Abstract:
An apparatus and method are provided for sensing the position of a point of contact through use of a sensor having a movable element, disposed between a sensing element and a source of magnetic flux, and having an aperture therein for allowing a portion of the magnetic flux from the source of magnetic flux to pass through the aperture and impinge on the sensing element. The aperture may have various shapes to provide a desired linear or non-linear relationship between the position of the movable element and the intensity of the flux that passes through the aperture and impinges on the sensing element for a given position of the movable element.

Description:
TECHNICAL FIELD OF THE INVENTION  
         [0001]    This invention relates to sensing the position of a surface or a datum, and more particularly to position sensors that determine the position and/or motion of a point of contact in relation to the position sensor by detecting the intensity of magnetic flux impinging on a sensing element within the position sensor.  
         BACKGROUND OF THE INVENTION  
         [0002]    In many mechanical devices, it is desirable or necessary to detect the position or movement of a movable member of the device in relation to another part of the mechanical device. It is also desirable in some cases to provide an output electrical signal that is indicative of the current position or movement of the movable member.  
           [0003]    One example of such a mechanical device is a brake booster for a vehicle, in which it is desirable to know the position of a movable diaphragm support located inside of the booster, in relation to a housing of the booster that completely encloses the diaphragm support and makes it impossible to view the diaphragm support external to the booster. It may be desirable to know the position of the diaphragm support in a booster to discern driver intent in a controlled brake system, or for providing a remote indication of a problem with the controlled brake system.  
           [0004]    One type of position sensor used for such purposes includes a magnetic flux sensing device, such as a Hall-effect sensor, to generate an electrical signal indicative of the intensity of a magnetic flux impinging on the sensing device. The sensing element is typically attached to a housing of the position sensor, along with a source of magnetic flux in the form of a permanent magnet, or an electromagnet. A movable element inside the position sensor has a connection adapted to bear against or be attached to a point of contact on a surface of a part whose position is to be detected. As the movable element moves with the point of contact, the position of the movable element in relation to the sensing element and the source of magnetic flux causes the magnetic flux impinging on the sensing element to vary as a function of the position of the movable element, in a manner that allows the sensing element to generate an electrical signal indicative of the position of the point of contact in relation to the housing of the sensor.  
           [0005]    While such position sensors are widely used, there are several areas in which an improved sensor design is desirable. It is sometimes difficult to achieve a true linear relationship between the position of the point of contact and electrical signal generated by the sensing element. Some prior position sensors addressed this problem by incorporating multiple sensing elements into the position sensor itself, or by using complex signal processing circuitry for converting a non-linear signal from the sensing elements into a signal having a linear relationship to the position of the point of contact.  
           [0006]    This has been particularly true in position sensors that are required to provide position detection over a wide range of movement, spanning a range of 0.5 to 2.0 inches or more of travel. The use of such additional sensing elements and signal processing circuitry is undesirable because the physical size and cost of the position sensor is significantly increased, and because the additional sensing elements and circuitry create more potential points of failure, thereby reducing the ruggedness and reliability of the sensor.  
           [0007]    What is needed, therefore, is an improved position sensor, and a method of sensing position, that provide a solution to one or more of the problems described above.  
         SUMMARY OF THE INVENTION  
         [0008]    Our invention provides an improved position sensor and method for sensing the position of a point of contact through use of a sensor having a movable element, disposed between a sensing element and a source of magnetic flux, and having an aperture therein for allowing a portion of the magnetic flux from the source of magnetic flux to pass through the aperture and impinge on the sensing element. The aperture may have various shapes to provide a desired linear or non-linear relationship between the position of the movable element and the intensity of the flux that passes through the aperture and impinges on the sensing element for a given position of the movable element.  
           [0009]    In one form of our invention, a position sensor includes a housing defining an axis of motion. A movable element, a sensing element and a magnet are attached to the housing. The movable element is formed of a ferromagnetic material, and is mounted in the housing for movement along the axis of motion. The movable element includes an aperture extending through the movable element in a direction transverse to the axis of motion, and has a connection thereof adapted for receiving a positional input from a point of contact. The sensing element is fixedly attached to the housing adjacent one side of the movable element for sensing magnetic flux passing through the aperture, and the magnet is fixedly attached to the housing adjacent an opposite side of the movable element.  
           [0010]    The movable element may have a width transverse to the axis of motion that varies in a linear or non-linear manner along the axis of motion, with respect to motion of the point of contact, to produce a corresponding desired change in intensity of magnetic flux impinging on the sensing element. The sensing element may be a single Hall-effect sensor, generating an electrical output signal indicative of the intensity of magnetic flux impinging on the sensing element. The magnet may also have a shape that enhances performance of the sensor in a manner that obviates the need for additional sensing elements and signal processing circuitry.  
           [0011]    Our invention may also take the form of a method for sensing the position of a point of contact by directing magnetic flux from a source of magnetic flux through an aperture in a movable element of ferromagnetic material operatively attached to the point of contact for movement therewith and onto a sensing element for determining an intensity of a magnetic flux impinging on the sensing element. The method may also include generating an output signal indicative of the intensity of the magnetic flux impinging on the sensing element.  
           [0012]    The foregoing and other features and advantages of our invention will become further apparent from the following detailed description of exemplary embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention rather than limiting, the scope of the invention being defined by the appended claims and equivalents thereof. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a longitudinal cross section of an exemplary embodiment of a position sensor according to our invention, in the form of a linear position sensor;  
         [0014]    [0014]FIG. 2 is a transverse cross section position sensor of FIG. 1, taken along lines  2 - 2  of FIG. 1, showing internal components of the sensor;  
         [0015]    [0015]FIG. 3 is an exploded view of the internal components of an exemplary position sensor, according to our invention; and  
         [0016]    [0016]FIGS. 4 a  and  4   b  show alternate configurations of the shape of an aperture in a movable element of a position sensor, according to our invention.  
     
    
     DETAILED DESCRIPTION  
       [0017]    [0017]FIGS. 1 and 2 depict a first exemplary embodiment of a position sensor  10 , having a housing  12  defining an axis of motion  14 . A movable element  16 , a sensing element  18 , and a magnet  20  are operatively attached to the housing  12  in the manner described in greater detail below, for measuring a linear position ‘d’ of a point of contact  28  on a translating wall  11 , in relation to a fixed wall  13 , to which the position sensor  10  is mounted.  
         [0018]    It should be noted that the terms “translating wall” and “fixed wall” are used herein for illustrative purposes in explaining our invention. Our sensor may be used for sensing relative movement or position of one body to another, i.e. not just walls. Also, in other embodiments contemplated within the scope of the appended claims, the sensor may be attached to a movable body, rather than a fixed body as is the case in the exemplary embodiment disclosed herein, with the point of contact being on either a movable or a fixed body.  
         [0019]    [0019]FIG. 3 is an exploded perspective view of the internal elements of a second embodiment of a position sensor  10  having some differences, as explained below, from the sensor  10  of FIGS. 1 and 2. The same reference numerals are used for similar elements in the following descriptions of the first and second exemplary embodiments, as shown in FIGS.  1 - 3 .  
         [0020]    The movable element is configured as a vane  16  of ferromagnetic material mounted in the housing  12  for sliding movement along the axis of motion  14 . The vane  16  includes an aperture  22  extending through the vane  16  in a direction transverse to the axis of motion  14 . A connection, in the form of pin  24 , extends from a first axial end  26  of the vane  16  and out of the housing  12  for receiving a positional input from a point of contact  28  with a surface  30  of a translating wall  11  external to the sensor  10 .  
         [0021]    The sensing element  18  is fixedly attached to the housing  12  adjacent one side of the vane  16  for sensing magnetic flux passing through the aperture  22 . In the exemplary embodiments shown, the sensing element is preferably a Hall-effect sensor  18 , configured to measure flux impinging on the sensing element  18  in a direction generally normal to the axis of motion  14 . The Hall-effect sensor  18  produces an electrical output signal indicative of the intensity of the magnetic flux impinging on the sensing element  18 , and delivers the output signal to an external circuit through an electrical connector  19  attached to the outside of the housing  12 .  
         [0022]    A helical compression spring  32  is operatively attached between an end cap  34  of the housing  12  and the vane  16 , for urging the vane  16  to return to an initial position along the axis of motion  14 , at a left end of the housing  12  as depicted in FIGS. 1 and 2. The first axial end  26  of the vane  16  is configured to rest against an inner wall  36  of the housing  12 , when the vane  26  is held in the initial position by the spring  32 . The opposite end  38  of the vane  16  is configured to receive the spring  32  and includes a stop portion  40  thereof that bears against a stop surface  42  of the end cap  34 , for limiting the maximum travel of the vane  16  within the housing  12  along the axis of motion  14 .  
         [0023]    The magnet  20  is fixedly attached to the housing  12  adjacent an opposite side of the vane  16 . In the exemplary embodiment shown in FIG. 1, the magnet  20  is a permanent magnet, and defines a magnetic flux axis  44 , extending from the magnet  20  and aligned to pass through the aperture  22 , during movement of the vane  16  along the axis of motion  14 . The magnet  20  in the exemplary embodiment of FIGS. 1 and 2, has a flat planar body  46  defining oppositely facing first and second faces  48 ,  50  of the magnet  20  extending normal to the magnetic flux axis  44 . The magnet  20  has a generally circular outer perimeter  52  disposed about the magnetic flux axis  44 . The outer periphery  52  defines a flat portion  54  thereof extending normal to the flux axis  44 , and oriented transverse to the axis of motion  14 .  
         [0024]    As shown in FIG. 3, the magnet  20  of the second exemplary embodiment of a linear position sensor  10  has a flat planar body with a periphery that is rectangular, rather than circular as is the case for the magnet  20  of the first embodiment shown in FIGS. 1 and 2. The shape of the magnet  20  has an effect on the linearity of an output signal generated by the sensing element  18 , and can be judiciously selected to fine tune the performance of the position sensor  10  in a manner that is desirable for various embodiments of our invention.  
         [0025]    The aperture  22  in the vane  16  has a width ‘w’ transverse to the axis of motion  14  that varies along the axis of motion  14 . The width ‘w’ can vary in either a linear or a non-linear manner along the axis of motion  14 , to allow the intensity of the magnetic flux passing through the aperture from the magnet  20  to the sensing element  18  to vary in relation to the position ‘d’ in a manner that produces a desired relationship between the position ‘d’ and the output electrical signal produced by the sensing element  18 .  
         [0026]    In the exemplary embodiments shown in FIGS.  1 - 3 , the aperture  22  is elongated along a longitudinal axis  56  of the aperture  22 , extending generally parallel to the axis of motion  14  from a first end  58  to a second end  60  of the aperture  22 , at opposite ends of the longitudinal axis  56  of the aperture  22 , and the width ‘w’ of the aperture  22  tapers inward from the first end  58  to the second end  60  of the aperture  22 . In the embodiment shown in FIG. 1, the sides of the aperture  22  between the first and second ends  58 ,  60  of the aperture  22  are straight, thereby providing a perimeter of the aperture  22  that is generally triangular shaped. In the embodiment depicted in FIG. 3, the sides of the aperture  22  between the first and second ends  58 ,  60  of the aperture  22  are curved inward, thereby providing a perimeter of the aperture  22  that is generally teardrop shaped, as shown in FIG. 4 a.    
         [0027]    In other embodiments, contemplated within the scope of our invention, the sides of the aperture  22  may be curved in another manner, or may be stepped, as shown in FIG. 4 b . The shape of the aperture  22  has a significant effect on the relationship between the amount of magnetic flux that passes through the aperture  22  and impinges on the sensing element  16  as a function of the position ‘d’ of the point of contact  28 . Multiple apertures may also be used for generating a digital output signal.  
         [0028]    While the embodiments of our invention disclosed herein are presently considered to be preferred, various changes and modifications can be made without departing from the spirit and scope of the invention. For example, other types of magnetic sensing elements may be used, and the magnet may be an electromagnet rather than a permanent magnet.  
         [0029]    It is also contemplated that the movable element of a sensor according our invention may be configured as a disk with an aperture extending along a circumferential axis of the aperture that turns about a rotational axis to provide a sensor that measures rotational or angular movement. The movable element may also be cylindrical in shape, and capable of both rotational and longitudinal movement. The aperture may also taper in the opposite direction from what is shown in FIGS.  1 - 3 .  
         [0030]    The connection of the movable element to the point of contact may be made at any point on the movable element, for example on a side or top surface of the movable element, rather than at one axial end as shown in the exemplary embodiment shown in FIGS.  1 - 3 . The connection between the vane and the point of contact may also be made directly or indirectly in a virtually unlimited number of ways using a cable or other tension elements, fasteners, and various mechanical, fluid, magnetic or electrical coupling elements, rather than the pin bearing against the point of contact shown in the exemplary embodiment of FIGS.  1 - 3 .  
         [0031]    Those having skill in the art will also appreciate that a single position sensor  10 , as described herein may be used in a variety of applications having different length of travel requirements by substituting a vane  16  having a shorter or longer aperture  22 . The pin  24  may be attached to the point of contact  28 , and the spring  32  may be oriented to urge the vane to move in the opposite direction along the axis of motion back to an initial position that is fully compressed, rather than fully extended as shown in FIGS.  1 - 3 . The spring may be provided in a number of other forms including, for example, a torsion spring, a leaf spring, a bellows, or as an element formed from a compressive or stretchable elastomer.  
         [0032]    The scope of the invention is indicated in the appended claims, and all changes or modifications within the meaning and range of equivalents are intended to be embraced therein.