Abstract:
A circuit and method of providing desired response from magnetic field sensors to a predetermined magnetic function. Typically, magnetic field sensors, such as magnetoresistive devices and Hall effect sensors, provide an output which is a characteristic function of the magnetic field density, and so they do not generate a linear response in relation to any predetermined magnetic function, such as is required within numerous position or angle resolving circuits. The present invention utilizes two or more magnetically sensitive devices to tailor the overall sensor output signal to any desired function of the magnetic field density. The devices are connected in such a way that they mutually effect each other&#39;s voltages or currents to render the final desired output characteristic.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not Applicable  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not Applicable  
         REFERENCE TO A MICROFICHE APPENDIX  
         [0003]    Not Applicable  
         BACKGROUND OF THE INVENTION  
         [0004]    1. Field of the Invention  
           [0005]    The present invention pertains generally to magnetic field sensors and more specifically to circuits and methods for generating a response according to a predetermined function of magnetic field density.  
           [0006]    2. Description of the Background Art  
           [0007]    Magnetic sensors are increasingly being utilized within electronic systems for the detection and measurement of parameters such as speed, position, and angle. Although a number of magnetic sensor types exist, the predominant forms are Hall effect sensors and magnetoresistive sensors. Often the output from a sensor is not in accord with the desired function of magnetic field density, because the output of a Hall effect sensor is a substantially linear response to the magnetic field density. For instance, employing a Hall effect sensor in a system to measure angular displacement results in an output which is decidedly non-linear in relation to the measured angle. Typically, the correction of these non-linearities in relation to the predetermined function requires the addition of circuitry to linearize the output, whereupon additional error, complication and expense arise.  
           [0008]    Therefore, a need exists for a circuit and method that can provide linear sensing of magnetic fields according to any desired function of magnetic field density. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed linearizing methods.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The invention generally comprises a circuit and method of magnetic sensing according to a predetermined function of magnetic field density. Multiple magnetic field sensors are combined in a single magnetic sense circuit to provide a tailored response according to the desired function of magnetic field density. The magnetic field sensors are chosen based on output characteristics and combined in configurations so as to provide offsetting sensor errors that essentially cancel out the errors within one another so as to provide a linear response to the predetermined function of magnetic field density.  
           [0010]    An object of the invention is to provide a magnetic sensor circuit which is capable of providing a linear output in response to a predetermined function of magnetic field density, such as a function of angle, or position.  
           [0011]    Another object of the invention is to provide a linear response to a predetermined function without the need of post measurement linearity correction.  
           [0012]    Another object of the invention is to provide a method of generating a linear response to a predetermined function of magnetic field density that is not dependent on a single type of magnetic sensing element.  
           [0013]    Another object of the invention is to provide a method of generating a linear response to a predetermined function of magnetic field density that is inexpensive to implement.  
           [0014]    Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention will be more fully understood by reference to the following drawings which are for illustrative purposes only:  
         [0016]    [0016]FIG. 1 is a schematic representation of a magnetoresistor shunt utilized with a Hall effect sensor according to an aspect of the present invention.  
         [0017]    [0017]FIG. 2 is a graph of circuit voltages for the circuit according to FIG. 1 in response to angular rotation shown with and without the magnetoresistor shunt.  
         [0018]    [0018]FIG. 3 is a schematic representation of a magnetoresistor utilized in series with a Hall effect sensor according to an aspect of the present invention.  
         [0019]    [0019]FIG. 4 is graph comparing circuit voltages for the Hall effect sensor circuit of FIG. 3 showing outputs from three configurations with and without magnetoresistors.  
         [0020]    [0020]FIG. 5 is a schematic representation of a Hall effect sensor within a Wheatstone bridge of four magnetoresistors according to an aspect of the present invention.  
         [0021]    [0021]FIG. 6 is a simplified layout for a monolithic circuit whose schematic is shown in FIG. 5.  
         [0022]    [0022]FIG. 7 is a schematic representation of a dual Hall effect sensor circuit according to an aspect of the present invention.  
         [0023]    [0023]FIG. 8 is a simplified layout for a monolithic circuit whose schematic is shown in FIG. 7.  
         [0024]    [0024]FIG. 9 is a schematic representation of magnetoresistors in a bridge configuration according to an aspect of the present invention.  
         [0025]    [0025]FIG. 10 is a graph of the two sets of resistance characteristics utilized within the magnetoresistors shown in FIG. 9.  
         [0026]    [0026]FIG. 11 is a graph of circuit voltage in response to magnetic field density for the circuit according to FIG. 9.  
         [0027]    [0027]FIG. 12 is a cross-section of a two layer integrated magnetoresistor and Hall effect sensor fabricated according to an aspect of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    Referring more specifically to the drawings, for illustrative purposes the present invention is embodied in the apparatus generally shown in FIG. 1 through FIG. 12. It will be appreciated that the apparatus may vary as to configuration and as to details of the parts, and that the method may vary as to the specific steps and sequence, without departing from the basic concepts as disclosed herein.  
         [0029]    By way of introduction, the present invention tailors the response of a magnetic sensor by using supplemental magnetic sensors chosen with suitable characteristics, and so combined to produce a substantially linear response according to any desired function of magnetic field density B. It should be appreciated that the inventive teachings herein are applicable to a variety of magnetic sensor embodiments in addition to the ones exemplified.  
         [0030]    Referring now to FIG. 1, an embodiment of a magnetic sensor circuit  10  in which a Hall effect sensor  12  is configured with a magnetoresistor shunt  14  is shown. A bias voltage V B    16  drives current from a constant current source  18  to the Hall effect sensor  12  which is connected to ground  20 . The magnetoresistor  14  is connected across the Hall effect sensor  12  so that the resistance of the magnetoresistor  14  provides a shunt across the Hall effect sensor  12 , and circuit output is provided at V OUT    22 .  
         [0031]    [0031]FIG. 2 is a graph of voltage outputs in response to angular position for the circuit of FIG. 1, plotted with and without the use of the magnetoresistor shunt. In measuring angular position, for instance within angle encoder applications, the magnetic field density follows a sinusoidal function. Curve  24  shows the relationship between Hall effect sensor output voltage and angle for an individual Hall effect sensor without a magnetoresistor shunt. The addition of a magnetoresistor shunt  14  to create the circuit of FIG. 1, tailors the response of Hall effect sensor  12  and results in a voltage response depicted by curve  26  in FIG. 2. Subject to low values of field density B of either polarity, the magnetoresistor exhibits a relatively low resistance and thereby diverts a substantial portion of the total drive current away from the Hall effect sensor. As the field strength increases the resistance of the magnetoresistor increases more readily than the resistance of the Hall effect sensor, such that less current is diverted thereby boosting the output of the Hall effect sensor accordingly. These offsetting effects in response to the sinusoidal magnetic field density act in concert to straighten the response curve, as can be appreciated from the two comparative plots within FIG. 2. The use of the magnetoresistive shunt  14  coupled with a Hall effect sensor, therefore, provides a significant improvement to the linearization of the Hall effect sensor  12  when resolving a magnetic field that follows a sinusoidal function.  
         [0032]    Implementing a magnetoresistor shunt across a Hall effect sensor is a single aspect of the invention, and it should further be appreciated that although the exemplified embodiments employ Hall effect sensors and magnetoresistors, various similar devices can be utilized for tailoring the magnetic field sensors according to the invention. These additional sensor types include, but are not limited to, sensors such as magnetodiodes and magnetotransistors. Furthermore, the magnetic field sensor devices employed may be fabricated from various materials and processes. For example a magnetoresistor can be fabricated from materials which include semiconductors such as InSb, ferromagnetic materials such as permalloy, or from newer materials which provide what has been termed a “giant” or “colossal” magnetoresistive effect.  
         [0033]    [0033]FIG. 3 is a magnetic sensor circuit  28  which provides tailored response characteristics from a Hall effect sensor in series with a magnetoresistor. A bias source V B    30  provides current through a magnetoresistor  32  to a Hall effect sensor  34  toward ground  36 . The response of the Hall effect sensor  34  to the applied magnetic field is exhibited at V OUT    38 . As the magnetic field strength B increases, the series magnetoresistor  32  reduces the series current flowing to the Hall effect sensor  34 .  
         [0034]    [0034]FIG. 4 is a graph of circuit output voltages V OUT  generated by three circuit configurations in response to magnetic field density B. It should be appreciated that the magnetic function B represented in FIG. 4 is a linear function and not that of angular position as depicted in FIG. 2. The linear response  40  of FIG. 4 is that of an individual Hall effect sensor shown to provide a baseline for comparison purposes. Shunting the Hall effect sensor with a magnetoresistor according to the circuit of FIG. 1 yields the response curve  42  of FIG. 4. It can be seen that the addition of the magnetoresistor shunt increases the voltage response of the circuit to the applied magnetic field. Conversely, the addition of the series magnetoresistor, as shown in FIG. 3, yields the flattened response curve  44  of FIG. 4. It will be appreciated that the response  44  produces reduced output voltage levels in response to increasing levels of magnetic field density B.  
         [0035]    [0035]FIG. 5 is a circuit  46  wherein the tailored magnetic response is provided in the form of a Wheatstone bridge. A voltage source V SS    48  provides current to a Hall effect sensor  50  through the voltage differentials provided by four bridge-connected magnetoresistors  52 ,  54 ,  56 ,  58  in reference to ground  60 . The output of the Wheatstone bridge is generated at V 2    64  of the Hall effect sensor. If MR 1    52  and MR 2    54  are less sensitive to magnetic field density than MR 3    56  and MR 4   58 , yet all four magnetoresistor elements exhibit the same temperature dependence, then the circuit can be configured so that the input voltage V 1    62  to the Hall effect sensor  50  is a quadratic function of the applied magnetic field. The Wheatstone bridge must be unbalanced at a zero field density level where the output voltage is V 1  (B=0). At low magnetic field densities the Hall effect sensor is therefore supplied with a constant voltage, and its output is linear with increasing magnetic field density, homologous to a Hall effect sensor without the added bridging magnetoresistors. At higher field density values where V 1  increases with increasing field density (B&gt;&gt;0 or B&lt;&lt;0), the voltage output of the Hall effect sensor increases rapidly in comparison with a linear output. The Wheatstone bridge circuit for a magnetic sensor with a tailored response is well suited for linearizing the output of angular position sensors, in which the magnetic circuit would produce a biasing field B=sin(θ) where θ is the angle to be measured. The output of a simple linear Hall sensor within the circuit of FIG. 5 is given by V HALL =αB+βB 2 . Through the proper adjustment of the values for MR 1 , MR 2 , MR 3 , and MR 4  the circuit can be configured to yield a response which resembles V 2 =α arc sin(B)=αθ. This configuration of circuitry is well suited to linearizing angular measurements such as found in angular position sensor applications.  
         [0036]    Packaging a magnetic sensor with tailored response according to the present invention is preferably accomplished by fabricating a single substrate utilizing a single set of processing techniques to produce low cost integrated circuit die. Furthermore, it should be recognized that a number of magnetic field sensors and integrated processing functions may be included within the integrated circuit. FIG. 6 exemplifies an embodiment of integrated circuit geometry utilizing deposited InSb films on a monolithic substrate  66  to implement the circuit shown in FIG. 5. The substrate  66 , preferably Gallium Arsenide (GaAs), contains magnetoresistive areas  68  and conductive areas  70  which include bonding pads  71 . The magnetoresistive areas  68  are preferably implemented by depositing Indium Antimony (InSb) onto the substrate  66  and the conductive areas  70 ,  71  are preferably created from depositing Gold (Au) onto the substrate  66 .  
         [0037]    [0037]FIG. 7 is a sensor circuit  72  which provides a tailored output from two Hall effect sensors  74 ,  76  whose output is tailored to yield a quadratic output in response to a magnetic field. The sensor circuit can provide a tailored response without the shunting resistors R 1    78  and R 2    80 , wherein the Hall effect sensor H 1    74  has an output that serves as an input to the second Hall effect sensor H 2    76 . The Hall effect sensors need not have identical characteristics and it should be recognized that through utilizing sensors with differing characteristics, a wide variety of magnetic field functions may be supported. The shunting resistors R 1    78  and R 2    80 , partially shunt H 1    74  and thereby add a linear term to the resultant response that can boost the sensitivity of the circuit to low amplitude magnetic fields. It will be appreciated that the use of magnetoresistors in place of the resistors R 1    78  and R 2    80 , would not provide a linear term and instead would provide a quadratic response to the magnetic field far in excess of what would otherwise be produced. FIG. 8 exemplifies a monolithic implementation  88  of the “nested” magnetic sensor circuit of FIG. 7. Conductive areas  90  are deposited on the substrate over Indium Antimony (InSb) or the like. Two different deposition thicknesses for the Indium Antimony (InSb) are reflected in area  92  of sensor H 1    74 , and area  94  of sensor H 2    76 .  
         [0038]    A bridging arrangement of magnetic sensors was described in reference to FIG. 5 which included a Hall effect sensor; however, a tailored response magnetic circuit can be implemented in various bridge arrangements. FIG. 9 illustrates an example of a Wheatstone bridge arrangement  96  connected between V SS    98  and ground  100  that does not require a Hall effect sensor. The Wheatstone bridge circuit comprises four magnetoresistors whose relative device characteristics determine the tailored magnetic response. Exemplifying the relative device characteristics, consider that the individual magnetoresistor elements MR 1    102  and MR 2    104  provide different responses as a function of magnetic field density than do the magnetoresistor elements MR 3    106  and MR 4    108 . FIG. 10 depicts response disparity as characteristic graphs of resistance R as a function of magnetic field density B, in which the characteristics of the magnetoresistors for both MR 1  and MR 2  are shown by the curve  112 , which indicates an increased response to magnetic field density in relation to the curve  114  for magnetoresistors MR 3  and MR 4    114 . FIG. 11 illustrates the response of the Wheatstone bridge embodiment of FIG. 9 whose tailored response provides a voltage output which is a decreasing function of magnetic field density. The output illustrated in FIG. 11  changes polarity at B=B 0 , the location of B 0  being dependent on the relative characteristics of MR 1 , MR 2 , MR 3 , and MR 4 . It should, therefore, be appreciated that the shapes of the curves, as shown in FIG. 10 and FIG. 11, and the cross-over points therein can be altered through magnetoresistor selection to suit specific magnetic field density functions.  
         [0039]    As can be seen, therefore, various embodiments of magnetic sensors which provide a tailored magnetic response to a function of magnetic field density according to the invention have been described in these configurations:  
         [0040]    H-M combination of Hall sensors and magnetoresistors  
         [0041]    H-H combination of two or more Hall effect sensors which may have different response curves  
         [0042]    M-M combining two or more magnetoresistors which have different response curves.  
         [0043]    Other circuit arrangements and sensor varieties responsive to magnetic field density may be utilized, as described previously, to produce circuits which provide any number of tailored response characteristics.  
         [0044]    The magnetic field sensing circuits according to the invention can in general be fabricated from discrete devices or integrated to a greater or lesser extent. In the case of MRs, they are typically made from narrow energy gap semiconductors with small electron effective masses, such as InSb or InAs. While MRs can be made from these materials in bulk crystal form, it is generally preferable to deposit these materials as thin films on an electrically insulating substrate, which may be a single crystal semiconductor such as Si, GaAs, or InP, or it may be a glass or ceramic material. Deposition onto a glass or ceramic substrate, often followed by a thermal annealing step, generally results in polycrystalline material with properties which are sufficient for a number of applications. If properly performed, deposition onto a single crystal produces single crystal (epitaxial) thin films which provide relatively high levels of crystallinity and high electron mobilities, as required for the fabrication of high sensitivity magnetoresistors. The properties of MRs are determined largely by the following factors which are generally recognized within the industry: geometry, film composition, thickness, surface preparation, doping, crystal growth conditions, composition variations with thickness, and particulars relating to the type and crystallographic orientation of the substrate. For example, the thin film is typically etched into a long mesa pattern which is then periodically covered with metal “shorting bars”. The length-to-width ratio of each MR element between the shorting bars, and the number of such elements that are put in series, are both important factors affecting the magnetic sensitivity of the MR element. Numerous alternative geometries can additionally be applied, such as a Corbino disk, which will be recognized by those of ordinary skill in the art.  
         [0045]    In several cases described above, at least two devices selected from the categories of Hall effect sensors and MRs are utilized in combination within a circuit. The benefits of integrating a large portion of the circuit on a single substrate will be appreciated, as this results in lower costs, increased reliability, and reduced size. It will be further appreciated that depositing an identical thin film on the substrate for the production of both MR elements and Hall effect sensors results in a simplified fabrication procedure. However, it is frequently the situation that a given film which is optimal for creating MRs is not optimal for creating Hall effect sensors. In particular, fabrication of an MR element requires deposition of a relatively thick film in comparison with fabrication of a Hall effect sensor. The disparity in optimum thickness is due largely to the requirement of an MR element for very high electron mobility in order for it to exhibit adequate sensitivity. If lower carrier mobility material is deposited when fabricating Hall effect sensors and MR sensors, the Hall effect sensors exhibit a slight reduction of sensitivity, while the MR sensors, fabricated from the same deposited film, exhibit a substantial sensitivity reduction. Doping further complicates fabrication, as one may dope the thin film toward being a substantially n-type material so as to decrease the dependence of electron density on temperature. Doping of the films, however, leads to a reduction in the sheet resistance of the deposited film and the Hall effect sensors so produced have a low input resistance, on the order of approximately 10 ohms to approximately 100 ohms, which is reflected as a relatively large power dissipation for any given bias voltage. It may be advisable, therefore, to reduce the film thickness when fabricating a Hall effect sensor and numerous methods exist for providing this reduction. One such method involves etching away a portion, or layer, of film to reduce the thickness in the region of the fabricated Hall effect sensor. Etching can reduce the mobility of the remaining current carriers, although, it is still capable of providing acceptable results for many applications. The desirability of having a choice of serviceable methods for thinning the depositions should be appreciated in view of the difficulties often encountered in controlling etch depth within a homogeneous film.  
         [0046]    A measure of control over the etching process can be gained by depositing the film in layers of differing composition, such as InSb over GaSb. FIG. 12 shows a partial cross-section view of a monolithic circuit  116 , having a region  118  upon which a magnetoresistor is fabricated, and a region  120  upon which a Hall effect sensor is fabricated. A substrate  122 , preferably GaAs, is shown covered by an ntype GaSb first layer  124  and an n-type InSb second layer  126 . The second layer was etched away in the region of the Hall effect sensor which substantially increases the sheet resistance in that region. In this example, the GaSb first layer  124  may be doped n-type, doped p-type, or nominally undoped, since that generally produces p-type material because of native defects in GaSb. The InSb second layer  126  layer requires an appreciable density of electrons to create a sensitive MR, since the electrons have a significantly larger mobility than holes within the material. Therefore, at least a part of the deposited thickness preferably comprises n-type, or alternately undoped, material. Undoped material may be utilized within some applications as the intrinsic electron density in InSb is often sufficient if the minimum device operating temperature is not excessively low. The InSb second layer  126  may comprise a more complex structure including a GaSb buffer layer (not shown). In addition, or alternatively, a buffer layer may be used whose lattice structure approximates that of InSb involving In 1−x Al x Sb, In 1−x Ga x Sb or InP x Sb 1−x , and would preferably be followed by an InSb layer whose doping levels are frequently varied during growth of the layer.  
         [0047]    Deposition layer options are discussed in an article by D. L. Partin et al. within Sensors and Actuators, volume 69, pages 39-45, 1998; along with U.S. Pat. No. 5,883,564 to Partin; and U.S. Pat. Nos. 5,184,106 and 5,153,557, both by Partin and Heremans. Each of the foregoing patents and publications are incorporated herein by reference.  
         [0048]    It should be appreciated that other materials, such as alloys of InSb or InAs, along with additional doping variations during growth can provide advantages when fabricating material for magnetoresistor sensors.  
         [0049]    Alternative approaches can be utilized to provide the effect of a deposited first layer. Considering the layers shown in FIG. 12, the first layer  124  may be alternatively created on the substrate  122 , by implanting or diffusing a thin surface layer of the substrate with dopants to convert it to n-type or p-type conductivity. Conversion can be performed by conventional ion implantation of a dopant impurity followed by thermal annealing, or by dopant diffusion. Thus, in the case of a GaAs insulating substrate, a donor impurity such as Si can be ion implanted and annealed. As the doped regions constitute a first layer, a second layer  126  may then be deposited thereon. It should be recognized that, alternatively, the first layer may be formed by deposition of an n-type or p-type epitaxial GaAs layer followed by the deposition of a second layer. In considering additional variations, the first or second layer may themselves comprise a number of layers in order to optimize material properties, while the addition of a third, and possibly subsequent layers are considered. Such layering is especially beneficial when MRs or Hall effect sensors are to be fabricated having two or more differing thicknesses in order to provide material property variation.  
         [0050]    Alternative materials may also be utilized, for example, the first layer  124  of FIG. 12 may comprise a layer of InP, InAs, InSb, GaAs, or an alloy thereof, such as ln 1−x Al x Sb; while the second layer  126  may comprise a layer of InAs, InSb, or an alloy of Sb, InSb, or InAs.  
         [0051]    Furthermore, the properties of an MR or Hall effect sensor can be varied, even if they are fabricated from the identical unaltered epitaxial film, by varying the processing geometry. For example, if an MR is fabricated from an extended mesa with metal shorting bars deposited periodically along its length, then the properties of the MR are dependant upon both the width of the mesas and the spacing between the shorting bars.  
         [0052]    The materials mentioned in connection with FIG. 12 are provided by way of example and not of limitation. Thereby, materials utilized in connection with one description, such as those mentioned and those which would be obvious to one of ordinary skill in the art, may in general be utilized with the other descriptions and obvious variations thereof.  
         [0053]    Accordingly, it will be seen that this invention provides tailored responses from magnetic sensor circuits and can be implemented with numerous variations based on the aforesaid descriptions, and variations which are obvious to one of ordinary skill in the art.  
         [0054]    Although the description above contains many specificities, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of this invention should be determined by the appended claims and their legal equivalents. Therefore, it will be appreciated that the scope of the present invention fully encompasses other embodiments which may become obvious to those skilled in the art, and that the scope of the present invention is accordingly to be limited by nothing other than the appended claims, in which reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural, chemical, and functional equivalents to the elements of the above-described preferred embodiment that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Moreover, it is not necessary for a device or method to address each and every problem sought to be solved by the present invention, for it to be encompassed by the present claims. Furthermore, no element, component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed under the provisions of 35 U.S.C. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for.”