Position sensor

• US 20040233178A1
• Filed: 05/06/2004
• Published: 11/25/2004
• Est. Priority Date: 05/21/2001
• Status: Active Grant

• Current Assignee: Synaptics (UK) Ltd. (Synaptics Incorporated)
• Original Assignee: Synaptics (UK) Ltd. (Synaptics Incorporated)
• Inventors: Ely, David T E, Silk, Christopher J, McCaughan, Gareth J, Collins, Ian, Haines, Julian, Errington, Andrew, Bolender, Robert, Foote, Geoffrey
• Granted Patent: US 7,511,705 B2
• Time in Patent Office: Days
• US Class Current: 345/179
• CPC Class Codes: G06F 1/3203 Power management, i.e. event-based initiation of a power-saving mode
• CPC Class Codes: G06F 1/3262 Power saving in digitizer or tablet
• CPC Class Codes: G06F 2203/0339 Touch strips, e.g. orthogonal touch strips to control cursor movement or scrolling; single touch strip to adjust parameter or to implement a row of soft keys
• CPC Class Codes: G06F 3/03545 Pens or stylus
• CPC Class Codes: G06F 3/03547 Touch pads, in which fingers can move on a surface
• CPC Class Codes: G06F 3/046 by electromagnetic means

Abstract

A low cost x-y digitising system is described for use in consumer electronic devices, such as portable digital assistants, mobile telephones, web browsers and the like. The digitiser includes a resonant stylus, an excitation winding for energising the resonant stylus and a set of sensor windings for sensing the signal generated by the stylus, from which the x-y position of the stylus is determined. A novel stylus design is described together with novel digitiser windings and novel excitation and processing circuitry.

Claims

1. A position indicator for use with a position detector, the position indicator comprising:

a housing;

a moveable nib extending from the housing and moveable relative to the housing between a retracted position and an extended position;

a sensing coil;

a first flux linkage element which extends at least 10 partially through the coil;

wherein said sensing coil and said first flux linkage element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib;

characterised by a second flux linkage element whose position is fixed relative to said sensing coil and which is arranged relative to the first flux linkage element so that the distance between said first and second flux linkage elements varies with the movement of said nib relative to the housing.

2. A position indicator according to claim 1, wherein said second flux linkage element comprises a washer.

4. A position indicator according to claim 2, wherein said washer has a non-circular central hole.

5. A position indicator according to claim 24, wherein said sensing coil is fixed within said housing, wherein means is provided for transmitting movement of said nib to said flux linkage element so that said nib and said flux linkage element are slidably mounted within said housing and wherein said movement transmitting means acts on said first flux linkage element through a central hole of said washer.

6. A position indicator according to claim 5, wherein said movement transmitting means is formed integrally with said nib.

7. A position indicator according to claim 2, wherein said washer comprises a magnetically permeable material.

8. A position indicator according to claim 7, wherein said magnetically permeable material is conductive.

9. A position indicator according to claim 1, comprising a spring for spring biasing said nib in said extended position.

10. A position indicator according to claim 9, wherein said spring applies said biasing force on said nib through said first flux linkage element.

3. A position indicator for use with a position detector, the position indicator comprising:

a housing;

a moveable nib extending from the housing and moveable relative to the housing between a retracted position and an extended position;

a sensing oil;

a flux linkage element which extends at least partially through the coil;

wherein said sensing coil and said flux linkage 35 element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib;

characterised by a magnetically permeable washer whose position is fixed relative to said sensing coil and which is arranged relative to the flux linkage element so that the distance between said washer and said flux linkage element varies with the movement of said nib relative to the housing.

11. A stylus for use with a position detector, the stylus comprising:

an elongate housing;

a moveable nib mounted at one end of the housing for axial movement relative thereto in a first direction from a retracted position to an extended position and in a second, opposite direction from the extended position to the retracted position;

a sensing coil;

a flux linkage element; and

wherein the sensing coil and the flux linkage element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib;

characterised in that the nib comprises a shank with an enlarged head at one end and a tip at its other end, the head comprising a first axially facing abutment surface facing towards the tip, and the housing comprising a second axially facing abutment surface facing away from said one end of the housing and co-operable with said first abutment surface to limit movement of the nib relative to the housing in said first direction.

12. A stylus according to claim 11, wherein said enlarged head comprises a third axially facing abutment surface facing away from said tip and wherein the coil or the flux linkage element comprises a fourth axially facing abutment surface facing towards said tip and co-operable with said third abutment surface to limit movement of the nib relative to the housing in said second direction.

13. A stylus according to claim 12, wherein said abutment surfaces are arranged so that the movement of said tip is less than 0.5 mm.

14. A stylus according to claim 12, further comprising a capacitor connected across the ends of said coil to form a resonant circuit, and wherein the movement of said tip changes the resonant frequency of said resonant circuit.

15. A stylus according to claim 14, wherein said change in resonant frequency is between 4% and 12% of said resonant frequency.

16. A stylus according to claim 15, wherein said sensing coil is fixed within said housing and wherein said stylus further comprises means for transmitting movement of said nib to said flux linkage element.

17. A stylus according to claim 16, wherein said movement transmitting means is integrally formed with said enlarged head of said nib.

18. A stylus according to claim 16, further comprising a magnetic washer whose position is fixed relative to said coil and wherein said movement transmitting means acts on said flux linkage element through a central hole of said washer.

19. A stylus according to claim 18, wherein said washer comprises a magnetically permeable material.

20. A stylus according to claim 11, further comprising a spring for spring-biasing said nib in said extended position.

21. A stylus according to claim 20, wherein the biasing force of said spring acts on said nib through said flux linkage element.

22. A stylus for use with a position detector, the stylus comprising:

an elongate housing;

a moveable nib mounted at one end of the housing for axial movement relative thereto in a first direction from a retracted position to an extended position and in a second, opposite direction from the extended position to the retracted position;

a sensing coil;

a flux linkage element; and

wherein the sensing coil and the flux linkage element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib;

characterised in that the nib comprises a shank with an enlarged head at one end and a tip at its other end, the head comprising a first axially facing abutment surface facing towards the tip, and the coil or the flux linkage element comprising a second axially facing abutment surface facing towards said tip and co-operable with said first abutment surface to limit movement of the nib relative to the housing in said second direction.

23. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energized, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonator frequency and which varies with the relative position of said first and second members;

excitation means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses, for energising said resonator; and

processing circuitry for processing said sensed signal to determine the relative position of said first and second members and to estimate the resonant frequency of said resonator;

characterised by control circuitry for controlling said excitation means and said processing circuitry to operate;

i) in a first stage in which said excitation means is operable to generate an excitation field comprising a first number of excitation pulses at a predetermined excitation frequency and in which said processing circuitry is operable to process the resulting sensed signal to estimate the resonant frequency of said resonator; and

ii) in a second stage in which said excitation means is operable to generate an excitation field comprising a second, greater number of excitation pulses substantially at the resonant frequency estimated in said first stage, and in which said processing circuitry is operable to process the resulting sensed signal to determine the relative position of said first and second members and/or to determine a more accurate estimate of the resonant frequency of said resonator.

24. A position detector according to claim 23, wherein said processing circuitry is operable to process said sensed signal over a period of time corresponding to a number of said excitation pulses and wherein said control circuitry is operable to control said processing circuitry so that in said first stage, said processing circuitry is operable to process said sensed signal over a period of time corresponding to a first number of said excitation pulses and, in said second stage, to cause said processing circuitry to process the sensed signal over a period of time corresponding to a second, greater number of said excitation pulses.

25. A position detector according to claim 23, wherein said second member comprises a plurality of said sensing means, each operable to generate a sensed signal which varies with time at said resonator frequency and which varies with the relative position of said first and second members, and wherein said processing circuitry is operable to process the sensed signal output from all of said sensing means.

26. A position detector according to claim 25, wherein said processing circuitry comprises:

a first mixer for mixing the sensed signal with a first time varying mixing signal having said excitation frequency to generate a first mixed signal;

a first integrator for integrating the first mixed signal over a time period corresponding to a number of said excitation pulses, to generate a first signal value;

a second mixer for mixing the sensed signal with a second time varying mixing signal having said excitation frequency, the second time varying mixing signal having a phase offset relative to said first time varying mixing signal, to generate a second mixed signal; and

a second integrator for integrating the second mixed signal over a time period corresponding to the number of said excitation pulses, to generate a second signal value.

27. A position detector according to any of claims 23, wherein said sensing means comprises a sensor winding for sensing a magnetic field generated by said resonator.

28. A position detector according to claim 27, wherein said sensor winding comprises at least one loop of conductor.

29. A position detector according to claim 28, wherein said sensor winding comprises first and second loops of conductor which are arranged in succession along a measurement direction and connected so that an EMF induced in the first loop by a common background alternating magnetic field opposes the EMF induced in the second loop.

30. A position detector according to claim 23, wherein said sensing means is formed as one or more conductive tracks on a printed circuit board.

31. A position detector according to claim 23, wherein said resonator comprises a coil and a capacitor.

32. A position detector according to claim 23, wherein said excitation means is capacitively coupled to said resonator.

33. A position detector according to claim 23, wherein said excitation means is magnetically coupled to said resonator.

34. A position detector according to claim 33, wherein said excitation means comprises one or more tracks of conductor on a printed circuit board.

35. A position detector according to claim 23, wherein said control circuit is operable to control said processing circuitry such that in said second stage, said processing circuitry processes the resulting sensed signal to determine the relative position of said first and second members and to determine a more accurate estimate of the resonant frequency of said resonator.

37. A position detector according to claim 23, wherein said control circuitry comprises means for comparing signals sensed by said sensing means in said first stage with a threshold and means for inhibiting the performance of said second stage in dependence upon a comparison result.

38. A position detector according to claim 23, wherein said control circuitry further comprises means for varying the number of excitation pulses transmitted during said second stage.

39. A position detector according to claim 23, wherein said processing circuitry is operable to determine an initial estimate of the relative position of said first and second members using the signal sensed by said sensing means during said first stage.

40. A position detector according to claim 23, wherein said first member comprises a plurality of states, each associated with a different resonant frequency of said resonator and wherein said processing circuitry is operable to compare said determined resonant frequency to determine a current state of said first member.

36. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energised, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonator frequency and which varies with the relative position of said first and second members;

excitation means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses, for energising said resonator; and

processing circuitry for processing said sensed signal over a period of time corresponding to a number of said excitation pulses, to estimate the resonant frequency of said resonator and to determine the relative position between, said first and second members;

characterised by control circuitry for controlling said excitation means and said processing circuitry to operate;

i) in a first stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses at a predetermined excitation frequency, in which said processing circuitry is operable to process said sensed signal over a period of time corresponding to a first number of said excitation pulses, to estimate the resonant frequency of said resonator; and

ii) in a second stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses substantially at the resonant frequency estimated in said first stage, and in which said processing circuitry is operable to process said sensed signal over a period of time corresponding to a second greater number of said excitation pulses, to determine a more accurate estimate of said resonant frequency of said resonator and/or to determine the relative position between said first and second members.

41. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energized, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonator frequency and which varies with the relative position of said first and second members;

excitation means for generating a time varying 25 excitation electromagnetic field comprising a sequence of excitation pulses at an excitation frequency, for energising said resonator; and

processing circuitry for processing said sensed signal to determine the relative position of said first and second members and to estimate the resonant frequency of said resonator, the processing circuitry comprising;

a first mixer for mixing the sensed signal with a first time varying mixing signal having said excitation frequency to generate a first mixed signal;

a first integrator for integrating the first mixed signal over a time period corresponding to a number of said excitation pulses, to generate a first signal value;

a second mixer for mixing the sensed signal with a second time varying mixing signal having said excitation frequency, the second time varying mixing signal having a phase offset relative to said first time varying mixing signal, to generate a second mixed signal;

a second integrator for integrating the second mixed signal over a time period corresponding to a number of said excitation pulses, to generate a second signal value;

first processing means for processing said first and second signal values to estimate the resonant frequency of said resonator; and

second processing means for processing at least one of said signal values to determine the relative position between said first and second members;

characterised by control circuitry for controlling said excitation means and said processing circuitry to operate;

i) in a first stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses at a predetermined excitation frequency, in which said first and second integrators are operable to integrate said mixed signals over a time period corresponding to a first number of said excitation pulses and in which said first processing means is operable to process the resulting signal values to estimate the resonant frequency of said resonator; and

ii) in a second stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses substantially at the resonant frequency estimated in said first stage, in which said first\ and second integrators are operable to integrate said mixed signals over a time period corresponding to a second, greater number of excitation pulses, in which said first processing means is operable to process the resulting signal values to determine a second, more accurate estimate of the resonant frequency of said resonator and in which the second processing means is operable to process the resulting at least one signal value to determine the relative position of said first and second members.

42. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energised, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

energising means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses at an excitation frequency, for energising said electromagnetic resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonator frequency and varies with the relative position of said first and second members;

processing means for processing said sensed signal to estimate the resonant frequency of said resonator and to determine the relative position between said first and second members; and

first varying means for varying said energising means so that in a subsequent excitation process the frequency of the excitation field generated thereby is substantially at the resonant frequency estimated during a current excitation process;

characterised by second varying means for varying the number of excitation pulses generated by said energising means during said subsequent excitation process.

43. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energised, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

energising means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses at an excitation frequency, for energising said electromagnetic resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonator frequency and varies with the relative position of said first and second members;

processing means for processing said sensed signal over a period of time corresponding to a number of said excitation pulses, to estimate the resonant frequency of said resonator and to determine the relative position between said first and second members; and

first varying means for varying said energising means so that in a subsequent excitation process the frequency of the excitation field generated thereby is substantially at the resonant frequency estimated during a current excitation process;

characterised by second varying means for varying the period of time over which said processing means processes said sensed signal during said subsequent excitation process.

44. A position detector comprising:

first and second relatively moveable members,

the first member comprising an electromagnetic resonator which, when energised, is operable to generate a resonator field which varies with time at a resonant frequency of the resonator;

energising means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses at an excitation frequency, for energising said electromagnetic resonator;

the second member comprising means for sensing the resonator field and for outputting a sensed signal in response, which sensed signal varies with time at said resonant frequency and varies with the relative position of said first and second members;

a first mixer for mixing the sensed signal with a 15 first time varying mixing signal having at said excitation frequency to generate a first mixed signal;

a first integrator for integrating the first mixed signal over a time period corresponding to a predetermined number of said excitation pulses, to generate a first signal value;

a second mixer for mixing the sensed signal with a second time varying mixing signal at said excitation frequency, the second time varying mixing signal having a phase offset relative to said first time varying mixing signal, to generate a second mixed signal;

a second integrator for integrating the second mixed signal over a time period corresponding to said predetermined number of said excitation pulses, to generate a second signal value;

first processing means for processing said first and second signal values to estimate the resonant frequency of said resonator;

second processing means for processing at least one of said signal values to determine the relative position of said first and second members; and

first varying means for varying said energising means so that in a subsequent excitation process the frequency of the excitation field generated thereby is substantially at the resonant frequency estimated during a current excitation process;

characterised by second varying means for varying the period of time over which said first and second integrators integrate said first and second mixed signals during said subsequent excitation process.

45. Excitation and processing electronics for use with a position detector used to determine the position of a resonator relative to at least one sensor, the resonator being operable to generate a time varying resonator signal at a resonant frequency of the resonator and the sensor being operable to sense the resonator signal and to output a sensed signal at said resonator frequency which varies with the relative position between said resonator and said sensor, the excitation and processing circuitry comprising:

excitation means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses, for energising said resonator;

processor circuitry for processing said sensed signal to estimate the resonant frequency of said resonator and to determine the relative position between said resonator and said sensor; and

control circuitry for controlling said excitation means and said processing circuitry to operate;

i) in a first stage in which excitation means is operable to generate an excitation field comprising a number of excitation pulses at a predetermined excitation frequency and in which said processing circuitry is operable to process the resulting sensed signal to estimate the resonant frequency of said resonator; and

ii) in a second stage in which said excitation means is operable to generate an excitation field comprising a second, greater number of excitation pulses substantially at the resonant frequency estimated in said first stage, and in which said processing circuitry as operable to process the resulting sensed signal to determine the relative position of said resonator and said sensor and/or to determine a more accurate estimate of the resonant frequency of said resonator.

46. Excitation and processing electronics for use with a position detector used to determine the position of a resonator relative to at least one sensor, the resonator being operable to generate a time varying resonator signal at a resonant frequency of the resonator and the sensor being operable to sense the resonator signal and to output a sensed signal at said resonator frequency which varies with the relative position between said resonator and said sensor, the excitation and processing circuitry comprising:

excitation means for generating a time varying excitation electromagnetic field comprising a sequence of excitation pulses, for energising said resonator;

processor circuitry for processing said sensed signal over a period of time corresponding to a number of said excitation pulses, to estimate the resonant frequency of said resonator and to determine the relative position between said resonator and said sensor; and

control circuitry for controlling said excitation means and said processing circuitry to operate;

i) in a first stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses at a predetermined excitation frequency, in which said processing circuitry is operable to process said sensed signal over a period of time corresponding to a first number of said excitation pulses, to estimate the resonant frequency of said resonator; and

ii) in a second stage in which said excitation means is operable to generate an excitation field comprising a number of excitation pulses substantially at the resonant frequency estimated in said first stage, and in which said processing circuitry is operable to process said sensed signal over a period of time corresponding to a second, greater number of said excitation pulses, to determine a more accurate estimate of said resonant frequency of said resonator and/or to determine the relative position between said first and second members.

47. A resonant position indicating member for use in a position detector, the resonant position indicator comprising:

a housing;

a moveable nib extending from the housing and moveable relative to the housing between a retracted position and an extended position;

a resonator;

a flux linkage element which extends at least partially through a coil of said resonator;

wherein said resonator coil and said flux linkage element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib to thereby vary the resonant frequency of said resonator;

characterised in that the resonant position indicator comprises means for limiting the movement of said nib to a distance which is less than 0.5 mm and wherein said coil and said flux linkage element are arranged so that the resonant frequency of said resonator varies by more than 5% of the resonator frequency between said extended and retracted positions.

48. A resonant position indicator according to claim 47, further comprising a magnetic washer whose position is fixed relative to said coil.

49. A resonant position indicator according to claim 48, wherein said flux linkage element is elongate and wherein one end of said flux linkage element is operable to move closer to or further away from said washer with the movement of said tip between said retracted and extended positions.

50. A resonant position indicator according to claim 49, wherein the end of said flux linkage element closest to said washer is further away from said washer when said nib is in said retracted position than when the nib is in said extended position.

51. A resonant position indicator according to claim 47, wherein said flux linkage element only partially extends through said coil of said resonator when said nib is in said retracted or in said extended position.

52. A position sensor including:

first and second members which are relatively movable along a measuring path, the first member carrying a transmitter which, in use, is electromagnetically coupled to a receiver carried by the other member, which transmitter and receiver are arranged so that in response to the transmission of a signal by said transmitter, there is induced in said receiver first and second output signals which vary with the relative position of the first and second members along said measuring path; and

means for determining the relative position of the first and second members along said measured path as a predetermined function of said first and second output signals;

wherein at least one of the transmitter and receiver comprises first and second circuits, each circuit having a plurality of primary sensing conductors which are separated from each other along said path, which cross said path and which are connected together to form at least one loop; and

wherein one or more of said primary sensing conductors of said first circuit have multiple bends whose positions are determined in order to compensate for position errors caused by the position of the primary sensing conductors of the second circuit through the operation of said predetermined function on said first and second output signals.

53. A position sensor according to claim 52, wherein one or more of said primary sensing conductors of each of said first and second circuits have multiple bends whose positions are determined in order to compensate for position errors caused by the position of the primary sensing conductors of the other circuit through the operation of said predetermined function on said first and second output signals.

54. A position sensor according to claim 52, wherein said primary sensing conductors are connected together to form at least two loops which are arranged along said measuring path and which are connected in series and arranged so that EMFs induced in adjacent loops by a common background alternating magnetic field oppose each other.

55. A position sensor according to claim 52, wherein the at least one loop of said second circuit is spatially separated along said measuring path from the at least one loop of said first circuit.

56. A position sensor according to claim 55, wherein said at least one loop of said second circuit is spatially separated along said measuring path by half the extent of the at least one loop of said first circuit.

57. A position sensor according to claim 52, wherein said primary sensing conductors are connected together to form at least one set of loops which are connected in series and arranged so that EMFs induced in 15 loops of the same set by a common alternating magnetic field add together.

58. A position sensor according to claim 52, wherein said transmitter comprises an electromagnetic device and wherein said second member comprises an excitation circuit for energising the electromagnet device.

59. A position sensor according to claim 58, wherein said electromagnetic device comprises an electromagnet resonator.

60. A position sensor according to claim 58, further comprising drive means for applying an input driving signal to said excitation circuit.

61. A position sensor according to claim 60, wherein said drive means is operable to apply a pulse of said driving signal during a first time interval and wherein said determining means is operable to determine the relative position of said first and second members from said first and second signals induced in said receiver during a second time interval after the first time interval.

62. A position sensor according to claim 52, wherein said predetermined function includes a ratio calculation of the first and second output signals.

63. A position sensor according to claim 62, wherein said predetermined function includes a trigonometric ratio calculation of the first and second output signals.

64. A position sensor according to claim 52, wherein said receiver comprises said first and second circuits.

65. A position sensor according to claim 64, wherein said primary sensing conductors are carried on a planar surface of said second member.

66. A position sensor according to claim 65, wherein said transmitter comprises a coil whose axis is generally perpendicular to said planar surface of said second member.

67. A position sensor according to claim 52, wherein said transmitter comprises said first and second circuits.

68. A position sensor according to claim 52, wherein said second member is fixed and wherein said first member is movable with respect to said second member.

69. A position sensor according to claim 52, wherein said measuring path is linear.

70. A position sensor according to claim 52, wherein said at least one of said transmitter and receiver further comprises third and fourth circuits, each having a plurality of primary sensing conductors which are separated from each other along a second measuring path which is substantially orthogonal to the other measuring path, whereby said position sensor is operable to determine the relative position of said first and second members in two dimensions.

71. A position sensor according to claim 52, wherein said measuring path is curved.

72. A position sensor according to claim 52, wherein said primary sensing conductors are connected together to form at least two sets of loops arranged 20 along said path, each loop extending along said path and said loops being connected in series and being arranged so that EMFS induced in loops of the same set by a common alternating magnetic field add together and so that EMFs induced in the first set of loops by a common alternating magnetic field oppose the EMFs induced in the second set of loops.

73. A position sensor according to claim 72, wherein each set of loops comprises the same number of loops.

74. A position sensor according to claim 72, wherein each set of loops is arranged to enclose substantially the same area.

75. A position sensor according to claim 52, wherein said first and second circuits are formed as conductive tracks on a printed circuit board.

76. A position sensor according to claim 75, wherein said printed circuit board is a two-layer printed circuit board.

77. A position sensor according to claim 75, wherein said printed circuit board is a flexible printed circuit board.

78. cancelled.

79. A method of manufacturing a resonant position indicator, the method comprising the steps of:

providing a housing;

inserting a nib into one end of the housing so that 25 the nib extends from the housing and is moveable relative to the housing between a retracted position and an extended position;

inserting a resonator into the housing, the resonator comprising a coil and a capacitor;

inserting a flux linkage element into the housing, the flux linkage element extending at least partially through the coil of said resonator;

wherein the resonator coil and the flux linkage element are mounted for relative movement with the movement of said nib, whereby the inductance of said coil is changed with the movement of said nib in order to change the resonant frequency of said resonator;

determining the resonant frequency of said resonator when said nib is in said retracted position and when said nib is in said extended position; and

comparing the resonant frequencies determined in said determining step with desired resonant frequencies.

80. A method according to claim 79, further comprising the step of discarding the current position indicator if said comparing step determines that said resonant frequencies are not within a predetermined amount of said desired resonant frequencies.

81. A method according to claim 79, further comprising the step of varying the resonant frequency of said resonator in dependence upon the results of said comparing step.

82. A method according to claim 81, wherein said varying step comprises the step of varying the positional relationship between the said flux linkage element and said coil when said nib is in said retracted position and said extended position.

83. A method according to claim 79, further comprising the step of measuring the capacitance of said capacitor and varying the number of turns of conductor forming said coil in dependence upon the sensed capacitor value.

84. A method according to claim 83, wherein said means for varying the number of turns of conductor of said coil is operable to vary the number of turns in accordance with a predetermined characteristic relating the number of turns to measured capacitor value in order to form a resonant circuit having a predetermined resonant frequency.

85. A method according to claim 79, further comprising the step of gluing a rear portion of said housing to a front portion of said housing after said determining and comparing steps.