MANUFACTURING METHOD OF A THERMALLY COMPENSATED OSCILLATOR

Description

[0001] The invention relates to a method of manufacturing an oscillator for a timepiece made from a balance wheel and at least two parts of a spring, arranged in parallel.

[0002] The regulation of mechanical watches relies on at least one mechanical oscillator, which generally comprises a flywheel, called a balance wheel, and a spirally wound spring, called a balance spring or simply a hairspring. The hairspring may be fixed at one end to the balance staff and at the other end to a fixed part of the timepiece, such as a bridge, on which the balance staff pivots. The hairspring in state-of-the-art mechanical watch movements is in the form of an elastic metal blade or a silicon blade, preferably rectangular in cross-section, most of which is wound around itself in a spiral. The balance wheel and hairspring oscillate around its equilibrium position (or dead center). When the balance wheel leaves this position, it winds the hairspring. This creates a restoring torque that acts on the balance wheel, tending to return it to its equilibrium position.As it gains speed, and therefore kinetic energy, the balance wheel passes its neutral point until a counter-torque from the balance spring stops it and forces it to rotate in the opposite direction. In this way, the balance spring regulates the period of oscillation of the balance wheel.

[0003] The accuracy of a mechanical watch depends on the regularity of the oscillations of its oscillator, formed by the balance wheel and balance spring. The frequency of these oscillations is determined by the inertia of the balance wheel and the stiffness of the balance spring, and there are several ways to precisely achieve a given frequency, for example, by using regulating elements that influence the spring's stiffness, or by properly matching a particular balance wheel with a specific balance spring. This matching also minimizes the necessary adjustments.

[0004] However, when the temperature varies, the respective thermal expansions of the balance spring and the balance wheel alter the properties of this oscillator, thus disrupting the accuracy of the watch.

[0005] There are state-of-the-art solutions that attempt to reduce, or even eliminate, the temperature-dependent variations in the operating behavior of an oscillator. One approach considers that the natural frequency f of such an oscillator depends on the ratio between the restoring torque constant C, corresponding to the angular stiffness of the balance spring, exerted by the spring on the balance wheel, and the moment of inertia of the latter, according to the following relationship: f = 1 2 π C I

[0006] By differentiating the previous equation with respect to temperature, we obtain the relative thermal variation of the oscillator's natural frequency, which is expressed as: 1 f df dT = 1 2 1 E dE dT + 3 α s − 2 α b where E is the Young's modulus of the oscillator's spiral, 1 f df dT is the thermal coefficient of the oscillator, also simply referred to by the acronym CT, 1 E dE dT is the thermal coefficient of the Young's modulus of the oscillator's spiral, also called by the acronym CTE, α s and α b are respectively the coefficients of thermal expansion of the spiral and the balance wheel of the oscillator.

[0007] Various state-of-the-art solutions seek to cancel the value of the thermal coefficient CT of the oscillator by choosing a CTE of the spiral adapted for this purpose, to thermo-compensate the oscillator.

[0008] Document CH705127A2 describes a thermo-compensated balance spring resonator. This balance spring is associated with two parallel spiral springs. Each spiral spring has a temperature-dependent thermal coefficient in the opposite direction to the other spiral spring, each spiral being designed to simultaneously achieve satisfactory thermo-compensation and elastic properties.

[0009] Document EP3088969A1 describes a thermo-compensated spiral consisting of alternating sections of a spiral made of different materials: monocrystalline silicon, silicon oxide, and polycrystalline silicon. It also discloses a variable cross-section of the spiral.

[0010] Document EP2063325A2 addresses the technical problem of balancing a balance spring. To this end, it describes a balance spring comprising two spiral springs arranged in the same plane and mounted in parallel.

[0011] The aim of the invention is to provide an oscillator for a watch part resulting from a method of combining its components which allows both to guarantee the thermo-compensation of the oscillator but also to improve the matching of the balance wheel and its angular return spring to reliably achieve a given oscillation frequency.

[0012] To this end, the invention is based on the association of an angular return spring comprising an arrangement of at least two distinct parts whose variations in angular stiffness as a function of temperature are different, with a balance wheel, whose inertia can be known, to form an oscillator of a timepiece, so that said oscillator of a timepiece can be thermo-compensated while oscillating at a given frequency without requiring adjustment, or at least minimizing the necessary adjustment operations.

[0013] Thus, the invention is based on a method of manufacturing an oscillator of a watch part made from a balance wheel and at least two parts of spring, arranged in parallel, according to the attached claim 1.

[0014] According to an embodiment close to the invention, the process comprises the following steps: Choose the materials for the balance wheel and the aforementioned parts of the spiral spring, these parts being able to be made of different materials; Choose the frequency f of the oscillator; Measure or consider the inertia I of the balance wheel; Calculate the angular stiffness Ci of each part i of the spring such that f = 1 2 π ∑ i C i I and which also cancel the equation 1 f df dT = 1 2 ∑ i C i CTE i + 3 α s , i ∑ i C i − 2 α bai ; Select parts i of the spring whose respective angular stiffnesses are close to the calculated values ​​Ci; Assemble said at least two selected parts of the spring to the balance wheel.

[0015] The invention is defined by the claims.

[0016] The objects, features and advantages of the present invention will be set forth in detail in the following description of particular embodiments made by way of non-limiting agreement in relation to the attached figure which schematically represents an oscillator of a timepiece comprising an angular return spring of the spiral spring type, with parallel arrangement of two spiral parts according to an embodiment of the invention.

[0017] The objective of the invention is to provide a temperature-compensated oscillator requiring little or no adjustment. To achieve this, a solution is sought that closely approximates a thermal coefficient (TC) value of zero for the oscillator, whose oscillations thus become independent or nearly independent of temperature. This solution also involves a combination of a flywheel, which we will call a "balancer," and a return spring to guarantee the required frequency, which is predetermined and therefore fixed.

[0018] The embodiment of the invention which will be described is based on the construction of an oscillator obtained by the association of at least two distinct angular return spring parts, chosen to form an angular return spring which can be coupled with a predefined balance wheel to obtain a thermo-compensated oscillator, and more specifically with a balance wheel whose inertia has been measured in such a way as to guarantee the chosen natural frequency of the oscillator regardless of the temperature.

[0019] The invention will be illustrated in more detail with an embodiment, represented by the figure 1in which a clockwork oscillator is presented in the form of a balance wheel and spring assembly. The angular return spring 10 of the oscillator is in the form of a "double spiral," comprising a parallel arrangement of two spirals 11, 12. By "parallel," we mean that each of the two spirals is attached on one side to the balance staff 5, for example by means of one or two ferrules 6, and on the other side to the balance bridge, for example by means of one or two pins connected to their respective peripheral ends 15, 16. This angular return spring acts on a flywheel or balance wheel 1, connected to the staff 5 by arms 2. The assembly forms a mechanical oscillator.

[0020] In this embodiment, each distinct part of the angular return spring is formed by a spiral or a portion of a spiral, comprising one or more turns or portions of turns. A turn is defined as a portion of the spiral extending along an angular arc of approximately 360°, and a portion of a turn as a portion extending along an angular arc of less than 360°. Furthermore, each spiral, turn, or portion of a turn in these embodiments of the invention is in the form of an elastic blade, preferably with a rectangular cross-section, wound upon itself in a spiral. We will denote e the thickness and h the height of this rectangular cross-section. In addition, we will denote L the curvilinear length of a spiral, turn, or portion of a turn. This curvilinear length is defined as the distance between two curvilinear abscissas on the neutral axis of the turn. Finally, we will denote r the radius of gyration of the balance wheel and m its mass.

[0021] A set of spirals consists of spirals, turns, or portions of turns made from the same material. A batch of spirals, turns, or portions of turns is a set of spirals, turns, or portions of turns considered identical except for slight geometric variations in manufacturing.

[0022] Here, "material" refers either to a single homogeneous material or to a composite material resulting from a combination of materials arranged in a given way. For example, a single-crystal silicon spiral coated on all sides with a silicon oxide of a given thickness is considered a composite material.

[0023] In the case of two spirals (or turns or portions of turns) arranged in parallel, the contribution of one spiral to thermo-compensation is weighted by its relative contribution to the restoring torque, therefore to the angular stiffness.

[0024] The restoring torque of the return spring is the sum of the torques of the two spirals. The natural frequency f of the oscillator can then be written by the following equation: f = 1 2 π C 1 + C 2 I where I is the inertia of the balance wheel and Ci the angular stiffness of a spiral i.

[0025] In the general case, equation (1) becomes: f = 1 2 π ∑ i C i I .

[0026] The moment of inertia I and the angular stiffness Ci, which is defined for pure bending, are calculated as follows: I = mr 2 et C i = E i e i 3 h i 12 l i where m is the mass of the pendulum, r the radius of gyration of the pendulum,

[0027] Ei the elastic modulus of the material of spiral i, ei the thickness of the blade of spiral i, hi the height of the blade of spiral i, and li the curvilinear length of spiral i.

[0028] By introducing the temperature-dependent terms I and Ci and differentiating equation (1) with respect to temperature, we obtain - after rearrangement - the following equation: 1 f df dT = 1 2 1 ∑ i C i ∑ i dC i dT − 2 r dr dT

[0029] Therefore, in the case of a spiral, equation (2) becomes: 1 f df dT = 1 2 1 ∑ i C i ∑ i C i 1 E i dE i dT + 3 e i de i dT + 1 h i dh i dT − 1 l i dl i dT − 2 r dr dT

[0030] Considering homogeneous and isotropic materials, or using a suitable apparent coefficient of expansion for each material, the coefficient of thermal expansion of the materials α = 1 / x*dx / dT is identical for the x directions specified above (r, L, e, and h). Furthermore, by defining the term CTE as the thermal coefficient of the elastic modulus 1 / E.dE / dT, the preceding equation can be simplified as follows: 1 f df dT = 1 2 ∑ i C i CTE i + 3 α s , i ∑ i C i − 2 α bal where α s,i and α bal are respectively the coefficients of thermal expansion of the spiral i and the balance wheel of the oscillator.

[0031] Since this first embodiment comprises two spirals (i=2), the previous equation becomes: 1 df f dT = 1 2 C 1 C 1 + C 2 CT E 1 + 3 α s , 1 + C 2 C 1 + C 2 CT E 2 + 3 α s , 2 − 2 α bal

[0032] Once the balance wheel material has been defined, the value of αbal is known, and the results satisfying equation (3) can be represented by a straight line in the plane [C1; C2]. There are therefore numerous pairs of solutions C1, C2 that will satisfy this equation. A skilled craftsman will be able to choose the materials for the balance wheel and for the balance spring(s) (or coils or portions of coils) appropriately to find solutions to this equation.

[0033] If the inertia of the balance wheel is known, it will then be possible, in the same plane, to draw, using equation (1), another line corresponding to its zero point. It will then suffice to choose sections of the balance spring whose angular stiffnesses C1 and C2 correspond to the intersection of these two lines to guarantee the realization of a temperature-compensated oscillator at the required frequency, and then assemble the two sections to form the balance spring, which can then be assembled with the balance wheel to form the oscillator.

[0034] As a point of note, in the case of an anisotropic material, such as silicon, the thermal conductivity varies according to the crystal direction of the material's stress and therefore varies along the length of the spiral (or turn or portion of a turn). Similarly, in the case of a heterogeneous material, such as oxidized silicon, the thermal conductivity varies within the cross-section of the blade. An equivalent or apparent thermal conductivity, known to those skilled in the art, can thus be considered for a spiral, or a turn or portion of a turn, formed from an anisotropic and / or heterogeneous material.

[0035] For example, if the balance wheel is made of CuBe2, and given that the thermal expansion of CuBe2 is positive (+17 ppm / °C), the materials for the two balance springs must be chosen so that at least one of the two terms CTE+αs is positive to cancel the equation; specifically, at least one of the two terms CTE+αs must be greater than at least twice αbal to cancel the equation. Then, simply adjusting the dimensions of the two balance springs, particularly their angular stiffness, will suffice to obtain the desired result, namely thermal compensation of the oscillator.

[0036] As an example, for a 4Hz oscillator, we first take a CuBe2 balance wheel with an inertia measured at 14.28 mg·cm 2. Then, we consider a batch of S1 spirals in monocrystalline silicon, cut in a {100} plane, with the following properties and dimensions: Emoyen=148 GPa, α=2.6 ppm / °C, CTE=-64.3 ppm / °C, height of 150 microns, active length of 150 mm and whose thickness is close to 36.5 µm. We also consider a second batch S2 of amorphous SiO2 spirals, with the following properties and dimensions: E=72.4 GPa, α=0.382 ppm / °C, CTE=210 ppm / °C, and of the same dimensions as the spirals of the first batch, which will be assembled in parallel on the balance wheel to make the oscillator.

[0037] The 4 Hz frequency will be obtained by selecting, from two sets of balance springs, a first spring S1a with an angular stiffness C1 of 5.97 × 10⁻⁷ Nm (corresponding to a thickness of 36,445 µm), and a second spring S2a with an angular stiffness C2 of 3.05 × 10⁻⁷ Nm (corresponding to a thickness of 36,975 µm). These two springs will be assembled to form a balance spring, which will in turn be assembled to the balance wheel to form a temperature-compensated oscillator with a frequency of 4 Hz.

[0038] For a composite material as defined above, the CTE is therefore calculated from the CTEs of each of the materials used, weighted by the geometric arrangement of the different materials, in a way known to the person skilled in the art.

[0039] The thermal coefficient of the oscillator, given by equation (3), allows, after rearrangement, the definition of the target value of CTE 2 + 3α 2: CTE 2 + 3 α s 2 = 2 α bal 2 πf 2 I − C 1 CTE 1 + 3 α s 1 2 πf 2 I − C 1

[0040] Thus, if we know the stiffness and temperature variation (CTE + 3α) of a balance spring, it becomes possible to select a second balance spring to cancel out the CT. These two balance springs can then be paired with a balance wheel of suitable inertia to achieve the target frequency.

[0041] As an example made of composite materials, for a 4 Hz oscillator consisting of a CuBe2 balance wheel with a measured inertia of 14.12 mg·cm², we consider a batch of single-crystal silicon spirals cut in the {100} plane, coated on all sides with a 3.5 µm thick layer of amorphous silicon dioxide (SiO₂) to form a composite spiral with a height of 157 microns, an active length of 150 mm, and a thickness close to 39 µm. The angular stiffness of these spirals is measured individually, and the total stiffness of all the spirals falls within a range close to 4.5 × 10⁻⁷ Nm.

[0042] The CTE of these composite spirals can be calculated from the CTE values ​​of silicon and amorphous oxidized silicon as well as the geometry of the spiral (respective thicknesses of silicon and oxide).

[0043] The 4 Hz frequency will be obtained by selecting from this batch of composite balance springs a first spring with an angular stiffness C1 of 4.8665·10⁻⁷ Nm and a second spring with an angular stiffness C2 of 4.0533·10⁻⁷ Nm. These two springs will be assembled to form a balance spring, which will in turn be assembled to the balance wheel to form a thermo-compensated oscillator with a frequency of 4 Hz.

[0044] A variant of this first embodiment relies on an angular return spring for an oscillator formed by the parallel arrangement of three spirals from at least two sets of spirals, forming three distinct parts of said return spring. This variant increases the number of possible combinations of spiral parts, thus eliminating the need for oscillator adjustment and / or allowing the use of spiral parts with very different properties and / or dimensions. It also enables second-order thermocompensation, useful when using materials whose thermal coefficient variations with temperature also have opposite signs.

[0045] Thus, the invention has the advantage of making it possible to manufacture a high-performance oscillator for a watch part, by increasing the flexibility of matching the balance wheel and the balance spring in order to precisely achieve a given oscillation frequency, making it possible to limit, or even eliminate, subsequent adjustments, while forming an oscillator that maintains its given oscillation frequency despite temperature changes.

[0046] Naturally, the invention is not limited to the embodiments described above, nor to the detailed examples, nor even to the simple equations mentioned earlier. In particular, it may comprise more than three distinct return spring parts.

[0047] On the other hand, the distinct parts of the return spring may or may not lie in the same plane. A blade of at least one distinct part of the return spring, for example, in the form of coils or portions of coils, may have a cross-section of any shape, not necessarily rectangular as in the previous embodiments. Moreover, this cross-section may remain constant along its entire length, or, conversely, vary. Finally, the distinct parts of the return spring may be in the form of a spiral, as considered in the previous examples, or alternatively in any other form, notably in the form of a straight blade.

[0048] Ultimately, by distinct parts of the return spring, we mean two or more separate elements positioned in the same arrangement to form the return spring. In this arrangement, these distinct parts contribute in a complementary way to the same return spring function, while also providing a thermal compensation effect for the oscillator. They are chosen to combine this thermal compensation effect with the precise selection of the oscillator frequency. These distinct parts can be joined together by any means of fastening, or simply positioned in close proximity. In all cases, these distinct parts are arranged so that they can cooperate with the same flywheel and form a single oscillator for the timepiece.These distinct parts are therefore not simply two zones of the same spring which would be a single piece, inseparable, and / or monolithic, even if these two zones may have different materials.

[0049] Once it is possible to combine at least two spirals to create a thermocompensated oscillator, it also becomes possible to combine said two spirals with a particular balance wheel to adjust the frequency of the oscillator.

[0050] The balance wheel, for example, is made of a copper-beryllium alloy, in a known manner (also simply called CuBe2 alloy), as illustrated in the preceding embodiments. Alternatively, other materials can be used for the balance wheel.

[0051] Advantageously, at least two distinct parts of the angular return spring are made of two different materials. One distinct part may be a single piece. It may be made of a single material, or it may comprise several different materials, for example, including areas made of different materials.

[0052] In all cases, the return spring of the invention comprises at least two distinct parts whose variations in angular stiffness Ci as a function of temperature, according to the term CTE+3αs, compensate for the thermal expansion of the associated balance wheel, so that said oscillator comprising this return spring of a timepiece is thermo-compensated.

[0053] Naturally, a separate part of the return spring can be made of any other material. Preferably, a material insensitive to magnetic fields is preferred, to avoid operating disturbances related to the residual magnetization of components subjected to a magnetic field.

[0054] At least one distinct part of the return spring may comprise all or part of monocrystalline silicon regardless of its orientation, polycrystalline silicon, amorphous silicon, quartz, amorphous silicon oxide, doped silicon regardless of the type and level of doping, porous silicon, an Fe-Ni based alloy having a positive Young's modulus thermal coefficient (hereinafter CTE), and / or an Nb-Zr-O alloy.

[0055] At least one distinct portion of the return spring may comprise one or more isotropic materials. Alternatively, it may comprise an anisotropic material, such as silicon, whose thermal coefficient varies according to the crystal direction of the material's stress and therefore varies along the length of the coil. The silicon may, 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 within the cross-section of the return spring leaf. The term dC / dT, or the equivalent or apparent CTE, known to those skilled in the art, is thus considered for a distinct portion of the return spring made of an anisotropic and / or heterogeneous material, and the preceding calculations remain applicable on this basis.

[0056] The embodiment described above defines several steps that are performed in the order described. However, the invention can alternatively be implemented using different approaches, including reversing the order of some of the steps described.

[0057] The described process includes the prior selection of a pendulum, whose inertia I is then measured.

[0058] Furthermore, the materials for the balance wheel and spring components are also advantageously chosen beforehand from among materials possessing natural properties known to those skilled in the art, so as to guarantee the successful implementation of the subsequent steps. Alternatively, however, the material can be chosen a posteriori or simultaneously with the selection of other parameters, such as the geometry, of the balance wheel and / or spring components.

[0059] Thus, many other embodiments can be defined, based on a method of manufacturing an oscillator of a watch part according to the attached claim 1.

[0060] The process advantageously includes a preliminary step consisting of choosing the materials for the balance wheel and spring parts.

[0061] The selection step (b) advantageously includes the selection of a pendulum with inertia I and spring sections with angular stiffnesses C i which satisfy the following equations: f = 1 2 π ∑ i C i I And 1 f df dT = 1 2 ∑ i C i CTE i + 3 α s , i ∑ i C i − 2 α bal = 0

[0062] Thus, the invention offers the advantage of enabling the manufacture of a high-performance oscillator for watch components, by increasing the flexibility in matching the balance wheel and balance spring to precisely achieve a given oscillation frequency. This flexibility is further enhanced by the ability to select the balance wheel and spring components from existing batches, eliminating the need for specific manufacturing for each new watch assembly.

Claims

1. A process for manufacturing a timepiece oscillator made up of a balance and of at least two spring portions that are arranged in parallel, characterized in that it comprises the following steps: a. choosing the frequency f of the oscillator; b. choosing a balance among an existing batch, which inertia I is then measured, and spring portions, by selecting the most appropriate existing spring portions among at least two batches or at least two sets of existing spring portions, so that the inertia of the balance and the angular stiffnesses of the spring portions allow an oscillator of chosen frequency f to be formed and so that the variations in angular stiffness of the spring portions as a function of temperature are able to thermo-compensate the oscillator; c. assembling said chosen spring portions with the chosen balance.

2. The process for manufacturing an oscillator as claimed in the preceding claim, characterized in that the choice of step b) comprises the individual measurement of the angular stiffness of existing spring portions among said at least two batches or at least two sets of existing spring portions.

3. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that it comprises a prior step consisting in choosing the materials of the balance and of the spring portions.

4. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that it comprises a prior step consisting in choosing at least two spring portions being made of different or same materials.

5. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that the choosing step, step (b), comprises choosing a balance of inertia I and spring portions of angular stiffnesses Ci that respect the following equations: f = 1 2 π ∑ i C i I and 1 f df dT = 1 2 ∑ i C i CTE i + 3 α s , i ∑ i C i − 2 α bal = 06. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that the balance is made of CuBe2.

7. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that step (b) of choosing the spring portions consists in selecting at least two spring portions having variations in their angular stiffness Ci as a function of temperature of opposite signs.

8. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that the spring portions are made of materials in particular chosen among single-crystal silicon whatever its crystal orientation, polysilicon, amorphous silicon, quartz, amorphous silicon oxide, doped silicon whatever the dopant type and concentration, an alloy based on Fe-Ni possessing a positive CTE, and / or an Nb-Zr-O alloy.

9. The process for manufacturing an oscillator as claimed in one of the preceding claims, characterized in that the spring portions take the form of a turn segment or of one or more turns, of rectilinear blades, or of a combination of turn segments, of turns and rectilinear blades, and / or in that one or more spring portions have a cross section that varies over their length.

10. The process for manufacturing an oscillator as claimed in one of claims 1 to 8, characterized in that the choosing step, step (b), comprises the following steps: - choosing a balance and measuring or estimating its inertia I; then - determining angular stiffnesses Ci for each portion i of the spring such that f = 1 2 π ∑ i C i I and that the following equation also equal zero 1 f df dT = 1 2 ∑ i C i CTE i + 3 α s , i ∑ i C i − 2 α bal = 0 ; then - choosing spring portions which respective angular stiffnesses are close to the angular-stiffness values Ci.