WO1998035167A1

WO1998035167A1 - Inexpensive rotary magnetic bearing with active centring along the rotation axis

Info

Publication number: WO1998035167A1
Application number: PCT/FR1998/000225
Authority: WO
Inventors: Charles Lambert; Christophe Bernus
Original assignee: Aerospatiale Societe Nationale Industrielle
Priority date: 1997-02-10
Filing date: 1998-02-06
Publication date: 1998-08-13
Also published as: JP2001511236A; CA2277780A1; EP0958459A1; FR2759434B1; FR2759434A1

Abstract

The invention concerns a 1-active-axis rotary magnetic bearing comprising: a first portion including, around a rotation axis, two ring-shaped radially internal (23) and radially external (24) polar parts and a ring-shaped magnet with permanent magnetisation (25) set between a first linking polar portion integrated with the radially external polar part and a second linking polar portion integrated with the radially internal polar part; a second portion comprising two closing ring-shaped polar parts (26, 27) axially arranged on either side of the first portion, and comprising ring-shaped sections (26A, 27A) axially arranged opposite the ring-shaped polar parts through ring-shaped air gaps. The ring-shaped magnet (25) has axial magnetisation, and the first and second linking polar portions (23A, 24A) between which the permanent magnet is placed are arranged transversal to the axis.

Description

Low-cost rotating magnetic bearing with active centering along the axis of rotation.

The present invention relates to a rotary magnetic bearing for magnetically active centering, along a centering axis, of a body movable in rotation relative to another body around this axis.

Magnetic bearings or magnetic suspension devices have already been proposed which are suitable for integration into systems comprising parts in relative motion, between which an absence of contact is required.

Relative movements can be translations; however, they are most often rotations. In such rotating magnetic bearings, it is necessary to ensure magnetic centering along the axis of rotation but also along two radial axes.

It has been known for a long time that it is impossible to obtain a stable magnetic suspension in three directions by the simple use of static fields such as fields generated by magnets: it is therefore necessary to associate with static magnetic fields (permanent magnet) fields variable magnetic (currents passing through windings) to allow the suspension of a body.

There are two main categories of rotating magnetic bearings: first of all there are rotating magnetic bearings in which the axial centering along the axis of rotation is active, i.e. controlled by variable fields, while centering along two radial axes is passively ensured, thanks to static fields generated by magnets, such bearings are sometimes referred to as magnetic 1-axis active bearings. There are also rotary magnetic bearings in which axial centering is done passively while centering along two transverse axes is done so. active, such bearings are sometimes referred to as active 2-axis magnetic bearings

An example of a magnetic bearing with two active centering axes is given in document EP-0,284,487

A magnetic bearing with 1 active axis has the particular advantage of providing a single control path, along this active axis, namely the axis of rotation. Such a bearing therefore requires simpler servo electronics than a magnetic bearing with two active axes for which a servo in two directions is required. An active 1-axis magnetic bearing however has the drawback of requiring the use of an annular magnet with radial magnetization. Such a magnet with radial magnetization is in practice made up of several sectors produced individually and then bonded, the assembly being finally remanufactured. It is understood that the cost of a radially magnetized magnet is much higher than the cost of an annular magnet of the same dimensions but axially magnetized. This cost difference can go up to a ratio of 7/1.

However, it is this configuration of rotary magnetic bearing with 1 active axis which seems best suited to the case where it is desired to produce small magnetic bearings.

By way of example, document FR-2,732,734 describes in particular a magnetic centering device, along an axis of rotation ZZ, of a second body movable relative to a first body, comprising - a rotating magnetic bearing magnetically active according to the axis of rotation, and - two annular magnetic centralizers offset axially, along this axis of rotation, on either side of the magnetic bearing.

This magnetic centering device, as shown diagrammatically in FIG. 1, comprises a first portion 1 and a second portion 2 which are mechanically independent and intended to be respectively secured to two bodies in relative movement A and B; They are adapted to have relative to each other a relative movement of rotation with respect to an axis of rotation Z-Z.

The first portion comprises, all around the axis of rotation, a radially external pole piece 3, on the one hand, and a radially internal pole piece 4, on the other hand, substantially magnetically insulated from the part radially outer pole, as well as an annular magnet 5 with permanent magnetization interposed between the radially outer pole piece and the radially inner pole piece. The second portion comprises two closing pole pieces 6 and 7 offset axially from one another and arranged axially on either side of the pole pieces of the first portion, each closing pole piece comprising a radially outer annular section 6A or 7A disposed axially opposite an annular surface of the radially external pole piece through a radially external annular gap and a radially internal annular section 6B or 7B disposed axially opposite an annular surface of the pole piece internal to through a radially internal air gap.

The annular magnet 5 is radially magnetized, and the two radially outer and inner pole pieces are cylindrical. The edges of these cylindrical pole pieces are axially facing the edges of the closure pole pieces which have a C shape.

On both sides axially of the annular magnet are arranged two coils 8 and 9 centered on the axis of rotation ZZ. They are axially projecting from the radially outer pole pieces and internal, until penetrating inside the closing pole pieces. These windings must therefore be capable of affecting their shape by themselves.

A position sensor 10 is provided for detecting the axial position of one of the portions 1 and 2 relative to the other, and servo electronics, not shown, determines the control currents to be applied to the aforementioned windings. To ensure good stability of the magnetic centering device even when the aforementioned elements are small, the aforementioned magnetic centralizers 11 and 12 are arranged radially outside the closure parts, and axially offset with respect to the other. These centralizers are in practice made up of crowns with permanent axial magnetization 11A, 11B, 12A and 12B arranged radially one inside the other.

Such an arrangement which is entirely satisfactory, however, presents certain implantation constraints since the centralizers must in practice have a diameter greater than that of the closure parts, and therefore a diameter greater than the maximum overall diameter of the magnetic bearing taken as a whole.

The subject of the invention is a axially active rotary magnetic bearing along the axis of rotation, but the cost of which is much lower than that of a conventional active 1-axis rotary magnetic bearing. In the alternative, the subject of the invention is a rotating magnetic bearing with an active 1-axis, the configuration of which makes it possible to combine a low cost and a small footprint, at least in the radial direction but preferably also in the axial direction, with a great flexibility of implantation, that is to say a certain flexibility in the choice of the overall shape of the bearing.

The basic idea of the invention is to obtain active centering along the axis of rotation using a permanent magnet magnetized parallel to this axis of rotation.

As is apparent from the above, this results in a very significant reduction in cost. The invention thus proposes a rotary magnetic bearing with an active axis comprising first and second independent portions adapted to have, relative to one another, a relative movement of rotation relative to an axis of rotation, the first portion comprising, around the axis of rotation, a radially outer annular pole piece, a radially inner annular pole piece substantially magnetically insulated from the radially outer annular pole piece, an annular magnet with permanent magnetization interposed between a first portion connecting pole secured to the radially outer annular pole piece and a second connecting polar portion secured to the radially inner annular pole piece, the second portion comprising two annular closing pole pieces axially offset from one another and arranged axially from on either side of the first portion, each closing pole piece included nt a radially external annular wafer disposed axially opposite an annular surface of the radially external annular pole piece through a radially external annular air gap and a radially internal annular wafer disposed axially opposite an annular surface of the pole piece radially internal annular through a radially internal air gap, this bearing being characterized in that the annular magnet is axially magnetized, and the first and second pole connecting portions between which the permanent magnet is interposed are arranged transversely to the '' axis and axially offset from each other

According to preferred arrangements of the invention, possibly combined one of the annular surfaces of the radially external annular pole piece which delimits the radially external air gaps is formed on said first connection portion,

the radially external annular pole piece generally has a C section, the concavity of which faces the axis of rotation, and has a cylindrical wall connected to two transverse flanges, one of which comprises said first pole portion,

the winding is arranged radially inside the radially external annular pole piece, the winding extends radially at least from the cylindrical wall to the second polar connection portion,

- the winding is formed by several elementary windings,

- the winding extends radially up to near at least one radially internal air gap, - the radially internal pole piece has a generally T-shaped section and comprises a cylindrical wall connected in an intermediate portion to a transverse annular flange which comprises said second polar connecting portion,

- The magnet is part of two annular magnets with axial magnetizations of opposite directions, arranged on either side of this transverse annular flange, the radially external annular pole piece has generally a C section whose concavity is turned towards the axis and has a cylindrical wall connected to two transverse flanges, each magnet extending axially to one of these transverse flanges, - the second connecting portion is a transverse flange with respect to which the bearing is substantially axially symmetrical.

Objects, characteristics and advantages of the invention appear from the following description, given by way of nonlimiting example, with reference to the appended drawings in which: FIG. 1 is a schematic view in axial section of a known magnetic centering device comprising a rotary magnetic bearing with 1 active axis of a known type,

FIG. 2 is a schematic view in axial section of a rotary magnetic bearing with an active 1-axis according to the invention,

FIG. 3 is a view in accordance with that of FIG. 2, on which the lines of static flux generated by the permanent magnet appearing in this bearing appear,

FIG. 4 is a view similar to that of FIG. 2, on which appear not only static flux lines generated by the magnet, but also variable flux lines generated by the winding,

- Figure 5 is a right half-view in axial section showing, in a bearing similar to that of Figure 2, but with a very small air gap, the magnetic flux lines generated by the permanent magnet, as defined by simulation by finite element,

- Figure 6 is a view similar to that of Figure 5, on which the global flux lines appear, resulting from the simultaneous action of the permanent magnet and the circulation of a current in the winding, as defined by simulation by finite element, FIG. 7 is a view in axial section of another rotary magnetic bearing according to the invention, and

- Figure 8 is an axial sectional view of yet another embodiment of a rotary magnetic bearing according to the invention.

The geometry of a first rotary magnetic bearing 20 with 1 active axis according to the invention is shown in Figures 2 to 4

This bearing conventionally comprises first and second mechanically independent portions 21 and 22 intended to be respectively secured to two bodies in relative movement A and B, for example a rotor B and a stator A These two parts 21 and 22 are adapted to have relative to each other a relative movement of rotation with respect to an axis of rotation ZZ

The first portion 21 comprises all around the axis of rotation, a radially external annular pole piece 23, on the one hand, and a radially internal annular pole piece 24, on the other hand This pole piece 24 is substantially magnetically isolated vis- with respect to the pole piece 23 This first portion 21 further comprises an annular magnet 25 with permanent magnetization interposed between a first pole connection portion 23A secured to the radially outer pole piece 23 and a second pole connection portion 24A secured to the radially inner pole piece 24

The second portion 22 comprises two closing pole pieces 26 and 27 offset axially from one another and arranged axially on either side of the pole pieces of the first portion Each closing pole piece 26 or 27 is a crown of which the current section is a C whose concavity is directed parallel to the axis Each closing pole piece 26 or 27 comprises a radially external annular section 26A or 27A disposed axially opposite an annular surface of the radially external pole piece 23, through a radially outer annular air gap and a radially inner annular edge 26B or 27B disposed axially opposite an annular surface of the radially inner pole piece through a radially inner air gap

Between the radially outer and inner pole pieces 23 and 24 are arranged two windings 28 and 29 centered on the axis of rotation Z-Z

A position sensor (not shown) is provided to detect the axial position of one of the portions 21 and 22 relative to the other, and a control electronics (not shown), quite conventional, determines the control currents to be applied. to the above windings

According to the invention, the annular magnet 25 has permanent axial magnetization, and the first and second pole connecting portions 23A and 24A between which this annular magnet is interposed are arranged transversely to the axis and offset axially with respect to each other

To do this, in the example shown, the radially external pole piece is a crown whose section has the shape of a C whose concavity is turned towards the axis, this pole piece 23 comprising a cylindrical wall 23B connected to two annular flanges 23A and 23C extending in the direction of the axis up to the proximity of the radially external annular sections 26A and 27A of the closing pole pieces 26 and 27

It is the radially internal ends of these annular flanges 23A and 23C which constitute the abovementioned annular surfaces which determine the radially external annular gaps in combination with the radially external annular sections 26A or 27A

On the other hand, it is one of these annular flanges, HERE the flange 23A, which constitutes the first pole connecting portion indicated above with respect to the annular magnet 25

The radially internal pole piece 24 is also a crown. Its section is a T, the bar of which is parallel to the axis while the rod is transverse to it. More specifically, this radially internal pole piece 24 comprises a cylindrical skirt 24B extending from one to the other of the two radially internal annular air gaps, and a transverse flange 24A situated in the intermediate position, here substantially at equal distance from the axial sections of the cylindrical skirt 24B.

It is this intermediate transverse flange 24A which constitutes the second pole connecting portion cited in connection with the annular magnet 25. The coils 28 and 29 are arranged in the annular space remaining between the radially inner and outer pole pieces 23 and 24 In the example shown, the coil 28 occupies the bottom of the radially outer pole piece, along the cylindrical skirt 23B, radially up to the outer edge of the transverse flange 24A and up to the outer edge of the magnet annular 25, while the winding 29 occupies the space remaining between the winding 28 and the cylindrical skirt 24B of the radially internal pole piece, on the other side of the magnet 25 relative to the transverse flange 24A

Of course, in a variant not shown, the coils 28 and 29 can be replaced by a single coil

Figures 3 and 4 show the flux lines likely to flow in the pole pieces of the bearing of Figure 2, depending on whether or not there is current flow in the windings 28 and 29

In Figure 3, only the static flux lines of the magnet 25 are shown

We observe that these static flux lines are divided between upper magnetic loops and lower magnetic loops. The upper magnetic loops circulate in the polar portions 23A, 23B, 23C, 26 then 24B, over half of its height and 24A. lower magnetic loops, much shorter, pass through the pole portion 24B in the thickness direction (therefore in the axial direction), the pole piece 27, the cylindrical skirt 24B over half its height and the transverse flange 24A

It can thus be seen that the flux lines of the upper magnetic loops travel a much larger magnetic path than the flux lines of the lower magnetic loops II results in greater leaks (which can range up to 30% of the total flux of the magnet ) than for the conventional solution represented for example in FIG. 1 (leaks of approximately 5% of the total flux of the magnet). However, this phenomenon can be very easily compensated for by increasing the volume of the magnet.

Furthermore, the structural asymmetry of the bearing with respect to a transverse plane results in a difference in axial thickness between the top and bottom air gaps, at zero of the magnetic forces, that is to say in the equilibrium configuration when only the magnet generates flux lines the upper air gaps will be a few microns weaker than the lower air gaps, when the spring is slaved

As is apparent from FIG. 4, the passage of a current through the coils 28 and 29 induces the appearance of large magnetic loops, since the resulting magnetic flux lines run through the cylindrical skirt 24B, the pole piece 26, the portions 23C, 23B and 23A of the radially external pole piece and the bottom closing pole piece 27

Of course the circulation in the windings of a current of opposite direction induces the appearance of lines of magnetic flux circulating in opposite direction

In the example shown in FIG. 4, it is understood that the passage of a current, in the chosen direction, through the windings, results in a decrease in the flow circulating in the upper air gaps and an increase in the magnetic flux passing through the air gaps This results in an axial relative force between the portions 21 and 22 If the part 21 is fixed to a stator A and if the portion 22 is fixed to a rotor movable relative to the stator A, the axial relative force between the elements 21 and 22 corresponds to the application of an upward axial force on the rotor B: the rotor rises

There is therefore indeed a rotating magnetic bearing with 1 active axis. FIGS. 5 and 6 visualize the magnetic flux lines, as they can be determined by simulation by finite elements in the case of a particular embodiment of a magnetic bearing according to the diagram in FIG. 2, in the case where the current flowing in the windings is zero (FIG. 5) and in the case (FIG. 6) where a current is applied, as in the example in FIG. 4 to the own winding to generate flux lines adding to the static flux lines of the magnet in the lower air gaps. The magnetic bearing considers has air gaps whose nominal thickness is 0.3 mm, the maximum diameter, as it can be measured along the outer surface of the cylindrical skirt of the radially outer pole piece, is 74 mm, while the total height, from the lower surface of the lower closing pole piece to the upper surface of the upper closing pole piece is 42 mm This bearing has an axial stiffness of 150 N / mm It can be noted that, in figure 5, there are six lines of flow which circulate in each of the air gaps, whether they are lower or higher On the other hand, in figure 6, there are only three lines of flow which actually cross the upper air gaps, while the flow lines which cross the lower air gaps are seven in number

It can be noted that the geometry of the rotary magnetic bearing of FIG. 2 allows a significant cost reduction compared to the configuration of FIG. 1 since it uses a magnet with permanent axial magnetization instead of a magnet with permanent radial magnetization as as indicated above, the cost reduction for the magnet can reach approximately 1/7. The configuration of the bearing in FIG. 2 also makes it possible, if desired, to use only one winding. control currents can be simplified

It is important to note, with regard to the windings, that these do not need to be self-supporting, that is to say that they do not need to have sufficient intrinsic rigidity to be able to guarantee maintaining their shape Figure 1, corresponding to a conventional magnetic bearing, however involves the use of such self-supporting windings

The winding of the invention can indeed be wound directly on the radially internal pole piece of the bearing on which the magnet will have been bonded beforehand. It is possible to give any desired shape to the winding. It may be noted that this winding is completely protected by the radially outer pole piece of the bearing Finally, it is interesting to note that thanks to its C shape, the radially external pole piece can have a radial bulk greater than that of the closing pole pieces. This has in particular the advantage of making it possible to independently choose the dimensioning of the closing pole pieces, d 'on the one hand, and that of the radially outer pole piece, on the other hand This is how, for example, the bearing in Figure 2 lends itself particularly well to cooperation with magnetic centralizers such as those shown in Figure 1 The result can be very compact Indeed, the configuration of FIG. 2 makes it possible to locate the closing pole pieces inside radially of such magnetic centralizers, while allowing that the radially external pole pieces can extend radially outwards , until occupying at best the volume existing axially between the magnetic centralizers

FIG. 7 shows an alternative embodiment in which the conventional principle of symmetry of the bearing with respect to a transverse plane is respected. This variant is essentially distinguished from the configuration of FIG. 2 by the fact that the single magnet 25 in FIG. 2 is HERE replaced by two magnets 35A and 35B with axial but opposite magnetizations, each disposed between the median transverse flange 34A of the radially inner pole piece 34 and the transverse lateral flanges 33A and 33C of the radially outer pole piece 33 A single coil 38, of generally rectangular section is provided, located radially between the bottom of this radially outer pole piece 33 and the two magnets 35A and 35B

This bearing, designated under the general reference 30, comprises elements whose reference numbers are deduced from those of FIG. 2 by adding the number 10

The structural symmetry of the bearing in FIG. 7 results in a symmetry of the air gaps, at zero of the magnetic forces

Figure 8 gives yet another alternative embodiment of a rotary magnetic bearing according to the invention this bearing 40 differs essentially from that of FIG. 7 by the fact that the overall outside diameter of the radially external pole piece is smaller than for the bearing of FIG. 7 The reference numbers are deduced from those of this FIG. 7 by adding the number 10 Le winding 38 interposed radially between the magnets 45A and 45B on the one hand, and the bottom of the radially outer pole piece 43 is therefore smaller However additional windings 39A and 39B are arranged radially between the magnets 45A and 45B and the cylindrical skirt 44B of the radially internal pole piece The combination of these three windings can turn out to have the same induction capacity as the single winding 38 of FIG. 7, in a space however smaller This example clearly shows the capacity of the configuration of the magnetic bearing rotary of the invention to best adapt to the available volume This configuration is therefore particularly well suited to the Readout of small but compact rotary magnetic bearings It goes without saying that the above description has been offered only by way of non-limiting example and that many variants can be proposed by those skilled in the art without go beyond the scope of the invention

Claims

CLAIMS 1 Rotating magnetic bearing with 1 active axis comprising first and second portions (21, 22, 31 32, 41, 42) independent adapted to have, relative to each other, a relative movement of rotation relative to has an axis of rotation (ZZ), the first portion comprising, around the axis of rotation, a radially external annular pole piece (23, 33, 43), a radially internal annular pole piece (24, 34, 44) substantially magnetically insulated from the radially outer annular pole piece, an annular magnet with permanent magnetization (25, 35A, 35B, 45A, 45B) interposes between a first pole connecting portion integral with the radially outer annular pole piece second pole connecting portion secured to the radially inner annular pole piece, the second portion comprising two annular closing pole pieces (26, 27, 36, 37, 46, 47) offset axially from each other and arranged ax ial on either side of the first portion, each closing pole piece comprising a radially outer annular section

(26A, 27A) disposed axially opposite an annular surface of the radially external annular pole piece through a radially external annular gap and a radially internal annular section disposed axially opposite an annular surface of the annular pole piece radially internal through a radially internal air gap, this bearing being characterized in that the annular magnet (25, 35A, 35B, 45A, 45B) is axially magnetized, and the first and second pole connecting portions (23A, 33A, 43A, 24A, 34A, 44A) between which the permanent magnet is interposed are arranged transversely to the axis and offset axially with respect to each other

2. Magnetic bearing according to claim 1, characterized in that one of the annular surfaces of the radially external annular pole piece which delimits the radially external air gaps is formed on said first connecting portion (23A, 33A, 43A).

3. Magnetic bearing according to claim 1 or 2, characterized in that the radially external annular pole piece (23, 33, 43) generally has a C section whose concavity is turned towards the axis of rotation, and has a wall cylindrical (23B, 33B, 43B) connected to two transverse flanges, one of which comprises said first pole portion.

4. Magnetic bearing according to claim 3, characterized in that the coil (28, 29, 38, 48, 49A, 49B) is arranged radially inside the radially external annular pole piece.

5. Magnetic bearing according to claim 4, characterized in that the winding extends radially at least from the cylindrical wall to the second pole connecting portion.

6. Magnetic bearing according to claim 5, characterized in that the winding is formed of several elementary windings.

7. Magnetic bearing according to any one of claims 1 to 6, characterized in that the winding extends radially until near at least one radially internal air gap.

8. Magnetic bearing according to any one of claims 1 to 7, characterized in that the radially internal pole piece generally has a T-section and has a cylindrical wall (24B, 34B, 44B) connected in an intermediate portion to a flange transverse annular which comprises said second pole connecting portion.

9. Magnetic bearing according to claim 8, characterized in that the magnet is part of two annular magnets with axial magnetizations of opposite directions, arranged on either side of this transverse annular flange, the radially external annular pole piece has generally a C section whose concavity faces the axis and has a cylindrical wall (33B, 43B) connected to two transverse flanges, each magnet extending axially to one of these transverse flanges

10 magnetic bearing according to any one of claims 1 to 9, characterized in that the second connecting portion is a transverse flange with respect to which the bearing is substantially axially symmetrical