Abstract:
Methods and apparatus for a magnetic sensor including an elliptical magnet to generate substantially circular concentric zones of similar flux density in a plane over and parallel to a surface of the magnet. The sensor can include a sensing element disposed a selected distance from the magnet and a substrate containing circuitry to process a signal from the sensing element to provide a sensor output.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Planar Hall elements and vertical Hall elements are known types of magnetic field sensing elements that can be used in magnetic field sensors. A planar Hall element tends to be responsive to (i.e., have a major response axis aligned with) magnetic fields perpendicular to a surface of a substrate on which the planar Hall element is formed. A vertical Hall element tends to be responsive to (i.e., have a major response axis aligned with) magnetic fields parallel to a surface of a substrate on which the vertical Hall element is formed. 
         [0002]    Other types of magnetic field sensing elements are known. For example, a so-called “circular vertical Hall” (CVH) sensing element, which includes a plurality of vertical magnetic field sensing elements, is known and described in PCT Patent Application No. PCT/EP2008/056517, entitled “Magnetic Field Sensor for Measuring Direction of a Magnetic Field in a Plane,” filed May 28, 2008, and published in the English language as PCT Publication No. WO 2008/145662, which application and publication thereof are incorporated by reference herein in their entirety. The CVH sensing element is a circular arrangement of vertical Hall elements arranged over a common circular implant region in a substrate. The CVH sensing element can be used to sense a direction (and optionally a strength) of a magnetic field in a plane of the substrate. A CVH sensing elements tends to be responsive to (i.e., have a major response axis aligned with) magnetic fields parallel to a surface of the substrate on which the CVH sensing element is formed. 
         [0003]    Various parameters characterize the performance of magnetic field sensing elements. These parameters include sensitivity, which is a change in an output signal of a magnetic field sensing element in response to a change of magnetic field experienced by the magnetic sensing element, and linearity, which is a degree to which the output signal of the magnetic field sensing element varies in direct proportion to the magnetic field. These parameters also include an offset, which is characterized by an output signal from the magnetic field sensing element not representative of a zero magnetic field when the magnetic field sensing element experiences a zero magnetic field. 
         [0004]    As described above, the CVH sensing element is operable, with associated circuits, to provide an output signal representative of an angle of a direction of a magnetic field. Therefore, as described below, if a magnet is disposed upon or otherwise coupled to a so-called “target object,” for example, a camshaft in an engine, the CVH sensing element can be used to provide an output signal representative of an angle of rotation, and/or a rotation speed, of the target object. 
         [0005]    Other conventional magnetic field sensor arrangements position a magnetic field sensor and associated magnetic field sensing element along an axis of movement of a line magnet configured to move linearly, the line magnet coupled to a target object. In these arrangements, the magnetic field sensor is disposed such that the major response axis of the magnetic field sensing element within the magnetic field sensor is parallel to the axis of movement of the line magnet and, like the conventional arrangement described above, parallel to a major surface of the line magnet. 
       SUMMARY OF THE INVENTION 
       [0006]    Exemplary embodiments of the invention provide methods and apparatus for sensors having elliptical magnets that have been magnetized along the major axis to generate substantially circular concentric zones of similar flux density. With this arrangement, a region in which a sensing element, for example, can be placed in relation to the elliptical magnet is increased as compared to a circular magnet that has been diametrically magnetized, which generates elliptical zones of flux density. In addition, less magnet material is needed for an elliptical magnet, as compared to a cylindrical magnet, to achieve comparable levels of angular position error. While exemplary embodiments of the invention are shown and described in conjunction with particular applications and configurations, it is understood that embodiments of the invention are applicable to magnetic applications in general in which circular concentric zones of similar flux density, less magnetic material, and/or greater offset tolerance are desirable. 
         [0007]    In one aspect of the invention, a magnetic sensor comprises an elliptical magnet to generate circular concentric zones of similar flux density in a plane over and parallel to a surface of the magnet, a sensing element disposed a selected distance from the magnet, and a substrate containing circuitry to process a signal from the sensing element to provide a sensor output. 
         [0008]    The sensor can further include one or more of the following features: a center of the concentric zones is aligned with a center of the magnet surface, a gradient of the flux density across the concentric zones is less than for a cylindrical magnet of the same volume, the sensor provides better accuracy than a magnetic sensor having a cylindrical magnet having the same volume as the elliptical magnet, the sensing element comprises a circular vertical hall (CVH) element, the elliptical magnet, a length to width ratio of the elliptical magnet is about 1.5, the elliptical magnet is not required to be keyed for orientation, the sensor senses angular position of a target, and/or the sensing element comprises at least one of a Hall element, an anisotropic magnetoresistance (AMR) element and/or a giant magnetoresistance (GMR) element. 
         [0009]    In another aspect of the invention, a magnetic sensor comprises a circular vertical hall element, and an elliptical magnet to generate circular concentric zones of substantially similar flux density in a plane parallel to a surface of the magnet, wherein the circular hall element is located in relation to the magnet. 
         [0010]    The sensor can further include one or more of the following features: a gradient of the flux density across the concentric zones is less than for a cylindrical magnet of the same volume, the sensor provides better accuracy than a magnetic sensor having a cylindrical magnet having the same volume as the elliptical magnet, a length to width ratio of the elliptical magnet is about 1.5, and/or the elliptical magnet is not keyed for orientation. 
         [0011]    In other embodiments, the elliptical magnet is keyed for orientation. It is understood that an elliptical shape is not completely symmetric across x and y axes which is useful from a safety standpoint since if the adhesive that holds the magnet in the holder fails, the magnet continues to rotate with the shaft and continues to provide useful data if the magnet does not fall out of the system. 
         [0012]    In a further aspect of the invention, a method comprises providing an elliptical magnet to generate concentric circular zones of similar flux density in a plane over and parallel to a surface of the magnet, providing a sensing element disposed a selected distance from the magnet, and providing a substrate containing circuitry to process a signal from the sensing element to provide a sensor output. 
         [0013]    The method can further include one or more of the following features: a gradient of the flux density across the concentric zones is less than for a cylindrical magnet of the same volume, the sensor provides better accuracy than a magnetic sensor having a cylindrical magnet having the same volume as the elliptical magnet, the sensing element comprises a circular vertical hall element, and/or a length to width ratio of the elliptical magnet is about 1.5. 
         [0014]    In a further aspect of the invention, a method comprises reducing an amount of material required to achieve a selected level of accuracy for a magnetic sensor as compared to a circular magnet by forming a magnet having an elliptical shape to generate circular concentric zones of substantially similar flux density in a plane parallel to a surface of the magnet. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing features of the invention, as well as the invention itself may be more fully understood from the following detailed description of the drawings, in which: 
           [0016]      FIG. 1  is a schematic representation of concentric zones of similar flux density in a plane above an elliptical magnet; 
           [0017]      FIG. 1A  shows the plane over an elliptical magnet; 
           [0018]      FIG. 1B  is a plan view of an exemplary elliptical magnet; 
           [0019]      FIG. 1C  is a schematic representation of flux density in a plane above an elliptical magnet; 
           [0020]      FIG. 2  is a schematic representation of elliptical zones of similar flux density in a plane above a prior art cylindrical magnet; 
           [0021]      FIG. 2A  is as schematic representation of an elliptical flux density zone rotations for a prior art cylindrical magnet; 
           [0022]      FIG. 3  is a graphical representation of accuracy error versus sensor magnet eccentricity for given volumes of elliptical and cylindrical magnets; 
           [0023]      FIG. 4  is a schematic representation of a circular vertical hall element having an elliptical magnet; 
           [0024]      FIG. 4A  is a schematic representation of a plurality of magnetic field sensing elements, for example, vertical Hall elements or magnetoresistance elements, upon a substrate; 
           [0025]      FIG. 5  is a graph showing an output signal that may be generated by a CVH sensing element having an elliptical magnet; 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0026]    Before describing exemplary embodiments of the invention, some introductory concepts and terminology are explained. As used herein, the term “magnetic field sensing element” is used to describe a variety of electronic elements that can sense a magnetic field. The magnetic field sensing elements can be, but are not limited to, Hall effect elements, magnetoresistance elements, or magnetotransistors. As is known, there are different types of Hall effect elements, for example, a planar Hall element, a vertical Hall element, and a circular vertical Hall (CVH) element. As is also known, there are different types of magnetoresistance elements, for example, a giant magnetoresistance (GMR) element, an anisotropic magnetoresistance element (AMR), a tunneling magnetoresistance (TMR) element, an Indium antimonide (InSb) sensor, and a magnetic tunnel junction (MTJ). 
         [0027]    As used herein, the term “sensor” is used to describe a circuit or assembly that includes a sensing element and other components. In particular, as used herein, the term “magnetic field sensor” is used to describe a circuit or assembly that includes a magnetic field sensing element and electronics coupled to the magnetic field sensing element. 
         [0028]    As is known, some of the above-described magnetic field sensing elements tend to have an axis of maximum sensitivity parallel to a substrate that supports the magnetic field sensing element, and others of the above-described magnetic field sensing elements tend to have an axis of maximum sensitivity perpendicular to a substrate that supports the magnetic field sensing element. In particular, planar Hall and some semiconductor based magnetoreistance elements tend to have axes of sensitivity perpendicular to a substrate, while anisotropic and giant magnetoresistance (AMR and GMR) elements and vertical Hall elements (including circular vertical Hall (CVH) sensing elements) tend to have axes of sensitivity parallel to a substrate. 
         [0029]    Magnetic field sensors are used in a variety of applications, including, but not limited to, an angle sensor that senses an angle of a direction of a magnetic field, a current sensor that senses a magnetic field generated by a current carried by a current-carrying conductor, a magnetic switch that senses the proximity of a ferromagnetic object, a rotation detector that senses passing ferromagnetic articles, for example, magnetic fields from regions of different magnetization of a ring magnet, and a magnetic field sensor that senses a magnetic field density of a magnetic field. 
         [0030]    While a circular vertical Hall (CVH) magnetic field sensing element, which has a plurality of vertical Hall magnetic field sensing elements, is described in examples below, it should be appreciated that the same or similar techniques apply to any type of magnetic field sensing element and to any type of magnetic field sensor. In particular, techniques apply to one or more separate vertical Hall elements or separate magnetoresistance elements, not arranged in a CVH structure, and with or without associated electronic circuits. 
         [0031]    In one aspect of the invention, a magnetic field sensor comprises a magnet with an elliptical shape to provide a highly uniform magnetic field in a plane parallel to the magnet surface. The magnetic field flux density comprises substantially circular concentric zones of substantially similar flux density. The benefits of a uniform magnetic field to reduce error levels in sensing applications will be readily apparent to one of ordinary skill in the art. In addition, a volume of magnetic material required for an elliptical magnet to produce a given accuracy level is less than for a comparable cylindrical magnet. While an elliptical magnet to generate a uniform magnetic field is well-suited for angle sensing applications that use a magnetic field to determine a position and/or direction of the target magnet, it is understood that embodiments of the invention are applicable to magnetic field sensing applications in general. 
         [0032]      FIGS. 1-1A  show exemplary flux density over a portion of an elliptical magnet M in a plane P ( FIG. 1A ) parallel to the magnet surface in accordance with exemplary embodiments of the invention. As can be seen, the flux density forms a number of substantially circular zones Z 1 - 4  where each zone is of substantially similar density. The zones Z 1 - 4  are concentric circles about a center C of the magnet. For magnets that are generally symmetrical and uniformly magnetized, the center of the zones coincides with the center of the magnet. It is understood that any number of flux density zones can be generated depending on the size and material of the magnet. 
         [0033]    In the illustrative flux density representation, the field is calculated by simulation for a 3 mm by 3 mm plane 1 mm from the surface of over the magnet, which is formed form a bonded NdFeB material (Br=0.60T, Hc=440 KA/m, and Bhamx=70KJ/m 3 ). The average field strength is about 560 G with a 30G gradient from the center of the magnet surface to the edge of the 3 mm by 3 mm edge. 
         [0034]      FIG. 1C  shows an elliptical magnet with cutout to retain the magnet within a holder for providing rotational friction/grip. It is understood that with inventive embodiments of an elliptical magnet, the shape orients itself and requires no cutouts. The elliptical shape of the magnet allows positioning without a cutout, which allows a more uniform magnetic field than may be achieved with some cutout geometries. 
         [0035]    As shown in  FIG. 2 , a conventional cylindrical magnet CM generates elliptical zones of flux density EZ at a plane above (or below) the magnet. The cylindrical magnet with no cutouts, will have non-uniform, oblong flux density zones centered on the magnet center MC. With no cutouts, the oblong flux density zones are centered on MC. With cutout (s), the center of the elliptical flux density zones is offset from the center MC of the magnet due to the asymmetry of the cutout. As can be seen in  FIG. 2A , as the cylindrical magnet or sensing element near the magnet surface rotates, a given elliptical zone of similar flux density rotates with an offset about the center of magnet. Thus, the (shaded) area in which an element can be placed to remain in a given zone of similar flux density is reduced due to the rotation of the magnet or rotation of the element in relation to the cylindrical magnet. This limits the tolerance for a pick and place machine, for example, to place an element in relation to the cylindrical magnet. 
         [0036]    In contrast, referring again to  FIG. 1 , an elliptical magnet M provides concentric circular zones Z 1 -N that have centers substantially aligned with the center of the magnet M. The circular zones allow the sensor to always have the same applied flux density. This increases the area of a region in which there is a zone of similar flux density in the face of rotation since the center of the similar flux density zones is substantially aligned with the magnet center. 
         [0037]    As shown in  FIG. 3 , an angle sensor having an elliptical magnet provides superior accuracy over cylindrical magnets for a given volume of magnet material.  FIG. 3  is a graphical representation of accuracy error versus sensor magnet eccentricity for given volumes of elliptical and cylindrical magnets. Sensor to magnet eccentricity refers to the distance between the physical center of a magnet and the center of the sensing element. Perfect alignment (zero eccentricity) would be defined as a configuration where the center of the magnet is located on the same axis as the sensing element. 
         [0038]    As can be seen, a first cylindrical magnet CM 1  having a 6 mm diameter and a volume of 84.8 mm 3  has a greater accuracy error than a first elliptical magnet EM 1  (7.3 mm by 4.9 mm) of the same volume. It is understood that accuracy errors refers to angular sensing error. For example, an absolute error of 3.6° of error out of 360° of rotation would be 1%. Angular error is determined by comparing the results of an angle measurement with the known physical position determined by external means, such as an encoder. For these illustrated accuracy plots, the sensor is oriented at the distance indicated on the graph in the X direction. Since the error is for a full rotation, the magnet, the error curve is the same for X displacement or Y displacement. 
         [0039]    Similarly, a second cylindrical magnet CM 2  has a greater accuracy error than a second elliptical magnet EM 2  of the same volume and a third cylindrical magnet CM 3  has a greater accuracy error that a third elliptical magnet EM 3  of the same volume. 
         [0040]    The benefits in reducing the volume of magnetic material to achieve a desired performance will be readily appreciated. The recent increases in the cost of magnetic materials is well known. In addition, an elliptical magnet achieves the same diameter of uniform field for a 360° rotation with less material than conventional magnets. 
         [0041]    It is understood that the length to width ratio of the magnet shape can vary to meet the needs of a particular application. In one particular embodiment, the length to width ratio of about 1.5 provides the optimal field for achieving circular, concentric zones of similar flux density. It is understood that the magnet is magnetized along the longest dimension, or the major axis of the elipse. In general, the thickness of the magnet primarily influences the magnitude of the field. 
         [0042]    It is understood that the eccentricity of the ellipse defining a shape of the magnet can vary to meet the needs of a particular application. In one embodiment, a magnet has a length of 9.8 mm, a width of 6.5 mm, for a length to width ratio of about 1.5, and a thickness of about 3 mm. 
         [0043]    Exemplary applications for inventive elliptical magnets to generate zones of similar flux density will be readily apparent to one of ordinary skill in the art. One application is magnetic sensor devices, such as angle sensors that use magnets in conjunction with sensing elements, such as Hall elements to sense perturbations in a magnetic field. 
         [0044]      FIG. 4  shows an exemplary circular vertical Hall (CVH) device  10  having an elliptical magnet  14  in accordance with exemplary embodiments of the invention. A circular vertical Hall (CVH) sensing element  12  includes a circular implant region  18  having a plurality of vertical 
         [0045]    Hall elements disposed thereon, of which a vertical Hall element  12   a  is but one example. Each vertical Hall element has a plurality of Hall element contacts (e.g., four or five contacts), of which a vertical Hall element contact  12   aa  is but one example. 
         [0046]    A particular vertical Hall element (e.g.,  12   a ) within the CVH sensing element  12 , which, for example, can have five adjacent contacts, can share some, for example, four, of the five contacts with a next vertical Hall element (e.g.,  12   b ). Thus, a next vertical Hall element can be shifted by one contact from a prior vertical Hall element. For such shifts by one contact, it will be understood that the number of vertical Hall elements is equal to the number of vertical Hall element contacts, e.g., 32. However, it will also be understood that a next vertical Hall element can be shifted by more than one contact from the prior vertical Hall element, in which case, there are fewer vertical Hall elements than there are vertical Hall element contacts in the CVH sensing element. 
         [0047]    A center of a vertical Hall element  0  is positioned along an x-axis  20  and a center of vertical Hall element  8  is positioned along a y-axis  22 . In the exemplary CVH  12 , there are thirty-two vertical Hall elements and thirty-two vertical Hall element contacts. However, a CVH can have more than or fewer than thirty-two vertical Hall elements and more than or fewer than thirty-two vertical Hall element contacts. 
         [0048]    In an exemplary embodiment, an elliptical magnet  14  has a south side  14   a  and a north side  14   b  disposed over the CVH  12 . The magnet  14  tends to generate a magnetic field  16  having a direction from the north side  14   a  to the south side  14   b.  Other magnets having other shapes and configurations are possible. 
         [0049]    In an exemplary embodiment, the elliptical magnet  14  is mechanically coupled to a rotating object (a target object), for example but not limited to, an automobile crank shaft or an automobile camshaft, a throttle position sensor, or the shaft of an electric motor, and is subject to rotation relative to the CVH sensing element  12 . With this arrangement, the CVH sensing element  12  in combination with an electronic circuit described below can generate a signal related to the angle of rotation of the magnet  14 . 
         [0050]    Since the elliptical magnet  14  generates concentric regions of similar flux density centered about a center of the magnet it will be appreciated that the accuracy of the CVH device will be enhanced as compared to conventional devices having cylindrical magnets that generate elliptical flux density regions. 
         [0051]    A center  24  of the CVH sensing element  12  is at a center of the entire CVH sensing element  12 . Since the CVH sensing element  12  has very little depth (into the page), the center  24  can be considered to be on the surface of the substrate  26 . 
         [0052]    Referring now to  FIG. 4A , a plurality of magnetic field sensing elements  30   a - 30   h  (or alternatively, sensors), in a general case, can be any type of magnetic field sensing elements. The magnetic field sensing elements  30   a - 30   h  can be, for example, planar Hall elements, vertical Hall elements, or magnetoresistance elements. These elements can also be coupled to an electronic circuit described below. For embodiments where the sensing elements  30   a - 30   h  are vertical Hall elements or magnetoresistance elements, there can also be a magnet the same as or similar to the magnet  14  of  FIG. 1 , disposed proximate to the sensing elements  30   a - 30   h  in the same way as is shown in  FIG. 4 . The group of sensing elements  30   a - 30   b  can be disposed upon a substrate  34 , for example, a silicon substrate, along with other electronics (not shown). 
         [0053]    A center  32  of the plurality of magnetic field sensing elements  30   a - 30   b  is at a center of the entire group of magnetic field sensing elements  30   a - 30   b.  Since the magnetic field sensing elements  30   a - 30   h  have very little depth (into the page), the center  32  can be considered to be on the surface of the substrate  34 . 
         [0054]    Referring now to  FIG. 5 , a graph  50  has a horizontal axis with a scale in units of CVH vertical Hall element position, n, around a CVH sensing element, for example, the CVH sensing element  12  of  FIG. 4 . The graph  50  also has a vertical axis with a scale in amplitude in units of millivolts. The graph  50  includes a signal  52  representative of output signal levels from the plurality of vertical Hall elements of the CVH taken sequentially with the magnetic field stationary and pointing in a direction of about forty-five degrees. 
         [0055]    Referring briefly to  FIG. 4 , as described above, vertical Hall element  0  is centered along the x-axis  20  and vertical Hall element  8  is centered along the y-axis  22 . In the exemplary CVH sensing element  12 , there are thirty-two vertical Hall element contacts and a corresponding thirty-two vertical Hall elements, each vertical Hall element having a plurality of vertical Hall element contacts, for example, five contacts. 
         [0056]    In  FIG. 5 , a maximum positive signal is achieved from a vertical Hall element centered at position  4 , which is aligned with the magnetic field  16  of  FIG. 4 , such that a line drawn between the vertical Hall element contacts (e.g., five contacts) of the vertical Hall element at position  4  is perpendicular to the magnetic field. A maximum negative signal is achieved from a vertical Hall element centered at position  20 , which is also aligned with the magnetic field  16  of  FIG. 1 , such that a line drawn between the vertical Hall element contacts (e.g., five contacts) of the vertical Hall element at position  20  is also perpendicular to the magnetic field. A sine wave  54  is provided to more clearly show the ideal behavior of the signal  52 . 
         [0057]    As will be appreciated, since the elliptical magnet has magnetic field that is more uniform than conventional circular magnets, the amount of error is reduced. In addition, due to the increased field uniformity the amount of flux density gradient over a given circular area of the elliptical magnet is decreased as compared to a cylindrical magnet. 
         [0058]    Full operation of a CVH sensing element  12  of  FIG. 4  and generation of the signal  52  of  FIG. 5  are described in more detail in the above-described PCT Patent Application No. PCT/EP2008/056517, entitled “Magnetic Field Sensor for Measuring Direction of a Magnetic Field in a Plane,” filed May 28, 2008, which is published in the English language as PCT Publication No. WO 2008/145662. 
         [0059]    It will be understood that, using techniques described above in PCT Patent Application No. PCT/EP2008/056517, a phase of the signal  52  (e.g., a phase of the signal  54 ) can be found and can be used to identify the pointing direction of the magnetic field  16  of  FIG. 1  relative to the CVH sensing element  12 . 
         [0060]    Exemplary embodiments of the invention comprise a magnetic sensor having an elliptical magnet to generate concentric zones of similar flux density in a plane above a surface of the magnet. Conventional cylindrical magnets generate elliptical zones of similar flux density. The inventive magnetic sensors having elliptical magnets have reduced accuracy error levels compared to cylindrical magnets of similar magnet volume. In addition less magnetic material is required for an elliptical magnet as compared to a cylindrical magnet to achieve a given accuracy level. It is understood that a variety of suitable magnet materials known to one of ordinary skill in the art can be used. 
         [0061]    Having described preferred embodiments, which serve to illustrate various concepts, structures and techniques, which are the subject of this patent, it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts, structures and techniques may be used. Accordingly, it is submitted that that scope of the patent should not be limited to the described embodiments but rather should be limited only by the spirit and scope of the following claims. All references cited herein are hereby incorporated herein by reference in their entirety.