U.S. patent application number 10/278984 was filed with the patent office on 2003-04-03 for position detector.
This patent application is currently assigned to Synaptics (UK) Limited. Invention is credited to Dames, Andrew N., Ely, David T.E., England, James M.C., Howe, Andrew R.L., Jones, Ross P., McKinnon, Alexander W., Pettigrew, Robert M..
Application Number | 20030062889 10/278984 |
Document ID | / |
Family ID | 27381074 |
Filed Date | 2003-04-03 |
United States Patent
Application |
20030062889 |
Kind Code |
A1 |
Ely, David T.E. ; et
al. |
April 3, 2003 |
Position detector
Abstract
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.
Inventors: |
Ely, David T.E.;
(Cambridgeshire, GB) ; Jones, Ross P.;
(Cambridgeshire, GB) ; England, James M.C.;
(Cambridgeshire, GB) ; McKinnon, Alexander W.;
(Cambridgeshire, GB) ; Pettigrew, Robert M.;
(Cambridgeshire, GB) ; Dames, Andrew N.;
(Cambridge, GB) ; Howe, Andrew R.L.; (Essex,
GB) |
Correspondence
Address: |
Nixon & Vanderhye P.C.
8th Floor
1100 N. Glebe Rd.
Arlington
VA
22201
US
|
Assignee: |
Synaptics (UK) Limited
|
Family ID: |
27381074 |
Appl. No.: |
10/278984 |
Filed: |
October 24, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10278984 |
Oct 24, 2002 |
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09627423 |
Jul 27, 2000 |
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6489899 |
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09627423 |
Jul 27, 2000 |
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09111885 |
Jul 8, 1998 |
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6249234 |
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09111885 |
Jul 8, 1998 |
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08737505 |
Dec 12, 1996 |
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5815091 |
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Current U.S.
Class: |
324/207.17 ;
324/207.15; 324/207.22; 324/207.23 |
Current CPC
Class: |
G06F 2203/04101
20130101; G06F 3/03545 20130101; G01D 5/2073 20130101; G06F 3/046
20130101; G06F 3/0362 20130101; G01D 5/208 20130101 |
Class at
Publication: |
324/207.17 ;
324/207.15; 324/207.23; 324/207.22 |
International
Class: |
G01B 007/14 |
Claims
1. 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
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 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 a second different
signal is generated in a second receive circuit which second signal
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 movable 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 movable members.
3. A position detector according to claim 2, wherein said relative
orientation of said two movable members causes a phase shift in
said sinusoidal variations.
4. A position detector according to claim 1, 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 path, wherein the second and fourth conductors are
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 in a third receive
circuit which third signal varies in dependence upon the relative
position and orientation of the third conductor and the magnetic
field generator and a fourth signal is generated in a fourth
receive circuit which fourth signal 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 movable members which is independent
of the relative orientation of said two movable members; and (ii) a
fine measurement value indicative of the relative position of said
two movable members which is dependent upon the relative
orientation of said two movable 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 the angle of the relative tilt between the
two movable 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 energising said
magnetic field generator.
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 measurement
path.
21. A position detector according to claim 20, wherein said first
receive circuit comprises said first conductor and wherein said
second receive circuit comprises said second conductor.
22. A position detector according to claim 21, wherein said third
receive circuit comprises said third conductor and wherein said
fourth receive circuit comprises said fourth conductor, and wherein
said processing means is operable to combine the signals from said
first and third receive circuits and to combine the signals from
said second and fourth receive circuits in order to derive said
position and orientation information.
23. A position detector according to claim 22, wherein each of said
receive signals varies 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 from the first
and third receive circuits and/or to combine the signals from the
second and fourth receive circuits 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 receive in said first and third receive circuits
and of measurements derived from the signals received in said
second and fourth receive circuits.
25. A position detector according to claim 24, wherein said
processing means is operable to combine the positional information
extracted from the signals from said first and third receive
circuits and the positional information extracted from said signals
from the second and fourth receive circuits, 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 26, when dependent upon
claim 4, wherein said excitation circuit comprises said first and
third conductors and wherein said first receive circuit comprises
said second conductor and said second receive circuit comprises
said fourth conductor.
28. A position detector according to claim 19, further comprising a
drive means for applying an energising signal to said excitation
circuit.
29. A position detector according to claim 28, wherein said drive
means is operable to energise both said first and third conductors
and wherein said processing means is operable to process the
signals received 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 received in said
second conductor when said first conductor is energised with the
signal received in said fourth conductor when said third conductor
is energised and to combine the signal received in said fourth
conductor when said first conductor is energised with the signal
received in said second conductor when said third conductor is
energised.
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 energising 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
received 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 mounted for relative movement in first and second
directions in a measuring plane; a first position detector
according to claim 1 for detecting the relative position and
orientation of the first and second members in the first direction;
a second position detector according to claim 1 for detecting the
relative position and orientation of said first and second members
in said second direction; 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
received 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 45, 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 received 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 45, 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 received 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 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
coincides with the centre 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 from the
signals induced in said receive windings by the three different
resonators.
55. 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
dependent upon the relative position and orientation of the first
conductor and the magnetic field generator and a second different
signal is generated in a second receive circuit which second signal
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 in
dependence upon said first and second spatial frequencies.
56. A position detector comprising: first and second members
mounted for relative movement 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;
characterised in that each group of windings 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.
57. 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 wire on a wiring loom in the
required manner so that the conductors extend in the measurement
path in a geometrically varying manner having the respective
characteristic dimension along the measuring path; and bonding the
wires to one or more substrates.
58. 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
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 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; and means for processing said first and
second signals to determine said relative position and orientation
a relationship between the respective characteristic dimension of
the geometrical variation of said two conductors.
59. 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
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.
60. A position detector according to claim 59, wherein said
combining means comprises sum and difference means for determining
the sum and difference of said first and second values.
61. An X-Y digitising system comprising: first and second members
mounted for relative movement 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.
62. Processing circuitry for processing signals obtained from a
position detector, the processing circuitry comprising all the
technical features of the processing circuitry used in the position
detector of claim 1.
63. An X-Y digitising tablet comprising a first group of windings
for determining the position of a moveable member relative to said
digitising tablet in a first direction and a second group of
windings for determining the position of said member relative to
said tablet in a second direction, wherein each of said windings
comprises a conductor which extends along the corresponding
measurement direction and which has a sensitivity to a magnetic
field having a predetermined spatial frequency; and wherein each
winding comprises 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.
64. A personal computer comprising a position detector according to
any preceding claim, wherein said second member comprising said
conductors and said receive circuits are located behind a display
of said computer and 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
said position detector are used to control information which is
displayed on said display.
65. A position detector comprising first and second members mounted
for relative movement along a measuring path; said first member
comprising a plurality of conductors each extending in a
geometrically varying manner having a different characteristic
dimension along the measuring path; said second member comprising
means for interacting with said conductors, such that in response
to an input driving signal applied to one of said conductors, there
is induced in the other said conductors an output signal, said
interacting means and said geometrically varying conductors being
arranged so that said output signal varies as a function of the
relative position between said first and second members along said
path; and means for processing said signal to derive said relative
position.
66. A method of detecting the relative position and orientation
between first and second members mounted for relative movement
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 or
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 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 a second different signal is generated
in a second receive circuit which second signal 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 from said receive circuits; and processing said
first and second signals to determine the relative position and
orientation of the two movable members using a relationship between
the respective characteristic dimension of the geometrical
variation of said two conductors.
67. A position detector comprising: first and second members
mounted for relative movement; 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 tilt of the first and second
members in two different directions; wherein each group of circuits
comprises at least two windings which are inductively coupled to
said magnetic field generator and wherein the magnetic coupling
between the at least two windings varies with different spatial
frequencies; and means for determining the relative rotation
between the first and second members using the sensed tilt in the
first and second directions.
68. A position sensor having two sets of windings each set
extending in different directions and each comprising at least two
windings magnetically coupled to a magnetic field generator, and
wherein the magnetic coupling between the magnetic field generator
and at least two of the windings in each set varies with different
spatial frequencies, and means for processing signals induced in
receive circuits as a result of the generation of a magnetic field
by the magnetic field generator in order to determine the relative
position and orientation of the magnetic field generator and the
sets of windings using said spatial frequencies.
69. A position sensor according to claim 68, wherein said magnetic
field generator comprises a resonator or a combination of
resonators.
70. A position sensor according to claim 68, wherein a plurality of
magnetic field generators are provided which are moveable relative
to said sets of windings, and each producing an identifiable signal
so as to allow the relative position and orientation of each
magnetic field generator and the sets of windings to be determined
from the signals induced in said receive circuits.
71. A position sensor according to claim 68, wherein said means for
processing said receive signals is arranged to determine the gap
between the magnetic field generator and the sets of windings.
72. A position sensor according to claim 68, wherein said magnetic
field generator generates two magnetic fields in two different
directions, and wherein said processing means is operable to derive
three degrees of freedom of the relative orientation of said
magnetic field generator and said sets of windings.
73. A board game comprising a plurality of playing pieces each
moveable over a playing surface and a position sensor according to
claim 70 for sensing the relative position and orientation of the
playing pieces and playing surface.
Description
[0001] This is a continuation in part application of application
Ser. No 08/737,505 filed on Dec. 12, 1996, the contents of which
are incorporated herein by reference.
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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. 4,848,490 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.
[0008] 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.
[0009] 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.
[0010] 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 a second different signal is
generated in a second receive circuit which second signal 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 in dependence
upon said first and second spacial frequencies.
[0011] The different spatial frequency variations of the output
signals are preferably achieved by shaping the conductors in a
predetermined manner over the measurement path. In particular, the
two conductors preferably extend in a geometrically varying manner
having different characteristic dimensions along the measurement
path. This can be achieved, for example, by using windings having a
different pitch along the measurement path. By using such windings,
a position measurement can be obtained across the entire
measurement path and an indication of the relative tilt of the two
members in the measurement direction can be obtained. This system
therefore avoids the need for having a large number of overlapping
windings which are spread out over the measurement path and
therefore does not suffer from the problems discussed above.
[0012] By providing a similar position detector for detecting the
relative position and orientation of the two members in a second
direction, the relative orientation in a plane containing the two
directions can be determined. Further still, by providing two or
more magnetic field generators on the first member, the complete
relative orientation of the two members can be determined from the
signals provided by the two or more magnetic field generators.
Therefore, a complete six degrees of freedom position detector can
be provided for detecting the position of an object over a planar
set of windings. The system does not require a set of windings in
two different planes which are inclined at an angle to each other.
This position detector is therefore suitable and convenient for
many applications especially children's toys and games and for use
in controlling a pointing device on a personal computer, where the
windings are embedded behind, for example, the LCD screen.
[0013] Exemplary embodiments will now be described with reference
to the accompanying drawings in which:
[0014] FIG. 1 schematically illustrates a computer system having a
X-Y digitising tablet for inputting data into the computer
system;
[0015] FIG. 2 schematically illustrates an exploded view of the
digitising tablet shown in FIG. 1, which generally shows two groups
of windings which form part of the digitising tablet and which are
used to sense the X-Y position of a stylus relative to the
digitising tablet;
[0016] FIG. 3 schematically illustrates the form of a stylus which
can be used with the X-Y digitiser tablet shown in FIG. 1;
[0017] FIG. 4a schematically illustrates the form of a first
periodic winding having a first period which forms part of a set of
windings used for sensing the X position of the stylus relative to
the digitising tablet;
[0018] FIG. 4b schematically illustrates the form of a second
periodic winding having the same period as and being in phase
quadrature to the winding shown in FIG. 4a, which also forms part
of the set of windings used for sensing the X position of the
stylus relative to the digitising tablet;
[0019] FIG. 4c schematically illustrates the form of a third
periodic winding having a period which is different from the period
of the windings shown in FIGS. 4a and 4b, and which also forms part
of the set of windings used for sensing the X position of the
stylus relative to the digitising tablet;
[0020] FIG. 4d schematically illustrates the form of a fourth
periodic winding having the same period as and being in phase
quadrature to the winding shown in FIG. 4c, which also forms part
of the set of windings used for sensing the X position of the
stylus relative to the digitising tablet;
[0021] FIG. 4e is a cross-sectional view of part of the X-Y
digitising tablet shown in FIG. 1;
[0022] FIG. 5 is a schematic representation of excitation and
processing circuitry used to determine the position of the stylus
shown in FIG. 3 relative to the X-Y digitising tablet shown in FIG.
1;
[0023] FIG. 6a illustrates the form of a time varying excitation
signal which is applied to some of the windings shown in FIG.
4;
[0024] FIG. 6b illustrates a time varying current which flows in a
resonator forming part of the stylus shown in FIG. 2, when the
excitation signal shown in FIG. 6a is applied to one of the
windings shown in FIG. 4;
[0025] FIG. 6c schematically illustrates the form of a signal
output from a mixer which forms part of the processing electronics
shown in FIG. 5;
[0026] FIG. 6d schematically illustrates the form of an output
voltage from an integrator/sample and hold circuit forming part of
the processing electronics shown in FIG. 5;
[0027] FIG. 7a shows a cross-section of part of the winding shown
in FIG. 4a and illustrates the relationship between the current
flowing in the winding and the resulting magnetic field which is
generated;
[0028] FIG. 7b schematically shows a vector representation of the
way in which a Z component of the magnetic field shown in FIG. 7a
varies along the X direction of the X-Y digitising tablet shown in
FIG. 1 and a corresponding approximation of the way in which this
vector representation varies with position along the X
direction;
[0029] FIG. 7c schematically shows a vector representation of the
way in which an X component of the magnetic field shown in FIG. 7a
varies along the X direction of the X-Y digitising tablet shown in
FIG. 1 and a corresponding approximation of the way in which this
vector representation varies with position along the X
direction;
[0030] FIG. 8 is a perspective view of an operator's hand holding
the stylus shown in FIG. 2, which illustrates the tilt of the
stylus longitudinal axis from the vertical direction;
[0031] FIG. 9 is a three-dimensional coordinate diagram relating
the axis of the stylus to the X, Y and Z coordinate system of the
digitising tablet shown in FIG. 1;
[0032] FIG. 10 is a coordinate diagram of the X-Z plane
illustrating the projection of the stylus axis shown in FIG. 9 in
the X-Z plane;
[0033] FIG. 11 is a coordinate diagram of the Y-Z plane
illustrating the projection of the stylus axis shown in FIG. 9 in
the Y-Z plane;
[0034] FIG. 12 is a plot illustrating the way in which two output
signals, derived by the processing electronics shown in FIG. 5,
vary in dependence upon the X position of the stylus relative to
the digitising tablet and illustrates the positional error caused
by the tilt of the stylus from the vertical direction;
[0035] FIG. 13 is a Cartesian plot showing the two values which an
angle can take if twice the angle is known;
[0036] FIG. 14 schematically illustrates the form of an electronic
game for a child;
[0037] FIG. 15 is a schematic representation of the form of a toy
car used in the electronic game shown in FIG. 14, which illustrates
the form of resonator used to detect the position of the car
relative to an X-Y digitising tablet forming part of the game shown
in FIG. 14;
[0038] FIG. 16 is a schematic view of the form of a two resonator
combination which can be used in the X-Y digitising systems
described with reference to FIGS. 1 and 14, which allows accurate
position calculations and orientation calculations to be made;
[0039] FIG. 17 illustrates the form of a three resonator
combination which can be used to provide complete orientation
information as well as the X, Y and Z position of an object
carrying the resonator combination relative to the X-Y digitising
tablet shown in FIG. 1 or 14;
[0040] FIG. 18 schematically illustrates the form of a two
resonator design which can be used to provide complete orientation
information as well as the X, Y and Z position of an object
carrying the resonator combination relative to the X-Y digitising
tablet shown in FIG. 1 or 14;
[0041] FIG. 19 schematically illustrates the form of a digitising
tablet having a periphery mounted excitation winding wound around a
set of receive windings;
[0042] FIG. 20a schematically illustrates the form of a winding
which, when energised, will produce a magnetic field which
sinusoidally varies along its length and which can be used in a
digitising tablet to sense position;
[0043] FIG. 20b schematically illustrates the form of a winding
which, when energised, will produce a magnetic field which linearly
varies along its length and which can be used in a digitising
tablet to sense position;
[0044] FIG. 21 is a perspective view of an electronic chess game
employing a X-Y digitising tablet for sensing the locations of the
playing pieces which form part of the chess game;
[0045] FIG. 22 schematically shows a cross-section of one of the
playing pieces of the chess game shown in FIG. 21;
[0046] FIG. 23 is perspective view of a personal computer having a
X-Y digitising system located behind its liquid crystal
display;
[0047] FIG. 24 schematically illustrates a cross-sectional view of
the display of the personal computer shown in FIG. 23, illustrating
the positional relationship between the windings of the digitising
system and the liquid crystal display;
[0048] FIG. 25a illustrates the form of a single period winding
which forms part of a set of windings used for sensing the position
of the stylus relative to the LCD display shown in FIG. 23; and
[0049] FIG. 25b illustrates the form of a second single period
winding having the same period as and being in phase quadrature to
the winding shown in FIG. 25a, which also forms part of the set of
windings used for sensing the position of the stylus relative to
the LCD display shown in FIG. 23.
[0050] FIG. 26 schematically illustrates the form of a stylus used
with the personal computer shown in FIG. 23;
[0051] FIG. 27 is a circuit diagram illustrating the electronic
components which form part of the stylus shown in FIG. 26;
[0052] FIG. 28a schematically illustrates a one-dimensional linear
position encoder;
[0053] FIG. 28b illustrates the form of a first periodic winding
forming part of the linear position encoder illustrated in FIG.
28a;
[0054] FIG. 28c illustrates the form of a second periodic winding
forming part of the position encoder shown in FIG. 28a which has
the same period as but is in phase quadrature to the winding shown
in 28b;
[0055] FIG. 28d illustrates the form a third winding forming part
of the position encoder shown in FIG. 28a which has a period
different to the period of the windings shown in FIGS. 28b and
28c;
[0056] FIG. 28e illustrates the form of a fourth winding forming
part of the linear position encoder shown in FIG. 28a which has the
same period as but is in phase quadrature to the winding shown in
FIG. 28d.
[0057] FIG. 1 schematically shows a computer system 1 having a
display 3, a main processing unit 5, a keyboard 7, an X-Y
digitising tablet 9 and a stylus 11. The X-Y digitising system
senses the current X-Y position of the stylus 11 over the tablet 9
and uses the sensed position to control the location of a cursor 13
on the display 3. FIG. 2 schematically shows an exploded view of
the digitising tablet 9. As shown, the digitising tablet comprises
a first group of windings 9-a, a second group of windings 9-b and a
base portion 9-c for supporting the two groups of windings 9-a and
9-b. The group of windings 9-a is used for determining the X
coordinate position of the stylus 11 and the group of windings 9-b
are used for determining the Y coordinate position of the stylus
11.
[0058] FIG. 3 shows in more detail the form of the stylus 11 shown
in FIG. 1. As shown, the stylus 11 comprises a coil 15 which is
connected in series, via a switch 16, to a capacitor 17 to form a
resonant circuit, generally indicated by reference numeral 18. The
coil 15 is wound around a ferrite core 19 so that the axis 21 of
the coil 15 coincides with that of the stylus 11. In this
embodiment, the switch 16 closes either when the tip 23 of the
stylus 11 is pressed against the top surface of the digitiser
tablet 9 or by the activation of a control button (not shown) on
the side of the stylus. Therefore, in this embodiment, the stylus
11 is passive in nature since it does not contain a power source
such as a battery or the like.
[0059] In operation, when the switch 16 is closed and when an
energising signal is applied to an energising winding (forming part
of the groups of windings 9-a and 9-b), the resonator 18 resonates
and induces signals in sensor windings (also forming part of the
groups of windings 9-a and 9-b). The arrangement of the excitation
winding, the sensor windings and the resonator 18 is such that the
signals induced in the sensor windings vary in dependence upon the
X-Y position of the resonator 18 relative to the digitising tablet
9. The current X-Y position of the resonator 18 can therefore be
determined by suitable processing of the signals induced in the
sensor windings. Additionally, the signals induced in the receive
windings also vary with the orientation of the stylus 11 and the
windings are arranged so that this orientation information can also
be determined by suitable processing of the received signals.
Further still, in this embodiment, the resonator 18 is in a fixed
position relative to the tip 23 of the stylus 11 and therefore, the
X-Y position of the tip 23 of the stylus can be determined from the
X-Y position of the resonator and the determined orientation.
[0060] In this embodiment, four separate windings are used for
determining the X position of the stylus 11 and four separate
windings are used for determining the Y position. In this
embodiment, the four windings used for determining the Y position
are the same as those used for the X position but rotated through
90 degrees. A detailed description of the form of the four windings
used for determining the X position will now be given with
reference to FIGS. 4a to 4d, which illustrate the form of these
windings. As shown, each of the windings 31 to 34 extends in the X
direction over the entire active length L.sub.X (which in this
embodiment is 300 mm) and in the Y direction over the entire active
length L.sub.Y (which in this embodiment is 300 mm) of the
digitising tablet 9. In this embodiment, the windings are arranged
to provide an output signal which sinusoidally varies with the
relative position of the stylus and the digitising tablet 9 along
the measurement path (the X-axis).
[0061] Referring to FIG. 4a, the winding 31 extends in the X
direction and comprises a repeating pattern of conductor. More
specifically, the winding 31 comprises five periods (31-1 to 31-5)
of the repeating pattern, with each period comprising two alternate
sense loops (a and b). As shown in FIG. 4a, loops a are formed by
winding the wire clockwise and loops b are formed by winding the
wire anti-clockwise. Since the five periods of the winding 31
extend over a length of 300 mm, the period or pitch
(.lambda..sub.1) of winding 31 is 60 mm. As a result of the
alternating sense of adjacent loops, the winding 31 is relatively
immune to electromagnetic interference (EMI) and does not itself
cause EMI in other electrical circuits because the magnetic field
generated by a current flowing in the winding falls off
approximately 55 dB per pitch from the winding (i.e. every 60 mm).
By making the extent of each loop (d.sub.1) equal to approximately
twice the spacing (d.sub.2) between adjacent loops, the output
signal varies approximately sinusoidally with the relative position
between the stylus and the digitising tablet, with an spatial
frequency (.omega.) equal to 2.pi./.lambda..sub.1.
[0062] The winding 32 shown in FIG. 4b is also formed by five
periods of alternating sense loops a and b and has the same pitch
.lambda..sub.1 as winding 31. However, as illustrated by the dashed
line 37, the loops of winding 32 are shifted along the X direction
by .lambda..sub.1/4, so that the windings 31 and 32 constitute a
phase quadrature pair of windings. In order that both windings 31
and 32 extend over the same length L.sub.X, the loops 38 and 39 at
the left and right hand end of winding 32 are both wound in the
same anti-clockwise direction but extend in the X direction for
only a quarter of the pitch .lambda..sub.1. This maintains the
balance between the number of and the area enclosed by each of the
two types of loops a and b. Winding 32 has also been rotated about
the X-axis by 180 degrees relative to winding 31, but this does not
affect its operation and facilitates the manufacture of the
digitising tablet 9.
[0063] Referring to FIG. 4c, winding 33 has the same general form
as winding 31 except that there are six periods (33-1 to 33-6) of
the repeating pattern which extend over the active length L.sub.X.
As with the windings 31 and 32, each period comprises two alternate
sense loops a and b. Since there are more periods of the repeating
pattern over the active length L.sub.X, the pitch .lambda..sub.2 of
winding 33 is smaller than the pitch .lambda..sub.1 of winding 31,
and in this embodiment .lambda..sub.2 is 50 mm. As shown in FIG.
4c, the output connection from winding 33 is located in the lower
right hand corner of the winding. As those skilled in the art will
appreciate, the connection point can be made at any position along
the length of the winding. The position of the connection point for
winding 33 has been chosen in order to separate it from the
connection points for windings 31 and 32.
[0064] As shown in FIG. 4d, winding 34 also comprises six, periods
of alternating sense loops a and b, but these are shifted by a
quarter of the pitch .lambda..sub.2 along the X direction relative
to those of winding 33. Therefore, like windings 31 and 32, the
windings 33 and 34 constitute a phase quadrature pair of windings.
Again, winding 34 has been rotated about the X axis by 180 degrees
relative to winding 33. This is in order to facilitate the
manufacture of the digitising tablet 9 and in order to separate the
connection points to the four windings 31 to 34.
[0065] In order to form the group of windings 9-a used for
determining the X position of the stylus 11, relative to the
digitising tablet 9, the windings 31 to 34 are superimposed on top
of each other. A similar set of windings rotated by 90 degrees, are
provided and superimposed over or under the windings 31 to 34 to
form the group of windings 9-b used for determining the Y
coordinate of the stylus 11 relative to the digitising tablet 9.
Therefore, in this embodiment, the digitising tablet 9 comprises a
total of eight separate windings.
[0066] In the remaining description, the quadrature pair of
windings 31 and 32 will be referred to as the sin A and the cos A
windings respectively and the windings 33 and 34 will be referred
to as the sin B and the cos B windings respectively. Similarly, the
corresponding windings used for determining the Y position will be
referred to as the sin C, cos C, sin D and cos D windings.
[0067] There are a number of ways that these windings can be
manufactured. Most commercial systems to date either employ screen
printing technology using conductive inks or printed circuit board
(PCB) technology. However, the screen printing technique suffers
from the disadvantage that the windings produced have a relatively
high resistance as compared with those produced by the PCB
technology, resulting either in low output signal levels if the
windings are used for sensing magnetic fields, or the necessity of
large transmitting powers in order to generate the required
strength of magnetic field if the windings are for generating
magnetic fields.
[0068] Although the windings produced using PCB technology have
lower resistance than those produced using screen printed inks, PCB
technology suffers from a number of disadvantages, including: (i)
existing PCB processing techniques are predominantly batch based
with maximum board dimensions of approximately 0.6 m; (ii) existing
PCB techniques typically employ multiple layers with through
connections (vias) which are difficult to manufacture, especially
with multi winding systems such as those used in the present
embodiment; and (iii) positional errors are generated in the output
signals because the conductors do not lie on a single layer but on
two or more separate layers.
[0069] Accordingly, in this embodiment, the windings of the
digitising tablet 9 are manufactured using wire bonding technology
which can alleviate some of these problems. Wire bonding is a
relatively well known technique in the art of printed circuit board
manufacture. The wire which is used to form the windings typically
has a diameter of between 0.1 mm to 0.5 mm and is usually made from
enamelled copper so that it can cross other wires in the same layer
without short circuiting. A suitable type of wire bonding
technology has been developed by, among others, Advanced
Interconnection Technology of Islip, Long Island, N.Y., USA. The
technique has existed for at least 20 years and the general
principle and structure of a suitable wire bonding apparatus is
described in, for example, U.S. Pat. No. 4,693,778, the contents of
which are incorporated herein by reference.
[0070] The applicant's copending International Application No.
______ filed on May 28, 1998 describes the way in which such a wire
bonding technique can be used to manufacture windings for use in
position sensors. More specifically, the windings are formed by
bonding an enamelled copper wire onto a suitable substrate in the
required pattern. In this embodiment, the eight windings of the
digitising tablet 9 are formed on a separate substrate which are
then superimposed on top of each other to form a multi layered
structure. More specifically, in this embodiment, the layered
structure is formed by firstly winding the wire onto a wiring loom
(not shown) in the required pattern in order to form a first one of
the eight windings. This winding is then sandwiched between first
and second substrates to trap the wires in place. Another winding
is then created using the wiring loom and then sandwiched between
the second substrate and a third substrate. This process is then
repeated until all eight windings have been sandwiched between two
substrates.
[0071] FIG. 4e shows a cross-sectional view along the X axis of the
digitising tablet 9 shown in FIG. 1. As shown, there are nine
substrate layers 45-1 to 45-9 which sandwich the eight separate
windings 41-1 to 41-8. The top substrate layer 45-1 also acts as a
protective layer which may have printed material on the top surface
depending on the application for the X-Y digitising tablet. As
shown, in this embodiment, the windings for the X position
measurement are arranged in alternating layers with those for the Y
position measurement. In order to provide mechanical stability, a
base layer 47 made from steel is provided. Since the steel base
layer 47 may interfere with the magnetic fields produced by
currents flowing in the digitiser windings 41, a magnetically soft
layer 49 is interposed between the base layer 47 and the last
substrate layer 45-9. The magnetically soft layer 47 effectively
shields the windings 41 from the steel base layer 47 and enhances
the performance by providing a permeable path from magnetic flux to
pass behind the windings. The magnetically soft layer may be made,
for example, from plastic or rubber containing iron or ferrite
powder, although any magnetically soft material may be used. This
material may be formed by extrusion in long lengths by, for
example, Anchor Magnets Ltd Sheffield UK, under the trade names
Ferrostrip and Ferrosheet, and is therefore suited to long length
systems. This material is minimally conductive, so that eddy
current losses are minimised. G40
[0072] The advantages of the wire bonding technology include: (i)
the windings have relatively low resistance (with a wire diameter
of approximately 0.15 mm, the resistivity is approximately 1 ohm
per meter); (ii) a high density of winding can be made--up to 6
wires per mm in two orthogonal directions (with a wire diameter of
0.15 mm), enabling higher complexity windings and increased winding
efficiency (because multiple turns can be used); and (iii) multiple
layers of wires can be used and wire crossings in the same layer
are possible.
[0073] A more detailed description will now be given of the way in
which the position of the stylus 11 relative to the digitising
tablet 9 is determined. In this embodiment, the excitation signal
is sequentially applied to the sin A winding twice, then twice to
the cos A winding, then twice to the sin C winding and finally
twice to the cos C winding. A short period of time is provided
between the energisation of each of these windings, during which
the signals received on the sin B winding, the cos B winding, the
sin D winding and the cos D winding are processed to extract the
stylus position relative to the digitising tablet 9. As will be
described in more detail below, in this embodiment, in addition to
determining the X and Y position of the stylus 11 relative to the
digitising tablet 9, the signals received in these windings are
processed to determine an estimate of (i) the height (Z) of the
stylus 11 above the digitising tablet 9; (ii) the angle (a) at
which the stylus is tilted from the vertical (i.e. from the
Z-axis); and (iii) the orientation (.theta.) of the stylus 11 in
the X-Y plane.
[0074] FIG. 5 illustrates the excitation and processing electronics
used to energise the excitation windings (sin A, cos A, sin C and
cos C) and to detect the signals received from the receive windings
(sin B, cos B, sin D and cos D). In this embodiment, the sin A
excitation winding is energised first and the signal received on
the sin B receive winding is processed. Then the sin A excitation
winding is energised again and the signal received on the cos B
winding is processed. A similar sequence of excitation and
processing is then performed for the excitation windings cos A and
the receive windings sin B and cos B and for the excitation
windings sin C and cos C and receive windings sin D and cos D. By
energising quadrature windings in this manner ensures that the
resonator is energised at all positions over the active area of the
digitising tablet 9. As shown, the excitation and processing
circuitry comprises a digital waveform generator 55 which generates
an appropriate excitation signal which is amplified by a MOSFET
amplifier 57 and applied to the appropriate excitation winding via
switch 59 and a respective output line 50-1 to 50-4. The digital
waveform generator 55 is controlled by a microcontroller 61 which
ensures that the frequency of the AC energising signal is suitable
for causing the resonator 18 in the stylus 11 to resonate. The
microcontroller 61 also controls the digital waveform generator and
the switch 59 so that the sin A, cos A, sin C and cos C windings
are energised at the right time and in the right order.
[0075] FIG. 6a, shows the form of the excitation signal which is
sequentially applied to the four excitation windings (sin A, cos A,
sin C and cos C) in this embodiment. As shown in FIG. 6a, the
excitation signal 52 comprises six periods of a square wave voltage
whose frequency matches that of the resonant frequency of the
resonator 18. In this embodiment, the resonant frequency of the
resonator, and hence that of the excitation signal, is 2 MHz,
although any frequency in the range of 10 kHz and 10 MHz would be
practical. When this excitation signal 52 is applied to one of the
excitation windings, a current flows in the excitation winding
which creates a magnetic field which couples with the resonator 18
and causes it to resonate. FIG. 6b illustrates the general form of
the resonator current 53 as a result of the energisation signal 52
being applied to one of the energising windings. As shown, the
resonator current gradually increases in magnitude from the time
that the excitation voltage is applied to the excitation winding.
The resonator current reaches a maximum value when the excitation
voltage is removed from the winding at time t.sub.1 and continues
to resonate for a short period of time (T) thereafter. As will be
explained below, in this embodiment, the processing circuitry is
arranged to process the received signals after time t.sub.2, i.e.
after the excitation signal has been removed from the excitation
winding. This is possible because the resonator continues to "ring"
after the excitation has been removed, and has the advantage that
it removes any error caused by direct coupling between the
excitation and receive windings.
[0076] The signals received from the receive windings (sin B, cos
B, sin D and cos D) are fed, via a respective input line 62-1 to
62-4 and switch 63, to an amplifier 65 which amplifies the received
signals. The signals which are received from the receive windings
are essentially an amplitude modulated version of the excitation
signal, in which the positional information of the stylus 11 is
encoded within the amplitude. The amplified signals are therefore
passed to a mixer 67 where they are synchronously demodulated by
multiplying them with a signal having the same fundamental
frequency as the excitation signal, which is provided by the
digital waveform generator 55 via line 69. More details of the
relationship between the excitation signal and the signal used to
demodulate the received signals can be found in the applicant's
earlier International application WO95/31696, the contents of which
are incorporated herein by reference.
[0077] FIG. 6c, shows the form of the signal 56 output by the mixer
67. As shown, the mixer output signal 56 starts at time t.sub.2
just after the excitation signal has been removed and comprises a
DC component which contains the positional information together
with unwanted high frequency AC components. The AC components are
removed from the mixer output signal 56 by integrating the signal
over a predetermined number of periods of the excitation signal
(since the integral of a sinusoidally varying signal over one or
more periods is zero). This integration of the mixer output signal
56 is performed by the integrator/sample and hold circuit 73 and
the integration period is controlled by the digital waveform
generator 55 via the control line 75. FIG. 6d, illustrates the form
of the output signal 58 of the integrator/sample and hold circuit
73. As shown, the output signal 58 increases with time and the
final value (V.sub.out) depends upon the total DC level of the
mixer output signal 56 during the integration period. The output
signal (V.sub.out) from the integrator/sample hold circuit 73 is
then converted from an analog signal into a digital signal by the
analog to digital converter 77 and passed to the microcontroller
61. As mentioned above, the excitation and processing procedure is
then repeated for different combinations of excitation and receive
windings and the microcontroller 61 uses the output signals
(V.sub.out) from the different combinations to derive the position
(X, Y, Z), tilt (.alpha.) and orientation (.theta.) of the stylus
11. This information is then passed, via line 79, to the main
computing unit 5, shown in FIG. 1, which uses the information to
control the position of the cursor 13 and other information
displayed on the display 3.
[0078] The way in which the X, Y and Z position, the tilt (.alpha.)
and the orientation (.theta.) of the stylus 11 is determined will
now be described. In order to do this, however it is necessary to
understand the form of magnetic field created by the energising
current flowing in the energising windings sin A, cos A, sin C and
cos C and how that magnetic field interacts with the resonator 18
in the stylus 11, to induce a signal in the receive windings sin B,
cos B, sin D and cos D, from which this positional and orientation
information can be determined. As will be appreciated by those
skilled in the art, the magnetic field generated by a current
flowing in a winding is a function of the shape of the winding and
the excitation signal which is applied to the winding, i.e.:
H.sup.winding(x, y, z, t)=f(SHAPE, E(t)) (1)
[0079] Similarly, the EMF induced in a winding located in an
alternating magnetic field is a function of the magnetic field and
a function of the shape of the winding, i.e.:
EMF.sub.winding(t)=f(SHAPE, H(x, y, z, t)) (2)
[0080] As mentioned above, in this embodiment, the shape of the
digitiser windings shown in FIG. 4 have been designed so that the
magnetic field generated by each winding, when energised, varies
substantially sinusoidally with position along the winding. The way
in which this is achieved will now be illustrated for the sin A
winding 31 shown in FIG. 4a, with reference to FIG. 7. FIG. 7a
shows a cross-section along the lines S-S of part of the sin A
winding 31 shown in FIG. 4a and in particular shows a cross-section
through period 31-3 and part of periods 31-2 and 31-4. The Figure
shows the situation where a constant current is applied to the sin
A winding 31. As can be confirmed by considering the path taken by
a current flowing through the sin A winding 31, the current in the
pairs of wires located next to each other, e.g. wires 81 and 82,
flow in the same direction, either into the paper or out of the
paper and the current flowing in an adjacent pair of wires, such as
wires 83 and 84, flow in the opposite direction. This is
illustrated in FIG. 7a by using dots to represent currents which
come out of the paper and by using crosses to represent currents
going into the paper. Therefore, the magnetic field created by each
pair of wires can be determined and these are approximated by the
circles 85-1 to 85-5. These magnetic fields 85 combine with each
other to create a resultant magnetic field which can be split into
a component in the Z direction, a component in the X direction and
a component in the Y direction. Each of these magnetic field
components will now be considered and in particular with regard to
how they vary in the X direction.
[0081] The Z component will be considered first. At the point A,
the magnetic field lines point horizontally to the left in the X
direction and therefore there is no Z component at point A. This is
represented by the dot 86 in the vector diagram shown in FIG. 7b.
However, in moving from point A to point B, the Z component of the
magnetic field increases in value to a maximum value at point B.
This is represented by the vectors 88 and 90, which increase in
size from point A to point B. Continuing this analysis along the X
direction, results in the complete vector drawing shown in FIG. 7b.
Beneath this vector drawing, there is an approximation of the way
in which the magnitude and direction of the vectors change with
position along the X direction. As shown, this approximation varies
in a sinusoidal manner with the period of the sinusoidal variation
matching the pitch .lambda..sub.1, of the sin A winding 31. As
those skilled in the art will appreciate, the variation of the Z
component will not be exactly sinusoidal and as a result, the
generated magnetic field will also include unwanted higher order
spatial harmonics. However, these higher order harmonics have
shorter pitches, and since their amplitudes drop off at a rate that
is proportional to their pitch, the magnetic field shape will
increasingly become a pure sinusoid with increasing gap between the
resonator 18 and the windings. In addition, the winding shapes can
be chosen to eliminate the lowest unwanted harmonics, which are the
ones that persist over larger gaps. For example, the spacing
between the neighbouring pairs of wires in FIG. 7 can be chosen to
eliminate the spatial third harmonic. By eliminating these low
spatial harmonics, the magnetic field shape very closely
approximates to a sinusoid at even small gaps.
[0082] A similar analysis on the horizontal X component of the
magnetic field results in the vector diagram illustrated in FIG.
7c, which, as shown, also approximates to a sinusoidally varying
function with X, having a period equal to the pitch .lambda..sub.1
of the sin A winding 31 but which is 90 degrees out of phase with
the Z component. A similar analysis can be performed for the
variation of the Y component of the magnetic field in the X
direction. However, since the wires are parallel to the Y
direction, they only produce magnetic field components in the X and
Z directions. As a result, the Y component of the magnetic field is
essentially zero, except at the ends of the coils in the Y
direction, where the wires parallel to the Y direction are joined
by wires parallel to the X direction. These latter wires produce
magnetic fields with components in the Y direction, but these drop
off rapidly with gap between the stylus and the windings.
[0083] As those skilled in the art will appreciate, the magnitude
of the magnetic field generated by the excitation of the sin A
winding decreases with distance from the winding in the Z
direction. It can be shown that this fall-off in the magnetic field
can be approximated by an exponential function, with the fall off
rate being approximately inversely proportional to the pitch
.lambda..sub.1 of the sin A winding, so that the larger the pitch
is the lower the fall-off rate is.
[0084] From the above analysis, a "shape factor" for the sin A
winding 31 can be defined as follows:
S.sup.sin A(x, y, z)=e.sup.(-.omega..sup..sub.A.sup.z) [-cos
(.omega..sub.Ax+.phi..sub.x), 0, sin (.omega..sub.Ax+.phi..sub.x)]
(3)
[0085] where .omega..sub.A=2.pi./.lambda..sub.1 and .phi..sub.x is
a system constant whose value depends upon the position of the
reference point used as the origin for the (X, Y, Z) position
measurements. Since the cos A winding 32 has the same general shape
as the sin A winding 31 but shifted by .lambda..sub.1/4 in the X
direction, a similar shape factor for the cos A winding 32 can be
defined as follows:
S.sup.cos A(x, y, z)=e.sup.(-.omega..sup..sub.A.sup.z) [sin
(.omega..sub.Ax+.phi..sub.x), 0, cos (.omega..sub.Ax+.phi..sub.x)]
(4)
[0086] Similar expressions can be derived for the shape factors for
the sin B and cos B windings 33 and 34, noting that the spatial
frequency (.omega.) will be different because the pitch
.lambda..sub.2 of the sin B and cos B windings is different from
that of the sin A and cos A windings 31 and 32. The shape factors
for the sin C, cos C, sin D and cos D windings can be obtained
through a similar analysis, noting that for these windings, the X
component of the magnetic field is essentially zero away from the
ends of the wires in the X direction, because the wires away from
these ends are all parallel to the X direction.
[0087] Consequently, when the excitation signal shown in FIG. 6a is
applied to the sin A winding, the following magnetic field is
generated around the digitising tablet 9:
H.sup.sin A(x, y, z, t)=S.sup.sin A(x, y, z).f(E(t)) (5)
[0088] As mentioned above, the excitation signal E(t) is a square
wave voltage which comprises a fundamental component, having a
frequency which is matched to the resonant frequency of the
resonator 18, and higher order harmonics. Since the exact nature of
the time variation of the excitation signal is not critical to the
following analysis, a more detailed description of the excitation
signal will be omitted. The generated magnetic field couples with
the resonator 18 and causes it to resonate. The current which is
caused to flow in the resonator by the generated magnetic field is
proportional to the component of the generated magnetic field along
the axis 21 of the resonator 18. During normal use, as illustrated
in FIG. 8, the user will usually hold the stylus 11 like a pen,
resulting in the axis 21 of the resonator 18 being tilted from the
vertical by some unknown angle (.alpha.).
[0089] FIG. 9, shows a three dimensional Cartesian plot with the
origin located at the centre of the resonator and with the axis of
the resonator pointing in the direction of the unit vector r (dx,
dy, dz). As shown in FIG. 9, the unit vector r is tilted from the
vertical by the angle .alpha. and rotated through an angle .theta.
from the X-axis. The angle .theta. represents a measure of the
orientation of the stylus 11 in the X-Y plane. Therefore, the
resonator current can be expressed by: 1 I RES sin A = - A z [ -
cos ( A x + x ) dx + 0 dy + sin ( A x + x ) dz ] f ' ( E ( t ) ) (
6 )
[0090] where
dx=r cos .theta. sin .alpha.=r.sub.x sin .alpha..sub.x (7)
dy=r sin .theta. sin .alpha.=r.sub.y sin .alpha..sub.y (8)
dz=r cos .alpha.=r.sub.x cos .alpha..sub.x=r.sub.y cos
.alpha..sub.y (9)
[0091] where, as illustrated in FIGS. 10 and 11, r.sub.x and
r.sub.y are the projections of the unit vector r in the X-Y and Y-Z
planes respectively and where .alpha..sub.x and .alpha..sub.y are
the respective angles between these projected vectors and the
Z-axis. This resonator current creates a resonator magnetic field
in the direction of the resonator axis 21 which in turn induces an
EMF in the sin B and cos B windings 33 and 34. The resonator
magnetic field will also induce a signal in the other windings,
however, these other signals are not used in the subsequent
calculations and will therefore be ignored. As a result of the
reciprocal nature of the magnetic coupling between the windings of
the digitising tablet 9 and the resonator 18, the EMF induced in
the sin B coil will have the following form: 2 EMF sin B sin A = -
B z [ sin ( B x + x ) dx + 0 dy + cos ( B x + x ) DZ ] f ( I RES
sin A ) ( 10 )
[0092] which, after being demodulated by the mixer 67 integrated by
the integrator/sample and hold circuit 73 and simplified has the
following from: 3 S A S B = Ar x 2 - x z [ sin ( A x + x - x ) sin
( B x + x - x ) ] ( 11 )
[0093] Where S.sup.AS.sup.B is the output signal V.sub.OUT
mentioned above when the sin A winding is energised and the signal
induced in the Sin B winding is processed, where
.omega..sub..SIGMA.x=.omega..sub.A+.omega..su- b.B and where A is a
coupling constant known in advance. In a similar manner, the signal
induced in the cos B winding by energising the sin A winding will
have the following form: 4 S A C B = Ar x 2 - x z [ sin ( A x + x -
x ) cos ( B x + x - x ) ] ( 12 )
[0094] Similar expressions can also be derived for the signals
induced in the sin B and cos B windings when the cos A winding is
energised and these are given by: 5 C A S B = Ar x 2 - x z [ cos (
A x + x - x ) sin ( B x + x - x ) ] ( 13 ) C A C B = Ar x 2 - x z [
cos ( A x + x - x ) cos ( B x + x - x ) ] ( 14 )
[0095] In this embodiment, the signals given in equations 11 to 14
are combined by the microcontroller 61 to form the following sum
and difference signals: 6 S x = S A C B + C A S B = Ar x 2 - x z
sin [ x x + 2 x - 2 x ] ( 15 ) C x = C A C B - C A C B = Ar x 2 - x
z cos [ x x + 2 x - 2 x ] ( 16 ) S x = S A C B - C A S B = Ar x 2 -
x z sin [ x x ] ( 17 ) C x = C A C B + S A S B = Ar x 2 - x z cos [
x x ] ( 18 )
[0096] which are derived using the well known expansions of sin
(A.+-.B) and cos (A.+-.B) in terms of sin A, sin B, cos A and cos
B. By taking the arc-tangent of the ratio of these components a
coarse position measurement phase and a fine position measurement
phase of the X coordinate position of the stylus 11 can be
determined. More specifically, the coarse position measurement
phase is determined by taking the four quadrant inverse tangent of
the sin and cos difference signals using the following equation: 7
arctan 2 ( S x , C x ) = x x = 2 ( N A - N B ) x L x ( 19 )
[0097] and the fine position measurement phase is determined by
taking the four quadrant inverse tangent of the ratio of the sin
and cos sum signals using the following equation: 8 arctan 2 ( S x
, C x ) = x x + 2 x - 2 x = 2 ( N A + N B ) x L x + 2 x - 2 x ( 20
)
[0098] where N.sub.A and N.sub.B are the number of periods of the
sin A and the cos B windings respectively over the active length
L.sub.X of the digitising tablet 9. As can be seen from equations
19 and 20, the tilt (.alpha.) of the stylus 11 has introduced a
positional error (2.alpha..sub.x) into the fine position
measurement phase given in equation 20. It does not, however,
affect the coarse position measurement phase.
[0099] A similar processing of the signals received from the sin D
and cos D windings when the sin C and cos C windings are energised
results in the following expressions for the coarse and fine
position measurement phases of the y position of the stylus 11
relative to the digitising tablet 9: 9 arctan 2 ( S y , C y ) = y y
= 2 ( N C - N D ) y L y ( 21 ) arctan 2 ( S y , C y ) = y y + 2 y +
2 y = 2 ( N C + N D ) y L y + 2 x + 2 x ( 22 )
[0100] FIG. 12 illustrates the way in which the coarse and fine
position measurement phases for the X position vary with the
position across the active length L.sub.X of the digitising tablet
9. As shown, the coarse position measurement phase 100 varies
linearly between -.pi. and .pi. across the entire active length
L.sub.X of the digitising tablet 9. This measurement therefore
gives an unambiguous measure of the X position over the entire
length L.sub.X. This is because N.sub.A-N.sub.B=1. The fine
position measurement phase 102 also varies linearly between -.pi.
and .pi.. However, the fine position measurement phase cycles
between -.pi. and .pi. eleven times across the length L.sub.X. This
is because N.sub.A+N.sub.B=11. Since the fine position measurement
phase contains a positional error .xi. caused by the tilt of the
resonator 18 it is not possible to retrieve the fine position
measurement. However, since the coarse position measurement phase
is derived from a difference signal, the effect of the tilt on the
sensed signals is cancelled out. Therefore, the coarse position
measurement can always be used to determine the current position of
the stylus 11 relative to the digitising tablet 9, regardless of
its tilt.
[0101] In this embodiment, the coarse position measurements for the
X and Y positions, are input into the respective equations 20 and
22 together with the known constants .phi..sub.x and .phi..sub.y to
give estimates of 2.alpha..sub.x and 2.alpha..sub.y. Given
2.alpha..sub.x and 2.alpha..sub.y, there are two possible values
for .alpha..sub.x and .alpha..sub.y which differ by 180 degrees.
This is illustrated in FIG. 13, which shows that the angle
2.alpha..sub.x between the Z-axis and the line 105 can be obtained
if .alpha..sub.x equals the angle between the Z-axis and the line
107 or the angle between the Z-axis and the line 109. However, if
the range of tilt is limited to -.pi./2 and .pi./2, then an
estimate of the value of .alpha..sub.x and .alpha..sub.y can be
determined, from which the tilt .alpha. of the resonator axis 21
from the Z-axis can be determined together with the orientation
.theta. of the stylus 11 in the X-Y plane, from the following
equations:
.alpha.=tan.sup.-1[{square root}{square root over
(tan.sup.2.alpha..sub.x+- tan.sup.2.alpha..sub.y)}] (23)
.theta.=arctan 2[tan (.alpha..sub.y), tan (.alpha..sub.x)] (24)
[0102] The height Z of the stylus 11 above the digitising tablet 9
can then be obtained from the amplitudes of the combined signals
given in equations 15 to 18 and in particular from either of the
following two amplitude terms: 10 A x = A x = ( C x ) 2 + ( S x ) 2
= A x = ( C x ) 2 + ( S x ) 2 = Ar x 2 - x z ( 25 ) A y = A y = ( C
y ) 2 + ( S y ) 2 = A y = ( C y ) 2 + ( S y ) 2 = Ar y 2 - y z ( 26
)
[0103] both of which vary with the height (Z) of the stylus 11
above the digitising tablet 9 and, to some extent, with the tilt
and orientation (because of r.sub.x.sup.2 and r.sub.y.sup.2) of the
stylus 11. However, since .alpha. and .theta. have been estimated
from equations 23 and 24, the value of r.sub.x.sup.2 and
r.sub.y.sup.2 can be determined using equations 7 to 9 and hence
the height Z of the stylus 11 above the digitising tablet 9 can be
determined from these amplitudes.
[0104] To summarise, in this embodiment, with an unknown angle of
tilt between the stylus and the Z-axis, a coarse position
measurement which is unaffected by the tilt has been obtained, from
which an estimate of (i) the tilt angle (.alpha.) and the
orientation (.theta.) of the stylus; and (ii) an estimate of the
height of the stylus above the digitising tablet, have been
determined. However, a fine position measurement of the stylus's
current position has not been obtained because it is corrupted with
a positional error caused by the tilt.
[0105] FIG. 14 illustrates the form of a second embodiment in which
the angle of tilt of the resonator axis relative to the Z-axis is
known. In particular, FIG. 14 schematically illustrates an
electronic game having a digitising tablet 9 (which is the same as
the one used in the first embodiment), a display 3 and a child's
toy 111 which is freely movable over the surface of the digitising
tablet 9. As shown in FIG. 15, the toy car 111 houses a resonator
18 whose axis is inclined by a known angle .alpha. from the Z-axis.
Provided the car is not lifted off the surface of the digitising
tablet 9, the angle between the axis 21 of the resonator 18 and the
Z-axis will be fixed at the value of a. Since the tilt angle
.alpha. is known in this embodiment, the orientation (.theta.) of
the car 111 in the X-Y plane can be determined from the ratio of
the amplitudes defined by equations 25 and 26 (provided that
.omega..sub..DELTA.x equals .omega..sub..DELTA.y), i.e. from: 11 A
x A y = r x 2 r y 2 = d x 2 + d z 2 d y 2 + d z 2 = sin 2 cos 2 +
cos 2 sin 2 sin 2 + cos 2 ( 27 )
[0106] In particular, the value of sin.sup.2.theta. or
cos.sup.2.theta. can be determined since .alpha. is known. However,
knowing only sin.sup.2.theta. or cos.sup.2.theta. allows for four
possible values of .theta., one in each quadrant. The best way to
determine the correct value of .theta. is: (a) to calculate
.alpha..sub.x and .alpha..sub.y from equations 23 and 24 for each
possible value of .theta.; then (b) use these values of
.alpha..sub.x and .alpha..sub.y to estimate the fine X and Y
position of the toy car 111 from equations 20 and 22; and finally
(c) identify which value of .theta. gives the smallest discrepancy
between the estimated fine position and the measured coarse
position obtained from equations 19 and 21. The orientation .theta.
which gives the smallest discrepancy is then used in order to
display an appropriate scene which might be viewed from the toy car
111 at its current position and orientation.
[0107] It should be noted, however, that the above method is not
the most robust way of determining the orientation (.theta.) of the
toy car 111, since errors in the coarse position measurement can
affect the way the fine position measurement is interpreted and can
result in errors in the choice of the orientation angle
.theta..
[0108] This problem can be overcome by providing two separate but
coincident resonators (i.e. having the same centre point) in the
toy car 111, each operating at a different frequency so that they
can be independently interrogated, with one resonator untilted and
the second resonator tilted at some known angle relative to the
other. FIG. 16 illustrates such a combination of resonators. As
shown, both resonators 18-1 and 18-2 have the same centre point 121
but the axis 21-2 of resonator 18-2 is tilted by a known angle
.alpha. from the axis 21-1 of the resonator 18-1. In practice the
resonator 18-2 can be formed by two series connected coils and a
capacitor, with one coil having the same axis as the coil used in
the untilted resonator 18-1, and with the other one having its axis
orthogonal to the other. When used in the toy car 111 of FIGS. 14
and 15, if the axis 21-1 of resonator 18-1 is arranged to point in
the Z direction, then .alpha..sub.x and .alpha..sub.y for resonator
18-1 will be zero. Therefore, both the coarse and the fine position
measurement phases, which are obtained by energising the resonator
18-1, can be used to determine the X and Y position of the centre
point 121 (and hence of the toy car 111) relative to the digitising
tablet 9. Further, since the resonators are coincident, their
positions are the same. Consequently, the fine position
measurements obtained from the signals from the untilted resonator
18-1, can be used to determine .alpha..sub.x and .alpha..sub.y for
the tilted resonator 18-2, using equations 20 and 22. These values
of .alpha..sub.x and .alpha..sub.y together with the known angle
.alpha. and the amplitude measurements defined by equations 25 and
26 can then be used to calculate the orientation .theta. of the toy
car 111. Therefore, by employing two coincident resonators 18-1 and
18-2, which can be independently interrogated, it is possible to
retrieve the fine position measurement and to use these to obtain a
more accurate estimate of the orientation of the toy car 111 in the
X-Y plane of the digitising tablet 9.
[0109] In the above embodiments, a technique has been described for
determining the X, Y and Z positions of a resonator or a pair of
resonators and for determining the orientation (.theta.) of the
resonator in the X-Y plane, and hence the position and orientation
of an object, such as a stylus or a child's toy, which carries the
resonator(s). However, in the above embodiments, it has been
assumed that the only rotation that the resonator is able to make
is in the X-Y plane. However, other rotations of the resonator(s)
are possible, for example, about the X-axis, which would lead to a
miscalculation of the resonator's position or orientation in the
X-Y plane. This is because the system lacks enough information to
determine both the fine position and the orientation of the
resonators from just the fine phase measurements. In particular,
from the four fine phase measurements there are only three
independent quantities, because the angle between the two resonator
axes is fixed, whereas the determination of the fine position and
the orientation requires the determination of five quantities. The
only possible source of extra information are the four amplitude
measurements, but as mentioned previously, these are not robust
quantities and using them to determine the fine position and the
orientation would compromise the accuracy of the system. (Note that
the coarse position measurements of the two resonators are
identical and give only the coarse position of both resonators in
the X and Y directions and so these provide no extra information.)
Furthermore, the orientation of just two resonators cannot be
uniquely determined by the system. This is because the system
cannot distinguish between a resonator and the same resonator with
its axis reversed, which can be achieved by rotating them through
180 degrees about an axis which is mutually perpendicular to both
of their axes. Therefore, in the above embodiments, it is not
possible to determine the complete orientation of an object which
carries the resonators.
[0110] An embodiment will now be described in which the complete
orientation of the object which carries the resonators can be
determined. In this embodiment, this is achieved by using three
coincident resonators, each operating at a different frequency so
that they can be independently interrogated and with the axis of
each resonator being tilted relative to the other two. A suitable
resonator combination is illustrated in FIG. 17. As shown, the
resonator combination comprises three resonators 18-1, 18-2 and
18-3, with the respective resonator axes 21-1, 21-2 and 21-3 being
tilted relative to each other by some known angles. In order to
avoid possible ambiguity with the resonators, there are two
configurations that must be avoided. In particular, two of the
resonator axes 21-1, 21-2 and 21-3 must not be both perpendicular
to the other axis and all three axes must not lie in the same
plane, since with these combinations of resonator, there are one or
more orientations which cannot be distinguished. For example, if
two of the axis are perpendicular to the third, then a rotation
through 180 degrees about the third axis reverses the direction of
the other two axes and these two orientations cannot be
distinguished. Similarly, if all three axis lie in the same plane,
then a 180 degree rotation about the line which is perpendicular to
that plane and which passes through the centre of the resonators,
reverses all three axes and again these two orientations cannot be
distinguished.
[0111] As described above, each resonator 18 produces a coarse
position measurement in the X and Y directions (defined by
equations 19 and 21), a fine position measurement of the X and Y
directions (defined by equations 20 and 22) and two amplitude
measurements (defined by equations 25 and 26). Since the coarse
position measurement only depends on the X and Y coordinates of the
resonator and since all three resonators are coincident, all three
resonators 18-1, 18-2 and 18-3 will therefore give the same coarse
position measurement. The amplitude measurements depend mostly on
Z, and to some extent on the orientation of the resonator axis with
respect to the Z-axis. However, this is not a robust source of
information for the resonator orientation, and so in this
embodiment, the amplitudes from the three resonators are used just
to give information about the height (Z) of the resonators above
the digitising tablet 9. The fine position measurement of the X and
Y position of the resonator combination still has to be determined
together with three parameters defining the complete orientation of
the resonator combination. One technique for determining these
measurements will now be described.
[0112] If the tilt angles of the three resonators 18-1, 18-2 and
18-3 are .alpha..sub.x and .alpha..sub.y, .beta..sub.x and
.beta..sub.y, .gamma..sub.x and .gamma..sub.y respectively, then
given estimates for X and Y (provided by the coarse position
measurements defined by equations 19 and 21) estimates for
2.alpha..sub.x and 2.alpha..sub.y, 2.beta..sub.x and 2.beta..sub.y,
2.gamma..sub.x and 2.gamma..sub.y can be calculated from the fine
position measurements. If the axis of one of the resonators 18 is
in the direction of the unit vector u (dx, dy, dz), then:
dx=dz tan .alpha..sub.x
dy=dz tan .alpha..sub.y (28)
[0113] and since u is a unit vector, dx.sup.2+dy.sup.2+dz.sup.2=1,
the unit vector u can therefore be defined as follows: 12 u = ( sin
2 x cos 2 x + 1 , sin 2 y cos 2 y + 1 , 1 ) [ sin 2 x cos 2 x + 1 ]
2 + [ sin 2 y cos 2 y + 1 ] 2 + 1 ( 29 )
[0114] Therefore, given estimates for the values of X and Y (from
the coarse position measurements), the unit vectors (u, v and w)
for the three resonator axes 21-1, 21,-2 and 21-3 can be
calculated. Since there are two possible directions for each of u,
v and w, this results in eight possible combinations of angles
between the resonator axes and in order to determine the correct
combination, these must be compared with the actual angles between
the resonator axes which are known in advance. This can be done
using a standard minimisation algorithm. For example, if a, b and c
are the unit vectors in the direction of the axes of the unrotated
resonator combination, then the quantity:
.lambda..sup.2=(u.multidot.v-a.multidot.b).sup.2+(v.multidot.w-b.multidot.-
c).sup.2+(w.multidot.u-c.multidot.a).sup.2 (30)
[0115] can be calculated for each of the eight possible
combinations of u, v and w, and the estimates for X and Y can be
varied to minimise .lambda..sup.2. The values of X and Y which
minimise .lambda..sup.2 are the best estimates for the resonator
position, and the choice of u, v and w which gives this minimum
value specifies the orientation of the resonator combination.
Although it may not be apparent from equation 30, the use of a
resonator triplet which does not form one of the two ambiguous
configurations discussed above, guarantees that .lambda..sup.2 will
be minimised for only one choice of u, v and w. It is possible that
there will be values of X and Y other than the resonator
co-ordinates for which .lambda..sup.2 reaches a local minimum, but
starting the minimisation algorithm with estimates of X and Y
derived from the coarse position measurements ensures that these
local minima are avoided.
[0116] An alternative resonator combination which can provide
complete orientation information of the resonator combination is
illustrated in FIG. 18. As shown, this resonator combination
comprises a pair of resonators separated by a fixed (known)
distance. In this case, for the orientation of the combination to
be unambiguous, the axes 21-5 and 21-6 of the two resonators 18-5
and 18-6 must not be parallel or perpendicular to the line 131
joining their centre points 133 and 135. If this condition is not
met, then there is a rotation about the line 131 which preserves or
reverses both axes, and these two orientations cannot be
distinguished. Additionally, the signals generated in the sensor
windings from each of the two resonators 18-5 and 18-6 must be
distinguishable from each other. This is most easily achieved by
using resonators having different resonant frequencies.
[0117] As with the resonator triplet discussed with reference to
FIG. 17, the coarse X and Y position of the combination can be
calculated from the coarse position measurement from either
resonator (or as an average of their coarse X and Y positions)
using equations 19 and 20. Similarly, the Z position of the
resonator combination can be calculated from the amplitude
measurements from either resonator (or again from an appropriate
average of the amplitudes of the signals from the two resonators).
This leaves the measurement of the fine X and Y position and the
orientation of the combination to be determined. In order to do
this, more information must be extracted from the coarse position
measurements. For example, since the coarse position measurements
indicate the X and Y positions of the two resonators 18-5 and 18-6,
the difference between the coarse positions will therefore indicate
the direction of the line 131 in the X-Y plane, i.e. the
orientation 0 of the resonator combination. Further, since the
distance between the centres 133 and 135 of the resonators 18-5 and
18-6 is known, there are only two possible directions for the line
131, depending upon which resonator 18-5 or 18-6 is higher.
Therefore, comparing the amplitude values from the two resonators
determines which resonator is higher, and therefore determines the
direction of the line 131. This therefore defines the overall
orientation of the resonator combination, except for the rotation
about the line 131. As in the case of the resonator triplet, this
last rotation and the fine position measurement of the X and Y
position can be determined using a standard minimisation
technique.
[0118] As will be apparent to those skilled in the art, the use of
a two resonator combination is advantageous over a resonator
triplet combination in a system in which a plurality of different
objects are to be tracked relative to the digitising tablet and
especially if the operating frequency bandwidth is limited.
However, this two resonator embodiment suffers from the problem
that the derivation of the fine position parameters involve the use
of coarse position measurements and amplitude measurements, which
may compromise the overall accuracy of the system.
[0119] A number of modifications which can be made to the above
digitising systems will now be described together with a number of
alternative applications.
[0120] In the above embodiments, periodic windings having a first
period were used to excite the resonator and periodic windings
having a second different period were used to receive the signal
generated by the resonator. FIG. 19 schematically illustrates the
form of a digitising tablet 9 which comprises the same windings
(generally indicated by reference numeral 161) as the digitising
tablets of FIGS. 1 and 14, together with a separate excitation
winding 151 mounted around the periphery of the windings 161. As
shown, in this embodiment, the excitation winding 151 is wound two
times around the other windings 161.
[0121] The general operation of this embodiment is similar to the
above embodiments. In particular, in this embodiment, an excitation
signal is applied to the winding 151 which energises a resonator
located within the stylus 11 and causes it to resonate, which in
turn induces signals in each of the windings 161. In this
embodiment, the signals induced in all of the eight windings are
used. It can be shown that, after demodulation, the signals induced
in the four windings used for determining the X position (i.e. the
sin A, cos A, sin B and cos B windings) have the following
form:
S.sup.A=A.sub.0e.sup.-.omega..sup..sub.A.sup.zr.sub.x sin
(.omega..sub.Ax+.phi..sub.x-.alpha..sub.x) (31)
C.sup.A=A.sub.0e.sup.-.omega..sup..sub.A.sup.zr.sub.x cos
(.omega..sub.Ax+.phi..sub.x-.alpha..sub.x) (32)
S.sup.B=A.sub.0e.sup.-.omega..sup..sub.B.sup.zr.sub.x sin
(.omega..sub.Bx+.phi..sub.x-.alpha..sub.x) (33)
C.sup.B=A.sub.0e.sup.-.omega..sup..sub.B.sup.zr.sub.x cos
(.omega..sub.Bx+.phi..sub.x-.alpha..sub.x) (34)
[0122] Where A.sub.0 is a coupling factor between the transmit
winding 151 and the resonator. Taking the arc-tangent of the ratio
of these signals provides a measurement of
.omega..sub.Ax+.phi..sub.x-.alpha..sub.x and
.omega..sub.Bx+.phi..sub.x-.alpha..sub.x. Similar measurements are
also provided by the sin C, cos C, sin D and cos D windings for use
in determining the Y position. Taking the sum and difference of the
arc-tangents for each of the X and Y direction signals yields the
coarse position measurements and the fine position measurements
defined by equations 19 to 22, as before. The X and Y position and
the orientation of the resonator can therefore be derived in the
same manner as before. However, in this embodiment, the overall
amplitude of the signals induced in the eight receive windings
depends on the coupling factor A.sub.0 between the transmit winding
151 and the resonator, which is a function of the position and
orientation of the resonator. However, since the orientation and
the X and Y position of the resonator has been calculated, these
can be combined with the amplitude values and the known field
pattern from the peripheral transmit winding 151 to determine the
height (Z) of the resonator above the digitising tablet 11. For a
simple transmit winding such as the one shown in FIG. 19, the
generated magnetic field pattern produced by energising the
transmit winding is essentially uniform over the operating region,
so that A.sub.0 only depends on the orientation of the resonator,
thereby simplifying the determination of Z.
[0123] In the above embodiments, the windings used to sense the
signal generated by the energised resonator comprise a plurality of
alternating sense conductive loops. As discussed above, this type
of winding is advantageous because it is relatively immune to
electromagnetic interference and does not itself cause much
interference to other electronic circuits. However, the use of such
windings is not essential. What is important is that the winding
generates a magnetic field which varies in a predetermined manner,
preferably sinusoidally. FIG. 20a illustrates the form of an
alternative winding which can be used. The periodic winding 171
comprises ten periods of alternating convolutions. By considering
the magnetic field generated by a current flowing in the winding
171, it can be shown that both the Z and X component of the
magnetic field generated by this winding, when energised, varies
sinusoidally in a similar manner to the magnetic field generated by
the windings shown in FIG. 4. Therefore, this winding may be used
in place of one of the windings shown in FIG. 4, but is more likely
to form one of a set of similar windings. However, the use of
winding 171 is not preferred because background electromagnetic
interference will couple into the winding and will produce errors
in the output signals. FIG. 20b illustrates the form of another
alternative winding 172 which can be used. As shown winding 172 is
formed from generally triangular shaped loops which narrow in from
their ends towards the central cross-over point. The shape of this
winding is arranged so that the output signal varies approximately
linearly in the measurement direction (i.e. in the X direction)
with the position and orientation of the stylus. By considering the
output signals from this winding and the output from, for example,
another similar winding having a different rate of narrowing of the
loops, the position and orientation can be determined. In order to
be able to determine the orientation of the stylus in this
embodiment, another similar winding would be required having for
example, a different taper so that the signals from each vary in a
different linear manner.
[0124] In the above embodiments, one or more resonators were
provided in the stylus or the toy car. The resonators used
comprised an inductor coil and a capacitor. Other forms of
resonator can be used, such as magnetostrictive resonators, ceramic
resonators or any combination of these. The use of resonators is
preferred in most applications because the stylus and the toy car
can be passive and the output signals generated by resonators are
much larger than those generated by, for example, conductive
screens or short circuit coils. Additionally, resonators allow the
use of a pulse-echo interrogation technique, like the one described
above which reduces interference caused by the direct coupling
between the excitation winding and the receive windings. However,
even if the signals on the receive windings are processed at the
same time as the excitation winding is excited, the signal from the
resonator can be distinguished from the signal from the excitation
winding because they are 90 degrees out of phase. The same is not
the case with a conductive screen or a short circuit coil. However,
a system using conductive screens or short circuit coils could, in
theory, be used. However, in such embodiments, it may be difficult
to derive the full orientation information, since it is difficult
to design different short circuit coil combinations and conductive
screen combinations which will produce distinguishable signals from
each.
[0125] An alternative possibility instead of a resonator is the use
of one or more powered coils. The coils can be powered by a battery
located within, for example, the stylus. In such an embodiment, the
stylus would also comprise a local oscillator for generating a
drive signal for application to the coil. Where more than one coil
is provided, a waveform generator would be required for generating
the different driving signals for the different coils, so that the
signals induced in the digitising tablet windings from the
different coils can be distinguished.
[0126] In the above embodiments, a single object was provided which
was moveable relative to the digitising tablet. FIG. 21 is a
perspective view of an electronic chess game 175 embodying the
present invention. The electronic chess game comprises a digitising
tablet 9 (which is the same as the digitising tablet used in the
embodiment described with reference to FIG. 1) which is used to
sense the position and orientation of the playing pieces 177
located on the chess board. In order to differentiate between the
signals from each of the different playing pieces 177, each piece
177 carries a resonator having a different resonant frequency.
Since there are 32 pieces in a chess game, this involves the use of
32 different resonant frequencies. If the bandwidth available is
limited, then the resonators used may comprise a ceramic resonator
in series with the coil and capacitor in order to improve the
frequency discrimination between the signals from the different
resonators. In this embodiment, the processing electronics must
energise and process the signals from each playing piece. This is
preferably performed sequentially, but can be performed
simultaneously if multiple processing channels are used.
[0127] In order to sequentially apply an appropriate energising
signal to the excitation windings, a digital waveform generator
which can be tuned to all the resonant frequencies of interest is
required. Continuous control of tuning around these frequencies of
interest is desirable to enable the computer control system (not
shown) to be able to optimise frequency and hence signal levels,
even in the case of poorly tuned resonators. This enables untuned
(cheap) resonators having high Q factors to be used. In order to
maximise signal levels, the computer can vary the frequency of the
energisation signal in order to gain maximum signal levels. It may
also detect both the in-phase and the quadrature phase return
signals from the resonator in order to detect the return phase of
the signal and align the phase with the optimum value. This control
of the phase, frequency and amplitude of the excitation signals can
be achieved, for example, by using a field programmable logic cell
array.
[0128] The limit on the number of playing pieces which can be
tracked is determined purely on the availability of different
resonator frequency values, given the Q of the resonators and
appropriate spacing between these frequencies to avoid crosstalk
between tracked resonators. In practice, resonators can easily be
obtained with 100 kHz increments from 100 kHz to 10 MHz, resulting
in the potential to have up to 100 uniquely trackable
resonators--the Q's being such that +/-10 kHz would be enough
isolation between channels. In this embodiment, it takes
approximately 4 ms to determine the position of a playing piece.
Therefore, it will take 128 ms to determine the current position of
all 32 pieces of the chess game, thus allowing the dynamic tracking
of the pieces.
[0129] FIG. 22 schematically illustrates the cross-section of one
of the playing pieces 177 of the chess game. As shown, in this
embodiment, a resonator 18-7 having an axis perpendicular to the
base 178 is provided. This ensure that when the piece is located on
the board game, the axis 21-7 of the resonator points in the
Z-direction. In an alternative game, such as a football game, where
the orientation of each playing piece is relevant to the game, each
playing piece 177 may carry a resonator combination comprising two
or more resonators, such as those shown in FIG. 17 or 18, from
which the complete orientation of the piece can be determined in
addition to its current position relative to the digitising tablet
using the techniques described above.
[0130] As those skilled in the art will appreciate, some
embodiments of the present invention can be used in a virtual
reality system, for example to track the movements of a 6D
joystick. Typically such systems use AC magnetic coupling to track
the position of objects. The digitising tablet system described
above can be used to mimic this function at lower cost and with a
more convenient planar, set of receive windings. However, since
this type of embodiment must be able to operate with relatively
large distances between the joystick and the digitiser tablet, and
since accuracy is not a key feature, windings having a single
period over the measurement area are preferably used (since the
fall-off of the magnetic field is inversely proportional to the
pitch of the windings). In such an application, instead of the two
or three resonator design illustrated in FIGS. 17 and 18, three
resonators having different resonant frequencies can be used which
are placed in different positions on the joystick. The twist of the
joystick perpendicular to the digitising tablet can then be
calculated from the relative positions of the three resonators and
the pitch and yaw of the joystick can be calculated from the
relative heights of the resonators above the digitising tablet.
Calibration for zero pitch and yaw may be performed by holding the
joystick vertical. In such an application, the joystick is
preferably powered either by a battery or by direct connection to
the processing electronics, since this increases the achievable
range, limits the electromagnetic emission and enables accurate
gap, pitch and yaw calculations based on signal levels alone
without recourse to ratios.
[0131] A further application for this type of position encoder is
to provide a position feedback measurement in a magnetic levitation
system. In such an application, balanced windings of the form shown
in FIG. 4 would be essential since such levitation systems use
large AC and DC magnetic fields which would interfere with the
windings if they are not balanced, i.e. if they do not comprise an
equal number of alternate sense loops. In order to speed up the
electronic processing in this embodiment (and in any of the other
embodiments), the signals from each of the sensor coils (sin B, cos
B, sin D and cos D) could be detected simultaneously with their own
processing channel, rather than using the time-multiplexed approach
illustrated in FIG. 5. However, this increases the complexity and
cost of the processing electronics and is only favoured where it is
essential to obtain the position measurements quickly.
[0132] FIG. 23 illustrates a further application of the X-Y
digitising system embodying the present invention. In particular,
FIG. 23 is a perspective view of a personal computer 181 which has,
embedded behind the liquid crystal display 183 thereof, a set of
windings for determining the X and Y position of a stylus 11
relative to the LCD display 183. In this embodiment, balanced
windings are used because they are relatively immune to
electromagnetic interference and because they cause little
interference to other circuits, and can therefore, be located
behind the liquid crystal display, without affecting its operation.
Existing display systems which have a touch screen ability use fine
coils printed onto the surface of the display. These have a high
resistance and therefore suffer from the same problem as screen
printed inks. The printed coils also reduce the transparency of the
screen. In contrast, the digitiser windings have a relatively low
resistance and can be placed behind the liquid crystal display.
[0133] FIG. 24 shows a cross-sectional view of the liquid crystal
display 183 shown in FIG. 23 taken through the line S-S. As shown,
the display comprises a protective top layer 191 overlaying the
liquid crystal layer 192 which is sandwiched between two layers of
electrodes 193 and 195. An insulating layer 197 is provided behind
the lower layer 195 of electrodes to electrically shield the
electrode layer from the digitiser windings 199 which are
sandwiched between two halves 201 and 203 of a substrate. In this
embodiment, the windings 199 are formed in a single layer. In order
to reduce the effect of any metal objects located behind the LCD
display, a layer 204 of magnetically soft material, such as rubber
containing ferrite powder, is provided behind the substrate layer
203.
[0134] FIGS. 25a and 25b schematically illustrate the form of a
quadrature pair of windings 211 and 213 used in this embodiment. As
shown, each of the windings 211 and 213 comprises a single period
of alternating sense loops, with each loop comprising four turns of
conductor. By increasing the number of turns in each loop, the
signal levels output by the windings 211 and 213 are increased. As
with the windings described with reference to FIG. 4, the windings
211 and 213 are designed to generate, when energised, a magnetic
field which sinusoidally varies in the X direction. Additionally,
the spacing (in the X direction) between the turns of conductor
forming the loops are arranged in order to try to reduce the higher
order spatial harmonics of this field mentioned above. The windings
211 and 213 are arranged to extend across the entire LCD display
183, and in this embodiment extend in the X direction by 250 mm and
in the Y direction by 180 mm. The fall off rate of the magnetic
field generated by windings 211 or 213 is therefore much less than
the fall off rate of the windings described with reference to FIG.
4. These windings can therefore be used to detect the position of
the stylus over a larger gap between the windings and the
stylus.
[0135] In addition to the two windings 211 and 213, a further two
windings which constitute a phase quadrature pair are required for
the X direction measurement. These other two windings may comprise,
for example, two periods of alternating sense loops, again with
each loop comprising four turns. Additionally, a further four
windings will be required for the Y direction measurement. If a
different number of turns are used to define each winding, then
different amplifications or weightings need to be applied to the
different received signals in order to compensate for this.
[0136] As in the embodiment described with reference to FIG. 1, the
stylus 11 may comprise a resonator which is energised by the
windings located behind the LCD display. However, in order to save
battery power of the personal computer 181, in this embodiment, the
stylus 11 is preferably powered by a replaceable battery. FIG. 26
illustrates such a powered stylus 11. As shown, the stylus
comprises a battery 221, an oscillator chip 223, a signal
processing chip 225, a coil 227 which is wound around a ferrite
core 229 and a user actuable control button 230. As shown in FIG.
27, the local oscillator 223 generates a local frequency signal
which is applied to the signal processing chip 223 which comprises
a signal generator 231 and an amplifier 233. The signal generator
231 generates an appropriate drive signal for application to the
coil 227 and the amplifier 233 amplifies this signal prior to
application to the coil 227. As shown in FIG. 27, the output signal
from the amplifier is applied to the coil 227 via switch 235 which
is controlled by the user actuable control button 230 shown in FIG.
26. By using the stylus described above with, for example, an AAA
alkaline battery, an operating life of around one thousand hours
can be achieved.
[0137] In the above embodiments, two dimensional X-Y digitising
systems have been described. Some aspects of the present invention
are not, however, limited to two dimensional position encoders. In
particular, some aspects of the present invention can be
incorporated into a one dimensional linear or rotary position
encoder. FIG. 28a illustrates the form of a one dimensional linear
position encoder 251 embodying the present invention. The encoder
comprises a support 253 which carries four separate windings 254-1,
254-2, 254-3 and 254-4 which are connected to an excitation and
processing circuit 255. The encoder is used for determining the
position of a resonant circuit 257 which is moveable in the X
direction, as represented by double headed arrow 259.
[0138] As shown in FIGS. 28b to 28e, each of the windings 254 is
formed by generally hexagonal shaped loops, with adjacent loops
being wound in alternate sense. As shown, windings 254-1 and 254-2
together form a phase quadrature pair and have five periods
(.lambda..sub.3) which extend over the measurement range.
Similarly, the windings 254-3 and 254-4 also constitute a phase
quadrature pair, but these windings extend for six periods
(.lambda..sub.4) over the measurement range. The shape of the
windings 254 are arranged so that the magnetic field generated by
an excitation signal applied to them varies sinusoidally with
position along the X direction. In this embodiment, windings 254-3
and 254-4 are energised by the excitation and processing circuits
255 and the signals induced in windings 254-1 and 254-2 by the
resonator 257 are processed to extract the resonator position in
the X direction. In addition to the resonator X position, an
estimate of the tilt of the resonator in the X-Z plane, i.e.
.alpha..sub.x can be derived using the coarse position measurement
and the fine position measurement phase, from equation 20 above.
Additionally, it is possible to obtain a rough estimate of the
height of the resonator 257 above the plane of the support 253.
However, since this height also depends on the overall orientation
of the resonator 257, the accuracy of the estimated height depends
upon the extent of the tilt of the resonator 257 in the Y-Z plane
(which cannot be determined from the measurements obtained in this
embodiment).
[0139] In the above embodiments, two phase quadrature pairs of
windings were used to determine the position of an object in each
of the directions to be measured, and the number of periods of one
pair of quadrature windings was one less than the number of periods
in the other pair of quadrature windings. With this configuration,
the coarse position measurement phase gives an absolute position
measurement of the object over the entire active length of the
digitising tablet. In an alternative embodiment, the number of
periods in each of the pairs of quadrature windings may differ by
more than one, in which case, the coarse position measurement will
not give an absolute measurement of the object's position. If
absolute position measurement is required, then this can be
achieved by defining a home position against which the object can
be registered in order to obtain an initial position, and then
absolute position measurement is achieved by tracking the object as
it is moved across the measurement area. However, this embodiment
is not preferred, since the absolute position of the object is lost
at power down and if the object is removed completely from the
sensing range of the windings.
[0140] An alternative solution is to provide a third set of
quadrature windings, again having a different pitch to the windings
of the other two sets, from which it is possible to perform a
Vernier type calculation in order to retrieve the absolute position
of the object. Additionally, in this embodiment, when a single
resonator is carried by the object, the signals induced in the
three sets of windings can be used to increase the accuracy of the
coarse position measurement. For example, if a ten period winding,
a seven period winding and a four period winding are provided, then
(i) the signals from the ten and the seven period windings can be
combined to give a coarse position measurement which does not vary
with the tilt of the stylus but which linearly varies three times
between -.pi. and .pi. over the measurement range; (ii) the signals
from the ten and the four period windings can be combined to give a
coarse position measurement which does not vary with the tilt of
the stylus but which linearly varies seven times between -.pi. and
.pi. over the measurement range; and (iii) these two coarse
position measurements can then be used in a Vernier type
calculation to determine more accurately the position of the stylus
in the measurement direction.
[0141] In the above embodiments, the height of the stylus above the
digitising tablet was determined from the amplitude values of the
signals induced in the receive windings. This measurement is,
however, prone to error due to variation in the resonant frequency
and the Q of the resonator, and temperature effects in the track
and processing electronics (which affects the constant value A in
equation 39). However, since these errors will cause the same
offset in the amplitude of the signals received from the different
receive windings, the height of the stylus above the digitising
tablet can be calculated more accurately by using relative
amplitudes of signals whose signal variation with gap is different.
In other words, by taking the relative amplitudes of signals
received from windings having different periods. More specifically,
in the embodiment which uses a periphery mounted excitation
winding, the height (Z) of the stylus above the digitising tablet
can be determined by taking the ratio of the amplitudes of the
signals received in the different period receive windings. In the
first embodiment, however, where an excitation winding having a
first period and a receive winding having a second different period
is used, it is not possible to determine such a relative amplitude
term as easily. In such an embodiment, three sets of periodic
windings would be required, each having a different period. The
signals from two sets of windings can be used to determine a first
amplitude value (from equation 25) and the signals received from
one of those two sets of windings and the third set of windings can
be used to provide a second amplitude value. The ratio of these two
amplitude values will provide an indication of the height (Z) of
the stylus above the digitising tablet which is unaffected by the
errors caused by the variation in the resonant frequency of the
resonator and the temperature effects of the track and the
processing electronics, provided the difference in the number of
periods between the windings which are used to provide the
amplitude measurement values is not the same.
[0142] In the above embodiments, the windings were wound around a
wiring loom and then sandwiched between two halves of a substrate
layer. In an alternative embodiment, the wires could be bonded onto
the substrate as they are being wound in the required
configuration. The bonding can be achieved by applying, for
example, ultrasonic energy to the wire which melts the substrate
and subsequently forms a bond with the wire when it cools.
[0143] In the above embodiments, the signals from the quadrature
sets of windings were used and phase measurements were obtained by
performing an arc-tangent calculation. It is possible to extract
the phase information from the received signals without performing
such an arc-tangent function. The applicants earlier International
application WO98/00921 discloses a technique for extracting the
phase information and converting it into a time varying phase. A
similar processing technique could be used to extract the phase
information from which the relative position of the stylus and the
digitising tablet can be determined together with the relative
orientation.
[0144] In each of the above embodiments, phase quadrature windings
were used. This is because the amplitude of the received signals
(which vary sinusoidally with the X or Y position) varies with the
height (Z) of the stylus above the windings, and therefore, by
taking the ratio of the quadrature signals, this amplitude
variation with height can be removed and the positionally varying
phase can be determined from a straight forward arc-tangent
function. In an alternative embodiment, two windings which are
shifted relative to each other in the measurement direction can be
used. However, this embodiment is not preferred, since a more
complex processing must be performed to extract the positionally
varying phase. Alternatively still, three windings shifted relative
to each other by one sixth of the winding pitch can be used, and in
particular can be used to regenerate the quadrature output signals.
Additionally, in an embodiment where, for example, the stylus is at
a fixed height above the windings, the provision of the second or
third shifted winding is not essential, because the amplitude of
the sinusoidal variation does not vary. Therefore the positional
information can be determined using the output signals from the
different period windings.
[0145] In the above two dimensional embodiments, the windings used
for determining the X an Y positions and the orientation, were
formed in mutually orthogonal directions. This is not essential.
All that is required in these embodiments is that there are two
groups of windings which measure the position and orientation in
two different directions, from which the X and Y positions can be
determined and from which the orientation can be determined.
[0146] In the first embodiment, windings having five and six
periods were used. The number of periods used is a design choice
and can be varied to optimise the resolution, accuracy and range of
the system. The resolution and accuracy can be improved with more
periods (up to a limit), but the practical operating range is
typically a third of the pitch of the windings. Therefore, in the
first embodiment six periods over an active length of 300 mm, the
maximum practical operating range is approximately 17 mm.
* * * * *