MECHANICAL OSCILLATOR WITH ISOCHRONISM CORRECTION

Description

technical field

[0001] The present invention relates to the field of mechanical oscillators for a watch movement. More particularly, the present invention relates to a mechanical oscillator for correcting the isochronism defect of the oscillator. State of the art

[0002] A flexible-guided oscillator consists of a pendulum (inertia) and a flexible pivot that serves the dual function of elastic return and rotational guidance of the pendulum. Such a flexible-guided oscillator is described, for example, in document EP3299905B1. The flexible pivot is composed of a plurality of flexible elements, often blades, connected to each other by rigid elements. The elastic blades are manufactured using processes such as deep reactive ion etching. Deep Reactive Ion Etching, DRIEIn the case of silicon, which allows for blade thicknesses accurate to ±0.5 µm, such a variation in thickness can influence not only the oscillator frequency but also its isochronism error (frequency variation when the oscillation amplitude varies). Since the thickness error is not systematic, it may be necessary to adjust the isochronism of the pivot. Furthermore, the blades of a flexible pivot can be distributed across several parts assembled manually or assembled at the wafer level (as with silicon). Assembly inaccuracies can also lead to an isochronism error. In all cases, this isochronism error manifests as a change in the relationship between the elastic restoring torque of the flexible pivot and the pivot's rotation angle.

[0003] Thus, correcting this isochronism defect involves acting on the elastic torque, typically by adding a spring in parallel with the flexible pivot. The isochronism can then be adjusted by changing the stiffness of this second spring or by preloading it. This correction is delicate because it is difficult to adjust the isochronism without degrading another property of the flexible pivot, such as its sensitivity to gravity. The oscillator's natural frequency can also be affected, but this can be adjusted subsequently, typically by modifying the inertia of the balance wheel. It should also be noted that isochronism is a fundamental property for an oscillator because it is powered by a mechanical torque that varies over time, which affects the amplitude of the balance wheel. If the oscillator is not isochronous, its frequency will vary over time, and the watch will therefore be inaccurate.

[0004] Document EP2273323 describes a mechanical oscillator that swings around a pivotless axis and comprises several elastic systems connecting the web and the mounting portion. This oscillator is characterized in particular by having at least one preload system for tensile or compressive tension on the blades of a return element. The preload system allows the oscillator's natural frequency to be adjusted via an additional spring that can be tensioned to varying degrees.

[0005] Document EP3722888 describes a mechanical oscillator including a compensation system for correcting the oscillator's isochronism. The compensation system is an additional spring system that can be adjusted using a sliding slider.

[0006] Document WO2017157870 describes a monolithic oscillator comprising a support and an inertial regulating element connected to the support by an elastic suspension. The elastic suspension includes an additional elastic adjustment link with one end adjustable relative to the support, so as to modify the overall stiffness of the elastic suspension and thus the oscillation frequency of the inertial regulating element.

[0007] Document CH715024 describes a mechanical oscillator comprising at least one base arranged to be fixed to a plate or bridge of a clock movement, and at least one inertial element arranged to oscillate about a virtual pivot axis fixed in position relative to the base, in a pivot plane perpendicular to the virtual pivot axis. Each inertial element is suspended from the base by several flexible links, each comprising at least one elastic blade, and the flexible links together define the virtual pivot axis.

[0008] Document EP3410229 describes a watch component comprising a moving part, a support, and a flexible pivot connecting the moving part to the support to guide the moving part's rotation relative to the support around a virtual axis of rotation. The flexible pivot is arranged to elastically return the moving part to a rest position.

[0009] The oscillator adjustment system described above acts asymmetrically and can unbalance the pivot axis. Brief summary of the invention

[0010] The present invention relates to a mechanical oscillator for a watch movement. The oscillator comprises a fixed part intended to be connected to a fixed element of the movement, a balance wheel coaxial with a pivot axis, and at least three flexible links configured so that the balance wheel can oscillate in a plane of oscillation substantially perpendicular to the pivot axis. Each flexible link extends radially from the pivot axis and connects the fixed part to the balance wheel. Each flexible link comprises a first flexible element and a second flexible element, substantially coplanar in an axial plane parallel to the pivot axis. One end of the first flexible element is connected to the fixed part, and one end of the second flexible element is rigidly connected to the balance wheel. The other end of the first and second flexible elements is connected by a rigid link.The oscillator includes a preload element configured to apply a preload to at least one of the rigid links in a substantially radial direction.

[0011] The preload applied by the preload element modifies the relationship between elastic torque and angle of rotation, minimizing the oscillator's isochronism defect within its normal operating amplitude range. The preload element acts symmetrically and does not unbalance the pivot axis. The oscillator's sensitivity to gravity is maintained when the preload is applied.

[0012] Unlike existing isochronism correction systems, the preload element adds virtually no extra bulk to the oscillator and is simple to manufacture. The preload element can be added to an existing oscillator without requiring complex assembly steps. Brief description of the figures

[0013] Examples of implementation of the invention are given in the description illustrated by the accompanying figures, in which: there figure 1 illustrates a mechanical oscillator comprising flexible links and a coupling frame with arcs, according to one embodiment; the figure 2 represents a detail of a portion of an arc connected to a flexible joint, as well as a preload element, according to one embodiment; and the figure 3 shows an isochronism curve of the oscillator including flexible links, not exhibiting a thickness defect (curve C1), exhibiting an uncorrected thickness defect (curve C2), and exhibiting a thickness defect corrected by the preload element (curve C3). Example(s) of an embodiment of the invention

[0014] There figure 1This illustrates a mechanical oscillator for a watch movement, according to one embodiment. The oscillator comprises a fixed portion 15 intended to be fixed to a fixed portion of the watch movement (not shown). The fixed portion 15 includes a fixed central portion 30. An inertial element 10 (for example, a balance wheel) is mounted coaxially with a pivot axis A. Such an oscillator is described in patent EP3299905B1 of the present applicant.

[0015] A rigid link 16, supporting the inertial element 10, extends radially between a movable central portion 31 and the inertial element 10. In the illustrated example, the fixed central portion 30 and the movable central portion 31 form a hub around the pivot axis A. The rigid link comprises three radially arranged arms 16, 17. More specifically, each arm comprises a first arm 16 connected in series with a second arm 17 via an annular structure 18. Other arrangements of the rigid link are also possible without changing the scope of application of the mechanical oscillator. For example, the rigid link may comprise only one or more arms 16 rigidly and directly connecting the movable central portion 31 to the inertial element 10.

[0016] The mechanical oscillator further includes an elastic restoring element 20 configured so that the inertial element 10 can oscillate in a plane of oscillation P o substantially perpendicular to the pivot axis A. According to the illustrated embodiment, the elastic restoring element has three flexible links 20. Each flexible link 20 extends radially with respect to the pivot axis A so as to connect the fixed part 15 to the inertial element 10 (via the fixed central portion 30).

[0017] More specifically, each flexible joint 20 comprises a first flexible element 21 and a second flexible element 22 substantially coplanar with the first flexible element 21 in an axial plane Pa parallel to the pivot axis A (and perpendicular to the oscillation plane Po). One end of the first flexible element 21, near the pivot axis A, is connected to the fixed central portion 30, and the end of the second flexible element 22 is connected to the rigid joint 16. The first and second flexible elements 21, 22 are connected together at their other ends by a rigid joint 50.

[0018] According to one embodiment, the mechanical oscillator includes a preload element 60 configured to apply a preload on at least one of the rigid links 50 in a substantially radial direction.

[0019] To minimize the oscillator's isochronism error, the stiffnesses of the first and second flexible elements 21, 22 must be identical. In practice, when the first and second flexible elements 21, 22 are blades, a difference in thickness between these blades, due to manufacturing tolerances, exists and produces a difference in stiffness that induces an isochronism error. This error must be minimized over the oscillator's typical operating amplitude. This difference in stiffness modifies the relationship between the elastic torque and the oscillator's angle of rotation. The applied preload allows for modification of this relationship, thus minimizing the oscillator's isochronism error within a range of normal operating amplitudes.

[0020] According to a form of execution illustrated in the figure 1The oscillator comprises a coupling frame 40 having three arcs 41 connected to each other at their ends. Each arc 41 includes an inner coupling blade 42 and an outer coupling blade 43 substantially coplanar in the oscillation plane Po. In this configuration, the preload element 60 is configured to preload at least one of the arcs 41. The preloaded arc 41 then applies the preload to at least one of the rigid links 50 in a substantially radial direction.

[0021] The preload element 60 can be configured to preload one or all of the arcs 41. Each preloaded arc 41 then applies the preload to at least one of the rigid links 50 in a substantially radial direction (towards the fixed central portion 30).

[0022] The preload element 60 can be configured to deform the outer coupling blade 43 in bending.

[0023] It should be noted that the coupling frame 40, even in the absence of the preload element 60, prevents the flexible links 20 from being twisted. Indeed, if the flexible links 20 twist, for example during the rotation of the inertial element 10, the oscillator's sensitivity to gravity is reduced.

[0024] There figure 2 represents a detail of a portion of an arc 41 showing the inner coupling blade 42 connected to the first flexible element 21 and the second flexible element 22 (not visible on the figure 2via the rigid connection 50. The preload element comprises an intermediate element 60 inserted between the inner coupling blade 42 and the outer coupling blade 43, forming the arc 41. The intermediate element 60 applies a preload to the arc 41 by bending the outer coupling blade 43. The intermediate element 60 can be a volume element with a constant cross-section. A lateral dimension of the volume element can be predetermined to adjust the degree of deformation of the outer coupling blade 43 and obtain a desired preload. For example, the intermediate element 60 can be a cylindrical element (such as a disc) with a predetermined diameter.

[0025] In the figure 2The continuous inline spacer 60 has a diameter that causes minimal deformation of the outer coupling blade 43 and applies a low preload. The larger diameter spacer 60', shown in dashed lines, produces greater deformation of the outer coupling blade 43' and applies a higher preload.

[0026] The spacer element 60 can be made of silicon or any other precision-machined material. Spacer elements 60 of varying diameters allow the arc 41 to be preloaded according to the magnitude of the isochronism error to be corrected. Once the correction has been made, the spacer element 60 can be attached to the oscillator.

[0027] Alternatively, the spacer element 60 may include a volume element having an adjustable lateral dimension. Such a spacer element 60 may include, for example, an eccentric screw. In this case, the preload can be adjusted when the spacer element 60 is inserted between the inner coupling blade 42 and the outer coupling blade 43. The spacer element 60 may also include a volume element with a variable cross-section, for example, a frustoconical volume. In this case, the applied preload can be adjusted by moving the spacer element 60 between the inner coupling blade 42 and the outer coupling blade 43, for example, in a direction substantially perpendicular to these blades 42 and 43.

[0028] According to one embodiment, the intermediate element 60 can be inserted at the rigid connection 50. In this configuration, the preload force, exerted by the intermediate element 60 on the arc 41, is transmitted to the first and second flexible elements 21, 22 via the rigid connection 50.

[0029] The spacer element 60 can be configured to allow the preload applied to each of the flexible links 20 to be modified independently. For example, the diameter of the spacer element 60 can be adjusted independently for each of the flexible links 20. Applying a substantially identical preload to each of the flexible links 20 is often preferable in order to avoid negatively impacting the oscillator's sensitivity to gravity. For example, a simple axisymmetric adjustment can be achieved with identical volume elements 60 for each flexible link 20 in order to avoid unbalancing the pivot and to maintain a gravity sensitivity identical to that of the pivot without preload. However, if the correction is small, it is possible to preload only one of the flexible links 20.

[0030] The intermediate element 60 can be configured to apply the preload to only one of the first and second flexible elements 21, 22 of the flexible link 20.

[0031] There figure 3Compare an isochronism curve of the oscillator comprising flexible links 20 with no thickness defect (curve C1) with an isochronism curve of the oscillator whose flexible links 20 have a thickness defect of ±0.5 µm and without correction (curve C2). Here, each of the first and second flexible elements 21, 22 comprises a blade. A thickness defect of ±0.5 µm corresponds to a defect relative to the target nominal thickness; for example, a blade of the first and / or second flexible elements 21, 22 of one of the flexible links 20 is 0.5 µm thicker, and a blade of the first and / or second flexible elements 21, 22 of another flexible link 20 is 0.5 µm thinner. The isochronism curve of the oscillator indicated by C3 corresponds to the oscillator having the same thickness defect as for curve C2 and including the preload element 60.The preload element 60 corrects the oscillator's isochronism defect so that the oscillator's rate variation is substantially zero (less than approximately 1%) for a range of oscillator amplitudes during normal operation. In the example of the... figure 3 , the oscillation amplitude range of the oscillator in normal operation is between 16° and 20°. Reference numbers used in the figures

[0032] 10 Inertial element, pendulum 15 Fixed part 16 First arm 17 Second arm 18 Annular structure 20 Elastic return element, flexible links 21 First flexible element 22 Second flexible element 30 Fixed central portion 31 Moving central portion 40 Coupling frame 41 Arc 42 Inner coupling blade 43, 43' Outer coupling blade 50 Rigid link 60, 60' Intermediate element A Pivot axis P a Axial plane P o Oscillation plane

Claims

1. Mechanical oscillator for a watch movement, the oscillator comprising: a fixed part (15) intended to be connected to a fixed element of the movement; an inertial element (10) coaxial with a pivot axis (A); at least three flexible links (20) configured so that the inertial element (10) can oscillate in an oscillation plane (Po) substantially perpendicular to the pivot axis (A); each flexible link (20) extending radially with respect to the pivot axis (A) and connecting the fixed part (15) to the inertial element (10), each flexible link (20) comprising a first flexible element (21) and a second flexible element (22), substantially coplanar in an axial plane (Pa) parallel to the pivot axis (A); a first end of the first flexible element (21) being connected to the fixed part (15) and a first end of the second flexible element (22) being rigidly connected to the inertial element (10); a second end of the first flexible element (21) and the second flexible element (22) being connected by a rigid link (50); characterized in that the oscillator comprises a preload element (60) configured to apply a preload to at least one of the rigid links (50) in a substantially radial direction.

2. The oscillator according to claim 1, comprising a coupling frame (40) having at least three arcs (41) connected to each other at their ends, each arc (41) comprising an inner coupling blade (42) and an outer coupling blade (43) substantially coplanar in the oscillation plane (Po), the second flexible element (22) being connected to the rigid link (50) substantially at the center of the arc (41); the preload element (60) being configured to apply the preload to at least one of the arcs (41) in the substantially radial direction.

3. The oscillator according to claim 2, wherein the preload element (60) is configured to flexibly deform the outer coupling blade (43).

4. The oscillator according to claim 3, wherein the preload element (60) comprises a spacer element (60) between the inner coupling blade (42) and the outer coupling blade (43).

5. The oscillator according to claim 4, wherein the spacer element (60) is configured to apply the preload independently to each of the flexible links (20).

6. The oscillator according to claim 4 or 5, wherein the spacer element (60) is configured to apply a substantially identical preload to each of the flexible links (20).

7. The oscillator according to any one of claims 4 to 6, wherein the spacer element (60) is configured to apply the preload to only one of the first and second flexible elements (21, 22) or both flexible elements (21, 22).

8. The oscillator according to any one of claims 4 to 7, wherein the spacer element (60) is inserted at the rigid link (50).

9. The oscillator according to claim 4 or 8, wherein the spacer element (60) comprises a volume element having a predetermined lateral dimension so as to obtain a desired preload.

10. The oscillator according to claim 9, wherein the volume element is of constant cross-section.

11. The oscillator according to claim 4 or 8, wherein the spacer element (60) comprises a volume element having an adjustable lateral dimension.

12. The oscillator according to claim 11, wherein the volume element has a variable cross-section.

13. The oscillator according to any of claims 4 to 12, wherein the spacer element (60) is made of silicon.