Abstract:
A linear sensor uses four spaced apart magnets arranged in a rectangular array and has an axis of symmetry. Each magnet has a staircase shape of at least two steps ascending towards a centerline. Each magnet of the array has a single N-S with magnets arranged as mirror images about the axis of symmetry having opposed poles, and magnets located on diagonals defined by the rectangular array having the same pole facing the axis of symmetry. The linear sensor employs a magnetic field sensor such as a programmable Hall effect sensor that is mounted for movement relative to the magnetic array along the axis of symmetry. The height of the steps defining the staircase shape of the magnets is selected to produce a magnetic field of selected linearity along a selected portion of the axis of symmetry.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    Not applicable. 
       STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates generally to position measurement devices in general and more particularly to devices used to sense linear motion using magnetic fields. 
         [0004]    It is often necessary to measure the position or displacement of two elements relative to each other. A particularly useful measurement is the linear displacement of a moving stage as it travels along a stationary base. This displacement can be measured with many different sensing technologies over a large range of accuracies, with different levels of complexity, and at a wide range of costs. 
         [0005]    Some common apparatus for measuring linear displacement employ linear encoders, capacitive sensors, eddy current sensors, a linear variable differential transformer, photoelectric or fiber optic sensors, or magnetic field sensors. Linear encoders use a glass or metal ruler that is made of a high stability material so that changes in temperature do not affect measurement accuracy. These materials, such as quartz, steel, invar, glass or ceramics generally require special machining techniques to manufacture and thus are more expensive. 
         [0006]    Capacitive sensors are used with both conductive and nonconductive target materials but are very sensitive to environmental variables that change the dielectric constant of the medium between the sensor and the target, usually air. Eddy current sensors contain two coils: an active coil that indicates the presence of a conducting target, and a secondary coil that completes a bridge circuit. A linear variable differential transformer (LVDT) sensor has a series of inductors in a hollow cylindrical shaft and a solid cylindrical core. The LVDT produces an electrical output that is proportional to the displacement of the core along the shaft. The size and mounting of these coils or cores and the sensitivity of measurement are competing design factors in the use of eddy current or LVDT sensors. 
         [0007]    Photoelectric and fiber optic sensors use beams of light to measure distance or displacement. The photoelectric sensor uses free-space transmission of light while the fiber optic sensor uses a pair of adjacent fibers to carry light to a target and receive reflected light from the object. Alignment of the fibers and the complexity of the optics needed to maintain the light path are difficulties in using this technology. 
         [0008]    Magnetic sensors such as the Hall effect sensor, GMR sensor, or an AMR sensor can be used with a linear array of teeth or alternating magnetic poles to produce a sinusoidal output indicative of the sensor&#39;s linear motion, however the initial position must be determined and each tooth or magnetic pole must be counted and phase data analyzed for greatest accuracy. A sensor that outputs voltage that is directly proportional to linear position has many advantages. One such sensor uses a pair of magnets with convex surfaces facing each other of the same magnetic pole. However, this type of sensor requires forming a nonlinear curve on the faces of the magnets, which, depending on the magnetic material used, can be costly. 
         [0009]    What is needed is a magnetic linear displacement sensor that produces direct correspondence between position and magnetic field strength that can be constructed with a simple magnet geometry. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is a linear magnetic sensor that employs four spaced apart magnets arranged symmetrically and positioned to form the corners of a first rectangle in a four pole array. The rectangle defines bisecting diagonals that define a center point with an axis of symmetry passing through the center point and bisecting angles defined by the diagonals. Each magnet is in the shape of a staircase having at least two steps. The magnets are arranged as mirror images about the axis of symmetry, with the steps facing the axis of symmetry and the step closest to the axis of symmetry arranged furthest from the center point. The steps define portions of the sides of at least one smaller rectangle within the rectangle and having a center coincidental with the center point. Magnets on a common diagonal having similar poles, and magnets symmetrically spaced about the axis of symmetry having opposite poles. This arrangement of magnets is used to create a magnetic field that varies substantially nearly linearly along the axis of symmetry. 
         [0011]    A programmable Hall effect sensor arranged with the sensor element parallel to the axis of symmetry detects linear motion of the Hall effect sensor along the axis of symmetry with respect to the magnetic array. The Hall effect sensor may be programmed with coefficients to substantially completely linearize the output of the Hall effect sensor, which output can then be directly correlated with relative linear motion between the Hall effect sensor and the magnetic array. Ferrous pole pieces on the sides of the magnets away from the axis of symmetry can be used to cause an increase in the magnetic field of each magnet of the magnetic array. The addition of a pole piece reduces the amount of magnet material needed to form the magnetic array of a selected magnetic field strength. 
         [0012]    It is a feature of the present invention to form a linear displacement sensor with a uniform linearly sloping magnetic field along the axis of displacement. 
         [0013]    It is a further feature of the present invention to form a linear displacement sensor utilizing magnets of simple geometry. 
         [0014]    It is a further feature of the present invention to form a linear displacement sensor that is less sensitive to displacement of the magnetic sensor off the sensing axis. 
         [0015]    Further features and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a side elevation view of a four magnet array and linear displacement sensor of the invention. 
           [0017]      FIG. 2  is an end elevation view of the four magnet array and linear displacement sensor of  FIG. 1 . 
           [0018]      FIG. 3  is an illustrative graphical view of the magnetic field strength along the axis of an array of four magnets of the type shown in  FIG. 1  at three different step heights. 
           [0019]      FIG. 4  is an illustrative graphical view of off axis magnetic field strength variation for an array of two magnets versus the array of four magnets of  FIG. 1 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring more particularly to  FIGS. 1-4 , wherein like numbers refer to similar parts, a linear motion sensor  20  is shown in  FIG. 1 . The linear motion sensor  20  is comprised of a magnetic field sensor  22  which senses magnetic field strength and polarity which is moved along a symmetry axis  24  of an array  26  of four symmetrically arranged magnets  28 ,  30 ,  32 , and  34 . Each of the magnets is arranged in one of the four quadrant of a rectangle  36 . Each magnet  28 ,  30 ,  32 , and  34  is in the shape of a staircase having at least two steps, a proximal step  38  with a pole surface  40  closest to the line of symmetry  24 , and a distal step  42  which is spaced further from the line of symmetry and has a pole surface  43 . Axial spacing between the steps of each magnet defines a step side  45 . The four step sides  45  together define a smaller rectangle  49  having a center coincident with the center point  47  of the larger rectangle  36  that lies on the line of symmetry  24 . Each magnet of the array of magnets  26  is arranged with a single N-S or S-N pole, as shown in  FIG. 1 . Magnets  28 ,  34  which are which lie along a first diagonal  44  of the rectangle  36  are arranged with the north pole facing the line of symmetry  24 . The magnets  32 ,  30  which lie along a second diagonal  46  of the rectangle  36  are arranged with the south pole facing the line of symmetry  24 . The diagonals  44 ,  46  cross at the center  47  of the rectangle  36  and the symmetry axis  24  passes through the center  47  bisecting angles formed between the diagonals. Magnets  28 ,  32 , which are arranged as mirror images with respect to the line of symmetry  24 , are arranged with unlike poles facing each other. Furthermore, magnets  30 ,  34  which are also arranged as mirror images with respect to the line of symmetry  24 , also are arranged with unlike poles facing each other; however, the arrangement of the polls N-S is reversed with respect to the poll arrangement S-N of the magnets  28 ,  32 . 
         [0021]    As shown in  FIG. 1 , the sensor  22  is moved along the line of symmetry  24  by either motion of the sensor or of the magnetic array  26 . The sensor  22  is preferably a Hall effect sensor with onboard logic that can be programmed to adjust to the output of the Hall affect device according to an onboard program. A suitable device is, for example, an HAL  855  available from Micronas GmbH of Freiburg, Germany. 
         [0022]      FIG. 3  shows a graph  48  of a simulation of the output of the Hall effect sensor as it is moved along the axis of symmetry  24  from one side to the other of the rectangle  36  of the magnetic array  26 . Also illustrated in  FIG. 3  is a graph  50  of a simulation of the output of the Hall effect sensor as it moves along the symmetry axis in an arrangement wherein the spacing of the magnets remains as illustrated in  FIG. 1  but the lower steps  42  of each magnet are completely eliminated. The graph  50  shows how the field strength increases as the sensor  22  approaches a point  52  of maximum field strength between opposed magnets  28 ,  32 , and decreases as the sensor approaches the center  47  of the rectangle  36 . As the sensor  22  continues to move toward a point  55  of maximum field strength between opposed magnets  30 ,  34 , the magnetic field continues to decrease because the opposed poles of the second pair of magnets  30 ,  34  are reversed. The arrangement of magnets without a step illustrated by the graph  50  shows that such an arrangement is unsuitable for a linear transducer because the field strength remains essentially constant for a considerable distance on either side of the center  47  and thus magnetic field strength could not be used to accurately determine position along symmetry axis  24 . 
         [0023]    In the magnetic array  26  illustrated in  FIG. 1 , each magnet  28 ,  30 ,  32 ,  34  has a step height of 4 mm as measured from a rear face  58  of each magnet. These distal steps  42  on the upper magnets  28 ,  30  extend toward each other and define an upper gap  54  between magnets  28 ,  30  above the axis of symmetry  24 , and centered above the center  47  of the rectangle  36 . The distal steps  42  the lower magnets  32 ,  34  extend toward each other and define a lower gap  56  between the lower magnets  32 ,  34  that is positioned below the center  47  of the rectangle  36 . As shown in  FIG. 3 , the graph  48  is very nearly linear between a point  60  between proximal steps  38  of the magnets  28 ,  32  of  FIG. 1  and a point  62  between proximal steps  38  of magnets  30 ,  34 . In particular, the slope of the magnetic field strength remains nearly constant as the sensor  22  crosses the center  47  and is between the upper gap  54  and the lower gap  56 . The magnet array  26  of  FIG. 1  provides a relatively steeply sloped linear change in magnetic field strength over a range of approximately 15 mm between point  60  and point  62 . A graph of magnetic field strength for a 2 mm step  64 , and a 5 mm step  66  are also shown in  FIG. 3  illustrating how step size can be varied to search for the most linear change in magnetic field strength within a magnet array similar to the array  26  shown in  FIG. 1 . Any remaining nonlinearity of the magnetic field strength slope can be corrected for by programming nonvolatile memory, which forms a part of the programmable Hall effect sensor  22 . Thus the output of the Hall effect sensor  22  may be read directly as linear position. Of course inherent accuracy is lost if the slope of magnetic field strength at any point approaches zero. 
         [0024]    The four magnet array  26  also provides nearly constant magnetic field strength for small deviations from the symmetry axis  24 .  FIG. 4  shows a simulation of off-axis magnetic field strength for points on the graph  48  where the magnetic field strength is approximately 800 gauss. The graph  68  shows a slope of zero in the immediate vicinity of the symmetry axis  24  that remains small for approximately 0.5 mm on either side of the symmetry axis  24 . This is in contrast to a two magnetic array comprising, for example the lower magnets  32 ,  34  of  FIG. 1 , where the graph  70  of the off axis change in magnetic field has a constant and relatively steep positive slope on either side of a line  72  in the same relative position as the symmetry axis  24 . 
         [0025]    The individual magnets  28 ,  30 ,  32 ,  34  as shown in  FIG. 1  can employ pole pieces  78  which increase the strength of the magnetic fields generated by the magnets. Because high-strength magnets are often made from relatively rare elements, the cost of the magnets can be reduced by decreasing the size of the magnets while maintaining the field strength through the use of the pole pieces  78  which are constructed from magnetically permeable material typically a low cost soft ferrous alloy. The pole pieces may be plates affixed to the rear surfaces  58  of each of the magnets  28 ,  30 ,  32 ,  24  of the array  26 . 
         [0026]    An alternative embodiment linear motion sensor  74  can be constructed where each magnet of the array has with three or more steps, as shown in phantom lines in  FIG. 1 . In the alternative embodiment, a third step  76  is situated intermediate in distance from the symmetry axis  24  having a spacing between the proximal step  38  and the distal step  42 . The use of a third step  76  is advantageous when a linear motion sensor with a relatively long sensing distance is required, for example a sensor with a substantially linear change in magnetic field strength over a distance of 30 mm was constructed using an array of four three-step magnets. 
         [0027]    In the design of the linear motion sensor  20  the following design variables can be used to achieve a desired shape of a graph of magnetic field strength versus linear position: varying the size of the rectangle  36  which defines the position of the magnets; varying the number of steps formed in each magnet and the distance each step is spaced from the symmetry axis; varying the width of the step along the symmetry axis; varying the width of the gap  54 ,  56  formed between the magnets along the symmetry axis; varying the type of magnetic material used in the size of the magnets; and employing pole pieces. 
         [0028]    Although generally the most linear graph of field strength versus distance is desired for uniform position sensing resolution, a magnetic field graph can vary significantly from the linear and still, by means of the programmable logic in the sensor, provide a linear output. In this arrangement, where the slope of the graph of magnetic field strength with respect to linear position is not constant, the variation in slope affects the inherent accuracy, which variation can be used to improve accuracy over a specific range by optimizing the magnets to have greater slope where greater resolution of linear position is desired, at the expense of somewhat lesser accuracy whether slope is less. 
         [0029]    In a practical application employing the sensor  20 , the four magnets  28 ,  30 ,  32 ,  34  are mounted in a housing which is mounted for linear motion, and the Hall effect sensor  22  is fixedly mounted to a circuit board, Typically the housing containing the four magnets is mounted to travel on rails or some similar arrangement which constrains the symmetry axis  24  to move over the Hall effect sensor  22 . 
         [0030]    It should be understood that the programmable sensor  22  includes a Hall effect device or sensor element  80  within the sensor package. The device or sensor element  80  is arranged parallel to the axis of symmetry and parallel to the magnetic pole surfaces  40  such that the magnetic field lines between the magnets of the array  26  are perpendicular to the Hall effect device. The sensing direction of the Hall effect device or sensor element  80  is perpendicular to its sensor element, and thus it is sensitive to magnetic field strength in a direction perpendicular to the axis of symmetry  24 . 
         [0031]    It should also be understood that the processing of the signal from the Hall effect device  80  may be done outside the package that houses the Hall effect device. Furthermore, it should be understood that other types of magnetic field sensors such as, for example, giant magnetoresistive (GMR) or Anisotropic Magneto-Resistive (AMR) sensors, with or without on-chip programmability could be used. In order to minimize the effect of external magnetic fields on the sensing system  20 , the magnetic field used by the sensor should be maximized because by utilizing the entire measurement range of the magnetic sensor, variation in the output of the sensor due to external magnetic noise is minimized. To minimize package size, magnets of high magnetic field strength may be used such as alnico (an aluminum-nickel-cobalt alloy), or samarium-cobalt (SmCo), and neodymium-iron-boron (NdFeB). Each magnet  28 ,  30 ,  32 ,  34  may be formed as a single piece or may be formed by combining magnets of simpler shape by, for example, bonding one rectangular magnet on to another. The pole surfaces  40 ,  43 , while generally planar and parallel to each other and the symmetry axis  24 , may incorporate such slight variations as do not substantially affect the benefits described. 
         [0032]    It should be understood that where the claims refer to relationships as being “substantially parallel”, “substantially aligned”, etc., it is meant to encompass such minor variations from a parallel state, alignment, etc. which still preserve the functionality of the device. 
         [0033]    The linear motion sensor  20  is designed to allow slight misalignments in the magnets in the sensor without introducing substantial errors in the output of the sensor. Furthermore, using the onboard logic it is possible to calibrate each linear motion sensor by controlled actuation accompanied by programming of onboard logic to linearize the output of the magnetic field sensor  22 , taking into account the measured deviance from linearity due to geometry misalignment or variations in the magnetic fields of the magnets. 
         [0034]    It should be understood that the arrangement of four similar or identical magnets at the corners of a rectangle, wherein magnets positioned across from each other along the line of symmetry have opposed poles and magnets connected by a diagonal have the same pole facing the symmetry axis, need not be limited to magnets having a staircase shape facing the line of symmetry. Thus the profile of the magnets, rather than having a staircase shape of two or more steps, can be a completely free variable that is optimized according to desired criteria of magnetic field strength along the axis of symmetry. 
         [0035]    It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described, but embraces all such modified forms thereof as come within the scope of the following claims.