Abstract:
A transducer is provided for use in a position sensor which senses the position of two relatively movable members. In one form, the transducer has a plurality of sensor windings having a number of loops arranged in succession and connected in series so that signals induced in adjacent loops by a common electromagnetic field oppose each other and an excitation winding having at least one loop super-imposed on but electrically isolated from the loops of the sensor windings.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for use in a position sensor which senses the relative position between at least two relatively movable members. The invention has application in linear and rotary inductive position sensors. 
     2. Related Art 
     WO95/31696 discloses an inductive position sensor in which one member carries an excitation winding and a number of sensor windings mounted on a sensor printed circuit board and the other member carries a resonator, also mounted on a printed circuit board. In operation, an alternating current is applied to the excitation winding which causes an alternating magnetic field to be set up in the region of the sensor printed circuit board. This field inductively couples with the resonator when it is placed in proximity to the sensor printed circuit board, to cause the resonator to resonate. This in turn generates its own spatially patterned alternating magnetic field in the vicinity of the sensor printed circuit board. This resonator magnetic field induces alternating signals in the sensor windings whose amplitude sinusoidally varies with the relative position between the two members. A similar system is disclosed in EP 0182085 which uses a conductive screen in place of the resonator. However, the use of a conductive screen has the disadvantages that the output signal levels are much smaller and that the signal induced in the sensor windings by the conductive screen has substantially the same phase but opposite polarity to any signal generated in the sensor windings directly from the excitation winding. In contrast, with a resonator, there is a 90° phase shift between the signal induced in the sensor windings by the resonator and any signal induced in the sensor windings by the excitation winding. Therefore, with a resonator, it is possible to employ synchronous detection to isolate the resonator signal from any excitation breakthrough. 
     The design of the sensor windings in both these systems is critical to the operation of the position sensor. In particular, they must be shaped so that there is the necessary sinusoidal coupling with the resonator and so that there is minimum coupling with the excitation winding. This is achieved by forming the sensor windings from a number of series connected loops arranged so that signals induced in adjacent loops by a common magnetic field oppose each other and by forming the excitation winding as one or more loops around the periphery of the sensor windings. 
     One aim of the present invention is to provide an alternative geometry of sensor and excitation windings. 
     The sensor windings and the excitation windings are preferably formed on a two-layer printed circuit board since these are cheap and easy to manufacture as compared with multi-layer circuit boards. For low cost, small size and optimum EMC performance, the electronics used to generate the excitation current and to process the signals induced in the sensor windings should be mounted on the same circuit board as the sensor windings. In the systems provided to date, these connections have been made by twisted pair flying leads, which are fiddly, expensive and can be unreliable. Connections can be made using conventional PCB track routing techniques. However, the connection tracks usually result (i) in a disturbance in the sinusoidal coupling relationship between the sensor windings and the resonator; and (ii) in the sensor windings being sensitive to the magnetic field generated by the excitation coil, both of which reduce the overall accuracy of the position sensor. 
     One solution to this problem is simply to use multilayer printed circuit boards. However, this approach increases cost and can reduce position sensing accuracy since several printed circuit board layers require accurate registration instead of just two. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect, the present invention provides a transducer for use in a position encoder comprising: a first circuit having at least two loops connected in series and being arranged so that signals generated in the loops by a common electromagnetic field oppose each other; and a second circuit having at least one loop which extends along the measurement path and which is superimposed on but electrically isolated from the loops of the first circuit. 
     Such a transducer provides the advantage that for a given size of circuit board, the area of the loops of the first circuit can be maximised regardless of the number of turns of the second circuit. 
     This aspect also provides an apparatus for use in a position encoder comprising: a first plurality of conductor tracks arranged along a measurement path; a second plurality of conductor tracks arranged along the measurement path; wherein the first and second plurality of conductor tracks are superimposed on each other to define (i) a first circuit having at least two loops along the measurement path, the loops being connected in series and being arranged so that signals generated in the two loops by a common electromagnetic field oppose each other; and (ii) a second circuit having at least one loop which extends along said measurement path; characterised in that the conductor tracks of said first plurality of conductor tracks which form part of said first circuit are arranged in first and second groups which are transversely spaced relative to said measurement path; the conductor tracks of said second plurality of conductor tracks which form part of the first circuit are arranged for connecting conductor tracks in the first group with conductor tracks in the second group; and the conductor track or tracks of the second plurality of conductor tracks which form said at least one loop of said second circuit are located between said first and second groups of conductor tracks. 
     According to another aspect, the present invention provides a transducer for use in a position sensor, comprising: two layers of conductor tracks insulated, at least at any crossovers, by an insulator, the conductor tracks being arranged so as to define (i) a first circuit having at least two loops which extend along a measurement path, said loops being connected in series and being arranged so that signals induced in said at least two loops by a common magnetic field oppose each other; and (ii) a second circuit having at least one loop which extends along said measurement path and which is superimposed on but electrically isolated from said at least two loops of said first circuit; wherein an electromagnetic coupling area between the at least one loop of said second circuit and one or more loops of said first circuit is varied to oppose the effect of a variation coupling between the at least one loop of the second circuit and a loop of the first circuit caused by an arrangement of connection tracks to said second circuit. 
     Preferably, in this aspect, the second circuit comprises a multi-start spiral winding, and wherein said variation in coupling areas is provided in the vicinity of the or each start to the spiral winding. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
     FIG. 1 schematically illustrates a rotating shaft having a position encoder mounted relative thereto, for encoding the position of the rotatable shaft; 
     FIG. 2 a  schematically shows one turn of a sensor winding forming part of the position encoder shown in FIG. 1; 
     FIG. 2 b  schematically shows the form of the winding shown in FIG. 2 a  when stretched out in a linear path; 
     FIG. 2 c  shows a printed circuit board upon which the winding shown in FIG. 2 a  is mounted together with three similar windings and an excitation winding; 
     FIG. 2 d  illustrates two of the windings shown in FIG. 2 c  when stretched out over a linear path and illustrates the way in which these two windings are connected in series to form a sensor winding; 
     FIG. 2 e  shows a top layer of printed conductors which form part of a printed circuit board; 
     FIG. 2 f  shows the bottom layer of conductors which, together with the top layer of conductors shown in FIG. 2 e , form a transducer having an excitation winding and two two-turn sensor windings in which the spacing between the turns of each sensor winding are arranged to reduce harmonic distortion; 
     FIG. 2 g  shows the two sensor windings shown in FIG. 2 c  when stretched out over a linear path; 
     FIG. 2 h  shows a top layer of conductors forming part of the printed circuit board shown in FIG. 2 c;    
     FIG. 2 i  shows the bottom layer of conductors forming part of the printed circuit board shown in FIG. 2 c;    
     FIG. 2 j  shows the winding shown in FIG. 2 a  together with part of the excitation winding and illustrates the way in which the excitation winding is connected to its connection pads without coupling to the sensor windings; 
     FIG. 3 a  shows the top layer of a resonator printed circuit board; 
     FIG. 3 b  shows the bottom layer of the resonator printed circuit board; 
     FIG. 4 is a schematic representation of excitation and processing circuitry used to energise the excitation winding shown in FIG. 2 c  and to process the signals received in the sense windings shown in FIG. 2 c;    
     FIG. 5 illustrates the way in which the peak amplitude of the signal induced in each sense winding varies with the angular position of the rotatable shaft; 
     FIG. 6 illustrates an alternative form of the excitation winding shown in FIG. 2 c ; and 
     FIG. 7 illustrates the form of an excitation and a set of sensor windings forming part of a linear encoder. 
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     FIG. 1 schematically shows a shaft  1  which is rotatable about its axis and which passes through a bearing  3  provided in a support wall  5 . A first printed circuit board  9  carrying a resonator (not shown) is mounted for rotation (as represented by arrow  7 ) with the shaft  1  via bushing  11  next to a second printed circuit board  15  (shown in cross-section) which carries a number of sensor windings (not shown) and an excitation winding (not shown). The second printed circuit board  15  is fixed to the support wall  5  and has a central hole  16  through which the rotatable shaft  1  passes. 
     The excitation winding, sensor windings and the resonator are arranged such that when an excitation current is applied to the excitation winding, the resonator is energised which induces signals in the sensor windings, the peak amplitudes of which vary sinusoidally with the angle of rotation of the shaft. The sensor windings are connected to processing electronics (not shown) which processes the induced signals to determine the rotational angle of the rotatable shaft  1 . 
     In this embodiment, two two-turn sensor windings are used which extend circumferentially around the circuit board  15 . Each turn of each sensor winding has the same geometric form and comprises three periods of windings, each period extending circumferentially over an angle of approximately 120°. FIG. 2 a  shows the form of one turn  21 - 1  of one of the sensor windings  21  and FIG. 2 b  illustrates the form of this turn  21 - 1  when it is stretched out over a linear path. As shown in FIGS. 2 a  and  2   b , sensor winding turn  21 - 1  comprises six hexagonal shaped loops  21 - 1   a  to  21 - 1   f  of series connected conductors, connected such that adjacent loops are wound in the opposition sense. Thus any EMF induced in one loop will oppose the EMF induced in an adjacent loop by the same magnetic field. In this embodiment, the area enclosed by each loop is approximately the same, thereby making each turn and hence each sensor winding relatively immune to background electromagnetic interference since EMFs induced in adjacent loops substantially cancel each other out. 
     In this embodiment, the sensor windings are formed on a two-layer printed circuit board having an approximate diameter of 25 mm. The conductors on the top side (which in use faces the support wall  5 ) are shown in FIGS. 2 a  and  2   b  as dashed lines, and the conductors on the bottom side (which in use faces the resonator circuit board  9 ) are shown in solid lines. As can be seen, the majority of the conductors are located on the side of the printed circuit board which, in use, will be nearest to the resonator printed circuit board  9 . This ensures that there is maximum coupling between the resonator and the sensor windings. As can be seen from FIGS. 2 a  and  2   b , the via holes  22  which connect the conductors on the top and bottom layers of the printed circuit board  15  are located at the inner and outer edges of the loops and the conductors connecting these edges are all on the bottom side of the printed circuit board. 
     In this embodiment, a connection is made from the sensor winding turn  21 - 1  to a pair of connection pads  23  and  25  by removing the via which would normally be at point A (for connecting conductors  24  and  26  which run on opposite sides of the printed circuit board) and by running a pair of connection tracks  28  from the ends of these conductors at point A to the connection pads  23  and  25 . As shown, the connection tracks  28  follow each other on opposite sides of the PCB, thereby minimising any magnetic coupling between the connection tracks  28  and the resonator (not shown) and between the connection tracks  28  and the excitation winding (not shown). 
     As mentioned above, in this embodiment, there are two two-turn sensor windings and an excitation winding on the printed circuit board  15 . FIG. 2 c  shows the printed. circuit board  15  with all these windings. As shown, the first turn  21 - 1  of sensor winding  21  is highlighted by use of thicker lines and the two turns of each sensor winding  21  and  27  are circumferentially spaced apart by 15° (one-eighth of a period) and the first turn  21 - 1  of sensor winding  21  is circumferentially spaced apart by 30° (one-quarter of a period) from the first turn  27 - 1  of sensor winding  27 . As shown, the second turn  21 - 2  of sensor winding  21  is connected to connection pads  23  and by a pair of connection tracks  32  which, like connection tracks  28 , follow each other on opposite sides of the printed circuit board  15 . Similarly, connection tracks  34  and  36  connect turns  27 - 1  and  27 - 2  respectively to connection pads  31  and  33  and  31  and  35  respectively. In this way, turns  21 - 1  and  21 - 2  are connected in series and turns  27 - 1  and  27 - 2  are also connected in series, and the connection tracks to these windings do not affect the magnetic coupling between the sensor windings  21  and  27  and the excitation winding and between the sensor windings  21  and  27  and the resonator. 
     FIG. 2 d  shows the two turns  21 - 1  and  21 - 2  which together form the sensor winding  21  when stretched out over a linear path. As shown, in this embodiment, the stagger or separation between the two turns is 15°. A two-turn sensor winding is used because it provides larger output signals and reduces spatial distortion. Lower spatial distortion means that the spatial coupling relationship between the resonator and the sensor windings is more nearly sinusoidal. This ensures a more accurate sensor. Ideally, the coil stagger angle should be in the region of one-sixth of the period (i.e. 20° in this embodiment), since this suppresses the major component of the harmonic error. The exact stagger angle for lowest harmonic distortion may not be exactly a sixth of a period and can be determined by experiment and or modelling. In this embodiment, a coil stagger angle of one-eighth of a pitch was chosen so as to avoid clashes between the conductors of each sensor winding due to lack of space. (An alternative approach is to stagger the positions of the radial lines closer to the optimum sixth of a period, while keeping the space-critical positions of the vias constant. The top and bottom layers of a printed circuit board which would form such sensor windings are shown in FIGS. 2 e  and  2   f .) 
     FIG. 2 g  shows the four turns  21 - 1 ,  21 - 2 ,  27 - 1  and  27 - 2  of windings which form the two sensor windings  21  and  27  of this embodiment, stretched out over a linear path. As shown, the separation or stagger angle between the two sensor windings  21  and  27  is 30°, which corresponds to a quarter of a period. Therefore, the signals output by the respective sensor windings will be in phase quadrature. 
     Referring again to FIG. 2 c , the connections between the conductors forming the sensor windings  21  and  27  on the top and bottom layers of the circuit board  15  are provided by two inner circles of vias  42  and two outer circles of vias  44 . FIG. 2 h  shows the bottom layer of conductors on the printed circuit board  15  (as seen from the top layer). As shown, conductor tracks between the inner circles of vias  42  and the outer circles of vias  44  are all made on this layer. Therefore, as shown in FIG. 2 c , the excitation winding  29  can be wound on the top layer between these inner and outer circles of vias without clashing with the sensor windings  21  and  27 . This is illustrated in FIG. 2 i , which shows the conductors which are formed on the top layer of the printed circuit board  15 . 
     As shown in FIG. 2 c , the excitation winding  29  comprises seven concentric loops formed by a two-start spiral winding. In particular, the excitation winding  29  extends anticlockwise in an increasing spiral from via  24  to point C, where the outer loop is connected by conductor  38  to via  26 . At via  26 , the excitation winding  29  continues in an anticlockwise increasing spiral until point D. As those skilled in the art will appreciate, each loop of each sensor winding will couple with the excitation winding  29 . However, since each loop is wound in the opposite sense, any signal induced from the excitation winding  29  in one loop will oppose the signal induced in an adjacent loop. Therefore, if the magnetic coupling between the excitation winding  29  and each loop of the sensor windings  21  and  27  is the same, then any signal induced in one loop by the excitation winding will substantially cancel with the signal induced in an adjacent loop and, therefore, there will be no net coupling between the excitation winding  29  and the sensor windings  21  and  27 . However, as will become apparent from the following description, in this embodiment, it is not possible to ensure that the magnetic coupling between the excitation winding  29  and each loop of each sensor winding will be the same because of the connection tracks  40  which connect the excitation winding  29  to the connection pads  37  and  39 . 
     As explained above, connections have been made to each turn of the sensor windings using connection tracks which have substantially no coupling with the resonator and the excitation winding  29 . This is because the connection tracks follow each other on either side of the printed circuit board  15  and because the magnetic field generated by the excitation winding and by the resonator are predominantly perpendicular to the surface of the circuit board  15 . As will be appreciated from FIGS. 2 h  and  2   i , it is not possible, in this embodiment with a two layer circuit board design, to run a similar set of connection tracks to the excitation winding  29  without clashing with the sensor windings  21  and  27 . In particular, with reference to FIG. 2 c , the connection tracks  40  which connect the excitation winding  29  to the connection pads  37  and  39  pass from these pads on either side of the printed circuit board  15  to point B, where the connection track on the top layer of the printed circuit board  15  passes to the left until it reaches the outer turn of the excitation winding  29  at point D, and the connection track on the bottom layer of the printed circuit board  15  passes to the right and connects to the inner turn of the excitation winding  29  at the via hole  24 . This divergence of the paths taken by the connection tracks  40  results in an imbalance in the coupling between the excitation winding  29  and the loops of the sensor windings  21  and  27 . 
     The reason for this imbalance will now be explained with reference to FIG. 2 j , which shows part of the excitation winding  29  and the first turn  21 - 1  of sensor winding  21 . As shown, where the connection tracks  40  diverge at point B, there is an additional area  43  of overlap between the excitation winding  29  and the loop  21 - 1   b  as compared with, for example, the area of overlap between the excitation winding  29  and loop  21 - 1   c . This additional overlap results in an increased magnetic coupling between the excitation winding  29  and loop  21 - 1   b , than that between the excitation winding  29  and loop  21 - 1   c . A similar imbalance is also created between the excitation winding  29  and the other sensor winding  27 . 
     In this embodiment, a further imbalance is deliberately added to counter the imbalance caused by the connection tracks. In particular, a similar imbalance is generated by creating a similar area to area  43  in a loop of the winding  21 - 1  which is wound in the opposite sense. In this embodiment, this deliberate imbalance is made by adding the additional overlap area  45  in loop  21 - 1   e , which is 180° spaced from the imbalance caused by the connection tracks in loop  21 - 1   b . As those skilled in the art will appreciate, the deliberate imbalance could have been made at any one of the loops  21 - 1   a ,  21 - 1   c  or  21 - 1   e . Alternatively, a smaller imbalance could have been added to each of these loops in order to counter the main imbalance caused by the connection tracks. 
     FIG. 3 a  shows the top layer of the resonator printed circuit board  9  and FIG. 3 b  shows the bottom layer of the resonator circuit board  9 , as it would be seen from the top layer. The printed circuit board  9  is formed from a circular disc having a diameter of approximately 25 mm and has two cutout regions  9 - a  and  9 - b  which receive securing pins  52 ,  54  and  56  which are mounted on the bushing  11  and which, in use, secure the circuit board  9  relative to the bushing  11 . As shown in FIG. 3 a , the top layer of circuit board  9  comprises three coil portions  53   a ,  55   a  and  57   a  which are circumferentially spaced apart by 120°. Similarly, the bottom layer of the resonator printed circuit board  9 , as shown in FIG. 3 b , comprises three coils  53   b ,  55   b  and  57   b  which are also circumferentially spaced apart by 120°, with coil  53   b  being located directly behind coil  53   a , with coil  55   b  being located directly behind coil  55   a  and with coil  57   b  being located directly behind coil  57   a . In this embodiment, in use, the top layer of the resonator circuit board  9  faces the sensor circuit board  15  with a separation therebetween of approximately 2 mm. 
     Each of the coils  53 ,  55  and  57  are connected in series and the ends of the coils are connected to a pair of capacitor mounts  61  and  63  which connect a pair of capacitors in series with the coils to form an LC resonator  50 . More specifically, the conductor track extends from capacitor mount  61  on the top layer of the resonator circuit board  9  in an anticlockwise decreasing spiral to define the coil  53   a  until it reaches via  65 , where the track passes through to the other side of the board  9 . From via  65 , the conductor track extends on the bottom layer of the board  9  in an anticlockwise increasing spiral to define coil  53   b  until it reaches via  67  where the track returns to the top layer of the circuit board  9 . The conductor track then continues in an anticlockwise decreasing spiral to define coil  55   a  until it reaches via  69  where it passes back through to the bottom layer of the circuit board  9 . The conductor track winds from via  69  in an anticlockwise increasing spiral to define coil  55   b  until via  71  where the conductor passes back to the top layer of the circuit board. From via  71 , the track winds in an anticlockwise decreasing spiral direction to define coil  57   a  until it reaches via  73 . At via  73 , the conductor passes back to the bottom layer and continues in an anticlockwise increasing spiral to define the coil  57   b  until it reaches via  75 , where the track is connected to the other capacitor mount  63 . 
     When the circuit board  9  is rotatably mounted on the bushing  11  and the circuit board  15  is fixed to the support wall  5  so that the inner edge  76  of the resonator circuit board  9  is concentric with the inner edge  16  of the sensor circuit board  15 , the coils  53 ,  55  and  57  are located adjacent the excitation winding  29  and the sensor windings  21  and  27 . When an excitation current is applied to the excitation winding  29 , the magnetic field generated by the excitation winding couples with the conductor tracks of the coils  53 ,  55  and  57  which are closest to the outer edge of the circuit board  9 , to induce an EMF in each of these coils. Since the coils are all wound in the same direction, these EMFs add and a resonator current is generated which flows through each coil  53 ,  55  and  57 , thereby generating a spatially patterned resonator magnetic field having three principal axes located at the centres of the coils  53 ,  55  and  57 . Since the resonator  50  has the same number of coils  53 ,  55  and  57  as there are periods of sensor winding, and since the separation between these coils corresponds to one period of the sensor windings, the asymmetries in the excitation winding  29  in regions  43  and  45  shown in FIG. 2 h , do not affect the coupling between the excitation winding  29  and the resonator  50  which remains constant for all angular positions of the resonator board  9 . 
     As will be appreciated by those skilled in the art, when the resonator  50  is resonating, the spatially patterned magnetic field generated by the resonator will interact with the sensor windings  21  and  27  to generate an output signal in each which will vary with the angular position of the resonator circuit board  9 . For example, when coil  53  is located over loop  21 - 1   a , coil  55  will be located over loop  21 - 1   c  and coil  57  will be located over loop  21 - 1   e . Since each of the coils  53 ,  55  and  57  are wound in the same direction, when the resonator is resonating, the same size and magnitude of EMF will be induced in each of these three loops  21 - 1   a ,  21 - 1   c  and  21 - 1   e  the magnetic field generated by the current flowing in these coils. Since these loops of the sensor winding  21  are wound in the same direction, these EMFs will add with each other to create an output signal. 
     When the resonator circuit board  9  rotates relative to the sensor windings so that the coils  53 ,  55  and  57  are located between adjacent sensor loops, the EMFs induced in the adjacent loops will cancel each other out to generate no resulting output signals in sensor winding turn  21 - 1 . As the resonator board rotates further, so that, for example, coils  53 ,  55  and  57  are located adjacent loops  21 - 1   b ,  21 - 1   d  and  21 - 1   f , the induced EMFs therein will all add together but will be of opposite polarity to the total EMF which was induced when the resonator board  9  was at an angle such that coils  53 ,  55  and  57  were opposite loops  21 - 1   a ,  21 - 1   c  and  21 - 1   e.    
     Therefore, as the resonator rotates relative to the sensor coils  21  and  27 , the output signals induced therein will vary periodically as the resonator board  9  rotates. Since the two sensor windings  21  and  27  are circumferentially spaced apart by a quarter of a period, the periodic variation of the signal induced in one sensor winding will be in phase quadrature with the periodic variation of the signal induced in the other sensor winding. Ideally, this periodic variation should be exactly sinusoidal. However, there are usually higher order harmonic variations which cause distortion in the output signals. As mentioned above, in an embodiment which uses multiturn sensor windings, it is possible to arrange for the spacing between the conductors of the different turns to reduce this harmonic distortion. The effect of this harmonic distortion also decreases as the gap between the resonator circuit board  9  and the sensor circuit board  15  increases. However, in this embodiment, with a 25 mm diameter device and a gap between the resonator circuit board  9  and the sensor circuit board  15  of 2 mm, most of these harmonic distortions can be ignored. 
     FIG. 4 shows the excitation and processing circuitry  101  used to energise the excitation winding  29  and to process the signals induced in the sensor windings  21  and  27 . In this embodiment, the excitation and processing circuitry  101  is mounted on the circuit board  15  and is connected to the excitation winding  29  at connection pads  37  and  39  and is connected to the sensor windings  21  and  27  at connection pads  25  and  30  and  33  and  35  respectively. As shown, the excitation and processing circuitry  101  comprises a digital waveform generator  103  which is operable to receive an oscillating input from a crystal oscillator  105  and to output the excitation signal which is applied to the excitation winding  29  via an excitation driver  107 . 
     In this embodiment, the excitation signal is a squarewave voltage having a fundamental frequency F 0  of 1 MHz, which is matched-to the resonant frequency of the resonator  50  mounted on the resonator circuit board  9 . The excitation current flowing in the excitation winding  29  generates an electromagnetic field in the vicinity of the resonator circuit board  9  which causes the resonator  50  to resonate. When resonating, the resonator  50  creates its own magnetic field which induces an EMF in each of the receive windings  21  and  27  whose peak amplitudes (Ê 21  and Ê 27 ) vary sinusoidally in phase quadrature with the rotation of the resonator  50 . The way in which these peak amplitudes vary with rotation angle (θ) is shown in FIG.  5 . As shown, the sinusoidal variation repeats every 120°, corresponding to the period or pitch of the sensor windings  21  and  27 . 
     As shown in FIG. 4, the EMF induced in the sensor windings  21  and  27  are input to mixers  109  and  111  respectively, where they are mixed with a 90° phase-shifted version of the excitation signal which is applied to the excitation winding  29 , in order to demodulate the induced EMFs. The 90° phase shift is required due to the operation of the resonator  50  when it resonates. The outputs from the mixers  109  and  111  will each comprise a DC component corresponding to the respective peak amplitudes of the EMFs induced in the corresponding sensor windings, together with high frequency time varying components. 
     The outputs from the mixers are then fed, one after the other, through switch  113  to a low pass filter  115  which removes these high frequency time varying components. These peak amplitude values are then converted from an analogue value into a digital value by the analogue-to-digital converter  117  and passed to the microprocessor  119  which determines the rotational angle of the resonator board  9  by using an arc-tangent function on these digital values. (As represented by the dashed lines  121  and  123 , the microprocessor  119  also controls the switching of the switch  115  and the signals generated by the digital waveform generator  103 .) As those skilled in the art will appreciate, the microprocessor  119  will be able to unambiguously determine the rotation angle of the resonator board over a shaft angle of 120°. This position encoder would, therefore, be suitable for determining the angular position of a throttle valve in an engine, which only rotates through 90°. Alternatively, if the microprocessor  119  counted the cycles, then the rotation of the resonator board  9  can be continuously tracked. 
     A brief overview has been given of the excitation and processing circuitry employed in the present embodiment. A more detailed explanation can be found in, for example, the applicant&#39;s earlier International Application WO95/31696, the contents of which are incorporated herein by reference. An alternative processing circuitry which does not use an arc-tangent function to determine the rotation angle can be found in the applicant&#39;s earlier International Application WO98/00921, the contents of which are also incorporated herein by reference. 
     The above embodiment has a number of significant advantages over the prior art systems described in the introduction. These include: 
     (1) As a result of winding the excitation windings over the loops of the sensor windings, efficient utilisation of printed circuit board space is achieved. In particular, for a given size of sensor printed circuit board, more turns of excitation winding can be provided whilst maintaining the largest possible size of sensor winding loops. In the prior art systems where the excitation winding is wound around the outside of the sensor loops, if the number of turns of the excitation winding is increased, then either the width of the sensor printed circuit board must be increased or the width of the sensor winding loops must be decreased. In some applications, it is not possible to increase the size of the sensor circuit board and reducing the size of the sensor winding loops decreases the output signal levels. 
     (2) Connections are made from each of the sensor windings to the processing electronics (a) without creating any significant disturbance in the sinusoidal coupling relationship between the sensor windings and the resonator; (b) without creating any significant net coupling between the sensor windings and the excitation windings; and (c) without making the sensor windings susceptible to background electromagnetic interference. 
     (3) A two-layer printed circuit board has been provided in which the excitation winding is wound inside the sensor windings and in which the excitation winding has been connected to the excitation electronics using printed circuit board tracks without creating a net magnetic coupling with any of the sensor windings. This is achieved by deliberately adding an imbalance to counter the imbalance caused by the excitation winding&#39;s connection tracks. 
     (4) Since connection pads to each of the sensor windings and to the excitation winding are located on the outside of the track patterns, connections can be made thereto in a simple manner. 
     (5) Since the loop area of the sensor windings has been maximised for a given size of printed circuit board, the device can operate over larger separations between the sensor circuit board  15  and the resonator circuit board  9 , because the rate of signal fall-off with this separation is slower. This is important where both parts are individually sealed for environmental protection since larger separations enable thicker layers of protective coating, which are generally cheaper and easier to apply and more robust. 
     (6) The outer radius of the excitation winding is maximised and the inner radius is minimised so that the coupling to the resonator is maximised. This is achieved in this embodiment (a) by winding the outer loop of the excitation winding as close as possible to the outer circles of vias  44 ; and (b) by making the radius of the inner loop of the excitation winding equal the distance from the centre of the resonator circuit board  9  to the centre point of the resonator coils  53 ,  55  or  57 . This is because the excitation winding mainly couples with the tracks of the coils  53 ,  55  and  57  which are at the outer edge of the circuit board  9 . As the width of the excitation winding increases towards the inner tracks of these coils, the overall coupling with the excitation winding decreases since the coupling in these inner tracks opposes the coupling in the outer tracks. 
     (7) A multi-turn excitation winding is provided which increases the inductance of the excitation winding, thereby allowing: (a) use of common sensor supply voltages (approximately three volts), thereby allowing the elimination of step-down voltage conversion circuitry; and (b) the use of a half-bridge or a full-bridge switching amplifier rather than a less efficient linear power amplifier for amplifying the signal output by the signal generator before it is applied to the excitation winding. 
     Modifications and Alternative Embodiments 
     In the above embodiment, a multi-turn excitation winding was used in which all the windings were wound in the same direction. Whilst this provides the advantages of increased inductance, it suffers from the disadvantage that there is a limit to how many turns the excitation winding can have before the coupling with the resonator begins to reduce. FIG. 6 illustrates an alternative form of excitation winding  131  having six series-connected turns of winding in which the outer three turns are wound in one direction and the inner three turns are wound in the opposition direction. This excitation coil has the advantage of having an increased magnetic coupling with the resonator since the outer turns will couple with the tracks at the outside edge of the resonator circuit board and the inner tracks will couple with the tracks at the inside edge of the resonator circuit board which will add to the coupling with the outside tracks. Since there is increased coupling between the resonator and the excitation winding  131 , this results in greater signal levels and hence improved accuracy and electromagnetic interference performance. FIG. 6 also shows the inner and outer circles of vias  42  and  44  associated with the sensor windings (not shown). Although not shown in FIG. 6, the connection tracks between the excitation circuitry and the ends of the excitation winding  131  can be made in a similar manner to the way these connections were made in the first embodiment, so that there will be no coupling between the excitation winding and the sensor windings. 
     A further benefit of the excitation winding shown in FIG. 6 is that is reduces the electromagnetic emission from the excitation winding which may interfere with other electronic circuits. Additionally, it is possible to make the excitation coil substantially immune to electromagnetic interference by including more turns on the inner coil so that the magnetic dipole of the inner and outer coils are equal and opposite. The disadvantage of such an embodiment is that the excitation winding will have reduced inductance and this will make it harder to drive. 
     In the first embodiment, the excitation winding comprised a two start spiral winding. It is possible to counter the imbalance caused by the connection tracks by creating a corresponding imbalance in, for example, the inner loop of a single start spiral winding. However, the multi-start spiral winding is preferred, because the imbalance can be made in a symmetric manner and if the excitation voltage is differentially applied to the excitation winding through a full bridge switching circuit, then the two-start spiral winding will reduce the effects of capacitive coupling between the excitation winding and the sensor windings. 
     In the first embodiment, it was mentioned that an imbalance can be added to several loops of the sensor winding in order to counteract the imbalance caused by the connection tracks. Preferably, the number of spiral starts to the excitation winding equals the number of imbalances (including the connection track imbalance), so that these imbalances remain symmetrical with each other. 
     In the above embodiments, two two-turn periodic sensor windings were provided having a period corresponding to a rotational angle of 120°. As those skilled in the art will appreciate, the above techniques can also be used in embodiments having any number of sensor windings having any number of periods wound circumferentially around the sensor circuit board and having any number of turns. For example, two one-turn single-period sensor windings could be provided which are circumferentially spaced apart around the circuit board by 90°. From the signals induced in these sensor windings, the position of the resonator can be unambiguously determined over 360°. However, multiple-turn sensor windings are preferred because of the advantages discussed above and should be used where space permits. 
     The above embodiments which have been described relate to rotary position encoders. Many of the features of the first embodiment will also apply to linear position encoders. FIG. 7 shows two three-period, 20 mm pitch two-turn sensor windings  135  and  137  in which the separation between the turns of each sensor winding is a sixth of the pitch (3.3 mm) and the separation between the first turn of winding  135  and the first turn of winding  137  is a quarter of the pitch (5 mm). As in the first embodiment, connections are made from the two turns of each sensor winding using connection tracks  139 ,  141 ,  143  and  145  which do not disturb the sinusoidal coupling relationship between the sensor coils and the resonator (not shown) and which do not couple with the excitation winding  147 . 
     As shown in FIG. 7, in this embodiment, the excitation winding  147  comprises four turns of conductor which pass through the centre of the sensor winding loops. In this embodiment, the connection to the excitation winding  147  can also be made using connection tracks  149  which follow each other on either side of the sensor circuit board (not shown), which will not couple with the sensor windings. Therefore, it is not necessary, in this embodiment, to deliberately provide an imbalance between the excitation winding  147  and the sensor windings  135  and  137  in order to counter any imbalance caused by the connection tracks. This is possible, because the connection can be made to the excitation coil at one end thereof. However, there may be applications where this is not possible and the connection must be made somewhere in the centre of the sensor windings  135  and  137 . In this case, an imbalance will be created by the connection tracks to the excitation winding  147  and therefore a further imbalance will have to be added in a similar manner to the way in which it was added in the first embodiment in order to counter this balance. 
     As those skilled in the art will appreciate, the linear embodiment shown in FIG. 7 also has many of the advantages described above for the rotary embodiment, including that the width of the sensor coils is maximised for a given sensor board width, thus increasing signal levels. This is especially important where there is pressure to reduce the overall width of the position sensor so that it can, for example, fit into a small diameter tube. 
     The above embodiments have been described with reference to forming the sensor windings and the excitation windings on a two-layer printed circuit board. Similar considerations will also apply where these windings are formed using thick or thin film technologies, in which, a first layer of conductive film is provided over a ceramic insulating substrate, followed by an insulating layer (which may be patterned so as to only insulate at the crossover points) followed by a second conductive film to generate the set of excitation and sensor windings. 
     In the above embodiment, a separate mixer was provided for demodulating the signals from the respective sensor windings and the demodulated signals were then passed, via a switch, to a common filter and analogue-to-digital converter and then to the microprocessor. In an alternative embodiment, separate filters and analogue-to-digital converters can be provided for the output of each of the mixers. Additionally, all this circuitry could be implemented using a digital ASIC (application specific integrated circuit), which would reduce the overall cost and complexity of the design. 
     In the first embodiment, the resonator was arranged to have a coil for each period of the sensor winding. This ensured that there was a constant coupling between the resonator and the excitation winding for all rotation angles of the resonator board. This is preferred but is not essential. In particular, if the coupling between the resonator and the excitation winding varies with position, this variation will be common to both signals induced in the sensor windings and will be removed due to the ratiometric calculation performed by the microprocessor. 
     In the above embodiment, a resonator was used to generate a magnetic field which varied as the resonator moved relative to the sensor windings. A similar magnetic field can be generated by other electromagnetic elements, such as from ferrite pole pieces or from short-circuit coils or metal screens. Such a short-circuit coil embodiment could use the coils  53 ,  55  and  57  shown in FIG. 3 but with the ends connected in series rather than connected through a capacitor. In an embodiment which uses a short-circuit coil or a metal screen, the coupling of magnetic flux between the excitation winding and the sensor windings will be governed by the presence or absence of the short-circuit coil or the metal screen, and a careful design of the location of the coils or screens gives rise to a similar set of sinusoidally varying output signals from the sensor windings. Such a position encoder has the advantage that it is cheaper than the resonator design. However, the resonator design is preferred because it provides much greater output signal levels, it allows a pulse-echo mode of operation to be used (in which a burst of excitation signal is applied to the excitation winding and the signals on the sensor windings are only processed after the burst has ended—this is possible because the resonator continues to “ring” after the excitation signal has been removed) and because the resonator signals are 90° out of phase with the excitation signals, making them easier to isolate from any excitation breakthrough signals. 
     In the above embodiments, an excitation signal is applied to the excitation winding and the signals induced in the sensor windings were used to determine the position of the resonator. In an alternative embodiment, the excitation signal could be applied to each of the sensor windings in turn, in order to energise the resonator and the signals induced in the excitation circuit by the resonator would then carry the position information. 
     In the first embodiment, the excitation winding did not extend beyond the centre points of the resonator coils, since this reduces the magnetic coupling between the excitation winding and the resonator. However, the effect of this reduces as the gap between the sensor circuit board and the resonator circuit board increases. This is because the effective radius of the excitation coil&#39;s field increases as this gap increases. The overall impact is a reduction in the dynamic range of signal level which is received as the resonator gap varies, and this is often of benefit to the processing electronics in applications where significant gap variations are experienced. Another benefit of this modification is that the maximum possible inductance of the excitation coil can be increased.