Abstract:
According to the intended use, an electrical conductor ( 1 ) having a magnetically sensitive surface ( 10 ) subjected to the magnetic force of a permanent magnet ( 11 ) is produced, the area of this surface varying according to its relative position with respect to the magnet, said relative position between the magnet and the magnetically sensitive surface is varied, which causes at least one physical characteristic of this conductor, to vary, and said variation in physical characteristic(s) of the electrical conductor is recorded, this variation being in correlation with the position of the magnet ( 11 ).

Description:
BACKGROUND OF THE INVENTION 
   The invention relates to the application of magnetoimpedance, also called here RMS (resistive magnetic saturation), in the field of sensors. 
   More precisely, the invention relates here to the use of this magnetoimpedance on a contactless position sensor, and to the sensor itself. 
   The physical phenomenon involved here is, in general, the following:
         an electrical conductor through which an AC electrical current flows sees this current distributed around the periphery of the conductor.       

   This phenomenon is commonly called the skin effect. 
   The depth of this skin is determined by the formula: 
           δ   =       ρ       πμ   0     ⁢     μ   r     ⁢   f               
where:
         δ is the skin depth;   ρ is the resistivity of the conductor in ohms.meter;   μ 0  is the magnetic permeability of free space (4π×10 −7 );   μ r  is the magnetic permeability of the material; and   f is the frequency in hertz.       
   Thus, the more the frequency f increases, the more the skin depth δ decreases. 
   Moreover, the electrical resistance of an electrical conductor is given by the formula: 
           R   =       ρ   ⁢           ⁢   L     A           
where:
         R is the resistance of the conductor in ohms;   ρ is the resistivity of the conductor in ohms.meter;   L is the length of the conductor in meters; and   A is the cross-sectional area of the conductor.       
   It should be noted that, in such a skin effect phenomenon, when the frequency f increases for a given electrical conductor (all other things being equal) the area A decreases and therefore the measured resistance R greatly increases. 
   If a magnetic field is applied to the electrical conductor made of an appropriate material, this modifies the magnetic permeability (μ r ) of the material. 
   This has the effect of making the measured resistance R drop compared with the same conductor not subjected to the external magnetic field in question. 
   This phenomenon has already been used to measure, typically with great precision, absolute magnetic fields, and sensors operating on the principle of magnetoimpedance, and especially on giant magnetoimpedance, have been developed. They operate at very high frequencies (greater than 1 GHz in the case of giant magnetoimpedance) and in particular use amorphous materials for forming the electrical conductor serving as sensitive element. 
   The problem posed here is that of how to widen the field of application of magnetoimpedance, on the basis of the abovementioned physical phenomenon, without necessarily having to measure very precisely the magnetic fields in question and without necessarily incurring the high costs imposed hitherto, in particular in the case of the aforementioned sensors, especially those developed in relation to giant magnetoimpedance. 
   SUMMARY OF THE INVENTION 
   It is within this context that it is proposed here to use this phenomenon of magnetoimpedance on a contactless position sensor, having the particular feature that:
         a) an electrical conductor having a magnetically sensitive surface subjected to the magnetic effect of a permanent magnet is produced, the area of this surface varying according to its relative position with respect to the magnet;   b) said relative position between the magnet and the magnetically sensitive surface of the electrical conductor is varied, which causes at least one physical characteristic of this conductor to vary; and   c) said variation in physical characteristic(s) of the electrical conductor obtained during step b) is recorded, this variation being in correlation with the position of the magnet.       

   Thus, the phenomenon of magnetoimpedance may be applied, especially in the automotive industry field, where the application conditions are different from those of the known prior sensors. 
   Preferably, during steps b) and c), the physical characteristic of the electrical conductor that varies, and which is recorded, is the electrical resistance of this conductor. 
   Specifically, this electrical resistance is modified by the variation in relative position between the magnet and the magnetically sensitive surface of the electrical conductor and recording this resistance is an easy and low-cost procedure, the more so as it is unnecessary for the recording to be of high precision. 
   Since the choice of using magnetoimpedance for producing such a position sensor is especially an economic one and since the objective of the present approach is not to result in an exact quantitative measurement, it is recommended that, during step b), the electrical conductor is subjected to an AC voltage alternating between a few kilohertz and at least around one hundred megahertz. 
   Although the effect produced by applying such a range of frequencies is smaller than in the case of the frequencies already known (above 1 GHz in the case of giant magnetoimpedance), this effect nevertheless remains quite sufficient to be detectable with the type of sensor concerned here. 
   As regards obtaining the variable surface area on the electrical conductor, it should now be noted that one or other of the following variants is recommended:
         either this variable surface area is obtained by varying the cross section of the electrical conductor;   or this surface area is obtained by varying the density of the electrical conductor subjected to the magnetic effect of the magnet.       

   The term “density” should be understood to mean the length of the segments (assumed to be of constant cross section) of the electrical conductor seen by the magnet in operation. 
   For example, the density will be higher if the magnet is moved facing three segments close together than if the same magnet, for the same displacement and for the same length of segments, sees only two segments spaced further apart. 
   Thus, the surface of the electrical conductor subjected to its magnetic field can vary according to the position of movement of the magnet, even if the cross section of the conductor remains constant. 
   It should be made clear that the relative displacement referred to between the magnet and the electrical conductor may in particular be a translation or a rotation. 
   In the latter case, it is proposed in particular, to meet the low-cost, effectiveness and reliability objectives, that:
         during step a), the conductor is wound so that it has, around the outside, segments developing along substantially a spiral; and   during step b), a relative rotation between the magnet and this spiral, which are facing each other, is imposed.       

   Apart from the use of the magnetoimpedance phenomenon as presented, the invention therefore applies to a position sensor as such, and in particular to a position sensor for a motor vehicle, comprising:
         a conducting track area having a surface sensitive to the magnetic field of a magnet, physical characteristics and terminals;   a permanent magnet for generating said magnetic field on the sensitive surface;   a power source for creating an AC voltage across said terminals of the conducting track area;   displacement means for making the relative position between the magnet and the sensitive surface vary; and   recording means for recording a variation in at least one of the physical characteristics of the conducting track area generated by said variation in relative position.       

   As already indicated, it is recommended that the recording means comprise means for recording the electrical resistance or the variation in electrical resistance of said conducting track area. 
   To meet the intended objectives of competitive cost, reliability and effectiveness, it is also recommended that the electrical conductor comprise or consist of (at least in its sensitive area) a paramagnetic material or a ferromagnetic material. 
   Moreover, to further enhance the effectiveness of the sensor, or even its sensitivity, it is furthermore recommended that:
         the magnet be movable with respect to the sensitive surface on a displacement direction; and   the conducting track area have, predominantly or exclusively, conducting track portions that are not perpendicular to the direction of displacement of the magnet.       

   For a similar objective, and complementarily or alternatively, it is proposed that the magnet be provided with pole pieces for concentrating the magnetic field. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the even more detailed description that follows, the figures illustrate exemplary embodiments in which: 
       FIG. 1  shows schematically an example of contactless position sensor according to the invention, designed on the abovementioned basis of magnetoimpedance; 
       FIG. 2  shows a side view of a position sensor with a magnet provided with pole pieces; and 
       FIGS. 3 ,  4 ,  5 ,  6 ,  7  and  8  show various geometries of electrical conductors (also called, above, conducting track areas). 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The object of the approach here is therefore a priori not to make an exact quantitative measurement (a measurement of the external magnetic field as was the case with GMI sensors). 
   In what follows, it will be moreover considered that the physical characteristic of the electrical conductor  1 , in the sensitive area  10  subjected to the effect of the magnet, which has to vary in order to allow the magnetoimpedance (or RMS) phenomenon to be used, is the impedance, and in particular the resistance R, of this conductor at the stimulation frequency. 
   Before specifically dealing with the figures, it should also be noted that the position sensor of the invention, therefore favorably based on the variation in resistance R of an electrical conductor, in particular made of a paramagnetic or ferromagnetic material, and subjected to an AC voltage of frequency f, is in general described as follows:
         a permanent magnet, generating a known magnetic field, is moved opposite the electrical conductor in question;   it “disturbs” and partially or completely eliminates the skin effect occurring in its affected area;   the resistance R of the conductor is therefore modified; and   measuring this resistance allows the position of the magnet to be located.       

   However, this is possible, for a given electrical conductor, only if its shape, and therefore the area of the surface subjected to the effect of the magnet, varies according to the position of this magnet. Thus, a linear conductor will have, locally, a resistance that varies under the effect of the magnetic field produced by the permanent magnet, but from an overall standpoint its resistance will be constant, even if it is much less than that measured in the absence of the magnet. 
   By making a conductor from a suitable material, and with an area of the surface subjected to the effect of the magnet that varies with the position of this magnet, the latter will mask different areas and the total resistance of the conductor will vary. 
   This may be favorably achieved in various ways. In particular:
         by varying the cross section of the conductor. This can be achieved easily with a conductor fastened to the PCB (printed circuit board) for example. It suffices to produce tracks (constituting the conductor) of different widths and the desired effect is obtained, and/or to vary the density of conductors subjected to the magnet. Taking the above example of a conducting track etched on a PCB, it suffices for example to produce loops or “meanders” of greater or lesser spacing in order to achieve the result.       

   For economic and effectiveness/cost ratio reasons, amorphous materials, which are very expensive, may advantageously be replaced with ferromagnetic materials such as for example μ-metal®. This allows the use of a low-cost magnet, but nevertheless excellent results are obtained. Thus, a film of μ-metal® may be applied to a PCB. The tracks are then etched. 
   Now, μ-metal® is a soft magnetic alloy having a crystalline structure that is relatively stable considering the various treatments occurring during its manufacturing process, and good thermal conductivity. It is an alloy based on nickel and iron (77% Ni, 14% Fe, 5% Cu and 4% Mo for example). 
   Permalloy® is another brand name of a known magnetic alloy that can be used, but other materials are available. For example, iron could be used, but it would then be necessary to modify the operating frequencies of the AC current associated with the sensor so as to achieve better performance. 
     FIG. 1  shows an electrical conductor  1  supplied by an alternating supply voltage, such as a source  3  of AC power, and grounded at  5 . 
   An alternating input voltage U 1  may then be applied between the terminals  1   a ,  1   b  of the conductor  1 . 
   In particular for a conductor  1  made of paramagnetic or ferromagnetic material, the frequency of this voltage U 1  will be between a few hundred Hz and less than one thousand MHz, advantageously between a few kHz and less than 100 MHz, and even more advantageously (for these materials) between 5 kHz and 50 MHz. 
   At the output (terminal  1   c ), the electrical conductor  1  may be connected to a voltmeter for measuring the amplitude of the output signal. 
   It will be possible to use an amplitude-modulated AC voltage source for further facilitating the electronics for measuring the output voltage as a function of the variation in resistance of the sensitive element. 
   The electrical conductor  1  defines an area  10  of conducting tracks, also called resistive tracks, constituting the sensitive part of the sensor with, at the point where the conductor passes, a magnetically sensitive surface (bounded by the outline  10 ) subjected to the magnetic effect of a permanent magnet  11  placed facing it. 
   As shown in  FIG. 2 , the conducting track area  10  may in particular be formed on a support substrate usually called a PCB (printed circuit board) commonly used for the production of electronic cards. 
   Facing these resistive tracks is the magnet  11  which, in  FIGS. 1 and 2 , can move translationally along a direction  11   a  parallel to the elongation direction of the tracks. 
   In the case of  FIG. 1  (but this is applicable to the other resistive track geometries shown in  FIGS. 3 to 8 ), this sensitive conducting element may be used in the manner in which a potentiometer would be used. However, it might be preferable to use a Wheastone bridge or one of its variants, or even other known means. Given the geometry of the conductor in the area  10 , it is the voltage ratio U 3 /U 4  that will vary depending on the position of the magnet  11 , U 3  and U 4  being the AC voltages between the terminals  1   a ,  1   c  and  1   b ,  1   c  respectively. 
   In  FIG. 1 , but also in  FIG. 3 , the movable magnet  11  extends over the entire width of the resistive track area  10  so as to make a sensitive surface as wide as possible. 
   The relative movement between the magnet and the sensitive area of the electrical conductor  1  may be generated by any appropriate member, or group of members, such as for example a motor shaft, a camshaft, a crankshaft (of an automobile engine), or else the clutch, the suspension, etc. 
     FIG. 6  shows schematically one end of a motor shaft  12  for rotating the magnet  11 , which is fastened to a free end. 
   In  FIGS. 1 and 3  to  5 , where the movement of the magnet is (or comprises) a translation, slideways can guide and orient this transverse displacement along the direction  11   a  (see the dotted lines  14   a  and  14   b  in  FIG. 3 ). 
   It should be noted that the electrical conductor  1  typically has, in its central part, an essentially linear segment  13  having a slope inclined very little to  11   a  and extending over the length L of the sensitive area  10 , so that, in this area, the electrical conductor  10  has, on either side of this essentially linear central linking segment, and therefore on either side of the central axis  11   a , two (approximately) similar areas  15   a ,  15   b , which are made up of crenellated or meandering segments, respectively increasing and decreasing, in one direction and in the opposite direction, along the axis  11   a , and defining substantially two opposed triangular areas. The terminal  1   c  is at one end of the segment  13  ( FIG. 1 ). 
   Again in  FIG. 1 , this segment is linear. The alternating output signal U 2  will therefore change with a constant slope, if the magnet moves along  11   a , as shown by the curve at the bottom of this  FIG. 1 . 
   In  FIG. 4 , the geometry of the resistive track  1  has been modified in the central part, where a flat has been created at the center, this flat appearing both on the inclined central segment  13  (at  13   a ) and, on each side of the place where the upper and lower adjacent crenellations occur, respectively  25   a  and  25   b  for both of them, a certain number of successive adjacent crenellations (here, three on each side) having the same area subjected to the effect of the magnet. 
   Thus, it is possible to obtain a flat, at  17 , on the curve U 2 =f(L) shown in the bottom of  FIG. 4 , this horizontal part of the curve making it possible, by comparison with the upstream slope  17   a  and downstream slope  17   b  respectively, both being constants, to identify a particular position, such as here a central position, of the magnet relative to the general area  10 . 
   More generally, it will be possible to obtain the curve of choice, according to the geometry of the resistive track  1  and the position of this geometric change along this track, by therefore adapting the variation in density of the conductor. 
     FIGS. 3 and 5  show two other geometrical shapes of the conducting track area  10  that are produced on the basis of a variation in density of the electrical conductor  1  subjected to the magnetic effect of the magnet, namely a crenellated and generally triangular shape in  FIG. 3 , and a crenellated shape with the gap between crenellations decreasing in one direction, along the direction of displacement  11   a , in  FIG. 5 . 
   It will also be possible, as indicated earlier, to work, complementarily or alternatively, on the cross section of the electrical conductor  1 , by increasing it or reducing it depending on whether it is desired to increase or reduce the signal in response, coupled with the effect produced by the moving magnet. 
   In  FIG. 3  it may be seen that the angle of inclination α between the direction  19   a  perpendicular to the axis of displacement  11   a  of the magnet  11  and the direction  19   b  parallel to which most (here, all) the lateral sides, such as  21  and  23  of the crenellations formed by the conductor  1  in the area  10  lie, where 0°&lt;α&lt;90°. 
   Here, the angle α ranges from about 2° to 10°. 
   Since the other segments, namely the bottom and end segments such as  25  and  27 , of this conducting track area are parallel or approximately parallel to the axis of displacement  11   a , there will therefore be predominantly, or in this case exclusively, a conducting track area having portions such as  21 ,  23 ,  25 ,  27  that are not perpendicular to the direction of displacement  11   a  of the magnet. 
   The effect obtained is to smooth the curve resulting from the measurement, which would exhibit otherwise, i.e. for a zero value of α, small jumps along the curve. 
   Also to enhance the sensitivity of the measurements, with a controlled cost, the magnet  11  may be favorably provided with pole pieces  29   a ,  29   b  as shown in  FIG. 2 , these being located on either side of its sides transverse to the axis of displacement  11   a.    
   In  FIGS. 6 ,  7  and  8 , the position sensor of the invention is used for measuring rotational displacements, such as for example in  FIG. 6  at the shaft end  12 , with here a square pattern of the resistive track area  10  and a rectangular magnet  11 , but it is possible for the geometry of the area  10  to correspond to that of  FIG. 4 , over a square area and not a rectangular area. 
   It is thus possible to obtain a variation in cos θ and sin θ, from which it is possible to extract the angle θ, from the formula θ=arctan(sin θ/cos θ). 
     FIGS. 7 and 8  show, respectively, two conducting tracks corresponding to the electrical conductors  30  and  31 . 
   To define the sensitive area  10 , each conductor  30 ,  31  is wound so as to have, on the outside, segments  30   a ,  30   b ;  31   a ,  31   b  respectively, for both of them in each case, these segments following substantially a spiral, as may be seen. Furthermore, it is facing this spiral that the magnet will rotate, in order to obtain the desired magnetoresistive sensor here.