Abstract:
A low current detector using magnetoresistive sensor is used in a ground fault detector that opens a current carrying circuit when an unbalanced or dangerous condition occurs in the circuit. The current circuit has windings wrapped around a toroidal member of magnetic material having a cross sectional gap. First magnetic flux lines induced in the toroidal member by the circuit project from the gap toward a permanent magnet having second emergent magnetic flux lines that bend the first flux lines in a flux line mesh zone when an unbalanced current exists in the current carrying circuit. The flux lines are non-existent in the gap of the toroidal member when the circuit is in a balanced or non-fault state. A magnetoresistive sensor is disposed in the mesh zone in a position producing a signal indicative of an unbalanced condition caused by a current fault. A circuit breaker, connected to flux sensor responds to the signal indicating an unbalanced condition by opening the circuit.

Description:
TECHNICAL FIELD 
     The invention relates generally to a low magnitude AC and DC current sensor, and more particularly to such devices employing magnetoresistive sensors. 
     BACKGROUND ART 
     One of the most important electrical safety devices is a Ground Fault Circuit Interrupter (GFCI). GFCIs are designed to provide protection against electrical shock from ground faults, which occur when the electrical current in a circuit, either a wire line or an appliance, strays outside the path where it should normally flow. This ground fault, or unintentional electric path between a source of current and a grounded member, occurs when current is leaking somewhere—in effect, electricity is escaping to ground. If a human body provides a path to ground for this leakage the person could be burned, severely shocked or electrocuted. Such a condition may be a shock hazard, even when the current flow is insufficient to trip an electrical circuit breaker associated with the current flow. 
     Modern world-wide electrical codes require that certain circuits in electrical wiring systems likely to be in contact with moisture include current interrupting devices which are designed to protect the user against shock by interrupting power when a current leakage is initially detected. More commonly, however, GFCI devices are incorporated into electrical receptacles that are designed for installation in bathrooms, kitchens, spas, garages and outdoors. GFCI devices enjoy widespread use in many countries around the world. GFCI devices are sometimes called Earth Leakage Circuit Breakers (ELCB) or Earth Leakage Switches as well as Residual Current Circuit Breakers (RCCB) or Residual Current Devices (RCD). 
     Conventional GFCI devices that are designed to trip in response to the detection of a ground fault condition typically employ one of two methods. In one approach a first current transformer senses the circuit line current and a second transformer senses the circuit neutral current and a comparison circuit is used to determine whether there is a remainder current as indicative of a ground fault. Another approach uses a summing transformer to surround both the line and neutral conductors and determines the presence of a ground fault when the resultant current is below a predetermined value. In either case, when such an imbalance is detected, a circuit breaker within the GFCI device is immediately tripped to an open condition, thereby opening both sides of the AC line and removing all power from the load. 
     Around the world the applications for GFCI&#39;s involve a wide variety of conditions. For example, in the United States a ground fault current in excess of 6 milliamperes cannot be permitted. However, in other countries the permissible ground fault current may be as high as 30 milliamperes. Accordingly, a GFCI for use in all international situations must be able to provide protection against ground fault currents in the range of 6-30 milliamperes. 
     Also, not all countries utilize 60 hertz AC power that is utilized in the United States. Therefore, a GFCI for international applications must be able to provide protection for a frequency range of 50-60 hertz. Further, this GFCI will operate in applications requiring other AC frequencies such as 400-Hz, which is the standard electrical system operating frequency of commercial aircraft. In come situations the GFCI must be able to respond to pulsating DC requirements. GFCIs in the art do not presently meet all of these requirements in a satisfactory manner. 
     GFCI devices using current transformers (“CT”) cannot sense DC current in power circuits for the reason that CTs only respond to AC current. At DC (zero) frequency, the output of a CT is zero so that a circuit incorporating a CT as a current measuring device has a 100% error. Even at frequencies of 30 Hz, prior art CT devices have a significant error in current measurement. A DC component in the AC mains will also cause a CT error. 
     An attempt to overcome prior art shortcomings is found in U.S. Pat. No. 5,986,444 to Powell. Powell teaches a device for detecting low magnitude electrical currents that may include leakage currents. A generally toroidally-shaped member made of magnetic material provides an air gap and a magnetoresistive device is located in the air gap for sensing a current flowing through a conductor that passes through the toroidal member. Magnetoresistive devices are resistive elements typically arranged in a Wheatstone or balanced bridge arrangement that changes resistance value in the presence of a magnetic field. In order to reduce damage due to overcurrents, the member has a portion of reduced cross-sectional area to cause saturation of the member. The apparatus measures the variation of magnetic field strength acting in the magnetoresistive sensor as a measure of current faults. 
     Saturated mode magnetoresistive sensors are popularly used as rotation speed sensors by detecting the existing magnetic flux bending when a gear, made of high permeability material like steel, is rotating nearby the surface. For example, see U.S. Pat. No. 6,194,893 to M. Yokotani et al. A permanent magnet is put under one side of the magnetoresistive sensor. When gear teeth fly by the other side of the sensor nearby, the unevenness of the gear surface causes a magnetic field to change its direction back and forth, which results in the change of resistance valus of magnetoresistor, causing a voltage signal to be generated. 
     Other patents of interest include U.S. Pat. No. 5,933,306 which use GMR sensors for use in a GFCI device. U.S. Pat. No. 5,923,514 shows use of a GM device within the gap of a toroid to measure magnetic field strength. U.S. Pat. No. 5,461,308 shows use of a GMR device in the airgap of a magnetic material for sensitive current measurement. 
     An object of the invention is to provide a reliable low current sensor without susceptibility to stray external magnetic fields, susceptibility to undesired saturation of the magnetic member due to current surges, and inability to measure a DC component while still providing protection to the consumer from hazardous leakage currents at all frequencies. 
     SUMMARY OF THE INVENTION 
     The above object has been met with a new current sensor that can be used in a frequency independent ground fault detector that substantially increases sensitivity, adjustability and reliability for low level leakage tripping. The new current sensing approach involves forcing magnetic field lines induced by an unbalanced portion of a circuit into a sensitive region of a magnetoresistive sensor. The unbalanced portion of the circuit is associated with a current fault in a pair of wires that are part of a current loop. The current fault is manifest due to magnetic flux lines in paired windings about a toroidal magnetic member, with non-cancelling magnetic flux lines, associated with the current fault, protruding from a gap in the toroidal magnetic member. At the same time, magnetic flux lines emerge from a nearby permanent magnet, with the two sets of flux lines permeating each other in a magnetic flux line mesh zone existing between the permanent magnet and the toroidal member. The magnetoresistive sensor has a sensitive region, which is normally planar, placed in the flux line mesh zone. Within the magnetic flux line mesh zone, the permanent magnet bends flux lines from the toroidal member into the sensitive plane of the sensor. By operating the magnetoresistive sensor in this manner, current through the sensor varies with changes in the balanced to unbalanced states of the toroidal member. Stability of operation and immunity to external electrical noise is promoted. The current signal produced by the sensor representing the unbalanced state is amplified, filtered and transmitted to a circuit interruption trip solenoid. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective plan view of the apparatus of the present invention operating in a balanced state. 
     FIG. 2 is a perspective plan view of the apparatus of FIG. 1 operating in an unbalanced state. 
     FIG. 3 is a diagram of a flux sensor employed in the apparatus of FIG. 1 with no external magnetic field applied to the sensor. 
     FIG. 4 is a diagram of the flux sensor of FIG. 3 with an external magnetic field applied to the sensor. 
     FIG. 5 is an electrical diagram of the apparatus of FIG.  1 . 
     FIGS. 6 and 7 are plan views of a detail of the apparatus of FIGS. 1 and 2, respectively. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, a ground fault detector  11  is shown as part of a circuit including AC power source  13  connected by means of a current loop to a load  15 . The principal component of the ground fault detector is the current sensor unit which senses an unbalanced condition between a hot side and a neutral side of the same current loop. The loop includes a neutral side  17 , as well as a hot side  19  which may be part of an AC or DC circuit. The present invention operates with both AC and DC current sensing and works equally well with single phase or multi-phase Circuits. 
     The current loop includes balanced coils  23  and  25  which are wrapped about a small toroid  27  made of magnetic material, such as soft iron. For purposes of illustration, the coils are shown to be spaced apart. The best practice is to wind the coils together as later described with reference to FIGS. 6 and 7. Current carried by the current loop flows into the coils which induce magnetic fields in the toroid with associated flux lines. Typically, the coils have the same number of turns and one coil can be wound over the other coil in order to maintain equal tension. The coils are wound in the same direction but current is arranged to flow in opposite directions to induce magnetic flux in opposite directions. When the circuit has no faults, opposing magnetic fields are balanced and will cancel, but in a fault state magnetic flux appears in the gap  29 . In other words, the flux lines travel around the toroid in the unbalanced current state and jump a small gap  29  in the cross-sectional dimension of the toroid, as seen in FIG.  2 . The flux lines which jump the gap give rise to a first distribution of flux lines  31  which protrude outwardly from the gap. 
     Referring to both FIGS. 1 and 2, a permanent magnet  33  is placed below the toroid in a spaced-apart relation with one pole  43  facing the toroid. The permanent magnet has a second distribution of flux lines  35  permeating the flux sensor  41 . Preferably, the flux sensor is a magnetoresistive sensor with sensor elements disposed in a Wheatstone Bridge configuration. An example of an integrated version of such a sensor is known as the HMC  1501  manufactured by Honeywell Sensor Products, with particular specifications at www.magneticseneors.com/spec 13  sheets/specs_ 1501 .html. The flux sensor  41  is disposed in the region where the distribution of flux lines  35  will permeate sensor  41 . Sensor  41  has a sensitive region, usually a plane in which the sensor elements lie. 
     In FIG. 2, the second distribution of flux lines  35  meshes with a first distribution of flux lines  31  emerging from toroid  27  in a mesh zone  37 . The second distribution interacts with the first distribution whereby at least a portion of the first distribution is forced into the sensitive region of the sensor. This condition exists only in the unbalanced current state. In the balanced current state there is no first distribution of flux lines. The only flux lines in the flux sensor for the balanced case arise from one pole, either north or south, pole of the permanent magnet and give rise to a symmetric or balanced magnetic field in the sensor. This balanced condition is sensed by flux sensor  41  which internally deploys resistors in a balanced bridge or Wheatstone arrangement. In such a balanced arrangement, the output signal from flux sensor  41 , taken on output line  42  is nominally a low value, or zero. Output line  42  is connected to a comparator  49 . Comparator  49  has a threshold adjustment trimmer  47  which is varied to manner such that output signal on line  42 , in a balanced condition, is always below the threshold set by the variable threshold level trimmer  47 . 
     In situation where an unbalanced flux situation is detected, as shown in FIG. 2, the balanced bridge is no longer balanced and an output on line  42  would exceed the threshold level and cause comparator  49  to produce an output signal on line  44  which is transmitted to actuator  53 . In the balanced condition shown in FIG. 1, actuator  53 , a solenoid, keeps the single pole switch  51  closed so that current may flow in the current loop circuit  21 . On the other hand, in the unbalanced circuit condition, arising because of contact by person, P, with the line associated with circuit  21 , an unbalanced circuit condition arises. The person, P, acts as a partial ground through a path  50  to ground contact  52 , with second coil  25  inducing a different amount of magnetic flux in toroid  27  than the first coil  23 . This asymmetry in flux lines causes a first distribution of flux lines  31 , as seen in FIG.  2 . The second distribution of magnet flux lines  35  is attracted or repelled by the first distribution of magnetic flux  31  generated in the gap of the magnetic member. The first distribution of flux lines  31 , is seen to be distorted. This distortion in flux lines  31  represents the attraction or repulsion caused by interaction with the second distribution of flux lines in the flux line mesh zone  37  and indicates an unbalanced condition sensed in the sensitive balanced resistive bridge region of the flux sensor  41 . The signal which is output on line  42  exceeds the level of the threshold signal and causes an output from comparator  49  which causes an actuator to open switch  51 , thereby breaking the current loop circuit  21  and stopping conduction through the circuit. 
     With reference to FIG. 3, a sensor  41  is seen with input terminal  32  and output terminal  34 . Direction of current flow is indicated by the vector I and an internal magnetic field is indicated by the vector M, parallel to vector I. The internal magnetic field is an inherent characteristic of the material of sensor  41 , preferably permalloy, as found in the Honeywell sensor previously described. The sensitive region of sensor  41  lies in the XY plane  36 , parallel to a major surface of sensor  41 . Internal magnetic field M is parallel to the current flow with no external magnetic field applied. In this situation, the internal field of the sensor, represented by arrow M has a vector alignment parallel to the vector alignment of the current I, therefore, the output of sensor is zero. 
     In FIG. 4, the sensor  41  is shown to be a planar sensor with an external magnetic field, H extending in the Y direction, perpendicular to the direction of current flow between terminals  32  and  34 , indicated by the arrow I. The resultant magnetic field vector, {overscore (M)}+{overscore (H)}, makes an angle a with reference to the current vector. It can be shown that the resultant resistance change with respect to angle α can be represented by the following equation: 
     
       
           R=R   0   +ΔR cos 3 α  (1) 
       
     
     Wherein R 0  and ΔR are material constants. So it can be seen that the resistance of the sensor has an angular dependence that arises from the applied external magnetic field in any direction in the sensor plane that is not the same as the current flow direction. 
     With reference to FIG. 5, the flux sensor  41  is seen to be sandwiched between the permanent magnet  33  and the toroid  27 . The physical construction of the apparatus involves placing all components in a single housing  78  which can fit in the same form factor as a conventional circuit breaker. This miniaturization of components allows the present invention to be interchangeable with GFI devices of the prior art. An external circuit may be connected at terminals  71   a  and  73   a  which are in common with terminals  71   b  and  73   b . The toroid  27  is placed in a plane parallel to the permanent magnet  33 . Magnet  33  has one pole adjacent to flux sensor  41  and the opposite pole of the two-pole magnet distal to the flux sensor. 
     The flux sensor  41  being packaged in a chip package is arranged with the plane of the chip parallel to permanent magnet  33  and toroid  27 . The projected view of the toroid  27  shows the orientation of gap  29  relative to the sensor  41 . The external magnetic vector in the flux line mesh zone is preferably perpendicular to the current vector in sensor  41  in the plane of the sensor  41 . The output line  74  carries an AC signal  82 , representing sensor output from an AC signal in the current loop. The sensor output on line  74  is fed to an AC amplifier  81 . The amplified AC signal  84  is fed. in turn, to a full-wave rectifier  83 , producing the rectified signal  86 . This output is fed to comparator  87  for comparison with the signal from the variable threshold device, namely trimmer  47 . A signal  88  exceeding the threshold trips an actuator, not shown, operating switch  51  in a manner previously described. While operation of the present invention has been described with reference to alternating current loops, the apparatus will also work with direct current loops with minor modifications. While the elements of the current sensor shown in FIG. 5 are illustrated in a sandwich relationship with each member of the flux sensor contacting an adjacent member, the elements can be spaced apart. By moving one element, such as the toroid, a predetermined distance away from the permanent magnet, the sensitivity and level of current detection can be changed. 
     With reference to FIG. 6, toroid portion  27 , having windings  28  and  30 , are shown spiraling around the toroid. The wires are parallel and tightly wound, with the same tension in the two wires  28  and  30  which are a part of the same circuit, but have current flowing in opposite directions. One direction is indicated by a dot and the other by an X. FIG. 6 illustrates the situation where there is no flux in gap  29  associated with balanced currents in windings  28  and  30 . Although current is flowing in the windings  28  and  30 , current flows in a balanced manner and the induced magnetic fields cancel each other out and so no flux appears in the gap. On the other hand, the permanent magnet  33  has magnetic flux lines  35  emerging from a north pole piece  43  in the Z direction of the sensor, perpendicular to the plane of the sensor. The magnetic flux lines penetrate the flux sensor  41  and loop around to the south pole with the magnet indicated by S. The field H of the sensor is parallel to the direction of current flow  1 . 
     In FIG. 7, there is an unbalanced current situation in windings  28  and  30  which causes a distribution of flux lines  31  in gap  29 . These flux lines intermesh with a second distribution of flux lines  35  associated with the magnet  33 , as previously described. There is a mesh zone  37  where the flux lines permeate each other, giving rise to flux lines in the plane of the sensor at an angle to the direction of current flow. The external flux has overcome the internal flux giving rise to an resistance at a vector angle a relative to the current flow. Part of the mesh zone resides in the flux sensor  41  where an unbalanced current condition exists in the Wheatstone bridge associated with the flux sensor, giving rise to an output signal indicating an unbalanced current exceeding a threshold. Thus, the field from the permanent magnet helps to guide the external field from magnetic member  27  such that at least a portion of the latter field lies in the plane of sensor  41  with a component of field strength perpendicular to the direction of current flow and strong enough to overcome the internal magnetic field. As the toroidal member exhibits different degrees of imbalance, the magnetic field strength in the gap of the toroid will change proportionally. The permanent magnet below the sensor will cause deflection of field lines emerging from the gap into the plane of the sensor with a changing field strength. The changing field strength must have a vector component changing in the plane of the sensor perpendicular to the direction of current flow. This change results in a proportional change in resistance of the sensor, giving rise to a change in a reference current through the sensor. Changes in the reference current are amplified and handled as explained with reference to FIG.  5 .