Patent Description:
The present invention provides a sensor system as defined in any of claims <NUM> and <NUM> to <NUM>. In another aspect, the invention provides a method as defined in any of claims <NUM> to <NUM>.

The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which:.

<FIG> is a circuit diagram illustrating an example of a magnetic field sensor <NUM> including a magnetic field sensing element <NUM> having bias misalignment compensation in accordance with illustrative embodiments. In embodiments, the magnetic field sensing element <NUM> can be positioned in relation to a magnet <NUM>, for example. In embodiments, the magnetic field sensing element <NUM> senses a ferromagnetic target <NUM>, for example, that causes changes in a magnetic field. A signal processing module <NUM> is coupled to the magnetic field sensing element <NUM> to process the signal from the sensing element. An output module <NUM> is coupled to the signal processing module <NUM> to provide an output signal for a device containing the magnetic field sensor.

In one embodiment shown in <FIG>, the magnetic field sensing element <NUM> of <FIG> comprises a GMR magnetic field sensor <NUM> in the form of a bridge. The bridge circuit <NUM> includes magnetic field sensing elements, such as GMR elements <NUM>, <NUM>, <NUM>, <NUM>, disposed on the respective branches of the bridge <NUM>. As shown, and described more fully below, the GMR elements <NUM>, <NUM>, <NUM>, <NUM> can be divided into two or more segments to provide misalignment compensation.

In the illustrative embodiment, one end of the GMR element <NUM> and one end of the GMR element <NUM> are connected in common to a power supply terminal Vcc via a node <NUM>, one end of the GMR element <NUM> and one end of the GMR element <NUM> are connected in common to ground via a node <NUM>. The other end of the GMR element <NUM> and the other end of the GMR element <NUM> are connected to a node <NUM>, and the other end of the GMR element <NUM> and the other end of the GMR element <NUM> are connected to a node <NUM>.

In the illustrated embodiment, node <NUM> of the bridge circuit <NUM> is connected to a differential amplifier circuit <NUM>. Node <NUM> is also connected to the differential amplifier circuit <NUM>. A first output of the differential amplifier circuit <NUM> is connected to an output module <NUM>. In embodiments, Vcc can be used to compensate for gain changes of the GMR elements over process and temperature. It is understood that the differential amplifier circuit <NUM> can include offset trim to correct for GMR sensor mismatch and/or sensitivity trim to adjust gain over temperature and process.

The magnetic field sensing planes of the GMR elements <NUM>, <NUM> and <NUM>, <NUM> react to a magnetic field by corresponding resistances changes. GMR elements <NUM>, <NUM> have maximum and minimum resistances at locations shifted in phase to that of GMR elements <NUM>, <NUM>. This is due to either how the magnetics of the system are configured and/or different pinning orientations of the elements. As a result, the voltages at the nodes <NUM>, <NUM> (mid-point voltages) of the bridge circuit <NUM> also change in a similar fashion.

Magnetoresistance refers to the dependence of the electrical resistance of a sample on the strength of external magnetic field characterized as: <MAT> where R(H) is the resistance of the sample in a magnetic field H, and R(<NUM>) corresponds to H = <NUM>. The term "giant magnetoresistance" indicates that the value δH for multilayer structures significantly exceeds the anisotropic magnetoresistance, which has a typical value within a few percent.

Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin-film structures composed of alternating ferromagnetic and non-magnetic conductive layers. The effect is observed as a significant change in the electrical resistance depending on whether the magnetization of adjacent ferromagnetic layers are in a parallel or an antiparallel alignment. The overall resistance is relatively low for parallel alignment and relatively high for antiparallel alignment. The magnetization direction can be controlled, for example, by applying an external magnetic field. The effect is based on the dependence of electron scattering on the spin orientation. A bridge of four identical GMR devices is insensitive to a uniform magnetic field and is reactive when the field directions are antiparallel in the neighboring arms of the bridge.

<FIG> illustrates a simplified GMR sensor <NUM> that can form a part of the magnetic field sensor <NUM> of <FIG> according to an embodiment. In <FIG>, the GMR sensor <NUM> includes a pinned layer <NUM>, a metal path <NUM>, such as copper, and a free layer <NUM>. The magnetic orientation of the pinned layer <NUM> is fixed. The magnetic orientation of the free layer <NUM> is maintained in a selected alignment through anisotropy or by the shown alternative second pinned layer <NUM>, each of which provide a pinning field, Han <NUM> (FIG. The magnetic orientation of the free layer <NUM> rotates <NUM> based on the applied field.

As shown, anisotropy may be used to creates a <NUM>° zero applied field orientation <NUM> of the free layer <NUM>, or a <NUM>° zero applied field orientation <NUM> may be provided with the second pinned layer <NUM>, which is <NUM>° to the pinned layer.

The sensitivity of a GMR (Giant Magnetoresistance) element used in a back bias environment depends on the magnetic bias (internal or external to the GMR structure). The bias induced by the magnet is typically not uniform and the GMR sensitivity changes with the position of the GMR with regards to the magnet such that placement tolerances may be a factor in the accuracy of the sensor.

Embodiments of the invention provide reduction in the effect of sensing element misalignment with respect to a magnet on GMR sensitivity by splitting a GMR structure into at least two segments and positioning a first GMR segment in a zone with positive bias and a second GMR segment in a symmetric zone with opposite bias. The zones are defined by one or more axes at least one of which may be aligned with a surface of the magnet and passing through a center of the magnet. In a configuration without misalignment, the first and second segments in zones of opposite bias will have the same magnitude of bias, therefore the same sensitivity. In the case of sensing element misalignment with respect to the field, the first segment will increase/decrease its bias with a consequent decrease/increase of sensitivity and the second segment will have the opposite behavior with decreasing/increasing bias with a consequent increase/decrease of sensitivity, as described more fully below. The first and second GMR segments attempt to compensate each other in sensitivity in the presence of misalignment of the GMR structure with respect to a bias magnet.

<FIG> show bias variations with respect to a magnet <NUM>. Positioning is described using x, y, and z coordinates as shown. <FIG> shows a configuration with a magnetic field sensing element, which can be provided as a GMR element, that is centered about the Y axis where the magnetic field sensing elements has first and second segments subject to opposite field polarity. The magnet <NUM> generates a bias on the Y axis that is symmetric to bias on the X axis. For the purposes of simplicity, the zero coordinate <NUM> is placed at the center of the magnet <NUM>. A point at coordinate "x1 ,y1" has a bias on Y axis equal and oppose to a point at coordinate "x1,-y1" so that the magnetic vector B1 is equal and opposite to magnetic vector B2. As can be seen the magnetic vector B1, B2 have the same length/magnitude and are located the same distance from the x-axis. The first segment can be located at "x1 ,y1" and the second segment can be located at "x1 ,-y1. " In embodiments, the sensing element is centered with respect to the magnet.

<FIG> shows magnetic field sensing element misalignment along the Y axis. The Y axis magnitude increases (in absolute value) moving from the (x,y) center of the magnet in the Y direction. Due to the misalignment, magnetic vectors B3 and B4 have the same distance between them but are offset in the Y direction compared to magnetic vectors Bl and B2, which correspond to no Y-axis misalignment. The magnetic vector B3 is positioned further from the center of the magnet compared to B1 and has a stronger bias on the Y axis compare to B1. The magnetic vector B4 is positioned closer to the magnet center compared to B2 and has a weaker bias on the Y axis compared to B2. In embodiments, center is defined as the intersection of the axis of symmetry of the magnet as shown in <FIG>. The fact that we have <NUM> field in the center is a consequence of the symmetry. Also if the magnet magnetization is tilted, there is no more <NUM> field point in the center.

It is understood that a GMR sensing element changes its resistance R roughly proportionally to the cosine of the magnetic vector according to this formula: <MAT> where Hx is the magnetic field on the X axis, and Hy is the magnetic field on the Y aix. Hx is typically the 'useful' field. In the illustrated embodiment, the target moves along the X-axis (from left to right or vice-versa).

It will be this appreciated that a single GMR structure is therefore relatively insensitive to position on the Y axis because its component on Y axis is changing. A misalignment on the Y axis between the GMR structure and the magnet moves the GMR magnetic vector from Bl to B3 which may significantly affect the response of the GMR.

In embodiments, the GMR elements are segmented and positioned to compensate for misalignment. For example, a first segment can be positioned at coordinate "x1,y1" and a second segment can be positioned at coordinate "x1,-y1. " Where there is no misalignment, the first and second GMR segments will have substantially the same response because they have the same distance from X, and thus, the same Hy field.

It is understood that the Hx magnetic field has been assumed to be similar in amplitude due to the same distance from the Y axis or due to the use of a magnet with a uniform field in the X axis. In case of misalignment of the GMR structure versus the magnet in the Y direction, the two segments will both move up or down. In such a case, when the first segment increases its bias on the Y axis the second reduces its bias on the Y axis to compensate for the first segment to a first approximation, and vice versa.

As shown above, a GMR element positioned to generate magnetic field vector B3 reduces its sensitivity due to the stronger field on the Y axis, but the GMR element positioned to generate magnetic vector field B4 increases its sensitivity due to the weaker field on the Y axis. The two sensitivities tend to compensate for the other so as to minimize the effect of y-axis misalignment.

<FIG> shows an example system <NUM>, not forming part of the claimed subject matter, having a GMR bridge <NUM> positioned with respect to a magnet <NUM> without misalignment. The magnetic field in the Z axis direction is shown by arrow <NUM>. Note that the field in the X direction is not shown. A first GMR element <NUM> includes a first segment 408a and a second segment 408b, a second GMR element <NUM> includes a third segment 410a and a fourth segment 410b, a third GMR element <NUM> includes a fifth segment 412a and a sixth segment 412b, and a fourth GMR element <NUM> includes a seventh segment 414a and an eight segment 414b.

As compared to a conventional GMR bridge, the first GMR element <NUM> is divided into the first and second segments 408a,b, each of which has substantially equal bias magnitude that is opposite in polarity since there is no y-axis misalignment. Similarly, the segments of the second, third and fourth GMR elements are subject to substantially equal bias of opposite polarity.

In the illustrated embodiment, a voltage supply VCC can be coupled to the first segment 408a of the first GMR element 408a and the fifth segment 412a of the third GMR element, and electrical ground GND can be coupled to the seventh segment 414a of the fourth GMR element <NUM> and the fourth segment 410b of the second GMR element.

<FIG> shows an equivalent electrical circuit showing the segment connections to VCC and GND. It is understood that VCC and GND can be coupled to the GMR elements in a variety of configurations to meet the needs of a particular application.

<FIG> shows an example heat map of the magnetic field Hy (along Y axis) produced by an example magnet of dimension (X;Y;Z)=(<NUM>;<NUM>;<NUM>)mm in the plane XY that is distant from the magnet by <NUM>. The rectangle indicates the position of the magnet in the XY plane.

Units are mm for X and Y and Oersted for the field. In the center, the map shows a gradient of Hy field along the Y axis with negative field for Y<<NUM> and positive field for Y><NUM>. The Hy field is relatively constant over the X axis.

<FIG> shows an example system <NUM>, in accordance with the invention, having a GMR bridge <NUM> positioned with respect to a magnet <NUM> with Y-axis sensing element/magnet misalignment. The magnetic field in the Z axis direction is shown by arrow <NUM>. A first GMR element <NUM> includes a first segment 508a and a second segment 508b, a second GMR element <NUM> includes a third segment 510a and a fourth segment 510b, a third GMR element <NUM> includes a fifth segment 512a and a sixth segment 512b, and a fourth GMR element <NUM> includes a seventh segment 514a and an eight segment 514b.

As can be seen, the distances D1, D2 from the x-axis to the first segment 508a and to the second segment 508b of the first GMR element <NUM> are different. In embodiments, the respective segments of the second, third, and fourth elements are also spaced at different distances from the x-axis.

Since the second segment 508b of the first GMR element <NUM> is closer to the center C of the magnet than the first segment 508a, the second segment has higher sensitivity to the magnetic field than the first segment. Where the first and second segments 508a, b are connected in electrical series, the total sensitivity of the first GMR element comprises the combined sensitivity of the first and second segments 508a,b.

In one embodiment, to a first approximation, the increased sensitivity of the second segment 508b of the first GMR element <NUM> compensates for the decreased sensitivity of the first segment 508a due to Y direction misalignment since δR=δHx/sqrt(Hx0<NUM>+Hy<NUM>) where δRis the amplitude of the signal, δHx is the variation of the field along X axis that generate the signal; Hx0 is the static field along X and Hy is the static field along Y. When Hy increases signal decreases, when Hy decreases, signal increases.

<FIG> shows an example sequence of steps for providing misalignment compensation in accordance with example embodiments. The steps of <FIG> can be understood in conjunction with the example embodiment of <FIG>. In step <NUM>, segments of a first magnetic field sensing element are positioned at locations of opposite field bias provided by a magnet. In embodiments, magnetic field sensing elements are configured in a bridge. In embodiments, the magnetic field sensing elements comprise GMR elements.

In step <NUM>, segments of a second magnetic field sensing element are positioned at locations of opposite field bias. In step <NUM>, segments of a third magnetic field sensing element are positioned at locations of opposite field bias. In step <NUM>, segments of a fourth magnetic field sensing element are positioned at locations of opposite field bias provided by a magnet. As described above, segments of a magnetic field sensing element at locations of opposite bias compensate for positional misalignment of the elements with respect to a magnetic field, which can be provided by a bias magnet.

While example embodiments are shown and described in conjunction with GMR elements, it is understood that other types of MR elements can be used, such as TMR elements.

<FIG> shows an example embodiment of a back bias 'speed' sensor having first and second bridges with three groups of GMR elements (left, center, right) with the GMR elements placed in relation to a magnet for removing sensitivity to the common mode field and alignment bias. The GMR elements are subject to opposite bias. The sensor provides speed and direction information of a target with first and second bridges having GMR elements located in different X positions, as shown. Due to the symmetry of the sensor, when the die and the magnet are perfectly aligned, a symmetric bridge has the GMR elements subject to same bias. Therefore, the sensor is immune to stray fields when the die and magnet are perfectly aligned.

<FIG> shows a sensor having GMR elements and <FIG> show alternative bridge constructions in which GMR elements are subject to opposite bias. <FIG> shows first and second GMR bridges each having four elements where each element has first and second segments. <FIG> shows the left bridge and right bridge for bridges in the sensor of <FIG>. Output signals in the left and right bridges can be used to determine speed and direction information. In one embodiment, subtraction and sum of signals in the left and right bridges outputs speed and direction information. The GMR elements are labeled such that subscription l refers to left, r refers to right, and c refers to center. Looking to <FIG>, yokes Ax and Cx may be referred to a outer yoke Bx may be referred to as central or inner yokes. From left to right in <FIG>, the GMR elements are listed as Al, Ac, Bl, Bcl, Bcr, Br, Cc, Cr. As seen in the left bridge of <FIG>, GMR element Al includes first and second segments Ala, Alb, the GMR element Bcl includes third and fourth elements Bcla, Bclb, and so on.

In this arrangement, the segments of the GMR elements do not experience the exact same bias conditions. For example, the bias field, which can be generated by a magnet, along Z axis (<FIG>) varies slightly between inner and outer yoke. This produces a difference in sensitivity of the elements, resulting in a global sensitivity to common mode field.

<FIG> shows symmetric and three point bridge configurations that may be useful for a back bias speed sensor. In an embodiment, the symmetric bridge of Al, Cc, Cr, and Ac, can be referred to as a direction channel and the three point bridge can be referred to as a speed channel. Yokes Ax and Cx can be referred to as outer yokes and yokes Bx can be referred to as central or inner yokes (see <FIG>).

In embodiments, yokes should be placed by pairs in a symmetric manner respective to the magnet. One yoke should be placed at a position +Yp and the second Ym=-Yp (assuming Y=<NUM> is at the center of the magnet). Then the spacing S (e.g., <NUM>*Yp) is selected high enough so that the bias due to the magnet is large enough to ensure a proper compensation of the misplacement along Y axis and stray field along that same axis and small enough to ensure the sensitivity is not too diminished for a far air gap signal. In embodiments, there is compensation for bias of the GMR and good tolerance to misplacements over airgap.

As shown in <FIG>, to promote substantially equal bias on the GMR elements, the GMR elements and segments can be dimensioned and/or spaced according to the iso-lines of bias field along the axis perpendicular to the reference direction, i.e. HyHy. As can be seen, in the example embodiment, there is a left GMR element, a center GMR element and a right GMR element, each having first and second segments. The left and right GMR elements are subject to different bias fields than the center elements as indicated by the iso-lines. The distance between the first and second segments of the left GMR element is εl and the length of the segment is δl. The distance between the segments of the center GMR element is εc and the length of the segment is δc. The distance between the first and second segments of the right GMR element is εr and the length of the segment is δr. In the illustrated embodiment of <FIG>, δl = δ r = δc and εl = εr = εc. In the illustrated embodiment of <FIG>, δl = δ r ≠ δc and εl = εr ≠ εc, as the center segment spacing and dimensions are different than the left and right segment spacing and dimension. In embodiments, δc is chosen so that HyHy at the bottom of the upper yokes is the same on all left, center and right yokes. And HyHy at the top of the upper yokes is the same on all left, center and right yokes. In embodiments, the yokes are all the same size and only the vertical spacing changes. The spacing is chosen so that the average HyHy value across the central yoke is the same as on the sides. <FIG> shows identical average bias on the elements.

It is understood that the length of the segments and distance between segments can be unique and can be configured to meet the needs of a particular application.

The arrangement of <FIG>, for example, distributes substantially equal bias through the GMR bridge elements. To keep the same sensitivity, central yokes and outer yokes should have the same resistance in absence of any magnetic field. As the square resistance of the stack forming the GMR elements is the same and the vertical length is fixed, zero field resistance can be adapted by adapting the GMR element width and/or adding a GMR segment placed right beside an existing element. Such elements can be connected either in series or in parallel with existing elements. To this end, the sensitivity in Ω/Oe can be made substantially equal between each GMR element (when both the die and the magnet are perfectly aligned).

In another aspect, a bias magnet can be shaped to decrease the sensitivity to the common mode field over X axis misplacement of the sensor elements. As shown in <FIG>, for a rectangular magnet cross section, bias HyHy can vary significantly when misplacement in the X axis occurs, as shown. Thus, due to sensor element misplacement, the sensor starts to be sensitive to stray field. The bias Hy is shown for the upper segment of the left element, the upper segment of the center element, and the upper segment of the right element for the embodiment of <FIG> in which the center element segments are spaced and dimensioned differently than the left and right element segments.

As shown in <FIG>, by changing the cross section of the magnet in the XY plane from square to, for example, a bevel perimeter shown in <FIG>, the slope of the bias Hx at which the GMR elements are located can be decreased, and may approach zero. When X-axis misplacement occurs, the bias does not change significantly. Therefore, sensitivity to stray field does not vary significantly through a range of X misplacement. In an ideal position, one is in the center and as soon as there is misalignment on X the <NUM> moves in such direction but because the profile is flat (slope is null) they still work as we are in the middle. Thus, there is good immunity to misalignment because there is no change. In embodiments, Hy depends on the distance to the top/bottom edge of the magnet. The closer to the edge, the higher Hy is. By reducing the height of the magnet on the sides we get these top and bottom edges closer to the yokes so that Hy increases again. That is how the function changes from something going down to <NUM> to something increasing again.

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 element can comprise, but is not limited to, a Hall Effect element, a magnetoresistance element, and/or a magnetotransistor. 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 semiconductor magnetoresistance element such as Indium Antimonide (InSb), a giant magnetoresistance (GMR) element, for example, a spin valve, an anisotropic magnetoresistance element (AMR), a tunneling magnetoresistance (TMR) element, a magnetic tunnel junction (MTJ), and a spin-valve. The magnetic field sensing element may be a single element or, alternatively, may include two or more magnetic field sensing elements arranged in various configurations, e.g., a half bridge or full bridge. Depending on the device type and other application requirements, the magnetic field sensing element may be a device made of a type IV semiconductor material such as Silicon (Si) or Germanium (Ge), or a type III-V semiconductor material like Gallium-Arsenide (GaAs) or an Indium compound, e.g., Indium-Antimonide (InSb).

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 elements tend to have axes of sensitivity perpendicular to a substrate, while metal based or metallic magnetoresistance elements (e.g., GMR, TMR, AMR) and vertical Hall elements tend to have axes of sensitivity parallel to a substrate.

As used herein, the term "magnetic field sensor" is used to describe a circuit that uses a magnetic field sensing element, generally in combination with other circuits. 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 domains of a ring magnet or a ferromagnetic target (e.g., gear teeth) where the magnetic field sensor is used in combination with a back-biased or other magnet, and a magnetic field sensor that senses a magnetic field density of a magnetic field.

<FIG> shows an exemplary computer <NUM> that can perform at least part of the processing described herein. The computer <NUM> includes a processor <NUM>, a volatile memory <NUM>, a non-volatile memory <NUM> (e.g., hard disk), an output device <NUM> and a graphical user interface (GUI) <NUM> (e.g., a mouse, a keyboard, a display, for example). The non-volatile memory <NUM> stores computer instructions <NUM>, an operating system <NUM> and data <NUM>. In one example, the computer instructions <NUM> are executed by the processor <NUM> out of volatile memory <NUM>. In one embodiment, an article <NUM> comprises non-transitory computer-readable instructions.

Processing may be implemented in hardware, software, or a combination of the two. Processing may be implemented in computer programs executed on programmable computers/machines that each includes a processor, a storage medium or other article of manufacture that is readable by the processor (including volatile and non-volatile memory and/or storage elements), at least one input device, and one or more output devices. Program code may be applied to data entered using an input device to perform processing and to generate output information.

The system can perform processing, at least in part, via a computer program product, (e.g., in a machine-readable storage device), for execution by, or to control the operation of, data processing apparatus (e.g., a programmable processor, a computer, or multiple computers). Each such program may be implemented in a high level procedural or object-oriented programming language to communicate with a computer system. However, the programs may be implemented in assembly or machine language. The language may be a compiled or an interpreted language and it may be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program may be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network. A computer program may be stored on a storage medium or device (e.g., CD-ROM, hard disk, or magnetic diskette) that is readable by a general or special purpose programmable computer for configuring and operating the computer when the storage medium or device is read by the computer. Processing may also be implemented as a machine-readable storage medium, configured with a computer program, where upon execution, instructions in the computer program cause the computer to operate.

Processing may be performed by one or more programmable processors executing one or more computer programs to perform the functions of the system. All or part of the system may be implemented as, special purpose logic circuitry (e.g., an FPGA (field programmable gate array) and/or an ASIC (application-specific integrated circuit)).

Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the scope of the appended claims.

Claim 1:
A sensor system, comprising:
a first magnetic field sensing element (<NUM>) and a magnet, the first magnetic field sensing element comprising first (508a) and second (508b) segments wherein the first and second segments are positioned, with respect to the magnet, at locations of opposite magnetic field bias, with respect to a sensing direction of the segments, provided by the magnet; and
a processing module (<NUM>) configured to receive signals from the first and second segments and further configured to combine the signals from the first and second segments to compensate for positional misalignment of the first magnetic field sensing element with respect to a magnetic field provided by the magnet;
characterized in that
the misalignment is due to the first and second segments being located at substantially non-equal distances from the center of the magnetic field.