processing described above. 
     To be implemented as part of an auto-calibrating current sensor, the integrated circuit  52  may be paired with a flux concentrator. For instance, the integrated circuit  52  could be arranged perpendicularly in a gap of a generally C-shaped core in the manner described above. Then, the integrated circuit  52  would be exposed to a magnetic flux density generated by the current flowing through a conductor passing through the flux concentrator. Alternatively, the integrated circuit  52  could be situated in close proximity to a conductor carrying the current to be sensed. The integrated circuit  52  would then be exposed to a magnetic flux density generated by the current flowing through this conductor. The operation for calibrating the sensor and sensing the current flowing through the conductor would be the same as that described above in connection the discrete component embodiments. 
     The current mode and voltage mode embodiments described above use a single supply  30 ,  131  for generating the bias current in the calibration coil  38 ,  138 . In another alternative, both embodiments could be modified to utilize a plurality of different supplies arranged to alternatively generate the bias current. These supplies would be different so as to generate different bias currents, such as I b ,  2 I b , and  3 I b . Thus, by monitoring V H1  and V H2  for each bias current, a multi-point calibration can be obtained instead of the two-point calibration described above. This multi-point calibration would provide a generally finer calibration and could also be used to include a calculation of the offset voltage in the calibration scheme. In particular, the multi-point calibration is used to include a more general calibration such as non-linear calibration. Similarly, in yet another embodiment, a single supply can be used to generate a plurality of current values to be used; this is obtained by building an electronic switch in the design of the current source. 
     The foregoing has described an auto-calibrating magnetic field sensor. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention as defined in the appended claims.