WO2025099418A1

WO2025099418A1 - Inductive encoder apparatus

Info

Publication number: WO2025099418A1
Application number: PCT/GB2024/052811
Authority: WO
Inventors: Richard John TAYLOR
Original assignee: Renishaw PLC
Current assignee: Renishaw PLC
Priority date: 2023-11-10
Filing date: 2024-11-06
Publication date: 2025-05-15
Anticipated expiration: 2026-05-10
Also published as: WO2025099421A1; WO2025099419A1; WO2025099420A1

Abstract

An inductive rotary encoder comprising first and second members relatively rotatable about an axis of rotation, configured such that: the first member has at least first and second scale tracks extending around a scale track axis, the second member has: i) an excitation coil through which an alternating current is passed so as to generate an alternating magnetic field which is manipulated by the at least first and second scale tracks; and ii) at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, and a second receiver coil for sensing the alternating magnetic field as manipulated by the second scale track, via which the relative rotational position of the first and second members about the axis of rotation can be measured. The inductive rotary encoder apparatus is also configured such that: the excitation coil and receiver coils extend around a coil axis at different radii to each other; and such that the excitation coil is a single direction excitation coil such that in use current flowing through the excitation coil around the coil axis does so in one direction only at any given instant in time.

Description

INDUCTIVE ENCODER APPARATUS

The present invention relates to an inductive encoder, in particular an inductive rotary encoder.

Inductive encoders are known and typically comprise: i) a first member (e.g. what is commonly referred to as a “rotor”, in the case of a rotary encoder) for mounting to a first part of a machine (e.g. such as a shaft that is rotatable relative to a static part of the machine about an axis); and ii) a second member (e.g. what is commonly referred to as a “stator”, in the case of a rotary encoder) for mounting to a second (e.g. static) part of a machine. Typically, one of the members (e.g. the second member/“ stator”) is an active/powered component and comprises transmit (or “excitation”) and receiver coils (and associated electronics) for creating and sensing an alternating magnetic/electromagnetic field. The terms “magnetic field” and “electromagnetic field” are used interchangeably herein because the magnetic fields referred to herein are created by an electric current and therefore can also be referred to as an electromagnetic field. Also, as will be understood, references herein to a magnetic field created by an excitation or transmit coil are references to an “alternating” magnetic field created thereby, and is often referred to herein simply as a “magnetic field” for brevity. Typically, the other member (e.g. the first member/“rotor”) is a passive/unpowered component comprising scale features which manipulate the electromagnetic field sensed by the second member’s (e.g. the stator’s) sensor coils such that the output(s) of the second member’s (e.g. the stator’s) receiver coil(s) are dependent on the relative position (e.g. relative rotational orientation about an axis, in the case of a rotary encoder) of the first and second members about the axis. Accordingly, the relative (e.g. rotational) position (and/or derivatives thereof) of the second member/stator and first member/rotor (and hence of the different parts of the machine) can be measured from the output(s) of the second member’ s/stator’s receiver coil(s).

The present invention relates to improvements in connection with inductive encoders. In particular the present document describes an inductive rotary encoder comprising first and second members relatively rotatable about an axis of rotation. The first member can have at least first and second scale tracks extending annularly around a scale track axis. The second member can have: i) an excitation coil through which an alternating current is passed so as to generate an alternating magnetic field which is manipulated by the at least first and second scale tracks; and ii) at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, and a second receiver coil for sensing the alternating magnetic field as manipulated by the second scale track, via which the relative rotational position of the first and second members about the axis of rotation can be measured. The inductive rotary encoder could be further configured such that the excitation coil and receiver coils extend annularly around a coil axis at different radii to each other.

Accordingly, in accordance with a first aspect of the invention there is provided an inductive rotary encoder comprising first and second members relatively rotatable about an axis of rotation, configured such that: the first member has at least first and second scale tracks extending annularly around a scale track axis; the second member has: i) an excitation coil through which an alternating current is passed so as to generate an alternating magnetic field which is manipulated by the at least first and second scale tracks; and ii) at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, and a second receiver coil for sensing the alternating magnetic field as manipulated by the second scale track, via which the relative rotational position of the first and second members about the axis of rotation can be measured; the inductive rotary encoder being further configured such that the excitation coil and receiver coils extend annularly around a coil axis at different radii to each other, and the excitation coil is a single direction excitation coil such that in use current flowing through the excitation coil does so in one direction only around the coil axis (e.g. clockwise or anticlockwise) at any given instant in time.

The at least first and second scale tracks can extend annularly around the scale track axis. The excitation coil can extend annularly around the coil axis. The receiver coil can extend annularly around the coil axis. As will be understood, references to the scale track, the excitation coil and/or the receiver coil extending annularly (around its respective axis), this includes both fully annularly, and substantially fully annularly (e.g. around at least 75% of the full annular extent, more preferably around at least 90% of the full annular extent).

Preferably, the excitation coil extends circularly around the coil axis at a constant radius. Preferably, the excitation coil extends circularly around the coil axis at a constant, and singular, radius.

Preferably, the excitation coil is a single-turn coil, and/or the first receiver coil is a single-turn coil, and/or the second receiver coil is a single-turn coil.

The first and second receiver coils can be located on opposing radial sides of the excitation coil. Accordingly, the excitation coil can sit radially between the first and second receiver coils. Accordingly, for example, the radius of the first receiver coil can be smaller than that of the excitation coil, which can be smaller than that of the second receiver coil.

The first and second receiver coils can be located on the same radial side of the excitation coil as each other. The first receiver coil can be radially separated from the excitation coil by the second receiver coil, for example. Accordingly, for example, the radius of the first receiver coil can be smaller than that of the second receiver coil, which can be smaller than that of the excitation coil.

The first member can comprise a third scale track extending (e.g. annularly) around the scale track axis. The second member comprises a third receiver coil extending (e.g. annularly) around a coil axis at a different radii to that of the first and second receiver coils and to that of the excitation coil, for sensing the magnetic field as manipulated by the third scale track, such that the relative rotational position of the first and second members about the axis of rotation can be measured from the first, second and third receiver coils. The third receiver coil can be located on the opposite radial side of the excitation coil to the first and second receiver coils. Accordingly, for example, the first and second receiver coils can be located on the radial inside of the excitation coil and the third receiver coil can be located on the radial outside of the excitation coil. Accordingly, for example, the excitation coil can sit radially between the second and third receiver coils. Accordingly, for example, the radius of the first receiver coil can be smaller than that of the second receiver coil, which can be smaller than that of the excitation coil, which can be smaller than that of the third receiver coil.

The alternating current can be passed through the excitation coil at a predetermined operating frequency. The excitation coil can comprise a capacitor configured such that excitation coil operates as a resonant circuit. The capacitor can be placed such that it is located in-line with the excitation coil’s annular/circular extent (i.e. at the same radius as the radius of a conductor wire of the excitation coil).

The inductive encoder, e.g. the first and/or the second member, can comprise a magnetic flux increaser within which a current (i.e. an electrical current) flow is generated by the excitation coil’s magnetic field, and which in turn itself generates a magnetic field which acts to increase, in the region of one or more of the receiver coils, the magnetic flux of the magnetic field sensed by said one or more receiver coils.

In a preferred embodiment, the excitation coil which generates the magnetic field which generates the current flow in the magnetic flux increaser is the same excitation coil which generates the (alternating) magnetic field which is manipulated by the scale tracks and sensed by the receiver coil(s) (and from which the relative position of the first and second members can be measured).

The magnetic flux increaser can comprise: at least one (electrical) conductor located radially outside of the first member’s scale tracks and the second member’s receiver and excitation coils, said at least one (electrical) conductor being hereinafter referred to as at least one “outer” conductor; and/or at least one (electrical) conductor located radially inside of the first member’s scale tracks and the second member’s receiver and excitation coils, said at least one (electrical) conductor being hereinafter referred to as at least one “inner” conductor. In preferred embodiments, the electrical conductivity of the outer and/or inner conductors is such that they have a resistivity of less than IxlO'⁶ ohm-metre (e.g. they could comprise copper which has a resistivity of about 1.7xl0'⁸ ohm-metre).

In a particularly preferred embodiment of the invention, the magnetic flux increaser comprises both an outer conductor and an inner conductor. In such an embodiment, the receiver and excitation coils could be located, in the radial dimension (e.g. with respect to the axis of rotation), between the outer and inner conductors. Similarly, the scale track(s) can be located radially between the outer and inner conductors.

Preferably such outer and inner conductors are separate. The outer and inner conductors can be physically separate. The outer and inner conductors can the outer and inner can be electrically-separate/insulated from each other.

The magnetic flux increaser, e.g. the at least one “outer” conductor, can comprise a closed annular (electrically) conductive loop. In preferred embodiments, the “outer” conductor has a substantially constant radius, i.e. has the shape of a regular circle.

The magnetic flux increaser, e.g. the “outer” conductor, can comprise a metal loop. Copper is one suitable metal, but as will be understood, other metals can be used. The “outer” conductor can comprise a wire-like loop. The “outer” conductor’s (e.g. closed) annular conductive loop can be centred on and extend annularly around an “outer conductor axis”.

The at least one “inner” conductor can comprise a closed annular (electrically) conductive loop or conductive disc. In preferred embodiments, the “inner” conductor has a substantially constant radius, i.e. has the shape of a regular circle.

The “inner” conductor can comprise a metal loop. Copper is one suitable metal, but as will be understood, other metals can be used. The “inner” conductor can comprise a wire-like loop. The “inner” conductor’s (e.g. closed) annular conductive loop or conductive disc can be centred on (and extend around) an “inner conductor axis”.

The inner conductor could be substantially planar. The outer conductor could be substantially planar. In preferred embodiments, the planes of the inner and/or outer conductors are substantially parallel. The inner and outer conductors can be substantially co-planar. Preferably the planes of the inner and/or outer conductors, the plane of the scale tracks, and the planes of the excitation and receiver coils are all substantially parallel to each other (e.g. such that the angles between any such planes is not more than 5°, more preferably not more than 3°, especially preferably not more than 2°, and for instance not more than 1°).

As will be understood (and as explained herein), the magnetic flux increaser (e.g. the inner conductor and/or outer conductor) need not necessarily be located within the same plane as (need not necessarily be co-planar with) the receiver and excitation coils, or within the same plane as (need not necessarily be co-planar with) the scale tracks.

Optionally, the first member comprises a magnetic flux increaser (e.g. at least one “outer” conductor and/or at least one “inner” conductor). In such a case, it can be preferred that the magnetic flux increaser (e.g. at least one “outer” conductor and/or said at least one “inner” conductor) is substantially planar with the at least one scale track. For example, in the case in which the outer and/or inner conductors are closed annular loops (described in more detail below), preferably the perpendicular distance between: i) the plane of the magnetic flux increaser (i.e. of the outer or inner conductor); and ii) the plane of the scale track(s) is not more than 7.5% (more preferably not more than 5%, especially preferably not more than 2%) of the diameter of the magnetic flux increaser, i.e. of the outer or inner conductor.

Optionally, the second member comprises a magnetic flux increaser (e.g. at least one “outer” conductor and/or at least one “inner” conductor). In such a case, it can be preferred that the magnetic flux increaser (e.g. at least one “outer” conductor and/or said at least one “inner” conductor) is substantially planar with at least one of the excitation and receiver coils. For example, in the case in which the outer and/or inner conductors are closed annular loops (described in more detail below), preferably the perpendicular distance between: i) the plane of the magnetic flux increaser (i.e. of the outer or inner conductor); and ii) the plane of the excitation and receiver coils is not more than 7.5% (more preferably not more than 5%, especially preferably not more than 2%) of the diameter of the magnetic flux increaser, i.e. of the outer or inner conductor.

Optionally, the first and second members each comprise a magnetic flux increaser (e.g. at least one “outer” conductor and/or at least one “inner” conductor).

Preferably the magnetic flux increaser does not extend/exist directly behind or directly in-front of the scale tracks, nor directly behind or directly in-front of the receiver and excitation coils.

For instance, the magnetic flux increaser (e.g. at least one “outer” conductor and/or said at least one “inner” conductor) can be provided on and located within the plane of the first member, and the receiver and excitation coils can be provided on and located within the plane of the second member (the first and second members and their respective planes being axially offset, with respect to the axis of rotation, and therefore are not coplanar).

As will be understood, references herein to the magnetic flux increaser (e.g. the inner conductor and/or outer conductor) being located “radially” with respect to the scale tracks, and/or excitation coil, and/or receiver coils, (e.g. “radially outside”, “radially inside”, or “radially between”) is not intended to limit the relative configuration/arrangement of such features in the axial dimension (with respect to the axis of rotation), but rather is intended only to serve as to indicate their relative configuration in the radial dimension (with respect to the axis of rotation). In other words, use of the terms “radially outside”, “radially inside”, or “radially between” is not intended to require the features referred to as being in the same plane as each other. For example, in an embodiment where the first member comprises a magnetic flux increaser comprising an “outer” (e.g. closed) annular conductive loop and “inner” (e.g. closed) annular conductive loop , and where the second member is axially offset with respect to the first member, the receiver and excitation coils (and any at least one magnetic field sensor) of the second member can still be located "radially between” the “outer” (e.g. closed) annular conductive loop and “inner” (e.g. closed) annular conductive loop, by virtue of their respective radial positions and dimensions. Indeed, this is the situation illustrated in the embodiment described above in connection with the Figures, in particular see Figure 7. Similarly, in the embodiment depicted in Figure 17(c), it can still be said that the scale tracks of the first member (rotor) are located radially between the outer and inner annular loops which are provided as part of the second member (stator).

As will be understood, another commonly used term in the field of inductive encoders for “excitation coil” is “transmit coil”. Accordingly, the term “transmit coil” could be used in place of the term “excitation coil” herein.

A scale track can comprise a series, e.g. an array, of (electrically) conduct! ve/non- conductive scale features. In other words, a scale track can comprise a series of alternating (electrically) conductive and non-conductive scale features. A scale track can comprise a periodic arrangement of scale features. Accordingly, a scale track can comprise a periodic arrangement of (electrically) conduct! ve/non- conductive scale features. In other words, a scale track can comprise a periodic series of alternating (electrically) conductive and non-conductive scale features. As will be understood, references herein to conductive (and non-conductive) relate to electrical conductivity, and for brevity will often be referred to as just conductive and non-conductive. As will be understood, the required or desired the level of resistivity of the conductive and non-conductive features can vary from application to application. Nevertheless, in connection with preferred embodiments of the invention, the electrically conductive scale features have a resistivity of less than IxlO'⁶ ohm-metre (e.g. copper has a resistivity of about 1.7xl0'⁸ ohm-metre), and the resistivity of non-conductive features is greater than IxlO'⁴ ohm-metre, and for example can be significantly higher than that, for example, is even greater than 1 ohm-metre, for example greater than IxlO³ ohmmetre.

As will be understood, a scale track can be configured to manipulate the magnetic field dependent on the relative position of the first and second members relatively rotatable about an axis of rotation.

A scale track can be configured such that its manipulating effect on the magnetic field varies in a spatially-cyclical manner. The spatial frequency of such spatially cyclical effect can be dependent on the period of the scale features.

The first and second scale tracks can be configured to manipulate the amplitude of the alternating magnetic field (generated by the excitation coil). Accordingly, a scale track can be configured to manipulate/modulate the amplitude of the (alternating) magnetic field dependent on the relative position of the first and second members relatively rotatable about an axis of rotation. A scale track can be configured to manipulate the amplitude of the (alternating) magnetic field in a cyclical manner the spatial frequency of which is dependent on the period of its scale features.

The at least first and second scale tracks (e.g. the features thereof) could be contained in a first plane. The excitation coil(s) could be contained in a second plane. The receiver coil(s) could be contained in a third plane. Preferably, the second and third planes can be substantially planar parallel, and optionally substantially co-planar. When assembled together, it can be preferred that the first and second/third planes are substantially parallel (e.g. such that the angles between any such planes is not more than 5°, more preferably not more than 3°, especially preferably not more than 2°, and for instance not more than 1°).

As will be understood, typically, and in preferred embodiments of the invention which comprise an inductive rotary encoder, it is configured such that, in use, it is intended/ configured to be arranged such that all of said axes (i.e. the scale track, excitation and receiver axes (or “coil axis”), and the axis of rotation) are substantially parallel to each other (at least to the extent such that during operation the first and second members do not physically come into contact with each other). As will be understood, the actual desired degree of parallelism will depend on various aspects of the inductive rotary encoder, such as its dimensions and the desired level of accuracy. Nevertheless, typically it will be desired that the angle between any of the scale track axis, the excitation and the receiver axes (or “coil axis”) is not more than 5°, more preferably not more than 3°, especially preferably not more than 2°, and for instance not more than 1°. Also, typically it will be desired that the angle between i) any of the scale track axis, the excitation axis and the receiver coil axis (or “coil axis”), and ii) the axis of rotation, is not more than 5°, more preferably not more than 3°, especially preferably not more than 2°, and for instance not more than 1°.

In preferred embodiments, the inductive rotary encoder is configured such that, in use, it is intended/configured to be arranged such that all of said axes (i.e. the scale track axis, excitation and receiver axes (or “coil axis”) and the axis of rotation can be (e.g. are) substantially coincident/coaxial (e.g. offset from each other by not more than 10% of the diameter of the receiver coil, more preferably by not more than 5% of the diameter of the receiver coil).

Similarly, in embodiments comprising an outer and/or inner conductor, preferably, when assembled, the scale track axis, the coil axis, the outer conductor axis and/or the inner conductor axis are all substantially parallel to each other, and optionally are all substantially coincident/coaxial.

Typically, in use, in the case of an inductive rotary encoder, the first member will be mounted on a rotating part of a machine and the second member will be mounted on a static part of machine (accordingly the first member could be referred to as a “rotor” and the second member could be referred to as a “stator”). Nevertheless, as will be understood, this need not necessarily be the case and the members could be mounted the other way around, or both of the first and second members could be mounted on rotatable parts.

As will be understood, the apparatus can be configured to determine from the output of the receiver coils, the rotational position of the first and second members about the axis of rotation, and/or a derivative thereof (such as velocity and/or acceleration). Such a rotational position could be an absolute relative rotational position of the first and second members about the axis of rotation. Accordingly, the apparatus can comprise means for determining from the receiver coils, information concerning the relative rotational position, and/or a derivative thereof, of the first and second members about the axis of rotation. Such means can comprise one or more “processing” or “processor” devices.

As will be understood, a “processing” or “processor” device can comprise a bespoke processing device configured for the specific application (e.g. a field programmable gate array “FPGA”) as well as a more generic processing device which can be programmed (e.g. via software) in accordance with the needs of the application in which it is used. Accordingly, a suitable “processing” or “processor” device can comprise, for example, a CPU (Central Processor Unit), FPGA (Field Programmable Gate Array), or ASIC (Application Specific Integrated Circuit), or the like.

The means, e.g. the “processing” or “processor” device(s), could be provided as part of one or both of the first or second members (in which case, preferably just the second member). But this doesn’t necessarily have to be the case and such means could be provided by one or more components separate to the first and second members. For instance, the inductive rotary encoder apparatus could comprise said first and second members and a separate interface unit comprising said means.

Accordingly, for example, the apparatus could comprise a controller comprising such “processing” or “processor” device(s). Said controller could be the controller of a machine on which the first and second members are, or are to be, mounted. Accordingly, for example, the apparatus can comprise a machine on which the first and second members are, or are to be, mounted.

As will be understood, said means for determining from the output of the receiver coil, the rotational position of the first and second members about the axis of rotation, and/or a derivative thereof (such as velocity and/or acceleration) could be shared/ split across the means provided on the first and/or second members, and another device (e.g. machine controller) that is separate to the first and/or second members.

According to a second aspect of the invention there is provided an inductive encoder comprising first and second members relatively moveable along a measurement direction (e.g. about an axis of rotation), wherein:

• the first member has at least a first scale track extending along the measurement direction (e.g. extending annularly around a scale track axis),

• the second member has: o an excitation coil for generating a magnetic field which is manipulated by the at least first scale track; and o at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, via which the relative rotational position of the first and second members along the measurement direction (e.g. about the axis of rotation) can be measured, • the inductive encoder apparatus is configured to pass an alternating current through the excitation coil at a predetermined operating frequency, and

• the excitation coil comprises a capacitor arranged such that excitation coil operates as a resonant circuit.

The excitation coil can extend substantially circularly about an excitation coil axis, and in which the capacitor is placed such that it is located in-line with the excitation coil’s circular extent. The excitation coil can be a single-turn coil, and/or the first receiver coil is a single-turn coil, and/or the second receiver coil is a single-turn coil. Features described above in connection with the first aspect of the invention are also applicable to the second aspect of the invention and so for the sake of brevity will not be repeated here.

According to a third aspect of the invention there is provided an inductive (e.g. rotary) encoder comprising first and second members relatively moveable along a measurement direction (e.g. rotatable about an axis of rotation), wherein:

• the first member has at least first and second scale tracks (e.g. extending annularly around a scale track axis),

• the second member has: o an excitation coil for generating a magnetic field which is manipulated by the at least first and second scale tracks; and o at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, and a second receiver coil for sensing the magnetic field as manipulated by the second scale track, via which the relative rotational position of the first and second members about the axis of rotation can be measured,

• the second receiver coil is (e.g. radially) separated from the excitation coil by the first receiver coil.

In embodiments in which the encoder is a rotary encoder, the excitation coil and receiver coils extend annularly around a coil axis. Features described above in connection with the first aspect of the invention are also applicable to the third aspect of the invention and so for the sake of brevity will not be repeated here. In particular, the magnetic flux increaser described above in connection with the first aspect of the invention can be particularly beneficial to the configuration of the third aspect of the invention.

Embodiments of the invention will now be described, by way of example only, with reference to the following drawings in which:

Figure 1 is a perspective view of an inductive rotary encoder according to the present invention;

Figure 2 is a schematic cross-sectional view of the inductive rotary encoder of Figure 1 mounted on a machine;

Figure 3 is a plan view of the rotor of the inductive rotary encoder of Figure 1;

Figure 4 is a plan view of the stator of the inductive rotary encoder of Figure 1;

Figure 5 is a plan view of a quadrant of the stator of Figure 4;

Figure 6 shows a plan view of a quadrant of the stator and rotor, wherein the stator’s substrate is transparent so that the rotor can be seen under the stator;

Figure 7 shows a schematic cross-sectional view of the stator and rotor taken along line C-C of Figure 6;

Figure 8 shows a plan view of the excitation coil of the stator, and the outer and inner conductive loops of the rotor, in isolation;

Figure 9 is a block diagram of the system for calculating a measure of the relative rotational position of the rotor and stator;

Figure 10 is a graph illustrating how the output of the Arc Tangent Calculator for each receiver coil varies with relative rotation of the stator and rotor;

Figures 11(a) and (b) show schematic, partial cross-sectional views of the stator and rotor, and serve to illustrates the effect of a change in the lateral/radial position of the stator and rotor (e.g. eccentricity) on the electromagnetic field as sensed by the stator’s sensors;

Figures 12(a), 12(b) and 12(c) show schematic, partial cross-sectional views of the stator and rotor, and serve to illustrates the effect of relative tilt on the electromagnetic field as sensed by the stator’s relative tilt sensors;

Figure 13 is a schematic plan view of the stator’s outer and inner quadrant sensors;

Figures 14(a) and 14(b) illustrate example Lissajous obtained from the outer and inner quadrant sensors;

Figures 15(a) and 15(b) are a plan views of alternative stator configurations;

Figures 16(a) and 16(b) respectively show isometric and plan views of an example excitation coil in isolation;

Figure 16(c) schematically illustrates how the magnitude of the component of magnetic flux density that is perpendicular to the scale (i.e. parallel to the axis D around which the scale extends) as created by the circular, single-direction excitation coil of Figures 16(a) and (b) varies across a bounded radial slice (indicated by the dashed-line in Figure 16(b)), both with (dashed line) and without (solid line) inner and outer conductor loops;

Figure 16(d) shows a plan view of an example excitation coil having a folded- rectangular loop; Figure 16(e) schematically illustrates how the magnitude of the component of the magnetic flux density that is perpendicular to the scale (i.e. parallel to the axis D around which the scale extends) created by the folded-rectangular loop of the excitation coil of Figure 16(d), varies across a bounded radial slice (indicated by the dashed-line in Figure 16(d)), both with (dashed line) and without (solid line) inner and outer conductor loops;

Figure 17(a) shows a front view of a stator (i.e. the side of the stator that will face the rotor when in use) and Figure 17(b) shows a front view of a rotor (i.e. the side of the rotor that will face the stator when in use), according to another embodiment of the invention; and

Figure 17(c) shows a cross-sectional view of the stator and rotor taken along the lines C-C of Figures 17(a) and (b).

Referring to Figures 1 and 2, an example inductive rotary encoder 100 according to the present invention comprises a rotor 200 and a stator 300. As schematically shown in Figure 2, in use, the rotor 200 is mounted to a part of a machine such as a shaft 500 that is rotatable relative to a static part 600 of the machine about an axis A, and the stator 300 is mounted to the static part of the machine. Similar to known inductive rotary encoders, the stator 300 is an active/powered component and comprises excitation and receiver coils (described in more detail below) and associated electronics for creating and sensing an alternating electromagnetic field. Also similar to known inductive rotary encoders, the rotor 200 is a passive/unpowered component comprising scale features which manipulate the electromagnetic field (and in the present example, manipulate/modulate the amplitude of the alternating electromagnetic field) sensed by the stator’s 300 receiver coils such that the particular outputs of the stator’s receiver coils are dependent on the relative rotational orientation of the stator 300 and rotor 200 about the axis A such that the relative rotational position (and/or derivatives thereof) of the stator 300 and rotor 200 (and hence of the static 600 and rotational 500 parts of the machine) can be measured therefrom. As will be understood, the rotor 200 and stator 300 could be configured the other way around (that is the rotor could have the active components, i.e. the excitation and receiver coils, thereon and the stator could have the passive components, i.e. the scale features, thereon) but due to the need for electrical connections it can be simpler and easier for the stator to be the active/powered component. Also, the rotor can be subject to high forces due to its motion and change in velocity, and so it can be beneficial to make the rotor the passive/unpowered component because the electronics on the active components can be susceptible to damage from such forces. As will be understood, in another embodiment, both of the members could rotate, that is both the “stator” and “rotor” could be rotatable, in which case both the “stator” 300 and “rotor” 200 could be referred to as “rotors”, e.g. “rotor 1” and “rotor 2”, or “active rotor” and “passive rotor”. Nevertheless, for the sake of simplicity and illustration, the embodiment described has a static “stator” 300 and a "rotor” 200 that is rotatable relative to the stator 300.

Referring to Figure 3, a plan view of the rotor 200 is shown. In the embodiment described, the rotor 200 comprises a disc-shaped substrate 202 on which scale features are provided. In the embodiment described, the substrate is made from fibreglass and has a generally circular shape and has a hole 203 extending through its middle such that the machine shaft (not shown in Figure 3) can be passed therethrough. A will be understood, the rotor substrate 202 could be made from other (e.g. non-conductive) materials, need not necessarily have a circular shape and need not necessarily have a hole extending therethrough.

The rotor’s 200 scale features are centred on and extend annularly around an axis B. In the embodiment described, the scale features are conductive (in this embodiment copper, and are formed on the substrate by etching/milling away a copper coating on the fibreglass substrate 202; although could be formed via other processes, such as copper plating). In an alternative embodiment, the substrate 202 could comprise a conductive material, wherein non-conductive features are formed thereon so as to provide the scale features. In the embodiment described, the rotor 200 comprises a first scale track 204, a second scale track 206 and a third scale track 208. Each scale track comprises a periodic series of scale features. The periods of the first 204, second 206 and third 208 scale tracks are different and configured such that each scale track has an integer number of periods per revolution and such that the phase relationship between the scale features of the three scale tracks at any angle about the axis B is mutually exclusive compared to any other angle of rotation. As shown, the first 204, second 206 and third 208 scale tracks are arranged concentrically with each other, and as such are all centred on the same axis B.

In the embodiment described, the rotor 200 also comprises an outer conductive loop 210 made from conductive material (in this embodiment, copper) and an inner conductive loop 212 made from conductive material (again, in this embodiment, copper). The outer 210 and inner 212 conductive loops are centred on and extend annularly around an axis D. As explained in more detail below, the outer 210 and inner 212 conductive loops help to increase the magnetic flux density in the radial space between them, thereby helping to increase the signal strength and system sensitivity of the stator’s receiver coils (described below). As shown most clearly in Figures 6 and 7, in the embodiment described, the outer conductive loop 210 is located radially outside all of the three scale tracks 204, 206, 208, and radially outside all of the coils on the stator (i.e. radially outside: i) the excitation coil, ii) the first, second and third receiver coils, and iii) the quadrant coils). Also, as shown most clearly in Figures 6 and 7, in the embodiment described the inner conductive loop 212 is located radially inside all of the three scale tracks 204, 206, 208, and radially inside of all of the coils on the stator (i.e. radially inside: i) the excitation coil, ii) the first, second and third receiver coils, and iii) the quadrant coils). Accordingly, in other words, the three scale tracks 204, 206, 208, and also all of the coils on the stator (i.e. : i) the excitation coil, ii) the first, second and third receiver coils, and iii) the quadrant coils) are located radially between the outer 210 and inner 212 conductive loops. In the embodiment described, the outer 210 and inner 212 conductive loops are closed, single-turn, plain circular copper loops. However, this need not necessarily be the case and the outer 210 and inner 212 conductive loops could take other forms. For instance, the outer 210 and inner 212 conductive loops could have a multiple-turn helical from (e.g. having a fixed radius but which extends axially through the substrate), could comprise a multiple-turn spiral form (e.g. having a constantly changing radius), could progress with a wave-like (e.g. sinusoidal) form/path around the axis. Optionally, multiple separate outer conductive loops 210 could be provided and/or multiple inner conductive loops 212 could be provided.

It has been found that the presence of the outer conductive loop 210 is particularly beneficial for those embodiments which comprise sensors for detecting relative tilt of the stator and/or rotor (described in more detail below). This is because as well as having the effect of increasing the signal strength for the receiver coils, it also makes the at least some of the sensors used in the tilt and/or lateral/radial position (e.g. eccentricity) detection substantially more sensitive to the relative tilt of the rotor and stator, thereby making it easier to distil relative tilt measurements from such sensors. In particular, as explained in more detail below, due to the outer conductive loop 210, it is possible to configure the inductive rotary encoder device such that the output of the outer quadrant coils (described below) are assumed to measure and relate only to the relative tilt of the rotor 200 and stator 300.

Additionally/alternatively, the presence of one or both of the outer 210 and/or inner 212 conductive loops can be particularly useful in those embodiments in which the number of excitation coils is fewer than the number of scale tracks and respective receiver coils (e.g. in order to improve magnetic flux density and hence signal strength thereof).

Additionally/alternatively, the presence of one or both of the outer 210 and/or inner 212 conductive loops is particularly useful for boosting the magnetic flux density perpendicular to the scale in regions radially distal to the excitation coil, which might be desirable where an increase in the radial width of a scale track and/or receiver coil is desired and/or for those embodiments in which a receiver coil and its corresponding scale track cannot be located directly radially next to the excitation coil (e.g. where there is another receiver coil and corresponding scale track radially between them and the excitation coil that generates the electromagnetic field which it is configured to sense) and/or where the excitation coil is configured such that it is present on only one radial side of a receiver coil and its corresponding scale track, such as due to it being a “single direction” excitation coil (described in more detail below). Indeed, all of these are the case in the embodiments of Figures 4 to 7, 15, 16(a), 16(b) and 17 where: i) a single direction excitation coil 330 is used, ii) the first scale track 204 and first receiver coil 304 have a relatively large radial width, and iii) the second receiver coil 306 and its corresponding second scale track 206 are located radially between the third receiver coil 308/third scale track 208 and the excitation coil 308. In this embodiment, the inner 212 conductive loop is particularly useful for boosting the magnetic flux density perpendicular to the scale in the region of the third scale track 208 and associated third receiver coil 308 which (due to the presence of the second scale track 206 and second receiver coil 306 lying radially between the third scale track 208 and third receiver coil 308 and the excitation coil 330) lie relatively radially far away from the excitation coil 330 compared to the first 204, 304 and second 204, 306 scale tracks and receiver coils. Similarly, the outer 210 conductive loop is particularly useful for boosting the magnetic flux density perpendicular to the scale in the radially distal region of the first scale track 204 and first receiver coil 308 (which are significantly radially wider than that of the second 206, 306 and third 208, 308 scale tracks and receiver coils and therefore can benefit from such boosting). This is explained in more detail below in connection with Figures 16(b) and 16(c).

As will be understood, regardless of the reason why, booting the magnetic flux density of the electromagnetic field in the region of the scale tracks increasing the eddy currents generated in the conductive scale features which in turn provides a greater amplitude modulating effect on the alternating electromagnetic fields sensed by the corresponding receiver coils.

Turning now to Figures 4 and 5, Figure 4 shows a plan view of the stator 300 and Figure 5 shows a detailed view of one quadrant of the stator 300. The stator 300 comprises a disc-shaped substrate 302 on which the excitation and receiver coils are provided, along with associated electronics which are provided on the other side of the substrate 302 to that shown. (In Figure 5, the shading of the stator substrate has been omitted to aid viewing of the other features of the stator). In the embodiment described, the substrate 302 of the stator is made from fibreglass and has a generally circular shape and has a hole extending through its middle such that the machine shaft 500 can be passed therethrough. In accordance with other embodiments of the invention, the stator substrate 302 could be made from other non-conductive materials (such as ceramic), need not necessarily have a circular shape and need not necessarily have a hole extending therethrough.

The stator 300 comprises a first scale receiver coil 304, a second scale receiver coil 306, and a third scale receiver coil 308, which are concentrically arranged and each of which are centred on and extend annularly around the axis D. As shown, the width as measured in the radial dimension (e.g. “radial width” R_w) of the first 304 receiver coil is larger than that of the second 306 and third 308 receiver coils. In the embodiment show, the radial width R_w of the first 304 receiver coil is about double that of the second 306 and third 308 receiver coils. As will be understood, because the receiver coils are progress around the axis D along a wave-like (e.g. sinusoidal) path, another appropriate term for radial width of a receiver coil is “coil amplitude”.

In the embodiment described, each of the first 304, second 306 and third 308 receiver coils actually comprise a first sinusoidally extending differential copper coil and a second sinusoidally extending differential copper coil. The first and second differential coils are overlapping and are phase shifted by 90° relative to each other. Accordingly, the first differential coil could be referred to as a SIN coil and the second differential coil could be referred to as COS coil.

Accordingly, each of the first 304, second 306 and third 308 receiver coils will actually output a pair of signals which are in quadrature (i.e. the signals are phase- shifted by 90°), and so, for example, can be labelled as SIN and COS signals. As will be understood, other configurations are possible, such as a three-phase system, comprising three sets of coils phase shifted by 60°. In another embodiment, each track/channel could comprise just a single coil, but such a system could be less accurate.

As shown in Figures 4 and 5, the first 304, second 306, and a third 308 scale receiver coils are sinusoidal in that they progress around the axis D along a substantially sinusoidal path. In fact, in the embodiment described, each of the SIN and COS coils of the first 304, second 306 and third 308 receiver coils are actually differential coils, having a clockwise and an anti -clockwise turn per period to cancel the residual magnetic field. The coil shape is sinusoidal for harmonic suppression reasons. Of course, the coil shape/path does not have to be sinusoidal. For instance, the coils could have a different periodic shape/path to pick up the modulation from the scale. Also, they don’t have to be differential coils. In the embodiment described, the period of the sinusoidal form of the first 304, second 306, and a third 308 scale receiver coils are different to each other (and in the example described are configured such that the period of the sinusoidal form of the first 304, second 306, and a third 308 scale receiver coils matches that of their corresponding scale track). As will be understood, this doesn’t have to be the case, although doing so can be beneficial from a processing and signal quality point of view.

In the embodiment described, each of the first 304, second 306 and third 304 receiver coil comprises a single-turn (or “single loop”) coil (in that they are wound around/extend around/turn around the axis D only once, rather than multiple (e.g. overlapping) times). In another embodiment, one or more of receiver coils could comprise a multiple-turn (or “multiple-loop”) coil, in which case they would wind around/extend around/turn around the axis D more than once, i.e. multiple (e.g. overlapping) times.

As explained in more detail below, when in use/assembled, the inductive encoder apparatus is configured such that: i) the first scale receiver coil 304 and the first scale track 204 are arranged directly in front of (i.e. “directly facing”) each other (in other words the first scale receiver coil 304 is arranged axially in-line with/directly above, in the orientation shown in Figure 7, the first scale track 204); ii) the second scale receiver coil 306 and the second scale track 206 are arranged directly in front of (i.e. “directly facing”) each other (in other words the second scale receiver coil 306 is arranged axially in-line with/directly above, in the orientation shown in Figure 7 the second scale track 206; and iii) the third scale receiver coil 308 and the third scale track 308 are arranged directly in front of (i.e. “directly facing”) each other (in other words the third scale receiver coil 308 is arranged axially in-line with/directly above, in the orientation shown in Figure 7, the third scale track 208).

The stator 300 also comprises an excitation coil 330 (which could also be referred to as a “transmit coil”) which is centred on and extends annularly around the axis D. In the embodiment described, the excitation coil 330 is located radially between the first scale receiver coil 304 and the second scale receiver coil 306.

In accordance with an aspect of the present invention, the excitation coil 330 can be a “single direction” excitation coil in that it comprises a conductor wire extending along a circular path around the axis D in a single direction, such that the current flows around the axis D in one direction only (e.g. clockwise or anticlockwise) at any given instant in time. In accordance with preferred embodiments of the invention, in the particular embodiment described, the conductor wire has a substantially constant, and singular, radius and therefore follows one substantially circular path. In the particular embodiment described, there is only one/single identifiable radius of the conductor wire. A suitable example excitation coil 330 having a conductor wire extending in a circular path around the axis D in a single direction, having a constant, and singular, radius is illustrated in isolation in Figure 16(a) and 16(b).

In the particular embodiment shown in Figure 16(a) and 16(b), the excitation coil 330 is a single-turn (or “single loop”) excitation coil. Accordingly, it is “wound around’7“ extends around’7“turns around” the axis D only once, rather than multiple (e.g. overlapping) times. In the embodiment described, the single- turn/loop excitation coil comprises two single-turn (copper) conductor wires 330a, 330b that extend circularly/annularly about the axis D once, which have the same radius “r” as each other and which are arranged in parallel on adjacent layers of the substrate 302 (and therefore are spaced apart along a direction parallel to the axis D as illustrated in Figure 16(a)), their respective ends being joined to a single pair of terminals (+/-). Accordingly, as configured, the two conductor wires 330a, 330b form a single direction, single-turn, circularly/annularly-extending excitation coil that extends around the axis D, and which is concentric with the first 304, second 306 and third 308 receiver coils. The benefit of using two conductor wires configured as described is that the resistance is reduced compared to an equivalent excitation coil that only has one conductor wire (of the same dimensions), which increases the quality (Q factor) of the excitation coil. As will be understood, in an equivalent embodiment, a similar reduction in the resistance of the excitation coil could be achieved with one conductor wire, by using a thicker conductor wire and/or a wire made from a material having reduced inherent resistance.

As will be understood, in other embodiments, the excitation coil 330 could comprise multiple-turns or “loops”, in which case the excitation coil’s conductor wire winds around the axis D more than once, e.g. adopting a helical form having a substantially constant radius. However, it can be preferred that the excitation coil is just a single turn or “loop” excitation coil, in order to minimise the inductance of the excitation coil, thereby relatively reducing the required drive voltage to achieve the desired magnetic field. However, as will be understood, for a given desired magnetic field to be created by an excitation coil, lowering the number of turns the excitation coil has, means that the level of current/amps required through the excitation coil needs to be increased. Whilst this could be achieved by providing a power supply/drive circuitry which directly provides the required level of current/amps to be passed through the excitation coil 330, the current (amp) requirement of the power supply/drive circuitry could be reduced by configuring the excitation coil to operate as a “resonant circuit” (also known as an “LC circuit”, “tank circuit” or “tuned circuit”), e.g. such that it stores energy that oscillates at the same frequency as the oscillating power supply /drive circuitry. In such a case the power supply/drive circuitry merely needs to supply sufficient current to overcome the losses in the excitation coil 330. An effective way of configuring the excitation coil 330 to operate as a resonant circuit is to configure the excitation coil 330 with a suitable capacitor 331 (which in such a circumstance can be referred to as a “resonant capacitor”), e.g. as schematically illustrated in Figures 16(a) and (b). In the embodiment described, the operating/drive frequency of alternating magnetic field/the stator’s excitation coil is approximately 3 MHz, and so the capacitor is selected accordingly, based on the inductance of the excitation coil’s circularly-extending conductor wires.

In the particular embodiment described, due to space constraints, the (“resonant”) capacitor 331 is provided on a different layer of the stator substrate 302 to that of the conductor wire(s) 330a, 330b of the excitation coil 330 (and therefore is illustrated in Figure 16(a) as being at a different position to the conductor wires 330a, 330b along a direction parallel to the axis D). However, in the embodiment described, the capacitor 331 is advantageously physically located to lie on substantially the same radius as that of the conductor wires 330a, 330b. This can be advantageous because the capacitor 331 contributes to the electromagnetic field produced by the excitation coil 330, and therefore placing it on substantially the same radius as that of the conductor wires 330a, 330b of the excitation coil helps to reduce a distortion in the electromagnetic field produced by the excitation coil 330 in the region of the capacitor 331. As will be understood, the use of a resonant capacitor is not limited to just the single turn, single direction embodiment described and shown in connection with Figures 16(a) and (b), but can be useful in other configurations of excitation coil too. For example, a resonant capacitor could be used in embodiments in which the coil comprises multiple turns, and also (as described in connection with Figure 16(d) below) can be used in embodiments in which the excitation coil extends/winds around the axis in both clockwise and anti-clockwise directions.

In the embodiment described, there is just one single excitation coil 330 which is shared between the three receiver coils (i.e. the one excitation coil 330 is used to generate the electromagnetic field for the three receiver coils). As will be understood, other excitation coil configurations are possible. For instance, more than one excitation coil could be provided; in such a case, ideally they would all be in resonance. As another example, the/each excitation coil could comprise a multiple-turn (or “multiple-loop”) coil. In another example, the/each excitation coil could comprise a “folded-rectangular loop”.

In use/during operation, a power supply/drive circuitry drives an alternating current through the excitation coil 330 which thereby generates an alternating electromagnetic field. Due to the configuration of the excitation coil 330 of the embodiment of Figure 16(a) extending around the axis D in a single direction only (i.e. it does not fold back on itself as is the case, for instance, in a “folded rectangular loop”), the current flows around the axis D in one direction only at any given instant in time. For example, as schematically illustrated by the arrows on the conductor wires 330a, 330b in Figure 16(a), at a first instant in time current flows through the conductor wires 330a, 330b in a clockwise direction only around the D axis. As will be understood, the direction of the arrows will point in the opposite at a different instant in time when the current is flowing through the conductor wires 330a, 330b in the opposite direction, due to the alternating current. The profile (schematically illustrated in Figure 16(c)) of the magnitude of the component of the magnetic flux density passing perpendicularly through the scale (i.e. parallel to the axis D) created by the single direction excitation coil 330 is to be contrasted with the profile of the magnitude of the component of the magnetic flux density passing perpendicularly into the scale (i.e. parallel to the axis D) as created directly by an excitation coil 430 configured as a folded rectangular loop (schematically illustrated in Figure 16(d)), wherein at any particular instant in time current flows in both clockwise and anti-clockwise directions around the D axis (illustrated by the arrows on the conductor wire 430a). As schematically illustrated by the solid line in Figure 16(e), in such a configuration, the magnitude of the component of the magnetic flux density passing perpendicular into the scale (i.e. parallel to the axis D) is significantly greater and more uniform inside the folded rectangular loop compared to outside it. In such a configuration, as schematically illustrated in Figures 16(d) and 16(e), the first receiver coil 304 can be located inside the folded rectangle loop, and the second 306 and third 308 receiver coils can be positioned on opposing radial sides of the folded rectangular loop. As will be understood, in this case, the radial position of the first 204, second 206 and third 308 scale tracks on the rotor will need to be changed/re- ordered accordingly. As schematically illustrated in Figure 16(d), the folded rectangular loop excitation coil 430 can optionally also comprise a resonant capacitor 431 if desired, and as schematically illustrated in Figure 16(d), it can be arranged to lie so as to be radially-in-line with the excitation coil’s conductor wire 430a (so as to minimise disturbances to the magnetic field produced thereby, although this is less important with a folded rectangular loop design due to the magnetic field not being uniform in the region of the folds/back-turns).

As mentioned above, the outer/inner conductive loops boost the (magnitude of the component of the) magnetic flux density passing perpendicularly into the scale, and this is schematically illustrated in Figures 16(c) and 16(e) by way of the solid and dashed lines which schematically illustrate the magnetic flux density perpendicular to the scale for equivalent systems both with and without outer/inner conductive loops. That is, in Figures 16(c) and 16(e) the solid lines schematically represent a representative profile of the magnetic flux density passing perpendicularly into the scale in a system without outer and inner conductive loops (210, 212), and the dashed lines schematically represent a representative profile of the magnetic flux density perpendicular to the scale in a system which is identical in all aspects excepted that it does have outer and inner conductive loops (210, 212). As will be understood, increasing the magnetic flux density passing perpendicularly into the scale features thereby increases the eddy currents generated therein which in turn provides a greater amplitude modulating effect on the electromagnetic fields sensed by the receiver coils.

Regardless of the particular configuration of the excitation coil 330, and regardless of the presence or absence of outer and/or inner conductive loops, the first 304, second 306, and a third 308 scale receiver coils are arranged to sense the electromagnetic field created by the excitation coil 330 and provide outputs in response thereto. As will be understood, and as described in more detail below, the electromagnetic field created by the excitation coil 330 is affected by (i.e. the electromagnetic field is changed by) the presence of the scale features of the first 204, second 206 and third 208 scale tracks on the rotor 200 (which in this embodiment is due to eddy currents created within the scale features, which have an amplitude-modulating effect on the alternating electromagnetic field created by the excitation coil 330), and this phenomenon is used to determine the relative rotational position of the rotor 200 and stator 300 about the axes B/D.

In the particular embodiment described, the stator 300 also comprises first 310, second 312, third 314, and fourth 316 outer quadrant coils and first 320, second 322, third 324, and fourth 326 inner quadrant coils. The angle subtended by each quadrant is substantially 90°. As will be understood, the length of each quadrant coil could be shorter such that the angle it subtends is less than 90°, but it can be preferred to maximise the angle each quadrant coil subtends (within the confines of a quadrant) in order to maximise their signal strength and avoid blind spots. As described in more detail below in connection with Figures 13 and 14, the inner and outer quadrant coils can be used to detect relative tilt and/or lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300. As illustrated by Figures 6 and 7, when assembled/in use, the outer quadrant coils (of which only the fourth outer quadrant coil 316 is shown in Figures 6 and 7) sit axially in-line with (e.g. directly above in the orientation shown in Figure 7) a blank region of the rotor’s substrate 202 and the inner quadrant coils (of which only the fourth inner quadrant coil 326 is shown in Figures 5, 6 and 7) sit axially in-line with/directly above (in the orientation shown in Figure 7) the third scale track 208.

As is also illustrated by Figures 6 and 7, the radial width of the third scale track 208 is selected such that both the third scale receiver coil 308 and the inner quadrant coils (of which only the fourth inner quadrant coil 326 is shown in Figures 6 and 7) sit axially in-line with (e.g. directly above in the orientation shown in Figure 7) the third scale track 208.

Referring now to Figure 8, the effect the outer 210 and inner 212 conductive loops have on the electromagnetic field produced by the excitation coil 330 will now be explained. In summary, the outer 210 and inner 212 conductive loops have the effect of “folding back” the electromagnetic field produced by the excitation coil 330 into the region in which the receiver coils are located so as to increase the magnetic flux density in the radial space between them, thereby helping to increase the magnetic field strength in-between the two coils.

In accordance with the above description, the excitation coil 330 is on the stator (not shown in Figure 8) and has an alternating current flowing in it, thereby generating an electromagnetic field in its vicinity. The arrow on the excitation coil 330 illustrates the direction of current flow through the excitation coil 330 at a particular instant in time (which in Figure 8 is shown in a clockwise direction). From the right-hand grip rule of electromagnetism, the excitation coil 330 produces a magnetic field with a 0 polarity inside the loop (into the page) and a • polarity outside the loop (out of the page). For ease of illustration, Figure 8 only shows the polarity for a single half-cycle of the alternating current/electromagnetic field. Also in accordance with the above description, the outer 210 and inner 212 conductive loops are provided on the rotor and are respectively situated radially outside and radially inside the rotor’s scale tracks (also not shown in Figure 8). Both the outer 210 and inner 212 conductive loops see a net magnetic flux of polarity 0 passing through them. By Lenz’s Law, they both induce a current (i.e. an electrical current) flow to oppose the magnetic field that created it (i.e. at the particular instant in time illustrated in Figure 8 is in an anti-clockwise direction as illustrated by the arrows on the outer 210 and inner 212 conductive loops). As result both the outer 210 and inner 212 conductive loops produce a • polarity inside their loop and a 0 polarity outside their loop. Accordingly, within the shaded areas 800, 802 of Figure 8 the magnetic flux from the outer 210 and inner 212 conductive loops oppose the magnetic flux from the excitation coil 330, reducing the magnetic flux density but, in contrast, within the non-shaded area 804 of Figure 8 (which is the area in which the scale tracks of the rotor and receiver coils of the stator are located) the magnetic flux from the outer 210 and inner 212 conductive loops are in phase with the excitation coil 330 and hence will act to increase the magnetic flux density in that area.

The operation of the inductive rotary encoder device 100 will now be described. As mentioned above, the excitation coil 330 has an alternating current flowing in it thereby generating an alternating electromagnetic field in its vicinity, and the first 304, second 306, and a third 308 scale receiver coils are arranged to sense the alternating electromagnetic field and provide outputs in response thereto. The alternating electromagnetic field is affected by (e.g. is changed by) the presence of the scale features of the first 204, second 206 and third 208 scale tracks on the rotor 200.

The effect (in the embodiment described, the amplitude-modulating effect) the first 204, second 206 and third 208 scale tracks on the rotor 200 have on the alternating electromagnetic field sensed by their respective first 304, second 306, and a third 308 scale receiver coils varies with relative rotation of the rotor 200 and stator 300 about the axes B/D. In particular, such (amplitude-modulating) effect on the alternating electromagnetic field varies in a cyclical manner, the spatial frequency of which depends on the period of the scale features. Furthermore, the effect the first 204, second 206 and third 208 scale tracks on the rotor 200 have on the alternating electromagnetic field, as sensed by their respective first 304, second 306, and a third 308 scale receiver coils, differs from each other due to their different scale periods.

Accordingly, the scale features of the first 204, second 206 and third 208 scale tracks on the rotor 200 impart cyclical variations in the (e.g. amplitude of the) alternating electromagnetic field at different spatial frequencies such that the spatial frequency of the cyclical variation in the electromagnetic field caused by the first scale track and as sensed by the first scale receiver coil 304, the spatial frequency of the cyclical variation in the electromagnetic field caused by the second scale track and as sensed by the second scale receiver coil 306, and the spatial frequency of the cyclical variation in the electromagnetic field caused by the third scale track and as sensed by the third scale receiver coil 308, are all different to each other. Accordingly, the cyclical variation in the signals produced from the first 304, second 306, and a third 308 scale receiver coils have different spatial periods/frequencies. The significance of this will be described in the following paragraphs with reference to Figures 9 and 10.

As explained above in connection with Figures 4 and 5, each of the first 304, second 306 and third 308 receiver coils actually comprise overlapping differential coils, having a sinusoidal form, and which are phase shifted by 90° relative to each other. Accordingly, each of the first 304, second 306 and third 308 receiver coils will actually output a pair of signals which are in quadrature (i.e. the signals are phase-shifted by 90°), and so, for example, can be labelled as SIN and COS signals. This is schematically illustrated in Figure 9, wherein a first one of the coils in a pair is labelled with a “s” suffix and the other of the coils in a pair is labelled with a “c” suffix. Accordingly, Figure 9 schematically illustrates that the first receiver coil 304 comprises a (differential) SIN coil 304_s and a (differential) COS coil 304_c, the second receiver coil 306 comprises a SIN coil 306_s and a COS coil 306_c, and the third receiver coil 308 comprises a SIN coil 308_s and a COS coil 308c.

As mentioned above, in the particular embodiment described, the first 204, second 206 and third 208 scale tracks on the rotor 200 impart cyclical variations in the amplitude of the alternating electromagnetic field (at different spatial frequencies). Accordingly, the (differential) SIN and COS coils 304_s, 304_c, 306_s, 306_c, 308_s, 308_c, sense amplitude modulated alternating electromagnetic signals. Accordingly, in accordance with this particular embodiment of the invention, those signals can be demodulated in order to obtain a demodulated (baseband) signal. Accordingly, in this embodiment, the stator comprises demodulators associated with each of the SIN and COS coils 304_s, 304_c, 306_s, 306_c, 308_s, 308_c for demodulating the signals sensed thereby. Therefore, in Figure 9, each of the SIN and COS signals output by each of the SIN and COS coils 304_s, 304_c, 306_s, 306_c, 308_s, 308_c is actually a demodulated (baseband) signal.

As illustrated, the outputs from each SIN and COS coil in a pair is provided to a corresponding Arc Tangent Calculator (i.e. a first Arc Tangent Calculator 304A/Tan, a second Arc Tangent Calculator 306A/Tan, and a third Arc Tangent Calculator 308A/Tan) which calculates an angle from the pair of SIN and COS signals (e.g. in the current embodiment, from the pair of demodulated/baseband SIN and COS signals). Figure 10 illustrates how the angles calculated by each of the first 304A/Tan , second 306A/Tan, and third 308A/Tan Arc Tangent Calculators vary with relative rotation of the stator and rotor about the axes B/D.

As mentioned above in connection with Figure 3, the periods of the first 204, second 206 and third 208 scale tracks are different. In particular, the first 204, second 206 and third 208 scale tracks and the first 304, second 306 and third 308 receiver coils are configured such that for each angle of rotation of the rotor and stator about the axes B/D the signal phases of the at least three separate signal channels have a unique combination. This enables an absolute rotational position to be determined from the outputs of the three Arc Tangent Calculators.

As will be understood, each of the scale tracks and associated scale receiver coils and electronics (e.g. the Arc Tangent Calculators) form a “signal channel”. In this embodiment: the first scale track 204, first receiver coil 304 and first Arc Tangent Calculator 304A/Tan, form a first signal channel; the second scale track 206, second receiver coil 306 and second Arc Tangent Calculator 306A/Tan, form a second signal channel; and the third scale track 208, third receiver coil 308 and third Arc Tangent Calculator 304A/Tan, form a third signal channel.

In this embodiment, the first scale track 204 has the highest number of integer periods per revolution, and could be referred to as the “Incremental Track” (and hence the channel associated therewith could be referred to as the “Incremental Channel”). The second 206 and third 208 scale tracks signal channels, have a lower number of periods per revolution compared to the first scale track 204. The second a third scale tracks (and hence the signal channels associated therewith) could be referred to as (first and second) “Vernier Tracks” (and hence the second and third signal channels associated therewith could be referred to as “Vernier Channels”). In the embodiment described the first 204, second 206 and third 208 scale tracks are configured such that such that the only common integer factor of the period count of all of the scale tracks is 1. As will be understood, other terms used in the art of inductive encoders for “period count” include “feature count” and “line count”.

Compared to a two-channel system, a three-channel system provides significantly greater freedom in the choice of the number of periods of each channel and provides a greater error tolerance in the calculation of the channel phase. This error tolerance allows a high number of incremental periods per revolution to be used and hence large diameter encoders are possible. The relatively large error tolerance on the phase detection allows small signal amplitudes to be used for the Vernier Channels with a lower signal to noise ratio. Indeed, it has been found that with a three-channel system limited or no filtering and automatic/dynamic signal correction need be applied to the Vernier Channels, thereby simplifying the design of and reducing the cost of the inductive rotary encoder system. Therefore, as shown in Figure 9, in the embodiment described, only SIN and COS signals from the first receiver coil 304 pass through an automatic/dynamic signal corrector 305, which in this embodiment performs automatic/dynamic gain correction (AGC), automatic/dynamic offset correction (AOC), automatic/dynamic balance correction (ABC), and automatic/dynamic phase correction (APC), on the output from the first receiver coils 304, before being passed to its corresponding Arc Tangent Calculator 304A/Tan.

Referring back to Figure 9, the signal from each of the three channels (i.e. the output from each of the first 304A/Tan, second 306A/Tan, and third 308A/Tan Arc Tangent Calculators) is passed to a position calculator 900 which calculates therefrom the absolute position of the stator and rotor about the axis of rotation. For instance, this could be done via a function or look-up table based on the values of each channel/outputs from the three Arc Tangent Calculators. However, in the embodiment described, the absolute position is calculated by a loop which increments through every possible incremental period and compares the actual Vernier phases to the expected Vernier phases for that period. Once the loop has been completed, there should only be one position detected. The use of an iterative loop for determining the position is particularly preferred over the use of other techniques, such as those that use a look up table. For instance, an iterative loop can be more efficient, especially from a memory point of view, and also more versatile (e.g. if the process is to be used across different sized inductive rotary encoders).

For example, the position calculator 900 can be configured to take the value output by the first Arc Tangent Calculator 304A/Tan, and then, for each of the scale periods of the first scale track/incremental channel (in the embodiment described for each of the sixty -four scale periods), identify what value you would expect to see from the second 306A/Tan and third 308A/Tan Arc Tangent Calculators. The position calculator 900 then compares the expected values with the actual values output by the second 306A/Tan and third 308A/Tan Arc Tangent Calculators, and checks if they match. Whether or not they match is recorded. As mentioned above, this is repeated for each of the scale periods of the first scale track/incremental channel (in the embodiment described for each of the sixty-four scale periods). There should be only one match, and if so then that matched position is provided to the output unit 902. If there are no matches, or more than one match, an error state is reported to the output unit 902. Output unit 902 can be any suitable unit for outputting/communicating the position information/error state to an external device, e.g. such as a position controller, including via a wired or wireless connection.

As will be understood, the Arc Tangent Calculators 304A/Tan, 306A/Tan, 308A/Tan and the position calculator 900 can comprise any suitable processing device, including, but not limited to bespoke processing devices configured for the specific application (e.g. a field programmable gate array “FPGA”) as well as a more generic processing devices which can be programmed (e.g. via software) in accordance with the needs of the application in which it is used. Accordingly, suitable processing devices include, for example, a microprocessor, CPU (Central Processor Unit), FPGA (Field Programmable Gate Array), or ASIC (Application Specific Integrated Circuit), or the like.

As mentioned above, in the embodiment described, the stator 300 comprises first 310, second 312, third 314, and fourth 316 outer quadrant coils and first 320, second 322, third 324, and fourth 326 inner quadrant coils. These are optional sensors and can be used to detect/provide a measure of the relative lateral/radial position (e.g. eccentricity), and/or relative tilt, of the rotor 200 and stator 300. In accordance with the present invention, it is not necessary or essential to provide such sensors, or other sensors which detect/provide a measure of the relative lateral/radial position (e.g. eccentricity), and/or relative tilt, of the rotor 200 and stator 300. Nevertheless, there can be advantages in doing so, as explained in more detail below.

As will be understood, relative eccentricity of the rotor 200 and stator 300 can be said to exist when the axis B about which the rotor’s scale tracks extend and are centred on is not coincident with the axis D about which the stator’s receiver coil extend and are centred on. In operation/use, such eccentricity could exist, for example, due to the rotor 200 not being arranged concentric to the machine shaft’s axis of rotation, and/or due to the stator 300 not being arranged concentric to the machine shaft’s axis of rotation, and/or due to the scale tracks and/or receiver coils not actually being centred on their assumed axis B/D. Accordingly, eccentricity of the rotor 200 and stator 300 is related to the lateral/radial position of the rotor 200 and stator 300 (i.e. their relative position in a degree of freedom/dimension/plane perpendicular to the axis of rotation (and hence perpendicular to the B and D axes)).

Relative tilt is a measure of how parallel the rotor 200 and stator 300 are. In operation/use, such relative tilt could be due to the rotor 200 being mounted to the machine such that its axis B (about which the rotor’s scale tracks extend and are centred on) is not parallel to the machine shaft’s axis of rotation A, and/or due to the stator 300 being mounted to the machine such that its axis D (about which the stator’s scale receiver coils extend and are centred on) is not parallel to the machine shaft’s axis of rotation A.

As will be understood, because the scale receiver coils 304, 306, 308 are full ring sensors (i.e. they extend substantially fully around the axis) the effect of any eccentricity of the rotor 200 and/or stator 300 on its own (i.e. in the absence of any relative tilt of the rotor 200 and stator 300) will cancel-out and therefore doesn’t cause an error, and similarly the effect of any relative tilt of the rotor 200 and stator 300 on its own (i.e. in the absence of any eccentricity) will cancel-out and therefore doesn’t cause an error. However, it has been found that the presence of both: i) eccentricity of the rotor 200 and/or stator 300; and ii) any relative tilt of the rotor 200 and stator 300, does result in a once per revolution system error being introduced into the rotational position measurement. In particular, the error could be expressed functionally as: Error = (Stator Eccentricity x Rotor Tilt) + (Stator Tilt x Rotor Eccentricity); (where “x” represents a vector cross product). Accordingly, for example, if the stator is tilted but the rotor has no eccentricity, then no error will result.

The once per revolution error can be calculated for a given value of relative tilt and eccentricity. Indeed, it has been found that the once per revolution error follows the equation:

7T e = -( x £

Where ( is the relative tilt vector and a is the relative eccentricity vector and where “x” represents a vector cross product.

Therefore, by measuring the relative tilt and the relative lateral/radial position (e.g. eccentricity) of the stator and rotor, it becomes possible to either compensate for the error or remove it during installation. To this end, as mentioned above, the stator 300 comprises two sets of quadrant coils on the stator 300. In particular the stator 300 comprises: a first set of quadrant coils comprising first 310, second 312, third 314, and fourth 316 outer quadrant coils; and a second set of quadrant coils comprising first 320, second 322, third 324, and fourth 326 inner quadrant coils.

An example of how these outer and inner quadrant coils can be used to detect relative tilt and/or the relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300 will now be explained with reference to Figures 11 to 14.

With regard to the detection of relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300, it should be noted that in this embodiment the first 320, second 322, third 324, and fourth 326 inner quadrant coils are arranged axially in-line with/directly above scale features (e.g. see Figures 6 and 7). In particular the first 320, second 322, third 324, and fourth 326 inner quadrant coils are arranged axially in-line with/directly above the third scale track 208. Furthermore, in this embodiment, the length of each of the first 320, second 322, third 324, and fourth 326 inner quadrature coils are an integer number of periods of the third scale track. The effect of this is that the axial separation between the stator 300 and rotor 200 does not substantially affect the strength of the magnetic field sensed by the first 320, second 322, third 324, and fourth 326 inner quadrature coils and therefore they are substantially immune/insensitive to any relative tilt of the stator 300 and rotor 200. This is due to eddy currents produced within the scale features which oppose the transmitted magnetic field. The opposing magnetic field produced by the eddy currents gets forced down through the gap between the scale features - therefore net magnetic field and sensed by the first 320, second 322, third 324, and fourth 326 inner quadrature coils remains the same for the expected range of axial separation of the stator and rotor during normal installation and operation circumstances.

However, as schematically illustrated by Figures 11(a) and 11(b), the first 320, second 322, third 324, and fourth 326 inner quadrant coils are sensitive to the relative lateral position of the rotor 200 and stator 300 (note that for ease of illustration, only the excitation coil 330 and inner quadrant coils 320, 322, 324, 326 of the stator 300, and only the third scale track of the rotor 200, are shown in Figures 11(a) and (b)). For instance, as illustrated in Figure 11(a) which illustrates a situation in which there is no relative eccentricity “s” between the rotor 200 and stator 300, the net magnetic flux (illustrated by the “magnetic field envelope” line) to which opposing inner quadrant coils are exposed is equal. In contrast, as illustrated in Figure 11(b) which illustrates a situation in which there is relative eccentricity “s” of the rotor 200 and stator 300, the net magnetic flux to which opposing inner quadrant coils are exposed differs. Accordingly, the difference in the outputs of opposing inner quadrant coils can be used to obtain a reliable measure of relative lateral/radial position (e.g. eccentricity) of the stator 300 and rotor 200. With regard to the detection of relative tilt of the rotor 200 and stator 300, it should be noted that in this embodiment the first 310, second 312, third 314, and fourth 316 outer quadrant coils are arranged axially in-line with/directly above a blank region of the rotor 200 (e.g. see Figures 6 and 7). With such a configuration, it has been found that the first 310, second 312, third 314, and fourth 316 outer quadrant coils are substantially insensitive to any relative eccentricity of the rotor 200 and stator 300 for the extent of eccentricity that would normally be expected of the rotor 200 and stator 300 during normal installation and operation. This is schematically illustrated in Figures 12(a) and 12(b) where it can be seen that there is little change in the state of magnetic flux to which the outer quadrant coils are exposed when there is a change in the extent of eccentricity of the rotor 200 and stator 300. In contrast, as illustrated in Figures 12(a) and 12(c), there is a significant change in the state of magnetic flux to which the outer quadrant coils are exposed when there is a change in the relative tilt of the rotor 200 and stator 300. Indeed, not only does the of flux density (represented by the number of magnetic flux lines) increase for the outer quadrant coils which become closer to the rotor due to the relative tilt, there is a decrease in the “angle” of incidence of the magnetic flux passing through the outer quadrant coils, which leads to a greater inductive effect in the quadrant coil(s) that are closer to the rotor 200. Accordingly, the difference in the outputs of opposing outer quadrant coils can be used to obtain a reliable measure of relative tilt of the stator 300 and rotor 200.

Note that for ease of illustration, only the excitation coil 330 and outer quadrant coils 310, 312, 314, 316 of the stator 300, and only the outer conductive loop 210 of the rotor 200, are shown in Figures 12(a), 12(b) and 12(c).

Bearing in mind the phenomena explained above in connection with Figures 11 and 12, reference is now made to Figure 13 which schematically illustrates the outer and inner quadrant coils and the excitation coil 330 of the stator 300. For ease of reference, the first 310, second 312, third 314, and fourth 316 outer quadrant coils have been labelled A, D, C and B respectively, and the first 320, second 322, third 324, and fourth 326 inner quadrature coils have been labelled E, K, J and F respectively.

By subtracting the outputs from opposite quadrant coils, a quadrature output can be obtained from each set. For example, the following signals can be obtained form the outer and inner quadrant coils: x_{1 =} (A - C) y_i = (B - D)

X₂ = (E -J) y₂ = F - K)

As illustrated in Figure 14(a), plotting xi against yi produces a Lissajous which is descriptive of the relative tilt of the rotor 200 and stator 300. The centre of the Lissajous of Figure 14(a) is due to tilt of the stator 300 (its effect is static), and the radius of the Lissajous of Figure 14(a) is due to tilt of the rotor 200. At any particular instant in time, the Lissajous position (the xi, yi values) describes the tilt error vector.

As illustrated in Figure 14(b), plotting X2 against y₂ produces a Lissajous which is descriptive of the relative eccentricity of the rotor 200 and stator 300. The centre of the Lissajous of Figure 14(b) is due to eccentricity of the stator 300 (its effect is static), and the radius of the Lissajous of Figure 14(b) is due to eccentricity of the rotor 200. At any particular instant in time, the Lissajous position (the X2, y₂ values) describes the eccentricity error vector. Accordingly, the values of xi, yi, X2, yi can be used to determine the current state of relative tilt and the lateral/radial position (e.g. eccentricity), which can be used to correct the position calculated by the position calculator 900. For instance, as illustrated in Figure 9, the inductive rotary encoder apparatus can comprise a tilt/ lateral position (e.g. eccentricity) calculator 901 which passes the xi, yi, X2, yi values to the position calculator 900 which can use them to correct the position provided to the output unit 902. As also illustrated by Figure 9, the xi, yi, X2, yi values could instead/additionally be passed to the output unit 902 for outputting to an external device. For example, the external device could use the xi, yi, X2, yi values to correct the position it has received. Additionally/altematively, rather than being used to correct a position value, the xi, yi, X2, yi values could be used to trigger a warning/error state or provide an indication of the relative tilt and/or lateral/radial position (e.g. eccentricity) of the stator and rotor. For instance, the indication could be a visual indication such that an installer can see whether the stator and rotor are properly configured. Optionally, an error signal could be produced if the xi, yi, X2, y values are indicative of relative tilt/lateral position (e.g. eccentricity) above a predetermined threshold. Optionally, such error signal could be used to control the machine the inductive rotary encoder is installed in a predetermined manner (e.g. cause the machine to shut down on receipt of such an error signal).

As is apparent from the above description, in the particular embodiment described the outer quadrant coils are primarily sensitive to and used to determine relative tilt of the rotor 200 and stator 300, whereas the inner quadrant coils are primarily sensitive to and used to determine relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300. As will be understood, this need not necessarily be the case, and for instance the outer quadrant coils could be configured such that they are primarily sensitive to and used to determine relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300, whereas the outer quadrant coils could be configured such that they are primarily sensitive to and used to determine relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300. Furthermore, in other embodiments, one or both of the outer and inner quadrant coils could be sensitive to both relative tilt and lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300. Whilst it would still be possible to distil relative tilt and lateral/radial position (e.g. eccentricity) measurements from such quadrant coils, this requires more complicated electronics and/or processing, and so it can be preferred that one of the inner and outer quadrant coils is primarily sensitive to relative tilt of the rotor and stator, and the other of the inner and outer quadrant coils is primarily sensitive to relative lateral/radial position (e.g. eccentricity) of the rotor and stator. In other embodiments, only one set of quadrant coils is provided. In such a case, the only one set of quadrant coils could be configured to be primarily sensitive to and used to determine the relative lateral/radial position (e.g. eccentricity) of the rotor 200 and stator 300, or could be configured to be primarily sensitive to and used to determine relative tilt of the rotor 200 and stator 300. In an optional embodiment, the only one set of quadrant coils could be configured to be sensitive to both relative tilt and lateral/radial position (e.g. eccentricity). Whilst it might not be possible to distil the measurements of the relative tilt and lateral/radial position (e.g. eccentricity) of the stator and rotor from only one set of quadrant coils that is sensitive to both relative tilt and lateral/radial position (e.g. eccentricity), the output from such a set of quadrant coils could still be useful to determine the presence of relative tilt and/or lateral/radial position (e.g. eccentricity). For instance, the output therefrom could be used during installation/setup and/or during operation to indicate/determine whether the rotor and stator are relatively tilted/eccentrically arranged which can be used as a warning that they are not setup/configured properly (e.g. it can serve as a go/no-go signal).

In the embodiments described above, the magnetic field sensed by the quadrant coils is generated by the same excitation coil as that which generates the magnetic field that is sensed by the stator’s receiver coils 304, 306, 308. However, this need not necessarily be the case. For instance, Figures 15(a) and (b) illustrate alternative embodiments in which the stator comprises first 330’ and second 330” excitation coils that are radially separated from each other. In these embodiments, the first excitation coil 330’ generates a magnetic field that is manipulated by the rotor’s scale tracks 204, 206, 208 and sensed by the stator’s receiver coils 304, 306, 308. The second excitation coil 330” generates a magnetic field that is sensed by the quadrant coils 310, 312, 314, 316, 320, 322, 324, 326 to determine relative tilt and/or the lateral/radial position (e.g. eccentricity) of the rotor 300 and stator 200. As will be understood, in this embodiment, the stator 200 will comprise one or more conductive features configured to manipulate the magnetic field generated by the second excitation coil 330’, in the regions of the quadrant coils, such that the outputs of the quadrant coils vary dependent on relative tilt and/or lateral position (e.g. eccentricity) of the stator 300 and rotor 200.

In the embodiments described above, the quadrant coils have been described in connection with detecting/determining the relative tilt and/or lateral/radial position (eccentricity) of the stator and/or rotor. As will be understood, the outputs of the quadrant coils can also be used to detect/determine surface flatness and/or noncircularity of the rotor. As will also be understood, in another embodiment, additionally or alternatively to the above described functionality, the apparatus could be configured to determine a measure of (or at least a signal indicative of) the axial separation of the stator and rotor members, e.g. via the signal strengths/amplitude of one or more of i) any one or more of the quadrant coils; and ii) any one or more of the receiver coils.

As will be understood, the above-described particular radial positions of the scale tracks 204, 206, 208 and the corresponding excitation 330 and scale receiver coils 304, 306, 308 are not essential and could be arranged to have different radial positions/combinations. For instance, the rotor 200 could be configured such that the first scale track 204 sits radially between the second 206 and third 208 scale tracks (and therefore the stator 300 could be configured such that the corresponding first receiver coil 304 sits radially between the second 306 and third 308 scale receiver coils).

In the embodiments described above, there are multiple scale tracks and multiple receiver coils. However, as will be understood, this need not necessarily be the case, and for instance just one scale track and one corresponding receiver coil could be provided.

As mentioned above, the first 310, second 312, third 314, and fourth 316 outer quadrant coils and first 320, second 322, third 324, and fourth 326 inner quadrant coils are optional sensors used to detect/provide a measure of the relative lateral/radial position (e.g. eccentricity), and/or relative tilt, of the rotor 200 and stator 300. Figure 17 depicts another embodiment of the invention (wherein like parts share like reference numerals with the other above-described embodiments). As shown in Figure 17(a), in this embodiment the optional outer 210 and inner 212 conductive loops are provided, in this case on the stator 300 instead of on the rotor 200. Also, in this embodiment, the stator 300 does not comprise any sensors used to detect/provide a measure of the relative lateral/radial position (e.g. eccentricity), and/or relative tilt, of the rotor 200 and stator 300. As shown, in this embodiment, the radial widths of the second 206 and third 208 scale tracks are the same as each other (and about half the radial width of the first scale track 204), and similarly the radial widths of the second 306 and third 308 receiver coils are the same as each other (and about half the radial width of the first receiver coil 304).

As shown in Figure 17(c), in this embodiment the rotor 200 comprises a planar disc-shaped member 202 made from a single layer of fibreglass, for example about 1.6 mm thick (e.g. it can be a printed circuit board “PCB” made from a single layer of FR-4). The scale features 204, 206, 208 are electrically conductive (in this embodiment they are copper, and are formed on the first planar discshaped member by etching/milling away a copper coating on the fibreglass material of the first planar member 202; although could be formed via other processes, such as copper plating). In an alternative embodiment, the first planar rotor member 202 could comprise an electrically conductive material, wherein non-conductive features are formed thereon so as to provide the scale features. As shown in Figure 17(c), in this embodiment, the stator 300 comprises a first planar disc-shaped member 302 on the front of which (i.e. on its side facing the rotor 200 when in use) the stator’s the excitation 330 and receiver 304, 306, 308 coils are provided. As shown in Figure 17(c) the excitation coil 330 comprises the parallel conductor wire configuration depicted in Figure 16(a).

The rear of the first planar disc-shaped member 302 is attached (via adhesive) to another/ second planar disc-shaped member 303, on the back of which (i.e. on its side facing away from the rotor 200 when in use) electronic components 320 for driving the excitation coil 330 and processing signals received from the receiver coils 304, 306, 308 are provided. In the embodiment of Figure 17, the first 302 and second 303 planar stator members are made from multiple layers (in this embodiment six layers) of fibreglass. In an alternative embodiment, the stator 300 comprises only one planar member 302/303 and the coils and electronic components are provided at/toward opposite faces thereof, for example.

As depicted in Figure 17(c), the receiver coils 304, 306, 308 can occupy more than one fibreglass/PCB layer. Although Figure 17(c) depicts the outer 210 and inner 212 conductive loops being provided in the same layer as the excitation 330 and receiver coils 304, 306, 308, this need not necessarily be the case.

As will be understood, the rotor 200 and/or stator 300 could be made from materials other than that described (such as ceramic).

As will be understood, the rotors 200 and stators 300 of the other above-described embodiments can comprise construction similar to that of Figure 17 (e.g. comprise one or more layers of fibreglass/PCB).

As will also be understood, the rotor 200 and/or stator 300 of the above-described embodiments need not necessarily have a circular or disc-like configuration, and need not necessarily have a hole extending therethrough, and need not even be planar, although the front surfaces on/at which the scale features 204, 206, 208 and excitation 300 and receiver coils 304, 306, 308 are provided are most preferably planar.

Claims

CLAIMS:

1. An inductive rotary encoder comprising first and second members relatively rotatable about an axis of rotation, configured such that:

• the first member has at least first and second scale tracks extending around a scale track axis,

• the second member has: o an excitation coil through which an alternating current is passed so as to generate an alternating magnetic field which is manipulated by the at least first and second scale tracks; and o at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, and a second receiver coil for sensing the alternating magnetic field as manipulated by the second scale track, via which the relative rotational position of the first and second members about the axis of rotation can be measured,

• the excitation coil and receiver coils extend around a coil axis at different radii to each other, and

• the excitation coil is a single direction excitation coil such that in use current flowing through the excitation coil does so in one direction only around the coil axis at any given instant in time.

2. An inductive rotary encoder as claimed in claim 1, in which the excitation coil extends circularly around the coil axis at a constant, and singular, radius.

3. An inductive rotary encoder as claimed in claim 1 or 2, in which the excitation coil is a single-turn coil, and/or the first receiver coil is a single-turn coil, and/or the second receiver coil is a single-turn coil.

4. An inductive rotary encoder as claimed in any preceding claim, in which the first and second receiver coils are located on opposing radial sides of the excitation coil, wherein the first and second receiver coils are radially separated from each other by the excitation coil.

5. An inductive rotary encoder as claimed in any of claims 1 to 3, in which the first and second receiver coils are located on the same radial side of the excitation coil as each other, wherein the first receiver coil is radially separated from the excitation coil by the second receiver coil.

6. An inductive rotary encoder as claimed in any preceding claim, in which:

• the first member comprises a third scale track extending annularly around the scale track axis, and

• the second member comprises a third receiver coil, extending around a coil axis at a different radii to that of the first and second receiver coils and to that of the excitation coil, for sensing the magnetic field as manipulated by the third scale track, such that the relative rotational position of the first and second members about the axis of rotation can be measured from the first, second and third receiver coils.

7. An inductive rotary encoder as claimed in claims 5 and 6, in which the third receiver coil is located on the opposite radial side of the excitation coil to the first and second receiver coils, wherein the third receiver coil is radially separated from the first and second receiver coils by the excitation coil.

8. An inductive rotary encoder as claimed in any preceding claim, configured such that an alternating current passes through the excitation coil at a predetermined operating frequency, and in which the excitation coil comprises a capacitor configured such that excitation coil operates as a resonant circuit.

9. An inductive rotary encoder as claimed in any claim 7, in which the capacitor is placed such that it is located in-line with the excitation coil’s circular extent.

10. An inductive rotary encoder as claimed in any preceding claim, comprising a magnetic flux increaser within which a current flow is generated by the excitation coil’s magnetic field, and which in turn itself generates a magnetic field which acts to increase, in the region of one or more of the receiver coils, the magnetic flux of the magnetic field sensed by said one or more receiver coils.

11. An inductive rotary encoder as claimed in claim 10, in which the magnetic flux increaser comprises: at least one conductor located radially outside of the first member’s scale tracks and the second member’s receiver and excitation coils, said at least one conductor being hereinafter referred to as at least one “outer” conductor; and/or at least one conductor located radially inside of the first member’s scale tracks and the second member’s receiver and excitation coils, said at least one conductor being hereinafter referred to as at least one “inner” conductor.

12. An apparatus as claimed in any preceding claim, in which the first and second scale tracks are configured to manipulate the amplitude of the magnetic field generated by the excitation coil dependent on the relative position of the first and second members about the axis of rotation.

13. An inductive encoder comprising first and second members relatively moveable along a measurement direction (e.g. about an axis of rotation), wherein:

• the second member has: o an excitation coil for generating a magnetic field which is manipulated by the at least first scale track; and o at least a first receiver coil for sensing the magnetic field as manipulated by the first scale track, via which the relative rotational position of the first and second members along the measurement direction (e.g. about the axis of rotation) can be measured,

• the inductive encoder apparatus is configured to pass an alternating current through the excitation coil at a predetermined operating frequency, and the excitation coil comprises a capacitor arranged such that excitation coil operates as a resonant circuit.

14. An inductive rotary encoder as claimed in claim 13, in which the excitation coil extends substantially circularly about an excitation coil axis, and in which the capacitor is placed such that it is located in-line with the excitation coil’s circular extent.

15. An inductive rotary encoder as claimed in claim 14, in which the excitation coil is a single-turn coil, and/or the first receiver coil is a single-turn coil, and/or the second receiver coil is a single-turn coil.

16. An inductive encoder comprising first and second members relatively moveable along a measurement direction, wherein:

• the first member has at least first and second scale tracks,

• the second receiver coil is separated from the excitation coil by the first receiver coil.