Patent Publication Number: US-2009217774-A1

Title: Torque sensing apparatus

Description:
This invention relates to the sensing of position or speed, and has particular, but not exclusive, relevance to a system for measuring torque and component parts thereof. 
     Torque measuring systems are employed, for example, in automobiles for measuring the torque applied to rotating members such as the steering wheel. In order to measure torque, relative rotary displacement between two points along the axis of rotation of a torsion bar is measured. 
     Inductive sensors have been used in the past for non-contact position measurement. The present invention addresses techniques for the incorporation of inductive position sensing in a torque sensor. 
     According to an aspect of the present invention, there is provided apparatus for generating rotary displacement information indicative of the torsion applied to a torsion bar which is rotatable relative to a housing, in which the electromagnetic coupling between a transmit aerial fixed relative to the housing and a receive aerial fixed relative to the housing varies in dependence upon first and second resonators fixed relative to respective spaced axial positions of the torsion bar. The first and second resonators have respective different resonant frequencies to enable the signal induced in the receive aerial by the first resonator to be distinguished from the signal induced in the receive aerial by the second resonator. 
    
    
     
       An embodiment of the present invention will now be described with reference to the attached figures in which: 
         FIG. 1  schematically shows a sectional view of a coupling arrangement between a steering wheel and a gear of a rack-and-pinion type steering mechanism; 
         FIG. 2  schematically shows the main components of a position sensor forming part of the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 3  shows a perspective view of a sleeve member, having mounted thereon a flexible printed circuit board, forming part of the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 4  shows a plan view of the flexible printed circuit board illustrated in  FIG. 3  when laid out flat; 
         FIG. 5  shows a perspective view of a first puck wheel, having mounted thereon a flexible printed circuit board, forming part of the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 6  shows a plan view of the first puck wheel illustrated in  FIG. 1 ; 
         FIG. 7  shows a plan view of the flexible printed circuit board illustrated in  FIG. 5  when laid out flat; 
         FIG. 8  shows a perspective view of a second puck wheel, having mounted thereon a flexible printed circuit board, forming part of the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 9  shows a plan view of the flexible printed circuit board illustrated in  FIG. 8  when laid out flat; 
         FIG. 10  schematically shows a perspective view of the positional relationship between the first and second puck wheels when mounted in the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 11  schematically shows an exploded view of the positional relationship between the sleeve, the first puck wheel and the second puck wheel; 
         FIG. 12  shows a perspective view of the positional relationship between the sleeve, the first puck wheel and the second puck wheel when mounted in the coupling arrangement illustrated in  FIG. 1 ; 
         FIG. 13  schematically shows the main components of an ASIC forming part of the position sensor illustrated in  FIG. 11 ; 
         FIG. 14  is a graph showing the relationship between a first detected phase angle and the absolute position of a first shaft forming part of the coupling arrangement illustrated in  FIG. 1 ; and 
         FIG. 15  is a graph showing the relationship between a second detected phase angle and the absolute position of the first shaft forming part of the coupling arrangement illustrated in  FIG. 1 . 
     
    
    
     In the illustrated embodiment of the invention, a car has a steering wheel which is connected to a gear forming part of a rack-and-pinion steering mechanism.  FIG. 1  shows a cross-sectional view of a coupling arrangement between the steering wheel and the gear. 
     A first elongate cylindrical shaft  1  is attached at one longitudinal end  5  to the steering wheel (not shown). As shown in  FIG. 1 , the first shaft  1  has a reduced diameter axial portion  3  extending from the longitudinal end  7  away from the steering wheel to a stepped region  9 . A second elongate cylindrical shaft  11  is attached at one longitudinal end  13  to the gear (not shown) of the rack-and-pinion steering mechanism, and has a hollow axial portion  15  extending from the longitudinal end  17  away from the gear, hereafter called the open end  17 . As shown in  FIG. 1 , the reduced diameter portion  3  of the first shaft  1  is mounted in the hollow portion  15  of the second shaft  11  with the first shaft  1  and the second shaft  11  axially aligned and the stepped region  9  of the first shaft  1  being adjacent the open end  17  of the second shaft  11 . A locking pin  19  fixes the first shaft  1  to the second shaft  11  towards the end  7  of the reduced diameter portion  3 . 
     For the remainder of this specification, the term axial direction refers to the direction of the common longitudinal axis of the first and second shafts  1  and  11 , the term radial direction refers to lines radiating perpendicularly away from the axial direction, and the term circumferential direction refers to a direction normal to both the axial direction and the radial direction. 
     The first shaft  1  and the second shaft  11  are rotatably mounted relative to a housing  21 , so that when a driver of the car turns the steering wheel both the first shaft  1  and the second shaft  11  rotate relative to the housing  21 . In particular, in this embodiment the range of rotational movement of the first and second shafts is two full revolutions, i.e. 720°, relative to the housing  21 . 
     In this embodiment, the steering mechanism is an electronic power-assisted steering mechanism in which electrical motors apply an assisting force which varies in dependence on the torque applied to the steering wheel by the driver. Accordingly, the torque applied by the driver must be monitored. 
     The torque applied to the steering wheel by the driver is transferred to the gear via the locking pin  19  which fixes the first shaft  1  to the second shaft  11 . However, the axial distance between the locking pin  19  and the junction between the stepped region  9  of the first shaft  1  and the open end  17  of the second shaft  11  results in a relative rotary displacement between the stepped region  9  and the open end  17  which varies in dependence on the applied torque. According to the invention, an inductive sensor measures the relative rotary displacement between the stepped region  9  and the open end  17 , and the applied torque is calculated from the measured relative rotary displacement. 
     The inductive sensor of the present invention has: an aerial member  23  which is mounted on an aerial guide  25  which is fixed relative to the housing  21 ; a first intermediate coupling element  27  (not shown in  FIG. 1 ) which is mounted on a first sleeve member  29  which is fixed relative to the first shaft  1 ; and a second intermediate coupling element  31  which is mounted on a second sleeve member  33  which is fixed relative to the second shaft  11 . The aerial member  23  has formed thereon a transmit aerial (not shown in  FIG. 1 ) which generates a magnetic field which varies around the circumference of the first shaft  1  and the second shaft  11 , and a receive aerial (not shown in  FIG. 1 ). The transmit aerial is balanced relative to the receive aerial so that in the absence of the first intermediate coupling element  27  and the second intermediate coupling element  31  no nett signal would be induced in the receive aerial by virtue of the magnetic field generated by the transmit aerial, but in the presence of the first intermediate coupling element  27  and the second intermediate coupling element  31   a  signal is induced in the receive aerial which depends on the rotary positions of the first shaft  1  and the second shaft  11 . 
     In this embodiment, for improved safety the inductive sensor has two independent sensing arrangements providing respective readings for the relative rotary displacement of the first shaft  1  and the second shaft  11 . In this way, if one sensing arrangement fails a measurement of the torque may still be calculated using the relative rotary displacement reading provided by the other sensing arrangement. As shown in  FIG. 1 , the inductive sensor has two ASICs  35   a ,  35   b , with each ASIC  35  being used by a respective different one of the two sensing arrangements. 
       FIG. 2  schematically shows the main components of the torque sensing circuitry. In  FIG. 2 , the first sensing arrangement  41   a  and the second sensing arrangement  41   b  are schematically indicated by dashed boxes. 
     Each independent sensing arrangement  41  has two associated excitation windings  43  (performing the transmit aerial function for that sensing arrangement) and one sensor winding  45  (performing the receive aerial function for that sensing arrangement). In particular, the first sensing arrangement  41  has first and second excitation windings  43   a  and  43   b  and a first sensor winding  45   a  which are formed on the aerial member  23  and are connected to the first ASIC  35   a . The first sensing arrangement also has a first resonant circuit  47   a  which is formed on the first intermediate coupling element  27  and a second resonant circuit  47   b  which is formed on the second intermediate coupling element  31 . Similarly, the second sensing arrangement  41   b  has third and fourth excitation windings  43   c  and  43   d  and a second sensor winding  45   b  which are formed on the aerial member  23  and are connected to the second ASIC  35   b , a third resonant circuit  47   c  which is formed on the first intermediate coupling element  27  and a fourth resonant circuit  47   d  which is formed on the second intermediate coupling element  31 . 
     In this embodiment, for the first sensing arrangement  41   a  the first and second excitation windings produce radial magnetic field components which vary through twenty cycles of the sine and cosine functions respectively around a full circumference, and the third and fourth excitation windings produce radial magnetic field components which vary through nineteen cycles of the sine and cosine functions respectively around a full circumference. The radial magnetic field components induce a signal in the first resonant circuit  47   a  which varies in accordance with the rotary position of the first shaft  1 , and induce a signal in the second resonant circuit which varies in accordance with the rotary position of the second shaft  11 . The signals induced in the first and second resonant circuits  47   a ,  47   b  induce corresponding signal components in the first sensor winding  45   a  which are processed by the first ASIC  35   a  to determine the relative rotary displacement between the first shaft  1  and the second shaft  11 . The second sensing arrangement  41   b  works in an analogous manner. 
     The ASIC  35  of each sensing arrangement outputs the calculated relative rotary displacement to a central control unit  49  of the car, which processes the relative rotary displacements to calculate the torque applied to the steering wheel. 
       FIG. 3  schematically shows a perspective view of the aerial member  23  and the aerial guide  25 . As shown, the aerial guide  25  is a cylindrical sleeve having an outer surface with a recessed circumferential portion in which the aerial member  23  is fixedly mounted. In this embodiment, the aerial member  23  is a rectangular sheet of two-layer flexible printed circuit board (PCB) material having a length which is longer than the circumference of the recessed portion of the aerial guide  25 . The aerial member  23  has conductive tracks deposited on either side which are connected, using via holes, to form the excitation windings  43  and the sensor windings  45 . Two sets of six electrical contacts  51  are provided, with each set of six electrical contacts  51  connecting the two excitation windings and the sensor winding of a sensing arrangement with the corresponding ASIC  35 . 
       FIG. 4  is a schematic plan view of the aerial member  23  when laid out flat, in which the conductive tracks formed on one side of the PCB are represented by solid lines whereas the conductive tracks formed on the other side of the PCB are represented by dashed lines. As shown in  FIG. 3 , the conductive tracks  61  associated with the first sensing arrangement  41   a  are spaced apart from the conductive tracks  63  associated with the second sensing arrangement  41   b  in the widthwise direction of the PCB (which corresponds to the axial direction when the aerial member  23  is mounted on the aerial guide  25 ). 
     In this embodiment, the excitation windings  43  and the sensor winding  45  for each sensing arrangement  41  include planar coil arrangements which extend over a length  65  of the PCB corresponding to the circumference of the recessed portion of the aerial guide  25 . The excitation windings produce magnetic fields having a magnetic field component perpendicular to the PCB which varies in accordance with multiple periods of the sine function and the cosine function respectively in substantially the same manner as the excitation windings described in UK Patent Application GB 2374424A (the whole contents of which are hereby incorporated herein by reference). Further, in this embodiment the sensor winding of a sensing arrangement is formed by a multi-loop planar coil extending around the whole of the length  61 . 
       FIG. 5  schematically shows a perspective view of the first intermediate coupling element  27  and the first sleeve member  29 . As shown, the first sleeve member  29  has a cylindrical recess  71  for receiving the first shaft  1 . The first sleeve member  29  also has a guide portion  73  on which the first intermediate coupling element  27  is mounted. The guide portion  73  has two opposing arc portions  75   a  and  75   b , which are centred on the axis of the cylindrical recess  71 , and two opposing connecting portions  77   a  and  77   b  which interconnect the two arc portions  75   a ,  75   b.    
       FIG. 6  shows a plan view of the first sleeve member  29  (i.e. looking along the axial direction when mounted to the first shaft  1 ) showing the cylindrical recess  71  and the guide portion  73 . As shown, the opposing arc portions  75   a ,  75   b  each extend approximately 70° around the cylindrical recess  71 , and the connecting portions  77   a ,  77   b  extend inside of the circle of which the outer surfaces of the two arc portions  75  form part of the circumference. 
     Returning to  FIG. 5 , the two arc portions  75  include projecting parts causing the arc portions  75  to have approximately twice the axial extent of the two connecting portions  77 . In this way, the side of the first sleeve member  29  including the projecting parts has a castellated appearance. 
     The first intermediate coupling arrangement includes a two-layer flexible PCB  79  having conductive tracks deposited on either side which are interconnected by via holes to form the inductors for the first and third resonant circuits  47   a ,  47   c .  FIG. 7  shows a schematic plan view of the flexible PCB  79  when laid out flat, with conductive tracks on one side of the PCB  79  being represented by solid lines and conductive tracks on the other side of the PCB  79  being represented by dashed lines. 
     As shown in  FIG. 7 , the PCB  79  has two end parts  91   a ,  91   b , whose dimensions match the dimensions of respective arc portions  75  of the guide portion  73 , and a connecting part  93  which is of reduced width and interconnects the two end parts  91 . The connecting part  93  separates the two end parts  91  by a distance which allows the two end parts  91  to be mounted to the outer surface of the two arc portions  75  of the guide portion  73 , with the connecting part  93  itself being mounted to one of the connecting portions  77  of the guide portion  73  (as shown in  FIG. 4 ). 
     The inductor for the first resonant circuit  47   a  is formed by the serial connection of eight periodically-spaced current loop structures  95   a - 95   h  and the inductor for the third resonant circuit  47   c  is formed by the serial connection of seven periodically-spaced current loop structures  97   a - 97   g . The current loop structures  95 ,  97  are arranged so that, when mounted to the sleeve member  29  as shown in  FIG. 5 , the current loops  95  for the first resonant circuit  47   a  are spaced apart in the axial direction from the current loops  97  of the third resonant circuit  47   c . In particular, the axial spacing between the current loops for the first and third resonant circuits  47   a ,  47   c  equals the axial spacing between the excitation/sensor windings for the first sensing arrangement  41   a  and the excitation/sensor windings for the second sensing arrangement  41   b.    
     As shown in  FIG. 7 , the current loop structures  95 , 97  are formed in the end parts  91  of the flexible PCB  79 . Two terminals  99   a  and  99   b  are formed in the connecting part  93  of the flexible PCB  79  to which a capacitor (not shown) is mounted to form the first resonant circuit  47   a  with the inductor formed by the current loop structures  95 , and two terminals  101   a  and  101   b  are formed in the connecting part  93  of the flexible PCB  79  to which a capacitor is mounted to form the third resonant circuit  47   c  with the inductor formed by the current loop structures  97 . The current loop structures  95  for the first resonant circuit  47   a  are mounted adjacent the projecting parts of the first sleeve member  29 . 
     In this embodiment, the first resonant circuit has a resonant frequency of 3.75 MHz and the third resonant circuit has a resonant frequency of 5 MHz. Further, the periodic spacing of the current loop structures  95  for the first resonant circuit  47   a  corresponds to an angular spacing of 18° (that is 360° divided by twenty), and the period spacing of the current loop structures  97  for the third resonant circuit  47   c  corresponds to an angular spacing of 18.95° (that is 360° divided by nineteen). 
       FIG. 8  schematically shows a perspective view of the second intermediate coupling element  31  and the second sleeve member  33 . As shown, the second sleeve member  33  has a substantially identical castellated shape to the first sleeve member  29 . 
     The second intermediate coupling element  33  is formed by a two-layer flexible PCB  111  in a similar manner to the first intermediate coupling element  29 .  FIG. 9  schematically shows a plan view of the flexible PCB  111  when laid out flat with the conductive tracks on one side being represented by solid lines and the conductive tracks on the other side being represented by dashed lines. Eight current loop structures  113   a  to  113   h  in the end portions of the flexible PCB  111  are connected in series with a capacitor (not shown) connected between terminals  115   a  and  115   b  to form the second resonant circuit  47   b , which in this embodiment has a resonant frequency of 1.875 MHz, and seven current loop structures  117   a  to  117   g  in the end portions of the flexible PCB  111  are connected in series with a capacitor (not shown) connected between terminals  119   a  and  119   b  to form the fourth resonant circuit  47   d , which in this embodiment has a resonant frequency of 2.5 MHz. 
     The periodic spacing of the current loop structures  113  for the second resonant circuit  47   b  corresponds to an angular spacing of 18° (that is 360° divided by twenty), and the periodic spacing of the current loop structures  117  for the fourth resonant circuit corresponds to an angular spacing of 18.95° (that is 360° divided by nineteen). The current loop structures  117  for the fourth resonant circuit  47   d  are mounted adjacent the projecting parts of the second sleeve member  33 . 
       FIG. 10  schematically shows a perspective view of the positional relationship between the first intermediate coupling element  27  and the second intermediate coupling element  31  when the coupling arrangement is assembled. As shown, the castellated ends of the first and second sleeve members interlock so that the projecting parts of the arc portions  75  of one sleeve member are located in the spaces adjacent the outside of the connecting portions  77  of the other sleeve member. In this way, the current loop structures of the first resonant circuit  47   a  are in the same position along the axial direction as the current loop structures of the second resonant circuit  47   b  but are spaced apart in the circumferential direction. Similarly, the current loop structures of the third resonant circuit  47   c  are in the same position along the axial direction as the current loop structures of the fourth resonant circuit  47   d  but are spaced in the circumferential direction. 
       FIG. 11  schematically shows an exploded view indicating how the first sleeve member  29  and the second sleeve member  31  are received in the aerial guide  25 , and  FIG. 12  shows schematically shows the first sleeve member  29  and the second sleeve member  31  when positioned within the aerial guide  25 . When assembled, the first sleeve member  29  and the second sleeve member  33  are rotatably mounted within the aerial guide  25 , with the current loop structures of the first and second resonant circuits  47   a ,  47   b  located in the same position along the axial direction as the first and second excitation windings  43   a ,  43   b  and the first sensor winding  45   a , and the current loop structures of the third and fourth resonant circuits  47   c ,  47   d  located in the same position along the axial direction as the third and fourth excitation windings  43   c ,  43   d  and the second sensor winding  45   b.    
       FIG. 13  schematically shows the main components of the first ASIC  35   a . A first quadrature signal generator  151   a  generates a quadrature pair of signals which in this embodiment have a frequency of 5 kHz (hereafter called the modulation frequency). A second quadrature signal generator  151   b  generates a quadrature signal at a first carrier frequency which is equal to the resonant frequency of the first resonant circuit  47   a , which in this embodiment is 3.75 MHz. A third quadrature signal generator  151   c  generates a quadrature signal at a third carrier frequency which is equal to the resonant frequency of the second resonant circuit  47   b , which in this embodiment is 1.875 MHz. 
     The quadrature pair of signals at the modulation frequency are input to a first modulating arrangement  153   a  which modulates the in-phase signal I 1  at the modulation frequency by the in-phase signal at the first carrier frequency to generate a signal I 1 (t) and modulates the quadrature signal at the modulation frequency by the in-phase signal I 1  at the first carrier frequency to generate a signal Q 1 (t). The quadrature pair of signals at the modulation frequency are also input to a second modulating arrangement  153   b  which modulates the in-phase signal at the modulation frequency by the in-phase signal I 2  at the second carrier frequency to generate a signal I 2 (t) and modulates the quadrature signal at the modulation frequency by the in-phase signal I 2  at the second carrier frequency to generate a signal Q 2 (t). 
     The signals I 1 (t) and I 2 (t) are then input into a first digital mixer  155   a  which combines the signals I 1 (t) and I 2 (t), and the resultant combined signal is amplified by a first coil driver  157   a . The amplified signal output by the first coil driver  157   a  is supplied to the first excitation winding  43   a . The signals Q 1 (t) and Q 2  (t) are input to a second digital mixer  155   b  and the resultant combined signal is amplified by a second coil driver  157   b  and supplied to the second excitation winding  43   b.    
     The signal components supplied to the first and second excitation windings  43   a ,  43   b  at around the first carrier frequency induce a resonant signal in the first resonant circuit  47   a  which varies in accordance with the radial position of the first shaft  1 . The resonant signal induced in the first resonant circuit  47   a  in turn induces a signal in the first sensor winding  45   a . Similarly, the signal components  3 Q supplied to the first and second excitation windings  43   a ,  43   b  at around the second carrier frequency induce a resonant signal in the second resonant circuit  47   b , which in turn induces a signal in the first sensor winding  45   a.    
     As set out in UK Patent Application GB 2374424A, when the signal induced in the first sensor winding  45   a  is input into a first synchronous detector  159   a  which performs synchronous detection using the quadrature signal Q 1  at the first carrier frequency, the resultant signal output by the first synchronous detector  159   a  has a component at the modulation frequency whose phase depends on the angular position of the first shaft  1 . This phase is detected by a first phase detector  161   a . Similarly, when the signal induced in the first sensor winding  45   a  is input into a second synchronous detector  159   b  which performs synchronous detection using the quadrature signal Q 2  at the second carrier frequency, the resultant signal output by the second synchronous detector  159   b  has a component at the modulation frequency whose phase depends on the angular position of the second shaft  11 . This phase is detected by a second phase detector  161   b.    
       FIG. 14  shows a graph indicating the relationship between the phase detected by the first phase detector  161   a  and the angular position of the first shaft  1 . As a result of the twenty periods of the excitation windings over a full revolution, as shown in  FIG. 14  the phase detected by the first phase detector  161   a  corresponds to twenty different absolute rotary positions of the first shaft  1 . Similarly, the phase detected by the second phase detector  161   b  corresponds to twenty different rotary positions of the second shaft  11 . The first ASIC  35   a  is therefore unable to determine the absolute rotary positions of the first and second shafts  1 , 11  (in this specification absolute rotary position refers to the rotary position of a shaft relative to a reference position; as the shafts can rotate around two full revolutions the absolute position does not unambiguously give the position of a shaft within its entire rotary range of movement). 
     Despite the first and second shafts  1 ,  11  being able to rotate over a range of approximately 720°, the relative rotary displacement between the first and second shafts  1 ,  11  is never more than a few degrees, which is well within one period of the readings. A processor  163  is therefore able to calculate and output the relative rotary displacement between the first and second shafts  1 ,  11 , thereby allowing the torque to be calculated by the central control unit  49 . 
     In the second sensing arrangement  41   b , the second ASIC  35   b  is substantially identical to the first ASIC  35   a  except that the first carrier frequency is set to 5 MHz and the second carrier frequency is set to 2.5 MHz. As discussed, the periodicity of the excitation windings and the resonant circuits in the second sensing arrangement  41   b  corresponds to nineteen periods over 3600. Therefore, as shown in  FIG. 15 , each phase reading corresponds to nineteen possible rotary positions. Again, as for the first sensing arrangement  41   a  although ambiguity exists in the absolute position measurement using the second sensing arrangement  41   b , the range of relative rotary displacement between the first shaft  1  and the second shaft  11  is significantly less than one period and accordingly the relative rotary displacement may be unambiguously calculated. 
     In this embodiment, the ASIC  35  of each sensing arrangement  41  outputs the respective calculated relative rotary displacement to a central control unit  49  of the car, and also outputs the detected phase angles to the central control unit  49  of the car. The central control unit  49  calculates the torque using the calculated relative rotary displacement. Further, although each individual detected phase angle can not be converted unambiguously to an absolute position measurement, due to the difference between the periodicity of the first sensing arrangement  41   a  and the second sensing arrangement  41   b  the central control unit  49  is able to determine an absolute position measurement using the phase readings form both sensing arrangements  41  using a Vernier-type calculation. 
     The particular arrangement of the excitation windings  43 , sensor windings  45  and resonant circuits  47  of the illustrated embodiment has a number of advantages. In particular:
     (1) By using plural periodically-spaced current loop structures in the resonant circuits the signal induced in each resonant circuit is increased in comparison with a resonant circuit having a single current loop structure.   (2) By arranging the current loop structures in a circumferential plane with each current loop structure having an opposing current loop structure, the sensitivity to any ambient electromagnetic field is reduced. Further, the sensitivity to slight misalignments, between the axis of rotation of the shafts and the centre of the circular path of the transmit and receive aerials is reduced. In addition, such a cylindrical geometry gives an increased tolerance to axial misalignment between the transmit/receive aerials and the resonators.   (3) By circumferentially spacing apart the current loops associated with each resonator in each sensing arrangement, noise caused by coupling between adjacent resonant circuits is reduced.   (4) By axially spacing the excitation windings, sensor winding and current loop structures of the resonant circuits for the first sensing arrangement from the excitation windings, sensor winding and current loop structures of the resonant circuits for the second sensing arrangement, noise caused by coupling between the sensing arrangements is reduced.   (5) By employing the described castellated arrangement, it has been found that the performance for a given axial distance over which the inductive sensor extends is improved.   

     MODIFICATIONS AND FURTHER EMBODIMENTS 
     As stated above, using plural periodically-spaced current loop structures in the resonant circuits has the advantage of increasing signal strength. While in the illustrated embodiment the periodic spacing of the current loop structures matches the period of the corresponding transmit aerial, the period of the resonant circuits could be any integer multiple of the period of the corresponding transmit coil. 
     It is not essential to use a plurality of current loop structures in each resonant circuit, and alternatively each resonant circuit could be formed by a single current loop structure. 
     In the illustrated embodiment, the excitation windings, the sensor windings and the current loop structures of the resonant circuits are arranged on circumferential surfaces, leading to advantage (2) above. However, this is not essential and the excitation windings, sensor windings and current loop structures could, for example, be formed on radial surfaces. In an embodiment, the radial surfaces for the current loop structures are provided by the surfaces of disks attached co-axially to the first and second shafts. 
     While the axial spacing of the sensing arrangements is preferred, it is not essential. In an alternative embodiment, the current loop structures for the first and third resonant circuits (which rotate with the first shaft) are located on the first shaft at a common axial position and the current loop structures for the second and fourth resonant circuits (which rotate with the second shaft) are located on the second shaft at a common axial position which is spaced axially apart from the common axial position of the first and third resonant circuits. In this alternative embodiment, the transmit aerials and receive aerials for the first and second sensing arrangements extend over an axial extent encompassing all the current loop structures. One advantage of such an arrangement is that it does not require interlocking castellated portions, and accordingly a full range of relative rotary displacement from −180° to +180° can be measured. Further, the current loop structures for resonant circuits formed in the same axial position need not be circumferentially spaced apart, and in an embodiment the resonant circuits could be formed by respective series of current loops which extend entirely around the respective shaft. 
     In the illustrated embodiment, in each sensing arrangement the associated resonant circuits are simultaneously energised, and the resultant signals induced in the sensor winding input into parallel processing paths to allow the phase angles at the modulation frequency associated with each resonant circuit to be measured in parallel. Alternatively, the resonant circuits could be alternately energised and the resultant signal induced in the sensor winding input into a single processing path in which the frequency of the synchronous detection is alternated in accordance with the energised resonant circuit. 
     In the illustrated embodiment, the excitation signal generating and sensed signal processing circuitry employs the general principles disclosed in GB 2374424A. However, alternative forms of excitation signal generating and sensed signal processing could be employed. For example, instead of having two excitation windings in the transmit aerial and detecting the phase of a signal induced in receive aerial formed by a single sensor winding, the transmit aerial could be formed by a single excitation winding and the receive aerial could be formed by two sensor windings, with the coupling between the excitation winding and the two sensor windings varying with rotary position. The general principles of such a rotary encoder are discussed in WO 95/31696. 
     In the described embodiment, carrier frequencies from 1.875 MHz to 5 MHz are used. It will be appreciated that the exact values of the carrier frequencies (and accordingly the resonant frequencies of the resonant circuits) is a design choice, although preferably the carrier frequencies are in the range 100 kHz to 10 MHz to achieve good signal coupling with comparatively cheap excitation and synchronous detection circuitry. The modulation frequency is also a design choice. 
     In the illustrated embodiment, the two sensing arrangements have a periodicity of twenty periods over 360° and nineteen periods over 360° respectively to enable absolute position measurement be carried out. It will be appreciated that alternative periodicities could be used. Further, if absolute position measurement was not required then the periodicities for the first and second sensing arrangements could be identical. 
     While in the illustrated embodiment absolute position measurement is obtained only with reference to the housing, it will be appreciated that the absolute position within the entire range of rotary movement could be measured by either employing an extra sensor to count revolutions or by continuously monitoring the rotary position in order to keep track of the revolutions. 
     Although separate ASICs are used for the two sensing arrangements in the illustrated embodiment for safety reasons, this is not essential and in many applications a common ASIC could be used for both sensing arrangements while still satisfying safety requirements. In some applications, the redundant sensing arrangement is not necessary and the redundant sensing arrangement accordingly need not be included. 
     In the illustrated embodiment, the output of each ASIC is representative of the relative rotary displacement between the first and second shafts and the central control unit determines the applied torque. It will be appreciated that the ASIC could perform linearisation and/or calibration processing. In an alternative embodiment, the ASIC could determine the applied torque. In another alternative embodiment, the output of each ASIC is representative of the detected phase angles associated with the first and second shafts, and the central control unit calculates both the relative rotary displacement and the applied torque. 
     It will be appreciated that there are many conventional signalling systems which could be used to transfer data between the ASICs and the central control unit, e.g. pulse width modulation or pulse code modulation. 
     Although ASICs are used in the illustrated embodiment, this is not essential and any other processing means could be employed, including using discrete electronic components. 
     In the illustrated embodiment, the aerials and resonant circuits are formed using PCB technology. This is not essential and other techniques for arranging conductive tracks, including arranging wire tracks, could alternatively be used. 
     In the illustrated embodiment, the torque sensor measures the torque applied to a steering wheel of a car. It will be appreciated that there are many other places in a car where a torque is applied and the inductive sensor according to the invention could be used. For example, the torque applied to a drive shaft could be measured. Further, the inductive torque sensor of the present invention also has application outside of the automotive industry. For example, the inductive sensor of the present invention could be used to measure the torque applied to a drill. 
     In the illustrated embodiment, a torsion bar arrangement is used in which two bars are fixed to each other, and relative rotary movement between the two bars is measured. This is generally advantageous in applications where the torsion bar must be made of a stiff material. However, in other applications a less stiff material may be acceptable, in which case the twisting of a single bar could be measured. In other words, the relative rotary displacement between two axial positions of the same member could be measured.