HK1106570B - Temperature compensated balance-spiral oscillator - Google Patents

Description

Temperature compensated balance wheel/hairspring oscillator

Technical Field

[01] The present invention relates generally to mechanical oscillators, and more particularly to a mechanical oscillator for a wristwatch, which includes a temperature compensation assembly formed by a hairspring and a balance wheel.

Background

[02] The mechanical oscillator of the timepiece, also called governor, consists of a flywheel called balance and a helical spring called balance spring, fixed on the one hand to the balance staff (staff) and on the other hand to a click bridge on which the balance staff pivots. The balance wheel/balance spring oscillates around its equilibrium position at a frequency which must be kept as constant as possible, since it determines the running of the clock. For a homogeneous and uniform balance spring, the oscillation period of this oscillator is given by the following expression:

[03] in the formula:

[04]J_bis the total moment of inertia of the balance wheel/balance spring;

[05]L_srepresents the working length of the balance spring;

[06]E_sis the elastic modulus of the balance spring; and

[07]I_sis the second moment of section (section) of the balance spring.

[08] The temperature change causes the change of the oscillation period to the first stage:

[09]i.e. a pair J_b，L_sAnd I_sExpansion effect of and a pair E_sThe thermoelastic effect of (a). As the temperature increases, the first three terms are generally positive (expansion of the balance, elongation of the balance spring and reduction of the young's modulus) and lead to losses, whereas the last term is negative (increase of the section of the balance spring) and leads to gains. In the past, to alleviate this problem, several approaches have been taken to compensate for temperature drift in frequency. In particular, attention may be paid to a compensation method by thermal correction of the moment of inertia of the balance (for example a bimetallic balance made of steel and brass), or by manufacturing the balance spring using a special alloy (for example a nickel-iron alloy) so as to have a very low thermoelastic coefficient. TheseThe process is still complex, difficult to implement and thus expensive.

[10] More recently, in european patent application EP 02026147.5, the applicant has described a method for thermal compensation of the spring rate constant of a coil spring, comprising thermal oxidation of a balance spring made of a silicon substrate. In the case of a balance spring made of steel of the nickel-iron alloy type, such as steel for housing, e.g. the alloy Nivarox-Far s.a., a coil spring made of oxidized silicon makes it possible to adjust the thermal behaviour of the spring itself, possibly with a slight overcompensation of a few ppm/deg.c. The limit of this overcompensation is due to the maximum oxide thickness produced in practice (typically less than 4 μm) and to the minimum allowable width of the cross section of the silicon hairspring (greater than 40 μm). Therefore, the balance must also be thermally compensated. This can be achieved, for example, by using an alloy of the "glucydur" type (a copper-beryllium alloy, also known as "beryllium"), or by using other alloys with very low coefficients of thermal expansion. This method is also complicated and, only otherwise more conventional, is not capable of correcting other isochronism defects, such as those arising from, for example, various friction effects on the oscillator, balance unbalance, decentering of the centre of mass of the balance spring, etc.

Disclosure of Invention

[11] It is an object of the present invention to overcome the drawbacks of the prior art described above by providing a balance spring for a clock oscillator, the technical solution of which relates to thermal vibrations that make it possible to keep the balance/balance spring assembly as much as possible less dependent on said thermal vibrations. More precisely, the balance spring of the invention is not only automatically compensated, but it can be made so as to also compensate for the thermal drift of the balance.

[12] Another purpose of the invention is to be able to also compensate for the isochronism drawbacks inherent in the structure of the balance/balance spring.

[13] These objects are achieved by an oscillator having the features defined in the claims.

[14] More specifically, the hairspring of the invention is made on a crystalline quartz substrate, the cut piece (cut) of which is selected in such a way that the assembly consisting of hairspring and balance is then thermally compensated.

[15] According to another characteristic of the invention, the shape of the balance spring is chosen so as to compensate for non-isochronous synchronization defects of the balance wheel/balance spring assembly.

[16] Quartz is well known in the field of electronic watches and research has been carried out to use quartz as an oscillator because of the piezoelectric phenomenon. According to the traditional horological vocabulary, the term oscillator is used, however, the term vibration mode applies more. The frequency reached is about 32 kHz. In the operating state, the properties of the quartz crystal used are not necessarily stable, and to overcome this disadvantage, the quartz crystal slices (tangential direction) are chosen such that the various vibration modes are combined, so that a completely stable performance is obtained.

[17] The spiral balance used in mechanical timepieces is now oscillating in nature and this phenomenon is purely due to mechanical reasons. The oscillation frequency is at most about 5Hz.

[18] The properties of quartz are quite different in the two application cases described above. There is no reason for those skilled in the art to use information derived from an electronic watch in a mechanical timepiece. The accumulated knowledge about the quartz oscillator used in the electronic watch cannot in fact be directly transferred to the helical spring.

[19] The thermal behavior of a quartz coil spring is essentially determined by the tilt angle of the slice with respect to the optical axis Z of the quartz crystal. As shown in fig. 1, the plane of the balance spring may be defined by a ZY/phi/theta double rotation (the sign of which is in accordance with the IEEE (institute of electrical and electronics engineers) standard), where phi is the longitude and theta is the latitude complement (the inclination of the axis of the balance spring with respect to the optical axis Z of the crystal).

[20] The rigidity of the crystal, both in tension and in shear, usually has an inverted hot spot (thermal point of inversion) close to 0 ℃ with a negative curvature. They become more rigid at low temperatures. Thus, their first thermal conductivity at room temperature, i.e., 25 ℃, is generally negative, having a negative curvature. It varies from tens to hundreds of ppm/c. Quartz is one of the rare crystals, which makes it possible to compensate for the first thermal conductivity of rigidity by means of the orientation of the slices, i.e. the structures, at room temperature, and even make it positive, with values of a few tens of ppm/c.

[21] Unlike a balance spring made of oxidized silicon or of a nickel-iron alloy steel, a quartz balance spring does not require a compensating balance of the glucydur type. This enables compensation of the thermal drift of most standard bottom-of-the-range (bottom-of-the-range) stainless steel balances, and even in some respects, better than that of 32kHz quartz tuning forks.

[22] The balance/balance-spring oscillator according to the invention also has all or some of the following features:

[23] -said balance spring is made on a quartz substrate, the slice of which is a double ZY/phi/theta rotation slice;

[24] -said balance spring is made on a quartz substrate, the section of which is a single X/theta rotation section;

[25] -said balance spring is made on a quartz substrate, the cut piece of which is a single Y/theta rotation cut piece;

[26] said angle θ is such that the first order thermal conductivity α of said balance spring compensates for the thermal drift of the balance;

[27] said angle θ is such that the curve representing the thermal drift of the balance/balance spring assembly remains contained within the horological model; and

[28] the thickness and, where possible, the pitch of the balance spring is amplitude modulated in order to compensate for isochronism defects of the balance.

Drawings

[29] Other objects, features and advantages of the present invention will become more apparent upon reading the following description, given by way of non-limiting example, and taken in conjunction with the accompanying drawings, in which:

[30] FIG. 1 shows a quartz plate which has undergone a ZY/φ/θ double rotation relative to the crystal axis;

[31] figures 2.a to 2.b show the behaviour of a first α, a second β and a third γ thermal conductivity coefficient of the rigidity of a balance spring manufactured on a plate such as that shown in figure 1, as a function of the angles θ and φ;

[32] figures 3.a to 3.c show horizontal curves of these same thermal conductivities;

[33] FIG. 4 shows a quartz plate which has undergone a single rotation about the X-axis;

[34] figures 5.a to 5.c show the variation of the thermal conductivity α, β and γ of the rigidity of a balance spring manufactured on the plate shown in figure 4;

[35] figure 6 shows the thermal drift of the frequency, in which the X/θ slice of the balance spring matches the coefficient α of the balance; and

[36] figure 7 shows an exemplary embodiment of a balance spring with non-isochronism compensation.

Detailed Description

[37]As indicated above, the thermal behaviour of a quartz balance spring is essentially dependent on the slicing of the plate on which it is made. Thus, for a ZY/φ/θ dual rotation slice, as shown in FIG. 1, the stiff primary, secondary and tertiary thermal conductivities α, β, γ of the balance spring are shown in FIGS. 2.a to 2.c, respectively, at 25 ℃. The vertical axes indicate α, β and γ in ppm/deg.C, ppb/deg.C, respectively²And ppt/. degree C³The values indicated. Fig. 3.a to 3.c show the horizontal lines of the graph in fig. 2. Considering in particular fig. 3.a, it relates to a first thermal conduction systemThe number α, it should be noted that the value of the latter is practically independent of the angle Φ, but varies with the angle θ. Since, moreover, the contribution of the second and third stage thermal conductivity proves to be negligible, it follows that a single rotation slice, for example an X/θ slice, is sufficient for making a balance spring according to the invention, that is to say capable of compensating not only the thermal drift itself but also the thermal drift of the balance associated therewith. A plate with such a slice is shown in fig. 4. It can be obtained by a single rotation theta around the optical axis X of the crystal. The balance spring manufactured on this type of plate will have one of the greatest elastic symmetries, namely the symmetry with respect to the YZ plane and the symmetry with respect to the axis of the balance spring (the Z' axis after rotation). These balance springs will thus be elastically balanced better than those made on a double rotating chipper plate and will have no limit on their thermal compensation capacity. It should be noted that this simple rotation can also be performed about the Y-axis.

[38] Fig. 5.a to 5.b show the variation of the thermal conductivity α, β and γ, respectively, of the rigidity as a function of the angle θ for a balance spring formed by a single rotation slice of X/θ. The coefficients are in practice symmetrical with respect to the axis θ being 0. If only the first coefficient α is considered (other higher-order coefficients have a lower and possibly negligible effect), it should be noted that it is equal to zero when θ is ± 24.0 °, and is the maximum when θ is 0. At this point, α is equal to 13.466ppm/° c, which is consistent with the maximum thermal compensation, which is possible for a balance spring made of quartz with X/θ ═ 0 slices. The thermal drift of the balance depends on the material from which it is made. Thus, the coefficient of thermal expansion of ordinary stainless steel typically varies between 10 and 15 ppm/deg.C, whereas for brass this coefficient has a value of 17 ppm/deg.C. Fig. 6 shows several examples of thermal compensation, which can be achieved by a balance spring with X/theta single rotation slices for a wide variety of balance materials. Curves C1 to C3 show the thermal drift of the frequency of the oscillator, which comprises various types of steel balance, while curve C4 corresponds to the thermal drift of the frequency of an oscillator with brass balance. It should be noted that such X/θ slices of the quartz balance spring may be found to enable compensation of drift of more common balances, such as steel balances, with respect to the timepiece template (frame R) on which the watch/chronograph is mounted (a frequency variation of less than ± 8 s/day in the temperature range of 23 ℃ ± 15 ℃). However, for a balance made of brass (curve C4), the maximum compensation of the quartz balance spring does not make it possible to fully satisfy this requirement of the timepiece template. Thus, for a given balance material, the angle θ of the slice of the quartz balance spring can be determined, which provides the maximum possible thermal compensation of the governor assembly.

[39]According to another characteristic of the invention, the quartz balance spring also makes it possible to compensate for isochronism defects of the oscillator. One of the main causes of non-isochronous synchronization is the variation of the amplitude of the balance wheel. This non-isochronous change may be on the order of a few ppm/degree of angle, typically 2 ppm/degree of angle, typically an angle change of + -25%. One known method for compensating for isochronism consists in acting on the curvature of the end of the balance spring close to the balance-wheel tip (stud) P. This method requires a particularly skilled person to carry out a conditioning step-this is not an optimal situation from an industrial point of view. According to a variant of the invention, it is proposed to act on the local rigidity of the turn (turn) by varying the width of its section. Amplitude modulation has the effect of increasing the inertia and local rigidity of the turns in the sector on the side opposite to the tip. The amplitude-modulation function ratio of the width of the section is, for example, kcos (theta)_m-theta), where k is a proportionality constant, theta represents the polar angle of the section in question, and theta represents the polar angle of the section in question_mIs the value of the polar angle at the balance end. When k is equal to 0.4, the non-isochronous compensation is about 1 ppm/degree of angle. The exact value of k for a given oscillator may be determined empirically or by numerical simulation. Fig. 7 shows a balance spring with such an amplitude modulation over the width of its cross section. The cross-sectional width amplitude modulation of the turns may be accompanied by a pitch amplitude modulation between the turns, so that the difference between the turns remains constant. The latter amplitude modulation (not shown) when large amplitudes are presentOut) enables to prevent adhesion between turns. The balance spring described above may be manufactured by any means known to those skilled in the art of quartz processing, such as wet (chemical) etching or dry (plasma) etching.

[40] Although the present invention has been described with respect to specific exemplary embodiments, it will be appreciated that modifications or variations are possible without departing from its scope. For example, other types of thickness modulation of the turns may be used, such as a linear variation of the thickness of the turns from the centre to the ends of the balance spring, whether or not this is accompanied by an increase in the pitch between the turns.

Claims (7)

1. A mechanical oscillator includes a balance spring and a balance wheel,

the method is characterized in that: the balance spring is made on a quartz substrate, the slice of which is a double ZY/phi/theta rotation slice, wherein the angle theta has a value between-24 deg. and +24 deg., so that the stiff first order thermal conductivity alpha of the balance spring compensates for the thermal drift of the balance wheel associated therewith.

2. The mechanical oscillator according to claim 1, wherein the angle θ is determined such that: the curve representing the thermal drift of the oscillator is still contained within the clock model.

3.A mechanical oscillator according to claim 1 or 2, characterised in that the thickness of the turns of the balance spring is amplitude-modulated to compensate for isochronous defects of the balance.

4. A mechanical oscillator according to claim 3, wherein the thickness amplitude modulation is kcos (θ)_m-theta), where k is a proportionality constant, theta represents the polar angle of the cross-section of the balance spring in question, and theta represents the polar angle of the cross-section of the balance spring in question_mIs the polar angle of the position of the balance spring tip.

5. The mechanical oscillator of claim 4, wherein the proportionality constant is equal to 0.4.

6. A mechanical oscillator according to claim 3, wherein the thickness amplitude modulation is a linear variation in thickness from the centre of the helix to its ends.

7. A mechanical oscillator according to claim 4, wherein the pitch of the turns of the balance spring is such that: the difference between two consecutive turns remains constant.