PATENT DOCUMENT

Publication Number: US-9752877-B2
Application Number: US-201514838075-A
Country: US
Kind Code: B2

Title: Electronic device having electronic compass with demagnetizing coil and annular flux concentrating yokes

Abstract:
An electronic device may be provided with an electronic compass. The electronic compass may include magnetic sensors. The magnetic sensors may include thin-film magnetic sensor elements such as giant magnetoresistance sensor elements. Magnetic flux concentrators may be used to guide magnetic fields through the sensor elements. To reduce offset in the electronic compass, the magnetic flux concentrators may be demagnetized by applying a current to a coil in the housing. The coil may be formed from loops of metal traces within a printed circuit or other loops of conductive paths. Magnetic flux concentrators may have ring shapes. A ring-shaped magnetic flux concentrator may be formed from multiple thin stacked layers of soft magnetic material separated by non-magnetic material.

Claims:
What is claimed is: 
     
       1. An electronic device, comprising:
 an electronic compass having thin-film magnetoresistance sensors and magnetic flux concentrators; 
 a demagnetization coil; and 
 control circuitry that applies current to the demagnetization coil to produce a magnetic field that demagnetizes the magnetic flux concentrators. 
 
     
     
       2. The electronic device defined in  claim 1  wherein the current comprises an alternating current signal and wherein the control circuitry applies the alternating current signal to produce an alternating current magnetic field that demagnetizes the magnetic flux concentrators and thereby reduces offset in the electronic compass. 
     
     
       3. The electronic device defined in  claim 2  wherein the control circuitry is configured to apply a direct current signal to the demagnetization coil. 
     
     
       4. The electronic device defined in  claim 3  wherein the control circuitry determines magnetic sensor sensitivity levels for the thin-film magnetoresistance sensors by making magnetic field measurements with the electronic compass while the direct current signal is being applied to the demagnetization coil. 
     
     
       5. The electronic device defined in  claim 1  wherein the demagnetization coil includes loops of metal traces on a printed circuit. 
     
     
       6. The electronic device defined in  claim 5  wherein the electronic compass is mounted on a printed circuit that is separate from the printed circuit that includes the loops of metal traces. 
     
     
       7. The electronic device defined in  claim 5  wherein the electronic compass is mounted on the printed circuit. 
     
     
       8. The electronic device defined in  claim 7  wherein the loops of metal traces surround the electronic compass. 
     
     
       9. The electronic device defined in  claim 7  wherein the electronic compass is located outside of the loops of metal traces. 
     
     
       10. The electronic device defined in  claim 7  wherein the electronic compass overlaps the loops of metal traces. 
     
     
       11. The electronic device defined in  claim 1  wherein the demagnetization coil comprises multiple layers of metal traces on a printed circuit and wherein each layer of metal traces includes loops of metal traces. 
     
     
       12. The electronic device defined in  claim 1  wherein the demagnetization coil comprises a packaged inductor and wherein the electronic compass and the packaged inductor are mounted on a printed circuit board. 
     
     
       13. The electronic compass defined in  claim 1  wherein the magnetic flux concentrators include at least one ring-shaped magnetic flux concentrator. 
     
     
       14. The electronic device defined in  claim 13  wherein the ring-shaped magnetic flux concentrator includes at least first and second magnetic layers separated by a non-magnetic layer. 
     
     
       15. A magnetic sensor, comprising:
 a substrate; 
 a strip of thin-film magnetoresistance sensor structures extending along an axis on the substrate and having a series of active areas; and 
 a series of magnetic flux concentrating yokes staggered on alternating sides of the strip of thin-film magnetoresistance sensor structures to direct magnetic flux through the active areas, wherein each yoke is formed from an elongated ring of magnetic material. 
 
     
     
       16. The magnetic sensor defined in  claim 15  wherein the magnetic flux concentrating yokes each include multiple stacked magnetic layers separated by a non-magnetic layer. 
     
     
       17. The magnetic sensor defined in  claim 15  wherein the strip of thin-film magnetoresistance sensor structures comprises a strip of thin-film giant magnetoresistance sensor structures. 
     
     
       18. A portable electronic device, comprising:
 a printed circuit; 
 an electronic compass on the printed circuit that has a magnetic sensor and a magnetic flux concentrator that directs magnetic flux through the magnetic sensor; and 
 a coil of metal traces in the printed circuit through which a signal is passed to reduce leakage flux in the magnetic sensor from remnant magnetization in the magnetic flux concentrator. 
 
     
     
       19. The portable electronic device defined in  claim 18  further comprising control circuitry that passes a direct current signal through the coil to make a magnetic sensor sensitivity level measurement on the magnetic sensor. 
     
     
       20. The portable electronic device defined in  claim 18  further comprising a display. 
     
     
       21. Apparatus, comprising:
 a substrate; 
 an elongated ring-shaped magnetic flux concentrating yoke on the substrate; 
 first and second elongated thin-film magnetoresistance sensors on the substrate extending along opposing sides of the elongated ring-shaped magnetic flux concentrating yoke. 
 
     
     
       22. The apparatus defined in  claim 21  further comprising:
 a demagnetizing coil on the substrate. 
 
     
     
       23. The apparatus defined in  claim 22  wherein the elongated ring-shaped magnetic flux concentrating yoke comprises at least first and second magnetic layers separated by a non-magnetic layer. 
     
     
       24. The apparatus defined in  claim 23  wherein the first and second magnetic layers each have a thickness of less than 2 microns. 
     
     
       25. A method of operating an electronic device having an electronic compass with magnetic sensors, comprising:
 with control circuitry in the electronic device, determining whether magnetic sensor sensitivity updates are desired for the magnetic sensors of the electronic compass; and 
 in response to determining that magnetic sensor sensitivity updates are desired, using the control circuitry to apply a direct current (DC) current to a coil in the electronic device and making calibrating magnetic sensor measurements with the magnetic sensors while the DC current is being applied. 
 
     
     
       26. The method defined in  claim 25  further comprising:
 demagnetizing magnetic flux concentrators in the magnetic sensors by applying a demagnetizing current to the coil.

Description:
This application claims the benefit of and claims priority to provisional patent application No. 62/151,628 filed on Apr. 23, 2015, which is incorporated by reference herein in its entirety. 
     BACKGROUND 
     This relates generally to electronic devices, and, more particularly, to electronic devices with electronic compasses. 
     Electronic devices such as cellular telephones may contain electronic compasses. An electronic compass includes magnetic sensors that detect the Earth&#39;s magnetic field. Compass readings may be used to provide orientation information to a navigation application or to other programs that use magnetic sensor data. 
     The magnetic sensors in electronic compasses may be formed from thin-film sensor structures. Magnetic flux concentrators are used to guide and amplify ambient magnetic fields, thereby enhancing the ability of thin-film sensors to detect weak fields such as the Earth&#39;s magnetic field. The magnetic flux concentrators are formed from soft magnetic materials. 
     Magnetic structures in a magnetic sensor such as the magnetic materials in a magnetic flux concentrator can become magnetized upon exposure to magnetic fields. For example, a magnetic flux concentrator may become magnetized when an external magnet or other source of a large external magnetic field is brought into the vicinity of the magnetic flux concentrator. The magnetization of a flux concentrator that has been exposed to magnetic fields in this way will relax to a remnant state upon removal of the external magnetic field. A remnant state will typically be characterized by a complex pattern of magnetic domains. This pattern of magnetic domains can give rise to a leakage flux that creates an undesired offset in the electronic compass. The offset can introduce inaccuracies in magnetic field readings and can limit the dynamic range of the electronic compass. 
     It would therefore be desirable to be able to provide improved magnetic compasses. 
     SUMMARY 
     An electronic device may be provided with an electronic compass. The electronic device may have a housing in which the electronic compass and control circuitry for operating the electronic compass are mounted. A display may be mounted to the housing. 
     The electronic compass may include magnetic sensors. The magnetic sensors may include thin-film magnetic sensor elements such as giant magnetoresistance sensor elements. Magnetic flux concentrators may be used to guide magnetic fields through the sensor elements. 
     To reduce offset in the electronic compass, the magnetic flux concentrators may be demagnetized by applying a current to a coil in the housing. The coil may be formed from loops of metal traces on a printed circuit or other loops of conductive lines. The electronic compass may be mounted on the same printed circuit as the demagnetizing coil or on a different printed circuit. When mounted on the same printed circuit as a coil formed from loops of metal traces, the electronic compass may be mounted inside or outside of the loops or may overlap the loops. 
     Magnetic flux concentrators may have ring shapes. A ring-shaped magnetic flux concentrator may be formed from multiple thin stacked layers of soft magnetic material separated by non-magnetic material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an illustrative electronic device having a magnetic sensor such as an electronic compass in accordance with an embodiment. 
         FIG. 2  is a cross-sectional side view of an illustrative thin-film magnetic sensor element in accordance with an embodiment. 
         FIG. 3  is a perspective view of an illustrative Z-axis magnetic sensor in accordance with an embodiment. 
         FIG. 4  is a top view of an illustrative X-axis or Y-axis magnetic sensor in accordance with an embodiment. 
         FIG. 5  is a schematic diagram of illustrative circuitry for measuring the resistance of magnetic sensor elements in accordance with an embodiment. 
         FIG. 6  is a top view of an illustrative configuration for demagnetizing an electronic compass in accordance with an embodiment. 
         FIG. 7  is a cross-sectional side view of an illustrative inductor formed from coils of traces in a printed circuit board in accordance with an embodiment. 
         FIG. 8  is a top view of an illustrative inductor and electronic compass that have been mounted on adjacent printed circuits in accordance with an embodiment. 
         FIG. 9  is a cross-sectional side view of an illustrative electronic compass and associated demagnetizing inductor coil mounted on a common printed circuit in accordance with an embodiment. 
         FIG. 10A  is a graph of an illustrative demagnetizing drive signal for use in demagnetizing an electronic compass in accordance with an embodiment. 
         FIG. 10B  is a graph illustrating how residual magnetization may be reduce by applying a time-varying degaussing magnetic field in accordance with an embodiment. 
         FIG. 10C  is a cross-sectional side view of a portion of a flux concentrator showing how demagnetization operations may reduce magnetic domain ordering in accordance with an embodiment. 
         FIG. 11  is a flow chart of illustrative steps involved in demagnetizing an electronic compass in accordance with an embodiment. 
         FIG. 12  is a flow chart of illustrative steps involved in measuring sensor sensitivities in an electronic compass in accordance with an embodiment. 
         FIG. 13  is a top view of an illustrative X-axis or Y-axis magnetic sensor having annular magnetic flux concentrators in accordance with an embodiment. 
         FIG. 14  is a perspective view of an illustrative Z-axis magnetic sensor with an annular flux concentrator in accordance with an embodiment. 
         FIG. 15  is a cross-sectional side view of an illustrative flux concentrator having a stack of magnetically coupled magnetic layers in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     An illustrative electronic device of the type that may be provided with magnetic sensor circuitry such as an electronic compass is shown in  FIG. 1 . As shown in  FIG. 1 , electronic device  10  may have control circuitry  16 . Control circuitry  16  may include storage and processing circuitry for supporting the operation of device  10 . The storage and processing circuitry may include storage such as hard disk drive storage, nonvolatile memory (e.g., flash memory or other electrically-programmable-read-only memory configured to form a solid state drive), volatile memory (e.g., static or dynamic random-access-memory), etc. Processing circuitry in control circuitry  16  may be used to control the operation of device  10 . The processing circuitry may be based on one or more microprocessors, microcontrollers, digital signal processors, baseband processors, power management units, audio chips, application specific integrated circuits, etc. 
     Input-output circuitry in device  10  such as input-output devices  12  may be used to allow data to be supplied to device  10  and to allow data to be provided from device  10  to external devices. Input-output devices  12  may include buttons, joysticks, scrolling wheels, touch pads, key pads, keyboards, microphones, speakers, tone generators, vibrators, cameras, sensors, light-emitting diodes and other status indicators, data ports, displays, etc. A user can control the operation of device  10  by supplying commands through input-output devices  12  and may receive status information and other output from device  10  using the output resources of input-output devices  12 . 
     Control circuitry  16  may be used to run software on device  10  such as operating system code and applications. During operation of device  10 , the software running on control circuitry  16  may control device  10  using information from sensors and other input-output devices. 
     Device  10  may be a tablet computer, laptop computer, a desktop computer, a display, a cellular telephone or other portable device, a media player, a wristwatch device or other wearable electronic equipment, part of an embedded system that includes a display and/or other components, or other suitable electronic device. 
     Input-output devices  12  may include one or more magnetic sensors. The magnetic sensors may be used to measure the Earth&#39;s magnetic field or other magnetic fields. With one suitable arrangement, which is sometimes described herein as an example, devices  12  include an electronic compass such as electronic compass  14  for measuring the Earth&#39;s magnetic field (and, if desired, other external magnetic fields). Electronic compass  14  may be, for example, a three-axis magnetic sensor having magnetic sensors  18  for three orthogonal directions (e.g., X-axis and Y-axis magnetic sensors for making magnetic field measurements along lateral X and Y dimensions and a Z-axis magnetic sensor for making magnetic field measurements along vertical dimension Z). 
     Magnetic sensors  18  may include thin-film magnetic sensor elements such as thin-film magnetoresistance sensor elements. Thin-film magnetoresistance sensor elements may be based on anisotropic magnetoresistance (AMR) effects, may be based on giant magnetoresistance (GMR) effects, or may be based on tunneling magnetoresistance (TMR) effects. Other types of sensors  18  may be used, if desired. Configurations in which the magnetic sensor elements for compass  14  are based on giant magnetoresistance effects are sometimes described herein as an example. This is, however, merely illustrative. Compass  14  may sense magnetic fields using any suitable type of magnetic sensor. 
     A cross-sectional side view of an illustrative giant magnetoresistance magnetic sensor element is shown in  FIG. 2 . As shown in  FIG. 2 , thin-film magnetic sensor element  20  (e.g., a giant magnetoresistance sensor element) may include a stack of thin-film structures formed on substrate  22 . Pinning layer  24  may be formed from a material such as FeMn, CrMn, or PtMn. Pinned layer  26  may be formed from a magnetic material such as NiFe or NiCo. Magnetic field  28  in pinned layer  26  has a fixed orientation that is established by pinning layer  24 . Magnetic layer  34  may be formed on top of layer  26 . A non-magnetic layer  32  such as a layer of Cu or Al—Cu may be formed between layers  34  and  26 . The resistance of layer  32  may be monitored at terminals  30 . Magnetic layer  34  is a free layer (sometimes referred to as a sense layer) having a magnetic field that reflects the state of external magnetic field  36 . When, for example, magnetic field  36  is oriented in direction  40 , the magnetic field in layer  34  will be oriented in direction  40  and will be parallel to magnetic field  28 . In this situation, the electrical resistance in layer  32  will have a first value. When external magnetic field  36  is oriented in direction  42 , the magnetic field in layer  34  will be oriented in direction  42  and will be antiparallel to magnetic field  28 . When the magnetic fields in layers  34  and  26  are antiparallel, the electrical resistance in layer  32  will have a second value that is greater than the first value. Changes in resistance in layer  32  may therefore be used to measure external magnetic field  36 . If desired, other configurations may be used for thin-film magnetic sensor elements in sensors  18 . The illustrative configuration of  FIG. 2  is presented as an example. 
     Magnetic sensor elements such as thin-film giant magnetoresistance magnetic sensor element  20  of  FIG. 2  may exhibit desirable attributes such as low power consumption. Magnetic flux concentrators that are formed from soft magnetic materials may be used to amplify ambient magnetic fields and to direct ambient magnetic fields through the thin-film sensor elements. The shape of the flux concentrators (which may sometimes be referred to a flux guides, flux directing structures, magnetic flux concentrating yokes, etc.) may be different for the different axes in compass  14 . Thin-film magnetic sensors that lie in the horizontal (X-Y) plane may use flux concentrators that redirect fields within the X-Y plane. Magnetic field measurements that are made on vertically oriented magnetic fields (i.e., magnetic fields running along vertical axis Z) may be made using a flux concentrator of the type shown in  FIG. 3 . 
     As shown in  FIG. 3 , magnetic sensor  18  may include sensor elements  20  and magnetic flux concentrator  38 . Magnetic flux concentrator  38  may redirect vertical magnetic fields  36  so that they pass horizontally through sensor elements  20  (i.e., parallel to dimension X in the example of  FIG. 3 ). Flux concentrator  38  may have an elongated rectangular box shape or other suitable shape. The length of flux concentrator  38  along axis Y may, for example, be tens or hundreds of microns or other suitable length. The width of flux concentrator  38  along axis Z may be 1-10 microns or other suitable width. The height of flux concentrator  38  along dimension Z may be 1-20 microns or other suitable height. 
     Magnetic sensor elements  20  may include first and second elongated thin-film magnetic sensor elements  20 A and  20 B. When oriented as shown in  FIG. 3 , sensor element  20 A may register an increase in resistance whenever sensor element  20 B registers a decrease in resistance. Sensor elements  20 A and  20 B may therefore sometimes be referred to as positive and negative sensor elements and may be placed in respective positive and negative arms of a resistive bridge circuit or other circuit to facilitate resistance measurements. 
     A top view of an illustrative flux concentrator of the type that may be used to direct and amplify magnetic field  36  when making magnetic field measurements on magnetic field  36  that is in the X-Y plane is shown in  FIG. 4 . In the example of  FIG. 4 , sensor  18  is an X-axis magnetic sensor. Incoming magnetic field  36  along axis X is directed through active region  20  of a strip of thin-film magnetoresistance sensor structures (strip  20 ′) by magnetic flux concentrators  38  (see, e.g., magnetic field  36 ′, which is measured by active region  20 ). Flux concentrators  38  may have a Z-shape or other suitable shape. 
     If desired, other flux concentrator designs may be used for forming the flux concentrator structures in sensors  18  of compass  14 . The configurations shown  FIGS. 3 and 4  are merely illustrative. 
       FIG. 5  is an illustrative resistive bridge circuit (i.e., a Wheatstone bridge) of the type that may be used to measure the resistance(s) of one or more magnetic sensor elements  20 . In the example of  FIG. 5 , resistance R 1  corresponds to a first magnetic sensor element (e.g., positive element  20 A of  FIG. 3 ) and resistance R 2  corresponds to a second magnetic sensor element (e.g., negative element  20 B of  FIG. 3 ). Reference voltages are applied to terminals  42  of bridge circuit  40 . Paths  44  may be used to convey signals from nodes  54  to measurement circuit  46  (e.g., part of control circuitry  16  of  FIG. 1 ). Circuit  46  may contain components such as differential amplifier  48  for producing an output that is proportional to the voltage difference across nodes  54 . Analog-to-digital converter  50  may convert this output to a digital output on path  52 . The output on path  52  will be proportional to the resistance of sensor elements  20  and will therefore reflect the strength of external magnetic field  36  that is being measured by the magnetic sensor elements in the bridge circuit. 
     When external magnetic fields of sufficient strength are applied to compass  14 , the soft magnetic material of the flux concentrators can become magnetized. Once the external magnetic field(s) is (are) removed, the flux concentrator returns to a “remnant” state. In the remnant state, the patterns of magnetic domains that are established in magnetized flux concentrators can lead to offsets in the readings of magnetic sensors  18  and therefore compass  14 . With one suitable arrangement, device  10  may include one or more coils that can create demagnetizing magnetic fields. The demagnetizing fields may be used to demagnetize flux concentrators  38  and thereby remove undesired offsets from compass  14 . 
     The coils (which may sometimes be referred to as loops, inductors, or inductive elements) may have loop-shaped signal paths formed from metal wire, metal traces on one or more layers of a printed circuit board or other substrate, structures in a package (e.g., a surface mount technology package or other suitable electrical component package), coils in a packaged inductor, or other suitable structures that can produce a magnetic field in response to application of a current. The signal lines in a demagnetizing coil of this type may have a plurality of turns (such as two or more turns or one hundred or more turns, or any other suitable number of turns). 
     In the illustrative configuration of  FIG. 6 , compass  14  (e.g., a packaged three-axis magnetic sensor having X, Y, and Z sensors  18  or other suitable magnetic sensor) may be mounted on substrate  56 . Coil  58  may have signal paths  62  that are organized in a series of concentric loops. The outline of the loops may be circular, elliptical, rectangular, or square, may have straight segments, curved segments, and/or combinations of straight and curved segments, or other suitable shapes. The cross-sectional shapes of paths  62  may be rectangular, circular, etc. 
     Current may be applied to terminals  60  by control circuitry  16  to generate a demagnetizing magnetic field that demagnetizes compass  14 . Compass  14  may be mounted within the interior of coil  58  so that all of compass  14  overlaps coil  58 , in a position that overlaps signal paths  62  of coil  58  (see, e.g., position  14 ′ in which part of compass  14  lies within coil  58  and part of compass  14  lies outside of coil  58 ), or in a position that does not overlap coil  58  but which is still sufficiently close to coil  58  to receive magnetic fields from coil  58  (see, e.g., position  14 ″ in which compass  14  is mounted outside of the loops of coil  58 ). 
     Coil  58  may be formed from metal traces that are formed on substrate  56 . For example, substrate  56  may be a printed circuit (e.g., a rigid printed circuit board formed from layers of printed circuit board material such as fiberglass-filled epoxy or a flexible printed circuit formed from a single-layer or multi-layer flexible polymer sheet such as a flexible polyimide layer) and coils  58  may be formed from one or more loops of metal traces in the printed circuit. 
     A cross-sectional side view of an illustrative printed circuit substrate that contains multiple interconnected loops of metal traces  62  for forming coil  58  is shown in  FIG. 7 . There are three loops of signal paths  62  in the respective layers of printed circuit  56  in the example of  FIG. 7 . This is merely illustrative. There may be fewer layers of metal traces, there may be more layers of metal traces, there may be two or more concentric loops of traces in each layer, or other configurations may be used for forming coil  58 . In the example of  FIG. 8 , coil  58  and compass  20  have been mounted on separate printed circuits in device  10 . Compass  14  has been mounted on printed circuit board  56 A. Coil  58  has been mounted on printed circuit board  56 B. If desired, coil  58  may be a packaged inductor having loops of wire or other conductive paths  62  (see, e.g., the cross-sectional side view of  FIG. 9  in which compass  14  and packaged inductor  58  have been mounted on printed circuit  56 ). 
     To demagnetize compass  14 , a demagnetizing drive current may be applied to coil  58 . The demagnetizing drive current may be, for example, an alternating current (AC) waveform with an exponentially decreasing envelope such as the illustrative signal of  FIG. 10A . The frequency of the AC waveform may be about 2000 Hz (or more than 1000 Hz, less than 3000 Hz, less than 10,000 Hz, less than 1000 Hz, 500-1000 Hz, more than 200 Hz, less than 700 Hz, etc.) and may have a duration of 10-15 ms, less than 50 ms, more than 5 ms, or any other suitable duration. The demagnetizing current that is applied to coil  58  generates an AC magnetic field that scrambles the magnetic domains within flux concentrator  38  and thereby reduces undesired offset. As shown in  FIG. 10B , for example, as applied magnetic field intensity is cycled back and forth during demagnetization operations, the amount of remnant magnetic flux from the magnetic domains within flux concentrator  38  decreases. Initially, concentrator  38  might be characterized by a relatively large magnetic flux density (see, e.g., point  100  of  FIG. 16 ). Following application of the demagnetizing current to coil  58 , the amount of magnetic flux density in flux concentrator  38  may decrease (see, e.g., point  102  of  FIG. 16 ). As shown in  FIG. 10C , flux concentrator  38  may initially have domains  104  that are relatively ordered and, following demagnetization, may have less regularly ordered domains  106 . 
     Illustrative steps involved in using coil  58  to remove offset from compass  14  are shown in  FIG. 11 . At step  64  (e.g., during design, testing, and manufacturing operations), an optimum position for coil  58  relative to compass  14  may be determined. This placement preferably helps remove offset from all axes of interest (e.g., X, Y, and Z for compass  14  of  FIG. 1 ) during demagnetization. At step  66 , device  10  is manufactured, including compass  14  and at least one appropriately located coil  58 . 
     At step  68 , a user of device  10  may operate device  10  normally. During operation, device  10  may sometimes not be exposed to significant external magnetic fields, so no change will take place in the offset of sensors  18  of compass  14 . As shown by line  70 , the user may continue to use device  10  normally in this situation. If, however, a significant offset is induced in one or more of the sensors  18  of compass  14  by exposure to a large external magnetic field, control circuitry  16  may apply a demagnetizing signal such as the signal of  FIG. 10  to coil  58  to demagnetize compass  14  (step  72 ). The operations of step  72  may be performed periodically, may be performed whenever compass  14  detects a magnetic field more than a predetermined threshold, may be performed in response to user input, or may be performed when other suitable criteria have been satisfied, after which device  10  can be operated normally (step  68 ), as indicated by line  74 . 
     It may be desirable to monitor the sensitivities of each of sensors  18 . Initially (e.g., during calibration as part of a manufacturing operation or at any other suitable time), the sensitivities of sensors  18  may be determined (step  76 ). Sensor sensitivity may be known from previous device characterization operations and/or coil  58  may produce a known magnetic field in response to application of a known direct current (DC) signal to coil  58 . The known magnetic field may be measured by each of sensors  18  and these measurements used to ascertain the sensitivity of each of sensor  18 . The initial sensor sensitivity levels for sensors  18  may be stored in device  10 . 
     A user of device  10  may use electronic compass  14  to gather measurements of the Earth&#39;s magnetic field or other magnetic fields at step  78 . The sensitivities of sensors  18  may be measured periodically, in response to the occurrence of one or more triggering events (e.g., measurement of a large magnetic field), in response to user input, or in response to the satisfaction of other suitable criteria. If no sensor updates are needed, processing may continue at step  78 , as indicated by line  80 . When sensor sensitivity updates are desired, control circuitry  16  may apply a small known DC current to coil  58  at step  82 . In response to the applied current, coil  58  may generate a known amount of magnetic field. The strength of the known magnetic field may be measured by sensors  18 . The known magnetic field strength and the known sensor readings may be processed to determine the sensitivity of each sensor  18  at step  84 . These sensitivity levels may be stored in memory in device  10  and used to calibrate future magnetic field measurements with compass  14 . Following step  84 , processing may loop back to step  78  (i.e., device  10  may be used normally), as shown by line  86 . 
     If desired, the stability of the magnetic domain pattern in flux concentrators  38  may be enhanced by using a loop-shaped (annular) flux concentrator configuration. Non-annular magnetic flux concentrators may be characterized by disorderly magnetic domain patterns after being exposed to large external magnetic fields. Ring-shaped flux concentrators, however, are characterized by stable magnetic domain patterns (e.g., all magnetic domains may be oriented in a loop that runs around the flux concentrator ring or in other well-ordered patterns). A ring-shaped flux concentrator will therefore be unlikely to acquire a magnetic domain pattern that produces an unexpected and undesired magnetic sensor offset in compass  14 . 
       FIG. 13  is a top view of an illustrative X-axis or Y-axis magnetic sensor for compass  14 . In the example of  FIG. 13 , sensor  18  has a series of elongated ring-shaped magnetic flux concentrators  38  that are located on alternating sides of an elongated strip of thin-film magnetoresistance sensor material (thin-film magnetoresistance sensor strip  20 ′). Active areas  20  of strip  20 ′ form magnetoresistance sensor elements that measurably change resistance in response to directed magnetic fields  36 ′ when an external magnetic field (field  36 ) is present. There are three ring-shaped flux concentrators  38  in the example of  FIG. 13  (i.e., two concentrators that are located along one edge of strip  20 ′ and one that is located along the opposing edge of strip  20 ′). This is merely illustrative. There may be two or more flux concentrators  38 , three or more flux concentrators  38 , four or more flux concentrators  38 , five or more flux concentrators  38 , etc. 
       FIG. 14  is a perspective view of an illustrative Z-axis magnetic sensor for compass  14 . In the illustrative configuration of  FIG. 14 , sensor  18  has positive elongated magnetic sensor element  20 A and negative elongated magnetic sensor element  20 B extending along opposing sides of ring-shaped magnetic flux concentrator  38  in parallel with the longitudinal axis of ring-shaped magnetic flux concentrator  38 . Flux concentrators  38  of  FIGS. 13 and 14  may be 10-100 microns long (or more than 20 microns long or less than 200 microns long) and 1-15 microns high and wide (or more than 1 micron or less than 20 microns). The width of the ring-shaped structure in concentrators  38  may be 1 micron, 0.5-2 microns, more than 0.8 microns, less than 1.5 microns, or other suitable size. The gap in the middle of the ring may be about 3 microns wide, 1-5 microns wide, more than 2 microns wide, or less than 5 microns wide (as examples). Magnetic flux concentrator  38  may have a ring shape such as the shape of a rectangular ring (e.g., a rectangular shape with slightly rounded corners as shown in  FIG. 14 ), an elliptical ring (see, e.g., curved ends  200  in  FIG. 14 ), ring shapes with ends and/or side segments of other curved and/or straight shapes, or other ring shapes. 
     The ring shapes of flux concentrators  38  such as the flux concentrators of  FIGS. 13 and 14  cause flux concentrators  38  to form stable closure domains (and avoid domain wall formation) so the magnetic domains in the flux concentrators are oriented around the ring in a stable fashion. Formation of an orderly and stable magnetic domain pattern in flux concentrators  38  can be further enhanced by forming flux concentrators  38  from a stack of thin magnetically coupled magnetic layers. The layers are preferably sufficiently thin to encourage domains to remain oriented within the plane of the flux concentrator ring. 
     A cross-sectional side view of an illustrative flux concentrator with a multilayer configuration of this type is shown in  FIG. 15 . As shown in  FIG. 15 , flux concentrator  38  may have a first magnetic layer such as magnetic layer  38 - 1 , a non-magnetic layer such as layer  90 , and a second magnetic layer such as magnetic layer  38 - 2 . Additional magnetic layers and non-magnetic layers may be included in the stack of magnetic layers for concentrator  38  if desired (e.g., flux concentrator  38  may have four or more layers of magnetic material). 
     Magnetic layers  38 - 1  and  38 - 2  may be formed from a magnetic material such as NiFe (permalloy), NiCo, CoFe, or other alloys or soft magnetic materials such as Ni, Fe, and Co. Non-magnetic layer  90  may be formed from a layer of aluminum oxide or other non-magnetic material. The thickness T 3  of non-magnetic layer  90  may be less than 0.2 microns, less than 0.1 microns, more than 0.01 microns, or other suitable thickness that allows layers  38 - 1  and  38 - 2  to magnetically couple. The thicknesses T 1  and T 2  of magnetic layers  38 - 1  and  38 - 2  are preferably less than 1 micron, although larger thicknesses may be used if desired (e.g., thicknesses T 1  and T 2  may be less than 2 microns, etc.). Thickness T 1  may be equal to thickness T 2  or may be slightly greater than thickness T 2  or other suitable thickness. 
     The foregoing is merely illustrative and various modifications can be made by those skilled in the art without departing from the scope and spirit of the described embodiments. The foregoing embodiments may be implemented individually or in any combination.

Metadata:
Filing Date: 20150827
Publication Date: 20170905
Grant Date: 20170905
Priority Date: 20150423
Inventors: BHATTACHARYYA MANOJ K
Balcells Christopher E.
GUO JIAN
HARTWELL PETER G.
Assignee: APPLE INC
CPC Classifications: [{"code": "G01R35/005", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01C17/02", "inventive": true, "first": true, "tree": "[]"}, {"code": "G01R33/0011", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R33/093", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R33/0011", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01R35/005", "inventive": false, "first": false, "tree": "[]"}, {"code": "G01R33/093", "inventive": true, "first": false, "tree": "[]"}, {"code": "G01C17/02", "inventive": true, "first": true, "tree": "[]"}]
Family ID: 57148604