Abstract:
A method and apparatus to detect non-cancelled magnetic field produced when current flows through an electric conductor are provided. The sensor includes multiple coils, which allow the sensor to be arbitrarily oriented and attach to the outside of an electrical power cable. Arbitrary orientation provides for easy of field installation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates generally to magnetic field sensors and more specifically the detection of whether a non-shielded multi-conductor power cable is energized. 
         [0004]    2. Background of the Invention 
         [0005]    A sensor may be used to monitor a power cable to detect whether current is flowing in the cable. The sensor may be connected to an alarm which becomes active when either current flows or alternatively when current ceases to flow. For example, a sensor may be used to activate an LED when current is running through the power cable. Alternatively, a sensor may be used to activate an alarm if current is cut from the power cable. 
         [0006]    To monitor current flow through the power cable, power first must be disconnected. With power disconnected, a resistive device may be placed in line with or a toroid placed around an electrical conductor in the cable to sense a current flow or a magnetic field. These monitors must break and/or isolate a single conductor from the power cable in order to monitor current through the power cable. 
         [0007]    Therefore, a need exists to monitor a power cable without interrupting electrical flow and without breaking the electrical connection. 
       SUMMARY 
       [0008]    Embodiments of the present invention provide a sensor and method to detect non-cancelled magnetic field produced when current flows through an electric conductor. The sensor includes multiple coils, which allows the sensor to be arbitrarily oriented and attach to the outside of an electrical power cable. Arbitrary orientation provides for easy field installation. 
         [0009]    Some embodiments of the present invention provide for a sensor to detect an electrical current in plurality of electrical wires, the sensor comprising: a first magnetic field sensing coil providing a first sensed signal and having a first port and a center axis; a first amplifier having an input port coupled to the first port of the first magnetic field sensing coil and an output port; a second magnetic field sensing coil providing a second sensed signal and having a first port and defining a center axis; a second amplifier having an input port coupled to the first port of the second magnetic field sensing coil and an output port; an amplitude detector having a first input port coupled to the output port of the first amplifier, a second input port coupled to the output port of the second amplifier, and an output port; and a comparator having a first input port coupled to the output port of the amplitude detector, a second input port couple to a source of a reference value, and an output port to provide an indication of a presence of the electrical current; wherein the center axis of the first coil and the center axis of the second coil form a positive angle. 
         [0010]    Some embodiments of the present invention provide for a method of detecting an electrical current in a plurality of electrical wires using a sensor, the method comprising: sensing a first sensed signal with a first magnetic field sensing coil, wherein the first magnetic field sensing coil defines a center axis; amplifying the first sensed signal; sensing a second sensed signal with a second magnetic field sensing coil, wherein the second magnetic field sensing coil defines a center axis, and wherein the center axis of the first magnetic field sensing coil and the center axis of the second magnetic field sensing coil define a first positive angle; amplifying the second sensed signal; determining a maximum amplitude from the first sensed signal and the second sensed signal; comparing the maximum amplitude to a reference value to form a comparison result; and indicating comparison result is greater than the reference value to provide an indication of a presence of the electrical current; wherein the center axis of the first coil and the center axis of the second coil form a positive angle. 
         [0011]    Some embodiments of the present invention provide for a sensor to detect an electrical current in plurality of electrical wires, the sensor comprising: a first means for sensing a first magnetic field to provide a first sensed signal; a first means for amplifying the first sensed signal; a second means for sensing a second sensed signal; a second means for amplifying the second sensed signal; a means for determining a maximum amplitude from the first sensed signal and the second sensed signal; a means for comparing the maximum amplitude to a reference value to form a comparison result; and a means for indicating the comparison result to provide an indication of a presence of the electrical current; wherein a center axis of the first sensing means and a center axis of the second sensing means form a positive angle. 
         [0012]    These and other aspects, features and advantages of the invention will be apparent from reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Embodiments of the invention will be described, by way of example only, with reference to the drawings. 
           [0014]      FIGS. 1A ,  1 B and  1 C show an electrical circuit and monitoring of the electrical connection. 
           [0015]      FIGS. 2 ,  3 ,  4  and  5  show electrical conductors and associated magnetic fields. 
           [0016]      FIG. 6  illustrates an electrical conductor, its associated magnetic field and an inductor. 
           [0017]      FIG. 7  illustrates a cross-section of conductors in an electrical power cable and a pair of inductors positioned relative to the conductors. 
           [0018]      FIGS. 8A ,  8 B,  8 C,  9 ,  10  and  11  illustrate sensors having a housing including two or more inductors and holding an electrical power cable, in accordance with embodiments of the present invention. 
           [0019]      FIGS. 12 and 13  show a prospective view of sensors having a housing holding an electrical power cable, in accordance with embodiments of the present invention. 
           [0020]      FIG. 14  is a block diagram of a sensor, in accordance with embodiments of the present invention. 
           [0021]      FIGS. 15A ,  15 B,  16  and  17  are schematic diagrams of a sensor, in accordance with embodiments of the present invention. 
           [0022]      FIG. 18  shows a flow diagram of a sensor, in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0023]    In the following description, reference is made to the accompanying drawings, which illustrate several embodiments of the present invention. It is understood that other embodiments may be utilized and mechanical, compositional, structural, electrical, and operational changes may be made without departing from the spirit and scope of the present disclosure. The following detailed description is not to be taken in a limiting sense. Furthermore, some portions of the detailed description that follows are presented in terms of procedures, steps, logic blocks, processing, and other symbolic representations of operations on data bits that can be performed in electronic circuitry or on computer memory. A procedure, computer executed step, logic block, process, etc., are here conceived to be a self-consistent sequence of steps or instructions leading to a desired result. The steps are those utilizing physical manipulations of physical quantities. These quantities can take the form of electrical, magnetic, or radio signals capable of being stored, transferred, combined, compared, and otherwise manipulated in electronic circuitry or in a computer system. These signals may be referred to at times as bits, values, elements, symbols, characters, terms, numbers, or the like. Each step may be performed by hardware, software, firmware, or combinations thereof. 
         [0024]      FIGS. 1A ,  1 B and  1 C show an electrical circuit and monitoring of the electrical connection.  FIG. 1  shows a source  10  of electrical power and a corresponding load  20  connected by a power cable including first electrical path using a first electrical conductor  110 A and its corresponding return path using a second electrical conductor  110 B.  FIG. 1B  shows a first example of an in-line sensor for a power cable including two conductors  110 A and  110 B. To install the sensor, power is first disconnected. Next, the first electrical conductor  110 A is broken and a low-impedance resistor R is inserted into the electrical path. A volt meter is placed across the resistor R. Once current passes through the power cable, a voltage drop across the resister R is measured by the volt meter. 
         [0025]      FIG. 1C  shows a second example of a sensor for a power cable including two conductors  110 A and  110 B. To install the sensor, power is first disconnected. Next, the first electrical conductor  110 A is broken and reconnected after passing through a toroid T. A resister R LOAD  and a volt meter are connected in parallel to the toroid. Once current passes through the power cable, a magnetic field imposes a current across the resister R LOAD  causing a voltage drop across the resister, which is measured by the volt meter. 
         [0026]      FIGS. 2 ,  3 ,  4  and  5  show electrical conductors and associated magnetic fields. In  FIG. 2 , two electrical conductors  110 A and  110 B are shown running parallel one another and carrying an alternating current. The figure illustrates a current at one moment in time during the cycle of the alternating current. The changing current generates a magnetic field as shown. In  FIG. 3 , a cross-section of the parallel electrical conductors of  FIG. 2 . In the first electrical conductor  110 A a current goes into the conductor (into the paper as shown by the tail of an arrow) and the second electrical conductor  110 B a current comes from the conductor (out from the paper as shown by the head of an arrow). Corresponding magnetic fields following the right-hand rule are also illustrated. The first electrical conductor  110 A generates a counter-clockwise magnetic field and the second electrical conductor  110 B generates a clockwise magnetic field as illustrated. 
         [0027]      FIG. 4  shows another cross section of the first and second electrical conductor  110 A and  110 B. In the figure an overall magnetic field from the combined effect of both currents is illustrated. At a first region near conductor  110 A and farthest away from conductor  110 B, maximum field leakage occurs. A corresponding point farthest away from conductor  110 A and near conductor  110 B, exhibits an equivalent maximum field leakage region. At a second region around a plane separating the two conductors  110 A and  110 B, a minimum field leakage occurs.  FIG. 5  shows a different cross-sectional view of three electrical conductors, for example from a three-phase power cable. A first electrical conductor  110 A, a second electrical conductor  110 B and a third electrical conductor  110  run parallel to one another. A three-phased alternating current runs in the power cable, which generates similar magnetic fields. The magnetic fields generate a combined leakage field with equivalent maximum and minimum regions. For example, in a first region away from conductors  110 A and  110 C and near conductor  110 B, a region of maximum field leakage occurs. In a second region between conductors  110 A and  110 B and away from  110 C, a first region of minimum field leakage occurs. 
         [0028]      FIG. 6  illustrates an isolated electrical conductor  110 , its associated magnetic field and an inductor L  210 . The electrical conductor  110  is shown going into the paper and separate from any other electrical conductors. The inductor L  210 , on the other hand, is shown away from and perpendicular to the electrical conductor  110 . This relative position between an electrical conductor and an inductor provides a maximum electro-magnetic coupling between the alternating current in the electrical conductor  110  and a circuit (not shown) electrically connected to the inductor L  210 . 
         [0029]      FIG. 7  illustrates a cross-section of conductors  110 A and  110 B from an electrical power cable and a pair of inductors  210 A and  210 B positioned relative to the conductors  110 A and  110 B and each other, in accordance with embodiments of the present invention. The electrical power cable includes a first electrical conductor  110 A and a second electrical conductor  110 B both going into the page. A first inductor L 1   210 A is positioned near and perpendicular to the first electrical conductor  110 A and away from the second electrical conductor  110 B. A second inductor L 2   210 B is also positioned perpendicular to but equally near both the conductors  110 A and  110 B and perpendicular to the first inductor L 1   210 A. 
         [0030]    As shown in  FIG. 4  and again here in  FIG. 7 , a first region near the first electrical conductor  110 A and away from the second electrical conductor  110 B provides maximum field leakage. A second region near both the first and second electrical conductors  110 A and  110 B provides minimum field leakage. The first inductor L 1   210 A is positioned in the first region of maximum field leakage and the second inductor L 2   210 B is positioned in the second region of minimum field leakage. When an alternating current is passing through the first and second electrical conductors  110 A and  110 B, the first inductor L 1   210 A provides maximum coupling between the first inductor L 1   210 A and the power cable. The second inductor L 2   210 B in the region of minimum field leakage has effectively no coupling between the circuits. 
         [0031]    By positioning the first and second inductors L 1   210 A and L 2   210 B perpendicular to one another, at least one of the two will be advantageously positioned relative to the conductors in a power cable. That is, one of the inductors will be positioned close to a single electrical conductor and farther away from the remaining one or more electrical conductors. The inductor so positioned will provide non-zero coupling between the power cable and the inductor. As such, a power cable may be arbitrarily positioned relative to an assembly of two or more inductors where the inductors are positioned at positive angles relative to one another. Though the inductors L 1   210 A and L 2   210 B are shown perpendicular to one another, having them spaced a distance away from each other and at a positive angle greater than zero degrees and less than 180° (e.g., 120°, 90° (that is, perpendicular as shown), 60°, or approximately at these angles at 120°±20°, 120°±10°, 120°±5°, 90°±20°, 90°±10°, 90°±5°, 60°±20°, 60°±10°, or 60°±5°). In each case, the inductors are positioned perpendicular or approximately perpendicular relative to a power cable and may be positioned in the same plane with one another. Such positioning between the assembly of inductors and the power cable will guarantee that at least one of the inductors has non-zero coupling with a power cable. 
         [0032]      FIGS. 8A ,  8 B,  8 C,  9 ,  10  and  11  illustrate sensors  200  having a housing  201  including two or more inductors and holding an electrical power cable  100 , in accordance with embodiments of the present invention. 
         [0033]    In  FIG. 8A , the electrical power cable  100  includes first and second electrical conductors  110 A and  110 B. The housing  201  includes the first inductor L 1   210 A and the second inductor L 2   210 B positioned perpendicular to one another. The housing  201  is attached to a locking mechanism  220 , such as a band, rubber band or belt. Alternatively, the housing  201  is formed to provide the functionality of the locking mechanism  220  as part of the housing  201 . For example, a housing may be formed in two parts that, when assembled together, wrap around the power cable. The two parts may be fastened together with one or more screws, clips, rubber bands, pieces of Velcro® material, or the like. The example in  FIG. 8A  shows a rubber band attached to two ends of a one piece sensor (e.g., at two corresponding posts) thereby holding the sensor  200  against the power cable. The locking mechanism  220  restrains the power cable to a position perpendicular to the inductors in the housing  201 . 
         [0034]    In  FIG. 8A , first inductor L 1   210 A is shown positioned close to the first electrical conductor  110 A but far away from the second electrical conductor  110 B where as the second inductor L 2   210 B is shown positioned equally close to the first and second electrical conductors  110 A and  110 B. In this relative position between housing  201  and the power cable  100 , the first inductor L 1   210 A is in a region of maximum magnetic leakage while the second inductor L 2   210 B is in a null or in a region of little magnetic leakage. Therefore, even though one of the inductors is in a leakage null, the other inductor is not in a leakage null.  FIG. 8B  shows the housing  201  and power cable  100  of  FIG. 8A , however, the power cable  100  is in a rotated position (45° from  FIG. 8A ) relative to the housing  201 . In this relative position, neither inductor is in a null region. Therefore, both inductors are positioned to sense a magnetic change in the power cable. 
         [0035]      FIG. 8C  shows a sensor  200  including a housing  201  having two inductors  210 A and  210 B at a 90° (right angle) and a locking mechanism  220 , in accordance with embodiments of the present invention. The two-wire power cable from  FIGS. 8A and 8B  has been replaced with a three-wire power cable  100 , which includes three electrical conductors  110 A,  110 B and  110 C. Because the sensor  200  includes two inductors  210 A and  210 B positioned apart and at relative angle to one another, at least one of the inductors ( 210 A or  210 B) power cable is positioned to be away from a minimal region of magnetic leakage. 
         [0036]      FIG. 9  shows a sensor  200  including a housing  201  including two inductors  210 A and  210 B at a 60-degree angle and a locking mechanism  220 , in accordance with embodiments of the present invention. The housing  201  is shaped in a wedge to assist in holding the power cable  100 . The locking mechanism may be provided by the housing  201  being hinged, semi-flexible or flexible. The first inductor  210 A is positioned away from the region of minimum magnetic leakage, therefore provides a maximum signal. The second inductor, on the other hand, is shown positioned directly in the region of a minimum magnetic leakage, therefore provides a minimum to no signal to the sensor. 
         [0037]      FIG. 10  shows a sensor  200  including three inductors at 60-degree angles, in accordance with embodiments of the present invention. The sensor  200  includes a housing  201  and three inductors  210 A,  210 B and  210 C oriented to surround or encompass a cross-section of the power cable  100 . Increasing the number of inductors helps to insure that one of the multiple inductors will be near a region of maximum magnetic leakage. The power cable shown creates a region of minimum magnetic leakage near the second inductor  210 B but a non-minimum regions of magnetic leakage are presented to the remaining inductors  210 A and  201 C. 
         [0038]      FIG. 11  shows a sensor  200  including a housing  201 A and  201 B and four inductors  210 A,  210 B,  210 C and  210 D, in accordance with embodiments of the present invention. The inductors described above have been presented in the form of a cylinder with or without a ferrous core. Alternatively, the inductors arched or otherwise shaped to better wrap around a power cable. The sensor  200  includes four inductors  210 A,  210 B,  210 C and  210 D and a housing including to sections  201 A and  201 B. A locking mechanism (not shown) may be used to secure the housing halves together, for example, by screws, snaps, bands, other fasteners, or the like. 
         [0039]      FIGS. 12 and 13  show a prospective view of sensors  200  having a housing holding an electrical power cable  100 , in accordance with embodiments of the present invention. In  FIG. 12 , a sensor  200  includes a housing  201 , a locking mechanism  220 , a pair of inductors  210 A and  210 B hidden in the housing  201 , and an indictor  350 . The locking mechanism  220 , which may be a rubber band or other mechanism, holds a power cable  100 , including electrical conductors  110 A and  110 B, against the inductors  210 A and  210 B in the housing  201 . The indicator  350  is coupled to electronic circuitry in the sensor  200  and may be used to indicate if current is detected, power is lost or power changes (between on and off or off and on). The indicator may provide an audio (e.g., a speaker) or visual (e.g., an LED) alert to an operator. Alternately, the indicator may provide a signal (such as an electrical signal) to external circuitry used for monitoring one or more power cables. 
         [0040]      FIG. 13  shows a sensor  200  having a housing comprising a first assembly  201 A and a second assembly  201 B. The first assembly  201 A contains the electronics, inductors and indicators. The electronics are further described below. The inductor  210 A is positioned on a ramp in the first assembly  200 A. The second assembly  201 B is a flat plate or a formed plate used as a locking mechanism to hold a power cable  100  against the inductors in the first assembly  201 A. Also shown are two indicators: a first indicators shows if the sensor  200  is receiving power; and a second indicator shows if current is flowing through a power cable. 
         [0041]      FIG. 14  is a block diagram of a sensor  200 , in accordance with embodiments of the present invention. The sensor  200  includes a first sensor  300 A, a second sensor  300 B, an amplitude detector  310 , a comparator  320  and an indicator  350 . The first sensor  300 A includes a first magnetic field sensing coil  210 A and a first amplifier (neither shown). Similarly, the second sensor  300 B includes a second magnetic field sensing coil  210 B and a second amplifier (neither shown). The coils may be air coils or filled coils such as by a ferrite core. The first sensor  300 A and second sensor  300 A sense non-minimum leakage magnetic field and each provide a sensed signal to the amplitude detector  310 . 
         [0042]    The amplitude detector  310  may simply be a maximum amplitude selector or switch that selects the maximum of the two sensed signals and provides that maximum signal to the comparator  320 . Alternatively, the amplitude detector  310  or signal paths leading to the amplitude detector  310  may pre-process the two sensed signals, for example, by filtering such as by RMS averaging, low pass filtering or the like. The amplitude detector  310  does not simply add or sum the signals from the coils but instead passes the larger of the input signals as an output signal. An adder has the unwanted ability to sum two large magnitude signals of opposite sign to become a negligible or null signal, which would mask the presence of an active power cable. Alternatively, an absolute value of the input signals may be taken to remove phase information and thus insuring signals do not cancel each other. 
         [0043]    The comparator  320  receives an output signal of the amplitude detector  310 . In some embodiments, the comparator  320  also includes an input for one or more reference values. The comparator  320 , based on the relative values between the maximum amplitude and the reference value, provides an indication of a presence of the electrical current. The first and second coils are positioned to form a positive angle, thereby providing at least one coil positioned to receive a signal from an arbitrarily positioned power cable. 
         [0044]      FIGS. 15A ,  15 B,  16 , and  17  are schematic diagrams of a sensor, in accordance with embodiments of the present invention. 
         [0045]      FIG. 15A  shows a sensor  200  that includes a first input signal path, a second input signal path, a capacitor  326  and a comparator  322 . In some embodiments, the sensor  200  also includes an indicator  350 . The first signal input path including an inductor  210 A, an amplifier  312 A and a diode  324 A coupled in series. The second signal input path, coupled in parallel to the first input signal path, also includes an inductor  210 B, an amplifier  312 B and a diode  324 B coupled in series. The first and second input signal paths may be coupled to a common source signal V 0  (e.g., a ground, a high voltage, a low voltage or a bias voltage). An amplitude detector  310  is implemented with a pair of diodes  324 A and  324 B. The output signals from each path is combine by the diodes  324 A and  324 B connected to act as an analog-OR circuit such that the maximum of the two signals is feed to the capacitor and to an input port of the comparator  322 . Thus, the two diodes  324 A and  324 B perform the function of the amplitude detector  310 . The comparator  322  compares the maximum signal to one or more reference signals or reference values (e.g., V REF ). For example, if the maximum signal is above the reference signal, then the output signal from the comparator  322  provides direction to the indicator  350  to alarm or signal as desired. 
         [0046]      FIG. 15B  shows a sensor  200  similar to the sensor of  FIG. 15A , however, the comparator  322  has been implemented with a comparator  322 A having a hysteresis circuit, such as by a Schmitt trigger. The comparator  322 A has a first input port coupled to the output port of the amplitude detector  310  and an output port connected to the input terminal of the indicator  350 . The hysteresis circuit is defined by a lower trigger point and a higher trigger point, which are additional input ports to the comparator. For example, the first input port is coupled to the output port of the amplitude detector  310 , a second input port is coupled internally to the lower trigger point, and a third input port is coupled internally to the higher trigger point. The hysteresis circuit uses its hysteresis trigger points to do the comparison function. When an input signal from the first and second input signal paths is below the lower trigger point, the output signal from the hysteresis circuit is low. When the input signal passes through the lower trigger point and then passed above an upper trigger point, the output signal from the hysteresis circuit goes high. While the input signal is above the higher trigger point, the output signal from the hysteresis circuit is high. Latter when the input signal decreases passing through the upper trigger point and then passes the below the lower trigger point, the output goes low. The hysteresis circuit forms a hysteresis band between the two trigger points, which keeps the output signal from chattering or jittering near any one input level. A circuit having a single trigger point or a single reference value may experience such jittering by small noise levels on the input signal when the input signal is near the single trigger point. The hysteresis circuit may be designed in hardware, for example by a Schmitt trigger, or alternatively may be designed in software, by a controller or other programmable hardware. As is known in the art, the specific design of the Schmitt trigger circuit sets these trigger points. 
         [0047]      FIG. 16  shows a similar sensor  200  including a first input signal path, a second input signal path, digital logic  340  and an indicator  350 . The first input signal path includes an inductor  210 A, an amplifier  312 A and an analog-to-digital converter  330 A. A second input signal path, coupled in parallel to the first input signal path, includes an inductor  210 B, an amplifier  312 B and an analog-to-digital converter  330 B. The digitized signals are passed to digital logic  340  or controller, such as a microprocessor (uP), microcontroller, dedicated logic, VLSI logic and/or the like. The digital logic  340  may include program and/or memory (e.g., internally or externally to a microcontroller) to execute and hold microcode as well as data, such as the reference value. The digital logic  340  may perform the functions of the amplitude detector  310  and the comparator  320  described above. The digital logic  340  provides an output signal to an indictor  350 , which may be part of or separate from the sensor  200 .  FIG. 17  shows the circuit of  FIG. 16 , however, the two analog-to-digital converter are replaced with a single analog-to-digital converter  330  shared by both signal paths via a multiplexer (MUX  360 ) and a control signal from the digital logic  340 . 
         [0048]      FIG. 18  shows a flow diagram of a sensor, in accordance with embodiments of the present invention. At  400 , the sensor senses a first signal with a first magnetic field sensing coil. At  402 , the sensor amplifies the first sensed signal. In parallel at  404 , the sensor senses a second sensed signal with a second magnetic field sensing coil. At  406 , the sensor similarly amplifies the second sensed signal. Steps  400 ,  402 ,  404  and  406  may be executed in series, in parallel or a combination of series and parallel. At  408 , the sensor determines a maximum amplitude from the first sensed signal and the second sensed signal. As described above, the maximum amplitude is a maximum from the two sensed signals. Prior to determining the maximum amplitude, the sensed signals may be pre-processed. For example, the sensed signals may be filtered such as by RMS averaging, low pass filtering or the like. At  410 , the sensor compares the maximum amplitude to a reference value (V REF ) to form a comparison result. At  412 , the sensor indicates the comparison result. 
         [0049]    The description above provides various hardware embodiments of the present invention. Furthermore, the figures provided are merely representational and may not be drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized. The figures are intended to illustrate various implementations of the invention that can be understood and appropriately carried out by those of ordinary skill in the art. Therefore, it should be understood that the invention could be practiced with modification and alteration within the spirit and scope of the claims. The description is not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention could be practiced with modification and alteration.