A system for sensing a rotor radial displacement in a rotating apparatus

Abstract

A system (1) for sensing a rotor (2) radial displacement in a high speed rotating apparatus, wherein the system (1) comprises a cylindrical hubless outer rotor (2) comprising a rotation axis (X) and a plurality of segments (4), wherein each segment (4) comprises at least one sensing target (6), wherein the sensing target (6) is arranged to cover at least partly the segment of a rotor inner surface (7). The system (1) further comprises two or more homopolar sensing elements (3) arranged along the circumference of the rotor (2) at fixed distances from each other. The homopolar sensing elements (3) are oriented towards the sensing targets (6) of the rotor (2); separated from the rotor (2) by an air gap (8); and comprise at least two coils configured to induce electromagnetic flux with the sensing targets (6) of the rotor (2). The homopolar sensing elements (3) are arranged to move the magnetic flux parallel to the rotation axis (X) inside the sensing targets (6). Also, a method is disclosed.

Description

[0001]A SYSTEM FOR SENSING A ROTOR RADIAL DISPLACEMENT IN A ROTATING APPARATUS FIELD OF THE INVENTION The present application relates generally to a system for sensing a rotor radial displacement in a rotating apparatus. More specifically, the present ap- plication relates to the system comprising a cylindrical outer rotor. BACKGROUND OF THE INVENTION An apparatus comprising a rotor equipped with magnetic bearing technology may require constant moni- toring of rotor position with sensors. Information re- ceived from the sensors may be used in a control system to accurately control the rotor position with electro- magnetic actuators. Rotor materials and sensor technol- ogy may be however further improved. SUMMARY This summary is provided to introduce a selec- tion of concepts in a simplified form that are further described below in the detailed description. This sum- mary is not intended to identify key features or essen- tial features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The scope of protection sought for var- ious embodiments of the present disclosure is set out by the independent claims. Example embodiments of the present disclosure enable using high speed apparatuses with cost affordable sensor solutions. This and other benefits may be achieved by the features of the independent claims. Fur- ther advantageous implementation forms are provided in the dependent claims, the description, and the drawings. According to a first aspect a system for sens- ing a rotor radial displacement in a high speed rotating apparatus is disclosed. The system may comprise a cy- lindrical hubless outer rotor, which may comprise a ro- tation axis and a plurality of segments, wherein each segment may comprise at least one sensing target, wherein the sensing target may be arranged to cover at least partly the segment of a rotor inner surface; and two or more homopolar sensing elements that may be ar- ranged along the circumference of the rotor at fixed distances from each other, wherein the homopolar sensing elements may be oriented towards the sensing targets of the rotor; separated from the rotor by an air gap; and may comprise at least two coils configured to induce electromagnetic flux with the sensing targets of the rotor, wherein the homopolar sensing elements may be arranged to move the magnetic flux parallel to the ro- tation axis inside the sensing targets. To overcome the sensing target material strength limitation, the rotor material may be divided in individual segments. Segmen- tation may reduce mechanical stresses within the sensing target material and allow to fully utilize properties of composite materials of the segment. The segment gaps may create challenges for magnetic bearing control sys- tems, especially at low speed ranges where the segment gaps may increase higher order harmonics. However, at higher speeds, these harmonics may become invisible for the control system. The homopolar sensing elements may allow most of the magnetic flux to pass through the segment gap. This way flux reduction may be much lower as the majority of the flux still has an appropriate path to flow compared to heteropolar sensing elements. The system may allow to use sensor technology in the very demanding high speed apparatuses and still have cost affordable sensor solutions. Also used raw materi- als may be cheaper and manufacturing methods may be simpler. Sensitivity of the sensor element may also be accurate enough to fulfill requirements of the high speed apparatuses. According to an embodiment of the first aspect, each of the homopolar sensing elements may comprise at least one excitation coil and at least one sensing coil. The sensing elements may comprise different number of different coils. According to an embodiment of the first aspect, sensing target material is ferromagnetic material. The sensing elements may require ferrite properties on the rotor side so that electromagnetic field may produce inductance depending on the sensor element. According to an embodiment of the first aspect, ferromagnetic material is at least one of the following: laminated electrical steel, soft magnetic composite ma- terial, and / or epoxy with ferrite particles. The ferro- magnetic material may be a combination of one or more of the said materials or it may be only one of them. Homopolar inductive sensor element design may be pro- vided in order to produce low cost sensor solutions for the active magnetic apparatuses equipped with the fiber composite rotor. It may allow to utilize common materi- als such as electrical steel and copper wire with ap- propriate electrical circuits to provide the excitation frequency. The soft magnetic composite materials may be used for their uniform 3D flux properties. According to an embodiment of the first aspect, the segment may further comprise structural support ma- terial, which may be carbon fiber reinforced polymer and / or nonmagnetic composite material. Yield and tensile strength properties of the structural support materials may be much higher than in the ferromagnetic materials. In order to utilize sensor technologies, fer- romagnetic or electrically conductive properties may need to be added to the structural support material of the rotor. This may be done by adding conductive coating on the structural support material rotor surface, bonding the ferromagnetic material to the rotor surface, or including the ferromagnetic material particles in the resin system, which may fulfill the sensor technology requirements. According to an embodiment of the first aspect, the sensing target may be embedded in the structural support material or the sensing target may be arranged to form a coating on the structural support material. According to an embodiment of the first aspect, the sensing element may comprise E shaped core, U shaped core, or I shaped core. The sensing element may comprise differently shaped cores. The core may have any shape geometry to reduce the reluctance in a flux path. How- ever, all the sensing elements may have identical core shape. According to an embodiment of the first aspect, a sensor outer arc length may be greater than a segment gap length. This may ensure that impact of flux reduc- tion is insignificant. According to an embodiment of the first aspect, a ratio of the segment gap length to the sensor outer arc length is from 0,001 to 0,01. The segment gap length may be significantly smaller than the sensor outer arc length. It may ensure that impact of flux reduction is insignificant. According to an embodiment of the first aspect, the sensor outer arc length is greater than a sensor inner arc length. The arc lengths may be adjusted ac- cording to the demand of the apparatus. The arc length may be adjusted to get better sensor sensitivity. According to an embodiment of the first aspect, segment side cutting angles may be arranged towards a geometrical centre of the rotor and may be the same for each segment for preventing separation of the segments. According to an embodiment of the first aspect, the system may be arranged to detect radial movements of the rotor and / or to provide information about radial position of the rotor. The system may be flexible and it may be used for several purposes. According to an embodiment of the first aspect, the high speed rotation apparatus may have a rotor sur- face velocity above 150 m / s. When using high rotor sur- face velocity harmonics may be less problematic as they may be designed to be pushed beyond the controller band- width and they may become invisible for the control system. According to an embodiment of the first aspect, the sensing element and an actuator of the rotating apparatus may be arranged in the same single unit. This may make the apparatus more compact. According to an embodiment of the first aspect, the actuator may be arranged to use the same sensing targets as the sensing element. This may make rotor design simpler. According to a second aspect a method for sens- ing a rotor radial displacement in a high speed rotating apparatus with a system is disclosed. The system may comprise a cylindrical hubless outer rotor, which may comprise a rotation axis and a plurality of segments, wherein each segment may comprise at least one sensing target, wherein the sensing target may be arranged at least partly to cover the segment of a rotor inner sur- face; and two or more homopolar sensing elements may be arranged along the circumference of the rotor at fixed distances from each other, wherein the homopolar sensing elements may be oriented towards the sensing targets of the rotor; may be separated from the rotor by an air gap ; and may comprise at least two coils, wherein the method may comprise inducing electromagnetic flux with the seg- mented sensing targets of the rotor by the at least two coils; and moving the magnetic flux parallel to the rotating axis inside the sensing targets by the homopo- lar sensing elements. To overcome the sensing target material strength limitation, the rotor material may be divided in individual segments. Segmentation may reduce mechanical stresses within the sensing target material and allow to fully utilize the properties of composite materials of the segment. The system may allow to use sensor technology in the very demanding high speed ap- paratuses and still have cost affordable sensor solu- tions. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are included to provide a further understanding of the invention and constitute a part of this specification, illustrate em- bodiments of the invention and together with the de- scription help to explain the principles of the inven- tion. In the drawings: Figure 1 illustrates an example of a rotor sensing system, according to an example embodiment; Figure 2 illustrates an example of a cross sec- tion B-B of the system of figure 1, according to an example embodiment; Figure 3 illustrates an example of a heteropo- lar sensing element arrangement, according to an example embodiment; Figure 4 illustrates an example of a homopolar sensing element arrangement, according to an example embodiment; and Figure 5 illustrates another example of a ho- mopolar sensing element arrangement, according to an example embodiment; Figure 6 illustrates an example of a signal quality variations between different topologies, ac- cording to an example embodiment; and Figure 7 illustrates an example of a method for sensing a rotor radial displacement in a rotating appa- ratus, according to an example embodiment. Like references are used to designate like parts in the accompanying drawings. DETAILED DESCRIPTION Reference will now be made in detail to embod- iments, examples of which are illustrated in the accom- panying drawings. The detailed description provided be- low in connection with the appended drawings is intended as a description of the present examples and is not intended to represent the only forms in which the pre- sent example may be constructed or utilized. The de- scription sets forth the functions of the example and the sequence of steps or operations for constructing and operating the example. However, the same or equivalent functions and sequences may be accomplished by different examples. According to an example embodiment, apparat- uses with composite rotors equipped with magnetic bear- ing technology may require constant monitoring of rotor position with sensors. Rotor position information may be used in a control system to accurately control the rotor position with electromagnetic actuators. The sen- sors may require ferrite properties on the rotor side so that electromagnetic field may produce inductance on the sensors. Composite rotor materials, such as carbon, aramid, or glass fiber may not offer ferromagnetic prop- erties and their conductivity may also be low, which may make them difficult to use. This is why ferromagnetic or electrically conductive properties may be added to the composite rotor material. However, if a continuous metal layer or coating may be applied on the rotor sur- face, these metal materials typically may start to limit possible surface velocity before the composite structure itself as their yield and tensile strength properties may be much lower than in the composite materials. According to an example embodiment, one way to overcome rotor material strength limitation is to divide the rotor in individual segments to reduce internal stresses within the material and allow it to fully utilize the properties of the composite materials. Ra- dial segment gaps between the segments may isolate the individual segments from each other. Segmented rotor material may introduce more challenges to the control system because these segment gaps between the segments tend to be visible as “notch” in a raw sensor signal because these gaps may be seen as “air” that has no electromagnetic properties. This may create challenges for magnetic bearing control systems especially at low speed ranges where the segment gaps may increase higher order harmonics, which may be unwanted as they are more complex to filter out. However, at higher speeds, these harmonics may be less problematic and may be designed to be pushed beyond the controller bandwidth and in that sense, they may become invisible for the control system. According to an example embodiment, a system is disclosed, which comprises a plurality of homopolar inductive sensor elements of a magnetic bearing for sensing a hubless, outer composite rotor radial dis- placement in a high speed rotating apparatus. A high speed rotation apparatus may have a rotor surface ve- locity above 150 m / s. The sensing target may be radially segmented. The stationary electromagnetic sensing ele- ment may face a rotating segmented sensing target. The sensing element may comprise two coils: an excitation coil and a sensing coil. The excitation coil may be excited by a high frequency current, which may create an electromagnetic circuit to link the excitation coil, the sensing target, and the sensing coil. The change in the segment gap in relation to the excitation coil may change the magnitude of the back electromagnetic force detected by the sensing coil. One or more sensing targets may be circumfer- entially arranged on the rotor inner surface. They may be segmented circumferentially to reduce the mechanical stresses of the sensing target resulting from the centrifugal force when the rotor may reach high rota- tional velocity. The stationary electromagnetic sensing element may be set up so that magnetic flux may not move in a circumferential plane through the segment gaps as in an heteropolar topology but transversely along the rotation axis as in homopolar topology. In the homopolar topology the magnetic flux may move in a radial plane through the segment gaps of the rotor. This way most of the magnetic flux may still pass through the segment gap and flux reduction may be much lower as the majority of the flux still may have an appropriate path to flow. The impact of the segment gap may be further reduced by optimizing the stationary side geometry of the sensor element. The segment gaps from the segmented sensing targets in ho- mopolar topologies may generate a much smaller increase of the reluctance of the magnetic circuit than in het- eropolar topologies as the entire magnetic flux may no longer need to pass through the segment gap. An example of figure 1 illustrates an example of a rotor sensing system 1, which may be located in a rotating apparatus. The system 1 may be used in a high speed rotating apparatus. The system may comprise a cy- lindrical hubless outer rotor 2 divided in a plurality of segments 4. The rotor 2 may comprise a rotation axis X. The rotor 2 may also have a geometrical centre O. Each segment 4 may comprise at least one sensing target 6. The sensing target 6 may be arranged to cover at least partly the segment of a rotor 2 inner surface 7. According to an example embodiment, the segment 4 further comprises structural support material, which is carbon fiber reinforced polymer and / or nonmagnetic composite material. The composite material may be aramid or glass fiber. According to an example embodiment, the sensing target 6 is embedded in the support material and / or the sensing target 6 is arranged to form a coating on the support material. According to an example embodiment, sensing target material comprises ferromagnetic material. Fer- romagnetic materials are those materials which may ex- hibit a spontaneous net magnetization at the atomic level. When placed in an external magnetic field, fer- romagnetic materials may be strongly magnetized in the direction of the field. According to an example embodiment, the ferro- magnetic material is at least one of the following: laminated electrical steel, soft magnetic composite ma- terial, and / or epoxy with ferrite particles. According to an example embodiment, segment side cutting angles α are arranged towards a geometrical centre O of the rotor 2 and are the same for each segment 4. The segment gap 12 between two segments 4 may be a radial segment gap. The system may also comprise two or more homo- polar sensing elements 3 arranged along the circumfer- ence of the rotor 2 at fixed distances from each other. The system may comprise four sensing elements 3, for example. The sensing elements 3 may be oriented towards the sensing targets 6 of the rotor 2 and separated from the rotor 2 by an air gap 8. The air gap 8 may be circumferential and it may separate the sensing elements 3 and the sensing targets 6 from each other. The homo- polar sensing elements 3 may comprise at least two coils configured to induce electromagnetic flux with the sens- ing targets 6 of the rotor 2 and may be arranged to move the magnetic flux along or parallel to the rotation axis X. Amount of the segments 4 in the rotor 2 may depend on a size of the rotor 2. The bigger rotor 2 may comprise more sensing targets. In an example of figure 1, the rotor 2 has been divided to 12 segments 4. According to an example embodiment, the homo- polar sensing elements 3 comprise at least one excitation coil 9 and at least one sensing coil 10. The excitation coil 9 may be excited by a high frequency current, creating an electromagnetic circuit linking the excitation coil 9, the sensing target 6, and the sensing coil 10. The difference between the sensing target 6 and the excitation coil 9 may change the magnitude of the back electromagnetic force detected by the sensing coil 10. The sensing element 3 may have a sensor arc surface 11 on the outer surface of the sensing element 3. The sensor arc surface 11 of the sensing element 3 may be located towards the air gap 8. A sensor outer arc length ALmay be the length of the sensor arc surface 11. The sensing element may have a sensor inner arc length IL.The sensor outer arc length ALof the sensing element 3 may be greater than the sensor inner arc length ILof the sensor element 3. An example of figure 2 illustrates a cross sec- tion B-B of the system 1 of figure 1. In the example of figure 2 the homopolar sensing element 3 has an E shaped core, wherein an excitation coil 9 and a sensing coil 10 have been wound around a centre leg of the E shaped core. The sensing coil 10 may be arranged to face the air gap 8 and the excitation coil 9 may be arranged next to it to face an inner part of the sensing element 3. Legs of the E shaped core may be located on top of each other in a direction of the rotation axis X. The E shaped core of the sensing element 3 may be arranged to face the segment target 6. According to an example embodiment, the sensing element 3 comprises an E shaped core, U shaped core, or I shaped core. All the sensing elements of the system 1 may have the same geometry and they may be identical to each other. An example of figure 3 illustrates a heteropo- lar sensing element arrangement. An E shaped core of the heteropolar sensing element 14 may be arranged in a horizontal position to face the segment target 6. This means that legs of the E shaped core may have been located adjacent to each other in a horizontal direction and perpendicular to rotation axis X. All the poles or ends of the legs may face to the sensing target 6. Windings of coils may be wound around the central leg of the E shaped core. The coils may be configured to induce electromagnetic flux with the sensing target 6 mounted on the rotor 2. The coils may comprise an exci- tation coil 9 and a sensing coil 10. The flux may flow from the center leg of the E shaped core through the air gap 8 towards the sensing target 6, wherefrom it may split into two flows and may travel through the air gap 8 back to the heteropolar sensing element 14 along the outer legs and finally return back to the center leg. The example of figure 3 shows that the flux has two magnetic flux paths 13, which may move the flux in a circumferential plane 15 or in a horizontal plane in relation to rotor 2. The flux may flow in a circumfer- ential plane 15 perpendicular to the axis X. However, when the flux path 13 reaches the segment gap 12, the flux may have a discontinuation point 17 because the segment gap 12 may not have electromagnetic properties and the flux may not be able to flow through the segment gap 12. This means that there may be a drastic drop in the flux path. This may create challenges for magnetic bearing control systems. The heteropolar sensing element arrangement illustrated in the example of figure 3 are called het- eropolar because polarity of the bias poles changes around the rotor circumference. This may be a conse- quence of the magnetic fluxes being constrained to the planes normal to the rotation axis. Magnetic flux lines may always form closed loops. If they exit at one pole they always may come back in the other, and in the case of heteropolar sensing elements all the poles may lie in the same horizontal plane. An example of figure 4 illustrates an example of a homopolar sensing element arrangement. In figure 4 example the homopolar sensing element 3 has an E shaped core, wherein an excitation coil 9 and a sensing coil 10 have been wound around a centre part of the E shaped core. Legs of the E shaped core may be located on top of each other in the direction of the axis X. The E shaped core of the sensing element 3 may be arranged in a vertical position to face the segment target 6. All the poles or ends of the legs may face to the sensing target 6. The coils may be configured to induce elec- tromagnetic flux with the sensing target 6 mounted on the rotor 2. The coils may comprise an excitation coil 9 and a sensing coil 10. The sensing coil 10 may be arranged to face the air gap 8 and the excitation coil 9 may be arranged next to it to face an inner part of the sensing element 3. The flux may flow from the center leg of the E shaped core through the air gap 8 towards the sensing target 6, wherefrom it may split into two flows and may travel through the air gap 8 back to the homopolar sensing element 3 along the outer legs and finally return back to the center leg. The example of figure 4 shows that the flux has two magnetic flux paths 13, which may move the flux in a radial plane 16 or in a vertical plane. An example of figure 5 illustrates another ex- ample of a homopolar sensing element arrangement seen from above. Magnetic flux paths 13 are coming from the segments 4 and may travel through the air gap 8 back to the homopolar sensing element 3 along the outer legs and from there they may return back to the center leg. When the flux path 13 reaches the segment gap 12, the flux may have a discontinuation point because the seg- ment gap 12 may not have electromagnetic properties and the flux may not be able to flow through the segment gap 12 and may have a discontinuation point 17. However, the magnetic flux may move in the radial plane 16, which has the effect that only the flux in the segment gap area has a discontinuation point 17 and it may not be able to flow through the segment gap 12, However, majority of the flux may still have an appropriate path to flow. This way only a small amount of the magnetic flux may not pass through the segment gap 12 and the flux reduc- tion may be much lower as the majority of the flux still has an appropriate path to flow. The impact of the seg- ment gap 12 may be further reduced by optimizing the stationary side geometry of the sensor element 3. The segment gaps 12 in the segmented rotor 2 in homopolar topologies may generate a much smaller increase of the reluctance of the magnetic circuit than in heteropolar topologies as the entire magnetic flux may no longer need to pass through the segment gap 12. According to an example embodiment, the segment gap 12 has a segment gap length GL.A sensor arc length ALmay be significantly greater than the segment gap length GL. A ratio of the segment gap length GLto the sensor arc length ALmay be from 0,001 to 0,01, for example. For example, the segment gap length GLis 1 mm and the sensor arc length ALis 100 mm. For example, the ratio of the segment gap length GLto the sensor arc length ALis GL / AL= 1 mm / 100 mm = 0,01. GL / ALmay be less than 0,01. The segment gap length GLmay be less than 1 % of the sensor arc length ALAccording to an example embodiment, the sensing element 3 and an actuator of the rotating apparatus are arranged in the same single unit. According to an example embodiment, the actu- ator is arranged to use the same sensing targets 6 as the sensing element 3. Figure 6 illustrates an example of a signal quality variations between different topologies. Figure 6 example shows signals for a heteropolar and a homopo- lar topology. When the sensing elements 3, 14 cross the segment gap 12 while the rotor 2 rotates, the sensed voltage may change or blip. For example, in the heter- opolar topology when the segment gap 12 is crossed, the heteropolar sensing element 14 may induce voltage in its sensing coil 10, which may go from -65.29 volts to - 51.61 volts. This may cause 20.95 percent voltage change, for example. However, in the homopolar topology when the segment gap 12 is crossed, the homopolar sens- ing element 3 may induce voltage in its sensing coil 10, which may go from -57.83 volts to -56.94 volts, for example. This means that when using the homopolar sens- ing elements 3 voltage difference may be only 1.53 per- cent, for example. The homopolar sensing elements 3 may be more effective than heteropolar sensing elements 14 because voltage difference may be smaller when the seg- ment gap 12 is crossed. Figure 7 illustrates an example of a method for sensing a rotor 2 radial displacement in a high speed rotating apparatus with a system 1. The system 1 may comprise a cylindrical hubless outer rotor 2 comprising a rotation axis X and a plurality of segments 4, wherein each segment 4 may comprise at least one sensing target 6, wherein the sensing target 6 may be arranged at least partly to cover the segment of a rotor inner surface 7. The system 1 may further comprise two or more homopolar sensing elements 3 arranged along the circumference of the rotor 2 at fixed distances from each other. The homopolar sensing elements 3 may be oriented towards the sensing targets 6 of the rotor 2, separated from the rotor 2 by an air gap 8, and may comprise at least two coils. At operation 700, the method may comprise in- ducing electromagnetic flux with the segmented sensing targets 6 of the rotor 2 by the at least two coils. At operation 710, the method may comprise mov- ing the magnetic flux parallel to the rotating axis X inside the sensing targets 6 by the homopolar sensing elements 3. Further features of the method directly result from functionalities of, for example, the system 1. Dif- ferent variations of the method may be also applied, as described in connection with the various embodiments. The system 1 may be configured to perform or cause performance of any aspect of the method described herein. Any range or device value given herein may be extended or altered without losing the effect sought. Also, any embodiment may be combined with another em- bodiment unless explicitly disallowed. Although the subject matter has been described in language specific to structural features and / or acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as examples of implementing the claims and other equiv- alent features and acts are intended to be within the scope of the claims. It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages. It will further be un- derstood that reference to 'an' item may refer to one or more of those items. The steps or operations of the methods de- scribed herein may be carried out in any suitable order, or simultaneously where appropriate. Additionally, in- dividual blocks may be deleted from any of the methods without departing from the scope of the subject matter described herein. Aspects of any of the embodiments de- scribed above may be combined with aspects of any of the other embodiments described to form further embodiments without losing the effect sought. The term 'comprising' is used herein to mean including the method, blocks, or elements identified, but that such blocks or elements do not comprise an exclusive list and a method or apparatus may contain additional blocks or elements. Although subjects may be referred to as ‘first, ‘second’, or ‘third’ subjects, this does not necessarily indicate any order or importance of the subjects. In- stead, such attributes may be used solely for the pur- pose of making a difference between subjects. It will be understood that the above descrip- tion is given by way of example only and that various modifications may be made by those skilled in the art. The above specification, examples and data provide a complete description of the structure and use of embod- iments. Although various embodiments have been described above with a certain degree of particularity, or with reference to one or more individual embodiments, those skilled in the art could make numerous alterations to the disclosed embodiments without departing from scope of this specification.

Claims

CLAIMS 1. A system (1) for sensing a rotor radial dis- placement in a high speed rotating apparatus, wherein the system (1) comprises a cylindrical hubless outer rotor (2) compris- ing a rotation axis (X) and a plurality of segments (4), wherein each segment (4) comprises at least one sensing target (6), wherein the sensing target (6) is arranged to cover at least partly the segment (4) of a rotor inner surface (7); and two or more homopolar sensing elements (3) ar- ranged along the circumference of the rotor (2) at fixed distances from each other, wherein the homopolar sensing elements (3) are oriented towards the sensing tar- gets (6) of the rotor (2); separated from the rotor (2) by an air gap (8); and comprise at least two coils (9, 10) configured to induce electromagnetic flux with the sensing targets (6) of the rotor (2), wherein the homopolar sensing elements (3) are arranged to move the magnetic flux parallel to the rotation axis (X) inside the sensing targets (6).

2. The system (1) according to claim 1, wherein each of the homopolar sensing elements (3) comprise at least one excitation coil (9) and at least one sensing coil (10).

3. The system (1) according to claim 1 or claim 2, wherein sensing target material is ferromagnetic ma- terial.

4. The system (1) according to claim 3, wherein the ferromagnetic material is at least one of thefollowing: laminated electrical steel, soft magnetic composite material, and / or epoxy with ferrite particles.

5. The system (1) according to any one of the preceding claims, wherein the segment (4) further com- prises structural support material, which is carbon fi- ber reinforced polymer and / or nonmagnetic composite ma- terial.

6. The system (1) according to any one of the preceding claims, wherein the sensing element (3) com- prises E shaped core, U shaped core, or I shaped core.

7. The system (1) according to any one of the preceding claims, wherein a sensor outer arc length (AL) is greater than a segment gap length (GL).

8. The system according to claim 7, wherein a ratio of the segment gap length (GL) to the sensor outer arc length (AL) is from 0,001 to 0,01.

9. The system (1) according to claims 7 or 8, wherein the sensor outer arc length (AL) is greater than a sensor inner arc length (IL).

10. The system (1) according to any one of the preceding claims, wherein segment side cutting angles (α) are arranged towards a geometrical centre (O) of the rotor (2) and are the same for each segment (4) for preventing separation of the segments (4).

11. The system (1) according to any one of the preceding claims, wherein the system (1) is arranged to detect radial movements of the rotor (1) and / or to pro- vide information about radial position of the rotor (1).

12. The system (1) according to any one of the preceding claims, wherein the high speed rotation appa- ratus has a rotor surface velocity above 150 m / s.

13. The system (1) according to any one of the preceding claims, wherein the sensing element (3) and an actuator of the rotating apparatus are arranged in a same single unit.

14. The system (1) according to claim 13, wherein the actuator is arranged to use the same sensing targets (6) as the sensing element (3).

15. A method for sensing a rotor radial dis- placement in a high speed rotating apparatus with a system (1), wherein the system (1) comprises a cylindrical hubless outer rotor (2) compris- ing a rotation axis (X) and a plurality of segments (4), wherein each segment (4) and at least one sensing target (6), wherein the sensing target (6) is arranged at least partly to cover the segment of a rotor inner surface (7); and two or more homopolar sensing elements (3) ar- ranged along the circumference of the rotor (2) at fixed distances from each other, wherein the homopolar sensing elements (3) are oriented towards the sensing tar- gets (6) of the rotor (2); separated from the rotor (2) by an air gap (8); and comprise at least two coils, wherein the method comprises inducing electromagnetic flux with the seg- mented sensing targets (6) of the rotor (2) by the at least two coils; andmoving the magnetic flux parallel to the ro- tating axis (X) inside the sensing targets (6) by the homopolar sensing elements (3).

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