Abstract:
A movable target having individually identifiable magnetic irregularities and a stationary magnetosensitive array and circuit for sensing the magnetic irregularities to thereby provide position of the target relative to a fixed initial location. The method of the present invention finds a present position of the target origin including the steps of: identifying a magnetic irregularity of the target; determining a first distance, Y, equal to a distance of the identified magnetic irregularity to a target origin; determining a second distance, X, equal to a distance of the identified magnetic irregularity to the array origin; and determining a distance, L, of the target origin with respect to the fixed initial location, according to a relation: L=L 0 +X−Y.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to magnetic position sensors and, more particularly, to magnetic position sensors using an array consisting of galvanomagnetic sensing elements.  
       BACKGROUND OF THE INVENTION  
       [0002]     Galvanomagnetic sensing elements, such as Hall generators and different types of magnetoresistors (MRs), are widely used in automotive and industrial position and speed sensors. They can operate in most environments as they are relatively unaffected by dirt, most chemicals, oils and other lubricants. They can operate up to reasonably high temperatures (150 or 200 degrees C.) depending on the sensing device material.  
         [0003]     The majority of these sensors use one, or at most two, sensing elements. Sensors with a single sensing element are the simplest, but also the least accurate. Sensors with two matched sensing elements spaced some distance apart from each other are used in a differential mode, whereby common mode disturbances are rejected. Two element sensors operating in differential mode provide better accuracy than single element sensors. Since they are capable of locating with high accuracy a particular feature of the sensed object, such as a tooth edge or a center of a slot, such differential sensors are often used as incremental (on-off) sensors, e.g., as crankshaft position sensors. The differential sensor, however, cannot maintain the same high accuracy if it is used as a linear sensor, providing a continuous analog output signal proportional to displacement. This is especially true where relatively large displacements, i.e., those on the order of five mm or higher, are measured.  
       SUMMARY OF THE INVENTION  
       [0004]     The present invention is a linear magnetic position sensor for determining the linear or angular present position of a first reference location of a ferromagnetic target, herein called the target origin, from an initially known second reference position of the target origin, herein called the initial position of the target origin. The sensor includes a stationary linear array of galvanomagnetic sensing elements mounted, preferably, upon a surface of a magnet fixedly mountable adjacent the target, wherein the target moves adjacent a surface of the array thereby generating a unique magnetic flux density pattern from excitation of the sensing elements of the stationary linear array. This pattern, in general, preferably consists of peaks and valleys. Any aspect of the target that results in a peak is generically referred to herein as a magnetic tooth or, simply, a tooth wherein any aspect of the target that results in a valley is generically referred to herein as a magnetic slot or, simply, a slot.  
         [0005]     In a first preferred embodiment of the present invention, the target includes a plurality of magnetic irregularities each of which being uniquely identifiable, as for example unique teeth and/or slots, such that a magnetic flux density resulting from excitation of the sensing elements of the stationary linear array consists of uniquely identifiable peaks and/or valleys directly corresponding to the uniquely identifiable teeth and/or slots of the target. The magnetic flux density pattern resulting from excitation of the sensing elements of the stationary linear array includes, at least, preferably, two peaks, or one peak and one valley, or two valleys, whereby a tooth and/or a slot is uniquely identifiable from at least the respective two peaks, or peak and valley, or two valleys of the magnetic flux density so as to thereby uniquely determine the position of a, preferably, tooth or slot with respect to the target origin.  
         [0006]     Appropriate signal processing algorithms identify the location of the tooth or slot with respect to a location on the stationary linear array, herein called the array origin, whose distance from the initial position of the target origin is known, whereby the linear or angular present position of the target origin from the initial position of the target origin can be determined.  
         [0007]     In a second preferred embodiment of the present invention, the target includes a plurality of uniquely identifiable magnetic irregularities, as for example uniquely identifiable teeth and/or slots, such that a magnetic flux density resulting from excitation of the sensing elements of the stationary linear array consists of uniquely identifiable peaks and/or valleys directly corresponding to the uniquely identifiable teeth and/or slots of the target. The magnetic flux density resulting from excitation of the sensing elements of the stationary linear array includes at least one peak or one valley, whereby a tooth and/or a slot is uniquely identifiable from at least the one peak or one valley of the magnetic flux density so as to thereby uniquely determine the position of a tooth or slot with respect to the target origin. Appropriate signal processing algorithms identify the location of the tooth or slot with respect to a location on the stationary linear array, herein called the array origin, whose distance from the initial position of the target origin is known, whereby the linear or angular present position of the target origin from the initial position of the target origin can be determined.  
         [0008]     A first circuit is used for exciting each of the sensing elements, and a second circuit is used for measuring a magnetic flux density value at each of the sensing elements. Each magnetic flux density value is associated with the magnetic flux density curve.  
         [0009]     Accordingly, it is an object of the present invention to provide a magnetic array position sensor and methodology of use therefor in which an array identifies a target feature of a target and obtains its corresponding distance, Y, to a target origin of the target; then algorithmically determines a distance, X, of the target feature relative to an array origin of the array, wherein the array is located a fixed distance, L 0 , from an initial position of the target origin; and then determines a distance, L, of a present position of the target origin from the initial position of the target origin, according to the relation: L=L 0 +X−Y.  
         [0010]     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  
       [0011]     The description herein makes reference to the accompanying drawings, wherein like reference numerals refer to like parts throughout the several views.  
         [0012]      FIG. 1  is a schematic view of an array sensor and target in accordance with the present invention.  
         [0013]      FIG. 2  is a circuit diagram including a circuit for exciting a Hall element sensor array and a circuit for measuring the resultant magnetic flux density through the sensing elements thereof.  
         [0014]      FIG. 3  is a circuit diagram including a circuit for exciting a magnetoresistor sensor array and a circuit for measuring the resultant magnetic flux density through the sensing elements;  
         [0015]      FIG. 4  is a schematic representation of a three point parabolic fit method.  
         [0016]      FIG. 5  depicts an example of determining the linear position or angular position of a target according to a first preferred embodiment of the present invention.  
         [0017]      FIG. 6  depicts an example of determining the linear position or angular position of a target according to a second preferred embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring now to the Drawing,  FIG. 1  is a schematic view of a magnetic linear array sensor  10  and target  24  in accordance with the present invention, wherein the magnetic linear array sensor is usable to measure either angular or linear position of the target relative to an initial position of the target according to the method of the present invention. The magnetic linear array sensor  10  includes a linear array  12  mounted on a bias magnet  18 , which is preferably of the permanent type, but may be, alternatively, of the electromagnetic type. The linear array  12  is linear and comprises a plurality of, preferably but not necessarily, generally identical and, preferably but not necessarily, equidistantly spaced galvanomagnetic sensing elements  14  on a single die  16 . Of course, more than one die  16  can also be used to form the linear array  12 . The sensing elements  14  of the linear array  12  can be Hall elements or magnetoresistive elements, by way of example. Details of the construction of one linear array  12  that can be used in the present invention are disclosed in U.S. Pat. No. 6,201,466, the entire contents of which is hereby incorporated herein by reference.  
         [0019]     In the example of  FIG. 1 , the linear array  12  has ten sensing elements  14  mutually spaced equidistantly apart by a (center-to-center) distance d along the length of the linear array  12 . The sensing elements  14  are identified as array element numbers  0  to  9 . The total (on-center) distance between the first and last sensing elements, array element numbers  0  and  9  in this example, is indicated by a distance D. In this example, the (on-center) position  20  of the first array element number  0  is, arbitrarily, chosen as the array origin O A  and is at a known distance L 0  from an initial position  22  of a target origin O T  of the target  24 , wherein movement of the target results in a present position of the target O T  at a new position  26  which is located at a distance L from the initial position  22  of the target.  
         [0020]     Target  24 , by way of preferred example, consists of a sequential series of magnetic irregularities in the form of magnetic teeth  28  and slots  30  movably supported above the linear array  12 . The target  24  can be one of a variety of configurations, as discussed in more detail below. The bottom of the target  24  is located above the top surfaces of the sensing elements  14 , thereby defining an air gap  19 . Although described as an “air gap”, the air gap  19  between the target  24  and the sensing elements  14  does not necessarily exist as empty space. An overmolding layer protecting the linear array  12  and a protective coating for the target  24  and target assembly, if used, are magnetically indistinguishable from air and comprise part of the air gap  19 . The teeth  28 , in this example, have a tooth width W that is narrower than the spacing d between the sensing elements  14  and move in the direction indicated by the arrows A and B in response to respective movement of the target  24  to which they are attached.  
         [0021]     The stationary magnetic linear array sensor  10  and target  24  can be one of a variety of configurations such that a magnetic flux density resulting from excitation of the sensing elements  14  of the linear array  12  consists of uniquely identifiable peaks and/or valleys directly corresponding to uniquely identifiable teeth  28  and/or slots  30  of the target. The exemplar magnetic tooth  28  or slot  30  will provide, when present at a position above the linear array  12 , a peak or valley, respectively, when viewing, as sensed by the sensing elements  14 , the associated magnetic flux density generated by the bias magnet  18 . This is because the position of the peak or valley, i.e., the location of the maximum or minimum voltage, is immune to air gap  19  variations. It is also preferable for a peak or valley to be roughly symmetrical about the location of its respective maximum or minimum.  
         [0022]     In the first preferred embodiment of the present invention, the linear array  12  consists of, preferably but not necessarily, generally identical, equidistantly spaced d galvanomagnetic sensing elements  14 , wherein the spacing d between adjacent galvanomagnetic sensing elements is known. The spacing d between adjacent galvanomagnetic sensing elements  14  is, preferably, stored in microprocessor  46 ,  56  memory, if necessary (see  FIGS. 2 and 3 ). The target  24  consists of teeth  28 , each tooth having a width W preferably narrower than the minimum spacing d between adjacent galvanomagnetic sensing elements  14 . Preferably, two adjacent teeth, for example,  32 ,  34  at a position above the linear array  12  result in two peaks in the magnetic flux density sensed by the sensing elements  14 ; or, for example, a tooth  32  and adjacent slot  36  at a position above the linear array result in one peak and one valley in the magnetic flux density sensed by the sensing elements; or, for example, two adjacent slots  36 ,  38  at a position above the linear array result in two valleys in the magnetic flux density sensed by the sensing elements. Accordingly, a tooth and/or slot is uniquely identifiable from at least the respective two peaks, or peak and valley, or two valleys, of the magnetic flux density so as to thereby uniquely determine the position of a, preferably, tooth or slot with respect to the target origin  26 .  
         [0023]     In  FIG. 1 , the target  24 , for example, may consist of identical teeth  28  separated by slots  30  wherein the spacing S of slots between adjacent teeth varies in a predetermined manner, available, for example, from a lookup table stored in microprocessor  46 ,  56  memory. As a result, the location of a tooth  32 , for example, from the present position  26  of the target origin O T  is uniquely identified by the distance between the highest points of two adjacent peaks in the magnetic flux density, one of which peaks, preferable the first peak, corresponds to the tooth  32 .  
         [0024]     Appropriate signal processing algorithms identify the location of the tooth  32 , for example, with respect to the array origin O A , whose distance L 0  from the initial position  22  of the target origin O T  is known, whereby the linear or angular present position  26  of the target origin O T  from the initial position  22  of the target origin can be determined, as will be detailed hereinbelow.  
         [0025]     In the second preferred embodiment of the present invention, linear array  12  consists of, preferably but not necessarily, generally identical equidistantly spaced galvanomagnetic sensing elements  14  wherein the spacing d between adjacent galvanomagnetic sensing elements is known. A tooth  28  having unique predetermined features or a slot  30  having unique predetermined features at a position above linear array  12  will result in a respective peak having unique predetermined features or valley having unique predetermined features in the magnetic flux density sensed by the sensing elements  14 . As a result, a tooth or a slot is uniquely identifiable from at least a peak or valley of the magnetic flux density, thereby uniquely determining the position of a tooth or a slot with respect to the present position  26  of the target origin O T .  
         [0026]     In  FIG. 1 , the target  24 , for example, may consist of teeth  28  having unique predetermined features separated by slots  30  having unique predetermined features, such that the location of a tooth  28  or a slot  30  from the target origin  26  is uniquely identified by a single peak or a single valley in the magnetic flux density. Locating, for example, the lowest point of the valley in the magnetic flux density identifies the distance of a slot  30 , for example, from the target origin  26 . Appropriate signal processing algorithms identify the location of the slot  30 , for example, with respect to the array origin O A , whose distance L 0  from the initial position  22  of the target origin O T  is known, whereby the linear or angular present position  26  of the target origin O T  from the initial position  22  of the target origin can be determined, as will be detailed hereinbelow.  
         [0027]     Processing circuitry is operatively connected to the linear array  12  according to known methods to excite the sensing elements  14 . The processing circuitry is also capable of scanning a voltage output of each sensing element  14  and digitizing each voltage output. The voltage output at each sensing element  14  is directly related to, and thus can be used to represent, the component of magnetic flux generated by the biasing magnet  18  at each sensing element. The processing circuitry can be a microprocessor or a digital signal processor (DSP), or the like, connected to the linear array  12  by electrical leads or integrated with the linear array  12  on the same die  16 . The processing circuitry preferably includes memory, but it could be connected to external memory capable of storing the digitized voltage output data of each sensing element  14  and storing a program including one or more algorithms, described in further detail herein, to determine the precise position of the target origin  26  from the initial position  22  of the target (i.e. the distance L). In this regard,  FIGS. 2 and 3  are two examples of processing circuitry  50 ,  40  that can be used to measure the voltage output of each sensing element  14 .  
         [0028]      FIG. 2  shows processing circuitry  50  that can be used when the linear array  12  comprises a plurality of sensing elements  14  in the form of Hall elements. There are n sensing elements, labeled Hall # 0 , Hall # 1 , . . . Hall #i, . . . Hall #n−1. Excitation of the sensing elements  14  can be performed by many different circuit designs. In  FIG. 2 , excitation is performed by a voltage supply  52 , wherein each sensing element  14  is connected to the voltage supply.  
         [0029]     The voltage output of each sensing element  14 , Hall # 0 , Hall # 1 , . . . Hall #i . . . . Hall #n−1, representing the component of magnetic flux generated by the biasing magnet  18  at each sensing element is input to respective channels, Channel  0 , Channel  1 , . . . Channel i . . . . Channel n−1, of a multiplexer  54 . The multiplexer  54  provides an output voltage associated with each channel number to a microprocessor  56 . The microprocessor  56  can be, for example, part of a standard engine controller. In any case, memory may be required for storing the output data.  
         [0030]     Of course, other processing circuitry known to those of skill in the art can be used to excite a magnetic element and measure magnetic flux density. For example,  FIG. 3  shows processing circuitry  40  that can be used when the linear array  12  comprises a plurality of sensing elements  14  in the form of magnetoresistive (MR) elements. As in  FIG. 2 , there are n sensing elements  14 , labeled MR 0 , MR 1 , . . . MR i , . . . MR n-1 . Excitation of the sensing elements  14  can be performed by any number of circuit designs. In  FIG. 3 , excitation is performed by one or more, preferably, matched current sources  42 . Each sensing element  14  is connected to a distinct current source  42  as depicted.  
         [0031]     The voltage output of each sensing element  14 , MR 0 , MR 1 , . . . MR i , . . . MR n-1 , representing the component of magnetic flux generated by the biasing magnet  18  at each sensing element is input to respective channels, Channel  0 , Channel  1 , . . . Channel i, . . . Channel n−1, of a multiplexer  44 . The multiplexer  44  provides an output voltage associated with each channel number to a microprocessor  46 . The microprocessor  46  can be, for example, part of a standard engine controller. In any case, memory may be required for storing the output data.  
         [0032]     With symmetric magnetic teeth  28 , for example, the highest point of a peak in the magnetic flux density is at the center of the tooth. Conversely, with symmetric magnetic slots  30 , for example, the lowest point of a valley of the magnetic flux density is at the center of the slot. The highest point of a peak or lowest point of a valley can be determined analytically by fitting a function having a peak or valley, e.g., cosine or a second-order or higher, even-order polynomial, to several of the voltage outputs obtained from sensing elements  14  closest to the highest point of a peak or the lowest point of a valley and then computing the location of the maximum (or minimum) of the function wherein the maximum of the function represents the highest point of the peak of the magnetic flux density and the minimum of the function represents the lowest point of the valley of the magnetic flux density.  
         [0033]     However, the fitting of some functions requires far more computation than that of others without improved accuracy. Testing shows that very accurate results can be obtained by fitting a parabola to just three points, herein referred to as the three point parabolic fit method. The three point parabolic fit method comprises three sequential values of voltage outputs of sensing elements  14  representing the component of magnetic flux density at each sensing element that include the highest voltage output of a sensing element when the magnetic flux density includes a peak or the lowest voltage output of a sensing element when the magnetic flux density includes a valley. In this case, the position of the highest point of a peak or lowest point of a valley can be computed directly, without using a regression method.  
         [0034]     As an example,  FIG. 4  is a schematic representation of a three point parabolic fit method to determine the highest point of a peak of the magnetic flux density representing the center of a tooth  28 , for example. The interpolated position P corresponding to the center of a tooth  28 , for example, representing the location of the highest point of a peak relative to the sensing element  0  (i.e. the array origin  20 ) is given by: 
 
 P= 0.5[ j   1   2 ( V   3   −V   2 )+ j   2   2 ( V   1   −V   3 )+ j   3   2 ( V   2   −V   1 )]/[ j   1 ( V   3   −V   2 )+ j   2 ( V   1   −V   3 )+ j   3 ( V   2   −V   1 )]  (1) 
 
 where j 1  is a first array element number in a sequence of three sensing elements  14 ; j 2  is a second array element number in a sequence of three sensing elements including array element number j 1 ; j 3  is a third array element number in the sequence of three sensing elements including array elements numbered j 1  and j 2 ; V 1  is a first voltage output associated with array element number j 1 ; V 2  is the highest second voltage output associated with array element number j 2  when the magnetic flux density includes a peak or the lowest second voltage output V 2  associated with array element number j 2  when the magnetic flux density includes a valley; and V 3  is a third voltage output associated with array element number j 3 . 
 
         [0035]     Several examples of the parabolic fit method can be provided using a linear array  12  with n sensing elements  14  where the first array element number is i=0 and the last array element number is i=n−1. If the first array element number  0  has the highest (or lowest) voltage output V 0 , array element numbers  0 ,  1  and  2  and their associated voltage outputs, for example, V 0 , V 1 , V 2 , can be used to determine the highest (lowest) point of a peak (valley). Similarly, if the last array element number n−1 has the highest (or lowest) voltage output V n-1 , then array element numbers n−1, n−2 and n−3 and their associated voltage outputs, for example, V n-1 , V n-2 , V n-3 , can be used to determine the highest (lowest) point of a peak (valley).  
         [0036]     In the example of  FIG. 4 , the highest measured voltage V 2  is associated with array element number i=2, that is, array element number  2 . V 1  is associated with array element number i−1, i.e., array element number  1 , and V 3  is associated with array element number i+1, i.e., array element number  3 . The position P of the highest point of the peak relative to the position of array element  0  can be determined from equation (1). The distance X of the highest point of the peak from array element  0  is determined by X=P·d, where d is the distance between adjacent sensing elements  14  of the linear array  12 . For example, if the distance d is a constant 160 micrometers between sensing elements  14  and the value of P from equation (1) is 1.67, X would be 267.2 micrometers. Notice, however, that there is more than one sequence of three array element numbers that include array element number  2  (i=2). Another sequence of three array element numbers that includes array element number i=2 also includes array element numbers i−1 and i−2, array element numbers  1  and  0 , respectively. Yet another sequence of three array element numbers that includes array element number i=2 also includes array element numbers i+1 and i+2, array element numbers  3  and  4 . It has been shown that even more accurate results can be obtained using the three point parabola fit method by calculating two positions P 1  and P 2  using two separate sequences, then averaging the two positions P 1  and P 2  to determine P. Although up to three sequences are available where the highest or lowest voltage output is somewhere other than the first sensing element  0  or last sensing element n−1, any additional accuracy due to the inclusion of the third sequence in the calculation of position does not appear to justify the additional computation required.  
         [0037]      FIG. 5  depicts an example of determining the linear or angular present position  26  of a target origin O T  of a target  24  from an initial position  22  of the target origin O T  according to the first preferred embodiment of the present invention.  
         [0038]     The linear array  12 , in this example, consists of roughly identical equidistantly spaced d galvanomagnetic sensing elements  14  wherein the spacing d between adjacent galvanomagnetic sensing elements is known. The target  24  consists of magnetic irregularities in the form of teeth  32 ,  34 , for example, preferably, each tooth having a width W that is narrower than the minimum spacing d between adjacent galvanomagnetic sensing elements  14  and slots  36 , wherein the spacing S of slots between adjacent teeth varies in a predetermined manner. The spacing S of the slots  36  is available, for example, from a lookup table stored in microprocessor  46 ,  56  memory, so that the location of a tooth  32 , for example, from the target origin O T  can be uniquely identified. Two adjacent teeth, for example teeth  32 ,  34 , at a position above the linear array  12  result in two peaks in the magnetic flux density sensed by the sensing elements  14 . The positions P 1  and P 2  which correspond to the centers of teeth  32 ,  34 , respectively, can be determined by techniques previously mentioned, as for example through the use of the parabolic fit method previously described. The spacing S between the teeth  32 ,  34  can be determined by S=d·(P 2 −P 1 ), whereby the distance Y of tooth  32  from the target origin O T  is uniquely identified through the use of the lookup table stored in microprocessor  46 ,  56  memory. The distance X of the tooth  32  from the array origin O A  is determined by the product of d times P 1 , that is, X=d·(P 1 ), wherein the distance L 0  of the array origin O A  from the initial position  22  of the target origin O T  is known. Thus, the linear or angular present position  26  of the target origin O T  from the initial position  22  of the target origin O T  is a distance L, given by the relation: 
 
 L=L   0   X−Y   (2). 
 
         [0039]      FIG. 6  depicts an example of determining the linear position or angular present position  26  of a target origin O T  of a target  24  from an initial position  22  of the target origin O T  according to the second preferred embodiment of the present invention.  
         [0040]     The linear array  12 , in this example, consists of roughly identical equidistantly spaced d galvanomagnetic sensing elements  14  wherein the spacing d between adjacent galvanomagnetic sensing elements is known and is, preferably, larger than the width W of a tooth  32  or  34 , for example. The target  24  consists of teeth  32 ,  34 , for example, preferably, narrower than the minimum spacing d between adjacent galvanomagnetic sensing elements  14  and slots  36  wherein, in this example, each tooth  32  having unique predetermined features, for example, is uniquely identifiable from a single peak of the magnetic flux density which thereby uniquely determines the position of each tooth with respect to the target origin O T . The identification of the tooth  32  and its distance Y from the target origin O T  is available, for example, from a lookup table stored in microprocessor  46 ,  56  memory. Position P corresponding to the center of tooth  32 , for example, can be determined by techniques previously mentioned, for example, through the use of the parabolic fit method previously described. The distance Y of the tooth  32  from the target origin O T  is uniquely identified through the use of the lookup table stored in microprocessor  46 ,  56  memory. The location X of the tooth  32  from the array origin O A  is determined by the product of d times P, that is, X=d·(P), whereas the distance L 0  of the array origin O A  from the initial position  22  of the target origin O T  is known. Thus, the linear or angular present position  26  of the target  24  from the initial position  22  of the target is a distance L, given by the relation: 
 
 L=L   0   +X−Y   (3). 
 
         [0041]     The sizes mentioned herein for the target, magnet, spacing d and length D are by example only. A linear array  12  with a long length D is more expensive. The smaller the spacing d, the more accurate the sensor  10  for the same length D of the linear array since it has more sensing elements  14 . However, the smaller the spacing d, the smaller the air gap  19  should be. Thus, assembly tolerances become an issue. The balance between tight tolerance requirements, accuracy and size, which equates directly to price, is application-specific and can be determined by one of skill in the art based upon the teachings herein.  
         [0042]     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.