Abstract:
A detachable current sensor provides an isolated and convenient device to measure current passing through a cable such as an AC power cable or non-metallic (NM) sheathed cable. Information about the magnitude and or phases of the currents passing through and/or voltages on the conductors is obtained by measuring the magnetic field at multiple circumferential positions around the cable using multiple semiconductor magnetic field sensors. A processing subsystem coupled to the multiple semiconductor magnetic field sensors determines information about the currents flowing in the conductors of the cable, including the current magnitude(s), and/or the phases and number of phases present in the cable, which can form part of a power measurement system that is used for energy monitoring, and/or for control purposes.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention is related to sensors providing input to power measurement systems, and more specifically to a non-contact current sensor that includes multiple semiconductor magnetic field sensors and/or voltage sensors that can be used to detect characteristics of currents flowing through, and/or electrical potentials on, multiple conductors in a cable. 
         [0003]    2. Description of Related Art 
         [0004]    A need to measure power consumption in AC line powered systems is increasing due to a focus on energy efficiency for both commercial and residential locations. In order to provide accurate measurements, the characteristics of the load must be taken into account along with the current drawn by the load. Information about the electrical potentials on conductors, i.e., the power line voltages, including magnitudes and/or phase, are also useful for determining power factor and thus actual power transfer to a load. 
         [0005]    Appliances and other devices are typically provided electrical power through multiple conductor cables both in wall-plug applications and in wiring systems that use non-metallic (NM) sheathed electrical cable. 
         [0006]    Therefore, it would be desirable to provide a sensor that can provide current draw information from a cable carrying AC line current to an appliance or other device. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    The invention is embodied in a current sensing device and its method of operation. The current sensing device includes multiple semiconductor magnetic field sensors integrated in a housing that can be detachably coupled to a multi-conductor cable and provides one or more outputs indicative of the currents passing through multiple conductors of the cable. 
         [0008]    The housing may be a clamshell that clamps around the cable, and the semiconductor magnetic field sensors embedded in the housing at positions around the circumference of an opening that accepts the cable. Ferrite or other high-permeability bodies may be included between the semiconductor magnetic field sensors and the cable to increase the magnetic field intensity in the vicinity of the sensors. A ferrite or other high-permeability material shield may be provided around the outer portion of the housing to shield the semiconductor magnetic field sensors from external magnetic field sources. 
         [0009]    The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]    The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of the invention when read in conjunction with the accompanying Figures, wherein like reference numerals indicate like components, and: 
           [0011]      FIG. 1A  and  FIG. 1B  are isometric views of a multi-conductor cable current sensor  10  according to an embodiment of the present invention. 
           [0012]      FIGS. 2A-2C  are cross-section views of sensors according to different embodiments of the present invention. 
           [0013]      FIG. 3  is a pictorial diagram depicting a magnetic field around a cable in which currents are measured by sensors  16  connected to a system according to an embodiment of the present invention. 
           [0014]      FIG. 4  is an electrical block diagram illustrating an electronic system in accordance with an embodiment of the present invention. 
           [0015]      FIGS. 5A and 5B  are cross-section view of sensors according to alternative embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0016]    The present invention encompasses a sensor and measurement techniques for performing non-invasive current measurements on multi-conductor cables that provide sensor measurements as input to power measurement systems, as well as for other uses. For example, the present invention can provide input to power monitoring equipment that monitors the power consumption of various appliances or branch circuits supplied by multi-conductor cables. 
         [0017]    Referring now to  FIGS. 1A-1B , a sensor  10  in accordance with an embodiment of the present invention is shown. Sensor  10  is formed by a housing  14  in which multiple semiconductor magnetic field sensors  16 , such as Hall effect sensors, are disposed in a circumferential arrangement around a multi-conductor cable  15  that is passed through an opening in sensor  10 . Housing  14  forms a clamshell arrangement that is hinged by a plastic hinge  12  and may include latching mechanisms or other mechanisms for securing sensor  10  in a closed position around cable  15 .  FIG. 1B  illustrates an appliance cord that may be attached to a wall outlet via a plug  11  and which connects an appliance  13  to a source of AC power. However, the present invention can be used in other current monitoring situations, in order to determine the relationship between, and in some embodiments the magnitude of, the currents within the conductors of multi-conductor cable  15 . 
         [0018]    Referring now to  FIG. 2A , details of sensor  10  of  FIGS. 1A-1B  are shown in cross-section, in accordance with an embodiment of the present invention. Within housing  14 , semiconductor magnetic field sensors  16  are disposed at various circumferential positions around the opening provided through housing  14 . By comparing the phase and magnitudes of the output voltages of semiconductor magnetic field sensors  16 , information about the phase and amplitudes of the conductors in a multi-conductor cable passing through the opening defined by housing can be obtained. 
         [0019]    Referring now to  FIG. 2B , details of another sensor  10 A in accordance with an alternative embodiment of the present invention are shown. Sensor  10 A may be used instead of sensor  10  in the depictions of  FIGS. 1A-1B . Sensor  10 A includes a plurality of bodies  18  formed from a high permeability material, such as ferrite, that cause the field around a cable inserted through the opening in housing  14 A to be concentrated in the vicinity of semiconductor magnetic field sensors  16 , improving the signal strength and signal-to-noise ratio (SNR) of the output voltages of sensors  16 . 
         [0020]    Referring now to  FIG. 2C , details of yet another sensor  10 B in accordance with another alternative embodiment of the present invention, that may be used instead of sensor  10  in the depictions of  FIGS. 1A-1B , is shown. Sensor  10 A also includes a plurality of high-permeability bodies  18  as in sensor  10 A of  FIG. 2B , that cause the field around a cable inserted through the opening in housing  14 A to be concentrated in the vicinity of semiconductor magnetic field sensors  16 , improving the signal strength and signal-to-noise ratio (SNR) of the output voltages of sensors  16 . Sensor  10 B also includes a shield  20  molded within or covering the exterior surface of housing  14  and formed from a high permeability material, which may ferrite, mu-metal or another suitable magnetic field shielding material. Alternatively, a shield may be formed from a lower permeability material such as steel. Shield  20  further improves the SNR of the output voltages of sensors  16 , since shield  20  prevents coupling from stray fields external to housing  14 . 
         [0021]    Referring now to  FIG. 3 , a magnetic field distribution around a pair of conductors  22 A, 22 B, that are carrying complementary currents within a cable  15 A, is shown. The fields  24  around conductors  22 A, 22 B cancel at the midpoint between conductors  22 A, 22 B, when measurements are taken at a point outside of cable  15 A. Therefore, sensors  16  that are located on the vertical centerline (i.e., sensors  16  at the “12 o&#39;clock” and “6 o&#39;clock” positions in the Figure) will have little or no signal output compared to sensors  16  at the other positions in the Figure. Similarly, sensor  16  at the “3 o&#39;clock” position has a much stronger coupling to conductor  22 A than to conductor  22 B, and so the field due to the current flowing in conductor  22 A will predominate in the output of the “3 o&#39;clock” sensor  16 . The “9 o&#39;clock” sensor will similarly have a strong coupling to conductor  22 B and a weaker coupling to conductor  22 A, and thus the field due to the current in conductor  22 B will predominate. The other sensors  16  will have output values between those of the sensors in the cardinal orientations mentioned above. 
         [0022]    Referring now to  FIG. 4 , a system in accordance with an embodiment of the present invention are shown. A multiplexer  101  receives signals from the individual magnetic field sensors  16  and selects a sensor for measurement, providing input to a magnetic field measurement circuit  108 , which is an analog circuit that appropriately scales and filters the output of sensors  16 . The output of magnetic field measurement circuit  108  is provided as an input to an analog-to-digital converter (ADC)  106 , which converts the output waveforms generated by current measurement circuit  108  to sampled values provided to a central processing unit (CPU)  100  that performs calculations in accordance with program instruction stored in a memory  104  coupled to CPU  104 . Alternatively, a separate magnetic field measurement circuit  108  and multiplexer  101 A may not be necessary, and sensors  16  may be coupled directly to ADC  106 . An input/output (I/O) interface  102  provides either a wireless or wired connection to an external monitoring system, such as a wireless local area network (WLAN) connection or wired Ethernet connection. An integrated display  105  may be additionally or alternatively provided to indicate a direct measure of current in a conductor. CPU  104  can perform computations to discover and map the phases of conductors in a cable, as the invention is not limited to 2-phase systems such as that depicted in  FIG. 3 . Further, cable configurations such as multiple conductors corresponding to a single return conductor may be measured and a net current magnitude value determined. The power measurement system depicted in  FIG. 4  may be integrated within or on an outside surface of sensor housing  14 , and all wiring from sensors  16  to multiplexer  101  integrated within sensor housing  14 . Alternatively, wiring may be routed from sensors  16  outside of sensor housing  14  and connected to an external enclosure and/or circuit board that contains the electronic circuits shown in  FIG. 4 . Power for the circuits shown in  FIG. 4  may be supplied from two conductors connected to an external battery or power supply. Alternatively, a battery may be mounted within sensor housing  14  or attached externally to sensor housing  14 . Alternatively, sensor  16  may use a high permeability conductive surface and obtain power supply current, e.g., to charge a power supply capacitor, from eddy currents generated in the conductive surface, which can, in turn, be used to power the system depicted in  FIG. 4 . 
         [0023]    The measurements made by the above-described system provides a signature of the currents in cable  15 A and/or a signature of the power consumed by an appliance connected via cable  15 A, and not necessarily an absolute current or power magnitude. While the “gain” of a particular sensor  16 , i.e., the ratio of the output of a sensor  16  to a current flowing in one of conductors  22 A,  22 B is unknown, measurements can be performed by observing the relative values obtained from each of sensors  16 , both in amplitude and in phase. For example, the strongest output from among sensors  16  may be taken as a measurement value and the values from the other sensors subtracted to eliminate noise and/or serve to detect faulty measurements. The waveforms generated by the outputs of each of sensors  16  may be processed, e.g., by performing fast-Fourier transforms (FFTs) on the sampled values to determine power line harmonics, power line noise, and time-dependent variations in load current(s) associated with conductors within a cable. 
         [0024]    Referring now to  FIG. 5A , details of another sensor  10 C in accordance with an alternative embodiment of the present invention are shown. Sensor  10 C has other details as illustrated in  FIGS. 1A-1B , although sensor  10 C incorporates voltage sensing elements  30 , in addition to magnetic field sensors  16 . Voltage sensing elements  30  may be conductive films or structures that may be provided by metal films adhered to or plated on the inner surface of high-permeability bodies  18  within housing  14 C, or if high-permeability bodies  18  are sufficiently conductive, high-permeability bodies  18  may be used as voltage sensing elements  30 . Further, in accordance with an alternative embodiment of the invention, high-permeability bodies  18  are omitted and voltage sensing elements are disposed directly on the inner surfaces of magnetic field sensors  16 . Connections from voltage sensing elements  30  are made to multiplexer  101 B of the system depicted in  FIG. 4  and provide voltages indicative of the electrostatic field around cable  15 , which provides further information about the position of the conductors of cable  15  within sensor  10 C and the phase and/or magnitude of the voltages on those conductors. 
         [0025]    Referring now to  FIG. 5B , details of another sensor  10 D in accordance with an alternative embodiment of the present invention are shown. Sensor  10 D has other details as illustrated in  FIGS. 1A-1B , although sensor  10 D incorporates only voltage sensing elements, and is for use in applications in which voltage sensing is needed, and current sensing is not needed, or is provided via other means. Sensor  10 D includes a plurality of voltage sensing elements  30 , which are conductive films or other structures, disposed around the inner surface of a housing  14 D, which may be molded from a plastic material. Connections from voltage sensing elements  30  are made to multiplexer  101 B of the system depicted in  FIG. 4  and provide voltages indicative of the electrostatic field around cable  15 , which provides further information about the position of the conductors of cable  15  within sensor  10 C and the phase and/or magnitude of the voltages on those conductors. 
         [0026]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.