MECHANICAL MOVEMENT WITH A MAGNETICALLY SWIVELING BALANCE WHEEL

Description

Technical field of the invention

[0001] The invention relates to the field of pivoting rotating elements in the watchmaking field. In particular, the invention relates to a mechanical watch movement comprising a mechanical resonator whose balance is magnetically pivoted, that is to say it is guided in rotation essentially by a magnetic device. Technological background

[0002] Document WO 2012 / 062524 discloses a method for pivoting a watch component, in particular a balance wheel, and a device for pivoting such a watch component. This document teaches how to orient a balance wheel along a given rotation axis using a magnetic pivoting guide device, formed from a pair of magnetic pivots. Figure 1a mechanical resonator 2 is shown formed of a balance 4 associated with a balance spring 3, this balance being formed of an inertial mass 4a and a shaft 6 which is provided with two magnetic pivots 6a, 6b to guide the balance in rotation, these two magnetic pivots being guided in rotation by two magnetic bearings formed respectively by two permanent magnets 10 and 12. These magnets are for example formed of a rare earth alloy, in particular SmCo or NdFeB. In particular, they are made of a material known under the reference VAC677_HR. The shaft 6 of the balance is made of a material having a high magnetic permeability, for example a steel. A carbon steel, known under the reference 20_AP, can advantageously be used.One of the two magnets is provided to be more powerful than the other, so that one of the two pivots 6a, 6b of the shaft 6 is in contact with the more powerful magnet 10 (or with an intermediate counter-pivot, for example a disk made of polished hard material such as a synthetic ruby), while the other pivot remains at a small distance from the less powerful magnet 12 (or from an intermediate disk protecting this magnet, in particular a usual counter-pivot) and therefore normally remains without contact with the corresponding magnetic bearing. Such an arrangement for magnetically guiding a sprung balance in rotation has major advantages, in particular by reducing friction relative to two usual mechanical bearings.

[0003] There Figure 2 gives the theoretical curve of a magnetic restoring force FM as a function of a radial displacement X of the balance 4 from the axis of rotation 8 of this balance for the magnetic guide device of the type shown in Figure 1 . The axis of rotation of the balance is defined by the geometric axis passing through the middle of the two magnets 10 and 12. It is observed that the function FM (X) is substantially linear for small displacements. Thus, with such an arrangement, the magnetic restoring force increases proportionally with a radial displacement of the balance from its given axis of rotation. This poses a problem of centering the balance 2, because the restoring force is weak for small displacements of the balance from its predefined axis of rotation. In addition, this problem of centering the balance is worsened by the fact that the usual magnets often have inhomogeneities on the millimeter scale. However, the diameter of the magnets 10, 12 is of the order of a millimeter. This centering problem poses a problem for the interaction of the balance with the escapement associated with this balance.

[0004] Document FR1314364A discloses, at the Figure 4, a clockwork mechanism for guiding the rotation of a pivoting element around an axis. This mechanism achieves magnetic suspension by repulsion, with only two magnets. Summary of the invention

[0005] The present invention aims to solve the problem of centering magnetic bearings guiding the rotation of small rotating elements, in particular in the watchmaking field and more specifically the problem of centering for magnetic guidance in rotation / pivoting of the balance of a mechanical resonator incorporated in a mechanical watch movement.

[0006] For this purpose, as defined in claim 1, the invention relates to a watch mechanism comprising a rotating element and a magnetic device for guiding the rotation of this rotating element, this magnetic device being arranged so as to exert a radial magnetic restoring force on the rotating element when a central axis of this rotating element undergoes a radial displacement relative to an axis of rotation which is predefined for the rotating element in the mechanism. According to the invention, the magnetic device comprises a pair of annular magnets of which the first annular magnet is carried by the rotating element, so that its center remains coincident with said central axis, and the second annular magnet is carried by a structure of the mechanism so that its center remains coincident with said axis of rotation.When the central axis of the rotating element coincides with the axis of rotation, the first annular magnet and the second annular magnet extend in general planes parallel and orthogonal to the axis of rotation, having an intermediate space between them, and the second annular magnet is at least partially axially superimposed on the first annular magnet, these first and second annular magnets being arranged in magnetic attraction so as to generate on each other a first axial magnetic force and, substantially as soon as the central axis of the rotating element deviates radially from the axis of rotation, a first radial magnetic force.

[0007] Thanks to the features of the invention, the magnetic guide device makes it possible to radially center the rotating element effectively, so as to limit, in normal operation, any radial displacement of the central axis of the rotating element relative to the axis of rotation. Indeed, two annular magnets superimposed and arranged in magnetic attraction make it possible to obtain a relatively intense radial restoring force already for small radial displacements of the rotating element relative to the axis of rotation provided in the mechanism for this rotating element.

[0008] According to an advantageous embodiment, the magnetic device further comprises at least one first end magnet which is arranged axially opposite a first end of a shaft of the rotating element, this shaft being formed at least partially by a soft ferromagnetic material or a magnetic material so that the first end magnet exerts a second axial magnetic force, in magnetic attraction, on the shaft and, when the central axis of the rotating element is radially distant from the axis of rotation, also a second radial magnetic force on this shaft, the second axial magnetic force having the same direction as the first axial magnetic force.

[0009] According to a preferred embodiment, the first annular magnet has, on the side of the second annular magnet, a first plurality of annular zones having alternating polarities and the second annular magnet has, on the side of the first annular magnet, a second plurality of annular zones having alternating polarities, at least two annular zones of the first plurality of annular zones being, when the central axis of the rotating element coincides with the axis of rotation, substantially entirely axially superimposed on two respective annular zones of the second plurality of annular zones and in magnetic attraction with these two respective annular zones. Brief description of the figures

[0010] The invention will be described below in more detail with the aid of the attached drawings, given as non-limiting examples, in which: there Figure 1, already described, is a side view showing a magnetic device for guiding a sprung balance according to the prior art; Figure 2 , already described, is a graph of the radial magnetic restoring force exerted by the magnetic device of the Figure 1 on the balance wheel as a function of a radial displacement of the latter; the Figure 3 is a partial perspective view of a first embodiment of a mechanism according to the invention; Figure 4 is a side view, partially in section, of the mechanism of the Figure 3 ; there Figure 5 is a partial perspective view of a second preferred embodiment of a mechanism according to the invention; Figure 6 represents in plan the first magnet and the second magnet of a pair of annular magnets forming the magnetic device for guiding the rotation of a sprung balance in a first variant of the second embodiment; Figures 7A and 7Bare graphs of a first radial magnetic restoring force exerted, in each of the first and second embodiments, by the pair of annular magnets on the balance as a function of a radial displacement of the latter and respectively for two separation distances between the first and second annular magnets; Figure 8 represents in plan the first magnet and the second magnet of a pair of annular magnets forming the magnetic device for guiding the rotation of a sprung balance in a second advantageous variant of the second embodiment; and the Figure 9 represents in plan the first magnet and the second magnet of a pair of annular magnets forming the magnetic device for guiding the rotation of a sprung balance in a third advantageous variant of the second embodiment. Detailed description of the invention

[0011] In reference to the Figures 3 and 4, we will describe a first embodiment of a watch movement, shown in part, according to the invention.

[0012] The watch movement 20 comprises a mechanical resonator, formed by a balance 22 having a shaft 24 defining a central axis 26 of the balance, and a magnetic device for guiding the balance in rotation / pivoting which comprises a pair of annular magnets, which is made up of a first annular magnet 32, arranged on a support 30 carried by the shaft 24, and a second annular magnet 34 arranged on a support 36, made of soft or non-magnetic ferromagnetic material, which is glued onto a cylindrical tube 38 driven, or fixed in another way, in an opening of a plate 40 of the watch movement. Note that the inertial mass of the balance and the balance spring of the mechanical resonator have not been shown in the Figures 3 and 4 (for these elements, see Figure 1). It will be noted that a space is left free between the flange supporting the inertial mass of the balance and the support 30, this for the arrangement of a device 72 for coupling the balance with an escapement, in particular a plate 74 carrying a pin 76 provided to interact in a conventional manner with an anchor of the escapement (not shown).

[0013] Generally, the first annular magnet 32 ​​is carried by the balance 22 so that its center remains coincident with the central axis 26, and the second annular magnet 34 is carried by a structure of the watch movement so that its center remains coincident with the axis of rotation 28, which is predefined for the balance in the watch movement. In the variant shown, the support 30 is an annular support made of a material with low magnetic permeability, in particular of a paramagnetic or diamagnetic material, which is fixedly mounted on the shaft 24. In a variant, the support 30 comprises a thin annular plate made of a soft ferromagnetic material arranged directly under the first annular magnet 32. It will be noted that, at Figure 4, the shaft 24, the support 30 and the first annular magnet 32 ​​are shown in side view, uncut, while the other elements are shown in section along a section plane including the axis of rotation 28. The balance 22 is shown in a centered position, as desired in normal operation, so that the central axis 26 of the shaft 24 coincides with the axis of rotation 28.

[0014] The magnetic device for guiding the balance in rotation is arranged so as to exert a radial magnetic restoring force FR (X) on this balance when the central axis 26 of the balance undergoes a radial displacement X relative to the axis of rotation 28. In the first embodiment, the second annular magnet 34 is parallel and substantially entirely axially superimposed on the first annular magnet 32 ​​when the central axis 26 of the rotating element coincides with the axis of rotation 28. These first and second annular magnets are arranged in magnetic attraction so as to generate on each other a first axial magnetic force and, substantially as soon as the central axis 26 of the balance has a radial difference with the axis of rotation 28, a first radial magnetic force FR (X). For this purpose, the inner diameter and the outer diameter of the first annular magnet and the second annular magnet are substantially identical, preferably identical.For example, the inner diameter of the two ring magnets is between 0.8 mm and 1.1 mm, and their outer diameter is between 1.5 mm and 2.2 mm.

[0015] In the first embodiment, each of the first and second annular magnets is a bipolar magnet, the two annular bipolar magnets having the same polarity (axial magnetic polarization, in the same direction) so as to attract each other. These two bipolar magnets are provided relatively thin, for example between 50 microns and 150 microns. However, in other particular embodiments, they can be provided thicker to increase the magnetic power. These magnets can in particular be laser cut from magnetized wafers perpendicular to the general plane of the wafer or be deposited in a thin layer. The first axial force and the first radial force depend in particular on the spacing / distance G between the two annular magnets.We will subsequently see graphs comparing the first radial force FR (X), which is used for the magnetic centering of the shaft 24 of the balance 22, for the first and second embodiments and two different spacings.

[0016] Then, similarly to the prior embodiment described in the technological background, the magnetic device further comprises a first end magnet 42 and a second end magnet 44 which are respectively arranged axially opposite a first end (first pivot) and a second end (second pivot) of the shaft 24 of the balance 22. The first end magnet 42 is arranged inside the cylindrical tube 38 which carries the second annular magnet 34 (annular magnet fixed relative to the structure carrying the first and second end magnets). Inside the cylindrical tube 38 is also arranged a pierced jewel 54 which forms a first radial safety stop for the shaft 24, thus to limit a radial displacement of this shaft in the event of an impact, more precisely of its first end.It will be noted that the first end magnet is covered with a plate 46 of hard material, a sort of counter-pivot stone which protects the first end magnet and which allows pivoting with less friction and relatively little wear. The second end magnet 44 is also arranged inside a cylindrical tube 50, which is driven, or fixed in another way, in an opening of a bridge 52, in particular a balance bridge. Inside the cylindrical tube 50 is also arranged a pierced stone 56 which forms a second radial safety stop for the shaft 24, thus to limit a radial displacement of this shaft in the event of an impact, more precisely of its second end. The second end magnet is also covered with a plate 48 of hard material. For example, the diameter of the two end magnets is between 0.8 mm and 1.0 mm and their height is between 0.4 mm and 0.6 mm.

[0017] The shaft 24 is formed by a soft ferromagnetic material so that the first end magnet 42 exerts a second axial magnetic force on the shaft, in magnetic attraction and in the same direction as the first axial magnetic force, and, when the central axis 26 of the balance is radially distant from the axis of rotation 28, also a second radial magnetic force on this shaft. The second end magnet 44 exerts a third axial magnetic force on the shaft, in magnetic attraction and in the opposite direction to the direction of the second axial magnetic force, and, when the central axis 26 of the balance is radially distant from the axis of rotation 28, also a third radial magnetic force on this shaft. The intensity of the third axial magnetic force is intended to be less than the total intensity of the first axial magnetic force and the second axial magnetic force.The total intensity of the second radial force and the third radial magnetic force exhibits a substantially linear dependence on the distance between the central axis 26 and the rotation axis 28, as shown by the curve of the . Figure 2 . The end magnets 42 and 44 have respective central axes which are aligned and which define the axis of rotation 28 for the balance 22, this axis of rotation being thus predefined in the watch movement 20. The axis of rotation remains in a given fixed position relative to the supporting structure of the balance 22, namely the plate 40 and the bridge 52.

[0018] It will be noted that the provision of the pair of annular magnets according to the invention makes it possible to significantly reduce the dimensions of the first and second end magnets, more particularly of the first end magnet which is provided to be larger than the second end magnet in the prior art (axial magnetic force exerted by the first end magnet on the shaft greater than that exerted by the second end magnet on this shaft in the prior art). Indeed, given the presence of the first axial magnetic force generated by the pair of annular magnets which is exerted in the same direction as the second axial magnetic force, the latter no longer needs to have an intensity greater than that of the third axial magnetic force.Furthermore, as will be seen later, the arrangement of the pair of annular magnets according to the invention also makes it possible to mainly reduce the second radial magnetic force and also, preferably to a lesser extent given the distance of the second end magnet 44 from the pair of annular magnets, the third radial magnetic force while having better magnetic centering of the mechanical resonator (balance spring).

[0019] The preceding description also makes it possible to design two particular embodiments not shown in the figures. In the first particular embodiment, only one end magnet is provided in addition to the pair of annular magnets 32 and 34. In a first variant, the only end magnet is the one located on the same side of the inertial mass as the pair of annular magnets and which produces an axial magnetic force in the same direction as this pair of annular magnets. In a second variant, the only end magnet is the end magnet which is located on the side of the inertial mass opposite to the side where the pair of annular magnets is located and which produces an axial magnetic force in the opposite direction to that of the axial magnetic force generated by the pair of annular magnets. In this second variant, the axial magnetic force of the pair of annular magnets is provided to be more intense than in the first variant.In the second particular embodiment, the magnetic device for guiding the rotation of the mechanical resonator incorporated in a mechanical movement consists only of the pair of annular magnets 32 and 34. A variant of this second particular embodiment advantageously comprises a pair of annular magnets according to the second embodiment which is described below. In the second particular embodiment, the counter-pivot jewel against which a pivot of the balance shaft presses in normal operation has a small flared cup for holding this pivot in a substantially central position, that is to say substantially on the axis of rotation provided for the balance. It will be noted that such a small flared cup can be advantageously provided in all the embodiments of the invention.

[0020] In reference to the Figures 5 , 6, 8 and 9, a second embodiment of a mechanism 60 according to the invention will be described below. The references already described previously will not be described again in detail. In a first variant embodiment shown in Figures 5 And 6, the second embodiment is distinguished from the first embodiment mainly by the fact that the first annular magnet 62 has, on the side of the second annular magnet 64, a first plurality of annular magnetized zones 62a, 62b and 62c having alternating polarities (N, S, N) and the second annular magnet 64 has, on the side of the first annular magnet 62, a second plurality of annular magnetized zones 64a, 64b and 64c having alternating polarities (S, N, S). Generally, at least two annular magnetized zones of the first plurality are, when the central axis 26 of the balance 22A coincides with the axis of rotation 28, substantially entirely axially superimposed on two respective annular magnetized zones of the second plurality and in magnetic attraction with these two respective annular zones.In the first variant, the first plurality comprises three annular magnetized areas which are respectively entirely superimposed on the three annular magnetized areas of the second plurality. In the partial perspective view of the . Figure 5 , the second annular magnet 64 is shown without its support, so that the outer face of this magnet is seen. On the other hand, at the Figure 6only the two ring magnets 62 and 64 are shown separately, seen from the intermediate space between these two ring magnets. The second embodiment further has a difference in the two end magnets 42 and 44. While in the first embodiment these two end magnets are provided with substantially the same dimensions and the same magnetic power, the first end magnet 42 is provided, in the second embodiment, with smaller dimensions which are smaller than those of the second end magnet 44, which is much further from the pair of end magnets than the first end magnet which is located relatively close to this pair of ring magnets.

[0021] The two annular magnets 62 and 64 are advantageously produced by a rare earth alloy vapor deposition process in a vacuum chamber, in particular by cathode sputtering. The thickness of the two multipolar annular magnets may for example be provided in the range 10 to 50 microns, in particular between 15 microns and 30 microns. The permanent magnetization of each layer of rare earth alloy deposited on a support may be carried out in an installation arranged to be able to generate an intense magnetic field which passes orthogonally through the deposited layer, with the vectors of this magnetic field mainly in one direction or an opposite direction depending on the different zones of the deposited layer, so as to obtain the magnetized annular zones with alternating polarities. Bipolar annular magnetized zones with axial magnetization are thus obtained.

[0022] The second embodiment makes it possible to significantly increase the magnetic centering force operated by the two annular magnets, as is evident from Figures 7A and 7B . There Figure 7A is a graph representing the radial magnetic restoring force FR (X) generated by the pair of annular magnets, having between them a distance G equal to 0.2 mm (200 microns), as a function of a radial displacement X of the central axis 26 of the balance relative to the axis of rotation 28, the curve 80 giving this function for the second embodiment while the curve 82 gives this function for the first embodiment (the dimensions of the annular magnets being identical). It will be noted that the force FR (X) is added to a force FE (X) which is a radial magnetic restoring force generated by the two end magnets (see Figure 2, however note that the slope of the linear function depends on the magnetic power of the end magnets, in particular their dimensions).

[0023] It is observed that the pair of annular magnets according to the invention quickly exerts a relatively large restoring force FR (X) towards the axis of rotation, the initial slope of the curves 80 and 82 being relatively strong over the first 50 microns, unlike the linear function FE (X). In addition, it is observed that this slope is much greater in the second embodiment than in the first embodiment (more than twice as large), which is very advantageous for obtaining efficient and precise magnetic centering.

[0024] There Figure 7B is a graph similar to that of the Figure 7A, but for a distance G between the two annular magnets which is equal to 0.1 mm (100 microns). Curve 84 relates to the second embodiment while curve 86 relates to the first embodiment. Firstly, we observe that the radial magnetic restoring force FR (X) has a maximum value (around X = 100 microns) much higher for the distance G = 0.1 mm than for the distance G = 0.2 mm (almost six times greater). This results in a gradient, over the first 50 microns, higher for curves 84 and 86 than respectively for curves 80 and 82, this gradient being much stronger at curve 84 than at curve 86 (almost five times stronger). This indicates that the second embodiment is particularly efficient and advantageous with a smaller distance between the two annular magnets, which are arranged parallel to each other when the central axis 26 is parallel to the rotation axis 28.

[0025] To Figures 8 and 9 , similar to the Figure 6 , two other variants of the second embodiment are shown. In the second variant of the Figure 8 , generally, the first annular magnet 62 comprises N annular magnetized zones (62a, 62b, 62c), N being greater than 1, and the second annular magnet 66 comprises N+1 annular magnetized zones (64a, 64b, 64c, 64d), the N annular magnetized zones of the first annular magnet 62 being, when the central axis 26 of the balance is coincident with the axis of rotation 28, respectively axially superimposed on N annular magnetized zones, including an inner zone (64a), of the second annular magnet 64, the second plurality of annular magnetized zones comprising an outer zone (64d) which extends radially beyond the first annular magnet 62. In the specific example shown in FIG. Figure 8, the integer N is equal to three, i.e. N = 3. This second variant makes it possible to increase the radial magnetic return force FR (X) relative to the first variant for the same first annular magnet 62 embedded on the rotating mobile element, namely on the balance 22A.

[0026] In the third variant of the Figure 9, generally, the first annular magnet 68 comprises N annular magnetized zones (62b, 62c), N being greater than 1, and the second annular magnet 66 comprises N+2 annular magnetized zones (64a, 64b, 64c, 64d), the N annular magnetized zones of the first annular magnet 68 being, when the central axis 26 of the rotating element (in particular the balance) coincides with the axis of rotation 28, respectively axially superimposed on N internal annular magnetized zones (64b, 64c) of the second annular magnet 66 which are located between an outer zone (64d) and an inner zone (64a) of the second plurality of annular magnetized zones of this second annular magnet, these inner and outer zones extending radially respectively on both sides of the first annular magnet 68. In the specific example shown in Figure 9, the integer N is equal to two, i.e. N = 2. This second variant makes it possible to obtain a relatively intense radial magnetic restoring force FR (X) for a first annular magnet, embedded on the balance, having smaller dimensions and thus a lower weight, taking into account the fact that the support of this first annular magnet also has smaller dimensions. This gives very effective magnetic centering by increasing the inertia of the rotating element relatively little, in particular the balance.

Claims

1. A horology mechanism (20; 60) comprising a rotary element (22; 22A) and a magnetic device for guiding this rotary element in rotation, this magnetic device being arranged so as to exert a radial magnetic return force (FR) on the rotary element when a central axis (26) of this rotary element undergoes a radial movement (X) relative to an axis of rotation (28) which is predefined for the rotary element in the mechanism; the magnetic device comprising a pair of annular magnets in which the first annular magnet (32; 62; 62; 68) is carried by the rotary element, so that its centre remains coincident with said central axis, and the second annular magnet (34; 64; 66; 66) is carried by a structure (40) of the mechanism so that its centre remains coincident with said axis of rotation; characterised in that, when the central axis of the rotary element is coincident with the axis of rotation, the first annular magnet and the second annular magnet extend in general planes that are parallel and orthogonal to the rotational axis, with an intermediate space between them, and the second annular magnet is at least partially axially superimposed with the first annular magnet, the first and second annular magnets being arranged in magnetic attraction so as to impart on one another a first axial magnetic force and, substantially as soon as the central axis of the rotary element deviates radially from the axis of rotation, a first radial magnetic force (FR (X)).

2. The horology mechanism according to claim 1, characterised in that the magnetic device further comprises at least one first end magnet (42) which is axially arranged facing a first end of an arbor (24) of the rotary element (22; 22A), this arbor being formed at least partially by a soft ferromagnetic material or a magnetic material so that the first end magnet exerts a second axial magnetic force on the arbor, to act as a magnetic attraction and in the same direction as the first axial magnetic force, and, when the central axis of the rotary element is radially remote from the axis of rotation, also a second radial magnetic force on this arbor.

3. The horology mechanism according to claim 2, characterised in that the magnetic device comprises a second end magnet (44) arranged facing a second end of the arbor (24), which is formed by a soft ferromagnetic material, the second end magnet exerting a third axial magnetic force on the arbor, as a magnetic attraction and in the opposite direction to the direction of the second axial magnetic force, and, when the central axis of the rotary element is radially remote from the axis of rotation, also a third radial magnetic force on this arbor, the intensity of the third axial magnetic force being lower than the total intensity of the first axial magnetic force and of the second axial magnetic force.

4. The horology mechanism according to claim 2 or 3, characterised in that the first annular magnet (32; 62; 68) is arranged on an annular rest (30) made of a material with a low magnetic permeability, in particular made of a paramagnetic or diamagnetic material, which is fixedly mounted on said arbor (24).

5. The horology mechanism according to any one of the preceding claims, characterised in that the first annular magnet (62; 62; 68) has, on the side of the second annular magnet (64; 66; 66), a first plurality of annular magnetised zones (62a, 62b, 62c; 62b, 62c) having alternate polarities and the second annular magnet has, on the side of the first annular magnet, a second plurality of annular magnetised zones (64a, 64b, 64c; 62a, 62b, 62c, 62d) having alternate polarities, at least two annular magnetised zones of the first plurality (62b, 62c) being, when the central axis (26) of the rotary element (22; 22A) is coincident with the axis of rotation (28), substantially entirely axially superimposed with two respective annular magnetised zones (64b, 64c) of the second plurality and in magnetic attraction with these two respective annular zones.

6. The horology mechanism according to claim 5, characterised in that each of said first plurality and said second plurality comprises three annular magnetised zones (62a, 62b, 62c; 64a, 64b, 64c).

7. The horology mechanism according to claim 5, characterised in that the first annular magnet (62) comprises N annular magnetised zones (62a, 62b, 62c), N being greater than 1, and the second annular magnet (66) comprises N+1 annular magnetised zones (64a, 64b, 64c, 64d), the N annular magnetised zones of the first annular magnet being, when the central axis (26) of the rotary element is coincident with the axis of rotation (28), respectively axially superimposed with N annular magnetised zones, including an internal zone, of the second annular magnet, the second plurality of annular magnetised zones comprising an external zone (64d) which extends radially beyond the first annular magnet.

8. The horology mechanism according to claim 5, characterised in that the first annular magnet (68) comprises N annular magnetised zones (62b, 62c), N being greater than 1, and the second annular magnet (66) comprises N+2 annular magnetised zones (64a, 64b, 64c, 64d), the N annular magnetised zones of the first annular magnet being, when the central axis (26) of the rotary element is coincident with the axis of rotation (28), respectively axially superimposed with N inner annular magnetised zones (64b, 64c) of the second annular magnet which are located between an external zone (64d) and an internal zone (64a) of the second plurality of annular magnetised zones, these internal and external zones extending radially respectively from both sides of the first annular magnet.