Abstract:
A displacement sensor for providing an indication of the position of a first body relative to a second body, the first body being displaceable relative to the second body in a displacement direction. The sensor is composed of: two magnets that are spaced from one another in the displacement direction to define therebetween a region containing a magnetic field; a magnetic field sensing element mounted in the region; and components for coupling at least one of the magnets to one of the bodies and the magnetic field sensing element to the other of the bodies to produce a relative displacement between the at least one magnet and the magnetic field sensing element in the displacement direction in response to displacement of the first body relative to the second body.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The benefit of priority is claimed of U.S. Provisional Application No. 60/234,341 filed in the U.S. Patent &amp; Trademark Office on Sep. 22, 2000, the complete disclosure of which is incorporated herein by reference. 
    
    
     GOVERNMENT LICENSE CLAUSE 
     The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of contract NAS 8-97238 with the National Aeronautics and Space Administration (NASA). 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to displacement, or position, sensors of the type having a movable element capable of being placed in contact with a component whose displacement is to be monitored, and particularly relates to displacement sensors constructed to monitor small amplitude displacements of the order of a fraction of an inch. 
     Displacement sensors of the type contemplated by the present invention are useful for a variety of purposes, one of which is the monitoring of movements experienced by parts of a structure for testing purposes. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention provides a displacement sensor for providing an indication of the position of a first body relative to a second body, the first body being displaceable relative to the second body in a displacement direction. The sensor comprises two magnets that are spaced from one another in the displacement direction to define therebetween a region containing a magnetic field, with a magnetic field sensing element mounted in the region. The sensor further comprises means for coupling at least one of the magnets to one of the bodies and the magnetic field sensing element to the other of the bodies to produce a relative displacement between the at least one magnet and the magnetic field sensing element in the displacement direction in response to displacement of the first body relative to the second body. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The accompanying drawings are incorporated in and constitute a part of the specification. The drawings, together with the general description given above and the detailed description of the preferred embodiments and methods given below, serve to explain the principles of the invention. In such drawings: 
     FIGS. 1,  2 , and  3  are simplified pictorial views of three embodiments of the present invention; 
     FIG. 4 is a cross-sectional view of a preferred embodiment of the invention; 
     FIG. 5 is a detailed view of one component of the embodiment of FIG. 4; and 
     FIG. 6 is a partial cross-sectional view of a further embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the presently preferred embodiments and methods of the invention. It should be noted, however, that the invention in its broader aspects is not limited to the specific details, representative devices and methods, and illustrative examples shown and described in this section in connection with the preferred embodiments and methods. The invention according to its various aspects is particularly pointed out and distinctly claimed in the attached claims read in view of this specification, and appropriate equivalents. 
     It is to be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     FIG. 1 shows a first embodiment of the invention composed of a housing  12  which contains a first permanent magnet  14 , a second permanent magnet  16  and a Hall effect sensor  18  interposed between magnets  14  and  16 . Magnet  14  and sensor  18  are fixed in position within housing  12 , while magnet  16  is mounted to be movable along the longitudinal axis of housing  12 , in the direction of arrows  20 . As is apparent from the drawing, magnet  16  is movable only in the direction of arrows  20 . 
     A compression spring  22  is installed between magnet  16  and sensor  18  in order to bias magnet  16  away from sensor  18 . 
     Magnets  14  and  16  are installed so that their magnetization directions are opposite to one another in order to produce a magnetic field at the location of sensor  18  that will vary as magnet  16  moves toward and away from magnet  14 . 
     Housing  12  is fixed to a body  26  which includes a movable component whose displacement in the direction of arrows  20  relative to body  26  is to be measured. This component or an extension  28  thereof is coupled to the end of magnet  16  that is remote from sensor  18 . Extension  28  may simply be in contact with magnet  16  to exert a pushing force thereon. Magnet  16  is held in position relative to extension  28  by the action of spring  22  as well as by the magnetic repelling force existing between magnets  14  and  16 . Any movement of the part whose displacement is to be measured in the direction of arrows  20  produces a corresponding movement of magnet  16 , thereby varying the distance between magnets  14  and  16 . This produces a corresponding variation in the intensity of the magnetic field at the location of sensor  18 . Sensor  18  produces a magnetic field sensing signal indicative of the distance between magnets  14  and  16 , and thus indicative of the displacement of the part being monitored of extension  28 . 
     In the embodiment illustrated in FIG. 2, magnet  16  is replaced by a permanent magnet  30  which is fixed in position in housing  12 , while sensor  18  is mounted to be movable within housing  12  in the direction of arrows  20 . Sensor  18  is connected to a linkage, here in the form of a rod  32 , which extends through a passage formed in magnet  30 . The end of rod  32  that is remote from sensor  18  is provided with an enlarged contact head  34  which engages extension  28  of body  26 . In order to maintain contact head  34  in contact with extension  28 , a compression spring  36  is provided between magnet  30  and head  34 . In this embodiment, both magnets  14  and  30  are fixed in position in housing  12 , so that the distance therebetween is fixed, and the configuration of the magnetic field between magnets  14  and  30  is such that the response of sensor  18  to its displacement between magnets  14  and  30  will be substantially linear over much of its displacement range. 
     In place of, or in addition to, spring  36 , contact head  34  can be provided with a magnet and extension  28  made in whole or in part of a magnetizable material. Alternatively, extension  28  can include a magnet and head  34  made of a magnetizable material. In either case, head  34  will be maintained in contact with extension  28  by a magnetic attractive force. According to a further possibility, spring  36  can be installed between magnet  14  and sensor  18 . 
     In the embodiment shown in FIG. 3, housing  12  is provided with an assembly of two magnets  40  and  42  which are joined together by a suitable connecting element  44 , such as a rod. Magnets  40  and  42  are shown having polarities (N and S) arranged end-to-end along a common longitudinal axis, with the polarities of magnet  40  being arranged opposite to the polarities of magnet  42 . Magnets  40  and  42  and element  44  are installed for movement as a unit within housing  12  in the direction of arrows  20 , while sensor  18  is fixed in position with respect to housing  12 . Magnet  42  carries a contact head  46  which bears against extension  28  and the assembly of magnets  40  and  42  and element  44  is biased toward extension  28  by a compression spring  48  located between the base of housing  12  and magnet  40 . This embodiment operates in much the same manner as the embodiment of FIG. 2 in that the spacing between magnets  40  and  42  is fixed and the position of sensor  18  in the magnetic field between those magnets will vary as a function of the displacement of extension  28 . 
     Two forms of construction of the embodiment of FIG. 3 are illustrated in FIGS. 4-6, which also illustrate the signal-carrying conductors  50  that are connected to sensor  18  and that extend through a suitable passage in housing  12 . Since sensor  18  is fixed in position relative to housing  12 , the feeding of conductors  50  out of housing  12  is a simple matter. 
     The embodiment shown in FIGS. 4 and 5 includes a simple support structure  43  to hold sensor  18  at the desired position and a retaining clip  51  installed at the base of the housing  12  for supporting spring  48 . 
     The embodiment shown in FIG. 6 is functionally similar to that of the embodiment of FIGS. 4 and 5. However, in the embodiment of FIG. 6, the housing is composed of a main body  52  and an end cap  54  which is screwed into a threaded passage in body  52 , with a seal  56  interposed between body  52  and cap  54  in order to hermetically seal the region enclosed by body  52 . An actuator  58  extends through a passage in the front end of the housing, which passage is sealed by two sealing rings  60  which surround actuator  58 . Actuator  58  is coupled to magnets  40  and  42  and the outer end of actuator  58  can be given any desired configuration in order to contact the component whose displacements are to be monitored. 
     In preferred embodiments of the present invention, and particularly those illustrated in FIGS. 2-6, sensor  18  can be made to operate over a linear range by an appropriate selection of the spacing between the two permanent magnets and the physical size, or dimensions, of each magnet. 
     Displacement sensors according to the present invention are particularly useful for measuring displacements which occur over a small range, one application being for measuring displacements that extend over a range of the order of 0.1 inch. According to one preferred embodiment of the invention, use can be made of magnets  40  and  42 , each having a diameter of 0.25 inch and a thickness of 0.06 inch with a spacing between the magnets of the order of 0.2 inch. With this arrangement, it was found experimentally that the magnetic field over a displacement range of 0.01 inch at a location midway between the magnets varied linearly with a deviation of less than 0.5% Each magnet  40 ,  42  can have a magnetic field strength that is typical for magnets made of conventional permanent magnetic material, it having been noted that the particular field strength employed will effect the magnitude of the output of sensor  18 , but not the linearity of the magnetic field variation. 
     In order to improve the response of a displacement sensor according to the present invention, use can be made of Hall effect sensors that are temperature compensated for the magnets with which they are associated, thus taking into account the fact that the magnetic field of any magnet varies roughly linearly with temperature. The Hall effect sensor may be programmed to compensate for this temperature variation in order to maintain linearity over a reasonable temperature range, for example, of ˜10 to 50° C. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.