WO2019166258A1

WO2019166258A1 - Rotor ring

Info

Publication number: WO2019166258A1
Application number: PCT/EP2019/053965
Authority: WO
Inventors: Walter Wyss
Original assignee: Admotec Precision Ag
Priority date: 2018-03-02
Filing date: 2019-02-18
Publication date: 2019-09-06

Abstract

A Rotor (10) for an angular position resolver comprises - a rotational body (1) defining an axis (3) of rotation and comprising ferromagnetic material, - an electrically conductive layer (2) covering a first region (4a) of a radially outer surface of said rotational body and leaving a second region (4b) of said radially outer surface uncovered, wherein at least a first circumference (5) of said rotational body crosses said first region and said second region. The invention is further directed to an angular position resolver comprising the rotor, to an angular position measuring arrangement comprising the angular position resolver and to a method of producing an angular position indicative signal, wherein the angular position measuring arrangement is used.

Description

Rotor ring

The invention addressed herein relates to a rotor for an angular position resolver. Under further aspects, the invention relates to an angular position resolver and to a method of operating the resolver.

In different applications resolvers for measuring an angular position or an angular velocity of e.g. a shaft are applied. One type of angular position resolvers uses an induced magnetic field between a stator and a rotor being rotatable around an axis with respect to the stator. The rotor may e.g. be arranged on the shaft, the angular position of which is to be measured. The rotation axis of the rotor defines a cylindrical coordinate system with an axial direction parallel to the axis, a radial direction orthogonal to the axis and an azimuthal direction along a circumference described by the rotation of the rotor around the axis. Such angular position resolvers, which determine the azimuthal angular position of the rotor with respect to the stator, have a rotor that creates or modifies a spatial distribution of a magnetic field in a way that is specific to the azimuthal position of the rotor. By measuring this magnetic field on the stator side, the azimuthal position of the rotor with respect to the stator can be determined. Such an arrangement may be seen as a sensor for measuring angular positions or revolutions using inductive coupling. An azimuthal position resolver for measuring an angular position using an induced magnetic field between a stator and a rotor is known e.g. from the document

EP 0 535 181 A1. The rotor of an azimuthal position

resolver according to this document comprises a loop of magnetic material extending around the axis of the rotor and being arranged along a geometric plane cutting the axis of the rotor under an oblique angle. This loop of magnetic material is placed between two hollow cylindrical bodies, which are made of non-magnetic material. The hollow

cylindrical bodies have faces running parallel to the geometric plane. These faces are in contact with the loop of magnetic material to hold the loop in place, such that the rotor as a whole has the form of a hollow cylinder having on its outer surface magnetic pole faces formed by the loop of magnetic material. A sinusoidally shaped form of this pole faces becomes apparent, if the cylinder surface in unrolled on a plane. Only the stator of an azimuthal position resolver according to EP 0 535 181 Al is wound with an exciting coil and with measuring coils.

Generally, angular position information of high angular precision is considered as valuable, particularly in the field of industrial automation, where angular position resolvers are used in the context of motion control of robots. Furthermore, a general trend towards higher rotational speed of electro-motors increases the need for angular position resolvers delivering highly precise signals over a wide range of rotational speeds. The object of the present invention is to provide an alternative angular position resolver, in particular to provide an angular position resolver alleviating or solving one or more of the problems of known angular position resolvers .

This object is achieved by a rotor according to claim 1.

The rotor according to the invention is a rotor for an angular position resolver. The rotor comprises a rotational body defining an axis of rotation and comprising

ferromagnetic material. The rotor further comprises an electrically conductive layer covering a first region of a radially outer surface of the rotational body and leaving a second region of the radially outer surface uncovered. At least a first circumference of the rotational body crosses the first region and the second region.

A circumference of the rotational body is defined as contour of a cross-section of the rotational body by a plane orthogonal to the axis of rotation of the rotational body. The first region (i.e. region covered by electrically conductive layer) and the second region (i.e. region not covered by electrically conductive layer) both may comprise unconnected parts or may comprise a single connected region. The electrically conductive layer may additionally cover other surfaces of the rotational body, e.g. end faces or a surface of a central bore. In addition to the first circumference, there may be a second circumference, which is spaced from the first circumference in axial direction and which crosses the first region and the second region, too. A pattern of first and second regions along the second circumference may be offset in azimuthal direction with respect to a pattern of first and second regions along the first circumference.

Such an offset in azimuthal direction may e.g. be 180°.

The electrically conductive layer may have a thickness in the range 50 micrometers to 500 micrometers, in particular in the range 100 micrometers to 300 micrometers. Generally, high electric conductivity of the electrically conductive layer is preferred. The electrically conductive layer may comprise at least one of copper, aluminum, silver and gold. The electrical conductivity of the electrically conductive layer may be selected to be higher than the electrical conductivity of the rotational body, but this is not necessary. The inventor has recognized that an eddy current shielding effect of the electrically conductive layer modifies the magnetic fields detected by the angular resolver enough to detect the angular position of the rotor according to the invention.

The inventor has recognized that with this rotor, a

component for an angular position resolver is provided, which is easy to fabricate. It provides a low-cost

alternative to previously known rotors. In particular, the rotor according to the invention may comprise ferromagnetic material, which is evenly distributed in the rotational body. Any angular dependence may be created by the geometry of the first and second region.

Embodiments of the rotor are defined by the features of claims 2 to 8.

In one embodiment of the rotor according to the invention, which may be combined with any of the embodiments still to be addressed unless in contradiction, the first

circumference has first sections crossing the first region and the first circumference has second sections crossing the second region, wherein pairs of adjacent first and second sections extend over an azimuthal angle a = 360°/n with respect to the axis of rotation. The number n is an integral number, i.e. n = 1, n=2, n=3, n=4, etc.

By selecting a specific number n, the rotor may be adapted to a corresponding stator in an angular position resolver having the same number n of pole pairs arranged on a circumference of the stator. In the case of n = 1, there exist exactly one first section and one second section of the first circumference and the first and the second section together cover the complete circumference, i.e. extend over 360°. For n = 2 and higher, a pattern of first and second sections is repeated n times over the complete circumference .

In one embodiment of the rotor according to the invention, which may be combined with any of the preaddressed embodiments and any of the embodiments still to be

addressed unless in contradiction, the second region is formed as windows in the electrically conductive layer and the windows are arranged along the first circumference.

Different shapes of the windows are conceivable, such as rectangular or trapezoidal shapes. At least the first circumference thus has sections inside the windows and sections, which are covered by the electrically conductive layer, outside the windows.

In one embodiment of the rotor according to the invention, which may be combined with any of the preaddressed

embodiments and any of the embodiments still to be

addressed unless in contradiction, each of the windows is delimited by an eye-shaped contour.

With this embodiment of the rotor, particularly clean electrical signals may be received in an angular position resolver. The windows in in the electrically conductive layer provide for a smooth transition from angular

positions, where the outer surface is completely covered by the electrically conducive layer, to angular positions, where the second region reaches its maximum axial

extension. The eye-shaped contour may as an example be defined by sections of sine functions along said

circumference. The specific geometry of the eye-shaped contour may be adapted such that the fraction of higher harmonics in a signal indicative for the angular position of the rotor is kept low. This may be achieved by

superposing higher harmonic sine functions, e.g. sine functions with periodicity 3 or 5 to the first order sine function. The superposed sine functions may be phase shifted with respect to the first order sine function. This way, a continuous curve defining the eye-shaped contour with high precision may defined. The adaptations to the eye-shaped contour may be developed in an iterative manner by determining undesired higher order harmonics from a realization of a resolver with the inventive rotor and by applying a correction to the eye-shaped contour of the windows, which define the second region. The correction may correspond in its form to the undesired harmonics, but has opposite phase. This contour may e.g. be produced by numerically controlled machine tools, such that a precisely defined continuous curve results.

embodiments and any of the embodiments still to be

addressed unless in contradiction, the rotational body has a central bore along the rotational axis. In particular, the rotational body may be a hollow cylinder. This

embodiment of the rotor is e.g. suited to be mounted on a shaft, the angular position of which is to be detected.

embodiments and any of the embodiments still to be

addressed unless in contradiction, the rotational body is configured to suppress eddy currents. In particular, the suppression of eddy currents may be achieved by arranging the ferromagnetic material in stacks of mutually isolated sheets. As another example, the suppression of eddy currents may be achieved by mutually isolated grains of the ferromagnetic material being arranged in a composite material .

With embodiment, the ferromagnetic material effectively guides alternating magnetic fields also for high

frequencies. Furthermore, heating of the rotor by

alternating magnetic fields is reduced.

embodiments and any of the embodiments still to be

addressed unless in contradiction, the electrically conductive layer has a thickness, which is larger than the skin depth in said electrically conductive layer at a frequency of 100 kHz.

An alternating magnetic field - as e.g. produced by an alternating electric current in the exciting coil

arrangement of an angular position resolver - decreases when entering into an electrical conductor with greater depth in the conductor. The skin depth is defined as the depth below the surface of the conductor at which the magnetic field is reduced to 1/e (approx. 0.37) times the field strength at the surface of the conductor. The skin depth depends on the resistivity p of the conductor, the magnetic permeability m of the conductor and the frequency f of the alternating magnetic field. The skin depth d is d =

fc

If, for example, the electrical conductor is copper, the skin depth at frequency f=100 kHz is approximately

200 micrometers. Thus, according to the present embodiment, the electrically conductive layer may be a copper layer of thickness t ³ 200 micrometers. When exchanging the material of the electrical conductive layer by a material different from copper and accordingly having different resistivity p and/or different magnetic permeability m, the thickness t of the new layer may be adjusted to a fraction or a

multiple of the skin depth of the other material

corresponding to the respective fraction or a multiple of the skin depth of the exchanged material. This way, the same eddy current shielding effect may be achieved with the other material, simply by adjusting the thickness t of the layer .

In this embodiment of the rotor, alternating magnetic fields having a frequency in the MHz range are nearly completely blocked by the electrically conductive layer in the first region. The shielding effect is already

significant for frequencies in the 10-kHz region.

embodiments and any of the embodiments still to be

addressed unless in contradiction, in the second region, the rotational body protrudes over a radially outer surface of the rotational body in the first region by a height equal to the thickness of the electrically conductive layer. This way, a smooth outer surface of the rotor may be formed .

Further in the scope of the invention lies an angular position resolver according to claim 9.

An angular position resolver according to the invention comprises a rotor according to the invention. The angular position resolver further comprises a stator. The rotor is rotatable around the axis with respect to the stator. The stator comprises

- an exciting coil arrangement configured to generate a magnetic field entering into the rotational body at the first circumference, propagating across at least a part of the ferromagnetic material and emanating from the rotational body,

- a first measuring coil arrangement with at least one first coil facing towards the axis and

- a second measuring coil arrangement with at least one second coil facing towards the axis.

As an example, the exciting coil arrangement, the first or the second measuring coil arrangement may comprise coils, e.g. saddle coils, defining areas each surrounded by a respective number of coil windings being an approximation to a sine or cosine function of the azimuthal position of the area multiplied by a factor common to all the coils of the respective coil arrangement. The inventor has recognized that this embodiment leads to a filtering out of harmonic distortions of higher order. This is particularly useful in the case, where first, second and higher order time derivatives are calculated based on the azimuthal position of the rotor to derive angular speed, angular acceleration and angular jerk. The latter guantities are highly sensitive to irregularities in the basic signal describing the azimuthal position. Particularly clean signals are achieved, if the coil windings approximate a sinusoidal distribution with respect the azimuthal

direction .

Embodiments of the angular position resolver are defined by the features of claim 10 and 11.

In one embodiment of the angular position resolver

according to the invention, which may be combined with any of the preaddressed embodiments and any of the embodiments still to be addressed unless in contradiction, the stator comprises a ring having a multitude of axially oriented grooves, two neighboring grooves defining in between them a projection facing towards the axis of the rotor. The coils of at least one of the exciting coil arrangement, the first measuring coil arrangement and the second measuring coil arrangement are arranged in the grooves and around the proj ections .

The projections and grooves may in particular provide a common support structure for all of the coil arrangements. The ring may comprise magnetic material, such that magnetic fields are efficiently guided through the coils of the coil arrangements .

In one embodiment of the angular position resolver

according to the invention, which may be combined with any of the preaddressed embodiments and any of the embodiments still to be addressed unless in contradiction, coils of at least one of the exciting coil arrangement, the first measuring coil arrangement and the second measuring coil arrangement are constructed as self-supporting air-core coils .

This embodiment makes possible a very compact design of the angular position resolver.

Further in the scope of the invention lies an angular position measuring arrangement according to claim 12.

Such an angular position measuring arrangement comprises

- an angular position resolver according to the invention,

- an alternating current power supply operatively connected to the exciting coil arrangement,

- at least a first voltage meter operatively connected to the first measuring coil arrangement, and

- a second voltage meter operably connected to the second measuring coil arrangement.

The invention is further directed to a method of producing an angular position indicative signal according to

claim 13. In the method, an angular position measuring arrangement according to the invention is used. The signal produced by the method is indicative for the angular position of the rotor with respect to the stator. The method comprises the steps of:

- applying an alternating current to the exciting coil arrangement, thereby generating a magnetic field entering into the rotational body at the first circumference, propagating across at least a part of the ferromagnetic material and emanating from the rotational body,

- measuring a first voltage signal induced by the magnetic field in the first measuring coil arrangement,

- measuring a second voltage signal induced by the magnetic field in the second measuring coil arrangement,

- evaluating amplitudes and phases of the first and second voltage signals to derive the angular position indicative signal .

The method may be executed by applying an alternating current having a frequency in the range from Hz to MHz. In Particular, the alternating current may have a frequency in the range from 2 kHz to 10 kHz.

The rotor, in particularly the thickness of the

electrically conductive layer may be adapted to the frequency of the alternating current, that will be applied when executing the method.

The invention shall now be further exemplified with the help of figures. The figures show: Fig. 1 a perspective view of a rotor according to the invention;

Fig. 2 a functional diagram of an angular position resolver;

Fig. 3 different views of a rotational body of an embodiment of the rotor in Figs. 3. a) to 3.d);

Fig. 4 an unrolled surface of an embodiment of the rotor together with a schematic configuration of coil windings on a stator of an angular position resolver;

Fig. 5 schematic configurations of coil winding on a stator of an embodiment of an angular position

resolver;

Fig. 6 a cross-section through an embodiment of an angular position resolver.

Fig. 1 shows a perspective view of an embodiment of a rotor according to the invention. The rotor 10 comprises a rotational body 1. In the embodiment shown here, the rotational body has the form of a hollow cylinder. The rotational body has an axis 3 of rotation corresponding to the cylinder axis and running through the central bore of the hollow cylinder. The axis 3 of rotation is indicated as dash-dotted line. The rotor comprises an electrically conductive layer 2. On the radially outer surface of the rotational body, there is a first region 4a, which is covered by the layer 2. There is also a second region 4b, which is not covered by the layer 2 and where the

rotational body 1 is directly visible. The directly visible surface of the rotational body is indicated by diagonal hatching. In the embodiment shown here, the layer 2, which is shown in white, covers the rotational body also on an end face and on the surface of the inner bore. The

rotational body comprises a ferromagnetic material. A first circumference 5 of the rotational body is indicated by a dashed line. This first circumference crosses the first region 4a as well as the second region 4b. In the

embodiment shown here, the second region comprises

unconnected parts of the region. These parts of the second region are formed as windows in the electrically conductive layer, which windows are arranged along the first

circumference. Specifically, in the embodiment shown here, these windows have an eye-shaped contour.

Fig. 2 shows a functional diagram of an angular position resolver. The angular position resolver comprises a rotor 10 and a stator 20. The rotor 10 is rotatable around axis 3, which in this view lies perpendicular to the plane of the figure. The stator comprises three types of coil arrangements: an exciting coil arrangement P, a first measuring coil arrangement SI and a second measuring coil arrangement S2. Each of these coil arrangements, which are symbolically indicated, has electrical ports, which are symbolically indicated as circles. At these ports, an alternating current power supply may be connected to said exciting coil arrangement P, a first voltage meter may be connected to said first measuring coil arrangement SI and a second voltage meter may be connected to said second measuring coil arrangement S2, such that an angular position measuring arrangement results.

The rotor 10 is rotatable around the axis 3 with respect to the stator 20. The angular position of the rotor (azimuthal position Q) is detected by angular position resolver due to the fact that the flux linkage of the coil arrangement is varied depending on the azimuthal position Q of the rotor relative to the stator. The rotor comprises magnetic material, here symbolized by the vertically hatched part.

Figs. 3. a) to 3.d) shows different views of a rotational body 1 of an embodiment of the rotor as shown in Fig. 1. Fig. 3. a) shows a front view of the rotational body 1;

Fig. 3.b) shows a side view of the rotational body 1;

Fig. 3.c) shows the radially outer surface of the

rotational body 1 unrolled into a plane;

Fig. 3.d) shows a perspective view of the rotational body 1

In Figs. 3. a) to 3.d), the rotational body is shown without the electrically conductive layer. The rotational body has substantially the shape of a hollow cylinder with an inner diameter ID and outer diameter OD. On its radially outer surface, three protrusions 6 slightly protrude over the radially outer surface. These protrusions are located, where the second region 4b (i.e. the region, which is left uncovered by the electrically conductive layer) is

established in the complete rotor. The height of the protrusions as shown here may be selected to be t, such that after applying an electrically conductive layer of having a thickness t in the first region, a smooth outer surface of the rotor results, which has an outer diameter of OD+2t . As an example, the height or the thickness, respectively, could be selected to be t = 300 micrometers.

The protrusions 6 define the regions 4b in the complete rotor. As apparent from Fig. 3.c), the first circumference 5 has first sections crossing said first region and second sections crossing said second region, wherein pairs of adjacent first and second sections extend over an azimuthal angle a = 360°/3 = 120°, thus being suited for a

configuration having n=3 pole pairs. The regions 4b have contours, which are defined by the contour 7 of the

protrusions 6. These contours have eye-shaped geometry. The contour may be defined by sections of sine functions along the circumference indicated by dashed lines. In the present case, these are two sine functions showing three complete oscillations over the complete circumference. A first sine function defines the contour by its positive sections, a second sine function defines the contour by its negative sections. The other sections of the sine functions are not displayed. The two sine functions have a mutual phase shift of 180°. Thus, same pattern is repeated three times. At their largest azimuthal extension, the protrusions 6 extend over an angle of 60°.

Fig. 4 shows in its lower part an unrolled radially outer surface of an embodiment of the rotor. The horizontal direction in the figure corresponds to the azimuthal direction with respect to the axis of the rotor. The first region 4a, shown in dark, is covered by the electrically conductive layer and the second region 4b, shown in white, is left uncovered. The second region 4b has an eye-shaped contour. This contour may e.g. be produced by numerically controlled machine tools. The eye-shaped contour may be adapted such that the fraction of higher harmonics in a signal indicative for the angular position of the rotor is kept low. In three rows in the upper part of the figure, schematic configurations of coil windings on a stator of an angular position resolver are shown, thereby Nref indicates number and winding direction of an exciting coil

arrangement P, Nsin indicates number and winding direction of a first measuring coil arrangement SI and Ncos indicates number and winding direction of a second measuring coil arrangement S2. The coil arrangements as shown here may be wound on projections on a stator ring. The present figure shows the configuration for n=2 pole pairs. The basic building block, which covers an azimuthal angle denoted by "360 °el" may be repeated several times over the complete circumference, such that n=2, 3, 4, 5, ... pole pairs result. According to the choice of n, the azimuthal angle "360°el" corresponds to a geometrical angle 360°/n, i.e. to 180°, 120°, 90°, 72°, etc. A rotation of the rotor by 360°/n results in a 360° phase shift of the electrical signals obtained by measuring the voltage induced in SI and S2. A further possibility would be to select "360°el" to cover a geometrical angle of 360°, i.e. the complete circumference of the rotor, which corresponds to n=l . In the latter case, only the left half of Fig. 4 describes the outer surface of the rotor and a suitable coil configuration of the stator. The coil distributions shown in Fig. 4 may be realized by winding coils around protrusions or notches on ring with grooves separating these protrusions. In the case shown, four protrusions cover an azimuthal angle corresponding to "360°el". The coil distributions can be seen as rough approximation to sine or cosine distributions. A better approximation to sine or cosine distribution may be

achieved by increasing the number of protrusions covering the azimuthal angle corresponding to "360°el" to, as an example, eight or twelve protrusions.

Fig. 5 shows schematic configurations of coil windings on a stator of an angular position resolver, similar to the ones shown in Fig. 4, but suited for coil arrangements that are constructed as self-supporting air-core coils. In each of the Figs. 5. a) to 5.c) a building block covering an

azimuthal angle "360 °el" corresponding to a geometrical angle 360°/n is shown. In the upper part of each figure, the pattern of winding number and winding direction is indicated. In the lower part of each figure, a schematic coil arrangement implementing the corresponding pattern. Building blocks of the coils arrangement may be connected in series by connecting the coil ends indicated as circles. Fig. 5. a) shows a building block suitable for an exciting coil arrangement P, Fig. 5.b) shows a building block suitable for a first measuring coil arrangement SI and Fig. 5.c) shows a building block suitable for a second measuring coil arrangement S2. A better approximation to sine or cosine distributions may be achieved in an analogous way as mentioned in connection with Fig. 4 for self-supporting air-core coils as well.

Fig. 6 shows a cross-section through an embodiment of an angular position resolver, cutting through a part of the stator 20, which carries coil arrangements P, SI, S2, and a part of the rotor 10. Crosses and dots indicate the winding directions of the individual wires of the coil

arrangements, which correspond to the coil arrangements shown in Figs. 5. a) to 5.c) . The cross-section is cut along the first circumference of the rotor. The rotor comprises a rotational body 1 comprising ferromagnetic material (shown diagonally hatched) and an electrically conducting layer 2, covering the rotational body 1 in some regions. Rotor and stator are separated by an air gap 21 and rotatable with respect to each other. A single field line of a magnetic field H is schematically shown. By applying an alternating current to the exciting coil arrangement P, such a magnetic field may be generated. The magnetic field H enters into the rotational body 1 at the first circumference 5, i.e. in the cutting plane of the present cross-section, the

magnetic field H propagates across at least a part of the ferromagnetic material and emanates from the rotational body. The magnetic field alternates as the alternating current applied to the exciting coil arrangement P and thereby induces voltages in the first and second measuring coil arrangements SI and S2. The amplitude of the induced voltage depends on the amount of electrically conductive layer material that is traversed by the magnetic field H, as the electrically conductive layer has an eddy current shielding effect. This amount of electrically conductive layer material depends on the thickness t of the

electrically conductive layer as well as on the position of the first and second regions of the rotor with respect to the exciting coil arrangement P and thus on the angular position of the rotor with respect to the stator.

List of reference signs

1 rotational body

2 electrically conductive layer

3 axis

4a first region (of radially outer surface)

4b second region (of radially outer surface)

5 first circumference

6 protrusion (on rotational body)

7 eye-shaped contour

10 rotor

20 stator

21 air gap

100 angular position resolver

360°el azimuthal angle corresponding to 360° electrical phase shift

ID inner diameter (of hollow cylinder)

OD outer diameter (of hollow cylinder)

t thickness (of electrically conductive layer)

P exciting coil arrangement

51 first measuring coil arrangement

52 second measuring coil arrangement

Nref number of coils of exciting coil arrangement

Ncos number of coils of first measuring coil arrangement Nsin number of coils of second measuring coil arrangement Q angular (azimuthal) position of the rotor

Claims

1. Rotor (10) for an angular position resolver, said rotor comprising

- a rotational body (1) defining an axis (3) of rotation and comprising ferromagnetic material,

- an electrically conductive layer (2) covering a first region (4a) of a radially outer surface of said rotational body and leaving a second region (4b) of said radially outer surface uncovered,

wherein at least a first circumference (5) of said

rotational body crosses said first region and said second region .

2. Rotor (10) according to claim 1, wherein said first circumference (5) has first sections crossing said first region and second sections crossing said second region, wherein pairs of adjacent first and second sections extend over an azimuthal angle a = 360°/n with respect to said axis (3) of rotation, wherein n is an integral number.

3. Rotor (10) according to claim 1 or 2, wherein said second region (4b) is formed as windows in said

electrically conductive layer (2) and wherein said windows are arranged along said first circumference (5).

4. Rotor (10) according to claim 3, wherein each of said windows is delimited by an eye-shaped contour (7) .

5. Rotor (10) according to any one of claims 1 to 4, wherein said rotational body (1) has a central bore along said rotational axis, and wherein said rotational body in particular is a hollow cylinder.

6. Rotor (10) according to any one of claims 1 to 5, wherein said rotational body (1) is configured to suppress eddy currents, in particular by said ferromagnetic material being arranged in stacks of mutually isolated sheets or by mutually isolated grains of said ferromagnetic material being arranged in a composite material.

7. Rotor (10) according to any one of claims 1 to 6, wherein said electrically conductive layer (2) has a thickness (t) , which is larger than the skin depth in said electrically conductive layer at a frequency of 100 kHz.

8. Rotor (10) according to any one of claims 1 to 7, wherein in said second region (4b), said rotational body protrudes over a radially outer surface of said rotational body in said first region by a height equal to the

thickness (t) of said electrically conductive layer (2).

9. Angular position resolver (100) comprising a rotor (10) according to any one of claims 1 to 8 and a stator (20), said rotor being rotatable around said axis (3) with respect to said stator, and wherein said stator comprises

- an exciting coil arrangement (P) configured to generate a magnetic field (H) entering into said rotational body at said first circumference (5), propagating across at least a part of said

ferromagnetic material and emanating from said

rotational body,

- a first measuring coil arrangement (SI) with at least one first coil facing towards said axis and

- a second measuring coil arrangement (S2) with at least one second coil facing towards said axis.

10. Angular position resolver (100) according to claim 9, wherein said stator (20) comprises a ring having a

multitude of axially oriented grooves, two neighboring grooves defining in between them a projection facing towards said axis and wherein coils of at least one of said exciting coil arrangement (P), said first measuring coil arrangement (SI) and said second measuring coil arrangement (S2) are arranged in said grooves and around said

proj ections .

11. Angular position resolver (100) according to claim 9 or 10, wherein coils of at least one of said exciting coil arrangement (P) , said first measuring coil arrangement (SI) and said second measuring coil arrangement (S2) are

constructed as self-supporting air-core coils.

12. Angular position measuring arrangement comprising

- an angular position resolver (100) according to any one of claims 9 to 11,

- an alternating current power supply operatively connected to said exciting coil arrangement (P),

- at least a first voltage meter operatively connected to said first measuring coil arrangement (SI), and

- a second voltage meter operably connected to said second measuring coil arrangement (S2) .

13. Method of producing an angular position indicative signal, wherein an angular position measuring arrangement according to claim 12 is used and wherein said signal is indicative for the angular position of said rotor with respect to said stator, the method comprising the steps of:

- applying an alternating current to said exciting coil arrangement (P), thereby generating a magnetic field (H) entering into said rotational body (1) at said first circumference (5), propagating across at least a part of said ferromagnetic material and emanating from said

rotational body (1),

- measuring a first voltage signal induced by said magnetic field in said first measuring coil arrangement (SI) ,

- measuring a second voltage signal induced by said

magnetic field in said second measuring coil arrangement (S2) , - evaluating amplitudes and phases of said first and second voltage signals to derive said angular position indicative signal .