HK1212477B - Magnetic centring device, watch movement and watch including a magnetic centering device - Google Patents

Description

Magnetic centering device, timepiece movement and timepiece comprising a magnetic centering device

Technical Field

The present invention relates to a magnetic centering device for small-sized components, in particular for centering a pivotally mounted component. A particular field of application of the invention relates to timepiece mechanisms.

Background

The introduction of active magnetic components, such as permanent magnets and components made of soft ferromagnetic materials, into timepiece mechanisms represents a significant technical challenge due to the extremely reduced dimensions and high spatial precision required to implement chronograph functions in a reliable manner.

Most natural or synthetic magnetic materials are heterogeneous on the millimeter scale or smaller, which makes it difficult to control magnetic field positioning and strength on that scale. In particular, permanent micro-magnets (e.g. SmCo or NdFeB micro-magnets) with the highest magnetic energy intensity are typically made from powders of chemical elements from the rare earth group, the granular structure of which has a size between 1 and 100 microns. The homogeneity of the magnetic field generally decreases as it approaches the grain size.

The use of magnetic assemblies to rotate elements of timepiece movements or mechanisms (as described for example in WO patent No.2012/0662524 and WO patent No. 2012/062523) or other micromechanical mechanisms (such as measuring or control instruments) is advantageous because it makes it possible to generate large positioned forces accompanied by low friction.

Disclosure of Invention

It is an object of the present invention to provide a magnetic centering device for small-sized components on the millimeter scale or smaller, which is accurate and reliable.

It is a particular object of the present invention to provide a magnetic centering device for a pivotally mounted assembly.

A particular object of the invention is to provide a magnetic centering device for a timepiece mechanism assembly, and a timepiece mechanism including a magnetic centering device.

It would be advantageous to provide a magnetic centering device that integrates bearings for rotating components with very low wear.

It would be advantageous to provide a magnetic centering device that integrates a high efficiency bearing.

It would be advantageous to provide a very compact and robust magnetic centering device.

It would be advantageous to provide a magnetic centering device that can be manufactured in a simple and economical manner.

The object of the invention is achieved by a magnetic centering device according to for micromechanical applications, comprising a magnet and a magnetic flux conducting means arranged on the magnet, said magnetic flux conducting means comprising a central magnetic flux conducting disc made of a magnetic material having a maximum high permeability equal to or greater than 100, and a peripheral region arranged around the central disc and separated from the central disc by a non-zero distance. Other preferred embodiments describe advantageous aspects of the invention. Preferably, the peripheral region is free of contact with the central disc. The peripheral region and the central wheel disc may be connected by one or more rods or by a connecting plate, the thickness of said rods or connecting plate being equal to or less than one tenth of the thickness of the central wheel disc. The peripheral region may be made of a magnetic material having a high maximum magnetic permeability equal to or greater than 100. The peripheral region may be made of the same material as the central wheel disc. The material of the magnet can be SmCo or NdFeB. The center disk may be separated from the peripheral region by a distance in a range between 0.45 times the average width of the magnets multiplied by the outer diameter of the peripheral region and 0.2 times the average width of the magnets multiplied by the outer diameter of the peripheral region. The peripheral region may form a closed circuit around the central wheel. The peripheral region may have a circular shape. The central disk may have a cylindrical shape or a truncated conical shape. The minimum distance between the central disc and the peripheral region may be equal to or greater than 0.2 mm. The central disk may be inscribed in a cylinder having a diameter between 5 and 30 microns and a thickness between 5 and 50 microns, and the peripheral region is inscribed in a cylindrical crown having an inner diameter between 0.5 and 2mm and a thickness between 5 and 30 microns. The central disk may be inscribed in a truncated cone having a first surface with a diameter equal to or less than 10 microns and a second surface with a diameter equal to or less than 50 microns and a height equal to or less than 50 microns, the second surface being arranged closer to the magnet than the first surface. The magnet may have a diameter between 0.5mm and 3mm and a thickness between 0.2mm and 1 mm. The magnetic flux conducting means may be formed on the surface of the magnet or the inner surface of the non-magnetic support by a deposition technique. The material of the central disc, and possibly of the peripheral region when the material is the same, may be made of nickel or cobalt or a nickel alloy or a cobalt alloy. The magnetic flux conducting means may be obtained by photolithography from a substantially uniform layer of material. The device may comprise a non-magnetic support made of a material having low permeability, the magnetic flux conducting device being arranged between the magnet and the non-magnetic support. The non-magnetic support may protect the magnetic flux conducting means and have a working surface against which the moving member can rest. The work surface may be made of a material selected from the group of materials including sapphire, ruby, and diamond. The surface of the non-magnetic support is recessed by laser machining, the material forming the central disc is then deposited in layers on the recessed surface by galvanic deposition and the magnetic flux conducting means are finally formed inside the recess by mechanical or chemical etching of the deposited material. The thickness of the non-magnetic support may be between 0.03mm and 0.6 mm. The device may be configured as a magnetic bearing for a pivoting or rotating movement member of a timepiece mechanism.

The object of the invention is achieved according to a timepiece movement including a magnetic centering device for micromechanical applications, comprising a magnet and a magnetic flux conducting device arranged on the magnet, the magnetic flux conducting device comprising a central magnetic flux conducting disc made of a magnetic material having a maximum high permeability equal to or greater than 100, and a peripheral region arranged around the central disc and separated from the central disc by a non-zero distance.

The object of the invention is achieved by a timepiece for a magnetic centering device for micromechanical applications, comprising a magnet and a magnetic flux conducting device arranged on the magnet, the magnetic flux conducting device comprising a central magnetic flux conducting disc made of a magnetic material having a maximum high permeability equal to or greater than 100, and a peripheral region arranged around the central disc and separated from the central disc by a non-zero distance.

The integration of a magnetic flux positioning element in an active magnetic assembly according to the invention improves and extends the use of magnetic assemblies, in particular magnetic timepiece assemblies.

Drawings

Other advantageous objects and aspects of the invention will appear upon reading the following detailed description of embodiments and the accompanying drawings, in which:

fig. 1 is a diagram of an embodiment of a magnetic centering device according to the present invention, fig. 1a illustrating a cross-sectional view and fig. 1b a top view of the device.

Fig. 2 is a diagram illustrating the magnetic field of a magnetic centering device according to one embodiment of the present invention.

FIG. 3 is a graph illustrating the magnitude of a magnetic field in the presence and absence of a magnetic flux positioning assembly of a magnetic centering device according to an embodiment of the present invention.

Detailed Description

With reference to the figures, in particular fig. 1a and 1b, a magnetic centering device 2 according to an embodiment of the invention comprises a magnet 6, a non-magnetic support 12 and a magnetic flux conducting device 4 arranged between the magnet and the non-magnetic support. The magnet 6 may be made of a material exhibiting a high magnetic energy density (e.g., SmCo or NdFeB), which is typically manufactured by a known method from a powder of a chemical element from the rare earth group. It is possible within the scope of the invention to omit the non-magnetic support in another embodiment.

For the applications envisaged by the present invention, the thickness Z6 of the magnet is of the order of millimeters or less, for example between 0.2 millimeters and 0.7 millimeters. For applications requiring very large magnetic forces, the height Z6 of the magnet may also be on the order of several millimeters, for example from 1 millimeter to 5 millimeters. For the applications envisaged by the present invention, the average width X6 of the magnet, or its diameter if it is a cylindrical magnet, is of the order of millimetres, typically less than 5 millimetres and in many applications of the order of millimetres or less. According to a preferred embodiment, the magnet 6 has a prismatic shape with a cross-section that is circular, polygonal, square and rectangular or other irregular shape as illustrated in fig. 1 b.

For most applications, the cross-section of the magnet is preferably circular.

The non-magnetic support 12 is made of a non-magnetic material, the choice of which depends among other things on the application envisaged for the magnetic centering device 2. The non-magnetic support 12 protects the magnetic flux conducting means 4 and has a working surface 16 against which a component (not shown) can rest.

One of the main applications of the magnetic centering means according to one embodiment of the invention consists of an axial bearing for rotating or pivoting the mobile part, which axial bearing also has the function of centering the rotating mobile member on the axis a defined by the magnetic flux conducting means 4. The magnetic centering device according to the invention can also be used for centering or positioning non-rotating and in particular stationary components. In such a case, it is also possible to envisage using the magnetic centering means according to the invention to define the reference point, for example for moving parts.

In applications where the non-magnetic support 12 has a working surface 16 for moving a support member (not shown), e.g. as an axial bearing for a rotating pivoting member, the material is selected from materials having good mechanical properties and in particular high deformation and fracture limits in addition to good tribological properties. Examples of materials responsive to these criteria include crystalline stones, such as sapphire, e.g., ruby or diamond. For mobile timepiece components, ruby would preferably be used as the material for the non-magnetic support 12.

In a preferred embodiment, the non-magnetic support is formed of a homogeneous material, for example, a non-magnetic support constructed of a sapphire disk (e.g., ruby). However, within the scope of the invention, the non-magnetic support may also be formed by a non-homogeneous structure formed by several layers of different materials. For example, in a variant, the non-magnetic support 12 may comprise a first material as the main material for the support body, a second material with high hardness and/or good tribological properties forming an outer layer comprising the work surface 16. For example, the first layer may be made of a material formed by deposition on the magnetic flux conducting means 4 using known deposition techniques, and the second layer may be made of another material. This second layer may be, for example, a diamond layer, also formed by a deposition technique, such as by plasma deposition or CVD (chemical vapor deposition).

The non-magnetic support may also be in the form of a single component such as a ruby or other cut stone, or a component made of a ceramic material.

The axial thickness Z12 of the non-magnetic support is preferably of the order of millimeters or less, preferably less than 0.5 millimeters. For most applications, the thickness Z12 is as low as possible, which takes into account, in addition to the limits of the manufacturing techniques used to produce the device, the pressure exerted on the non-magnetic support and the resistance of the material from which the non-magnetic support is made. This makes it possible to obtain the smallest possible axial distance separating the working surface 16 from the magnetic flux conducting means 4 for the highest possible magnetic field strength.

Work surface 16 may be planar or substantially planar. However, in a variant, the work surface may have a non-planar shape, for example a convex shape or a concave shape, which depends among other things on the application envisaged for the magnetic centering device according to the invention.

The convex shape may be useful, for example, when the magnetic centering means is arranged on the end of a pivot pin or when a member (not shown) resting on the work surface has a flat or concave surface, in order to form a contact point between the member and the work surface.

In one embodiment, the width of the non-magnetic support may be the same as or substantially match the width X6 of the magnet. However, it is possible in a variant to have a non-magnetic support 12 having a width and/or a cross-section different from the width and cross-section of the magnets. For example, in a variant, the non-magnetic support may form an integral or integrated part of a larger support element having other functions and/or forming part of a member of another device.

The magnetic flux conducting means 4 comprises a central magnetic flux conducting disc 8 made of magnetic material M1 and a peripheral region 10 made of material M2, said material M2 depending on the variant may be identical to the material M1 of the central disc or the peripheral region is made of a different material than the material of the central disc.

The peripheral region 10 is disposed about and separated from the central disc by a distance R10. In an advantageous embodiment, the peripheral region is not in contact with the central disc. In a variant, the peripheral region and the central wheel disc are connected by one or more bars, spokes or bridges of different shape. The peripheral region and the central wheel disc may also be connected by a bar or web having a thickness equal to or less than one tenth of the thickness of the central wheel disc. The distance R10 may be on the order of half the width X6 of the magnets minus the radial thickness Xp of the material forming the peripheral region and the radius of the central disc 8 (X8/2). In one embodiment, the peripheral region is proximate to an outer periphery 20 of the magnet 6 or proximate to an outer periphery 22 of the non-magnetic support 12. The distance R8 separating the central disk from the peripheral region is in the preferred embodiment in the range between 0.45 times the average magnet width X6 and 0.2 times the average magnet width X6. If the magnets have a width of about 1mm, the distance separating the peripheral region from the central disk is preferably between 0.45 mm and 0.2 mm.

In a preferred embodiment, the peripheral region 10 forms a closed, preferably circular, circuit around a central wheel disc, which is preferably arranged at the centre of the peripheral region 10 within the limits of manufacturing tolerances, although the peripheral region may also have the shape of a triangle or a regular polygon or a square. In a variant (not shown) the peripheral region 10 may also be formed by a non-closed circuit around the central disc, or comprise a plurality of dots, arcs or sections of material M2 which are discontinuous but distributed around the central magnetic flux conducting disc 8.

In a preferred embodiment, the material M2 of the peripheral region 10 is advantageously a magnetic flux conducting material, in particular having a high magnetic permeability. In this embodiment, material M2 may advantageously be the same magnetic flux conducting material used for the central magnetic flux conducting disc 8. In a variant, however, it is possible to make the material M2 with a low permeability, in particular a non-magnetic material and in this case the peripheral region with a support and/or spacer function defining an axial distance Z10 between one surface of the magnet 6 and the inner surface 14 of the non-magnetic surface 12.

In a preferred embodiment, the magnetic flux conducting means 4 is formed on the surface 18 of the magnet or on the inner surface 14 of the non-magnetic support by a deposition technique. Various known prior art techniques of deposition techniques may be used and will therefore not be described in detail. One of the known methods that can be advantageously used in one embodiment of the invention consists of depositing a layer of material M1 forming the central disc, possibly made of another material M2 forming the peripheral region if the material of the peripheral region is different from that of the central disc, followed by a photolithographic method to form the central disc and a spacing R8 separating the central disc 8 from the peripheral region 10. According to a variant, the method of manufacturing the central disc and the peripheral region may comprise laser machining the layers of material forming the central disc and the peripheral region. According to a variant, the non-magnetic support 12 may comprise a central recess (holow), for example a laser machined recess in a hard material such as sapphire or ceramic component, in which the magnetic material of the central disc is deposited, for example by galvanic deposition, possibly accompanied by a mechanical or chemical attachment method to form the final shape of the central disc and of the peripheral region. The same method can also be used to form the peripheral region by forming a recess for the peripheral region. The recess formed in the support may also include a central wheel disc and a peripheral region, the space between the central component and the peripheral region being machined, removed by chemical etching or by laser methods. In a preferred embodiment, the recess in the non-magnetic support 12 may be inscribed in a truncated cone having the smallest possible diameter towards the bottom of the recess.

The space separating the peripheral region from the central disc may be filled with a gas or, in a preferred variant, with a non-magnetic solid material, i.e. a material having a low permeability (for example close to the value 1).

The material M1 of the central disc 8, and in the appropriate case of the peripheral region 10 (when this is the same material), can advantageously be made of nickel or cobalt or of a nickel alloy or cobalt alloy. In one embodiment, material M1 and/or material M2 are formed entirely of nickel. In another variant, these elements are made entirely of nickel-phosphorus with a phosphorus percentage less than or equal to 11%. According to yet another variant, the flux conducting element is formed entirely of cobalt. According to another variant, the flux conducting element may be made entirely of a magnetically soft material consisting of a coercive field of less than 5kA/m_cAnd a maximum magnetic permeability mu equal to or greater than 100_RAnd (5) characterizing.

Fig. 2 illustrates magnetic field lines generated by the magnetic centering device according to fig. 1. The field lines are located in the region of the central disc 8 and are hardly visible on a practical scale. The central disc 8 has a diameter equal to 10 microns and a thickness equal to 10 microns in this example. The peripheral region 10 has a circular crown shape with an outer diameter equal to 1mm and an inner diameter equal to 0.8mm and a thickness equal to 10 microns. The disk and the peripheral region are made of nickel. The magnetic flux conducting means 4 are interposed between a permanent magnet made of NdFeB with a diameter equal to 1mm and a thickness equal to 0.5mm and a ruby with the same diameter and a thickness equal to 0.05 mm.

The residual field of the permanent magnet is 1T and its magnetization is inhomogeneous and 0.2mm off-center with respect to the center of the magnet. The magnetic field on the outer surface of the ruby is shown in fig. 3. In fig. 3, the magnetic field magnitude is measured in the direction of the x-axis in a cross-section along the axes y =0, z =0.31mm, with and without the magnetic flux conducting element B. The maximum magnetic field corresponds to the central nickel disk, which acts as a conductor and locator for the magnetic flux leaving the magnet. In the absence of the conductive element, the maximum magnetic field on the gemstone will be displaced by approximately 0.2mm due to the eccentric magnetization of the permanent magnet.

According to an advantageous embodiment, the magnetic flux conducting element comprises a circular central disc having a diameter equal to or less than 10 microns and having a thickness equal to or less than 7 microns.

According to an advantageous embodiment, the magnetic flux conducting element comprises a central disc which can be inscribed in a truncated cone having a first surface with a diameter equal to or less than 10 microns and a second surface with a diameter equal to or less than 50 microns and a height equal to or less than 50 microns, the largest surface being the surface closest to the magnet 6.

According to an advantageous embodiment, the minimum distance between the central disc and the peripheral zone is equal to or greater than 0.2 mm.

According to an advantageous embodiment, the peripheral region has a circular crown shape.

According to an advantageous embodiment, the peripheral region is completely not in communication with the central disc.

According to a second embodiment, a non-magnetic connecting element is present between the peripheral region and the central disc.

According to one embodiment, the peripheral region and the wheel disc are made of the same material and are connected by a bar or web having a thickness equal to or less than one tenth of the thickness of the wheel disc.

Preferably, the magnet is a permanent rare earth magnet.

According to an advantageous embodiment, the materials M1 and M2 are identical.

According to an advantageous embodiment, the flux conducting element is made entirely of nickel.

According to a variant, the magnetic flux conducting means are entirely made of nickel-phosphorus with a phosphorus percentage lower than or equal to 11%.

According to a variant, the magnetic flux conducting means are made entirely of cobalt.

According to a variant, the magnetic flux conducting means are made entirely of a magnetically soft material, which has a coercive field of less than 5kA/m_cAnd a maximum magnetic permeability mu of 100 or less_RAnd (5) characterizing.

According to another embodiment, the material M1 is magnetic and the material M2 is non-magnetic.

According to another embodiment, the non-magnetic support is omitted.

According to an advantageous manufacturing method, the magnetic flux conducting means are created by deposition on the surface of the magnet.

According to another advantageous manufacturing method, the magnetic flux conducting means are created by deposition on the surface of the non-magnetic support.

According to an advantageous manufacturing method, the magnetic flux conducting means are obtained by photolithography from a uniform layer of material.

According to an advantageous manufacturing method, the surface of the non-magnetic support is recessed by laser machining, the material M1 is then deposited uniformly on the recessed surface by current deposition and the magnetic flux conducting means are finally formed inside the recess by mechanical or chemical etching of the deposited material M1. According to a preferred manufacturing method, the recess in the magnetic support may be inscribed in a truncated cone having the smallest possible diameter towards the bottom of the recess.

In horological applications, the magnetic centering device according to the invention can be advantageously used for:

mechanical timepiece movements, in particular for a sprung balance and an escapement;

-a magnetic pivot for various moving components, such as a gear train, escapement or balance;

-timers (oscillator, movement and anti-flutter);

-date mechanism (semi-instant and instant);

-striking mechanisms (hammer, pin barrels, regulators);

tuning fork timepieces (in particular with Clifford magnetic escapement).

The invention can also be used in measuring or control instruments and in other instruments comprising rotating micromechanical components, such as miniature gyroscopes.

Among its advantages, the magnetic centering device according to the invention:

-ensuring the positioning of the magnetic flux of the magnetic assembly off the horological dimensions with an accuracy of less than 10 microns;

can be integrated in all active magnetic components (permanent magnets, soft ferromagnetic, antiferromagnetic components, etc.),

regardless of the magnetic energy density, remanence, and other magnetic properties of the assembly;

can be integrated directly in components already developed in the watchmaking industry, for example in magnetic pivots;

ensuring its magnetic flux localization function in a very high temperature range (in the range greater than-200 ℃/+150 ℃, depending on the material chosen), regardless of the various climatic conditions;

-ensuring the magnetic flux localization regardless of its intensity;

robust to shocks.

REFERENCE LIST

Magnetic centering means 2

Magnetic flux conducting means 4

Central magnetic flux conducting disk 8

Magnetic material M1

Peripheral region 10

Material M2

A transverse periphery 22

Non-magnetic support 12

Inner surface 14

Outer surface 16 (work surface)

A magnet 6

Inner surface 18

A lateral periphery 20

Claims (25)

1. Magnetic centering device (2) for micromechanical applications, comprising a magnet (6) and a magnetic flux conducting device (4) arranged on the magnet, the magnetic flux conducting device comprising a central magnetic flux conducting disc (8) made of a magnetic material (M1) having a maximum high permeability equal to or greater than 100, and a peripheral region (10) arranged around the central disc and separated from the central disc by a non-zero distance (R10),

wherein the device comprises a non-magnetic support (12) made of a material having a low permeability, the magnetic flux conducting device being arranged between the magnet and the non-magnetic support.

2. The device of claim 1, wherein the peripheral region does not contact the central disk.

3. Device according to claim 1, characterized in that the peripheral region and the central wheel disc are connected by one or more rods or by connecting plates, the thickness of said rods or connecting plates being equal to or less than one tenth of the thickness of the central wheel disc.

4. A device according to claim 1, characterized in that the peripheral region is made of a magnetic material (M2) having a high maximum magnetic permeability equal to or greater than 100.

5. The device of claim 4, wherein the peripheral region is made of the same material as the central disk.

6. The device of claim 1, wherein the material of the magnet is SmCo or NdFeB.

7. The device according to claim 1, characterized in that the distance (R10) separating the central disc from the peripheral region is in the range between 0.45 times the average width of the magnets of the outer diameter of the peripheral region (X6) and 0.2 times the average width of the magnets of the outer diameter of the peripheral region (X6).

8. The device of claim 1, wherein the peripheral region forms a closed circuit around the central disk.

9. The device of claim 1, wherein the peripheral region has a circular shape.

10. The device of claim 1, wherein the central disk has a cylindrical shape.

11. The device of claim 1, wherein the central disk has a truncated cone shape.

12. The device of claim 1, wherein the minimum distance between the central disk and the peripheral region is equal to or greater than 0.2 mm.

13. The device of claim 1, wherein the central disk is inscribed in a cylinder having a diameter between 5 and 30 microns and a thickness between 5 and 50 microns, and the peripheral region is inscribed in a cylindrical crown having an inner diameter between 0.5 and 2mm and a thickness between 5 and 30 microns.

14. The device of claim 1, wherein the central disk is inscribed in a truncated cone having a first surface and a second surface, the first surface having a diameter equal to or less than 10 microns, the second surface having a diameter equal to or less than 50 microns and a height equal to or less than 50 microns, the second surface being disposed closer to the magnet than the first surface.

15. The device of claim 1, wherein the magnet has a diameter of between 0.5mm and 3mm and a thickness of between 0.2mm and 1 mm.

16. Device according to claim 1, characterized in that the magnetic flux conducting means are formed on the surface (18) of the magnet or on the inner surface (14) of the non-magnetic support by means of a deposition technique.

17. Device according to claim 1, characterized in that the material (M1) of the central disc and possibly of the peripheral region when the material is the same is made of nickel or cobalt or a nickel alloy or a cobalt alloy.

18. A device according to claim 1, characterized in that the magnetic flux conducting means are obtained by photolithography from a substantially uniform layer of material.

19. Device according to claim 1, characterized in that the non-magnetic support protects the magnetic flux conducting device and has a working surface (16) against which the moving member can rest.

20. The device of claim 19, wherein the working surface is made of a material selected from the group of materials consisting of sapphire, ruby, and diamond.

21. A device according to claim 1, wherein the surface of the non-magnetic support is recessed by laser machining, the material forming the central disc is then deposited in layers on the recessed surface by galvanic deposition and the magnetic flux conducting means is finally formed inside the recess by mechanical or chemical etching of the deposited material.

22. The device according to claim 1, characterized in that the thickness of the non-magnetic support is between 0.03mm and 0.6 mm.

23. Device according to claim 1, characterized in that it is configured as a magnetic bearing for a pivoting or rotating movement member of a timepiece mechanism.

24. Timepiece movement comprising a magnetic centering means for micromechanical applications, the magnetic centering means comprising a magnet (6) and a magnetic flux conducting means (4) arranged on the magnet, the magnetic flux conducting means comprising a central magnetic flux conducting disc (8) made of a magnetic material (M1) having a maximum high permeability equal to or greater than 100, and a peripheral region (10) arranged around the central disc and separated from the central disc by a non-zero distance (R10), wherein the means comprise a non-magnetic support (12) made of a material having a low permeability, the magnetic flux conducting means being arranged between the magnet and the non-magnetic support.

25. Timepiece comprising a magnetic centering device for micromechanical applications, the magnetic centering device comprising a magnet (6) and a magnetic flux conducting device (4) arranged on the magnet, the magnetic flux conducting device comprising a central magnetic flux conducting disc (8) made of a magnetic material (M1) having a maximum high permeability equal to or greater than 100, and a peripheral region (10) arranged around the central disc and separated from the central disc by a non-zero distance (R10), wherein the device comprises a non-magnetic support (12) made of a material having a low permeability, the magnetic flux conducting device being arranged between the magnet and the non-magnetic support.