Abstract:
A position sensor comprising a primary winding ( 26 ) for generating a magnetic flux, a first secondary winding ( 24.1 ) the number of turns of which increases in one direction, a second secondary winding ( 24.2 ) the number of turns of which increases in the opposite direction to that of the first secondary winding. Both secondary windings ( 24.1, 24.2 ) are subjected to the magnetic flux generated by the primary winding ( 26 ) whereby voltages are induced in the secondary windings. The sensor further comprises an element movable with respect to the secondary windings. The element is of a magnetic or conductive material and distorts, in its vicinity, the magnetic flux generated by the primary winding ( 26 ). By providing third and fourth secondary windings at right angles to the first and secondary windings it is possible to detect position in two dimensions.

Description:
[0001]     This application is a national stage filing of international application PCT/ZA2004/000101 filed 20 Aug. 2004, published in English under PCT Article 21(2), which claims benefit from and is a Paris Convention filing of U.S. Provisional Patent Application Ser. No. 60/496,554 filed 20 Aug. 2003, the specifications of which are hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention relates to position sensors.  
         [0004]     2. Description of the Related Art  
         [0005]     A linear variable differential transformer (LVDT) is a device which is used for sensing the position of objects in industrial machinery. For example, in machines which produce steel pressings (such as car body panels), there is a requirement to check the dimensions of the pressings after production (a process called gauging). This is done using an array of LVDTs, mounted in a frame, to measure the panel&#39;s dimensions and shape. A typical LVDT, see  FIG. 1  of the accompanying drawings, consists of a rod  10  (the moving part) and a cylinder  12  (the fixed part).  
         [0006]     The end of the rod may have a nut fastening  14  as shown or a contact probe. The cylinder  12  contains a primary transformer winding  16  (see  FIG. 2 ) and two secondary transformer windings  18 ,  20  which are used to detect the position of a core  22  mounted on the end of the rod  10 .  
         [0007]     LVDTs have an accuracy of as little as one micron (a thousandth of a millimetre) and can be constructed as large as is necessary. LVDTs which can accommodate displacements of a meter are not uncommon.  
         [0008]     Applicant is aware of many patents which disclose position sensors of the LVDT type discussed above. Applicant is also aware of a non-LVDT type of position sensor which has recently been disclosed in U.S. Specification 6,489,899B1.  
         [0009]     The present invention seeks to provide a sensor which is an improvement of the LVDT and non-LVDT types of position sensor discussed above.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     According to the present invention there is provided a position sensor comprising a primary winding for generating a magnetic flux, a first secondary winding the number of turns of which increases in one direction, a second secondary winding the number of turns of which increases in the opposite direction to that of the first secondary winding, both secondary windings being subjected to the magnetic flux generated by the primary winding whereby voltages are induced in said secondary windings, and an element movable with respect to the secondary windings, the element being of a magnetic or conductive material and distorting, in its vicinity, said magnetic flux generated by the primary winding.  
         [0011]     Each secondary winding can comprise a plurality of side-by-side sections, each intermediate section having more turns than the section to one side thereof and less turns than the section on the other side thereof, one end section having less turns than the intermediate section next to it and the other end section having more turns than the intermediate section next to it.  
         [0012]     Portions of the secondary windings transverse to said directions can be skew to said directions.  
         [0013]     The outputs from the secondary windings are preferably connected to a circuit comprising a differential amplifier and a demodulator to provide a d.c. output. For best results, the difference in the signals from each winding should be divided by the sum of the signals, to provide a so-called ratiometric output. This technique reduces the effect of common-mode changes in the magnetic circuit, and is well understood in the art.  
         [0014]     To enable position to be sensed in two dimensions, the present invention provides a position sensor comprising a primary winding for generating a magnetic flux, a first secondary winding the number of turns of which increases in one direction, a second secondary winding the number of turns of which increases in the opposite direction to that of the first secondary winding, both secondary windings being subjected to the magnetic flux generated by the primary winding whereby voltages are induced in said secondary windings, a third secondary winding the number of turns of which increases in a direction transverse to said one direction, a fourth secondary winding the number of turns of which increases in the opposite direction to that of the third secondary winding, the third and fourth secondary windings being subjected to the magnetic flux generated by the primary winding whereby voltages are induced in said secondary windings, and an element movable with respect to the secondary windings, the element being of a magnetic or conductive material and distorting, in its vicinity, said magnetic flux generated by the primary winding.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]     The above and other aspects, features and advantages of the invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:  
         [0016]      FIG. 1  shows a typical LVDT.  
         [0017]      FIG. 2  shows primary transformer winding of cylinder  12  in  FIG. 1 .  
         [0018]      FIG. 3  is a plan view of a transformer winding;  
         [0019]      FIG. 4  is an “exploded” view of the windings of a position sensor which senses position in one dimension;  
         [0020]      FIG. 5  is a pictorial view of a conductive or magnetic element in conjunction with two secondary windings which are shown spaced apart;  
         [0021]      FIG. 6  illustrates a position sensor which senses position in two dimensions;  
         [0022]      FIG. 7  illustrates the output signal of a position sensor which operates in one dimension; and  
         [0023]      FIG. 8  is a circuit diagram.  
     
    
     DETAILED DESCRIPTION  
       [0024]     Referring to  FIG. 3 , the transformer winding  24  illustrated is on a planar substrate and comprises four transformer sections designated A, B, C and D. Each section is in the form of a winding of rectangular configuration. Section A is illustrated as having four turns, section B as having three turns, section C as having two turns and section D as having one turn. In practice, of course, there will be many more turns but the pattern of a decreasing number of turns from section to section is maintained.  
         [0025]     Magnetic flux lines perpendicular to the plane of the drawings and passing through section A, B, C and D induce current flow in the winding  24 . By the laws of magnetic induction, the current generated in each section is dependant on the number of turns. Hence, in the illustrated embodiment, flux passing through section A will generate four times as much current as the same flux passing through Section D.  
         [0026]     A transformer wound so that the number of turns varies from one end to the other is hereinafter referred to as having a “tapered winding”.  
         [0027]     A tapered winding can be created using the techniques employed to produce printed circuit boards.  
         [0028]     Turning now to  FIG. 4 , the position sensor illustrated comprises two windings  24 . 1 ,  24 . 2  each of the form shown in  FIG. 3 . The number of turns of the winding  24 . 1  increases in the longitudinal direction, that is, from left to right and the number of turns of the winding  24 . 2  increases from right to left in the longitudinal direction. The windings thus taper in opposite directions to one another. The transverse lengths of the windings are shown as being skew to the longitudinal direction thereby to provide a smooth transition from section to section rather than a step-like transition.  
         [0029]     A rectangular primary winding is shown at  26 . The winding  26  generates a magnetic flux to which the windings  24 . 1  and  24 . 2  are subjected, thereby generating an alternating current in each winding  24 . 1 ,  24 . 2 .  
         [0030]     In  FIG. 5  the windings  24 . 1  and  24 . 2  are illustrated as well as an element  28  which moves back and forth from left to right and the position of which is to be sensed. The element  28  can be electrically conductive or can have a different magnetic permeability to the medium, usually air, which surrounds it. Dimensions X, X 1  and X 2  are designated in  FIG. 5  for the purposes of the following explanation.  
         [0031]     The following is a simple analysis of a one-dimensional position sensor in accordance with the present invention. Ignoring the effect of the short end sections, the transformer windings are modelled as two sets of parallel, co-located conductors placed a distance Z apart. The length of the longest conductor is X, and all of the m conductors of the primary winding  26  are the full length X. The conductors of one secondary winding  24 . 1  run from x=0 to iX/n, where i is the endpoint of the ith of n conductors. The conductors of the other secondary winding  24 . 2  run from x=(n-i)X/n to X. A circulating current Ip running through each conductor of the primary induces currents in the secondaries S 1  and S 2  whose magnitudes are:  
               I   S1     =           I   P     ⁢   m   ⁢           ⁢   A     X     ⁢     (       X   n     +       2   ⁢   X     n     +   ⋯   +         (     n   -   1     )     ⁢   X     n     +   X     )               (   1   )             and                           I   S1     =           I   P     ⁢   m   ⁢           ⁢   A     X     ⁢     (       X   n     +       2   ⁢   X     n     +   ⋯   +         (     n   -   1     )     ⁢   X     n     +   X     )               (   1   )             
 
 where A is a coupling factor representing the driving frequency and magnetic coupling (mutual inductance) between the primary and the secondaries. Both these currents are equal. If the number of secondary windings, n, is large, the summation can be approximated by an integral expression, as will be seen in Equations 3 and 4. 
 
         [0032]     If the coupling factor between the primary winding and the secondary windings changes because the local permeability changes, usually from that of air to that of a conductive or magnetic medium, it is possible to model the nonuniformity of the permeability as occurring in a slice which is perpendicular to the transformer plane. The permeabilities could be Ao for air and A 1  for the non uniform element. Ao could be in the range 0&lt;x&lt;x1 and x2&lt;x&lt;X, and A 1  in the interval x1&lt;x &lt;x2. In this circumstances, the current induced in the secondary windings is, using an integral representation as follows:  
               I   S1     =       I   P     ⁢     mn     X   2       ⁢     (         A   0     ⁢       ∫   0     x   1       ⁢     x   ⁢           ⁢     ⅆ   x           +       A   1     ⁢           ⁢       ∫     x   1       x   2       ⁢     x   ⁢           ⁢     ⅆ   x           +       A   0     ⁢       ∫     x   2     X     ⁢     x   ⁢           ⁢     ⅆ   x             )               (   3   )             and                           I   S2     =       I   P     ⁢     mn     X   2       ⁢     (         A   0     ⁢       ∫   0     x   1       ⁢       (     X   -   x     )     ⁢     ⅆ   x           +       A   1     ⁢       ∫     x   1       x   2       ⁢       (     X   -   x     )     ⁢           ⁢     ⅆ   x           +       A   0     ⁢       ∫     x   2     X     ⁢       (     X   -   x     )     ⁢           ⁢     ⅆ   x             )               (   4   )             
 
         [0033]     It is then possible to evaluate the difference between the currents and this reduces to a product of several components:  
                 I   S2     -     I   S1       =           I   P     ⁢   mn       X   2       ⁢     (       A   0     -     A   1       )     ⁢     (       x   2     -     x   1       )     ⁢     (     X   -     x   1     -     x   2       )               (   5   )             
 
         [0034]     The first bracketed component of this product represents the difference in coupling caused by the difference in permeability, the second bracketed component represents the size of the area of differing permeability, and the third bracketed component represents the location on the x axis of the area of differing permeability. The current difference is thus linear with respect to the position of the zone of differing permeability.  
         [0035]     To enable position to be sensed in two dimensions at right angles to one another, the arrangement shown in  FIG. 6  is used.  
         [0036]     Four tapering windings  30 . 1 ,  30 . 2  and  32 . 1 ,  32 . 2  are used in conjunction with a single primary winding (not shown). The windings  30 . 1  and  30 . 2  taper in opposite directions but in the same dimension. Windings  32 . 1 ,  32 . 2  also taper in opposite directions to one another but in a dimension at right angles to the dimension in which windings  30 . 1 ,  30 . 2  taper.  
         [0037]     The primary winding extends around the periphery of the stack of windings  30 . 1 ,  30 . 2 ,  32 . 1 ,  32 . 2  and is rectangular, for example, square.  
         [0038]     The primary winding generates a reasonably uniform magnetic flux density perpendicular to the plane of the secondary windings, and passing through the secondary windings. An element which creates a local nonuniformity in the magnetic field affects the flux passing through a varying number of secondary turns, depending on where the element is positioned. The differential nature of the windings means that the difference in current (induced in each winding of the pair) indicates how far from the centre point of the array of windings the element is located.  
         [0039]     The equations discussed above apply to each pair of windings  30 . 1 ,  30 . 2  and  32 . 1 ,  32 . 2  and hence the position of the element in two dimensions can be sensed.  
         [0040]      FIG. 7  is a graph illustrating the output of a position sensor comprising two primary windings tapering in opposite directions. For illustrative purposes the sensor is assumed to have a sensing range of 90 mm. The output is substantially linear in the measuring range. Non-linearity can result from misalignment of the primary and secondary windings.  
         [0041]     Turning finally to  FIG. 8 , this illustrates a circuit for rectifying the output signals of the secondary windings and providing a demodulated d.c. output signal.  
         [0042]     The primary winding  34  is driven by an oscillator  36  which is also connected to a demodulator  38 . The secondary windings  40 . 1 ,  40 . 2  are connected to a differential amplifier  42  and the output of the amplifier  42  constitutes the input to the demodulator  38 . The output of the demodulator  38  is fed to a low pass filter  44 , the output of the filter  44  being the d.c. signal illustrated in  FIG. 7 .  
         [0043]     The pattern of the windings can be varied to give particular effects that are required. For example, if there is a greater need for accuracy in a specific area, the windings can be such as to be more sensitive in that area.  
         [0044]     To improve sensitivity, it is possible to provide a position sensor on each side of the element  28 . This construction is particularly suitable when the element  28  is relatively large. An example is that the position of a drive shaft can be detected by position sensors located on each side thereof.