CH708068B1 - of self-compensating balance spring watch. - Google Patents

Abstract

Spiral spring (1) intended to equip a mechanical pendulum oscillator of a timepiece movement or other precision instrument, and comprising a rod (2) of silicon comprising an outer surface (3), and having a first thermal coefficient of the module Young (CTE), characterized in that the spiral spring (1) further comprises material for at least partially compensating for the first CTE of silicon and characterized in that the material is added either in the form of a coating ( 8) comprising a metal or an alloy and at least partially covering the outer surface (3) of the spiral spring (1); under the outer surface (3) of silicon of the spiral spring (1).

Description

Technical area

The present invention relates to a spiral for mechanical oscillator balance-spiral watch movement or other precision instrument. More particularly, the present invention relates to an autocompensated spiral spring.

State of the art

The regulating organ of the mechanical watches is conventionally composed of a flywheel, called balance and a spiral spring, called spiral or spiral spring, fixed by one end on the axis of the balance and by the another end on a bridge, called cock, in which pivots the axis of the balance. More specifically, the spiral spring equipping, to date, the movements of mechanical watches is an elastic metal blade of rectangular section wound on itself spiral Archimedes and comprising from 12 to 15 turns.

The sprung balance oscillates around its equilibrium position (or dead point). When the pendulum leaves this position, it arms the hairspring. This creates a return torque which, when the balance is released, returns it to its equilibrium position. As he has acquired a certain speed, therefore a kinetic energy, he exceeds his dead point until the opposite pair of the hairspring stops him and forces him to turn in the other direction. Thus, the spiral regulates the period of oscillation of the balance.

The accuracy of the mechanical watches depends on the stability of the natural frequency of the oscillator formed of the sprung balance. When the temperature varies, the thermal expansions of the balance spring and balance, as well as the variation of the Young's modulus of the spiral, modify the natural frequency of this oscillating assembly, disturbing the accuracy of the watch.

Most of the methods proposed to compensate for these frequency variations are based on the consideration that this natural frequency depends exclusively on the ratio between the constant of the return torque exerted by the balance spring on the balance and the moment of inertia of the latter. , as shown in the following relation: F = 1 / 2π (C / 1) <0> <.> <5> (1) or: F = the natural frequency of the oscillator, C = the constant of the restoring moment exerted by the spiral of the oscillator, and I = the moment of inertia of the pendulum of the oscillator.

For example, since the discovery of Fe-Ni based alloys having a thermal coefficient of Young's modulus (hereinafter CTE) positive, the thermal compensation of the mechanical oscillator is obtained by adjusting the CTE of the spiral in function of the coefficients of thermal expansion of the spiral and the balance. Indeed, by expressing the torque and the inertia from the characteristics of the balance spring and the balance, then by deriving the equation (1) with respect to the temperature, one obtains the thermal variation of the natural frequency: 1 / F dF / dT = 1/2 (1 / E dE / dT + 3cs-2cb) (2) or: 1 / E dE / dT = CTE: is the thermal coefficient of the Young's modulus of the hairspring, cs = the coefficient of thermal expansion of the hairspring, and cb = the coefficient of thermal expansion of the pendulum.

By adjusting the self-compensation term A = 1/2 (CTE + 3cs) to the value of the coefficient of thermal expansion of the pendulum cb, it is possible to cancel equation (2). Thus, the thermal variation of the natural frequency of the mechanical oscillator can be eliminated. In equation (2), the CTE of the hairspring is in practice much higher than its coefficient of thermal expansion, and this coefficient can be neglected.

Currently, complex alloys are used, both by the number of components and by the metallurgical processes used for the purpose of obtaining a self-compensation of the variations of the modulus of elasticity of the metal by combining two contrary influences: that of the temperature. and that of magnetoconstriction (contraction of magnetic bodies under the effect of magnetization).

[0009] However, the spirals made of these alloys are difficult to manufacture. First of all, because of the complexity of the processes used to make the alloys, the intrinsic mechanical properties of the metal are not constant from one production to another. Then, the setting of the regulating organ, which is the technique to ensure that the watch indicates at all times the most accurate time, is tedious and long. This operation requires many manual interventions and many defective parts must be eliminated. For these reasons, production is expensive and maintaining consistent quality is an ongoing challenge.

In JP 6 117 470, a spring-shaped spiral is made of monocrystalline silicon. It is dimensioned to have a constant return torque, to provide a very accurate electrical measuring device. However, this document is silent as to the thermal stability of the constant of the return torque of this spring. It can not be used directly as a spiral spring in a timepiece.

In DE10 127 733, a spiral spring is made of monocrystalline silicon coated with silicon dioxide so as to obtain good stability of the spiral shape with temperature variations. The thermal stability of the constant of the return torque of this spring is also not mentioned in this document.

The silicon CTE is strongly influenced by the temperature and a compensation of this effect is necessary for its use in horological applications. Indeed, the silicon CTE is of the order of -60 x 10 <-> <6> / ° C and the thermal drift of a spiral spring made of silicon is thus about 155 seconds / day, for a variation temperature of 23 ° C +/- 15 ° C. This makes it incompatible with watchmaking requirements which are of the order of 8 seconds / day.

EP1 422 436 discloses a spiral spring cut into a plate (001) of monocrystalline silicon. The hairspring comprises a layer of SiO 2, having a CTE opposite to that of silicon and formed around the outer surface of the hairspring, in order to minimize the thermal drift of the hairspring assembly.

However, the presence of a relatively thick layer, around 6% the width of the spiral spring, results in a spiral with a dark and matte surface state, having an unsightly appearance.

Brief summary of the invention

An object of the present invention is to provide a spiral spring device without limitations of the state of the art.

Another object of the invention is to provide a spiral spring for equipping a mechanical oscillator balance-spiral watch movement or other precision instrument, formed of a spiral spring silicon having an outer surface and having a first temperature coefficient of Young's modulus (CTE) compensated.

According to the invention, these objects are achieved in particular by a spiral spring further comprising material for at least partially compensating for the first thermal coefficient of the Young's modulus of silicon, the material being added under the outer surface of the bar of silicon.

According to another embodiment of the invention, the material is added in the form of one or more oxide layers, for example SiO 2, under the outer silicon surface of the spiral spring.

Still according to another embodiment of the invention, the material is added in the form of one or more layers of doped silicon, under the outer silicon surface of the spiral spring.

According to another embodiment of the invention, the compensated spiral spring comprises a diamond coating or DLC (diamond-like carbon) covering at least partially its outer surface.

This solution has the advantage over the prior art to obtain a silicon spiral whose sensitivity to thermal variations and magnetic fields is minimized. In addition, the silicon spirals are easily machinable and conformable, and allow low manufacturing costs. The spiral spring of the invention does not require the presence of a relatively thick layer of oxide on its outer surface can have an aesthetic surface appearance.

Brief description of the figures

Examples of implementation of the invention are indicated in the description illustrated by the appended figures in which:

FIG. 1 illustrates a spiral spring according to the invention;

FIG. 2a shows a segment of the spiral spring in longitudinal section according to one embodiment of the invention;

FIG. 2b shows a segment of the spiral spring in cross-section according to one embodiment of the invention;

FIG. 3 shows a segment of the spiral spring according to another embodiment of the invention; and

FIG. 4 shows a segment of the spiral spring still according to another embodiment of the invention.

Example (s) of embodiment of the invention

The spiral of the invention, shown in Figs. 1 to 4, comprises a bar 2 comprising an outer surface 3, and spirally cut in a silicon wafer using a wet or dry machining method, such as plasma machining or by the DRIE method. (Deep Reaction Ion Etching). The silicon may be monocrystalline, with an orientation such as (001), (111) or other, or be polycrystalline. Silicon has a negative first thermal coefficient of Young's modulus (CTE) and is non-magnetic. The bar preferably has a rectangular section.

In the example of FIGS. 2a and 2b, which show a segment of the spiral spring 1 in longitudinal and transverse section, respectively, of the material is added in the form of a coating 4 composed of a metal or a metal alloy, and covering at least partially the outer surface 3 of the spiral spring 1. In this example, the coating 4 is composed of a Fe-36% Ni alloy having a second CTE substantially positive so as to compensate for the first CTE of the silicon. Preferably, the alloy is of Invar <®> type and contains certain impurities such as, for example, carbon and chromium. The coating 4 has a second CTE of opposite sign to the first CTE of the silicon so as to compensate, at least partially, the first CTE of the silicon.

The coating 4 may also be composed of other types of alloys. For example, an alloy of the Elinvar type (Fe-36% Ni-12% Cr), or Kovar type (Fe-28% Ni-18% Co) can be used, insofar as the coating 4, consisting of one of these alloys makes it possible to compensate at least partially the first CTE of silicon.

In one example, the spiral spring 1 of silicon is coated at least partially on its outer surface 3 of an alloy based on niobium and zirconium, for example, Nb-5% -25% Zr.

In another example, the spiral spring 1 is coated at least partially on its outer surface 3 with a coating 4 made of stainless and non-magnetic metal such as gold, platinum, rhodium, palladium, etc., having a second substantially positive CTE.

The alloy or the metal can be produced by a vapor phase deposition process such as PVD deposition, but can also be carried out using various known methods, such as sputtering, ion implantation or deposition. electrolytic.

In another embodiment of the invention illustrated in FIG. 3, which shows a segment of the spiral spring 1 in longitudinal section, the spiral spring 1 comprises added material in the form of one or more metal layers 5, two in the example of FIG. 3, located in the volume of the spiral spring 1, inside the outer surface 3 of silicon, in other words, under the outer surface 3 of silicon. In this configuration, each of the metal layers 5 is separated from the other by silicon 6. The metal layers 5 have a second CTE of opposite sign to that of the silicon CTE so as to at least partially compensate for the first CTE of the silicon. The number and thickness of the metal layers 5 can be determined so as to at least partially compensate for the first CTE of the silicon.

In yet another embodiment of the invention, the spiral spring 1 comprises added material in the form of one or more oxide layers 7 according to the configuration of FIG. 3. Each of the oxide layers 7 has a second CTE of opposite sign to the first CTE of the silicon, so as to compensate, at least partially, the first CTE of the silicon. In this configuration, the oxide is inside the outer surface 3, allowing the spiral spring 1 to have a free silicon surface 3 of a thick and unsightly oxide layer.

[0036] Alternatively, an oxide coating 8 may also be added so as to at least partially cover the outer surface of the spiral spring 1.

The oxide may be SiO 2, zirconium oxide, or any other oxide to obtain one or more layers 7 or a coating 8 capable of at least partially compensating for the first CTE of the silicon. The layer or layers 7 or coatings 8 may also be silicon nitride or silicon carbide.

The oxide may be formed using various known techniques including, for example, thermal oxidation, vapor phase deposition, ion implantation, flame hydrolysis, and the like.

For example, the spiral spring 1 having the configuration of FIG. 3 can be formed by alternately depositing silicon with an LPCVD process, and oxidizing with a PECVD process. Silicon can also be formed by a PVD process.

In one variant of the embodiment, the oxide is doped so as to modify the second CTE of the oxide, by adjusting the doping level thereof. The doping element may comprise a non-metallic element such as boron, phosphorus, nitrogen, carbon, etc., or a metal element, or a mixture of these elements. The doping can be carried out by a chemical diffusion process or by ion implantation. An annealing step may be performed to densify the layer 7 or doped oxide coating 8.

In yet another embodiment of the invention, the spiral spring 1 comprises added material in the form of a doping element so as to produce one or more doped silicon layers 9 according to the configuration of the invention. fig. 3. The doping level of the silicon can be adjusted to modify the second CTE of the doped silicon so as to compensate, at least partially, the first CTE of the silicon.

In one variant of the embodiment, the silicon may also be doped in a region at least partially covering the outer surface of the spiral spring 1.

In another variant of the embodiment, all the silicon of the spiral spring 1 is doped.

The silicon may be doped with a non-metallic element comprising, for example, boron, phosphorus, nitrogen, carbon, etc., or a metal element, such as Fe, Ni, Co, Zr, or others, or a mixture of these elements. The doping can be carried out by a chemical diffusion process or by ion implantation.

Still in another embodiment illustrated in FIG. 4, which shows a segment of the spiral spring 1 in longitudinal section, the spiral spring 1 is manufactured, in a first step, by machining a first silicon layer (not shown) by a known technique so as to create one or more cavities ( not shown), for example non-through, in the thickness of this first layer. In a second step, the cavities are filled with a doped metal or alloy, oxide or silicon, so as to produce structures having a second CTE of opposite sign to the first CTE of the silicon. In a third step, a second silicon layer (not shown) is deposited on the first silicon layer comprising the structures 10. The cavities may be machined with a rectangular shape, spherical or otherwise, using a method of known machining. The above steps can be repeated more than once, so as to produce a composite spiral spring 1 having several structures 10, two rows of several structures 10 in the example of FIG. 4, in a silicon matrix, capable of compensating, at least partially, the first CTE of silicon. The structures 10 are disposed inside the outer surface 3 of the spiral spring 1.

In yet another embodiment not shown, the spiral spring 1 comprises layers 5, 7, 9 made of different materials, for example, an alternation of layers 7 composed of an oxide of different nature of a layer. to the other, or layers of oxides 7 alternated with metal layers 5, etc.

The spiral spring 1 of the invention may also comprise a coating of diamond or DLC (not shown) at least partially covering the outer surface 3 or the coating 4, 8. Such diamond coating or DLC is advantageous for its mechanical properties such as hardness, low coefficient of friction, impact resistance, as well as for decorative purposes. For example, the diamond-coated or DLC-coated spring will thus be less brittle. The surface of the diamond coating or DLC can be oxygenated, hydrogenated or fluorinated, or doped with one or a mixture of the elements already mentioned above. The diamond or DLC coating may be deposited directly on the silicon or on an intermediate layer of oxide, for example SiO 2.

It goes without saying that the present invention is not limited to the embodiment which has just been described and that various modifications and simple variants can be envisaged by the skilled person without departing from the scope of the present invention. .

For example, other materials than those mentioned above may also be added in the forms and configurations described above or according to other configurations; to the extent that these materials and configurations are capable of at least partially compensating for the first silicon CTE.

Still in another embodiment of the invention not shown, the spiral spring 1 is made of a composite material comprising at least two materials having a first and second CTE opposed, respectively where the added material takes, for example, the shape of a frame in the silicon matrix.

The spiral spring 1 may also be composed of a metal matrix consisting of an alloy such as Invar <®>, Elinvar or Kovar, and comprising particles of silicon and / or an oxide , such as SiO2, silicon nitride or silicon carbide. The particles, having a CTE substantially opposite that of the matrix, can take the form of spheres, lamellae or have any other forms.

Reference numbers used in the figures

[0052] <Tb> 1 <September> spiral <Tb> 2 <September> Bar <tb> 3 <SEP> outer surface of the bar <tb> 4 <SEP> metallic coating <tb> 5 <SEP> metal layer <Tb> 6 <September> silicon <tb> 7 <SEP> oxide layer <tb> 8 <SEP> oxide coating <tb> 9 <SEP> doped silicon layer <Tb> 10 <September> structure <tb> CTE <SEP> Young's modulus thermal coefficient

Claims (15)

1. Spiral spring (1) intended to equip a mechanical pendulum oscillator clockwork movement or other precision instrument, and comprising a rod (2) of silicon comprising an outer surface (3), and the bar having a first thermal coefficient of Young's modulus CTE, characterized in that the spiral spring (1) further comprises material for at least partially compensating for the first thermal coefficient of the Young's CTE module of silicon and characterized in that the material is added under the outer surface (3) of the bar (2) of silicon. 2. Spiral spring according to claim 1, characterized in that the material is added at least partly in the form of at least one layer (5, 7, 9), under the outer surface (3) of silicon. 3. Spiral spring according to claim 1, characterized in that the material is added at least partly in the form of structures (10) filling cavities formed under the outer surface (3) of silicon. 4. Spiral spring according to claim 3, characterized in that the structures (10) are spherical or rectangular. 5. Spiral spring according to one of claims 1 to 4, characterized in that the added material comprises a metallic material. 6. Spiral spring according to claim 1, characterized in that the added material comprises an oxide. Spiral spring according to Claim 6, characterized in that the oxide is SiO 2. 8. Spiral spring according to one of claims 6 to 7, characterized in that the oxide is also doped by at least one doping element. 9. Spiral spring according to claim 1, characterized in that the added material comprises silicon with at least one doping element therein. 10. Spiral spring according to claim 8 or 9, characterized in that the doping element is a non-metallic or metallic element, or a mixture of these elements. Spiral spring according to one of Claims 8 to 10, characterized in that the doping level of the oxide or silicon is adjusted so as to modify the CTE of the doped oxide or silicon. 12. Spiral spring according to one of claims 1 to 11, comprising a diamond coating or DLC covering at least partially the outer surface (3) of the spiral spring (1). 13. Timepiece comprising a spiral spring according to the features of one of claims 1 to 12. 14. A method of manufacturing a spiral spring according to one of claims 6 to 9, comprising a step of forming the oxide by a thermal process or a chemical vapor deposition process. 15. A method of manufacturing a spiral spring according to one of claims 8 to 11, comprising a doping step performed by a chemical diffusion process or by ion implantation.