WO2018114004A1

WO2018114004A1 - Resolver

Info

Publication number: WO2018114004A1
Application number: PCT/EP2016/082619
Authority: WO
Inventors: Walter Wyss
Original assignee: Admotec Precision Ag
Priority date: 2016-12-23
Filing date: 2016-12-23
Publication date: 2018-06-28

Abstract

Azimuthal position resolver (10) comprising a rotor (1) being rotatable around an axis (3) with respect to a stator (2), wherein - said rotor comprises at least one loop (4) of magnetic material extending around said axis and being arranged along a geometric plane cutting said axis under an oblique angle; - said stator comprises - an exciting coil arrangement P generating a magnetic field H entering said loop, propagating along at least a part of said loop and emanating from said loop, - a first measuring coil arrangement S1 with at least one first saddle coil facing towards said axis and - a second measuring coil arrangement S2 with at least one second saddle coil facing towards said axis; and wherein - said first and second saddle coils have an opening angle α₁, α₂of 120° in azimuthal direction with respect to said axis. The invention is further directed to an azimuthal position measuring arrangement and to a method of producing an azimuthal position indicative signal.

Description

Resolver

The invention addressed herein relates to an azimuthal position resolver. Under further aspects, the invention relates to an azimuthal position measuring arrangement and to a method of producing an azimuthal position indicative signal .

In different applications resolvers for measuring an angular position or an angular velocity of e.g. a shaft are applied. One type of azimuthal 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 azimuthal position resolvers 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 Al . 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, azimuthal position information of high angular precision is considered as valuable, particularly in the field of industrial automation, where azimuthal 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 azimuthal position resolvers delivering highly precise signals over a wide range of rotational speeds. The object of the present invention is to provide an alternative azimuthal position resolver, in particular to provide an azimuthal position resolver alleviating or solving one or more of the problems of known azimuthal position resolvers.

This object is achieved by an azimuthal position resolver according to claim 1. Such an azimuthal position resolver comprises a rotor being rotatable around an axis with respect to a stator. The rotor comprises at least one loop of magnetic material extending around the axis and being arranged along a geometric plane cutting the axis under an oblique angle. The stator comprises

- an exciting coil arrangement generating a magnetic field entering the loop, propagating along at least a part of the loop and emanating from the loop,

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

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

According to the invention, the first and second saddle coils have an opening angle of 120° in azimuthal direction with respect to the axis.

The inventor has recognized that an azimuthal position resolver according to the invention provides particularly clean signals. The saddle coils having opening angles of 120° in azimuthal direction are immune against induction of signals, which have a third order harmonic dependency on the azimuthal position of the rotor. A magnetic flux distribution having a third order harmonic dependency (i.e. a combination of cos(3-9) and sin(3-9), Θ being the

azimuthal position of the rotor) creates a zero net

magnetic flux through the saddle coils having opening angles of 120° in azimuthal direction. Thus, according to the invention a signal having third order harmonic

dependency on Θ is filtered out by virtue of the specific coil design of the saddle coils in the first and second measuring coil arrangements.

The inventor has further recognized that the azimuthal position resolver according to the invention delivers clean signals of time derivatives of the azimuthal position of the rotor. Generally, first, second and higher order time derivatives calculated based on the azimuthal position of the rotor to derive angular speed, angular acceleration and angular jerk are with increasing order increasingly

sensitive to irregularities in the basic signal describing the azimuthal position. The azimuthal position resolver according to the invention keeps such irregularities at a low level.

Embodiments of the inventions are defined by the features of claims 2 to 14. In one embodiment of the azimuthal position resolver according to the invention, which may be combined with any of the embodiments still to be addressed unless in

contradiction, at least one of the first and the second measuring coil arrangements comprises a group of coils with at least two of the respective saddle coils connected in series .

Magnetic flux crossing two or more saddle coils leads to higher voltages at the terminals of the measuring coils arrangements and to increased signal to noise ratio. The suppression of third order harmonic described for a single saddle coil having an opening angle of 120° in azimuthal direction is effective for a combination of such coils connected in series. Thus, using saddle coils having an opening angle of 120° in azimuthal direction as building block, more complex measuring coil arrangements may be constructed, which suppress the third order harmonic effects .

In one embodiment of the azimuthal 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 above- mentioned group of coils comprises mutually overlapping of the saddle coils connected in series.

With this embodiment it possible to create a measuring coil arrangement having different sensitivity to magnetic flux in different areas. In areas where several saddle coils overlap the sensitivity is highest. Thus by connecting coils to a group according to this embodiment, sensitivity of the measuring coil arrangement may be matched to the spatial distribution of the magnetic flux to be picked up, e.g. approximating a sine shape distribution along the azimuthal direction.

In one embodiment of the azimuthal 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, saddle coils of the group of coils are connected in series and are pairwise arranged on opposite azimuthal sides of the stator, in particular spaced by 180° in azimuthal direction with respect to the axis. With this embodiment magnetic flux entering on one

azimuthal side from the stator into the rotor and emanating on the opposite side may be picked up and converted

efficiently into an induced voltage signal.

In one embodiment of the azimuthal 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

geometric form of the second measuring coil arrangement corresponds to the geometric form of the first measuring coil arrangement and wherein the position of the second measuring coil arrangement is rotated by 90° around the axis with respect to the first measuring coil arrangement. With this embodiment complementary signals are generated by the first and second measuring coil arrangement, such as a signal dependent on the cosine of the azimuthal position of the rotor and a signal dependent on the sine of the

azimuthal position of the rotor.

azimuthal position resolver comprises a third measuring coil arrangement, wherein the geometric forms of the second and third measuring coil arrangements correspond to the geometric form of the first measuring coil arrangement and the position of the second and third measuring coil

arrangement are rotated by +120° and -120°, respectively, around the axis with respect to the first measuring coil arrangement .

With this embodiment an arrangement similar to a so-called syncro is achieved. Signals corresponding to the

alternating current of a three-phase rotary current are measured with this embodiment of the azimuthal position resolver. Having a third signal increases the reliability of the azimuthal position resolver.

In one embodiment of the azimuthal 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 at least a first ring with a first number of equally spaced projections on an internal or external circumference, the first number being divisible by three, and wherein the first and or second saddle coils each surround a separate group of the projections comprising a second number of neighboring projections, the second number being equal to the first number divided by three, in particular wherein the first number equals twelve and the second number equals four. The projections in this embodiment may be formed similar to teeth of a gear-wheel, either as inner toothing on an internal circumference or as outer toothing on an external circumference of the ring. Winding the coils of the

measuring coils arrangements around a third of the equally spaced projections leads in a particularly simple way to saddle coils having an opening angle of 120° in azimuthal direction with respect to the axis. An embodiment having twelve projections, or grooves respectively, on its circumference makes possible to arrange the second

measuring coil arrangement in a position rotated by 90° with respect to the first measuring coil arrangement by using corresponding grooves, which are spaced by three proj ections .

In one embodiment of the azimuthal 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 second ring having a geometric form corresponding to the geometric form of the first ring and being axially displaced from the first ring, and wherein the first and or second saddle coils surround a

corresponding group of projections of the first and of the second ring.

In one embodiment of the azimuthal 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 first and/or second ring comprises a magnetic material.

The magnetic material concentrates the magnetic flux in the projections and thereby increases the voltage induced in the measurement coil arrangement.

In one embodiment of the azimuthal 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 exciting coil arrangement comprises a circular coil coaxial to the axis and having a center coinciding with a center of the loop .

The exciting coils arrangement according to this embodiment is particularly simple to build. Depending on the direction of current applied to the circular coil, it generates a magnetic field along the axis and on a larger radius pointing towards the axis on a first axial side of the circular coil and pointing away from the axis on the opposite axial side of the circular coil. Thus when

arranged e.g. concentric with the loop of the rotor, it generates a magnetic field entering the loop on the first axial side, propagating along at least a part of the loop and emanating from the loop at the opposite axial side.

When alternating current is applied to the coil, first and opposite sides switch roles according to the current flow.

In one embodiment of the azimuthal 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, at least one of said first and second measuring coil arrangements comprises saddle coils defining areas each surrounded by a respective number of coil windings being an approximation to the sine function of the azimuthal position of the area multiplied by a factor common to all the saddle coils.

The inventor has recognized that this embodiment leads to a filtering out of harmonic distortions of higher order (i.e. a combination of cos (η·θ) and sin(n-G), Θ being the

azimuthal position of the rotor and n > 2) . If e.g. twelve areas distributed equally along the azimuthal axis are defined by saddle coils, harmonic distortions of order 2 to 12 may be efficiently suppressed. The single saddle coils may be wound around projections of rings, as described in the context of another embodiment. The azimuthal position of the areas may be defined by the center of the areas. The areas may be defined by saddle coils overlapping each other, thus some area may be defined by the overlap area of one or several of the saddle coils. Depending on the sense of winding of the surrounding coil windings, positive and negative winding numbers may be added up to a total winding number for the respective area. A coil winding number n± for an area Ai at azimuthal position ai may be calculated as n± = round ( N-sin( ai ) ) , wherein "round ()" means rounding to the next whole number and N is a maximum coil winding number.

In one embodiment of the azimuthal 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 first and a second ring spaced from each other and coaxially with the axis, each ring having a respective multitude of axially oriented first and second grooves, two neighboring grooves defining a respectively first or second projection in between them,

wherein at least one of the exciting coil arrangement, the first measuring coil arrangement and the second measuring coil arrangement comprises

- first coils arranged in the first grooves and around at least one of the first projections and

- second coils arranged in the second grooves and around at least one of the second projections.

This embodiment enables a particularly simple and

production process of the coil windings of the three coil arrangements comprised in the azimuthal position resolver. The exciting coil arrangement and/or the first measuring coil arrangement and/or the second measuring coil

arrangement may be produced as a first subassembly

comprising coil windings arranged in grooves of the first ring only and a second subassembly comprising coil windings arranged in grooves of the second ring only. Handling a single ring during the coil winding process is simpler than handling a complete stator with all its geometric

restrictions. Then, the two rings carrying the

subassemblies may be arranged in or on the stator. Finally, the coil windings of the first and the second subassembly are connected in series to complete the coil arrangements.

In this embodiment the first and second ring may be made of magnetic material, thus being part of a magnetic structure guiding the magnetic field lines toward the rotor. The two ring may be axially spaced by a further ring made of magnetic material. The first and second ring may have identical geometry.

In one embodiment of the azimuthal 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 rotor comprises a ring groove along a plane perpendicular to the axis and completely within the loop. This embodiment leads to a smooth characteristic of the dependency of the induced voltage in the measuring coil arrangement on the azimuthal position of the rotor.

This embodiment may e.g. be realized with a rotor

comprising two non-magnetic positioning elements on both axial sides of the loop made of magnetic material and in contact with the loop. The positioning elements and the loop may define a common radially outer surface with first and second coaxial cylindrical sections being separated by a circumferential ring groove, which runs in the loop only, i.e. at no point crossing the border between the magnetic and the non-magnetic material. Narrow tips of magnetic material potentially leading to spikes in the signal picked up with the measuring coil arrangements are thus avoided.

In one embodiment of the azimuthal 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 rotor comprises a non-magnetic sleeve being in contact with an outer circumference of the loop. In particular, the sleeve may be a hollow cylindrical sleeve. Further in particular, the sleeve may comprise titanium or a titanium alloy.

The inventor has realized that his embodiment may be used in applications leading to very high rotational speed of the rotor. Even at high rotational speeds a stable

azimuthal position indicative signal may be received. The sleeve has the effect of mechanically stabilizing the rotor and in particular the ring on the rotor. The sleeve

furthermore prevents a mechanical destruction of the rotor at very high rotational speeds. With this embodiment the material of the ring may be selected mainly on the grounds of the magnetic properties of the ring. E.g. an alloy having high magnetic permeability but low tensile strength may be selected, as the sleeve provides the mechanical stability needed.

Further in the scope of the invention lies an azimuthal position measuring arrangement according to claim 15. Such an azimuthal position measuring arrangement comprises

- an azimuthal 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 azimuthal position indicative signal according to claim 16. Such a method of produces an azimuthal position indicative signal being indicative for an azimuthal position of a rotor being rotatable around an axis with respect to a stator. The signal produced by the method is free of third order harmonic dependence on the azimuthal position. I.e. the signal shows no dependency on cos(3-9) and sin(3-9), Θ being the azimuthal position of the rotor. The method comprises the steps of:

- providing the rotor with a loop of magnetic material extending around the axis and being arranged along a geometric plane cutting the axis under an oblique angle, - providing the stator with a first measuring coil

arrangement with at least one first saddle coil facing towards the axis and having an opening angle of 120° in azimuthal direction with respect to the axis,

- providing the stator with a second measuring coil arrangement with at least one second saddle coil facing towards the axis and having an opening angle of 120° in azimuthal direction with respect to the axis,

- applying alternating current to an exciting coil

arrangement being arranged on the stator, thereby

generating a magnetic field entering the loop, propagating along at least a part of the loop and emanating from the loop,

- 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, and

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

The effect that the signal produced by the method is free of third order harmonic dependence on the azimuthal position results from the mechanism as described above in the context of the geometry of the saddle coils. The applied alternating current may e.g. have a frequency in the range of 5 kHz to 50 kHz.

Coming back to the magnetic properties of the materials used in the azimuthal position resolver, we clarify herewith that in the context of the present patent

application we understand under a magnetic material a material that is ferromagnetic. Under a non-magnetic material we understand a material that is not ferromagnetic and thus not able to significantly guide magnetic flux. Materials being paramagnetic, diamagnetic, ferri-magnetic or anti-ferromagnetic are therefore referred to as non^¬ magnetic materials. Non-magnetic materials may e.g. be titanium, aluminum or austenitic steel (e.g. most types of stainless steel) . The discrimination between magnetic and non-magnetic shall be made under the temperature conditions in which the azimuthal position resolver is used. The loop of magnetic material may consists of magnetically soft material, in particular of high permeability magnetic material. This may e.g. be a soft magnetic iron-silicon alloy, e.g. an alloy comprising approximately 97% Fe and 3% Si, or an iron-nickel alloy, e.g. an alloy comprising approximately 51% Fe, 48% Ni, Mn and Si. The loop may be made of several sheets stacked upon each other. The stator may comprise magnetic elements e.g. to guide magnetic flux generated by the exciting coil arrangement towards the rotor .

The invention shall now be further exemplified with the help of figures. The figures show:

Fig. 1 a schematic perspective view of the azimuthal position resolver according to the invention; Fig. 2 schematic flattened views of measuring coil arrangements in Fig. 2. a) to 2.d) according to

embodiments ;

Fig. 3 a perspective, partially cut-away view of a rotor and a stator according to an embodiment;

Fig. 4 a functional diagram of an azimuthal position resolver;

Fig. 5 a partially cut-away view of a rotor according to an embodiment;

Fig. 6 a partially cut-away view of a rotor according to a further embodiment;

Fig. 7 shows in Fig. 7. a) to 7.d) in schematic

flattened views four variants of elementary building blocks of coils windings;

Fig. 8. a) shows in schematic flattened view an

embodiment with coil windings surrounding areas;

8.b) shows numbers of coil windings wound around areas according to an embodiment and Fig. 8.c) the

corresponding sine and cosine functions.

Fig. 1 shows schematically and simplified, an azimuthal position resolver according to the invention. The azimuthal position resolver 10 comprises a rotor 1 and a stator 2. The rotor 1 is rotatable around an axis 3. The rotor comprises a loop 4 made of magnetic material shown

vertically hatched. The loop 4 extends around the axis 3 and along a geometric plane cutting the axis under an oblique angle, such that the axial position of the loop varies around the circumference of the rotor. In the position shown in this figure, this axial position varies from a lower axial position on the left side in the figure to an upper axial position on the right side in the figure. Schematically and simplified the position of the stator 2 is indicated by dashed lines. The stator comprises an exciting coil arrangement P. When provided with a current, the exciting coil arrangement creates a magnetic field H, the direction of which is indicated by arrows at some selected positions and for a given direction of current - here the direction is such that the magnetic field has a field component upward in the region of the rotor. The magnetic field H generated by the exciting coil enters the loop, propagates along a part of the loop and emanates from the loop. The stator further comprises a first measuring coil arrangement SI and a second measuring coil arrangement shown as thick black and fine double lines to facilitate distinguishing them in the overlapping region on the right side of the figure. First and second measuring coil

arrangement comprise each a saddle coil facing the axis 3. This way, the saddle coils pick up magnetic flux entering or emanating radially from the loop 4 on the rotor. The saddle coils both have an opening angle oti or ot2 ,

respectively, being 120° in azimuthal direction.

Indicated by dotted lines a possible connection from an alternating current power source 20 to the exciting coil arrangement P, and from the measuring coil arrangements SI and S2 to respective voltage meters 21 and 22 is shown. By operatively connecting these elements to the azimuthal position resolver an azimuthal position measuring

arrangement according to the invention results.

Fig. 2. a) shows in a schematic flattened view, i.e.

unrolled on a plane, first SI and second S2 measuring coil arrangements, each having a single saddle coil with opening angle αι = 120°, or a,2=120° respectively, in azimuthal direction. The saddle coils displayed in a simplified manner may comprise a multiplicity of coil windings. The azimuthal direction with respect to the axis of the

azimuthal position resolver is indicated by the horizontal arrow denoted as a. First and second measuring coil arrangements are offset with respect to each other by 90° in azimuthal direction. This way, the signal induced in the first and second measurement arrangement shows a cosine or sine, respectively, dependency on the azimuthal position of the rotor. The situation of the measurement coil

arrangement shown in Fig. 2. a) corresponds to the situation shown in Fig . 1.

Fig. 2.b) shows a first measuring coil arrangement SI according to an embodiment in a similar view as Fig. 2. a) . The first measuring coil arrangement comprises a group of coils with at least two, in this particular case three, saddle coils connected in series. The saddle coils in the group mutually overlap. Each of the saddle coils has an azimuthal opening angle of ai 120°. For better visibility of the individual saddle coils, they are slightly offset in their height in the figure. This offset does not need to be translated into a corresponding axial offset of the coils on the stator. For simplicity the second measuring coil arrangement is not shown in this figure. The second

measuring coils arrangement may comprise a similar group of coils offset in azimuthal direction a, e.g. offset by 90°. Fig. 2.c) shows a first measuring coil arrangement SI according to a further embodiment in a similar view as Fig. 2. a) . A pair of saddle coils is connected in series. Both saddle coils of the pair are arranged on opposite sides of the stator. In this case they are spaced by 180° in

azimuthal direction a .

Fig. 2.d) shows a first measuring coil arrangement SI according to a combination of the embodiments shown in Fig. 2.b) and Fig. 2.c) in a similar view as Fig. 2. a) . The measuring coil arrangement comprises two groups of mutually overlapping saddle coils. All saddle coils are connected in series. The saddle coils are pairwise arranged on opposite azimuthal sides of the stator, in particular spaced by 180° in azimuthal direction. The relative winding sense of the coils in a pair of coils being arranged on opposite

azimuthal sides of the stator may be selected such that the voltage induced in the individual coils by a magnetic flux traversing the stator adds up in the series connection of the coils. A measuring coil arrangement as shown in Fig. 2.d) may be wound on ring structure of the stator having twelve equally spaced axial grooves on a circumference. As an example, the saddle coil arrangement shown here defines ten different areas surrounded by coil windings. Fig. 3 shows parts of an azimuthal position resolver according to an embodiment in a perspective, partially cut^¬ away, view. A rotor 1 is rotatable around axis 3. Half of a stator 2 is cut-away giving sight into the interior of the azimuthal position resolver. A loop 4 of magnetic material, marked by vertical hatching, extends around the rotor 1. The stator 2 is arranged around the rotor 1 defining in some regions a narrow air gap 5. An exciting coil

arrangement P in form of a circular coil around the axis is part of the stator. They're may be elements holding the exciting coils arrangement in place, which are not shown. A part of a saddle coil of a first measuring coil arrangement SI is visible. The saddle coil faces towards the axis 3. Other parts of the first and a second measuring coil arrangement may be present, but are not shown in this figure. The stator comprises a part of magnetic material marked by diagonal hatching in the cutting plane. This part of magnetic material guides the magnetic field generated by the exciting coil arrangement towards the regions of the loop 4 on the rotors being in the uppermost or lowermost axial position. The magnetic part of the stator is formed as two concentric and axially spaced first 6' and 6' ' ring with and further ring in between them. The rings 6, 6' ' - in upper and lower position in the present figure - have grooves 7 in axial direction defining protrusions 8 between them. Windings of saddle coils may be arranged in these grooves 7 and be wound around these protrusions 8, as representatively shown by the displayed saddle coil of the first measuring coil arrangement SI. Fig. 4 shows a schematic functional diagram of an azimuthal position resolver. A rotor 1 is rotatable around an axis 3, which her lies perpendicular to the figure. The rotor comprises a magnetic part, which is not rotationally symmetric, here symbolized by the vertically hatched part. Note that in an azimuthal position resolver according to the invention, this part is the loop of magnetic material, the rotational asymmetry of which might not be visible in this particular view. A stator 2 is arranged radially outside the rotor 1 and spaced from the rotor by an air gap 5 enabling a rotation of the rotor with respect to the stator. An exciting coil arrangement P, a first measuring coil arrangement SI and a second measuring coils

arrangement S2 are parts of the stator. Depending on its azimuthal position Θ, the rotor modifies the inductive coupling between the exciting coil arrangement P and the first measuring coil arrangement SI, e.g. such that the coupling is proportional to cos (Θ) . Similarly, depending on its azimuthal position Θ, the rotor modifies the inductive coupling between the exciting coil arrangement P and the second measuring coil arrangement S2, e.g. such that the coupling is proportional to sin(9) . By applying an

alternating current to the exciting coil arrangement and by measuring amplitude and phase of a voltage induced in the first and second measuring coil arrangements SI and S2, the azimuthal position Θ can be determined. An ambiguity resulting from the fact that e.g. a certain cosine value may be caused by two different values of Θ is resolved by additionally considering the signal on the second measuring coil arrangement providing e.g. the sine value of Θ. An azimuthal position resolver according to the invention may be seen as a variable reluctance transformer having as primary coil the exciting coil arrangement P and having at least two secondary coils, namely first and second

measuring coil arrangements SI and S2. The rotor modulates the reluctance of each of a magnetic loop through the primary and each of the secondary coils according to its azimuthal position relative to the stator.

Fig. 5 shows a partially cut-away view of a rotor according to an embodiment. In the upper half of the figure, a cross section through the rotor 1 is shown. An inner hollow cylinder 13, a first 11 and a second 12 axial positioning element, all made of non-magnetic material and marked by diagonal hatching in the cross-section, hold in place a ring 4 made of magnetic material and marked by horizontal hatching. The lower half of the figure shows a view onto the rotor with hidden lines shown a dotted lines. A ring groove extends around the rotor along a plane perpendicular to the axis 3. The ring groove is completely in the loop of magnetic material. The width of the groove, the width of the loop and the oblique angle, under which the plane defining the orientation of the loop cuts the axis 3 are combined such that the ring groove runs between the

rightmost axial position of the left border of the ring 4 and the leftmost axial position of the right border of the ring 4. The azimuthal orientation of the rotor as displayed in this figure is selected such that the extreme axial points of the loop 4 lie on the uppermost and lowermost rim.

Fig. 6 shows a partially cut-away view of a rotor according to an embodiment. In the upper half of the figure, a cross section through the rotor 1 is shown. Similar to the embodiment shown in Fig. 5, an inner hollow cylinder 13, a first 11 and a second 12 axial positioning element, all made of non-magnetic material and marked by diagonal hatching in the cross-section, hold in place a ring 4 made of magnetic material and marked by horizontal hatching. In addition to the embodiment shown in Fig. 5, a sleeve 15 of non-magnetic material is in contact with an outer

circumference of the loop 4. The sleeve 15 sits on the outermost radius of the rotor. The sleeve shown here is a hollow cylindrical sleeve. It may e.g. comprise titanium or a titanium alloy. The lower half of the figure shows a view onto the rotor with hidden lines shown a dotted lines.

Fig. 7 shows in Fig. 7. a) to 7.d) in schematic flattened views four variants of elementary building blocks of coils windings that may be used to build various of the

embodiments. In all four figures, projections 8, around which coil windings may be wound, are schematically

indicated by squares. The situation of projections arranged on two rings 6', an upper ring, and 6' ', a lower ring respectively, is shown. In this flattened view, the

projections belonging to one ring are arranged in a

horizontal row. The projections are separated by grooves 7. The respective elementary build block of a coil winding is denoted as 9. In all cases shown, at free ends of the coil windings shown a neighboring coil winding may continue directly continue. Alternatively, such free ends may lead to a region outside the coil winding region and be

connected in series with other building blocks of the same measuring coil arrangement or exciting coil arrangement, respectively. Depending on the embodiment under discussion, there may be more or less projections 8 on a circumference of a ring 6', 6' ' than the number of projection shown here.

Fig. 7. a) shows a coil winding 9 defining a saddle coil around a single projection 8 in azimuthal direction a on the first ring 6' and around a corresponding projection on the second ring 6' ' . The number of windings of such a coil winding may be selected according to the azimuthal position of the coil winding in order to approximate a sine or cosine distribution according to an embodiment. As a special case, a coil winding of the type shown here may have an opening angle of 120° in azimuthal direction, if the single projection around which it is wound covers a third of the circumference. The type of coil winding shown in Fig. 7. a) may be used as building block in the first and second measuring coils arrangements of embodiments of the invention . Fig. 7.b) shows a coil winding 9 defining a saddle coil around a several, in the case shown, around four

projections 8 in azimuthal direction a on the first ring 6' and around the same number of projections on the second ring 6' ' . A coil winding of the type shown here may have an opening angle of 120° in azimuthal direction, if there are twelve projections along the circumference of each ring. The type of coil winding shown in Fig. 7.b) may be used as building block in the first and second measuring coils arrangements of embodiments of the invention.

Fig. 7.c) shows two coil windings 9 defining a separate saddle coil around single projections 8, one of the coil windings being arranged on the first ring 6' only and the other being on the second ring 6' ' only. This type of coil windings is useful if first and second ring carrying respective coil windings are built as prefabricated

subassemblies. The number of windings around each

individual projection may be selected such that a sine to cosine distribution is approximated, as defined in an embodiment. The type of coil winding shown in Fig. 7.c) may be used as building block in the first and second measuring coils arrangements of embodiments of the invention as well as building blocks of an exciting coil arrangement. In the latter case, the coil winding 9 on the first 6' and the second 6' ' ring may be connected in series with respective polarities, such that the generated radial magnetic flux on one ring points outward and on the other ring points inward. Furthermore in the latter case, the number of windings around the single projections may be equal on all projections carrying coil windings. In particular, all projections may carry coil windings being connected in series to form an embodiment of the exciting coil

arrangement . Fig. 7.d) shows two coil windings 9 each defining a

separate saddle coil around several projections 8 of the same ring, one of the coil windings being arranged on the first ring 6' only and the other being on the second ring 6' ' only. A coil winding of the type shown here may have an opening angle of 120° in azimuthal direction, if there are twelve projections along the circumference of each ring. This type of coil windings is useful if first and second ring carrying respective coil windings are built as

prefabricated subassemblies. The type of coil winding shown in Fig. 7.c) may be used as building block in the first and second measuring coils arrangements of embodiments of the invention as well as building blocks of an exciting coil arrangement of embodiments of the invention.

Fig. 8. a) shows schematically, in a flattened view, a first measuring coil arrangement SI having saddle coils defining areas Ai, A₂, each area being surrounded by a number n'i, n'₂ of coil windings approximating a multiple of the sine of the azimuthal position of the respective area. Such an arrangement of saddle coils may e.g. be built by using building blocks as shown in any of the figures Fig. 7. a) to Fig. 7.d) . Different numbers of coil windings are

symbolized by different line width. Fig. 8.b) shows for an embodiment having twelve possible positions on the circumference the numbers of coil

windings. Numbers n'i, n'₂ n ' 12 denote the numbers of coil windings of the saddle coils at each of the twelve azimuthal positions of a first measuring coil arrangement SI having saddle coils as shown in Fig. 8. a) . The

distribution of these numbers approximate a multiple of a sine function. Similiarly, for a second measuring coil arrangement S2, numbers of coil windings n' 'i, n' '2 n' ' i2 of the saddle coils are given. The distribution of these numbers approximate a multiple of a cosine function. Numbers 1 to 12 along the horizontal axis denote the positions of the coil windings. Note that on position 6 and 12 the first measuring coil arrangement SI has zero windings and that on positions 3 and 9 the second measuring coil arrangement S2 has zero windings. The numbers of coil windings n'i, n'₂ , n ' 12 are in the case shown:

25, 43, 50, 43, 25, 0, -25, -43, -50, -43, -25, 0.

The numbers of coil windings n' '1, n' '2 n ' ' 12 are in the case shown:

50, 43, 25, 0, -25, -43, -50, -43, -25, 0, 25, 43.

Negative numbers indicate inverse sense of winding.

Fig. 8.c) shows the sine and cosine functions of azimuthal position a multiplied by the factor 50. By rounding the values of these functions read at the twelve vertical grid lines to whole numbers, the numbers of coil windings as shown in Fig. 8.b) can be obtained. Note that position 0 corresponds to position 12, i.e. to the azimuthal position 0° being equal to the azimuthal position 360°. List of reference signs

1 rotor

2 stator

3 axis

4 loop (of magnetic material)

5 air gap

6 ring

6', 6' ' first / second ring

7 groove

8 projection

9 coil winding

10 azimuthal position resolver

11 first axial positioning element

12 second axial positioning element

13 hollow cylinder

14 ring groove

15 sleeve

20 alternating current power supply

21, 22 voltage meter

Ai, A2 areas (surrounded by saddle coils)

H magnetic field

n'i, n'2,..., n ' 12 number of windings

n' '1, n' '2,..., n ' ' 12 number of windings

P exciting coil arrangement

SI first measuring coil arrangement

S2 second measuring coil arrangement

a azimuthal direction

oti , ot2 opening angles in azimuthal direction

Θ azimuthal position of the rotor

Claims

1. Azimuthal position resolver (10) comprising a rotor (1) being rotatable around an axis (3) with respect to a stator (2), wherein

- said rotor comprises at least one loop (4) of magnetic material extending around said axis and being arranged along a geometric plane cutting said axis under an oblique angle ;

- said stator comprises

- an exciting coil arrangement (P) generating a magnetic field (H) entering said loop, propagating along at least a part of said loop and emanating from said loop,

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

- a second measuring coil arrangement (S2) with at least one second saddle coil facing towards said axis; and wherein

- said first and second saddle coils have an opening angle ( 011 , 012 ) of 120° in azimuthal direction with respect to said axis.

2. Azimuthal position resolver according to claim 1, wherein at least one of said first and said second

measuring coil arrangements comprises a group of coils with at least two of said respective saddle coils connected in series .

3. Azimuthal position resolver according to claim 2, wherein said group of coils comprises mutually overlapping of said saddle coils connected in series.

4. Azimuthal position resolver according to claim 2 or 3, wherein saddle coils of said group of coils are connected in series and are pairwise arranged on opposite azimuthal sides of said stator, in particular spaced by 180° in azimuthal direction with respect to said axis.

5. Azimuthal position resolver according to any one of claims 1 to 4, wherein the geometric form of said second measuring coil arrangement corresponds to the geometric form of said first measuring coil arrangement and wherein the position of the second measuring coil arrangement is rotated by 90° around said axis with respect to said first measuring coil arrangement.

6. Azimuthal position resolver according to any one of claims 1 to 4, comprising a third measuring coil

arrangement, wherein the geometric forms of said second and third measuring coil arrangements correspond to the

geometric form of said first measuring coil arrangement and wherein the position of said second and third measuring coil arrangement are rotated by +120° and -120°, respectively, around said axis with respect to said first measuring coil arrangement.

7. Azimuthal position resolver according to any one of claims 1 to 6, wherein said stator comprises at least a first ring with a first number of equally spaced

projections on an internal or external circumference, said first number being divisible by three, and wherein said first and or second saddle coils each surround a separate group of said projections comprising a second number of neighboring projections, said second number being equal to the first number divided by three, in particular wherein said first number equals twelve and said second number equals four.

8. Azimuthal position resolver according to claim 7, wherein said stator comprises a second ring having a geometric form corresponding to the geometric form of said first ring and being axially displaced from said first ring, and wherein said first and or second saddle coils surround a corresponding group of projections of the first and of the second ring.

9. Azimuthal position resolver according to claim 7 or 8, wherein said first and/or second ring comprises a magnetic material .

10. Azimuthal position resolver according to any one of claims 1 to 9, wherein the exciting coil arrangement comprises a circular coil coaxial to said axis and having a center coinciding with a center of said loop.

11. Azimuthal position resolver according to any one of claims 1 to 10, wherein at least one of said first and second measuring coil arrangement comprises saddle coils defining areas ( Ai , A₂ ) each surrounded by a respective number of coil windings (n' i , n'₂, n' i₂ , n' ' i , n''₂, n' ' i₂ ) being an approximation to the sine function of the

azimuthal position of the area multiplied by a factor common to all said saddle coils.

12. Azimuthal position resolver according to any one of claims 1 to 11,

wherein said stator comprises a first and a second ring spaced from each other and coaxially with said axis, each ring having a respective multitude of axially oriented first and second grooves, two neighboring grooves defining a respectively first or second projection in between them, wherein at least one of said exciting coil arrangement, said first measuring coil arrangement and said second measuring coil arrangement comprises

- first coils arranged in said first grooves and around at least one of the first projections and

- second coils arranged in said second grooves and around at least one of the second projections.

13. Azimuthal position resolver according to any one of claims 1 to 12,

said rotor comprising a ring groove (14) along a plane perpendicular to said axis and completely within said loop.

14. Azimuthal position resolver according to any one of claims 1 to 13,

wherein said rotor comprises a non-magnetic sleeve (15) being in contact with an outer circumference of said loop, in particular wherein said sleeve is a hollow cylindrical sleeve, further in particular wherein said sleeve comprises titanium or a titanium alloy.

15. Azimuthal position measuring arrangement comprising - an azimuthal position resolver according to any one of claims 1 to 14,

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

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

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

16. Method of producing an azimuthal position indicative signal being indicative for an azimuthal position of a rotor being rotatable around an axis with respect to a stator and said signal being free of third order harmonic dependence on said azimuthal position, the method comprising the steps of:

- providing said rotor with a loop of magnetic material extending around said axis and being arranged along a geometric plane cutting said axis under an oblique angle, - providing said stator with a first measuring coil arrangement with at least one first saddle coil facing towards said axis and having an opening angle of 120° in azimuthal direction with respect to said axis,

- providing said stator with a second measuring coil arrangement with at least one second saddle coil facing towards said axis and having an opening angle of 120° in azimuthal direction with respect to said axis,

- applying alternating current to an exciting coil

arrangement being arranged on said stator, thereby

generating a magnetic field entering said loop, propagating along at least a part of said loop and emanating from said loop,

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

- measuring a second voltage signal induced by said

magnetic field in said second measuring coil arrangement,

- evaluating amplitudes and phases of said first and second voltage signals to derive said azimuthal position

indicative signal.