A THIN MAGNETORESISTIVE CURRENT SENSOR SYSTEM

Abstract: A thin magnetoresistive current sensor system having a direction of magnetic field sensitivity parallel to the larger surfaces of its lamination or laminations. The sensor consists of one or just a few thin laminations of a high permeability magnetic material or nickel-iron or like metal, forming a loop having a gap, and a magnetoresistive-magnetic field sensor situated in the gap. The current sensor structure avoids the expensive and bulky numerous laminations of the related art current sensors. The thin lamination or structure of the present magnetic field sensor is preferably formed on a single substrate or PC that contains the processing electronics for the current sensor system.

Description: A THIN MAGNETORESISTIVE CURRENT SENSOR SYSTEM BACKGROUND The invention pertains to current sensing. Particularly, it pertains to magnetic closed-loop, current sensors, and more particularly to those sensors using magnetoresistive technology. A method in current sensing is to pass the current to be sensed via a conductor through a metallically closed path (except for the gap) . This closed path is made of metal laminations with an air gap at one point in the loop. In this gap, a magnetic sensor is placed. The laminations cause the magnetic field of a passing current to be concentrated in the air gap. Thus, a measure of the current's magnetic field in the gap provides a measure of the current itself. However, in a closed loop system, the amount of magnetic feedback needed to null out the current's magnetic field would provide a measure of the current. A method of measuring this field is through the use of a Hall effect, magnetic field sensor. This technique requires the metal loop, or core, to be relatively thick or to be made of numerous laminations, since the principles of this technique require the plane through which the magnetic flux pass to be perpendicular to that of the laminations. This geometry and the sensitivity of Hall effect sensors require many laminations to provide the flux across large enough area needed by the Hall effect sensor to be effective. These laminations are bulky, heavy and costly. Of course, a cost-effective form of a closed-loop, current sensor is one made of just a few thin laminations. Magnetoresistive technology relies on a magnetic field in the plane of the substrate on which it is made. The conductor carrying the current to be measured is perpendicular to the plane of the core. The conductor or wire is fed through the center of the opening that the lamination encircles. Because the MR sensing plane is thin compared to the large area necessary for a Hall effect device, this enables the construction of a current sensor with a single, thin lamination, thus reducing the size and cost of a magnetic field type of current sensor. In an open loop (i.e., no feedback) a thin lamination would generally saturate, but a closed loop nulls the fields in the lamination preventing them from saturating with so little magnetic field strength. SUMMARY OF THE INVENTION This invention consists of one or, at most, only a few, flat laminations of a magnetic metal through which the current to be sensed travels. The magnetic field of this current is concentrated m the laminations. A gap m the lamination loop exists so that the magnetic field lines m the metal can cross this air gap while circling the current. The magnitude of the magnetic field in the air gap is an indication of the amount of current passing through the closed path in an open loop sensor. In the closed loop configuration, the amount of current is indicated by the magnitude of feedback needed to null out the magnetic field A magnetoresistive, magnetic sensor is in the air gap to measure the magnetic field strength. Magnetoresistive based magnetic sensors can easily be made to sense a magnetic field in the plane of a substrate or printed circuit board on which it is fabricated. By requiring a magnetic field only in the plane of the laminations and not m an area perpendicular to the laminations, the need for numerous laminations is eliminated, thereby resulting m a current sensor much smaller than the Hall effect current sensor. The thickness of the lamination stack for the present magnetoresistive current sensor can be less than 0.2 millimeter (mm) . The thickness of lamination stack of the Hall effect current sensor is typically greater than 0.2 centimeter (cm) . BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the direction of magnetic field sensitivity for a Hall effect, magnetic field sensor. Figure 2 shows the direction of magnetic field sensitivity for a magnetoresistive, magnetic field sensor. Figure 3 is a depiction of a related art Hall-effect current sensor. Figure 4 is a depiction of the at least one lamination magnetoresistive current sensor. Figure 5a is an end view of a current carrying conductor and the associated magnetic field. Figure 5b illustrates a closed magnetic path of a closed loop current sensor. Figure 6 is a schematic of the resistor bridge and its differential output amplifier for the magnetoresistive current sensor system. Figure 7 is a perspective of an embodiment of the present system as a closed loop sensor. DESCRIPTION OF THE EMBODIMENTS A Hall effect based sensor 11 is shown in figure 1. Arrow 33 shows the direction of magnetic field sensitivity. The sensitive plane (i.e., the larger surface) of sensor 11 is perpendicular to the direction of its sensitivity, therefore, requiring many laminated layers for a field concentrator. Active area 46 is in the sensitive plane of sensor 11. The stack of laminated layers 15 is thicker than the height of the larger and primary surface or plane of sensor 11, as shown in figure 3. Sensor 12 is a magnetoresistive-based sensor shown in figure 2. Arrow 34 shows the direction of magnetic sensitivity for sensor 12. Its sensitive surface or plane is parallel to the direction of its sensitivity and is parallel to the primary and larger surface or plane of a substrate or PC board 20, as shown in figure 4. That permits sensor 12 to be constructed with significantly fewer laminations. The lamination height is decreased by the difference in sensitive area heights between sensors 11 and 12, since sensor 12 ' s larger surface is parallel to the larger surfaces of the laminations 19. Fewer laminations 19 for sensor 17 do not significantly decrease the performance of current sensor 12. The active area in sensitive direction 34 involves thickness 47, which is about 20 nanometers (nm) maximum. This is the thickness of the Permalloy material on the die. The bridge of resistors 22, 23, 24 and 25, are in the Permalloy material on the die. The resistor configuration in figure 2 does not necessarily represent the actual geometry of the bridge. Different resistor configurations may be implemented. Active area 46 of sensor 11 is about 0.13 mm square (i.e., dimensions 48 and 49) . The advantage of sensor 12 over sensor 11 is the difference in height (i.e., the y-value) . The difference between 0.13 mm and 20 nm is 0.12998 mm. The height of the sensitive area 46 of sensor 11 is about 6,500 times larger than the height of the sensitive area of sensor 17. Typical laminations for this application are about 0.2 mm. A stack of 12 laminations 15 can be reduced to 11 laminations 19 with no decrease in useful performance. It also means that for a single lamination 19, MR sensor 12 has much better performance than that of Hall effect sensor 11. Figure 3 is a drawing of a metallic closed-path Hall effect current sensor system 13 made of many layers of laminated, magnetic material 15. A Hall-based, magnetic field sensor 11 is disposed in a space or gap 35 in laminations 15. The Hall sensor 11 sensitivity is primarily perpendicular to the plane of laminations 15. Lamination stack 15 is situated on substrate 16. Sensor 13 requires many laminations 15 to function adequately. Figure 4 is a drawing of a metallic closed magnetic path, magnetoresistive, current sensor system 17 made of just one or a few thin layers of laminated, magnetic material 19. In a closed-loop approach, a feedback coil 41 would also be placed on the closed path, as shown in figure 5b. A magnetoresistive magnetic field sensor 12 is in a space or gap 36 of laminations 19. Laminations 19 and sensor 12 are situated on a substrate or PCB 20. The sensitive surface of sensor 12 has a low height and its sensitivity is parallel to the larger surfaces of laminations 19 and substrate or PCB 20. The height of lamination stack 19 ranges from less than 0.2 mm to 1 mm. The height of lamination stack 15 of Hall effect sensor 11 is at least 2 mm. Each lamination layer is close to 0.2 mm. Laminations 19 are stamped from large sheets composed of a nickel-iron alloy. Laminations 19 are annealed for their magnetic properties. In a single lamination magnetic path, such as of sensor system 17, a closed loop having a feedback coil on lamination 19 would be incorporated. The magnetic feedback would null the magnetic field in the closed magnetic path in the lamination and prevent magnetic saturation of thin lamination 19. Figure 5a shows the end view of a wire 39 carrying a current 40, which emanates a magnetic field 44. It is field 44 that is sensed to determine the magnitude and direction of current 40. In figure 5b, magnetic field flux lines 45 form a closed path through lamination 19 and gap 36. Gap 36 has no magnetic material. The current through magnetic feedback coil 41 produces magnetic field 45. Magnetic field 45 nulls out magnetic field 44 from the current 40 of conductor 39. Magnetic field 44 is sensed by MR sensor 12, which via electronics 21 provides a signal 29 to indicate the amount of current needed in coil 41 to produce magnetic field 45 for nulling out field 44. Conductor or wire 39 with a given current 40 can be positioned in various places in the opening within lamination 19 without changing the constancy of the flux in gap 36. "Closed path" refers to the closed magnetic path of lamination 19 and gap 36. "Closed loop" refers to the electronics 21 and feedback coil with magnetic field feedback 45 used to null out field 44 from current 40. "Open loop" refers to a current sensor system where there is no feedback coil 41 to generate magnetic field 45 to null out magnetic field 44 from conductor 39. The magnetic field flux lines in the magnetic closed path of lamination 19 and gap 36 are from current 40 in conductor 39. This magnetic field is not nulled out. The measurement of current 40 is based on the actually sensed magnetic flux 44, not on the amount of current that is required for coil 41 to produce a magnetic field 45 sufficient to null out field 44. Figure 6 shows basic magnetoresistor electronics 21 for magnetoresistive, magnetic field or current sensor system 17 of Figure 4. Electronics 21 consist primarily of a Wheatstone bridge 30, a differential amplifier 26, feedback coil 41 and load resistor 42. Bridge 30 is a part of sensor 12 and is made of four magnetoresistive resistors 22, 23, 24 and 25, connected end-to-end in series, with diagonally opposite nodes 37 and 38 connected to a first voltage 27 and a second voltage 28, e.g., ground, respectively. Resistor bridge 30 constitutes sensor 12. Resistors 22 and 24 are designed to either increase or decrease simultaneously in resistance in the presence of a magnetic field along direction 34 of sensitivity. Resistors 23 and 25 are also designed to either decrease or increase simultaneously, but do so oppositely than resistors 22 and 24, respectively, in the presence of the same applied magnetic field. The resistors typically form a one K ohm Wheatstone bridge 30. Bridge 30 has low hysteresis and low repeatability error. The bridge also exhibits low linearity error. This design produces a maximum differential or difference in voltages between the other set of opposite nodes 31 and 32. Resistors 22, 23, 24 and 25 are fabricated from Permalloy (NiFe) . Signals from nodes 31 and 32 go to the inverting and non-inverting terminals, respectively, of differential amplifier 26. The output of amplifier 26 is a signal 29, which has a magnitude that is proportional to the magnetic field's strength sensed m direction 34, parallel to the plane of lamination stack 19. In an open loop system, the magnitude of signal 29 is m turn proportional to the magnitude of the current producing the field. Signal 29 would typically be measured as a voltage. In a closed loop system, signal 29 goes through feedback coil 41 and load resistor 42. Coil 41 is situated on the closed magnetic path of the lamination 19 and air gap 36 as shown for example by coil 41 in figure 5, 6 or 7. The coil provides a magnetic signal to null out the magnetic field of current 40. Signal 29 here is noted as a magnitude of current, particularly as required for this nulling, which is an indication of the strength of the magnetic field of current 40 in conductor 39. In turn, the magnitude of signal 29 indicates the magnitude of current 40. In the closed loop configuration, the magnetic field in the closed magnetic path caused by the current is driven towards zero, despite the amount of current. This avoids the magnetic saturation of a thin lamination or laminations 19. A conductor 39 carries current 40 through the center opening of the loop of lamination stack 19. The direction of current 40 is perpendicular to the larger surface or plane of lamination stack 19. Figure 7 reveals the mechanical details of sensor system 17. Lamination 19 is situated on PC board 20. A coil 41 is wound on bobbin 43, situated in the lamination 19 loop, for providing magnetic feedback to null out magnetic field 44 from conductor 39 by sensor 12. Sensor 12 and gap 36 may instead be in the closed magnetic path within bobbin 43. Pins 33 are for output signals 29 from sensor 17 and for providing electrical power (e.g., ± 15 volts) to sensor 17 for operation. The bottom side of PC board 20 contains the portions of electronics 21 for processing signals from sensor 12. Although the invention has been described with respect to a specific preferred embodiment, many variations and modifications will become apparent to those skilled in the art upon reading the present application. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.

Claims: THE CLAIMS 1. A thin magnetoresistive current sensor system, comprising; a substrate having a surface; a loop of a stack of at least one metal lamination, having a gap in the loop, situated on said substrate; and a magnetoresistive sensor, having a direction of sensitivity to magnetic fields parallel to its surface that is situated on the surface of said substrate and in the gap of said loop. 2. The current sensor system of claim 1, further comprising electronics, connected to said magnetoresistive sensor, for processing detection signals from said magnetoresistive sensor. 3. The current sensor system of claim 2, wherein: said magnetoresistive sensor is for measuring a magnetic field of an electric current; the electric current is conveyed by a conductor approximately perpendicular to the surface of said substrate and through a center portion of said loop of the stack of at least one metal lamination. 4. The current sensor system of claim 3 , wherein said resistors are fabricated from Permalloy. 5. The current sensor system of claim 4 further comprising: a coil wound around a portion of said loop; and wherein said coil is connected to said electronics. 6. The current sensor of claim 5, wherein the detection signals from said electronics goes to said coil to produce a magnetic field in said loop, to null the magnetic field from the electric current. 7. A thin magnetoresistive current sensor system, comprising : a layer of metal forming a closed path in a loop, wherein the closed loop is situated in a plane and has a gap; and a magnetoresistive circuit situated in the gap. 8. The system of claim 7, wherein: said magnetoresistive circuit is sensitive to a magnetic field; and said magnetoresistive circuit has a direction of sensitivity parallel to the plane. 9. The system of claim 8, wherein said magnetoresistive circuit is a Wheatstone bridge of four magnetoresistive resistors . 10. The system of claim 9, further comprising electronics having an input connected to said bridge and having an output . 11. The system of claim 10, wherein: a conductor carrying a current through a center portion of the loop of said layer of metal produces a first magnetic field; the first magnetic field results in flux in a first direction in the closed path of said layer of metal; and the output of said electronics has a first signal that indicates a magnitude of the first magnetic field sensed by said magnetoresistive circuit; 12. The system of claim 11 wherein the first signal from the output of said electronics indicates a magnitude of the current in the conductor. 13. The system of claim 11, further comprising a coil would about a portion of said layer of metal, and connected to the output of said electronics. 14. The system of claim 13, wherein: the first signal goes to the coil; the coil provides a second magnetic field that results in flux in a second direction in the closed path of said layer of metal; the second direction is opposite of the first direction; the flux in the second direction nulls the flux in the first direction; and a magnitude of the first signal sufficient to result in nulling of the flux in the first direction is an indication of a magnitude of the current in the conductor. 15. The system of claim 14, wherein the four magnetoresistive resistors are made from Permalloy. 16. A thin magnetoresistive current sensor system comprising : a sensor comprising at least one element that changes in resistance upon being affected by a first magnetic field; a loop of metal sheet; and electronics connected to said sensor and having a first output signal that indicates changes in resistance of the at least one element; and wherein: said loop has a gap; said sensor is situated in the gap; said loop, sensor and gap form a magnetic path. 17. The system of claim 16, wherein: a portion of a conductor is encircled by the magnetic path; a current in the conductor results in a first magnetic field in the magnet path, which affects the resistance of the at least one element of the sensor; and the first output signal that indicates changes in resistance of the at least one element, in turn indicates changes in magnitude of the current in the conductor. 18. The system of claim 17, wherein the first output signal is processed by said electronics into a second output signal to indicate the magnitude of the current . 19. The system of claim 18 further comprising an inductive device proximate to said magnetic path. 20. The system of claim 19, wherein the second output signal is input to said inductive device to provide a second magnetic field to null out the first magnetic field. 21. A means for current sensing, comprising: path means for providing a magnetic path in a loop of a thin metal lamination having a gap; sensing means for magnetoresistive sensing of a magnetic field situated in the gap; and processing means connected to said sensing means for processing signals; and wherein : the magnetic field is emanated by a current in a means for conducting the current; the magnetic field has a first direction in the magnetic path; the sensing means has a resistance that is affected by the magnetic field; said processing means obtains signals indicative of the resistance from said sensing means and processes the signals into signals indicative of current magnitude . 22. The means for current sensing of claim 21, further comprising : feedback means, connected to said processing means and coupled to said magnetic path; signals indicative of the resistance from said sensing means go to said feedback means which provides a magnetic field having a second direction in the magnetic path; the magnetic field having the second direction nulls out the magnetic field having the first direction; said feedback means provides a magnetic field sufficient to null out the magnetic field having the first direction until signals indicative of the resistance from said sensing means indicate no magnetic field present; a magnitude of the magnetic field having the second direction sufficient to null out the magnetic field having the first direction, is an indication of the magnitude of the current being sensed. 23. The means for current sensing of claim 22, wherein the direction of the magnetic field sufficient to null out the magnetic field emanated by the current is indicative of a direction of the current.

Inventors: FRENCH HOWARD B, GEDDES ALEX, LENZ JAMES E, PHEE JOHN, SANDQUIST DAVID A, FRENCH, HOWARD, B., GEDDES, ALEX, LENZ, JAMES, E., PHEE, JOHN, SANDQUIST, DAVID, A.
Applicants: FRENCH HOWARD B, GEDDES ALEX, HONEYWELL INC, LENZ JAMES E, PHEE JOHN, SANDQUIST DAVID A, FRENCH, HOWARD, B., GEDDES, ALEX, HONEYWELL INC., LENZ, JAMES, E., PHEE, JOHN, SANDQUIST, DAVID, A.
Application year: 2000
Main classification: G01R31
IPC codes: G01R15