Abstract:
A rotary position sensor featuring a robust and simply constructed magnetic field assembly which provides a uniform magnetic field in the working air gap for a conventional magnetosensitive device located thereinside. The magnetic field assembly includes a plastic casing having a blind hollow, magnet pocket located at the blind end of the hollow, and a pair of diametrically opposed pole piece pockets oriented in upstanding relation to the magnet pocket. A permanent magnet is snapped into the magnet pocket, and a pole piece is respectively snapped into each of the pole piece pockets. The pole pieces may be rectangular or arc shaped, wherein the space therebetween forms the working air gap into which the magnetosensitive device is placed. The magnetic field established in the working air gap is transverse to the axis of the blind hollow, and is generally uniform.

Description:
TECHNICAL FIELD  
         [0001]    The present invention relates to rotary position sensors used to sense rotary movement using a sensor including a magnetosensitive device, such as a Hall effect device, and more particularly to a magnetic assembly therefor.  
         BACKGROUND OF THE INVENTION  
         [0002]    Rotary position sensors utilize a magnetic field and a magnetosensitive device, such as a Hall effect device or a magnetoresistor located within the magnetic field. To detect rotational movement as between a first article (such as for example a rotatable throttle shaft of an air control valve) and a second article (such as for example a stationary base), the magnetic field is oriented transverse in relation to the axis of rotation of the first article, and the magnetosensitive device is located inside the magnetic field. The member providing the magnetic field is connected to one of the articles, and the magnetosensitive device is connected to the other article. As the articles rotate relative to each other, the magnetosensitive device is caused to change its angular position relative to the magnetic field direction, resulting in a change of output signal from the magnetosensitive device responsive to its angle with respect to the magnetic field direction. This change in signal is indicative of the angular position as between the first and second articles.  
           [0003]    [0003]FIGS. 1 and 2 depict a typical configuration of a prior art rotary position sensor  10 . A shaft  12  supports a magnetic assembly  14  including two mutually opposed permanent magnet arcs  16 ,  18  and a keeper ring  20 . A working air gap  22  is provided between the magnet arcs  16 ,  18 , wherein a magnetic field B is provided therebetween having a direction D locally defined by the magnetic lines L. The magnet arcs  16 ,  18  are glued or bonded into place on the keeper ring  20 . A magnetosensitive device  24  (as for example an AISC chip, such as a Melexis MLX90215 or Alegro ATS635LSB) is placed into the working air gap  22 , and is connected to a base  26  by at least one peg  28 . FIG. 3 depicts the magnetic field strength as a function of distance along the cross-section line  30 , and indicates the magnetic field is nonuniform in that it drops in field strength on either side of the center of the working air gap.  
           [0004]    What remains needed in the art is a magnet assembly for a rotary position sensor which is robust, yet simply constructed, and which provides a uniform magnetic field in the working air gap.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention is a rotary position sensor featuring a robust and simply constructed magnetic field assembly which provides a uniform magnetic field in the working air gap for a conventional mangetosensitive device located thereinside.  
           [0006]    The magnetic field assembly according to the present invention includes a plastic casing having a blind hollow, magnet pocket located at the blind end of the hollow, and a pair of diametrically opposed pole piece pockets oriented in upstanding relation to the magnet pocket. A permanent magnet is snapped into the magnet pocket, and a pole piece is respectively snapped into each of the pole piece pockets. The pole pieces may be rectangular or arc shaped, wherein the space therebetween forms the working air gap into which the magentosensitive device is placed. The magnetic field established in the working air gap is transverse to the axis of the blind hollow, and is generally uniform.  
           [0007]    The aforesaid snapping feature is provided preferably by a plurality of nibs located at the periphery of the magnet and pole piece pockets which interferingly engage the respective magnet and pole pieces so as to retain them fixedly seated with respect thereto.  
           [0008]    The casing may be configured to interface with either a stationary or rotatable article, as for example via a shaft cavity for being press-fit onto an end of a shaft.  
           [0009]    Accordingly, it is an object of the present invention to provide a rotary position sensor including a simply constructed, snap together magnet assembly.  
           [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]    [0011]FIG. 1 is a partly sectional side view of a prior art rotary position sensor which is press-fit onto a shaft end.  
         [0012]    [0012]FIG. 2 is an end view seen along line  2 - 2  of FIG. 1.  
         [0013]    [0013]FIG. 3 is a graph of magnetic field strength versus cross-sectional position of the prior art rotary position sensor of FIGS. 1 and 2.  
         [0014]    [0014]FIG. 4 is a partly sectional side view of a magnet assembly according to the present invention, shown press-fit onto a shaft.  
         [0015]    [0015]FIG. 5 is a partly sectional side view of a rotary position sensor having a magnet assembly according to the present invention.  
         [0016]    [0016]FIG. 6 is a partly sectional view seen along line  6 - 6  of FIG. 5.  
         [0017]    [0017]FIG. 7 is a view of the working air gap environs of the magnet assembly as shown at FIG. 5, depicting the magnetic field.  
         [0018]    [0018]FIG. 8 is a graph of the magnetic field strength versus position for the working air gap of FIG. 7.  
         [0019]    [0019]FIG. 9 is a plan view of an alternate rotary position sensor according to the present invention  
         [0020]    [0020]FIG. 10 is a partly sectional view seen along line  10 - 10  in FIG. 9.  
         [0021]    [0021]FIG. 11 is a graph of the magnetic field strength versus position for the working air gap of FIG. 9.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]    Referring now to the Drawing, FIGS. 1 through 11 depict examples of a rotary position sensor according to the present invention.  
         [0023]    Turning attention firstly to FIGS. 4 through 8, aspects of a first rotary position sensor  100  utilizing a first magnet assembly  100 ′ according to the present invention are depicted. The first magnet assembly  100 ′ includes a plastic casing  102  having a blind hollow  104  formed therein defined by a sidewall  106   a  and a blind end  106   b . A magnet pocket  108  and a pair of diametrically opposed pole piece pockets  110   a ,  110   b  are formed in the casing  102  at the sidewall and blind end  106   b  of the blind hollow  104 .  
         [0024]    The magnet pocket  108  is formed in the blind end  106   b , having a preferably rectangular recess cooperatively defined by mutually opposed magnet pocket walls  112   a ,  112   b  and the aforesaid blind end. At each of the magnet pocket walls  112   a ,  112   b  is formed at least one nib  114  which protrusively overhangs the magnet pocket  108 .  
         [0025]    A permanent magnet  116  having a shape complementary to that of the magnet pocket  108  is snapped into the magnet pocket by being pressed past the nibs  114 , which resiliently give and then overhang the permanent magnet upon its being seated at the magnet pocket.  
         [0026]    The pole piece pockets  110   a ,  110   b  are formed in the sidewall  106   a  of the casing  102  in upstanding relation to the magnet pocket  108  and in perpendicular relation to the magnet pocket walls  112   a ,  112   b . Each pole piece pocket  110   a ,  110   b  is defined by a preferably rectangular recess cooperatively defined by mutually opposed pole piece pocket walls  118   a ,  118   b , a bottom wall  120  preferably in plane with the blind end, and a back wall  122  which is perpendicular to each of the pole piece pocket walls and the bottom wall. The pole piece pocket walls  118   a ,  118   b  are offset in relation to their respectively adjacent the magnet pocket walls  112   a ,  112   b , thereby forming a pair of abutment shoulders  124 . At each of the back walls  122  is formed at least one nib  114 ′ which protrusively overhangs the respective pole piece pocket  110   a ,  110   b.    
         [0027]    A pair of pole pieces  126 ,  128 , are provided for placement into the pole piece pockets  111   a ,  110   b , each being composed of a ferromagnetic material, and each being complementarily shaped with respect to its pole piece pocket. Each pole piece  126 ,  128  is snapped into its respective pole piece pocket  110   a ,  110   b  by being pressed past the nibs  114 ′, which resiliently give and then overhang the respective pole piece upon its being seated at the respective pole piece pocket. When seated, each pole piece  126 ,  128  has a face  130 ,  132  which is in good contact with a respective permanent magnet pole  134 ,  136  thereby providing minimal reluctance to the magnetic circuit (shown at FIG. 7) at the interface therebetween.  
         [0028]    The permanent magnet  116  is fixed in place relative to the casing  102  by the nibs  114  versus the blind end  106   b , the opposed magnet pocket walls  112   a ,  112   b  and the two pole pieces  126 ,  128 . Each pole piece  126 ,  128  is fixed in place relative to the casing  102  by the nibs  114 ′ versus the bottom wall  120 , the opposed pole piece walls  118   a ,  118   b , and the back wall  122  versus the permanent magnet  116  and the abutment shoulders  124 .  
         [0029]    As shown at FIGS. 7 and 8, the magnetic field B′ which is provided in the working air gap  138  between the pole piece faces  126   f ,  128   f  is substantially uniform along the axial centerline  140 . Accordingly, the signal output of a magnetosensitive device  24 ′ placed into the working air gap is accurately responsive to its orientation with respect to the direction of the magnetic field B in the working air gap.  
         [0030]    The casing  102  is configured to be associated with an article which is rotatable with respect to the base  142  (which may, for example, be a circuit board) upon which the magnetosensitive device  24 ′ is connected. For example, the article may be a shaft  12 ′ having an end which is press-fit into a shaft cavity  144  formed in the casing  102  opposite the bind hollow  104 .  
         [0031]    Turning attention next to FIGS. 9 through 11, aspects of a second rotary position sensor  200  utilizing a second magnet assembly  200 ′ according to the present invention are shown. A plastic casing  202  has a blind hollow  204  formed therein defined by a sidewall  206   a  and a blind end  206   b . A magnet pocket  208  and a pair of diametrically opposed pole piece pockets  210   a ,  210   b  are formed in the casing  202  at the sidewall and blind end of the blind hollow  204 .  
         [0032]    The magnet pocket  208  is formed in the blind end  206   b , having a preferably rectangular recess cooperatively defined by mutually opposed magnet pocket walls  212   a ,  212   b  and the aforesaid blind end. At each of the magnet pocket walls  212   a ,  212   b  is, preferably, formed at least one nib  214  which protrusively overhangs the magnet pocket  208 .  
         [0033]    A permanent magnet  216  having a shape complementary to that of the magnet pocket  208  is snapped into the magnet pocket by being pressed past the nibs  214 , which, if present, resiliently give and then overhang the permanent magnet upon its being seated at the magnet pocket.  
         [0034]    The pole piece pockets  210   a ,  210   b  are formed in the sidewall  206   a  of the casing  202  in upstanding relation to the magnet pocket  208  and in perpendicular relation to the magnet pocket walls  212   a ,  212   b . Each pole piece pocket  210   a ,  210   b  is defined by a preferably rectangular recess having a leg pocket  215  cooperatively defined by mutually opposed pole piece pocket walls  218   a ,  218   b , a bottom wall  220  at the leg pocket  215  preferably in plane with the blind end, and a back wall  222  which is perpendicular to each of the pole piece pocket walls and the bottom wall. The pole piece pocket walls  218   a ,  218   b  are offset in relation to their respectively adjacent the magnet pocket walls  212   a ,  212   b , thereby forming a pair of abutment shoulders  224 . At each of the back walls  222  is formed at least one nib  214 ′ which protrusively overhangs the respective pole piece pocket  210   a ,  210   b.    
         [0035]    A pair of pole pieces  226 ,  228 , are provided for placement into the pole piece pockets  210   a ,  210   b , each being composed of a ferromagnetic material, and each being complementarily shaped with respect to its pole piece pocket, including a leg  225  for placement into its respective leg pocket  215 . Each pole piece  226 ,  228  is snapped into its respective pole piece pocket  210   a ,  210   b  by being pressed past the nibs  214 ′, which resiliently give and then overhang the respective pole piece upon its being seated at the respective pole piece pocket. When seated, each pole piece  226 ,  228  has a face  230 ,  232  at the leg  225  which is in good contact with a respective permanent magnet pole  234 ,  236  thereby providing minimal reluctance to the magnetic circuit at the interface therebetween.  
         [0036]    Each of the pole pieces  226 ,  228  overlies the permanent magnet  216  and has a mutually facing concave pole piece face  226   f ,  228   f  which define the working air gap  238 . The permanent magnet  216  is fixed in place relative to the casing  202  by the overlying pole pieces  126 ,  128  and nibs  214  (if present) versus the blind end  206   b , the opposed magnet pocket walls  212   a ,  212   b  and the leg  225  of the two pole pieces  226 ,  228 . Since the pole pieces  126 ,  128  overlie the permanent magnet, the nibs  214  are not mandatory to hold the permanent magnet in place.  
         [0037]    Each pole piece  226 ,  228  is fixed in place relative to the casing  202  by the nibs  214 ′ versus the bottom wall  220 , the opposed pole piece walls  218   a ,  218   b , and the back wall  222  versus the permanent magnet  216  at the leg  225  and the abutment shoulders  224 .  
         [0038]    The magnetosensitive device  24 ″ for placement in the working air gap  238  is preferably sandwiched by a pair of semi-circular ferromagnetic (steel) flux shapers  235   a ,  235   b . The flux shapers  235   a ,  235   b  are fixed in relation to the magnetosensitive device  24 ″ and collectively form a sensor package  245 .  
         [0039]    As shown at FIG. 9, the magnetic field B″ which is provided in the working air gap  238  is substantially uniform along the axial centerline  140 . Accordingly, the signal output of a magnetosensitive device  24 ″ placed into the working air gap is accurately responsive to its orientation with respect to the direction of the magnetic field B in the working air gap.  
         [0040]    The casing  102  is configured to be associated with an article which is rotatable with respect to the base  142  (which may, for example be a circuit board) upon which the magnetosensitive device  24 ′ is connected. For example, the article may be a shaft  12 ′ having an end which is press-fit into a shaft cavity  144  formed in the casing  102  opposite the bind hollow  104 .  
         [0041]    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.