Abstract:
One embodiment relates to a sensing system that includes a magnetic encoder wheel having alternating pole magnetic domains along a circumference thereof. The magnetic encoder wheel is configured to rotate about a first axis. The sensing system further includes a magnetic field sensing element in spatial relationship with the magnetic encoder wheel that is oriented to sense magnetic field components extending generally in a direction parallel to a second axis that is perpendicular to the first axis. The sensing system also includes a magnetic flux influencing element configured to influence magnetic field components associated with the alternating pole magnetic domains of the magnetic encoder to reduce magnetic field components associated with the first axis.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of U.S. patent application Ser. No. 12/796,276 filed Jun. 8, 2010 entitled “Through Bias Pole for IGMR Speed Sensing” and is hereby incorporated by reference in its entirety. 
     
    
     FIELD 
       [0002]    The present disclosure relates to generally to magnetic sensing systems such as rotational speed, acceleration and rotational direction of a rotating element such as a shaft or gear in automotive applications or other type applications. 
       BACKGROUND 
       [0003]    In many applications, it is useful to detect changes in magnetic field to track translational motion, rotational motion, proximity, speed and the like. Accordingly, magnetic field sensors are used in a wide variety of applications to detect subtle (or drastic) changes in magnetic field. 
         [0004]    Magnetic field sensors are often used in large scale industrial applications, such as in automobiles. For example, magnetic field sensors are often used to detect the angle of a crankshaft or camshaft, and can also be used to measure tire speed rotation and a host of other conditions. Magnetic field sensors are also used in small-scale devices, such as computers. For example, magneto resistive sensors are currently the leading technology used for read heads in computer hard disks. Due to the wide range of applications, improvements in magnetic field sensors are a valuable contribution to the marketplace. 
       SUMMARY 
       [0005]    The following presents a simplified summary of the disclosure in order to provide a basic understanding of some aspects of the disclosure. This summary is not an extensive overview of the disclosure, and is neither intended to identify key or critical elements of the disclosure nor to delineate the scope of the disclosure. Rather, the purpose of the summary is to present some concepts of the disclosure in a simplified form as a prelude to the more detailed description that is presented later. 
         [0006]    A sensing system is provided and comprises a magnetic encoder wheel comprising alternating pole magnetic domains along a circumference thereof, wherein the magnetic encoder wheel is configured to rotate about a first axis. The system further comprises a magnetic field sensing element in spatial relationship with the magnetic encoder wheel and oriented to sense magnetic field components extending generally in a direction parallel to a second axis that is perpendicular to the first axis. The system further comprises a magnetic flux influencing element configured to influence magnetic field components associated with the alternating pole magnetic domains of the magnetic encoder to reduce magnetic field components associated with the first axis. 
         [0007]    The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the disclosure. These are indicative of but a few of the various ways in which the principles of the disclosure may be employed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  illustrates a giant magnetoresistance sensing element. 
           [0009]      FIGS. 2   a  and  2   b  illustrate how scattering probability explains different magnetoresistances for two differing sensed external magnetic fields. 
           [0010]      FIG. 3  is a graph illustrating different magnetoresistances for differing amounts of a particular magnetic field component. 
           [0011]      FIG. 4  is a perspective view of a magnetic sensor system employing a magnetic encoder wheel and a magnetic field sensing element. 
           [0012]      FIG. 5  shows a fragmentary view of a magnetic field sensor in proximity to a magnetic encoder wheel providing a magnetic field, and the various field components illustrated as flux lines. 
           [0013]      FIG. 6  shows an axially displaced magnetic field sensing element in proximity to a magnetic encoder wheel. 
           [0014]      FIG. 7  is a fragmentary view of a magnetic field sensing means that is tilted off-axis in proximity to a magnetic encoder wheel providing a magnetic field, and the various field components illustrated as flux lines. 
           [0015]      FIG. 8  shows fragmentary views of a magnetic field sensor in proximity to a magnetic encoder wheel providing a magnetic field, and a magnetic field sensor having a through pole associated therewith, and the various field components illustrated as flux lines that are re-directed to a preferred axis according to one embodiment of the invention. 
           [0016]      FIGS. 9   a  and  9   b  are isometric views illustrating a through pole piece wrapped around an integrated circuit containing a magnetic field sensor, and the through pole piece standing alone according to one embodiment of the invention. 
           [0017]      FIG. 10  shows fragmentary views of a tilted magnetic field sensor in proximity to a magnetic encoder wheel providing a magnetic field, and a tilted magnetic field sensor having a through pole associated therewith, and the various field components illustrated as flux lines that are re-directed to a preferred axis according to one embodiment of the invention. 
           [0018]      FIG. 11  is a graph illustrating the improvement in jitter performance provided by the pole piece for situations where the magnetic field sensing element experiences axial displacement with respect to its nominal position. 
           [0019]      FIG. 12  is a cross section diagram illustrating an integrated circuit package having a die containing a magnetic field sensing element thereon, and a pole piece integrated into the package according to one embodiment. 
           [0020]      FIG. 13  is a cross section diagram illustrating an integrated circuit die having a magnetic field sensing element formed thereon and a pole piece integrated into the integrated circuit according to another embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The present disclosure will now be described with reference to the drawings wherein like reference numerals are used to refer to like elements throughout, and wherein the illustrated structures are not necessarily drawn to scale. Although various illustrated embodiments are described and illustrated as a hardware structure, the functionality and corresponding features of the present system can also be performed by appropriate software routines or a combination of hardware and software. Thus, the present disclosure should not be limited to any particular implementation and can be construed to cover any implementation that falls within the spirit and scope of the claims. Nothing in this detailed description is admitted as prior art. 
         [0022]    Sensing systems using a magnetic field sensor such as a giant magnetoresistance (GMR) sensor are numerous. One exemplary sensing system using such a magnetic field sensor is a magnetic encoder application. In a magnetic encoder application, a magnetic encoder wheel is used to detect the rotational speed, position, acceleration and rotational direction of a shaft or gear. The encoder wheel has several magnetic domains (e.g., small permanent magnets) along the wheel circumference, and such domains generate a magnetic field pattern. If the encoder wheel rotates, causing the magnetic domains to pass local to a magnetic field sensor, the sensor will detect the change in magnetic field density. 
         [0023]    Various types of magnetic field sensors exist, and all such magnetic field sensors are contemplated as falling within the scope of the present invention. One type of magnetic field sensor is the GMR sensor, a simplified portion of such a GMR sensor being illustrated in  FIG. 1  at reference numeral  10 . The GMR sensor  10  comprises at least three layers, and typically multiples of such layers, such layers comprising a pinned layer  12 , a spacer layer  14  and a free layer  16 . The pinned layer  12 , also sometimes called the hard layer, has a fixed magnetic orientation, the direction of which cannot be altered by an externally applied magnetic field. The sensor  10  further comprises the free layer  16 , sometimes referred to as a soft layer that exhibits a variable magnetic orientation that substantially follows the direction of an externally applied magnetic field. The pinned layer  12  and the free layer  16  are separated and magnetically decoupled from one another by the non-magnetic spacer layer  14  disposed therebetween. 
         [0024]    The GMR sensor  10  of  FIG. 1  operates based on the GMR effect, which is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and non-magnetic layers. It can be understood as interface scattering of conducting electrons when moving through the stack of magnetic layers separated by non-magnetic layers. As illustrated in  FIG. 2   a  and  FIG. 2   b , each electron  20  has a spin that can be directed either “up” or “down” (based on spin rotation). The scattering probability of the electrons depends on the orientation of the spin and the magnetic moment of the layer. A parallel orientation (parallel alignment between the pinned layer  12  and the free layer  16 ) yields a low scattering probability (i.e., a low resistance) as seen in  FIG. 2   a , while an anti-parallel orientation illustrated in  FIG. 2   b  leads to a high scattering probability and therefore to a high electrical resistance. The structure highlighted in  FIG. 1  is sometimes referred to as a spin valve GMR. 
         [0025]    As can be appreciated, the electrical resistivity of the structure varies based on the magnetization orientation of the pinned layer  12  with respect to the free layer  16 . Thus the direction of an externally applied magnetic field will vary the electrical resistivity of the sensor based on its direction, or the vector component direction. In the above manner the GMR is a magnetoresistance, the value of which reflects a detected direction of an external magnetic field. More particularly, the electrical resistivity can be characterized more accurately in  FIG. 3 . As can be seen, for an externally applied magnetic field +Bk oriented in the generally opposite direction (anti-parallel) to the pinned layer  12  orientation (oriented in the −x direction), the resistance reaches a maximum amount (Rmax), after which the resistance saturates. Similarly, for an externally applied magnetic field that is generally parallel to the pinned layer  12  in the −x direction, the resistance reaches a minimum, non-zero amount (Rmin), after which the resistance also saturates. Between such saturation ranges (−Bk&lt;Bx&lt;+Bk) the magnetoresistance resistance profile of the sensor  10  is generally a linear function. 
         [0026]    Referring generally to  FIG. 4 , a sensing system  30  having a magnetic encoder wheel  32  and a magnetic field sensing element  34  is provided. For purposes of explanation, a Cartesian coordinate axis system is provided to aid in explaining how the various field components of the magnetic fields are sensed by the magnetic field sensing element. As can be seen in  FIG. 4 , the magnetic encoder wheel includes alternating pole magnetic domains  36  along a circumference thereof. In one embodiment, the magnetic domains  36  comprise permanent magnets oriented so that their respective poles alternate about the circumference of the wheel. The magnetic encoder wheel rotates about a first axis  38 , for example, in a clockwise direction  40  as shown. In this particular example, the first axis  40  corresponds to the y-axis and points into the page. 
         [0027]    As can be clearly seen in the figure, the alternating magnetic domains  36  result in magnetic fields  42  illustrated more particularly as magnetic field lines or flux lines  44 . As shown, the magnetic field lines  44  originate on the first or north poles  46  and terminate or end on the second or south poles  48 , respectively. The magnetic field sensing element  34  is in spatial proximity to the encoder wheel  32  such that the magnetic fields lines  44  are sensed thereat as the wheel  32  rotates about the first axis  38  in the clockwise rotation  40 . In this example, a single magnetic field sensing element is illustrated within a sensor package  50 , such as an integrated circuit package, however, multiple sensors may be employed (in a single IC package or in separate packages) and such alternative embodiments are contemplated as falling within the scope of the present invention. 
         [0028]    As illustrated in  FIG. 4 , the magnetic field components of the magnetic fields  42  at the sensor element  34  are primarily in a direction parallel to the second axis (e.g., the x-axis) or a third axis (e.g., the z-axis), wherein the first, second and third axes are mutually perpendicular or orthogonal to one another. Therefore the axis of rotation of the wheel is parallel to the first or y-axis, while the poles  36 , at the point they pass by the magnetic field sensor element  34 , are generally moving tangential to and in a direction parallel to the second or x-axis. 
         [0029]    Ideally, the magnetic field  42  sensed at the magnetic field sensor  34  has only an x-axis component Bx, while other field components, By=Bz=0. Thus, as the wheel rotates, for example, at a constant speed, the time dependence of Bx sensed at the magnetic field sensing element  32  is sinusoidal. As the rotation speed changes, such changes are detected in changes in the phase and/or frequency of the resultant signal. Ideal conditions, however, do not exist, and By and Bz do not equal 0. In particular, the non-zero By magnetic field component that is sensed by the magnetic field sensing element results in jitter, which denigrates the measurement sensitivity of the sensing system  30 . More particularly, the inventor the present invention has discovered that several positioning issues of the sensing magnetic field sensor  34  with respect to the encoder wheel  32  affect system performance. 
         [0030]      FIG. 5  illustrates the sensing system  30  of  FIG. 4  rotated by 90 degrees such that the second axis (i.e., the x-axis) is now directed into the page. This rotation of the system  30  helps better appreciate the role of the location of the magnetic sensing element  34  with respect to the encoder wheel  32 . In the illustration of  FIG. 5 , the magnetic field sensing element  34  resides in a nominal position, wherein the sensor  34  is aligned with the center of the encoder wheel  32  in the x-direction, centered in the y-direction, and nominally spaced (with respect to air gap) in the z-direction. In such a nominal position, the Bx component and the Bz component are approximately equal, and By is approximately zero (due to the symmetric cancellation due to superposition). 
         [0031]      FIG. 6  illustrates a system condition where the magnetic field sensing element  34  is displaced in the y-direction, such that the sensing element  34  is not aligned in the y-direction with respect to the encoder wheel  32 . More particularly, as can be seen in  FIG. 6 , the nominal sensor position is drawn in phantom while the new, offset location is displaced from the nominal position in the +y direction. Such axial displacement will impact on how the magnetic fields generated by the encoder wheel  32  affect the sensor  34 . Compared to the fields from the nominal position (e.g., Bx 0 , By 0 , Bz 0 ), the field components detected by the sensor  34  at the displaced position are Bx, By and Bz, wherein Bx&lt;Bx 0 , By&gt;&gt;By 0 , and Bz&lt;Bz 0  (as absolute values). Depending on various factors, such as encoder wheel design such as wheel height, such displacement can increase phase jitter, thereby degrading performance. Such displacement can also negatively impact speed and direction signal data. 
         [0032]      FIG. 7  illustrates a system condition where the magnetic field sensing element  34  exhibits a rotational error about the x-axis. This rotational error affects the magnetic field components detected by the sensor (Bx, By, Bz) as follows: Bx≈Bx 0 , By&gt;&gt;By 0 , and Bz&lt;Bz 0 . Due to the illustrated tilt, or x-axis rotation error, the nominal position&#39;s z-component is now in the y-plane, which leads to a significant increase in the y-component By. As stated supra, the increase in By negatively affects phase jitter and potentially the speed and direction signals. 
         [0033]    The inventor of the present invention appreciated the above problems and further appreciated that a reduction in the By field components despite axial displacement and sensor tilt would provide significant improvements in such a magnetic field sensor. The present invention employs a pole in conjunction with the magnetic field sensing element to urge or re-direct the problematic y-axis field components to the preferred x-axis. 
         [0034]      FIG. 8  illustrates a magnetic field sensing element  34  having a pole  60  that abuts, and in one embodiment surrounds, a portion of the sensor package. In the example illustrated in  FIG. 8 , the pole  60  extends along the sensor in the x-direction (into the page), while surrounding the sides of the package in the z-direction. Thus one embodiment of the invention provides for a ferrous pole material with a z-axis surround that is added to the backside of the magnetic sensor package that operates to conduct or transfer the problematic By filed components to the preferred x-direction. As can be seen in  FIG. 8 , on the left hand side is a picture of a sensing element  34  without a pole piece  60 . In the figure one can see the magnetic field lines  44  and how they drift in the positive and negative y-directions. As can be seen in the right hand side of the figure, the pole piece  60  causes the magnetic field lines  62  to become elongated and the y-components of the fields are drawn to the desired x-axis. This re-direction of the magnetic fields results in better air gap capability and improved jitter performance. In this embodiment, the performance of the pole is optimized by extending the full length  64  of the sensor (e.g., GMR sensor) within the package  66 , and the pole piece  60  or shroud runs the full width of the IC package (into the page). Alternatively, the pole piece  60  may extend the full length of the package and have other shapes or geometries. 
         [0035]      FIGS. 9   a  and  9   b  are isometric views of the magnetic sensing element in a package  70  having a pole piece  60  engaging the package, for example, surrounding a portion of the package.  FIG. 9   b  illustrates solely the pole piece  60  in isometric view. In the example provided in  FIGS. 9   a  and  9   b , the ferrous pole piece  60  is added to the backside of the integrated circuit and is physically affixed thereto. Alternatively, any pole piece in physical proximity to the magnetic field sensor that is operable to re-direct the y-component fields toward the x-axis is contemplated as falling within the scope of the present invention. 
         [0036]      FIG. 10  illustrates the effect the pole piece  60  has on the magnetic field lines  62 , and how the x-axis tilt effects are mitigated by the present invention. As can be seen, the magnetic field lines  62  are elongated and directed toward the dense, preferred x-axis. 
         [0037]      FIG. 11  is a graph that illustrates jitter performance for an encoder wheel sensor system for varying amounts of offset or axial displacement. The jitter performance graph provides for two different conditions, one for the magnetic field sensing element without a pole piece  100 , and one for the magnetic field sensing element with a pole piece  102 . As can be seen by the two graphs, the pole piece provides for a significant improvement in jitter performance when the sensor is in its optimal location (no offset) as well as at all amounts of offset. In fact, as the amount of offset or axial displacement increases, the amount of improvement over the no pole solution increases, wherein at a displacement of ±2 mm, the jitter performance is approximately 3-4× better with the through pole  60  than without it. 
         [0038]    In the embodiments discussed supra and illustrated, for example, in  FIGS. 9   a  and  9   b , the pole piece  60  was affixed or positioned external and adjacent to the magnetic sensing element that resides within an integrated circuit package. Alternatively, a pole piece or other type magnetic flux influencing element may reside within the integrated circuit package. For example, as illustrated in  FIG. 12 , an integrated circuit package  110  is illustrated. While a typical package configuration is illustrated in  FIG. 12 , a different type of package is illustrated in  FIG. 9   a , and it should be understood that any type of integrated circuit package configuration is contemplated as falling within the scope of the present invention. 
         [0039]    The package  110  has a mold compound  112  or ceramic or other type lid that covers an integrated circuit die  114 . In one embodiment the die has one or more magnetic field sensing elements integrated thereon. Bonding wires  115  electrically connect circuit from the die  114  to respective leads on a leadframe  116 . Alternatively, the bonding wires  115  may be replaced with solder balls or other attachment elements. In one embodiment, an electrically insulating layer  118  is attached or otherwise formed to a backside of the leadframe  116  and a ferrous material comprising a pole piece  120  is attached thereto, thereby electrically insulated from the leadframe  116 . In the above manner, the pole piece  120  may be customized with respect to its shape and reliably positioned with respect to the one or more magnetic field sensing elements so as to optimize sensor performance. Note that in  FIG. 12 , the view is a cross section, consequently the pole piece tabs or ear portions are not illustrated. If the package were rotated by 90 degrees, such tabs or ear portions, in at least one embodiment would be visible, rising up and covering a side portion of the sensor die  114 . 
         [0040]    In another embodiment of the structure of  FIG. 12 , the leadframe structure  116  itself is formed of or covered with a ferrous material, either completely or partially. With this structure the pole piece  120  becomes an integral part of the leadframe  116 , thereby reducing the total number of parts and providing flexibility in fashioning its shape and structure. 
         [0041]    In another alternative embodiment, a pole piece is integrated into the die containing the magnetic field sensing element, as illustrated in  FIG. 13 .  FIG. 13  shows a plan view of a die  130  having a semiconductor or other type substrate  132  on which one or more magnetic field sensing elements  134  are built, along with other support circuitry, if desired. An insulating dielectric layer is formed over and around the substrate  132  and circuitry  134 . In the dielectric layer, one or more pole pieces  138  are formed, comprised of a ferrous material. In the illustrated example, the pole piece  138  is formed like a shield, cap or shroud that surrounds the magnetic field sensing element  134 . Alternatively, the pole piece  138  simply approximates the dimensions of the one or more sensors. Further, while  FIG. 13  illustrates the pole piece  138  on a front or top side of the die, an alternative arrangement may be employed with a flip chip type configuration with the top side of the die flipped upside down and coupled to a leadframe or printed circuit board (PCB) via solder balls, for example. In such an arrangement, a dielectric layer  136  may contain or encompass the pole piece  138  on or in the backside portion of the die, as may be desired. Any such type of integration alternative is contemplated as falling within the scope of the present invention. 
         [0042]    In regard to the various functions performed by the above described components or structures (units, nodes, devices, circuits, systems, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component or structure which performs the specified function of the described component (e.g., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the disclosure. In addition, while a particular feature of the disclosure may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.