Abstract:
One embodiment relates to a sensor. The sensor includes a first magnet having a first surface and a second magnet having a second surface. A differential sensing element extends alongside the first and second surfaces. The differential sensing element includes a first sensing element and a second sensing element. In addition, a layer of ferromagnetic or paramagnetic material runs between the first and second magnets and spaces the first and second magnets from one another. Other apparatuses and methods are also set forth.

Description:
FIELD OF INVENTION 
     The present invention relates to methods and systems for magnetic field sensing. 
     BACKGROUND 
     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. 
     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 
     The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention, and is neither intended to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, the purpose of the summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later. 
     One embodiment relates to a sensor. The sensor includes a first magnet having a first surface and a second magnet having a second surface. A differential sensing element extends alongside the first and second surfaces. The differential sensing element includes a first sensing element and a second sensing element. In addition, a layer of ferromagnetic or paramagnetic material runs between the first and second magnets and spaces the first and second magnets from one another. Other apparatuses and methods are also set forth. 
     The following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. 
    
    
     
       DRAWINGS 
         FIG. 1  depicts a magnetic field sensor in accordance with one embodiment; 
         FIG. 1A  depicts the magnetic field sensor of  FIG. 1  with magnetic field lines superimposed thereon; 
         FIG. 2  depicts a three-dimensional depiction of a magnetic field sensor in accordance with one embodiment; 
         FIG. 3  depicts a three-dimensional depiction of a differential sensing element that include a pair of giant magneto resistance (GMR) sensing elements; 
         FIG. 4  depicts another embodiment of a three-dimensional depiction of a magnetic field sensor; 
         FIGS. 5-7  depicts magnetic field sensors during operation with a tooth-wheel and tone-wheel; 
         FIG. 8  is a flowchart showing one embodiment of a method of sensor operation; and 
         FIG. 9  is a flowchart showing one embodiment of a method of sensor manufacture. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention 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. 
       FIG. 1  shows one embodiment of a magnetic field sensor  100  that includes a first magnet  102  and a second magnet  104 . A differential sensing element  106 , which includes a first sensing element  108  and a second sensing element  110 , is positioned under first and second surfaces  116 ,  118 , respectively associated with the first and second magnets  102 ,  104 . A field accumulator  112 , which can be realized as a layer of ferromagnetic or paramagnetic material in some embodiments, separates the first magnet  102  from the second magnet  104 . 
     During operation, the first and second sensing elements  108 ,  110  provide output signals S 1 , S 2  to analysis circuitry  120 . These output signals S 1 , S 2  are indicative of the magnetic field detected by the sensing elements. By comparing the output signals S 1 , S 2 , the analysis circuitry  120  can determine information about the object to be detected (e.g., speed, proximity, shape, composition, position, or rotation information related to the object.) 
     In some embodiments, the first and second sensing elements  108 ,  110  are magneto resistive sensors, such as anisotropic magneto resistive (AMR) sensors or giant magneto resistive (GMR) sensors. Magneto resistance is the property of a material to change its electrical resistance as a function of a magnetic field applied to it. Thus, magneto resistive sensors provide a resistance that varies in a predictable manner as a function of magnetic field. When the first and second sensing elements  108 ,  110  comprise magneto resistive sensors, the sensors are sensitive to x-component changes in magnetic field. 
       FIG. 1A  shows magnetic field lines  114  in the absence of an object to be detected. Under this condition, the field accumulator  112  perpendicularly guides the magnetic field lines  114  from the magnets&#39; first and second surfaces  116 ,  118 , respectively through the first and second sensing elements  108 ,  110 , respectively, thereby putting the magneto resistive sensors into an unsaturated, magnetically neutral state. When an object to be detected passes nearby one of the magneto resistive sensors (not shown), the magnetic field lines respond by altering their orientation from the un-saturating y-axis to the saturating x-axis, thereby putting the magneto resistive sensor into a saturated high or low resistive state, depending on the negative or positive component of the x-axis input. In this manner, the first and second sensing elements  108 ,  110  continuously track magnetic field changes (i.e., resistance changes) and provide output signals S 1 , S 2  to the analysis circuitry  120 . 
     Referring now to  FIG. 2-3 , one can see one embodiment of a three-dimensional depiction of the magnetic field sensor  100 . As shown, the first magnet  102  includes a first surface  116  associated with a magnetic pole of a first magnetic polarity (e.g., north (N)) and a third surface  122  associated with a magnetic pole of a second magnetic polarity (e.g., south (S)). Similarly, the second magnet  104  includes a second surface  118  associated with a magnetic pole of the first magnetic polarity (e.g., N) and a fourth surface  124  associated with a magnetic pole of the second magnetic polarity (e.g., S). 
     In the illustrated embodiment, the differential sensing element  106  includes a first GMR sensor  126  and a second GMR sensor  128 . The first and second GMR sensors  126 ,  128  are sensitive to x-component changes in magnetic field. 
       FIG. 3  shows the first and second GMR sensors  126 ,  128  in more detail. The first and second GMR sensors  126 ,  128  include a first ferromagnetic layer  130  and a second ferromagnetic layer  132 , which are separated from one another by a non-magnetic layer  134 . An anti-ferromagnetic layer  136  is also included each GMR sensor. In other embodiments, additional alternating ferromagnetic and non-magnetic layers could also be added. 
     In one embodiment the first ferromagnetic layer  130  (which may also be referred to as a free layer) and the second ferromagnetic layer  132  (which may also be referred to as a pinned layer) can comprise a layer of ferromagnetic material having a thickness ranging from about 0.6 μm to about 5.0 μm. In some embodiments, the ferromagnetic material could comprise: CoFe, AuFe, or AlFe. The non-magnetic layer  134 , which may also be referred to as a spacer layer, can have a thickness ranging from about 0.4 μm to about 3.0 μm; and can comprise Ru, Au, or Cu. The anti-ferromagnetic layer can comprise PtMn, FeMn, or URu 2 Si 2 . As will be appreciated by one of ordinary skill in the art, however, these layers could have other thicknesses and could be made of other materials in other embodiments. 
     In the absence of an object to be detected, the field accumulator  112  axially perpendicularly guides the magnetic field lines from the first and second surfaces  116 ,  118  into the first and second GMR sensors  126 ,  128 , respectively. Thus, under this condition, the axially perpendicular magnetic field has no impact on the direction of magnetization of the free layer  130  and pinned layer  132  due to a weak anti-ferromagnetic coupling between them—this causes a median (neither high nor low) resistance. When an object to be detected comes in close proximity to the GMR sensors, however, the magnetization of the free layer  130  either aligns in a parallel or anti-parallel manner to the pinned layer  132  creating a low or high resistance respectively. 
     While  FIGS. 2-3  show one embodiment of a three-dimensional sensor in the context of GMR sensors, alterations and/or modifications may be made to this embodiment without departing from the spirit and scope of the appended claims. For example,  FIG. 4  shows another embodiment where the field accumulator  112  is characterized by a recess  138  relative to the first and second surfaces  116 ,  118 ; and a recess  140  relative to the third and fourth surfaces  122 ,  124 . These recesses  138 ,  140  may provide somewhat improved perpendicular guidance of the magnetic field lines, depending on the materials used in the magnetic field sensor  100 . 
     In other un-illustrated embodiments more complex geometries could also be used for the magnets, field accumulator, and sensing elements. For example, although the first and second magnets  102 ,  104 , field accumulator  112 , and differential sensing element  106  are illustrated as cube-like structures, in other embodiments these structures could be other polyhedral structures having any number of flat faces and straight edges. Further, in still other embodiments these structures could have curved faces and/or curved edges, and could be irregularly shaped. Although all such structures are contemplated as falling within the scope of the present disclosure, the illustrated cube-like structures may be advantageous in that the first and second magnets  102 ,  104  can be easily manufactured and do not need to be machined to complex shapes. This potentially reduces costs and improves manufacturing yields. In addition, although the field accumulator is shown as a single continuous layer, in other embodiments it could comprise multiple layers with the same or different compositions. 
     Referring now to  FIGS. 5-7 , one can see some two examples of how the magnetic field sensor  100  can be used.  FIG. 5 , for example, shows an embodiment of a tooth-wheel  500  that includes ferromagnetic or paramagnetic teeth  502  which rotate about a central axis  504 . As the teeth rotate, the magnetic field lines from the first and second magnets  102 ,  104  change correspondingly. Because the first and second sensing elements  108 ,  110  are spaced apart by some distance, the magnetic fields at the first and second sensors  108 ,  110  are phase shifted relative to one another. 
       FIG. 7  shows signals S 1 , S 2  as provided by the sensing elements  108 ,  110 , respectively. Thus, assuming the first and second sensing elements  108 ,  110  are perfectly matched (which need not be the case), when a tooth  502  is equidistant between the first and second sensing elements  108 ,  110  the signals S 1  and S 2  should be equal (i.e., the magnetic field measured by sensing elements is equal). This corresponds to point  702  in  FIG. 7 , where the magnitude of S 1  is equal to the magnitude of S 2 . As the tooth  502  proceeds along its radial path in time, one of the signals (e.g., S 2 ) will increase while the other signal (e.g., S 2 ) decreases. The analysis circuitry associated with the magnetic field sensor can compare these two signals to obtain the differential signal S diff . 
     Because the differential signal S diff  is obtained by comparing two signals, it efficiently ignores magnetic field variations due to unwanted influences. For example, the Earth&#39;s magnetic field could change slightly over time. However, because the first and second sensing elements  108 ,  110  would both experience this slight change in a similar manner (e.g., would be both be level shifted by the same amount), the differential signal S diff  would still accurately represent the change in magnetic field due to only the tooth-wheel  500 . The same discussion holds true for the tone-wheel in  FIG. 6 , which shows magnets  602  positioned around an outer perimeter of the tone-wheel  600 . 
     Now that several examples of several magnetic field sensors and systems have been discussed, a method is now described with reference to  FIG. 8 . The analysis circuitry and magnetic field sensor include suitable circuitry, state machines, firmware, software, logic, etc. to perform this method  800  as well as other functions illustrated and described herein. While the methods illustrated and described herein are illustrated and described as a series of signal patterns, acts, or events, it will be appreciated that the present invention is not limited by the illustrated ordering of such acts, or events. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein, in accordance with the invention. In addition, not all illustrated steps may be required to implement a methodology in accordance with the present invention. 
     Referring now to  FIG. 8 , one can see that method  800  starts at  802 , where an object to be detected is passed by first and second sensing elements. 
     In  804 , the first sensing element continuously or intermittently measures the magnetic field as the object passes by. The first sensing element returns a signal S 1  that is indicative of the time-varying magnetic field it experiences. 
     In  806 , the second sensing element continuously or intermittently measures the magnetic field as the object passes by. The second sensing element returns a signal S 2  that is indicative of the time-varying magnetic field it experiences. 
     In  808 , the signals S 1  and S 2  are compared to obtain a differential signal S diff , which can be analyzed to determine information about the object, such as translational motion, rotational motion, speed, shape, proximity, and/or composition associated with the object. 
     While both AMR and GMR sensors can be used to monitor magnetic fields, these typically sensors differ in the amount by which change in resistance corresponds to change in magnetic field. For example, AMR sensors typically exhibit a change in resistance of about 3-5%, while GMR sensors typically exhibit a change in resistance of about 6-10%. In addition, it will be appreciated that other types of sensors could be used based on tradeoffs between cost, performance, and other customer requirements. For example, in other embodiments, Hall sensors could be used for the first and second sensing elements  108 ,  110 . 
       FIG. 9  shows a method of manufacturing a magnetic field sensor in accordance with another embodiment. Again, these acts may be carried out in different orders than illustrated, and some acts may be carried out concurrently or may comprise multiple sub-acts. At  902 , a layer of paramagnetic or ferromagnetic material is provided. At  904 , first and second magnets are adhered to opposing sides of the layer of paramagnetic or ferromagnetic material. Because the field accumulator  112  causes the magnets  102  and  104  to attract one another (in spite of the poles tending to repel one another in the absence of the field accumulator), the field accumulator and magnets could be assembled without the use of epoxy in some embodiments. In other embodiments, however, it is advantageous to wait until later in the manufacturing process to magnetize the magnets, and in these embodiment epoxy or some other adhesive could be used to adhere the magnets to the field accumulator. In  906 , a differential magnetic field sensor is adhered to a first surface of the first magnet and a second surface of the second magnet. Analysis circuitry may be integrated along with any of these various components, or may be separated connected to the assembled sensor. 
     In regard to the various functions performed by the above described components or structures (blocks, units, assemblies, 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 invention. In addition, while a particular feature of the invention 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”.