EP2373901A1

EP2373901A1 - Electrodynamic actuator

Info

Publication number: EP2373901A1
Application number: EP09830660A
Authority: EP
Inventors: Torbjörn Lembke
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-12-02
Filing date: 2009-11-30
Publication date: 2011-10-12
Also published as: SE535378C2; WO2010064978A1; EP2373901A4; SE1150597A1

Abstract

An electrodynamic actuator (1) comprises a stator actuator part (21) having a stator magnetic circuit member (22), and a rotor actuator part (11) having a rotationally symmetric rotor magnetic circuit member (12). The actuator further comprises a magnet (31) inducing a magnetic flux (32) through a magnetic circuit (30) comprising the stator and rotor magnetic circuit members. A first side (15) of the rotor magnetic circuit member faces a first side (25) of the stator magnetic circuit member, both exhibiting a variable reluctance or a variable magnetization in a radial direction (3). The stator actuator part further comprises an electrically conducting loop (40) encircling a portion (28) of the stator magnetic circuit member, which portion is arranged for conducting a magnetic flux through the electrically conducting loop. The portion is arranged for causing the flux to change when the rotor and stator actuator parts are moved radially relative each other.

Description

ELECTRODYNAMIC ACTUATOR

TECHNICAL FIELD

The present invention relates in general to bearing arrangements and methods and in particular to such arrangements and methods involving magnetic interactions. BACKGROUND

Magnetic forces have since long been used in different kinds of bearing equipment. In the European Patent Publication EP O 594 033 B1, a magnetic bearing arrangement is disclosed, in which pairs of magnets are positioned in corresponding locations on opposite sides of a thin electrically conducting material. The thin electrically conducting material is in relative motion with respect to the magnet pairs and has a thickness allowing the magnetic field to penetrate, As long as the thin electrically conducting material is perfectly centred between the pair of magnets, the magnetic fields will almost compensate each other and almost no eddy currents are produced in the thin electrically conducting material. However, if the thin electrically conducting material is offset in any direction, a resulting magnetic field at the thin electrically conducting material will be different from zero and eddy currents are produced in the thin electrically conducting material, operating for counteracting the displacement. However, even though compensating magnetic fields are used, a small resulting field will always be present, resulting in energy losses also at ordinary non-disturbed operation.

In the Published International Patent Application WO 98/32981, a device for magnetic suspension is disclosed. A rotationally symmetric magnetic field is provided around an electrically conducting, nonmagnetic, rotor. When the rotation axis of the rotor coincides with the symmetry axis of the magnetic field, the entire rotor will experience the same magnetic fields at all times. However, when an off- magnetic-axis rotation is performed, different parts on the rotor will experience a magnetic field that varies with time. Eddy currents will be produced which tend to drive the rotor back to a symmetric position. However, the reactive force as created from the eddy currents produced in the rotor itself are generally not directed towards the centre, but has typically also a perpendicular component. This results in vibrations in the rotor system. Furthermore, the restoring force is dependent only of the magnitude of the displacement, which means that no damping is present.

The US patent US 6,304,015 discloses a magneto-dynamic bearing. Shortened loops of conductors are provided in the rotor and permanent magnets are provided in the stator at positions corresponding to the rotor loops. A deviation from alignment between rotor and stator will at a location of the shortened loops of conductors rotating with the rotor result in a fluctuating magnetic field, giving rise to eddy currents, which in turn strive to move the rotor back. The shortened loops of conductor thus operate as a spring bearing the rotor in radial direction. These loops do, however, not provide any damping effect. In one embodiment, additional shortened loops of conductors are provided in the stator. At the rotor, at a radial position in the vicinity of the additional shortened loops of conductors, magnets are provided. If a sideward directed motion takes place, eddy currents are produced in the additional shortened loops of conductors tending to counteract the motion, which gives a small 5 damping action. A disadvantage with this arrangement is, however, that the available damping action is very small since the velocity of the sideward motion typically is very small and thus also the time derivate of the magnetic flux passing the additional shortened loops of conductors.

In the international patent application publication WO01/84693, a bearing system is disclosed, whicho combines an axial bearing based on electromagnetic control of the axial position with a passive radial magnetic bearing. The passive radial magnetic bearing comprises multiple concentric radially spaced apart axially magnetized ring magnets at a stator and corresponding aligned pole rings on the rotor. A restoring force appears when the rotor is displaced from alignment. Axial forces between the magnets and the pole rings are balanced by the controlled axial bearing. One disadvantage with such a bearing 5 system is that the radially operating restoring forces are undamped, which may cause oscillations.

SUMMARY

A general problem with prior art magnetic bearings is that sufficient damping in the radial direction is difficult to provide. Often, separate additional arrangements are provided in order to supply bearing stiffness and bearing damping functionality, respectively. Still, if vibration levels are too high the o amplitude needs to be reduced using external touch down bearings to avoid damage.

A general object of the present invention is thus to improve damping of a rotating bearing arrangement in an energy efficient manner. A further object for preferred embodiments is to provide possibilities to actively control the damping properties, and preferably also to integrate touch down bearing 5 functionality when desired.

These objects are achieved by actuator arrangements, rotating machines and methods according to the enclosed patent claims. In general words, in a first aspect, an electrodynamic actuator comprises a stator actuator part having a stator magnetic circuit member, which comprises magnetic material, and a o rotor actuator part having a rotor magnetic circuit member, which comprises magnetic material. The stator actuator part and the rotor actuator part have an axis of intended rotation relative each other. The electrodynamic actuator further comprises at least one magnet inducing a magnetic flux through a magnetic circuit that comprises the stator magnetic circuit member and the rotor magnetic circuit member. The rotor magnetic circuit member is essentially rotationally symmetric with respect to the 5 axis. A first side of the stator magnetic circuit member exhibits at least one of a variable reluctance and a variable magnetization in a radial direction with reference to the axis. A first side of the rotor magnetic circuit member, facing the first side of the stator magnetic circuit member, exhibits at least one of a variable reluctance and a variable magnetization in a radial direction with reference to the axis. The stator actuator part further comprises at least one electrically conducting loop encircling a respective portion of the stator magnetic circuit member. That portion of the stator magnetic circuit member comprises magnetic material and is arranged for conducting a magnetic flux having a non-zero component through the at least one electrically conducting loop. The portion of the stator magnetic circuit member is arranged for causing the non-zero component to change when the stator actuator part and the rotor actuator part are moved relative each other in a radial direction.

In a second aspect, a rotating machine comprises a stator, a rotor and at least one electrodynamic actuator according to the first aspect. The rotor actuator part is thereby attached to the rotor and the stator actuator part is attached to the stator.

In a third aspect, a method for operating an electrodynamic actuator according to the first aspect comprises rotating of the stator actuator rotor part and the rotor actuator part relative to each other around the axis, whereby any discrepancy from alignment between the stator actuator part and the rotor actuator part gives rise to eddy currents in the electrically conducting loop acting as vibration damper.

One advantage with the present invention is that improved damping is provided to rotating bearing systems in a simple and energy efficient manner. The damping can be utilized as a passive damping, but allows also for different active damping solutions. Further advantages are discussed in connection with different embodiments described further below. BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1A is a cross-sectional view of an embodiment of an electrodynamic actuator according to the present invention;

FIG. 1 B is another cross-sectional view of the embodiment of Fig. 1 A;

FIG. 1C is yet another cross-sectional view of the embodiment of Fig. 1A;

FIG. 1D is a cross-sectional view of the embodiment of Fig. 1A, when a rotor actuator part and a stator actuator part are displaced from alignment; FIG. 2 is a cross-sectional view of an embodiment of a rotating machine comprising an electrodynamic actuator according to the present invention;

FIG, 3 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having electrically conducting loops oriented along the axis; FIG. 4 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having permanent magnets at a first side of a rotor actuator part; FIG. 5 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having permanent magnets at a first side of a stator actuator part; FIG. 6 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having gradually changing magnetic reluctance in a radial direction;

FIG. 7 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention where a stator actuator part faces a rotor actuator part from two sides; FIG. 8A is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having a symmetric arrangement in axial direction; FIG. 8B is another cross-sectional view of the embodiment of Fig. 8A;

FIG. 9A is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having an alignment position with intentional offset between stator actuator part and rotor actuator part; FIG. 9B is another cross-sectional view of the embodiment of Fig. 9A; FIG. 10 is a cross-sectional view of yet another embodiment of an electrodynamic actuator according to the present invention having a symmetric arrangement in axial direction; FIG. 11 is a diagram illustrating induced and controlled currents through electrically conducting loops; FIG. 12 is a schematic drawing illustrating a rotating machine with a control unit for controlling currents through electrically conducting loops; FIG. 13 is a flow diagram of steps of an embodiment of a method according to the present invention;

FIG. 14 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having double rotor actuator parts;

FIG. 15 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention providing additional restoring forces in a radial direction; FIG. 16 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having the rotor shaft as part of the magnetic circuit;

FIG. 17 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention having a three-part rotor actuator part; FIG. 18 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention with repulsive permanent magnets; and

FIG. 19 is a cross-sectional view of another embodiment of an electrodynamic actuator according to the present invention with air gaps directed in radial direction. DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

In the present disclosure, a bearing arrangement is defined as a bearing arrangement, typically axial, providing at least one of bearing stiffness and bearing damping in a radial and/or axial direction. A member of such a bearing arrangement providing at least a part of any such functionality based on any electrodynamic interaction is in the present disclosure denoted an electrodynamic actuator.

Fig. 1A schematically illustrates an embodiment of an electrodynamic actuator 1 according to the present invention. The electrodynamic actuator is preferably used as or in connection with a rotating bearing of a rotating machine, and is preferably integrated as a part of the rotating bearing. The electrodynamic actuator is arranged for providing a damping operation on movements in radial directions. The actuator 1 is to a part attached to a rotor 10 and to a part to a stator 20. A rotor actuator part 11 is thereby fastened to the rotor 10 for following the rotor 10 when being rotated. A stator actuator part 21 is likewise fastened to the stator 20 for being stationary regardless of any rotation of the rotor 10. The stator actuator part 21 and the rotor actuator part 11 thereby have an axis 4 of intended rotation relative each other. A direction along the axis, as illustrated by an arrow, is in the present disclosure denoted as an axial direction 2. Likewise, a direction perpendicular to the axial direction 2 and in a same plane as the axial direction 2 is denoted as a radial direction 3.

The actuator 1 comprises a magnetic circuit 30, comprising a stator magnetic circuit member 22 and a rotor magnetic circuit member 12. The stator magnetic circuit member 22 is comprised in the stator actuator part 21 and the rotor magnetic circuit member 12 is comprised in the rotor actuator part 11. The stator magnetic circuit member 22 as well as the rotor magnetic circuit member 12 comprises magnetic material 13, 23 in order to define a magnetic flow path. Preferably, the magnetic material has a relative magnetic permeability of at least 100. Since the magnetic circuit 30 has members in both the stator 20 and the rotor 10, a magnetic flux 32 will pass both the stator actuator part 21 and the rotor actuator part 11. The magnetic circuit 30 also comprises at least two gaps 33 between the stator actuator part 21 and the rotor actuator part 11. The magnetic circuit 30 comprises at least one magnet 31 inducing the magnetic flux 32 through the stator magnetic circuit member 22 and the rotor magnetic circuit member 12. In the present embodiment, the magnet 31 is a permanent magnet 34 comprised in the stator magnetic circuit member 22. The magnet 31 can in alternative embodiments be positioned at other positions along the magnetic circuit 30. Also multiple magnets 31 are possible to use. In an alternative embodiment, the magnet flux 32 can instead be induced by an electromagnet arrangement.

5 The rotor magnetic circuit member 12 is essentially rotationally symmetric with respect to the axis 4 of intended rotation. A first side 15 of the rotor magnetic circuit member 12 exhibits a variation of magnetic properties in the radial direction 3 with reference to the axis 4. In the present embodiment, this variation of magnetic properties is realised by providing a variable reluctance in the radial direction 3, The first side 15 of the rotor magnetic circuit member 12 is here provided with protrusions 16 ando recesses 17 extending in the axial direction 2. These geometric structures give a variable reluctance for a magnetic flux in the axial direction 2.

Similarly, in the present embodiment, a first side 25 of said stator magnetic circuit member 22 exhibits a variation of magnetic properties in the radial direction 3 with reference to the axis 4. Also here, the5 variation of magnetic properties is realised by providing a variable reluctance, by means of geometrical structures comprising protrusions 26 and recesses 27 extending in the axial direction 2. The first side 15 of the rotor magnetic circuit member 12 faces the first side 25 of the stator magnetic circuit member 22. The first side 25 of the stator magnetic circuit member 22 and the first side 15 of the rotor magnetic circuit member 12 are thereby magnetically interacting over the gap 33. In the present embodiment, 0 the protrusions 16, 26 are positioned to face each other, and the recesses 17, 27 are positioned to face each other, giving rise to areas with a narrow gap and areas with a wide gap. The variation of magnetic properties in the radial direction 3 has the effect that the magnetic flux is concentrated to areas having a low magnetic reluctance in the axial direction 2. In the present embodiment, almost all flux 32 of the magnetic circuit 30 will pass the narrow gap areas. When the rotor 10 rotates without any 5 radial displacements, the magnetic flux passing between the stator magnetic circuit member 22 and the rotor magnetic circuit member 21 is constant, due to the rotational symmetry of the rotor magnetic circuit member 12.

The stator actuator part 21 further comprises at least one electrically conducting loop 40 encircling a o respective portion 28 of the stator magnetic circuit member 22. In this particular embodiment, the electrically conducting loops 40 are placed essentially in a plane having a normal directed in the axial direction 2, i.e. the electrically conducting loops 40 are essentially extended in radial and/or tangential directions. The electrically conducting loop 40 comprises electrically conducting material, preferably with an electrical conductivity exceeding 10 MS/m and more preferably higher than 30 MS/m. This 5 portion 28 of the stator magnetic circuit member 22 comprises magnetic material 23. This portion 28 is further arranged for conducting a magnetic flux 32 having a non-zero component through the corresponding electrically conducting loop 40. As will be explained more in detail further below, the portion 28 of the stator magnetic circuit member 22 is arranged for causing the non-zero component to change when the stator actuator part 21 and the rotor actuator part 11 are moved relative to each other in the radial direction 3.

A partial, cross-sectional, view of the embodiment illustrated in Fig. 1A taken along the line A-A is illustrated in Fig. 1B. The protrusions 26 and recesses 27 are easily seen. The protrusions 26 are in this embodiment interrupted by slits 29 at different tangential positions, and the electrically conducting loops 40 are placed through these slits 29. In the present embodiment, each electrically conducting loop 40 is a separate loop and encircles a respective portion 28 of the protrusions 26. In alternative embodiments, the electrically conducting loop 40 may encircle more than one portion 28 and may overlap with each other. The electrically conducting loops 40 may also be electrically interconnected to each other in different configurations.

Fig. 1C illustrates a partial, cross-sectional, view of the embodiment illustrated in Fig. 1a taken along the line B-B. Here the rotational symmetry of the protrusions 16 and recesses 17 is seen.

Fig. 1D illustrates a part of the actuator 1 of Fig, 1A, in a situation where the rotor actuator part 11 and the stator actuator part 21 are displaced relative to each other by a distance 5 in the radial direction.

Due to the displacement with the distance 5, the width of the narrow gap areas in the gap 33 is reduced. This means that the magnetic reluctance for the entire magnetic circuit 30 is increased and the magnetic flux crossing the gap 33 is reduced. There will be a magnetic force between the rotor actuator part 11 and the stator actuator part 21. An axial component of this magnetic force 6 tries to close the gap between the rotor actuator part 11 and the stator actuator part 21. At small distances 5, this force 6 will be about the same as is present in the aligned condition. A radial component of the magnetic force tries to reduce the distance 5. This magnetic force thus operates according to similar principles as disclosed in WO 01/84693.

If the situation in Fig. 1 D is steady, i.e. the magnetic forces are compensated in some manner, the flux passing the gap 33 and the portions 28 encircled by the electrically conducting loops 40 is constant due to the rotational symmetry of the rotor magnetic circuit member 12. Hence, no eddy currents are induced in the electrically conducting loops 40. However, during the displacement between the situations of Fig. 1A and Fig. 1D, respectively, the flux will change, i.e. the flux has a non-zero time derivative. The same is valid if the magnetic forces are allowed to reduce the distance 5. During such events, an eddy current will be induced in the electrically conducting loops 40. This current will, according to the general principles of eddy currents, counteract the flux changes causing the eddy current, i.e. counteract the displacement change, regardless of in which direction the displacement is changed. A damping functionality is thus achieved.

5 The restoring magnetic force in the radial direction is dependent only on the momentary size of the displacement, i.e. the distance 5. Such a dependence thus acts as a spring trying to move the rotor actuator part 11 and the stator actuator part 21 relative each other towards an equilibrium point. However, since only the displacement is of importance, no damping is present. At the contrary, the counteracting forces created by the eddy currents in the electrically conducting loops 40 are instead o depending on the rate of displacement change. If the displacement doesn't change, the force produced by the electrically conducting loops 40 is zero. Furthermore, even if the displacement is directed towards the perfect alignment of the rotor actuator part 11 and the stator actuator part 21 , the force produced by the electrically conducting loops 40 will also counteract such changes. In other words, all forms of motion are counteracted, i.e. a damping force is provided. 5

By utilizing the variation of magnetic properties in the radial direction 3, changes in the magnetic flux through the electrically conducting loops 40 can be enhanced, whereby a strong damping can be achieved, compared to prior art solutions. The portion 28 of the stator magnetic circuit member 22 is thereby arranged for causing the non-zero component to change when the stator actuator part 21 and o the rotor actuator part 11 are moved relative to each other in the radial direction 3.

Fig. 2 illustrates schematically a rotating machine 9, comprising a stator 20 and a rotor 10. The rotating machine 9 comprises at least one electrodynamic actuator 1 according to the principles presented in the present disclosure. The rotor actuator part 11 is attached to the rotor 10 and the stator actuator 5 part 21 is attached to the stator 20.

There are many variations in the detailed configuration of the stator actuator part 21 and the rotor actuator part 11 , all giving a technical effect in relation to the prior art presented above.

0 In Fig. 3, another embodiment of an electrodynamic actuator 1 is illustrated. In this embodiment, an electrically conducting loop 40 is encircling the stator magnetic circuit member 22 at a portion connecting two groups of protrusions 26, between which a magnetic flux is flowing. In this embodiment, the magnetic flux passes the electrically conducting loop 40 in a radial direction 3. However, any changes in the geometrical relationship between the protrusions 26 at the stator actuator part 21 and the protrusions 16 at the rotor actuator part 11 also give a change in the flux passing through the electrically conducting loop 40. An advantage with this embodiment is that there is typically more space for an electrically conducting loop 40 in such a position. In Fig. 4, another embodiment of an electrodynamic actuator 1 is illustrated. In this embodiment, the first side 15 of the rotor magnetic circuit member 12 also exhibits a variation of magnetic properties in the radial direction 3 with reference to the axis 4. However, in the present embodiment, this variation of magnetic properties is realised by providing a variable magnetization in the radial direction 3. Permanent magnets 34 are provided at the surface rotor actuator part 11 facing the first side of the stator actuator part 21. Permanent magnets 34 are brittle and are therefore preferably mechanically supported in the radial direction by solid non-magnetic material 39. An advantage with such an embodiment is that the rotor actuator part 11 and the stator actuator part 21 can be made smaller, since both the magnetization and the variation in radial direction is provided by on and the same structure.

It is also possible to utilize permanent magnets 34 for providing the variation of magnetic properties at the stator actuator part 21, as illustrated by an embodiment of Fig. 5. This is typically an advantage if the rotating machine to which the present actuator 1 is connected operates at high speed, since there will be no centrifugal forces on any permanent magnets in the stator actuator part 21.

In a further alternative embodiment, magnets for inducing the magnetic flux in the magnetic circuit 30 can also be provided both in the rotor actuator part 11 and the stator actuator part 21. Furthermore, even if permanent magnets are to prefer in most applications, also electromagnets can be utilized.

Fig. 6 illustrates yet another principle of producing a variation of magnetic properties in the radial direction 3. Here the first surface 15, 25 of the rotor actuator part 11 and the stator actuator part 21, respectively, are flat. However, the composition of the material varies in the radial direction so as to give rise to a variable magnetic reluctance. This is indicated be the varying hatching in the surface region. This embodiment has the advantage that the surfaces rotating close to each other are smooth which means that there is a reduced risk for damages if the bearing action in the axial direction fails temporarily.

So far, the illustrated embodiments have shown gaps in a single plane in the radial direction. However, as mentioned further above, the pure magnetic forces between the rotor actuator part 11 and the stator actuator part 21 tends to attract to each other. A strong force in the axial direction thus appears. This can be used as an advantage in applications where the rotor is relatively heavy and the axis of rotation is vertical. The magnetic forces in the axial direction may then assist in lifting the rotor. A change in distance between the rotor actuator part and the stator actuator part also influences the magnitude of the magnetic flux passing the gap, which means that also such changes are damped by the through the electrically conducting loops. However, since these forces are not restoring forces, just having a damping effect, an axial bearing has to be provided for anyway.

Even more, for many applications, e.g. where the rotor is very light or where the axis is horizontal, the axially directed magnetic forces are a disadvantage.

Fig. 7 illustrates another embodiment of an electrodynamic actuator 1, which presents a reduced contribution of the axial magnetic forces when the rotor actuator part 11 and the stator actuator part 21 are perfectly centred with respect to each other. Still, an axial bearing is needed, which provides restoring forces when the axial position becomes offset, however, the magnetic bias force from the electrodynamic actuator 1 is significantly reduced. In this embodiment, the stator actuator part 21 comprises two portions, 47, 48, situated at opposite sides, in axial direction 2, of the rotor actuator part 11. In this embodiment, the two portions 47, 48 are magnetically coupled to each other through a stator actuator bridge 49. Two magnetic gaps 33, 35 are present at a respective side of the rotor actuator part 11, where both gaps 33, 35 are parts of the magnetic circuit 30. The magnetic forces try to close the gaps and operate thereby against each other, thus reducing the total influence by the magnetic axial forces.

The portion 48 is situated with a narrow gap 35 to the rotor actuator part 11. This can be further utilized in applications operating with a pressure difference between different ends of the rotor axis. Examples of such applications are e.g. vacuum pumps. In the present embodiment, a surface of portion 48 facing the rotor actuator part is provided with a grooved structure 71. The grooves have a spiral form and influences gas present in the gap 35 to move towards the axis 4 when the plane rotor actuator part 11 rotates fast relative the stator actuator part 21. A pumping action is thus achieved according to pumping principles known as such in prior art. However, this type of pump is advantageously combined with the narrow plane gaps creating beneficial synergetic functions. In order not to significantly influence the magnetic properties of the actuator as a whole, the groove structure 71 could be made in a non-magnetic material, e.g. a polymer. In alternative embodiments, the grooved structure 71 could instead be provided at the rotor actuator part 11 interacting with a plane stator actuator surface instead.

In Fig. 8A, these ideas are further developed. In this embodiment, both the portions, 47, 48 are provided with variable reluctance in the radial direction facing matching structures at the rotor. In other words, the rotor magnetic circuit member 12 has a second side 19, directed opposite, in an axial direction, to said first side 15. A second side 46 of the stator magnetic circuit member 22 faces the second side 19 of the rotor magnetic circuit member 12, separated by the gap 35. The second side 46 of the stator magnetic circuit member 22 thereby magnetically interacts with the second side 19 of the rotor magnetic circuit member 12 over the gap 35. The second side 46 of the stator magnetic circuit member 22 is in magnetic contact with the first side 15 of the stator magnetic circuit member 22. Moreover, the second side 46 of the stator magnetic circuit member exhibits at least one of a variable 5 reluctance and a variable magnetization in a radial direction 3 with reference to the axis 4 and the second side 19 of the rotor magnetic circuit stator member 12 exhibits at least one of a variable reluctance and a variable magnetization in a radial direction 3 with reference to said axis 4.

Electrically conducting loops 40 are provided also at the second side 46 of the stator magnetic circuito member 22, in order to pick up changes in the magnetic flux 32 also at this part of the magnetic circuit 30. Furthermore, an electrically conducting loop 41 is also provided around the rotor magnetic circuit member 12, in a radial direction 3. This electrically conducting loop 41 is positioned inside the stator actuator bridge 49 and therefore picks up all flux changes in the gaps 33, 35. 5 In this way, electrically conducting loops 40, 41 are provided for efficiently detecting and counteracting any relative movements between the rotor actuator part 11 and the stator actuator part 21 in the radial direction 3. However, relative movements between the rotor actuator part 11 and the stator actuator part 21 in the axial direction 2 will essentially not change the magnetic flux, since a decrease in gap distance on one side of the rotor actuator part 11 is compensated by a corresponding increase in gap o distance on the other side. This is the result of that it is the same magnetic flux passing both gaps 33,

35. Similarly, also the magnetic forces in the axial direction are mutually compensating. A motion in the axial direction will indeed decrease one of the gaps 33, 35, but will instead increase the other gap with the same amount. The resulting magnetic forces in the axial direction are therefore essentially compensated. 5

Also in this embodiment, grooved structures 71 can be provided to achieve a pumping action. To this end, the volumes between the protrusions at the rotor actuator part 11 are filled with non-magnetic material and the groove structure 71 is provided on top of these, utilizing the stator actuator part 21 as the opposite plane pumping structure. Note that the grooves at the gaps 33 and 35, respectively, are o spirally formed in opposite directions to achieve a combined pumping effect. Also here an alternative by changing the position of the groove structures 71 and the plane surfaces between rotor and stator can be useful.

Fig. 8B illustrates a cross-sectional view of the same embodiment as in Fig. 8A, but perpendicular to5 the rotational axis, along the line C-C. Here it can be noted that also the electrically conducting loops

40, 41 encircling the portion 28 of the stator magnetic circuit member 22 also encircles the rotational axis. The electrically conducting loops 40, 41 are therefore in the present embodiment preferably realized as solid copper rings soldered into the recesses of the stator actuator part 21.

An advantage with the design of Fig. 8B is that losses are minimized due to the rotationally symmetric magnetic fields. Another advantage is that the configuration is relatively easy to manufacture.

With reference again to Fig. 8A, it is worth noting that since the geometrical structures of the rotor actuator part 11 and the stator actuator part 21 in the present embodiment are in registry with each other, a maximum magnetic flux 32 through the magnetic circuit 30 is achieved when the rotor actuator part 11 and the stator actuator part 21 are perfectly aligned in the radial direction. Any displacement from this condition will reduce the magnetic flux 32. With such a design, all relative radial movements of the rotor actuator part 11 and the stator actuator part 21 will result in either increased or constant reluctance in the gaps in all parts of the magnetic circuit or decreased or constant magnetic reluctance in the gaps in all parts of the magnetic circuit. In other words, there is no possible radial motion with the configuration of the present embodiment that causes the magnetic reluctance to increase in one part of the magnetic circuit and to decrease in other parts. This is the reason why the electrically conducting loops 40, 41 are allowed to encircle the rotational axis 4.

The magnetically restoring forces in the radial direction are quite strong in the above embodiment. In yet another embodiment, illustrated in Fig. 9A, the magnetically restoring forces in the radial direction are reduced at the same time as the damping function is essentially the same or better. This is caused by deliberately introduce a mismatch between the geometrical structures of the rotor actuator part 11 and the stator actuator part 21. Note that Fig. 9A is drawn in the intended alignment position between the rotor actuator part 11 and the stator actuator part 21. When the rotor 10 and stator 20 are rotating in an aligned relation, the protrusions 26 of the stator actuator part 21 are situated at a smaller radius than the protrusions 16 of the rotor actuator part 11. The magnetic forces at one point along the actuator 1 will try to displace the stator radially outwards. However, such a movement will increase the mismatch at the opposite side of the actuator 1. The magnetic forces thereby strive in opposite directions and will consequently compensate each other, at least to a certain degree.

At the same time, a damping function is maintained. If a motion in one direction is created, a change in the magnetic flux will appear at both sides of the rotor. At one side, the magnetic flux will increase, since the reluctance is reduced when the protrusions 16 and 26 comes closer to a matching position. At the opposite side, however, the magnetic flux will decrease, since the reluctance is increase when the protrusions 16 and 26 are moved further apart. However, if each electrically conducting loop 40, 41 only cover a part of the stator actuator part 21 along a tangential direction, both these changes can be detected and used for creating damping eddy currents. In Fig. 9B, a cross-sectional view D-D perpendicular to the axis 4 of one embodiment is illustrated. The electrically conducting loops 40 are here provided by a solid copper plate provided with holes for the portions 28. In other words, the electrically conducting loops 40 comprise rigid non-magnetic electrically conducting metal pieces. In this embodiment, four cross-connections 55 are provided in a radial direction, allowing four essentially independent eddy currents to run through the copper plate. The currents in two opposite sides of the copper plate will typically run in the opposite directions compared to each other. But the currents in the different parts will anyway both strive to counteract any motion at all.

The interruption of the protrusions 26 has the advantage that it prohibits any magnetic field to take a path along the protrusions 26 instead of passing the gaps, if there are any differences between different sides of the stator actuator part 21. The damping thereby becomes more efficient. However, the interruption of the rotationally symmetric magnetic fields causes some losses. This can be mitigated be providing a narrow pole piece at the surface of the stator actuator part 21 on top of the electrically conducting loops 40, 41. A further alternative is to provide cross-connections by instead providing holes through the protrusions 26. The size of such holes is determined as a compromise between the wish to have the rotationally symmetric magnetic properties and the wish to prohibit magnetic fields to propagate along the protrusions and to have a sufficient cross-connection area for the electrically conducting loops 40.

Alternatively, the cross-connections could be provided through the magnetic material itself, i.e. through the protrusions. If the width of the protrusions 26 is small enough, the electrical resistance in the radial direction is sufficiently small for enabling an electrically conducting loop 40 to comprise a part of the protrusions 26. In other words, the electrically conducting loop 40 comprises to a part a part of the stator magnetic circuit member.

Based on Fig. 9A, in a further alternative embodiment, wires are provided as electrically conducting loops. Since the design itself results in differently directed currents in opposite parts of the stator actuator part, the wires are connected with the electrically conducting loops at the opposite side, however, in an opposite direction. In other words, the wires are provided in windings wound clockwise around one sector of the stator actuator part and anti-clockwise around an opposite sector, to enhance the radial damping effects, in particular the ground tone of any oscillations.

Preferably, an electrically conducting loop 41 encircling all the gaps between the rotor actuator part 11 and the stator actuator part 21 is also provided. Such an electrically conducting loop 41 will have a limited effect at the ground tone of any oscillations, however, it will instead efficiently damp out a first overtone of any oscillations.

The damping effect is dependent on several parameters, e.g. the width of the gap and the rate of the 5 changing magnetic properties in the radial directions. In cases where geometrical shapes are used, the width of the protrusions is of high importance. In general, the change in magnetic flux over the gap is larger for designs having many narrow protrusions as compared to designs having fewer broad ones. Furthermore, the effect will also increase with decreasing gap distance. Small geometrical features in the actuator will be more sensitive to relative displacements. In such a way, the actuator can be o designed to give almost any requested damping power.

Fig. 10 illustrates another embodiment of an electrodynamic actuator 1, which presents another solution of the two-side concept. Here, the stator actuator part 21 has portions 47, 48 on opposite sides of the rotor actuator part 11. However, instead of having a stator actuator bridge, the magnetic5 circuit is closed by two magnetic paths through the rotor actuator part 11. Such an embodiment can be made slimmer in the radial direction and still maintaining the same total number of gaps. Magnets can be provided at one portion of the stator actuator part 21 or both, and/or in the rotor actuator part 11.

One way to control the damping of the actuator is to provide a certain design concerning width of o protrusions, gaps etc. However, in some applications, the damping effect may for instance not be enough or is requested to vary with time. The damping functionality depends on the generation of eddy currents in the electrically conducting loops. Since the electrically conducting loops are situated at the stator side, the eddy current is a quantity that is relatively easy to measure. By connecting one or several of the electrically conducting loops 40 to a control unit 90, as illustrated in Fig. 12, the eddy 5 currents can be monitored as a function of time. Fig. 11 is a diagram illustrating the time evolution of such an eddy current 101. Such monitoring also opens up for controlling the damping force actively.

If the damping force supplied spontaneously by the eddy currents of an actuator system is considered to be too small, the time evolution of the eddy currents anyway gives information about when and how o current through the electrically conducting loops give a damping effect. Such timing information may be difficult to achieve in other ways. By not only monitoring the eddy currents, but also controlling them thus gives a possibility to control the damping efficiency and/or the stiffness of the bearing. The control unit 90 (Fig. 12) is thereby arranged for controlling currents through the electrically conducting loops. If the control unit adds a current through the electrically conducting loops so that a total current 102 5 having the same phase as the pure eddy current 101 but a higher amplitude is supplied to the electrically conducting loops, the clamping effect will be increased. Shifting the phase of the added current, as in curve 104, may also play an important role, especially if the coil inductance is high.

Similarly, if the damping generated by the pure eddy currents is too large, the damping may be mitigated by adding a current in the opposite direction to the eddy current. A total current may then look like curve 103. In an extreme case, the damping may even be controlled to be negative, i.e. a total current flowing through the electrically conducting loops flows in opposite direction compared to any uncontrolled eddy currents. A vibration in a rotating machine can in such a case be intentionally increased.

As anyone skilled in the art realises, the ways of controlling the damping efficiency can be varied in many respects. Since an electrical connection to the electrically conducting loops already exists, it is preferable to use the electrically conducting loops themselves also for the controlled currents. However, it is also possible to provide additional conducting loops, preferably in parallel to the original electrically conducting loops, in which a correction current is conducted. The total effect on the rotating machine will be essentially the same, however, if the controlled current is opposite to the eddy current, the total heat generated in the loops will increase.

If the electronically conducting loops 41 are connected in different direction at opposite sides of the stator, as discussed earlier in connection with Fig. 9A, a proper control of the current can give rise to a resulting force in the radial direction. This possibility is most prominent if non-aligned stator and rotor geometries are utilized.

Fig. 13 illustrates a flow diagram of steps of an embodiment of a method according to the present invention. The method for operating an electrodynamic actuator according to the principles presented in the present disclosure starts in step 200. In step 210 the stator actuator part and the rotor actuator part are rotated relative to each other around an axis. In step 220, vibrations are damped by inducing eddy currents in the electrically conducting loops. This takes place spontaneously as a result of changing the magnitude of a discrepancy from alignment between the stator actuator part and the rotor actuator part. In step 230, currents through the electrically conducting loops are controlled for controlling at least one of damping efficiency and stiffness. The method is ended in step 299.

The steps of Fig. 13 should not be interpreted as a strict flow diagram, but merely an indication of the existence of the different steps. The steps 210-230 should therefore be considered as being possible to perform continuously and/or simultaneously. The eddy currents give information about the relative motion in a radial direction of the rotor and stator. By integrating this signal, utilizing an equilibrium position as a fix position, information about the absolute position is possible to achieve. It is thereby also possible to control the sideward position. The same possibilities are provided by a position sensor.

Fig. 14 illustrates another embodiment of an electrodynamic actuator 1. In this case, the rotor actuator part 11 comprises two portions 61, 62, magnetically connected through the rotor 10. The stator actuator part 21 has four portions 63, 64, 65, 66, facing each rotor actuator part portion 61 , 62 in pairs. The two pairs are magnetically connected by a permanent magnet 34. One electrically conducting loop 70 per rotor actuator part portion 61, 62 is provided, encircling the respective portion 61, 62 in radial direction. The magnetic circuit thus involves both pairs of stator actuator part portions 63, 64, 65, 66, both rotor actuator part portion 61, 62, a part of the rotor 10 and the permanent magnet. The magnetic circuit also has two branches at each rotor actuator part portion 61, 62, one from above (as illustrated in the figure) and one from below. These two paths are thus entering into the region inside the electrically conducting loop 70 in opposite directions. The flow also enters one of the rotor actuator part portions 61 and leaves the other one 62.

The electrically conducting loop 70 does, however, not operate as a damping means, since the net change in magnetic field flow through the electrically conducting loop 70 is zero, regardless of the radial motion.

The electrically conducting loops 70 are controllable according to similar ideas as presented here above. By changing the current through the electrically conducting loops 70, the magnetic flow to/from the rotor actuator part portion 61, 62 can be differentiated between the respective upper and lower stator actuator part portions 63, 64, 65, 66. This means that it is possible to deliberately e.g. increase the flow to the lower stator actuator part portions 64, 66 and decrease the flow to the upper stator actuator part portions 63, 65, or the opposite. A net force will in such cases be directed in the axial direction, i.e. a controllable axial force is provided at the same time as a damping in the radial direction is achieved. This is just one example of how the basic concept of the present invention can be combined into new configurations, having interesting features.

Electrically conducting loops 40 are also provided around parts of each stator part portion 63-66.

These electrically conducting loop 40 provide a damping effect. To avoid damping also the axial control as provided by the electrically conducting loop 70, the electrically conducting loop 40 are preferably wound in opposite directions at opposite parts of the stator actuator part, as discussed in the alternative embodiment discussed in connection with Fig. 9A. Furthermore, electrically conducting loop 40 of the upper and lower stator part portions, respectively, should be provided by opposite polarity.

Another possibility to increase the utilization of the present invention is to combine the basic ideas with 5 different prior art concepts. Since the magnetic flow in the rotor actuator part 11 already is differentiated in radial direction, this differentiation can e.g. be utilized for providing also radially restoring forces. Fig. 15 illustrates such an embodiment. The rotor actuator part 11 there further comprises at least one electrically conducting rotor loop 69 encircling a respective portion 68 of the rotor magnetic circuit member 12. The portion 68 of the rotor magnetic circuit member 12 compriseso magnetic material and is arranged for conducting a magnetic flux having a non-zero component through the electrically conducting rotor loop 69. The portion 68 of the rotor magnetic circuit member 12 is arranged for causing the non-zero component to change when the rotor actuator part 11 and the stator actuator part are displaced from an alignment relationship in a radial direction with respect to each other. The stator magnetic circuit member 22 is essentially rotationally symmetric with respect to5 the axis 4.

In this configuration, a displacement in radial direction causes the electrically conducting rotor loop 69 on the rotor actuator part 11 to create a restoring force. A motion in the radial direction, i.e. a non-zero time derivate of a displacement, instead causes the electrically conducting loop 40 on the stator o actuator part 21 to create a damping force.

Fig. 16 illustrates an embodiment of an electrodynamic actuator 1 according to the present invention having the rotor shaft 76 as part of the magnetic circuit 30. The magnetic flux passes from the stator actuator part 21 over to the rotor actuator part 11 over the gaps 33 and 35. The flux 32 then is 5 conducted out to the rotor shaft 76, where magnetic conductors 77 are separating the flux 32 into two parts. Each part flux passes an additional gap 78 over to a magnetic conductor 72 at the stator actuator part 21. An electrically conducting loop 70 provided around essentially the entire magnetic circuit 30 can be utilized also here for regulating an axial position. By utilizing the rotor shaft 76 for conducting magnetic flux in two different directions, the arrangement can be made more compact in 0 the axial dimension.

Fig. 17 illustrates another embodiment of an electrodynamic actuator according to the present invention having a three-part rotor actuator part 11. The upper and lower ones are operating as in Fig. 14. The middle part 73 of the rotor actuator part 11 instead operates as a pure damper, e.g. similar to the one shown in Fig. 8A. The arrangement becomes extended in the axial direction. However, this can also be utilized since this also to a part prohibits gas or other fluids to pass through the arrangement. A so called labyrinth sealing is thereby formed. This labyrinth structure can also be enlarged by providing additional shielding rings 74 in available volumes in order to further increase the flow path distance through the arrangement. The shielding rings 74 protrude from the first side and/or the second side 47 of the stator magnetic circuit member 22. The rings 74 are preferably made of the same material as the conducting rings 40. The rings 74 can also be surface treated with a low friction coating in order to be able to act as radial touch down bearings, thus eliminating the need for additional external bearings.

Fig. 18 illustrates another embodiment of an electrodynamic actuator according to the present invention, here with repulsive permanent magnets 75. The magnetic flux 32 of the main magnetic circuit 30 is caused by ring magnets 31 at the rotor 10, fastened e.g. by a bandage 79. By adding magnets 75 also at the stator actuator part, directed in parallel direction to the magnets 31 at the rotor

10, a repulsive force is created, acting as a conventional magnetic bearing. The introduction of the additional magnets 75 will not significantly change the damping properties of the rest of the arrangement.

In the embodiments above, the rotor magnetic circuit member and the stator magnetic circuit member are magnetically interacting over a gap that extends in the axial direction 2. Fig. 19 illustrates another embodiment of an electrodynamic actuator according to the present invention with air gaps 33, 35 instead directed in radial direction 3. A motion in radial direction will cause a change in magnetic flux through the electrically conducting loops 40 and a damping will be performed. Typically, the damping can be made very efficiently. However, such solutions instead tend to be somewhat more instable.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. In particular most illustrated embodiments having a configuration with geometrical structures of the rotor actuator part and the stator actuator part aligned are typically easily modified to provide non-aligned configurations. Surfaces providing pumping actions or labyrinth sealings can also be combined with most embodiments. The scope of the present invention is, however, defined by the appended claims.

Claims

1. Electrodynamic actuator (1), comprising: a stator actuator part (21) having a stator magnetic circuit member (22), comprising magnetic material (23); and

5 a rotor actuator part (11) having a rotor magnetic circuit member (12), comprising magnetic material (13); said stator actuator part (21) and said rotor actuator part (11) having an axis (4) of intended rotation relative each other; at least one magnet (31) inducing a magnetic flux (32) through a magnetic circuit (30) o comprising said stator magnetic circuit member (22) and said rotor magnetic circuit member (12); said rotor magnetic circuit member (12) being essentially rotationally symmetric with respect to said axis (4); a first side (25) of said stator magnetic circuit member (22) exhibiting at least one of a variable reluctance and a variable magnetization in a radial direction (3) with reference to said axis (4);5 a first side (15) of said rotor magnetic circuit member (12), magnetically interacting with said first side (25) of said stator magnetic circuit member (22), exhibiting at least one of a variable reluctance and a variable magnetization in said radial direction (3); said stator actuator part (21) further comprises at least one electrically conducting loop (40) encircling a respective portion (28) of said stator magnetic circuit member (22); o said portion (28) of said stator magnetic circuit member (22) comprises magnetic material

(23) and is arranged for conducting a magnetic flux (32) having a non-zero component through said at least one electrically conducting loop (40); said portion (28) of said stator magnetic circuit member (22) being arranged for causing said non-zero component to change when said stator actuator part (21) and said rotor actuator part (11) are 5 moved relative each other in said radial direction (3).

2. Electrodynamic actuator according to claim 1, characterised in that said first side (25) of said stator magnetic circuit member (22) is provided with protrusions (26) and recesses (27) extending in an axial direction (2), causing at least a part of said variable reluctance.

3. Electrodynamic actuator according to claim 1 or 2, characterised in that said first side (15)0 of said rotor magnetic circuit member (12) is provided with protrusions (16) and recesses (17) extending in said axial direction (2), causing at least a part of said variable reluctance.

4. Electrodynamic actuator according to any of the claims 1 to 3, characterised in that said first side (15) of said rotor magnetic circuit member (12) and said first side (25) of said stator magnetic circuit member (22) are magnetically interacting over a gap in the axial direction.

5. Electrodynamic actuator according to any of the claims 1 to 3, characterised in that said first side (15) of said rotor magnetic circuit member (12) and said first side (25) of said stator magnetic circuit member (22) are magnetically interacting over a gap in the radial direction.

6. Electrodynamic actuator according to claim 5, characterised by shielding rings (74) protruding from said first side (25) of said stator magnetic circuit member (22), said shielding rings (74) being surface treated with a low friction coating in order to be able to act as radial touch down bearings.

7. Electrodynamic actuator according to any of the claims 1 to 6, characterised in that said first side (25) of said stator magnetic circuit member (22) exhibits a variable magnetization in a radial direction (3).

8. Electrodynamic actuator according to any of the claims 1 to 7, characterised in that said first side (15) of said rotor magnetic circuit member (12) exhibits a variable magnetization in a radial direction (3).

9. Electrodynamic actuator according to any of the claims 3 to 8, in turn dependent on claim 2, characterised in that said at least one electrically conducting loop (40) is provided in said recesses

(27).

10. Electrodynamic actuator according to any of the claims 1 to 9, characterised in that said at least one electrically conducting loop (40) to a part comprises a part of said stator magnetic circuit member (22).

11. Electrodynamic actuator according to any of the claims 1 to 10, characterised in that said rotor magnetic circuit member (12) has a second side (19), directed opposite to said first side (15); and a second side (46) of said stator magnetic circuit member (22), magnetically interacting with said second side (19) of said rotor magnetic circuit member (12).

12. Electrodynamic actuator according to claim 11, characterised in that said second side (46) of said stator magnetic circuit member (22) is in magnetic contact with said first side (19) of said stator magnetic circuit member (22).

13. Electrodynamic actuator according to claim 11 or 12, characterised in that said second side (46) of said stator magnetic circuit member (22) exhibits at least one of a variable reluctance and a variable magnetization in said radial direction (3) and said second side (19) of said rotor magnetic circuit stator member (15) exhibits at least one of a variable reluctance and a variable magnetization in said radial direction (3).

14. Electrodynamic actuator according to any of the claims 1 to 13, characterised in that said rotor actuator part (11) further comprises at least one electrically conducting rotor loop (69) encircling a respective portion (68) of said rotor magnetic circuit member (12); said portion (68) of said rotor magnetic circuit member (12) comprises magnetic material (13) 5 and is arranged for conducting a magnetic flux (32) having a non-zero component through said at least one electrically conducting rotor loop (69); said portion (68) of said rotor magnetic circuit member (12) being arranged for causing said non-zero component to change when said stator actuator part (21) and said rotor actuator part (11) are displaced relative each other in said radial direction (3) compared to an aligned position; o said stator magnetic circuit member (22) being essentially rotationally symmetric with respect to said axis (4).

15. Electrodynamic actuator according to any of the claims 1 to 14, characterised by an electric control unit (90) connected to said electrically conducting loop (40) and arranged for controlling currents through said electrically conducting loops (40). 5

16. Rotating machine (9), comprising: a stator (20); a rotor (10); and at least one electrodynamic actuator (1) according to any of the claims 1 to 15; wherein said rotor actuator part (11) being attached to said rotor (10) and said stator actuator o part (21) being attached to said stator (20).

17. Method for operating an electrodynamic actuator (1) according to any of the claims 1 to 14, said method comprising the steps of: rotating (210) said stator actuator part (21) and said rotor actuator part (11) relative to each other around said axis; and 5 damping (220) vibrations between said stator actuator part (21) and said rotor actuator part

(11) by use of eddy currents in electrically conducting loops (40) caused by any relative movement between said stator actuator part (21) and said rotor actuator part (11).

18. Method according to claim 17, characterised by the further step of controlling (230) currents through said electrically conducting loop for controlling at least one of damping efficiency and stiffness.0