CH714706A2 - Angular return spring for thermo-compensated oscillator. - Google Patents

Abstract

The invention relates to an angular return spring (20) for an assembly with a flywheel for forming an oscillator of a timepiece, characterized in that it comprises an arrangement of at least two distinct parts. (21, 22), assembled in series, whose variations in angular stiffness Ci as a function of the temperature are different, so as to thermo-compensate said oscillator of a timepiece. The invention also relates to an oscillator and a timepiece comprising such a spring, as well as its method of manufacture.

Description

CH 714 706 A2

Description: The invention relates to an angular return spring for an oscillator of a timepiece, as well as an oscillator, a timepiece movement and a timepiece as such which include a such an angular return spring, in particular a spiral spring. Finally, it also relates to a method of manufacturing such a return spring.

The regulation of mechanical watches is based on at least one mechanical oscillator, which generally comprises a flywheel, called a pendulum, and a coil-shaped spring, called a spiral spring or more simply a spiral spring. The hairspring can be fixed at one end to the pendulum axis and at the other end to a fixed part of the timepiece, such as a bridge, called a cock, on which the pendulum axis pivots. The spiral spring equipping the mechanical watch movements of the prior art is in the form of an elastic metal blade or a silicon blade preferably of rectangular section, the major part of which is wound on itself in Archimedes' spiral. The balance-spring oscillates around its equilibrium (or neutral) position. When the balance leaves this position, it arms the balance spring. This creates a return torque which acts on the pendulum to tend to bring it back to its equilibrium position. As it has acquired a certain speed, therefore a kinetic energy, the pendulum exceeds its neutral point until an opposite torque of the hairspring stops it and forces it to turn in the other direction. In this way, the balance spring regulates the period of oscillation of the balance.

The precision of a mechanical watch depends on the regularity of the oscillations of its oscillator formed by the balance and the balance spring. When the temperature varies, the variation of the Young E modulus of the balance spring as well as the thermal expansions of the balance spring and of the balance modify the properties of this oscillator, thus disturbing the precision of the watch. There are solutions of the prior art which try to reduce, or even eliminate, the variations in the operation of an oscillator with temperature. One approach considers that the natural frequency f of such an oscillator depends on the ratio between the constant of the return torque C, corresponding to the angular stiffness of the balance spring, exerted by the balance spring, and the moment of inertia I, more commonly called inertia I, of the latter, by the following relation:

Deriving the previous equation with respect to temperature, we obtain the relative thermal variation of the natural frequency of the oscillator, which is expressed by:

i df 1 1dE fdT 2ÏE dT ^JJ J [0006] Where E is the Young's modulus of the oscillator hairspring, iç / fdT is the thermal coefficient of the oscillator, also simply called by the acronym CT, d £

ΣέΤ is the thermal coefficient of the Young's modulus of the balance spring of the oscillator, also called by the acronym CTE, a _s and a _b are respectively the coefficients of thermal expansion of the balance spring and the balance of the oscillator.

Different solutions of the state of the art seek to cancel the value of the thermal coefficient CT of the oscillator by choosing a CTE of the balance spring adapted for this purpose, to thermo-compensate the oscillator.

[0008] Thus, prior art solutions are based on the use of several materials with different properties distributed in the same hairspring so as to obtain a thermo-compensated oscillator of the oscillator comprising this hairspring.

For example, document EP 1 422 436 describes a solution based on a silicon hairspring comprising an oxide layer. This solution requires a thick oxide layer. Its manufacture requires treating the hairspring for a significant time at very high temperature, which represents a drawback. In general, existing thermally compensated hairsprings are complex to manufacture.

The object of the invention is to provide another return spring solution for a timepiece which allows thermo-compensation of an oscillator in a simple and reliable manner.

To this end, the invention is based on an angular return spring intended for assembly with a flywheel to form an oscillator of a timepiece, characterized in that it comprises an arrangement of at least two separate parts, assembled in series, whose variations in angular stiffness as a function of the temperature are different, so that said oscillator of a timepiece can be thermo-compensated.

The invention is more precisely defined by the claims.

CH 714 706 A2 These objects, characteristics and advantages of the present invention will be explained in detail in the following description of particular embodiments made without implied limitation in relation to the attached figures among which:

Fig. 1 schematically represents an angular return spring in the form of an arrangement in parallel of two hairsprings to constitute an oscillator of a timepiece according to a first embodiment of the invention.

Fig. 2 schematically represents an angular return spring in the form of a series arrangement of two parts of hairspring to constitute an oscillator of a timepiece according to a second embodiment of the invention.

Fig. 3 schematically represents an angular return spring in the form of a variant of an arrangement in parallel of two hairsprings to constitute an oscillator of a timepiece according to the first embodiment of the invention.

The objective of the invention is to provide a thermo-compensated oscillator. For this, a solution is sought which approaches a maximum value of the thermal coefficient (CT) zero for the oscillator, the oscillations of which thus become independent or almost independent of temperature.

The embodiment of the invention is based on a construction of a particular angular return spring, obtained by an arrangement of at least two separate parts, chosen to form an angular return spring which can be coupled with a predefined flywheel to obtain a thermo-compensated oscillator.

The invention will be illustrated in more detail with two embodiments in which an oscillator for a timepiece is in the form of a balance-spring assembly. Each angular return spring of the oscillator comprises an arrangement of two separate parts in series, which can optionally further comprise at least two separate parts in parallel. In these embodiments, each separate part forms a hairspring or a portion of hairspring, comprising one or more turns or portions of turns. We define by turn a portion of the hairspring extending along an angular arc of around 360 °, and by turn portion a portion extending along an angular arc less than 360 °. In addition, each hairspring, turn or portion of turn of these embodiments of the invention is in the form of an elastic blade preferably of rectangular section, wound on itself in an Archimedean spiral. We will denote by e the thickness and h the height of this rectangular section. In addition, we will call L the curvilinear length of a hairspring, turn or portion of turn. This curvilinear length is defined as the distance between two 2 curvilinear abscesses, on the neutral fiber of the coil. Finally, we will call I the inertia of the balance, equal to the product of its mass m by the square of the radius of gyration of the balance r.

The first embodiment is based on an angular return spring 10 of an oscillator, shown in FIG. 1, formed by the parallel arrangement of at least two hairsprings 11, 12, forming at least two distinct parts of said return spring. By the expression “in parallel”, we mean an attachment of each of the at least two hairsprings on the one hand to the balance shaft 5, for example by means of one or more ferrules 6 connected at their central ends, and on the other hand to the balance bridge, not shown, for example by means of one or two pegs connected to their respective peripheral ends 15, 16. This angular return spring acts on a flywheel or balance 1, connected to the shaft 5 by arms 2. The assembly forms a mechanical oscillator. In this embodiment represented by FIG. 1, the two hairsprings 11, 12 form two separate parts which extend generally in two parallel planes. These parallel planes are also parallel to the plane of the pendulum 1 and perpendicular to the pendulum shaft 5. In addition, the two hairsprings 11, 12 are perfectly superimposed. Their two respective ends are superimposed in a direction parallel to the pendulum shaft 5. Alternatively, these ends could be offset. According to another variant, the two parallel hairsprings could only be partially superimposed. They can each include a number of equal or different turns. At least one of these two hairsprings in parallel further comprises at least two parts, not shown, assembled in series.

[0018] FIG. 3 illustrates an alternative embodiment of this first embodiment, in which the two hairsprings 11 ′, 12 ′ are always arranged in parallel, but in the same plane. Their respective two central ends 13 ′, 14 ′ are fixed to the same section of the balance shaft 5 ′ (in the same plane perpendicular to the balance shaft), in two diametrically opposite connection zones. The two peripheral ends 15 ′, 16 ′ are likewise diametrically opposite relative to the balance shaft 5 ′. The two hairsprings 11 ′, 12 ′ are thus entirely positioned in the same plane perpendicular to the pendulum shaft 5. They are nested one inside the other. Their turns are angularly offset by 180 degrees. As a variant, the points of attachment to the balance shaft 5 ′ can be offset by an angle other than 180 and / or the two peripheral ends 15 ′, 16 ′ can be positioned respectively with an angle other than 180 °. At least one of these two hairsprings in parallel further comprises at least two parts, not shown, assembled in series.

In the case of two hairsprings (or turns or portions of turns) arranged in parallel, the contribution of a hairspring to the thermo-compensation is weighted by its contribution relative to the return torque, therefore to the angular stiffness.

CH 714 706 A2 The return torque of the angular return spring is the sum of the couples of the two hairsprings. The natural frequency f of the oscillator can then be written by the following equation:

VS

I = mr ² and where I is the inertia of the balance and Ci the angular stiffness of a hairspring i.

The inertia I of the balance wheel and the angular stiffness Ci of the hairspring i, which is defined for pure bending, are calculated as follows:

Eie ^ hj l _t where m is the mass of the pendulum, r the radius of gyration of the pendulum,

El the elastic modulus of the hairspring material i, ei the thickness of the hairspring blade i, hi the height of the hairspring blade i, and II the curvilinear length of hairspring i.

By introducing the terms I and Ci dependent on the temperature and by deriving the equation (1) with respect to the temperature, we obtain - after rearrangement - the following equation:

df _ 1/1 y / 1 dEi 3 de ^ 1 dh, 1 dl _t ~ fdT ~ 2 l Σί J ^C '\ Ë [~ dT ⁺ ëi ~ dT ⁺ lïi ~ dr ~ T _i dT

By considering homogeneous and isotropic materials, or by using for each material a suitable apparent coefficient of expansion, the coefficient of thermal expansion of the materials a = 1 / x * dx / dT is identical for the directions x explained above. (r, I, e and h). On the other hand, by defining the term CTE as being the thermal coefficient of the elastic modulus 1 / E.dE / dT, the previous equation can be simplified as follows:

Idf

7DT

Σ ^c ^ ^{CTEi +} ~ ^{2α & αι} 'i where a _s , i and a _ba i are respectively the coefficients of thermal expansion of the hairspring i and the pendulum of the oscillator.

In the particular case of the first embodiment comprising two hairsprings (i = 2), the above equation becomes:

idf fdT

{CTE ^ 3a ^) ^

---— (CTE, + 3a ,, + c / ² * ²

(2) The thermal expansion a _ba i of the balance being known and depending on the material used, the materials of the two hairsprings are chosen so as to cancel this equation (2).

Note, in the case of an anisotropic material, for example silicon, the thermal coefficient varies according to the crystal direction of the stress of the material and therefore varies over the length of the hairspring (or of the turn or portion of, turn). Likewise, in the case of a heterogeneous material, such as oxidized silicon, the thermal coefficient varies within the section of the blade. An equivalent or apparent CTE, known to a person skilled in the art, can thus be considered for a hairspring, or a turn or a portion of a turn, formed from an anisotropic and / or heterogeneous material.

In the case of a CuBe2 pendulum, knowing that the thermal expansion of CuBe2 is positive (+ 17 [ppm / ° C]), it is necessary to choose the materials of the two hairsprings so that at least one of the two terms CTE + 3a _s is positive to cancel the equation, in particular that at least one of the two terms CTE + 3a _s is greater than at least twice a _ba i to cancel the equation. It will then suffice to adjust the dimensions of the two hairsprings, in particular their angular stiffness, to obtain the expected result, namely thermo-compensation of the oscillator.

For example, for an oscillator at 4 Hz, we first take a CuBe2 pendulum with an inertia of 14 mg cm ² . Then, we consider a first spiral in monocrystalline silicon, cut in a plane {100}, of properties and following dimensions: Emoyen = 148 MPa, a = 2.6 ppm / ° C, CTE = - 64.3 ppm / ° C, height of 150 microns, active length of 150 mm. We also consider a second hairspring arranged in parallel, in amorphous SiO ₂ , and with the following properties and dimensions: E = 72.4 MPa, a = 0.382 ppm / ° C, CTE = 210 ppm / ° C, and of the same height and same active length as the first hairspring. The frequency of 4 Hz is obtained by choosing C1 + C2 = 8.84 10 ^-7 Nm and the

CH 714 706 A2 thermo-compensation is then ensured by the choice of blade thicknesses at 36.20 microns for the silicon of the first hairspring and at 36.73 microns for the silicon oxide of the second hairspring.

As a second example, still for a 4 Hz oscillator, if we take a CuBe2 balance of inertia 4.7 mg cm ² and a height of 120 microns and a length of 110 mm for each of the two hairsprings, in the same materials as above, equation (2) gives thicknesses of 24.44 microns for the first balance spring in silicon and of 24.80 microns for the second balance spring in amorphous SiO ₂ .

There are of course a multitude of possibilities for arranging two hairsprings in parallel, the dimensions of which are different, so as to cancel equation (2) above and to obtain a thermo-compensated oscillator.

The second embodiment is based on an angular return spring 20 of an oscillator, shown in FIG. 2, formed by the series arrangement of at least two turns or set of turns or portions of turns, which we will simply call at least two separate parts 21,22. By "in series" we mean that the at least two separate parts are joined to each other by one of their ends. A first part 21 can be attached to the balance shaft 5, by its central end 23, by means of a ferrule 6, and at least a second part 22 can be attached to the bridge by its peripheral end 26, by means of 'a piton 27. The two distinct parts 21, 22 are fixed to each other by their respective peripheral 25 and central 24 ends. The angular return spring 20 thus obtained takes the form of a single continuous hairspring, extending in a plane perpendicular to the pendulum shaft 5.

The angular stiffness in pure bending of a blade of non-uniform stiffness over its length L is described by the following equation:

c = ----------------- ¹² Jo £ - (/) e ³ (Z) / i (0 ^dl [0032] For several distinct parts i arranged in series, this equation bECOMES:

i

12ζ ~~ Ξ ~ Τ ^Li Ci [0033] The frequency f of the oscillator corresponds to the equation:

Idf _ 1 / ~ fdT ~ 2I £

1 ^{f 2π} ^ Σ. £ Finally, the derivative of this equation with respect to temperature results in:

~ f ÇCTE _i + 3a _s j) - 2a _bal [0035] As the embodiment comprises two distinct parts, this equation can be written:

Idf 1 / Cj X ^C <, \ 1 In the particular case of a hairspring formed by the arrangement in series of two turns or set of turns or portions of turns, the contribution of each part to thermo-compensation is weighted by the relative stiffness of the other party.

The thermal expansion “bay of the pendulum being known for a given material, the materials of the two distinct parts are chosen so as to cancel this equation (3), in a similar manner to the first embodiment.

For example, if the balance is made of CuBe2, knowing that the thermal expansion of CuBe2 is positive (+ 17 [ppm / ° C]), it is necessary to choose the materials of the two hairsprings so that at least one of the two terms of form CTE + 3a _s is positive to cancel equation (3), in particular that at least one of the two terms CTE + 3a _s is greater than at least twice "bai to cancel equation (3) . It will then suffice to adjust the dimensions of the two separate parts to obtain the expected result, namely thermo-compensation of the oscillator.

CH 714 706 A2 By way of example, we here use a 4 Hz oscillator provided with a CuBe2 balance wheel with an inertia of 14 mg cm ² , as already described above. We consider moreover two distinct parts in series, comprising an identical section of 150 microns in height and 40 microns in thickness, one in monocrystalline silicon cut in a plane {100}, of properties Emoyen = 148 MPa, a = 2.6 ppm / ° C and CTE = -64.3 ppm / ° C, and the other in amorphous SiO ₂ , with properties E = 72.4 MPa, a = 0.382 ppm / ° C and CTE = 210 ppm / ° C. Equation (3) gives curvilinear lengths of turns portions of 88.62 mm and 22.15 mm respectively for each of the two distinct parts of the angular return spring in series.

We consider a second example still at a frequency of 4 Hz with a CuBe2 balance, the inertia of 4.7 mg cm ² and therefore an angular stiffness of the hairspring of 2.9688 10 ^-7 N m. We impose as a constraint that the section (thickness and height) of the two turns or portions of turns is constant and that the total active length is 110 mm. The height of the two turns is 120 microns. In this case, in order to obtain perfect thermo-compensation according to equation (3), the thickness must be 29.88 microns for each of the turns, and the lengths are 88.01 mm for the first portion of silicon. and 21.99 mm for the second portion of the silicon oxide coil.

In a third example, we consider the same pendulum but by imposing a constant bending stiffness along the hairspring, as well as an equally uniform height of 120 microns over the entire length of the hairspring. The total length of the balance spring is also defined at 110 mm. To obtain thermo-compensation of the 4 Hz oscillator, the silicon part must have a thickness of 28.05 microns and a length of 72.81 mm, while the amorphous oxide part must have a thickness of 35.60 microns and a length of 37.19 mm.

This sizing variant has the advantage of giving the hairspring a development during the oscillation equivalent to that of a flat hairspring of constant section and composed of a single material, which is well known.

Naturally, the invention is not limited to the two embodiments described above, nor to the detailed examples, nor even to the simple equations mentioned upstream. Each term of these equations, in particular the CTE, can be replaced by its equivalent taking into account spatial or structural variations (heterogeneous material, anisotropic, non-rectangular section, non-regular section along the length, height of the blade) It appears that there are a multitude of possibilities for dimensioning a return spring for a timepiece according to the invention, making it possible to thermo-compensate an oscillator.

In particular, it is of course possible, in alternative embodiments, to produce an oscillator comprising more than two separate parts, in particular three or four separate parts, of which at least two separate parts are assembled in series. In addition, these separate parts can be arranged in parallel and / or in series, in order to best configure the thermal behavior of the return spring and the oscillator. On the other hand, each separate part can take different forms: a hairspring, one or more turns or portions of turns, these parts may include rectilinear portions. In the case where each separate part is formed by one or more turns or portions of turns, the return spring can be qualified as multi-turns.

On the other hand, the separate parts of the return spring may or may not be in the same plane. A blade of at least a separate part of the return spring, for example in the form of turns or portions of turns, may have a section of any shape, not necessarily rectangular as in the previous embodiments. In addition, this section can remain constant over its entire length, or on the contrary vary. Finally, the return spring can be in the form of a hairspring, as envisaged in the previous examples, or alternatively in any other form.

We finally mean by separate parts of the return spring two separate elements, which are positioned in the same arrangement to together form the return spring. In this arrangement, these separate parts thus participate in a complementary manner in the same return spring function, while providing a thermocompensation effect of the oscillator. These separate parts can be assembled to each other by any means of attachment, or simply positioned nearby. In all cases, these separate parts are arranged so as to be able to cooperate with the same flywheel, and form a single timepiece oscillator. These separate parts are therefore not simply two zones of the same spring which would be in one piece, inseparable, and / or monolithic, even if these two zones may have different materials.

By definition of these separate parts, the invention therefore allows the implementation of a method of manufacturing a return spring which may include a separate, independent manufacturing step, of at least two separate parts, then a step of assembling these at least two distinct parts, optionally by a connection or fixing to one another, in the same arrangement, to form a return spring, more particularly an angular return spring, intended to cooperate with the same flywheel inertia. Alternatively, the invention allows the implementation of a method of manufacturing a return spring in one piece resulting, for example, from the engraving of two wafers of two separate materials previously joined together.

Advantageously, at least two separate parts of the angular return spring are made of two different materials. A separate part can be in one piece. It can be made of a single material, or comprise several different materials, for example comprising zones made of different materials.

Claims (11)

CH 714 706 A2 In all cases, the return spring of the invention comprises at least two separate parts whose variations in angular stiffness Ci as a function of the temperature, CTE + 3a _s , are different, so that said oscillator comprising this return spring of a timepiece is thermally compensated. Naturally, a separate part of the return spring can be found in any other material. Preferably, a material insensitive to magnetic fields is preferred, in order to avoid disturbances in walking linked to the residual magnetization of the components subjected to a magnetic field. At least a separate part of the return spring may include all or part of the monocrystalline silicon, polycrystalline silicon, amorphous silicon, quartz, amorphous silicon oxide, doped silicon, porous silicon, an alloy based on Fe-Ni having a positive Young's modulus (hereinafter CTE) thermal coefficient, and / or an Nb-Zr-O alloy. At least a separate part of the return spring may include one or more isotropic materials. As a variant, it may comprise an anisotropic material, for example silicon, the thermal coefficient of which varies according to the crystal direction of the stress of the material and therefore varies over the length of the hairspring. Silicon can, for example, be coated with a layer of silicon oxide. In the case of a heterogeneous material such as oxidized silicon, the thermal coefficient varies inside the section of the leaf of the return spring. An equivalent or apparent CTE, known to a person skilled in the art, is thus considered for a separate part of the return spring formed from an anisotropic and / or heterogeneous material, and the above calculations remain applicable on this basis. The invention also relates to a timepiece oscillator, a timepiece movement and a timepiece, such as a watch, for example a wristwatch, comprising a return spring as described. previously. The balance is for example made of a copper-beryllium alloy, in a known manner (also called simply CuBe2 alloy), as has been illustrated in the previous embodiments. Alternatively, other materials can be used for the pendulum. claims 1. Angular return spring (10, 10 '; 20) intended for assembly with a flywheel (1) to form an oscillator of a timepiece, characterized in that it comprises an arrangement of at least two distinct parts (11, 12; 11 ', 12'; 21, 22), assembled in series, whose angular stiffness variations Ci as a function of the temperature are different, so as to thermo-compensate said oscillator d 'a timepiece. 2. angular return spring (10, 10 '; 20) according to the preceding claim, characterized in that the at least two separate parts (11, 12; 11', 12 '; 21, 22) are made of two different materials or different combinations of several materials. 3. Angular return spring (10, 10 '; 20) according to one of the preceding claims, characterized in that at least two of the separate parts (11, 12; 11', 12 '; 21, 22) have variations in the angular stiffness Ci as a function of the temperature of opposite signs. 4. Angular return spring (10,10 '; 20) according to one of the preceding claims, characterized in that it comprises at least two separate parts (11, 12; 11', 12 '; 21, 22) in materials chosen in particular from monocrystalline silicon, polycrystalline silicon, amorphous silicon, quartz, amorphous silicon oxide, doped silicon, porous silicon. 5. Angular return spring (10, 10 ') according to one of the preceding claims, characterized in that it further comprises at least two separate parts (11, 12; 11 ’, 12') assembled in parallel. 6. Angular return spring (10 ') according to the preceding claim, characterized in that it comprises at least two separate parts (11', 12 ') arranged in parallel comprising blades located in the same plane. 7. angular return spring (10, 10 '; 20) according to one of the preceding claims, characterized in that the at least two distinct parts (11, 12; 11', 12 '; 21, 22) are present under the form of spiral springs, one or more turns, rectilinear blades, or a combination of spiral springs and rectilinear blades. 8. Angular return spring (10,10 '; 20) according to one of the preceding claims, characterized in that it comprises at least one separate part which has a section varying along the length of said separate part. 9. Oscillator for a timepiece comprising an angular return spring (10, 10 '; 20) according to one of the preceding claims. 10. Oscillator according to the preceding claim, characterized in that it comprises a balance and an angular return spring (10,10 ') comprising at least two distinct parts (11,12; 11', 12 ') arranged in parallel and having angular stiffness Ci which substantially respect the following two equations: CH 714 706 A2 1 Ec \ 2, -rJ l ~ ⁼ i (ς? ^{Σ + 3tï} - ·) - ^2a ^ ô where f is the ^natural frequency of the oscillator, I is the inertia of the pendulum, CTE, represents the thermal coefficient of the elastic modulus of the distinct part i, a _s , i represents the coefficient of thermal expansion of the distinct part i, otbai represents the coefficient of thermal expansion of the pendulum of the oscillator. 11. Oscillator according to claim 9 or 10, characterized in that it comprises a balance (1) and an angular return spring (10, 10 '; 20) comprising at least two distinct parts arranged in series (21, 22) and having angular stiffness Ci which substantially respect the following two equations: 1 df 1/1 v- »1 \ 7 ïït ~ 2 (τζΣ c; ^ ^{CTEi + 3a} ^ ~ ^2abal \ Li ri / \ H /