Position detector

A position detector is provided for detecting the relative movement of first and second members which are mounted for relative movement along a measuring path. One of the members comprises a magnetic field generator for generating a magnetic field and the other member comprises first and second conductors which are inductively coupled to said magnetic field generator. The arrangement of the first and second conductors and the magnetic field generator is such that output signals are generated in a first and second receive circuits whose position varies with the relative movement between the two members. In addition to carrying information relating to the relative position between the two members, the signals induced in the receive circuits also comprise information defining the relative orientation of the two movable members, and by suitable processing of the received signals the relative orientation of the two members can also be determined. In a preferred form of the invention, the system operates to define the relative position and orientation of the two movable members in first and second directions from which the relative orientation of the two members in a plane containing the two directions can be determined. The signals induced in the receive circuits can also be processed to give an indication of the gap between the two circuits and to provide an indication of the full relative orientation of the two members.

 

What is claimed is:

1. A position detector comprising:

first and second members which are moveable relative to each other along a measuring path;

said first member comprising a magnetic field generator for generating a magnetic field;

said second member comprising first and second conductors which are inductively coupled to said magnetic field generator, the first conductor extending in a geometrically varying manner having a first characteristic dimension along the measuring path and the second conductor extending in a geometrically varying manner having a second different characteristic dimension along the measuring path, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated which varies in dependence upon the relative position and orientation of the first conductor and the magnetic field generator and a second different signal is generated which varies in dependence upon the relative position and orientation of the second conductor and the magnetic field generator; and

means for processing said first and second signals to determine the relative position and orientation of the two moveable members using a relationship between the respective characteristic dimension of the geometrical variation of said two conductors.

2. A position detector according to claim 1, wherein said conductors and said magnetic field generator are arranged so that said first and second signals vary substantially sinusoidally with the relative position of the two moveable members.

3. A position detector according to claim 2, wherein said relative orientation of said two moveable members causes a phase shift in said sinusoidal variations.

4. A position detector according to claim 2, wherein said second member further comprises third and fourth conductors which are inductively coupled to said magnetic field generator, the third conductor extending in a geometrically varying manner having the same characteristic dimension as the first conductor and the fourth conductor extending in a geometrically varying manner having the same characteristic dimension as the second conductor, wherein the first and third conductors are shifted relative to each other along the measuring shifted relative to each other along said measuring path and wherein in response to a magnetic field generated by said magnetic field generator, a third signal is generated which varies in dependence upon the relative position and orientation of the third conductor and the magnetic field generator and a fourth signal is generated which varies in dependence upon the relative position and orientation of the fourth conductor and the magnetic field generator.

5. A position detector according to claim 4, wherein said first and third conductors are spaced along said measuring path so as to form a phase quadrature pair.

6. A position detector according to claim 4, wherein said second and fourth conductors are spaced along said measuring path so as to form a phase quadrature pair.

7. A position detector according to claim 1, wherein said processing means is operable to process said first and second signals to provide a first value which depends upon said relative position and orientation and a second different value which depends upon said relative position and orientation.

8. A position detector according to claim 7, wherein said processing means is operable to determine said relative position and orientation by performing a weighted combination of said first and second values and wherein the weighting applied depends upon the characteristic dimension of the geometrical variation of said conductors.

9. A position detector according to claim 1, wherein said conductors are periodic and wherein said characteristic dimension of the respective conductors comprises the pitch.

10. A position detector according to claim 1, wherein said conductors are tapered, narrowing in from their ends towards a central cross-over point to define a number of substantially triangular shaped loops, and wherein said characteristic dimension comprises the taper of each of the said conductors.

11. A position detector according to claim 1, wherein said processing means is arranged to process said first and second signals to provide (i) a coarse measurement value indicative of the relative position of said two moveable members which is independent of the relative orientation of said two moveable members; and (ii) a fine measurement value indicative of the relative position of said two moveable members which is dependent upon the relative orientation of said two moveable members.

12. A position detector according to claim 11, wherein said relative orientation causes an apparent shift in the fine measurement value relative to the coarse measurement value.

13. A position detector according to claim 12, wherein said shift is approximately twice in the angle of the relative tilt between the two moveable members along the measuring path.

14. A position detector according to claim 12, wherein said angle of tilt is known and used to determine the relative position of said members from said fine measurement value.

15. A position detector according to claim 1, wherein said magnetic field generator comprises a powered coil.

16. A position detector according to claim 1, wherein said magnetic field generator comprises at least one of: a resonator, a short circuit coil and a conductive screen.

17. A position detector according to claim 16, wherein said magnetic field generator comprises an inductor and capacitor resonant circuit.

18. A position detector according to claim 16, wherein said magnetic field generator comprises a ceramic resonator.

19. A position detector according to claim 16, wherein said second member further comprises an excitation circuit for energizing said magnetic field geneator.

20. A position detector according to claim 19, wherein said excitation circuit is arranged to have a substantially constant coupling with said magnetic field generator over the measuring path.

21. A position detector according to claim 20, wherein said first signal is output from said first conductor and wherein said second signal is output from said second conductor.

22. A position detector according to claim 4, wherein said second member further comprises an excitation circuit for energizing said magnetic field generator, which excitation circuit is arranged to have a substantially constant coupling with said magnetic field generator over the measurement path, wherein said first signal is output from said first conductor, said second signal is output from said second conductor, said third signal is output from said third conductor and said fourth signal is output from said fourth conductor, and wherein said processing means is operable to combine the signals output from said first and third conductors and to combine the signals output from said second and fourth conductors in order to derive said position and orientation information.

23. A position detector according to claim 22, wherein each of said signals varies substantially sinusoidally with said relative position along the measuring path and wherein the peak amplitude of said sinusoidal variation varies in dependence upon the gap between said magnetic field generator and said conductors, and wherein said processing means is operable to combine the signals output from the first and third conductors and/or to combine the signals output from the second and fourth conductors to determine an indication of the gap between said first and second members.

24. A position detector according to claim 22, wherein said processing means is operable to extract said positional information by determining a ratiometric arc-tangent of measurements derived from the signals output from said first and third conductors and of measurements derived from the signals output from said second and fourth conductors.

25. A position detector according to claim 24, wherein said processing means is operable to combine the positional information extracted from the signals output from said first and third conductors and the positional information extracted from said signals output from the second and fourth conductors, to provide a coarse position measurement value which does not depend upon the orientation of said two moveable members, and to provide a fine measurement value which does depend upon said relative orientation.

26. A position detector according to claim 19, wherein said excitation circuit comprises one of said first and second conductors.

27. A position detector according to claim 4, wherein said second member further comprises an excitation circuit for energizing said magnetic field generator and wherein said excitation circuit comprises said first and third conductors and wherein said first signal is output from said second conductor and said second signal is output from said fourth conductor.

28. A position detector according to claim 19, further comprising a drive means for applying an energizing signal to said excitation circuit.

29. A position detector according to claim 27, further comprising a drive means for applying an energizing signal to said excitation circuit, which drive means is operable to energize both said first and third conductors and wherein said processing means is operable to process the signals output from said second and fourth conductors as a result of the excitation of said first and third conductors.

30. A position detector according to claim 29, wherein said processing means is operable to combine the signal output from said second conductor when said first conductor is energized with the signal output from said fourth conductor when said third conductor is energized and to combine the signal output from said fourth conductor when said first conductor is energized with the signal output from said second conductor when said third conductor is energized.

31. A position detector according to claim 30, wherein said combination includes obtaining the sum and difference of said signals.

32. A position detector according to claim 30, wherein said processing means is operable to extract positional information from said combined signals by determining a ratiometric arc-tangent of the combined signals to provide a coarse position measurement value which does not depend upon the orientation of said two moveable members, and to provide a fine position measurement value which does depend upon said relative orientation.

33. A position detector according to claim 28, wherein said drive means is operable to apply a pulse of said energizing signal to said excitation circuit during a first time interval and wherein said processing means is operable to process said induced signals during a subsequent second time interval after said first time interval.

34. A position detector according to claim 1, wherein said conductors are arranged to form at least two loops arranged in succession along said measuring path, each loop extending along said path and said loops being connected in series and being arranged so that EMFs induced in adjacent said loops by a common background alternating magnetic field oppose each other.

35. A position detector according to claim 34, wherein said loops have a generally rectangular shape.

36. A position detector according to claim 34, wherein said loops have a generally hexagonal shape.

37. A position detector according to claim 34, wherein each loop comprises one or more turns of conductor.

38. A position detector according to claim 1, wherein said first and second signals are time varying signals whose amplitude varies in dependence upon said relative position and orientation of the two moveable members.

39. A position detector according to claim 38, wherein said processing means comprises a demodulator for demodulating the time varying signals.

40. A position detector according to claim 1, wherein said conductors are formed by wires bonded onto one or more substrates.

41. A position detector according to claim 1, wherein said first and second conductors are formed substantially in the same plane or in substantially parallel planes.

42. A position detector according to claim 1, wherein said second member is fixed and said first member is moveable.

43. A position detector according to claim 1, arranged to detect the relative position and orientation of a plurality of first members, each having a respective magnetic field generator characteristic of the first member.

44. A two dimensional position detector comprising:

first and second members which are moveable relative to each other in first and second directions in a measuring plane;

said first member comprising a magnetic field generator for generating a magnetic field;

said second member comprising:

(i) first and second conductors which are inductively coupled to said magnetic field generator, the first conductor extending in a geometrically varying manner having a first characteristic dimension along said first direction and the second conductor extending in a geometrically varying manner having a second different characteristic dimension along the first direction, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated which varies in dependence upon the relative position and orientation of the first conductor and the magnetic field generator and a second different signal is generated which varies in dependence upon the relative position and orientation of the second conductor and the magnetic field generator; and

(ii) third and fourth conductors which are inductively coupled to said magnetic field generator, the third conductor extending in a geometrically varying manner having a third characteristic dimension along said second direction and the fourth conductor extending in a geometrically varying manner having a fourth characteristic dimension along said second direction different from said third characteristic dimension, as a result of which, in response to a magnetic field generated by said magnetic field generator, a third signal is generated which varies in dependence upon the relative position and orientation of the third conductor and the magnetic field generator and a fourth different signal is generated which varies in dependence upon the relative position and orientation of the fourth conductor and the magnetic field generator;

means for processing said first and second signals to determine the relative position and orientation of the two moveable members in said first direction using a relationship between the respective characteristic dimension of the geometrical variation of said first and second conductors;

means for processing said third and fourth signals to determine the relative position and orientation of the two moveable members along said second direction using a relationship between the respective characteristic dimension of the geometrical variation of said third and fourth conductors; and

means for combining the relative orientations in said first and second directions to determine the relative orientation of said first and second members in said measuring plane.

45. A position detector according to claim 44, wherein said first member comprises first and second magnetic field generators which are operable to generate respective different magnetic fields in substantially different directions, and wherein said processing means is operable to distinguish the signals from the two magnetic field generators to determine said relative position and orientation in said plane.

46. A position detector according to claim 45, wherein the first magnetic field generator is operable to produce a magnetic field substantially in a direction perpendicular to said plane, and wherein said processing means is operable to process the signals from the first magnetic field generator to determine a fine and a coarse position measurement of the relative position of said first and second members, and wherein said second magnetic field generator is operable to generate a magnetic field substantially in a direction which is inclined at a predetermined angle to said plane, and wherein said processing means is operable to process the signals from said second magnetic field generator to determine the relative orientation of said first and second members in said plane.

47. A position detector according to claim 46, wherein said first and second magnetic field generators are coincident with each other.

48. A position detector according to claim 48, wherein said first and second magnetic field generators are separated from each other by a predetermined distance, and wherein said processing means is operable to process the signals from said first and second magnetic field generators to determine the complete relative orientation of said first and second members.

49. A position detector according to claim 48, comprising three coincident magnetic field generators each arranged to generate a magnetic field in different directions and arranged such that said processing means can process the signals from said magnetic field generators and derive the complete relative orientation of said first and second members.

50. A position detector according to claim 44, wherein said magnetic field generators comprise a powered coil and/or a resonator.

51. A position detector according to claim 50, wherein each of said magnetic field generators comprises an inductor and a capacitor resonant circuit.

52. A position detector according to claim 44, wherein the windings used in the two different directions have substantially the same form.

53. A resonator combination for use in a position detector according to claim 1, the resonator combination comprising first and second different resonators each comprising an inductive coil and a capacitor, the combination being such that the centre point of each resonator coil is substantially the same and so that the axis of said coils are inclined relative to each other.

54. A resonator combination according to claim 53, further comprising a third resonator comprising an inductive coil and a capacitor, wherein the centre point of the third resonator coil substantially coincides with the center point of the coils of the other two resonators, wherein the axis of the third resonator coil is tilted to the axis of the other two resonator coils so as to allow said position detector to determine the complete orientation of an object carrying said resonator combination from the signals induced in receive windings forming part of the position detector by the three different resonators.

55. A position detector comprising:

first and second members which are moveable relative to each other in a measuring plane;

said first member comprising a magnetic field generator for generating a magnetic field;

said second member comprising first and second groups of circuits for sensing the relative position and orientation of the first and second members in two different directions in said plane; and

means for determining the relative orientation of the first and second members in said plane using the relative orientations in said first and second directions;

characterized in that each of groups of circuits comprises first and second conductors which are inductively coupled to said magnetic field generator, the first conductor extending in a geometrically varying manner having a first characteristic dimension along the corresponding direction and the second conductor extending in a geometrically varying manner having a second different characteristic dimension along said corresponding direction.

56. A method of manufacturing a plurality of shaped conductors for use in a position detector according to claim 1, the method comprising the steps of:

winding a plurality of conductive wires on a wiring former so as to form a plurality of conductors each extending along a path in a geometrically varying manner having a respective characteristic dimension along the path; and

bonding the wires to one or more substrates.

57. A position detector comprising:

first and second members which are moveable relative to each other along a measuring path;

said first member comprising a magnetic field generator for generating a magnetic field;

said second member comprising first and second circuits each comprising a conductor which is inductively coupled to said magnetic field generator, the conductor of said first circuit extending in a geometrically varying manner having a first characteristic dimension along the measuring path and the conductor of said second circuit extending in a geometrically varying manner having a second different characteristic dimension along the measuring path, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated in said first circuit and second different signal is generated in said second circuit, the first and second signals both varying in dependence upon the relative position and orientation of the two moveable members; and

means for processing said first and second signals to determine said relative position and orientation of said first and second members using a relationship between the respective characteristics dimension of the geometrical variation of said two conductors.

58. A position detector comprising

first and second members which are moveable relative to each other along a measuring path;

said first member comprising a magnetic field generator for generating a magnetic field;

said second member comprising first and second periodic windings which extend along the measuring path and which are inductively coupled to said magnetic field generator, the period of said first winding being different to the period of said second winding, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated in said first circuit and a second different signal is generated in said second circuit, the first and second signals both varying in a substantially sinusoidal manner in dependence upon the relative position and orientation of the two moveable members;

means for processing said first and second signals to provide a first value which depends upon said relative position and orientation and a second different value which depends upon said relative position and orientation; and

means for combining said first and second values to determine said relative position and orientation in dependence upon a relationship between the two periods of said windings.

59. A position detector according to claim 58, wherein said combining means comprises sum and difference means for determining the sum and difference of said first and second values.

60. An x-y digitizing system comprising:

first and second members that are moveable relative to each other in the x-y direction;

said first member comprising a first magnetic field generator for generating a magnetic field substantially in a first direction and a second magnetic field generator for generating a magnetic field substantially in a second direction different from said first direction;

said second member comprising two sets of periodic windings, each set comprising first and second periodic windings, which extend along the measuring path and which are inductively coupled to said first and second magnetic field generators, the period of said first winding being different to the period of said second winding, as a result of which, in response to a magnetic field generated by each of said magnetic field generators, a first signal is generated in said first circuit and a second different signal is generated in said second circuit, the first and second signals both varying in dependence upon the relative position and orientation of the two moveable members;

means for processing said first and second signals from each of said magnetic field generators to provide a first value which depends upon said relative position and orientation and a second different value which depends upon said relative position and orientation; and

means for combining said first and second values to determine said relative x-y position and to determine said relative orientation in dependence upon a relationship between the two periods of said windings.

61. A personal computer comprising a position detector according to claim 1, wherein said second member comprising said conductors are located behind a display of said computer, wherein said first member comprises a pointing device for pointing to positions on said display, and wherein the relative position of said stylus and said display, determined from position detector, are used to control information which is displayed on said display.

62. A method of detecting the relative position and orientation between first and second members that are moveable relative to each other along a measuring path, the method comprising the steps of:

providing a magnetic field generator for generating a magnetic field on said first member;

providing first and second conductors which are inductively coupled to said magnetic field generator on said second member, the first conductor extending in a geometrically varying manner having a first characteristic dimension along the measuring path and the second conductor extending in a geometrically varying manner having a second different characteristic dimension along the measuring path, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated which varies in dependence upon the relative position and orientation of the first conductor and the magnetic field generator and a second different signal is generated which varies in dependence upon the relative position and orientation of the second conductor and the magnetic field generator;

generating a magnetic field using said magnetic field generator;

receiving the first and second signals generated in response to the generation of said magnetic field by said magnetic field generator; and

processing said first and second signals to determine the relative position and orientation of the two moveable members using a relationship between the respective characteristic dimension of the geometrical variation of said two conductors.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to position sensors generally. The invention has particular although not exclusive relevance to non-contact linear and rotary position encoders. The invention is particularly suited for use in systems where the object whose position is being sensed can be tilted relative to the measurement direction.

Many types of non-contact linear and rotary position encoders have been proposed for generating signals indicative of the position of two relatively moveable members. Typically, one of the members carries one or more sensor coils and the other carries one or more magnetic field generators. The magnetic field generators and the sensor coils are arranged such that the amount of magnetic coupling between them varies as a function of the relative position between the two members. This can be achieved by, for example, designing the sensor coils so that their sensitivity to magnetic field varies in a predetermined manner along the measurement path. Alternatively, the magnetic field generators can be designed so that the magnetic field which they generate varies in a predetermined manner along the measurement path.

One example of this type of position encoder is the Inductosyn, which comprises a contactless slider which is arranged to detect the field generated by a stationary track, or vice versa. The stationary track comprises a repeating pattern of conductors which generates a magnetic field of substantially sinusoidal variation in the measurement direction when a current is applied to them. This magnetic field is detected by the moving slider, which comprises sin and cos detector tracks. The position of the two relatively moveable members is then determined from the spatial phase of the signals detected by these two detector tracks.

The applicant has proposed in its earlier International Application WO95/31696, a similar type of position encoder in which one member carries an excitation coil and a number of sensor coils and the other member carries a resonator. In operation, the excitation coil energises the resonator which in turn induces signals in the sensor coils which sinusoidally vary 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 the conductive screen in place of the resonator has the disadvantages that the output signal levels are much smaller and that the system cannot be operated in a pulse-echo mode of operation, in which a short burst of excitation current is applied to the excitation winding and then, after the excitation current has ended, detecting and processing the signals induced in the sensor coils.

A problem common to all of these known position sensors is that a positional error is introduced into the measurements if the moveable member is tilted relative to the other member. In some applications, such as machine tool applications, it is possible to physically restrict the movement of the two relatively moveable members, e.g. by using guide rails or the like. However, sometimes this is not possible. For example, in an X-Y digitising tablet, such as the one described in U.S. Pat. No. 4,848,496, the moveable member (the stylus) is moved by a human operator and its tilt relative to the tablet varies considerably during normal use.

Most digitising tablets which have been proposed to date employ a large number of overlapping but separate excitation and sense coils which are spread over the active area of the digitising tablet. The system identifies the current position of the stylus by detecting the excitation and sensor coil combination, which provides the greatest output signal levels. Some systems, such as the one disclosed in U.S. Pat. No. 4,848,496 mentioned above, perform a quadratic type interpolation to try to determine more accurately the current position of the stylus. However, this type of system suffers from the problem that it requires a large number of excitation coils, which must be individually energised, and a large number of sensor coils, which must be individually monitored for each energised excitation coil. There is therefore a trade off between the system's response time and the accuracy of the tablet. In particular, for high accuracy, a large number of excitation and sense coils are required, however, as the number of excitation coils and sensor coils increases, the system's response time decreases. The number of excitation and sense coils used in a given system is therefore governed by the required application.

EP-A-0680009 discloses such a digitising tablet system which is also arranged to process the signals from the different sensor coils in order to determine the orientation of the stylus in the X-Y plane.

SUMMARY OF THE INVENTION

The present invention aims to at least alleviate some of these problems with the prior art position sensors and to provide an alternative technique for determining the orientation of, for example, a stylus relative to a digitising tablet.

According to one aspect, the present invention provides a position detector comprising first and second members mounted for relative movement along a measuring path; said first member comprising a magnetic field generator for generating a magnetic field; said second member comprising first and second conductors which are inductively coupled to said magnetic field generator, the magnetic coupling between said first conductor and said magnetic field generator varying with a first spatial frequency and the magnetic coupling between said second conductor and said magnetic field generator varying with a second different spatial frequency, as a result of which, in response to a magnetic field generated by said magnetic field generator, a first signal is generated in a first receive circuit which first signal varies in dependence upon the relative position and orientation of the first conductor and the magnetic field generator and