Abstract:
A single die MR sensor having three MR elements, each being preferably composed of a number of serially connected MR segments for use in linear position sensing schemes. The MR sensor is, generally, aligned in the direction of movement of a magnetic target. The middle MR element is the actual position sensor. The two outer MR elements serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, B MAX , corresponding to a position X MAX , and the other MR element is always exposed to some minimum magnetic field, B MIN , corresponding to a position X MIN , and wherein a portion of the middle MR element is exposed to B MAX  and another portion of the middle MR element is exposed to B MIN , wherein the position, X, of the target is computed assuming uniformity of the middle MR element along its length.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to magnetoresistor devices used for magnetic position sensors.  
         BACKGROUND OF THE INVENTION  
         [0002]    The use of magnetoresistors (MRs) and Hall devices as position sensors is well known in the art. For example, a magnetically biased differential MR sensor may be used to sense angular position of a rotating toothed wheel, as for example exemplified by U.S. Pat. Nos. 4,835,467, 5,731,702, and 5,754,042.  
           [0003]    In such applications, the magnetoresistor (MR) is biased with a magnetic field and electrically excited, typically, with a constant current source or a constant voltage source. A magnetic (i.e., ferromagnetic) object moving relative and in close proximity to the MR, such as a toothed wheel, produces a varying magnetic flux density through the MR, which, in turn, varies the resistance of the MR. The MR will have a higher magnetic flux density and a higher resistance when a tooth of the moving target wheel is adjacent to the MR than when a slot of the moving target wheel is adjacent to the MR.  
           [0004]    Increasingly more sophisticated spark timing and emission controls introduced the need for crankshaft sensors capable of providing precise position information during cranking. Various combinations of magnetoresistors and single and dual track toothed or slotted wheels (also known as encoder wheels and target wheels) have been used to obtain this information (see for example U.S. Pat. Nos. 5,570,016, 5,731,702, and 5,754,042).  
           [0005]    The shortcoming of MR devices is their temperature sensitivity. They have a negative temperature coefficient of resistance and their resistance can drop as much as 50% when heated to 180 degrees Celsius. Generally, this led to the use of MR devices in matched pairs for temperature compensation. Additionally, it is preferable to drive MR devices with current sources since, with the same available power supply, the output signal is nearly doubled in comparison with a constant voltage source.  
           [0006]    To compensate for the MR resistance drop at higher temperatures, and thus, the magnitude decrease of the output signal resulting in decreased sensitivity of the MR device, it is also desirable to make the current of the current source automatically increase with the MR temperature increase. This is shown in U.S. Pat. No. 5,404,102 in which an active feedback circuit automatically adjusts the current of the current source in response to temperature variations of the MR device. It is also known that air gap variations between the MR device and ferromagnetic materials or objects will affect the resistance of MR devices with larger air gaps producing less resistance and decreased output signals.  
           [0007]    Single element magnetic field sensors composed of, for example, an indium antimonide or indium arsenide epitaxial film strip supported on, for example, a monocrystalline elemental semiconductor substrate, are also known. The indium antimonide or indium arsenide film is, for example, either directly on the elemental semiconductor substrate or on an intermediate film that has a higher resistivity than that of silicon. A conductive contact is located at either end of the epitaxial film, and a plurality of metallic (gold) shorting bars are on, and regularly spaced along, the epitaxial film. Examples thereof are exemplified by U.S. Pat. Nos. 5,153,557, 5,184,106 and 5,491,461.  
           [0008]    Many kinds of measurements cannot be performed with common magnetic sensors comprising a single sensing element. However, compound semiconductor MRs, such as those manufactured from InSb, InAs, etc. are simply two-terminal resistors with a high magnetic sensitivity and, thus, are very suitable for the construction of single die MR sensors (in most cases one terminal of all the MR elements can be common).  
           [0009]    Ultimately, such MR sensors could be integrated on the same die with appropriate processing circuitry. For example, if the MR array was fabricated on a Si substrate then the processing circuitry would be also Si based. For higher operating temperatures, silicon-on-insulator (SOI) could be used. A potentially lower cost alternative to the SOI approach would be to take advantage of the fact that MRs are currently fabricated on GaAs, a high temperature semiconductor, and thus, to fabricate the integrated processing circuitry from GaAs (or related InP) using HBT (Heterojunction Bipolar Transistor) or HEMT (High Electron Mobility Transistor) structures. This technology is now easily available and inexpensive through the explosive growth of the cellular phone industry.  
           [0010]    Accordingly, what remains needed is a compact and inexpensive die having three magnetic sensing elements and configured to provide a linear position sensor capable of self compensation over wide ranges of temperature and air gaps, including tilts.  
         SUMMARY OF THE INVENTION  
         [0011]    The present invention is a compact and inexpensive single die having three MR elments, wherein each MR element thereof is preferably composed of a number of serially connected MR segments.  
           [0012]    The present invention is a magnetoresistor linear position sensor incorporated on a single die capable of self compensation over wide temperature ranges and air gaps, including tilts. It employs three MR elements with (preferably) one common bias magnet. The MR sensor is, generally, aligned in the direction of movement of a magnetic target. The middle MR element is the actual linear position sensor. The two outer MR elements serve as reference sensors which sense the magnetic field at the limits of the position sensing range. The cooperating magnetic target assures that one of the two outer MR elements is always exposed to some maximum magnetic field, B MAX , corresponding to a position X MAX , and the other outer MR element is always exposed to some minimum magnetic field, B MIN , corresponding to a position X MIN , and wherein the middle MR element has a portion exposed to B MAX  and another portion exposed to B MIN  wherein the relative proportion of the portions vary with the position, X, of the target. The effective resistance of the second MR element is proportional to the linear position of the target. Thus, the present invention provides an MR sensor composed of three MR elements for sensing linear displacement of a selected target.  
           [0013]    According to a preferred method of fabrication, an indium antimonide epitaxial film is formed, then masked and etched to thereby provide epitaxial mesas characterizing the MR elements. Shorting bars, preferably of gold, are thereupon deposited, wherein the epitaxial mesa not covered by the shorting bars provides the MR segments. The techniques for fabricating epitaxial mesas with shorting bars are elaborated in U.S. Pat. No. 5,153,557, issued Oct. 6, 1992, U.S. Pat. No. 5,184,106, issued Feb. 2, 1993 and U.S. Pat. No. 5,491,461, issued Feb. 13, 1996, each of which being hereby incorporated herein by reference.  
           [0014]    Accordingly, it is in object of the present invention to provide an MR die comprising three MR elements capable of detecting one-dimensional position of a magnetic target along an alignment axis of the MR elements.  
           [0015]    This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 depicts an example of the preferred environment of use of the present invention.  
         [0017]    [0017]FIG. 2A is a schematic representation of a single die MR sensor according to the present invention.  
         [0018]    [0018]FIG. 2B is a detailed depiction of a single die composed of multiple MR elements according to the present invention.  
         [0019]    [0019]FIG. 2C is a detail view of an MR element, seen at circle  2 C of FIG. 2B.  
         [0020]    [0020]FIG. 3 shows a first example of an analog circuit implementing the present invention.  
         [0021]    [0021]FIG. 4 shows a second example of an analog circuit implementing the present invention.  
         [0022]    [0022]FIG. 5 shows an example of a circuit employing a digital processor implementing the present invention.  
         [0023]    [0023]FIG. 6 is a flow diagram for the digital processor of FIG. 5. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    [0024]FIG. 1 depicts an example of the preferred environment of use of the present invention. The MR sensor  10 , preferably stationary, employs an MR die  12  comprised of three magnetoresistor elements, MR 1 ′, MR 2 , and MR 3 ′, which are biased by a permanent magnet  14 , wherein the magnetic flux  16 ,  18 , and  20  emanating therefrom are represented by the dashed arrows. The magnetic flux  16 ,  18 , and  20  pass from the permanent magnet  14  through the magnetoresistors MR 1 ′, MR 2 , and MR 3 ′ and through the air gaps  22  and  24  to the target  30 . The length of the air gap  22  is, typically, 0.1 to 0.2 mm for a minimum tooth height  28  of, approximately, 0.5 mm wherein the range (X MAX -X MIN ) corresponds, preferably, to the length  42  on the order of 1 to 3 mm of MR 2 .  
         [0025]    The target  30  is made of a magnetic material, having, in this example, a tooth  32 , tooth edge  26 , and a space  34 , and is designed through the use of the small air gap  22  and tooth height  28  to have a steep slope  40  to the magnetic field profile  36  thereby approximating a step function at the tooth edge  26  which is conveyed with the target as the target moves. The target  30  may have other configurations besides that shown in FIG. 1 and may be appropriately shaped to provide any desirable magnetic field profile similar to the magnetic field profile  36 . The target  30  moves in the X direction  38  and is constrained to move in a known range having a maximum value X MAX  and a minimum value X MIN  wherein the range (X MAX -X MIN ) corresponds, preferably, to the length  42  of MR 2 . The magnetic profile  36  and the range of movement of the target between X MAX  and X MIN  ensure that MR 1 ′ is always exposed to B MAX  and MR 3 ′ is always exposed to B MIN  whereas the portion of MR 2  between X MAX  and X is exposed to B MAX  and the portion of MR 2  between X MIN  and X is exposed to B MIN  where X designates, in this example, the relative position of the tooth edge  26  with respect to the length  42  of MR 2  and (X MAX −X) designates the length of MR 2  exposed to the magnetic field B MAX  (i.e. the effective length of MR 2 ). If the range (X MAX -X MIN ) corresponds to the length  42  of MR 2 , a simpler coordinate system  38 ′ may be chosen which is normalized to the length of MR 2  wherein the origin is taken at X MAX . In this case, X′ designates the relative position of the tooth edge  26  with respect to the length  42  of MR 2  as well as the fraction of the length of MR 2  exposed to the magnetic field B MAX  (i.e. the effective length of MR 2  is X′) wherein the value of X′ is less than one.  
         [0026]    [0026]FIG. 2A is a schematic representation of a single die  60  MR sensor  50  according to the present invention. The MR sensor  50  consists of three serpentinely configured MR elements  52 ,  54 , and  56  representing MR 1 ′, MR 2 , and MR 3 ′, respectively, wherein the lengths  44  and  46  are, preferably but not necessarily, the same with equal spacing  62 . The contact pads may be separated for each of the MR elements, or may be combined (as depicted) between MR elements  52  and  54  and between MR elements  54  and  56 .  
         [0027]    Since MR 1 ′ and MR 3 ′ only serve to provide reference values for the computation of X, the resistance of MR 1 ′, proportional to the length  44 , and the resistance of MR 3 ′, proportional to the length  46 , can be a small fixed portion of the resistance of MR 2 , proportional to the length  42 , in order to save die  60  area and allocate most of the die area to MR 2  which does the actual position sensing. For example, if MR 1 ′ and MR 2  (elements  52  and  54 ) are exposed to the maximum magnetic field B MAX , the resistance of MR 1 ′ is chosen to be k*R MR2MAX  and if MR 2  and MR 3 ′ (elements  54  and  56 ) are exposed to the minimum magnetic field B MIN , the resistance of MR 3 ′ is chosen to be p*R MR2MIN  where k and p are constant coefficients whose values are, preferably, less than one and k may be equal to p wherein R MR2MAX  is the maximum resistance of MR 2  and R MR2MIN  is the minimum resistance of MR 2 . If the values of k and p are both one, then the resistance of MR 1 ′, R MR1′ , would be R MR2MAX  and could be designated simply as R MR1  whereas the resistance of MR 3 ′, R MR3′ , would be R MR2MIN  and could be designated simply as R MR3 . The use of a single die  60  for the MR elements  52 ,  54 , and  56  ensures that the sensing elements have matched thermal and magnetic sensitivities.  
         [0028]    For purposes of exemplification, FIGS. 2B and 2C show details of an MR die  60 ′ composed of and MR sensor  50 ′. Structurally, the MR die  60 ′ consists of a plurality of MR elements wherein each MR element is composed of a number of MR segments  62  demarcated by uniform shorting bars  64  which are, preferably, gold. The MR segments  62  are each uniformly matched to the others (that is, the MR segments are identical).  
         [0029]    By way of preferred example, each MR segment  62  is composed of indium antimonide (InSb) epitaxial film mesas. Each epitaxial film mesa is provided, by way of preferred example, by forming an indium antimonide epitaxial film, then masking and etching it. The shorting bars  64 , which demarcate the MR segments  62 , are composed of gold bars deposited upon the MR segments. Bonding pads (or terminals)  66 , preferably also of gold, are provided, in this example, for every MR element.  
         [0030]    Referring back to FIG. 1, using the coordinate system  38 ′ the resistance of MR 2 , R MR2 , can be expressed as:  
           R   MR2   =R   2MAX   +R   2MIN   (1)  
         [0031]    where R 2MAX  is the resistance of the portion of MR 2  exposed to B MAX  and R 2MIN  is the resistance of the portion of MR 2  exposed to B MIN . Due to the steep slope  40  of the magnetic profile  36 , R 2MAX =X′*R MR2MAX  and R 2MIN =(1−X′)*R MR2MIN  by which equation (1) can be written as:  
           R   MR2   =X′*R   MR2MAX −(1− X ′)* R   MR2MIN .  (2)  
         [0032]    Using R MR1′ =k*R MR2MAX  and R MR3′ =p*R MR2MIN , the position X′ in equation (2) can be expressed as:  
           X ′=( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )  (3)  
         [0033]    or  
           X ′=( R   MR2   −R   MR3 )/( R   MR1   −R   MR3 )  (4)  
         [0034]    wherein the variables have been previously defined.  
         [0035]    [0035]FIG. 3 shows a first example of an analog circuit  70  implementing the present invention. V S  is the power supply voltage and i 1 , i 2 , and i 3  are matched constant current sources such that i 1 =i 2 =i 3 . V 1 , V 2 , and V 3  are given by:  
           V   1   =i   1   *R   MR1′   (5)  
           V   2   =i   2   *R   MR2   (6)  
         [0036]    and  
           V   3   =i   3   *R   MR3′ .  (7)  
         [0037]    The output V 4  of amplifier  72  (i.e. an OP-AMP) having a gain of (1/k) and the output V 5  of amplifier  74  (i.e. an OP-AMP) having a gain of (1/p) are given by:  
           V   4   =V   1   /k=i   1   *R   MR1   /k   (8)  
         [0038]    and  
           V   5   =V   3   /p=i   3   *R   MR3   /p.   (9)  
         [0039]    The output V 6  of differential amplifier  76  (i.e. an OP AMP) and the output V 7  of differential amplifier  78  (i.e. an OP AMP) are given by:  
           V   6   =V   4   −V   5   =V   1   /k−V   3   /p=i   1   *R   MR1′   /k−i   3   *R   MR3′   /p   (10)  
         [0040]    and  
           V   7   =V   2   −V   5   =V   2   −V   3   /p=i   2   *R   MR2   −i   3   *R   MR3′   /p   (11)  
         [0041]    whereby the output V OUT  of analog divider  80  is:  
           V   OUT   =C *( V   7   /V   6 )= C *( i   2   *R   MR2   −i   3   *R   MR3′   /p )/( i   1   *R   MR1′   /k−i   3   *R   MR3′   /p )  (12)  
         [0042]    or, since i 1 =i 2 =i 3 ,  
           V   OUT   =C *( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )= C *( R   MR2   −R   MR3 )/( R   MR1   −R   MR3 )  (13)  
         [0043]    where C is the gain of analog divider  80  and is adjusted for maximum sensitivity or C is adjusted to satisfy other system requirements. For example, C may be adjusted such that V OUT  has a value of zero when MR 2  is at the position X MIN  and a value of 5 volts when MR 2  is at the position X MAX . Hence,  
         ( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )=( R   2   −R   3 )/( R   1   −R   3 )= V   OUT   /C   (14)  
         [0044]    and equations (3) and (4) may be expressed as:  
           X′=V   OUT   /C   (15)  
         [0045]    Thus, since the gain C is known, the position X′ can be determined from the voltage V OUT  from which the position X of coordinate system  38  of FIG. 1 can be ascertained.  
         [0046]    [0046]FIG. 4 shows a second example of an analog circuit  70 ′ well suited for the integration on the MR die  60  implementing the present invention. V′ S  is the power supply voltage and i′ 1 , i′ 2 , and i′ 3  are weighted constant current sources such that i′ 1 =i′ 2 /k and i′ 3 =i′ 2 /p. V′ 1 , V′ 2 , and V′ 3  are given by:  
           V′   1   =i′   1   *R   MR1′ =( i′   2   /k )* R   MR1′   (6)  
           V′   2   =i′   2   *R   MR2   (17)  
         [0047]    and  
           V′   3   =i′   3   *R   MR3′ =( i′   2   /p )* R   MR3′ .  (18)  
         [0048]    The output V′ 6  of differential amplifier  76 ′ (i.e. an OP AMP) and the output V′ 7  of differential amplifier  78 ′ (i.e. an OP AMP) are given by:  
           V′   6   V′   1   −V′   3 =( i′   2   /k )* R   MR1′ −( i′   2   /p )* R   MR3′   (19)  
         [0049]    and  
           V′   7   =V′   2   −V′   3   =i′   2   *R   MR2 −( i′   2   /p )* R   MR3′   (20)  
         [0050]    whereby the output V′ OUT  of analog divider  80 ′ is:  
           V′   OUT   =C ′*( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )= C ′*( R   MR2   −R   MR3 )/( R   MR1   −R   MR3 )  (21)  
         [0051]    where C is the gain of analog divider  80 ′ and is adjusted for maximum sensitivity or C′ is adjusted to satisfy other system requirements. For example, C′ may be adjusted such that V′ OUT  has a value of zero when MR 2  is at the position X MIN  and a value of 5 volts when MR 2  is at the position X MAX . Hence,  
         ( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )=( R   2   −R   3 )/( R   1   −R   3 )= V′   OUT   /C   (22)  
         [0052]    and equations (3) and (4) may be expressed as:  
           X′=V′   OUT   /C′   (23)  
         [0053]    Thus, since the gain C′ is known, the position X′ can be determined from the voltage V′ OUT  from which the position X of coordinate system  38  of FIG. 1 can be ascertained.  
         [0054]    [0054]FIG. 5 shows an example of a circuit  90  employing a digital processor  92  (i.e. digital signal processor, micro controller, microprocessor, etc.) implementing the present invention. V″ S  is the value of the supply voltage and is implicitly known to the digital processor  92 , for example, as an input or stored in the digital processor&#39;s memory. The position range, X MIN , and X MAX  as well as the parameters p and k are, preferably, stored in memory also. The values of V A  and V B  are input to the digital processor  92  and can be expressed as:  
           V   A   =V″   S *( R   MR2   −R   MR3 ′)/( R   MR1   ′+R   MR2   +R   MR3 ′)  (24)  
         [0055]    and  
           V   B   =V″   S   *R   MR3 ′/( R   MR1   ′+R   MR2   +R   MR3 ′).  (25)  
         [0056]    V MR1 , V MR2 , and V MR3  are the values of the voltages across MR 1 ′, MR 2 , and MR 3 ′, respectively, whereas i is the current through MR 1 ′, MR 2 , and MR 3 ′, and can be expressed as:  
           V   MR1′   =V″   S   −V   A   i*R   MR1 ′  (26)  
           V   MR2   =V   A   −V   B   =i*R   MR2   (27)  
         [0057]    and  
           V   MR3′   =V   B   =i*R   MR3 ′  (28)  
         [0058]    The value of the output voltage V″ OUT  is computed by the digital processor  92  and can be expressed as:  
           V″   OUT   =C ″*( V   MR2   −V   MR3′   /p )/( V   MR1′   /k−V   MR3′   /p )  (29)  
         [0059]    or from equations (26), (27), and (28)  
           V″   OUT   =C ″*( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )  (30)  
         [0060]    wherein C″ is the gain and is adjusted for maximum sensitivity or C″ is adjusted to satisfy other system requirements. For example, C″ may be adjusted such that V″ OUT  has a value of zero when MR 2  is at the position X MIN  and a value of 5 volts when MR 2  is at the position X MAX . Hence,  
         ( R   MR2   −R   MR3′   /p )/( R   MR1′   /k−R   MR3′   /p )=( R   2   −R   3 )/( R   1   −R   3 )= V″   OUT   /C″   (31)  
         [0061]    and equations (3) and (4) may be expressed as:  
           X′=V″   OUT   /C″   (32)  
         [0062]    Thus, since the gain C″ is known, the position X′ can be determined from the voltage V″ OUT  from which the position X of coordinate system  38  of FIG. 1 can be ascertained.  
         [0063]    [0063]FIG. 6 is a flow diagram for the digital processor  92  of FIG. 5. The procedure starts at block  100  where initialization of the digital processor  92  is accomplished. At block  102  the values of V A  and V B  are entered into the digital processor  92  and the values of V MR1′ , V MR2 , and V MR3′  are computed at block  104  according to equations (26), (27), and (28). The gain C″ is selected at block  106  and the output voltage V″ OUT  is computed and output at block  108 . V′ OUT  is computed according to equation (29) using the stored values of k and p. If at (optional) decision block  110  the procedure is not done, then control passes to block  102 . Otherwise the procedure ends at block  112 . If desired, the value of X′ may be computed according to equation (32) and output as well. The method of accomplishing this would involve another computation block being implemented in FIG. 6 and is well known to those skilled in the art.  
         [0064]    To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.