Abstract:
A sensor includes a Hall-effect probe fastened only by way of its connection pins and includes a sensing element, a magnet having a cavity having a base, and in which cavity the sensing element is housed, and a cylindrical hole having an axis and which extends from the base toward the interior of the magnet, the Hall-effect probe being capable of moving inside the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application of PCT/FR2013/050303 filed Feb. 14, 2013, which claims priority to FR 1251433 filed Feb. 16, 2012. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable 
     THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT 
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     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB) 
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     STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINT INVENTOR 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a sensor comprising a magnet and a Hall-effect probe and intended to detect if a mobile metal object is in front of the Hall-effect probe, as well as a magnet for such a sensor. 
     Description of Related Art 
     To check the presence of a mobile metal object, it is known to use a sensor comprising a magnet coupled to a Hall-effect probe. 
     Such a sensor is used for example to detect the position of the axis of the dog clutch of a gearbox, or else the position of the camshaft.  FIG. 1  shows a curve  10  that represents the variations in magnetic induction as a function of the position of the mobile object with respect to the Hall-effect probe. The value of the magnetic induction decreases when the mobile object moves away from the Hall-effect probe. 
     Currently, manufacturers in particular of motor vehicles, require that the position of the mobile object with respect to the Hall-effect probe be determined from the start-up of the sensor. This function is called TPO (True Power On.) 
     This function thus makes it possible to determine at start-up whether or not the mobile object is facing the Hall-effect probe. To do this, a magnetic induction threshold value  12  is set in the factory. 
     Thus, when the sensor starts up, if the value of the magnetic induction seen by the Hall-effect probe is above the threshold value  12 , this is interpreted as the fact that the mobile object is facing the Hall-effect probe. If, on the contrary, when the sensor starts up, the value of the magnetic induction seen by the Hall-effect probe is below the threshold value  12 , this is interpreted as the fact that the mobile object is not facing the Hall-effect probe. 
     Over the lifetime of the sensor, it can happen that the Hall-effect probe moves with respect to the magnet and then the magnetic induction value changes. The curve  14  is the curve representing the variations in magnetic induction after movement of the Hall-effect probe. 
     As can be seen in  FIG. 1 , the threshold value  12  is no longer reached after the movement of the Hall-effect probe and the TPO function is no longer fulfilled. 
     To solve this problem, it is known practice to fasten the Hall-effect probe in the sensor and therefore to make the Hall-effect probe immobile with respect to the magnet. The fastening is performed for example by casting resin in the sensor, thereby encircling the Hall-effect probe. 
     Such a solution has other disadvantages. For example, the fastening of the Hall-effect probe leads to the appearance of mechanical stresses due to the thermal expansions in the Hall-effect probe and in particular at the level of the welds. Furthermore, the placing of the resin is burdensome and requires the use of an appropriate machine. 
     From the document U.S. Pat. No. 5,637,995 a sensor is known composed of a probe including a sensitive element, a permanent magnet, a casing and a hood. The permanent magnet has a recess and a through hole running along the central axis of the permanent magnet. The through hole communicates with the recess. The probe runs along the through hole and into the recess, the sensitive element being housed at the level of the recess. The permanent magnet and the probe are held in place by the casing and the hood. The probe is held in place in the magnet in order to come permanently in contact with the hood. 
     BRIEF SUMMARY OF THE INVENTION 
     An object of the present invention is to propose a sensor comprising a magnet and a Hall-effect probe that do not have the drawbacks of the prior art. 
     For this purpose, a sensor is proposed including:
         a Hall-effect probe including a sensitive element,   a magnet having a recess having a bottom and wherein the sensitive element is housed, and a cylindrical hole with an axis and which runs from said bottom to the inside of the magnet,
 
the Hall-effect probe being liable to move inside the recess.
       

     The cylindrical hole makes it possible to lower the magnetic induction, particularly in such a way that the latter reaches a value of around zero Gauss at the level of the sensitive element of the Hall-effect probe. 
     The cylindrical hole can have a circular transverse cross section. 
     The bottom of the recess can define a transverse plane, in particular perpendicular to the axis of the cylindrical hole. 
     The Hall-effect probe can be housed in the recess. 
     This plane can form an end stop for the movement of the Hall-effect probe along the axis of the cylindrical hole. 
     In other words, the movement of the Hall-effect probe inside the recess can involve no movement of the latter inside the cylindrical hole. 
     During its movement inside the recess, the probe can remain outside the hole. The dimension, measured perpendicular to the axis of the cylindrical hole of the probe, is in particular greater than the dimension of the cylindrical hole, perpendicular to the axis of the cylindrical hole. 
     The movement of the probe can take place exclusively outside the hole. 
     Advantageously, the Hall-effect probe does not house, even partially, in the cylindrical hole. 
     Advantageously, the cylindrical hole is blind, i.e. only one of the ends of the hole along its axis is open, the other being closed. 
     The recess can open out onto the outside of the magnet via two apertures arranged in parallel with the axis of the hole. These apertures can be made in the hole or not. 
     The cylindrical hole can open out onto the outside of the magnet via two apertures arranged in parallel with the axis of said hole. 
     Owing to these apertures, the existence of thin walls of the magnet arranged in parallel with the axis of the hole and around said hole can be avoided. 
     The apertures can be diametrically opposed about the axis of the hole. 
     In an exemplary embodiment of the invention, the hole is blind and opens out onto the outside of the magnet via two apertures arranged in parallel with the axis of said hole. In other words, according to this exemplary embodiment of the invention, one of the ends of the hole along its axis is closed and two apertures are made on either side of said axis, the apertures being in particular diametrically opposed. 
     Advantageously, the dimensions of the magnet are such that during the movements of the Hall-effect probe, the value of the magnetic induction perceived by the sensitive element remains substantially constant. 
     The dimensions of the magnet are in particular such that the variations in the magnetic induction perceived by the sensitive element during the movements of the Hall-effect probe are not liable to move the curve representing the magnetic field with respect to the threshold value corresponding to the TPO. The TPO function therefore remains operational. 
     In other words, the dimensions of the magnet can be such that, within the tolerance range of the movements of the Hall-effect probe, the value of the magnetic induction perceived by the sensitive element remains substantially constant. 
     Thus, it is then not necessary to fasten the Hall-effect probe with resin and there is therefore no appearance of mechanical stresses on the Hall-effect probe and the TPO function remains operational. 
     The recess can have a parallelepipedal shape. The probe can then have a shape allowing it to be received in the recess. 
     The sensor can comprise a support and the Hall-effect probe can be fastened to this support, for example solely by way of connection leads. 
     These leads can be flexible and this flexibility of the connection leads of the probe can allow the probe to move in the recess. 
     The magnet and the Hall-effect probe can each be linked, directly or otherwise, to one and the same support. The magnet and the Hall-effect probe can each be joined onto this same support. The support can be a casing of the sensor. 
     The magnet and the Hall-effect probe can be not rigidly coupled together, contrary to what would be the case if a resin was used to immobilize the magnet with respect to the Hall-effect probe. 
     The magnet and the Hall-effect probe can thus both be linked to one and the same support without the magnet and the Hall-effect probe being rigidly coupled together. 
     The Hall-effect probe can move in the recess in a direction parallel with the axis of the hole and/or in a plane perpendicular to said axis. 
     The invention also proposes a magnet for a sensor according to one of the preceding embodiments, said magnet having a recess having a bottom and intended to house a sensitive element of a Hall-effect probe, and a cylindrical hole with an axis and which runs from said bottom to the inside of the magnet. 
     Advantageously, the cylindrical hole is blind. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       The features of the invention mentioned above, as well as others, will become more clearly apparent upon reading the following description of an exemplary embodiment, said description being made with reference to the attached drawings, among which: 
         FIG. 1  represents a curve of the variations in a magnetic field of a sensor of the prior art, 
         FIG. 2  shows a sensor according to the invention in a so-called front view, 
         FIG. 3  shows the sensor in  FIG. 2  in a top view, 
         FIG. 4  shows a magnet for the sensor in  FIG. 2  in a front view, 
         FIG. 5  shows the magnet in  FIG. 4  in a side view, and 
         FIG. 6  shows a curve of the magnetic field for a sensor according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  shows a sensor  20  according to the invention in a front view and  FIG. 3  shows the sensor  20  in a top view. The sensor  20  comprises a magnet  30  and a Hall-effect probe  21 . 
     The sensor  20  also includes a base, not represented, on which the magnet  30  is fastened. The Hall-effect probe  21  includes a sensitive element  23  and it is fastened onto a printed circuit board (not represented) or onto tabs by way of its connection leads  22 . The printed circuit board is itself fastened onto the base, which can form a protective casing wherein the magnet  30 , the Hall-effect probe  21  and the printed circuit board are housed. 
     The magnet  30  has an open recess  32  with a bottom  31  and wherein the Hall-effect probe  21  and more particularly the sensitive element  23  are housed. 
     In the example described, the recess  32  takes the shape of a parallelepiped which is limited by the bottom  31  and which is open on the face opposite the bottom  31 . The parallelepiped is limited by the magnet  30 , along two of its other parallel faces, and it is open along the two last parallel faces. 
     The Hall-effect probe  21  is sunk into the recess  32 . A part of the magnet  30  is facing two of the parallel edges of the Hall-effect probe  21 . 
     The magnet  30  also has a cylindrical hole  33  with an axis  34  and which runs from the bottom  31  to the inside of the magnet  30 . The axis  34  is perpendicular to the bottom  31 , i.e. to the plane in which the Hall-effect probe  21  extends. 
     The cylindrical hole  33  is blind. The dimensions of the hole  33  can depend on the dimensions of the magnet  30 , particularly on its dimension along the axis  34  and on those in a plane perpendicular to this axis  34 , and/or on the dimensions of the recess  32 . 
     The magnet  30  is globally symmetrical with respect to one of the planes passing through the axis  34 . 
     The preferred position of the sensitive element  23 , when the sensor  20  is assembled, is such that the sensitive element  23  is housed in the recess  32  and positioned on the axis  34 . 
     The Hall-effect probe  21  is not fastened by way of resin. In the example described, it is only fastened by way of its connecting leads  22  which leave the recess  32  via one of the two last open faces of the parallelepiped. The Hall-effect probe  21  is therefore liable to move with respect to the magnet  30  in a tolerance range around the preferred position. The tolerance range is defined by the manufacturing tolerances of the components, the tolerances of assembly of the components amongst themselves and the movements that occur over the lifetime of the sensor  20 . 
     The sensitive element  23  is thus liable to move in the recess  32  in a first direction parallel with the axis  34  and in a plane parallel with the bottom  31  about the axis  34 . The length of the bottom  31  is in particular determined to allow the introduction of the Hall-effect probe  21  and limit the movements of the Hall-effect probe  21  in the recess  32 . 
     The dimensions of the magnet  30  are such that the variations in the magnetic induction perceived by the sensitive element  23  during the movements of the Hall-effect probe  21  are not liable to move the curve representing the magnetic field with respect to the threshold value corresponding to the TPO. The TPO function therefore remains operational. 
     In other words, the dimensions of the magnet  30  are such that, in the tolerance range of the movements of the Hall-effect probe  21 , the value of the magnetic induction perceived by the sensitive element  23  remains substantially constant. 
     Thus, it is not necessary to fasten the Hall-effect probe  21  with resin and there is therefore no appearance of mechanical stresses on the Hall-effect probe  21  and the TPO function remains operational. 
       FIG. 6  is a graph showing the magnetic induction perceived by the sensitive element  23  as a function of the position of the sensitive element  23  along the axis  34 . 
     The zero of the abscissa axis corresponds to the theoretical position of the sensitive element  23  as it has been defined during the design of the sensor  20 , i.e. when the sensitive element  23  is on the axis  34  and when the Hall-effect probe  21  is at the bottom of the recess  32 . Each division of the abscissa axis corresponds to 0.5 mm. Due to the fact of its possible movement while remaining in the tolerance range, the real position of the Hall-effect probe  21  varies from 0 mm (position at the bottom of the recess  32 ) to −0.5 mm (position detached from the bottom of the recess  32 ). 
     The zero of the ordinate axis corresponds to 0 G. Each division of the ordinate axis corresponds to 500 G. 
     The curve  61  corresponds to the magnetic induction perceived by the sensitive element of a Hall-effect probe of a sensor of the prior art. 
     The curve  62  corresponds to the magnetic induction perceived by the sensitive element  23  of the Hall-effect probe  21  of the sensor  20  according to the invention. 
     In the case of the sensor of the prior art, the variation in the magnetic induction within the range of movement of the sensitive element is of around 300 G. 
     In the case of the sensor  30  according to the invention, the variation in the magnetic induction within the range of movement of the sensitive element  23  is in the order of 10 G. 
     The magnet  30  is for example obtained by molding. In a variant, the magnet is a compressed, sintered or machined magnet. 
       FIG. 4  and  FIG. 5  show a particular embodiment of the magnet  30  in a front view and a top view in the case where the Hall-effect probe  21  is a probe from INFINEON and bears the reference TLE 498x. 
     The magnet  30  is composed of a block of a height (measured along the axis  34 ) of 5.5 mm. The block is a portion of a cylinder of 10 mm in diameter centered on the axis  34 . The cylinder is limited by two parallel planes symmetrical with respect to the axis  34 . The distance between the two planes is of 4.92 mm. 
     The recess  32  possesses a rectangular shape centered on the axis  34  and perpendicular to the planes limiting the cylinder. The length of the bottom  31  parallel with the planes is of 5.47 mm and the width of the bottom  31  perpendicular to the planes is of 4.92 mm. The height of the recess  32  parallel with the axis  34  is of 1.08 mm. 
     The cylindrical hole  33  has a diameter of 4.9 mm and a height (measured along the axis  34 ) with respect to the bottom  31  of 1.75 mm. In the example under consideration the wall limiting the cylindrical hole  33  opens out onto each of the planes limiting the cylinder via a slit perpendicular to said plane and with a width of 2.6 mm distributed symmetrically on either side of the axis  34 . 
     Of course, the present invention is not limited to the examples and embodiments described and represented, but it is open to many variants available to those skilled in the art.