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    • 专利摘要:
    A silicon balance spring (100) as a biasing member for an oscillator of a mechanical timepiece and having an oscillation frequency, said member exerting a return torque comprising a spiral spring body (110) having a number N of turns and an inner end (115) intended to be engaged in a rotating inertial element by a ferrule (117), and an outer end intended to be engaged in a stationary element of the cock type, and has a width (140), a height and a total arc length; wherein the spiral spring body (110) comprises a core formed of a monocrystalline silicon wafer oriented along the <110> crystallographic axis; and wherein the body of the spiral spring (110) has at least one peripheral coating of a material having a thermoelastic constant different from that of the core of the spiral spring body (110) to maintain the oscillation frequency of an oscillator including the element exerting a return torque substantially insensitive to changes in ambient temperature.
    公开号:CH710795A2
    申请号:CH00161/16
    申请日:2016-02-08
    公开日:2016-08-31
    发明作者:Ching Ho；Hang Ko Pui
    申请人:Master Dynamic Ltd；
    IPC主号:
    专利说明:

    Technical area
    [0001] The invention deals with balance springs for balance wheels, in particular balance springs for timepieces made of silicon and a particular construction thereof.
    Technical background of the invention
    [0002] The balance spring is a key component of a mechanical timepiece. It is one of the two main components of an oscillator, the other being formed by the balance. The oscillator of a timepiece provides a means of regulating time through a simple harmonic movement of it. The balance acts as an inertial element and is attached to an inner end of a coil-shaped balance spring. The outer end of the balance spring is typically rigidly attached to a fixed eye. In an ideal oscillator, the balance spring provides a return torque to the balance which is proportional to the angular displacement of the balance wheel from a position of equilibrium, and equations of motion describe it as a second order linear system. .
    [0003] The equilibrium position is defined as the angular position of the balance in which when the oscillator is static, the net torque applied by the balance spring to the balance is zero. An oscillator is considered isochronous when its natural frequency is independent of its amplitude and other external factors, such as temperature variations, magnetic fields, etc. Since the precision of a timepiece is largely determined by the stability of the natural frequency of the oscillator, isochronism is one of the most important properties of a mechanical timepiece.
    [0004] Historically, the balance spring has been regarded as one of the most difficult components to manufacture for a timepiece, especially for mechanical movements used in watches. It is required that it tends and relaxes continuously at a frequency typically between three and five Hertz, this frequency range constituting that of a modern oscillator for a mechanical timepiece, over the entire duration of life of the timepiece before maintenance, typically several years. The balance spring is also one of the smallest components of a mechanical movement comprising a hairspring band with a thickness typically in the range of 30 to 40 microns.
    [0005] A balance spring must also be made of materials which resist the effects acting on their intrinsic properties due to temperature variations, and more particularly their Young's modulus, in order to maintain a correct measurement of time and to minimize fluctuations.
    [0006] Furthermore, due to the increased number of electric and magnetic fields due to the proliferation of the number of electronic devices, a modern balance spring must also be able to withstand or significantly minimize the effect that magnetic fields can. exercise on this one. To this end, the precision and constancy of the rigidity of a balance spring prove to be demanding parameters, since even 0.1% variation in rigidity can result in an inaccuracy of up to one minute per day for the part. watchmaking, which would be unacceptable for the watchmaking industry in charge of manufacturing such a part. This is the reason why much effort has historically been made in the watch industry to provide balance springs which minimize such effects by means of manufacturing technologies or particular shapes.
    [0007] Traditional balance springs are made of metal alloys, starting with the hardened steel used by John Harrison almost 300 years ago, until the Elinvar invented by Charles Guillaume in 1919 and more recently Nivarox invented by Dr. HC Reinhard Straumann. Almost all modern balance springs consist of some variations of Nivarox, which is an alloy based on iron and nickel. Balance springs are made using a wire drawing process in which the strip of material is stretched to form a thin wire. The straight strips are then wound into a hairspring before being processed to stabilize the shape of the hairspring. However, this process has many disadvantages including: (a) The fact that a wire drawing process is typically not a very precise technique, its tolerance being around a few microns, i.e. a significant percentage of the thickness of the wire. the band of the balance spring typically being in the range of 30 to 40 microns, which results in irregularities in the stiffness; (b) The fact that metal alloys such as Nivarox have an inherent tendency to creep and deform slightly with use under prolonged stress, such that the metal balance spring cannot maintain its original shape in balance spring after more than a year of continuous operation, which may require adjustment and inevitably affects the accuracy of time measurement, and (c) Although the thermoelastic constant and magnetic susceptibility of Nivarox materials have been substantially reduced in Comparison of earlier metal balance springs by intensive doping with trace elements such as chromium, these problems and drawbacks could not be completely eliminated.
    [0008] In order to deal with or minimize the above-mentioned problems relating to Nivarox and other metal alloys as well as their manufacturing methods for balance springs, the last decade has seen the introduction of the use of silicon and micro-fabrication techniques for the fabrication of balance springs.
    [0009] Silicon balance springs are made using a micro-fabrication process which achieves sub-micron precision providing a significantly better degree of precision than with conventional metal shaping techniques such as wire drawing. The use of silicon includes the following advantages: (a) Such a material is not subject to creep or oxidation over time compared to most metal alloys, thus making it possible to maintain its mechanical properties and its integrity, (b) Such a material is completely non-magnetic, and (c) Temperature sensitivity can be minimized or considerably eliminated under normal conditions of use, by a balance spring with a silicon core covered with 'a thin layer of silicon dioxide, such that the net thermoelastic constant of the balance spring approaches zero.
    [0010] The use of the technology of manufacturing silicon balance springs has consequently demonstrated several advances during the last decade, including that disclosed in document DE 10 127 733 of June 7, 2001 in which is disclosed the The use of micromechanical silicon springs, where the silicon is monocrystalline and lies in the <100> or <111> plane, each of these two orientations being considered as appropriate in an equivalent manner. The spring is a spiral spring with good resistance to large-scale temperature stresses as well as good shape stability. It is also disclosed therein that a coating of silicon dioxide can also cover the springs.
    [0011] Document EP 1 422 436 of June 25, 2002 describes a method for reducing the thermal drift of a balance spring for a timepiece taken in isolation, in order to obtain a temperature coefficient approaching zero. The method and device use a spiral spring intended to equip the balance of a mechanical timepiece and is formed by a hairspring cutting rod from a monocrystalline silicon wafer <100> having thermoelastic constants of first and second order, the turns of the coil of the spring having a width "w" and a thickness "t", and where a coating of silicon dioxide makes it possible to minimize the thermal coefficients of the spring constant of the spiral spring. The spiral spring described thus ideally comprises a modulation of the width of the spring.
    [0012] Document EP 2 224 293 of April 29, 2004 discloses a timepiece movement comprising a regulating device comprising a balance and a planar balance spring which may be formed from silicon. The planar balance spring comprises, at its outermost turn, a reinforced portion arranged such that the deformations of the turns are substantially concentric. The reinforced portion ends before the outer end of the balance spring, and is characterized in that the spacing between the end portion of the outer end of the outermost coil and the penultimate coil of the balance spring is sufficiently large so that the penultimate turn can remain free radially during the expansions of the balance spring up to amplitudes corresponding substantially to the maximum angle of rotation of the balance in the movement. This helps to maintain the concentricity of the balance spring in use, and thus maintains good timing properties. It is disclosed therein that the silicon balance spring can be formed using the method of patent document EP 0 732 635. This patent, in the name of Patek Philippe, is known to disclose the structure of its Spiromax® balance spring, as discussed below.
    [0013] In 2006, Patek Philippe publicly unveiled its Spiromax® balance spring made of Silinvar®. This balance spring is obtained by a vacuum oxidation process which compensates for temperature variations. The concentric nature (the symmetrical expansions and contractions of the balance spring relative to its center) is made possible by a terminal curve which is not coiled but rather has a noticeably thicker region at its outer end, as described and claimed in patent EP 2 224 293.
    Document EP 2 215 531 of November 28, 2007 describes a mechanical horological oscillator, comprising a spiral spring formed of monocrystalline silicon (Si) oriented along the crystallographic axis <111>, and which has a coating selected to obtain a variation as a function of temperature, of the resistance torque of the spiral spring, compensating for the variation as a function of temperature at the level of the moment of inertia of the balance. This document uses a monocrystalline silicon material of axis <111> in the same way as that with which monocrystalline silicon <001> is used in EP 1 422 436, and a coating to provide a balance spring insensitive to the shock. temperature, also in the same way that the Spiromax balance spring is formed from Silinvar with an oxide coating to provide a balance spring unaffected by temperature changes.
    [0015] Newer silicon balance springs are also available, however, their commercial use is limited, and in view of the limited use of silicon balance springs for a limited period of time spanning less than Over ten years uncommon compared to metal alloy balance springs, which have been used prolifically for several decades, the long-term reliability and integrity of silicon-formed balance springs have not had the possibility of being estimated and compared with those of the industrial standards which are the balance springs in metal or in metal alloy, such as the balance springs in Nivarox metal alloy.
    Object of the invention
    [0016] An objective of the present invention is to provide a silicon balance spring which makes it possible to fill, or at least reduce, certain deficiencies associated with the solutions of the prior art.
    Brief summary of the invention
    [0017] According to a first aspect, the present invention provides an element exerting a return torque for an oscillator of a mechanical timepiece having an oscillation frequency, said element exerting a return torque comprising a spiral spring body having a number N of turns with an inner end intended to be engaged in a rotary inertia element) by means of a ferrule, and an outer end intended to be engaged in a stationary element of the rooster type, and has a width, a height and a total arc length; the spiral spring body comprising a core formed of a monocrystalline silicon wafer oriented along the crystallographic axis <110>; and the body of the spiral spring comprising at least one peripheral coating of a material having a thermoelastic constant different from that of the core of the body of the spiral spring in order to maintain the oscillation frequency of an oscillator including the element exerting a torque of reminder substantially insensitive to variations in ambient temperature.
    Preferably, the body of the spiral spring is substantially of rectangular section. Preferably, the body of the spiral spring has a width of between 20 μm and 60 μm, a height of between 100 μm and 400 μm, and a total arc length of between 50mm and 200mm. Preferably, the body of the spiral spring has a number of turns between 5 and 20.
    The peripheral coating of the balance spring is formed from silicon dioxide and preferably has a thickness of between 3 μm and 6 μm.
    [0020] In one embodiment, the angle of orientation of the <110> monocrystalline silicon wafer produces slight variations in the overall stiffness of the balance spring which can be used for fine-tuning the stiffness. The body of the spiral spring preferably has a width varying cyclically along at least a portion of the total arc length based on the coil in order to compensate for variations in section stiffness of the balance spring due to the anisotropy of the coil. Young's modulus in the plane of the wafer. The width for the <110> monocrystalline silicon wafer may vary depending on the equation:
    Here, S11, S12, and S44 are elements of the compliance matrix of monocrystalline silicon defined as being equal to 7.68, –2.14, and 12.6, respectively, in these units expressed in 10 <–> <12> Pa <- > <1>. The term θ is the angle of orientation in the plane of the wafer. The coil spring body may be formed by a fabrication technique including Deep Reactive Ion Etching (DRIE).
    According to another embodiment, the longitudinal ridges formed at the intersection of the faces of the height of the spiral spring body and of the faces of the width of the spiral spring body have a chamfer which extends at least over one portion of the total arc length. The chamfer provides a reduction in the structural stress concentration at said ridges during elastic deformation of the coil spring body with use. Chamfers can be formed by wet etching.
    According to a second aspect, the present invention provides an oscillator for a timepiece, said oscillator comprising an element exerting a return torque according to the first aspect, and a balance fixed to the inner end of the element exerting a booster couple.
    According to a third aspect, the present invention provides a method of forming an element exerting a return torque according to the first aspect, the spiral spring body being formed by an engraving manufacturing technique including a manufacturing technique by Deep Reactive Ion Etching (DRIE).
    According to a fourth aspect, the present invention provides a method of forming an element exerting a restoring torque according to the first aspect, in which the width for the single crystal silicon wafer <110> varies according to the equation:
    According to a fifth aspect, the present invention provides a mechanical oscillator for a timepiece, the mechanical oscillator comprising an element exerting a restoring torque comprising a spiral spring body having a number N of turns with an inner end intended to be engaged in a rotating inertial element by means of a ferrule for rotation about an axis, and an outer end intended to be engaged in a stationary element of the rooster type, and has a width, a height and a total length arc; as well as a rotary inertial element in which is engaged the inner end of the spiral spring, rotating about this axis; wherein the spiral spring body has a core formed of a monocrystalline silicon wafer oriented along the <110> crystallographic axis; and wherein the body of the spiral spring has at least one peripheral coating of a material having a different thermoelastic constant than the core of the body of the spiral spring in order to maintain the oscillation frequency of the oscillator including the exerting member. return torque substantially insensitive to variations in ambient temperature.
    [0027] Preferably, the element exerting a return torque according to the first aspect.
    Brief description of the drawings
    [0028] An example for the implementation of the invention will now be described with reference to the accompanying figures, in which: FIG. 1 shows a diagram of a generic element for exerting a return torque as a balance spring for a mechanical timepiece, in accordance with the present invention, in an idle state. All turns except the outer peripheral turn consist of a constant pitch Archimedean spiral; Fig. 2 shows a sectional view of a first embodiment for a silicon balance spring according to the present invention, consisting of a silicon core covered with a coating of silicon dioxide of constant thickness according to the present invention; Fig. 3 shows a cross-sectional view of a second embodiment for a silicon balance spring according to the present invention, consisting of a silicon core covered with a silicon dioxide coating of constant thickness and chamfered longitudinal edges according to the present invention; Fig. 4 shows a plot of Young's modulus as a function of the in-plane orientation angle of the wafer for monocrystalline silicon <100>, <110>, and <111>; Fig. 5 shows a graph of the oscillation rate versus temperature for five different thicknesses of silicon dioxide; and FIG. 6 shows variations in the width of the silicon balance spring as a function of the angular position in order to compensate for the variation in the level of curvature caused by the Young's Modulus anisotropy of a monocrystalline silicon <110> in accordance with the present invention .
    Detailed description of the drawings
    [0029] The present invention uses monocrystalline silicon <110> for the formation of the element exerting a return torque, in particular for a balance spring for mechanical timepieces. The present invention and specific features relating thereto are described with reference to Figures 1 to 3, and the advantages provided by the present invention and the parameters relating thereto are described and explained with reference to Figures 4 to 6.
    [0030] Referring to FIG. 1, it can be seen that there is shown a generic element exerting a return torque as a balance spring 100 according to the present invention. The balance spring 100 is used in an oscillator having an oscillation frequency and has a spiral spring body 110 having a main section with an inner end 115, an outer section 120, and an outer end 125. The balance spring has a width 140, a height in the direction of the depth of the page and a total arc length, with turns 170.
    [0031] The main body of the spiral spring body 110 forms an Archimedean spiral at constant pitch; its inner end 115 intended for engagement is connected to a ferrule 117. The ferrule 117 is in turn rigidly attached to the balance; which, although not shown in this figure, will be understood by those skilled in the art as constituting a rotating inertial element.
    The outer section 120 has a significantly enlarged pitch to leave room for the arrangement of a piton and an outer end 125 intended to be engaged in a stationary rooster.
    The balance spring 100 is formed of a monocrystalline silicon wafer oriented along the crystallographic axis <110>, and the balance spring 100 can be formed from micro-manufacturing techniques, such as a technique fabrication by etching including a deep reactive ionic etching (DRIE) fabrication technique.
    The body of the spiral spring 110 has a shape of substantially rectangular section. According to the invention, the body of the spiral spring 110 may have a width 140 of between 20 µm and 60 µm, a height 150 of between 100 µm and 400 µm, and a total arc length 160 of between 50mm and 200mm, with a number of turns between 5 and 20. According to the particular example shown, the number of turns is 13.5. Such dimensions and sizes make the balance spring 100 applicable to timepieces such as wristwatches or the like.
    [0035] Referring to FIG. 2, it can be seen that a section of a balance spring 100a according to a first embodiment, shown at an arbitrary position at the level of the coils of the balance spring, is illustrated. The balance spring 100a here has a width 140a and a height 150a; the characteristics and sizes of the generic element exerting a return torque of FIG. 1 are analogous to those of this embodiment and inherently like reference numerals refer to like features.
    [0036] The coil spring body 110a includes a core 180a formed by a monocrystalline silicon wafer oriented along the crystallographic axis <110>. The coil spring body 110a also includes a peripheral coating 190a of a material having a different thermoelastic constant from that of the core of the coil spring body. The peripheral coating allows the balance spring 100a to be thermo-compensated and insensitive to changes in ambient temperature. Furthermore, the provision of such a peripheral coating makes it possible to use the balance spring 100a in a regulator for a timepiece, the balance spring then being engaged in the balance, such that the oscillation frequency of the oscillator is maintained and the oscillator is substantially insensitive to variations in ambient temperature.
    As can be seen, the balance can have its own thermal expansion profile which can alter its moment of inertia when the ambient temperature changes, and an adequate selection of the material for the peripheral coating 190a of the balance spring 100a allows to take into account and compensate for changes in the balance due to temperature so that the regulator is substantially insensitive to changes in ambient temperature, so that a constant oscillation frequency can be maintained, thus providing a measure of time correct and reliable to a timepiece incorporating such a regulator.
    [0038] According to the present invention, the peripheral coating 190a can be a coating of silicon oxide, applied by thermal oxidation. The silicon balance spring is heated to around 1000 ° C in an oxidative environment, so that the peripheral coating 190a is made. Typically, a peripheral coating 190a having a thickness between 3 μm and 6 μm is used in the context of the present invention. Such a peripheral coating 190a allows, when the latter is made of monocrystalline silicon <110> for a balance spring 100a having the geometric parameters as described with reference to FIG. 1, thermal regulation and compensation for a regulator of a timepiece comprising a balance spring and a balance. Thus, the balance spring 100a of the present invention provides temperature stability and accurate time measurement independent of fluctuations and changes in temperature when the latter is incorporated into an oscillator for a timepiece.
    [0039] According to the invention, the cross section of the body of the spiral spring 110a of the balance spring 100a can either:(i) be substantially constant over the entirety of the total arc length, or(ii) may vary along at least a portion of the total arc length.
    By using a <110> monocrystalline silicon wafer as defined in the context of the present invention, due to the variance of Young's modulus as discussed in more detail in the following, it is possible to use the small variations in overall stiffness of the balance spring to perform fine adjustment of the stiffness, which is one of the advantages of the present invention over prior art solutions employing <100> single crystal silicon wafers and <111>. This is also discussed in more detail in the following.
    According to the embodiments of the present invention in which the cross section of the body of the spiral spring is not constant throughout the length of the latter, the cross section of the body of the spiral spring 110a of the balance spring 100a may vary depending on required shape parameters. Due to the fact that a <110> monocrystalline silicon wafer is anisotropic, and is an embodiment within the scope of the present invention, the width and therefore the cross section of the body of the spiral spring 110a can be varied. In embodiments of the present invention, the cross section of the coil spring body 110a may have a cyclically varying width 140a. This can be applied along at least a portion of the main part of the coil spring body 110a, or substantially along the full length of the arc, but not necessarily along the last outer turn. The width 140a of the spiral spring body 110a may vary depending on the equation:
    [0042] Referring now to FIG. 3, there is seen a sectional view of a second embodiment of a balance spring 100b according to the present invention. As shown in this figure, the core 180b has chamfers 185b present along the longitudinal ridges formed at the intersection of the 150b height faces of the spiral spring body 110b and the 140b width faces of the spiral spring body. 110b. Chamfers can extend along at least a portion of the total arc length.
    [0043] The chamfers 185b provide a reduction in the concentration of structural stresses at the ridges during elastic deformation of the hairspring body during use, which reduces the likelihood of fatigue failure of a balance spring 100b to use. Chamfers 185b can be formed by wet etching, and this is discussed in more detail below.
    In order to put the context back, it is recalled that the material used in the context of the present invention is described and explained in particular by comparison with other similar materials, and the advantages provided by the use of monocrystalline silicon <110> in a balance spring for a timepiece are then put forward.
    There are two different types of silicon wafers, those called monocrystalline and those called polycrystalline. Monocrystalline silicon consists of a single crystal arranged uniformly over the entire wafer. Monocrystalline silicon wafers can then be categorized as <100>, <110>, and <111> depending on the orientation of the crystal. Polycrystalline silicon consists of many microscopic crystals (on a scale from nanometers to micrometers) arranged randomly on the wafer. In general, single crystal silicon wafers have more uniform material properties within the wafer compared to polycrystalline silicon wafers.
    [0046] The uniform arrangement of single crystal silicon crystals provides virtually uniform material properties throughout all of it in particular orientations. However, the random arrangement of the microscopic polycrystalline silicon crystals means that the material properties are highly dependent on the smoothness of the mixing such that the macroscopic effect is substantially uniform. Moreover, polycrystalline silicon has a visible grain limit which depends on the size of the individual crystals, which negatively impacts the aesthetics of a mechanical timepiece where visual attractiveness is highly valued.
    The essential difference between the monocrystalline silicon wafers <100>, <110>, and <111> is their Young's modulus which, among other material properties, depends on their orientation in the plane as shown in FIG. . 4. The <111> silicon wafer can be described as isotropic within the plane of the wafer because Young's modulus is independent of orientation. Conversely, silicon <110> and <100> wafers are anisotropic within the plane of the wafer because Young's modulus varies from 130.2 to 187.5 GPa for silicon wafers <110>, and from 130.2 to 168.9 GPa for <100> silicon wafers. Each type of wafer has its strengths and weaknesses as a material to be used in the manufacture of silicon balance springs.
    [0048] The isotropic nature of the <111> silicon monocrystalline wafer makes the design of a silicon balance spring much simpler; this state of affairs is validated by the academic literature for micromechanical systems in general. Since the silicon balance spring never bends out of the wafer plane, the uniform material properties in that plane make it easy to predict strain when the balance spring is under stress.
    [0049] However, the particular orientation of the silicon wafer <111> also makes cutting and polishing it more difficult, and its manufacture requires more work, which is why it is the most expensive and the most rare of the three types of monocrystalline silicon wafers.
    On the contrary, the monocrystalline silicon <100> and <110> wafers are much simpler and cheaper to produce because the orientation of the crystals is better aligned with the cutting and polishing plane, more especially for the <100> silicon wafers whose crystal structure is perfectly aligned with Cartesian coordinates relative to the plane of the wafer.
    However, the anisotropic nature of the silicon wafers <100> and <110> make the modeling of silicon more complex because the Young's modulus and therefore the stiffness of the section of a strip of material is sensitive to the orientation.
    The design of a silicon balance spring is a complex process which takes into account several constraints. Regarding the core, the rigidity of the balance spring must correspond to the inertia of the balance in order to produce the desired natural frequency according to the following equation:
    Here the parameters ωn, E, b, h, L, I, k, and lb correspond respectively to the natural frequency of the oscillator, the Young's modulus of the balance spring, the width of the cross section of the spring of balance, the height of the straight section of the balance spring, the total arc length of the balance spring, the second surface moment of inertia of the cross section of the balance spring, the stiffness of the balance spring, and the moment of inertia of the balance. There are also other additional constraints for the radial positions of the outer and inner ends of the balance spring.
    The width ratio between that of the band and that of the pitch (spacing between the turns) of the balance spring has a lower limit in order to avoid any collision between the bands when the hairspring contracts under the effect of its rotation, more particularly for a conventional shape of constant cross section which lacks concentricity during its deformation.
    The difference in angular position between the outer end and the inner end also has an effect on the stability of the stiffness for varying amplitudes of oscillation. The accumulation of these requirements places extremely severe constraints on the freedom of balance spring design, such that the geometry of almost all balance springs of similar stiffness is substantially identical.
    The aforementioned restriction on the freedom of design of a balance spring does not cause a serious problem for metal balance springs because the tendency of the metal to creep and deformation over time also gives it its robustness. and its flexibility. Despite the relative lack of precision in the manufacturing process for metal balance springs, the latter can be easily bent and cut as part of a post-processing process to fine-tune the stiffness in order to correspond to the inertia of the balance on a case-by-case basis.
    The silicon balance springs on the contrary confer better structural stability over time, and the microfabrication process constitutes a more precise manufacturing technique. The downside is the limited flexibility for post-processing to adjust the length and geometry of the balance spring by fine-tuning the stiffness. Typically, any offset from the desired stiffness for the balance spring would require a complete redesign which is both technically difficult and inefficient.
    The use of monocrystalline silicon wafers <100> and <110> allows fine adjustment of the rigidity of the balance spring by changing the angle of rotation of the modeling in the wafer, so that the orientation on the wafer is different. This is possible because the Young's modulus of silicon depends on the angle of orientation.
    [0059] Assuming that the geometry of the balance spring is mainly that of an Archimedean balance spring, the rigidity of the balance spring rigidity can be described by the following equation:
    Here, the parameters θ0, θF, and a respectively represent the angular position of the inner and outer ends of the balance spring and the pitch of the Archimedean hairspring. Note that Young's modulus is a function of the orientation angle θ, and a change in the orientation of the balance spring means a change in the angles θ0 and θF while keeping their difference constant. Therefore, the equation demonstrates that a change in the orientation of the balance spring on the wafer can change its stiffness.
    By comparing the use of <100> and <110> monocrystalline silicon wafers, the present invention has identified that the use of <110> monocrystalline silicon wafers is preferable for fine tuning a balance spring. .
    [0062] The Young's modulus of silicon wafers <100> and <110> as a function of the interior of the plane can be defined as follows.
    Here, the parameters S11, S12, and S44 are elements of the compliance matrix for monocrystalline silicon defined as being equal to 7.68, –2.14, and 12.6, respectively, in units of 10 <–> <12> Pa <–> <1>. E <100> varies between 130.2 and 168.9 GPa, compared to the range between 130.2 and 187.5 GPa for E <110> over an orientation interval of 90 degrees.
    This means that the sensitivity of the rigidity of the balance spring towards its angle of orientation on the wafer is greater than for a wafer made of monocrystalline silicon <110> and thus confers greater flexibility for the fine adjustment of the rigidity, in accordance with the solution of the present invention.
    Another advantage identified by the present invention and which is conferred by the use of a wafer of monocrystalline silicon <110> compared to a wafer of monocrystalline silicon <100> is that the wafer of monocrystalline silicon <110> tends to form chamfers when exposed to wet etching techniques, which acts as a strain relief mechanism.
    A balance spring in a mechanical timepiece typically experiences large cyclic loads when it has to bend at a frequency of 3 to 5 Hertz for years. A small reduction in load stresses on the balance spring can significantly extend its life. This assertion is particularly true for a balance spring covered with a thermal oxide for thermo-compensation reasons, as shown in Figures 2 and 3.
    [0066] One of the weak points of a silicon balance spring covered with a coating layer is the boundary between the silicon core and the silicon dioxide coating. The crystal structure of each of the materials is different and undergoes adhesion stresses deteriorated by thermal stresses due to the different coefficients of thermal expansion when the balance spring cools from the temperature of over 1000 ° C of the process. coating. If the stresses are high enough, the thermal oxide can separate from the silicon core, and the thermo-compensation effect is lost, thus resulting in a timepiece lacking good regulating and time-measuring characteristics.
    [0067] An effective technique as provided by the present invention in terms of stress reduction by eliminating the sharp edges which cause the concentration of stress stresses is described in relation to FIG. 3.
    [0068] The manufacture of silicon balance springs typically involves the process of deep reactive ion etching (Deep Reactive Ion Etching - acronym DRIE) to create the coarse structure. However, depending on the type of monocrystalline wafer, such as that provided by the present invention, the chamfers can be made at the ridges of the balance springs by wet etching processes. Wet etching involves the use of alkaline solvents to remove silicon by immersion.
    [0069] Depending on the exact type of solvent used, the result of the etching can be strongly anisotropic depending on the crystal orientation; the selectivity of the crystal orientation of <100> can be up to 400 times higher than for the <110> or <111> directions. On a <110> or <111> monocrystalline silicon wafer, wet etching alone can produce trapezoidal cross-sectional structures.
    [0070] Used as a post-processing method of a DRIE, wet etching can produce chamfers on a section that would otherwise be rectangular, and thus eliminate sharp edges and reduce stress concentration, something that does not This is not possible with the <100> monocrystalline silicon wafer because the wet etching would then mainly etch in the side wall of the cross section of the silicon balance spring, and thus could not provide the advantages conferred by the present invention.
    A silicon balance spring must have a silicon dioxide coating to perform the thermocompensation, and it can be shown that this mechanism works with a <110> monocrystalline silicon wafer like that used in accordance with the present invention.
    The purpose of the silicon dioxide coating is to compensate for the reduction in the oscillation frequency occurring with a rise in temperature as a consequence of the positive thermal expansion coefficient of the balance wheel and the negative thermoelastic constant of the silicon core of the balance spring. The dependence of the moment of inertia of the balance (which is also referred to as the balance wheel) on temperature can be described as follows:
    Here, the parameters lbo, α, and ΔT are respectively the moment of inertia of the balance at a nominal temperature, the coefficient of thermal expansion of the material of the balance, and the temperature difference from the nominal temperature.
    [0074] The coefficient of thermal expansion of beryllium copper, a material typically used for balance wheels, is approximately 16 ppm / K. The temperature range typically used by the watch industry to check thermocompensation is around 23 ° C +/– 15 ° C.
    To deduce the equation of the sensitivity of the rigidity of a silicon balance spring coated with a layer of silicon dioxide with respect to the temperature, one must first determine the equivalent Young's modulus for the composite structure at nominal temperature.
    Here, the parameters ς, E0, ESi, 0, ESi02.0, b, and h respectively represent the oxide thickness, the nominal Young's modulus of the composite balance spring band, the Young's modulus nominal silicon, nominal Young's modulus of silicon dioxide, total width, and total height of the cross section of the balance spring band.
    [0077] It should be noted that the value of the parameter ESi, 0 depends on the angle of orientation on the wafer and varies between the silicon wafers <100> and <110>. The value of ES102.0, is approximately 72.4 GPa. If the temperature is taken into account, the equation changes as follows:
    Here, the parameters εSi; and εSi02 represent the thermoelastic constant of silicon and silicon dioxide, the values of which are approximately –60 ppm / K and 215 ppm / K, respectively. The stiffness of the balance spring can then be defined as follows:
    [0079] By combining this with the equation for the moment of inertia of the balance we obtain the following oscillation frequency:
    If the oscillation frequency is normalized as being equal to the desired frequency at the nominal temperature, it is possible to numerically determine the thickness of oxide required to perform the thermocompensation to meet the required tolerances typically set at + - 1 second / day / Kelvin.
    [0081] FIG. 5 shows a graph of oscillation frequency versus temperature difference for five different oxide layers using a <110> monocrystalline silicon wafer according to the present invention.
    [0082] The width and height of the band of the balance spring varies between 35 and 40 µm and between 200 and 210 µm, respectively, depending on the thickness of oxide used. The total arc length of the total balance spring is approximately 130 mm, and the moment of inertia of the balance is approximately 1.65 g * mm <2>.
    It can be noted that for an oxide thickness of 4.5 μm, the oscillation frequency varies by less than 0.1 second per day and per Kelvin over the entire temperature range extending from 5 ° C to 40 ° C , which is well within standard thermocompensation tolerances when using a <110> monocrystalline silicon wafer in accordance with the present invention.
    The anisotropic properties of the material of the monocrystalline silicon <110> wafer impose a challenge for the control of the degree of bending of the balance spring band at each section since the transverse stiffness depends on the orientation angle .
    In order to maintain a constant transverse rigidity throughout the length of the arc, according to embodiments of the present invention, the width of the balance spring can be varied according to the variation of the Young's modulus in order to neutralize the net variation in stiffness.
    [0086] Considering that the transverse stiffness of the balance spring is proportional to Young's modulus and to the cube of the width of the strip, the width can be calculated as varying according to the following equation for a silicon wafer <110>.
    [0087] Using the binomial approximation theorem, the above equation can be approximated as follows:
    [0088] Here, b0 is the nominal width of the band of the silicon balance spring. The width varies cyclically for each half turn of the silicon balance spring. The equation assumes that the nominal Young's modulus is 1 / Sn or 130.2 GPa. Fig. 6 graphically represents the variation of the width of a silicon balance spring <110> as a function of the angle of orientation, using a silicon monocrystalline <110> wafer in accordance with the present invention.
    The present invention confers, through the use of a monocrystalline silicon <110> wafer in accordance with the present invention, several advantages over the prior art, including monocrystalline silicon <111> and <100> , among which: (i) a material easier to manufacture and less expensive than silicon <111> is used within the scope of the present invention, which allows a simple and more profitable manufacture thereof; (ii) a material with greater sensitivity to make adjustments than <100> silicon is provided by the present invention, which allows more precise adjustments to be made; (iii) a material making it possible to form a chamfer at the level of its peripheral edges is used, which confers the advantages of:reduce the interfacing stresses between the edges of the silicon balance spring and the layer of silicon dioxide, consequently reducing their risk of separation at their adhesion layer, and thus the loss of the thermal compensation effect ;reduce stress concentration factors and local stress increases, reducing the likelihood of failure due to balance spring fatigue;provide a balance spring exhibiting greater resistance to breakage against inherent edge defects, thereby imparting longer resistance to fatigue, which is an essential parameter for ensuring the longevity of balance springs in timepieces;thanks to the fact that the localized stresses are reduced, thus resulting in a lower probability of fatigue failure, such a balance spring can be used for high frequency applications, such as between 8 and 10 Hz for example; andSilicon balance springs have not been used for such extended periods of time as traditional balance springs made of metal or traditional alloys. Intrinsically, the longevity of such items, for example 50 years or more, cannot yet bedetermined for now. Thus, by providing a balance spring with less localized stresses, one provides a balance spring having a greater propensity to meet the durability requirements of a balance spring compared to those of the prior art including the monocrystalline silicon <100> and <111>.
    The present invention overcomes the drawbacks of balance springs made both from monocrystalline silicon <111> and <100>, in terms of manufacture, selectivity of design parameters and longevity / fatigue including both the longevity of the adhesion of a thermocompensation layer; and the longevity of the silicon core of a balance spring.
    权利要求:
    Claims (18)
    [1]
    1. Element exerting a return torque for the oscillator of a mechanical timepiece having an oscillation frequency, said element exerting a return torque comprising:a spiral spring body (110, 110a, 110b) having an N number of turns (170) with an inner end (115) intended to be engaged in a rotating inertial member by means of a ferrule (117), and an end outer (125) intended to be engaged in a stationary rooster, and has a width (140, 140a, 140b), a height (150a, 150b) and a total arc length;the spiral spring body (110, 110a, 110b) comprising a core (180a, 180b) formed of a monocrystalline silicon wafer oriented along the <110> crystallographic axis; andthe body of the spiral spring (110, 110a, 110b) comprising at least one peripheral coating (190a) of a material having a thermoelastic constant different from that of the core of the body of the spiral spring (180a, 180b) in order to maintain the frequency d oscillation of an oscillator including the element exerting a restoring torque substantially insensitive to variations in ambient temperature.
    [2]
    2. Element exerting a return torque according to claim 1, the body of the spiral spring (110, 110a, 110b) being substantially rectangular in section.
    [3]
    3. Element exerting a return torque according to claim 1 or 2 having a width (140, 140a, 140b) between 20 µm and 60 µm, a height (150a, 150b) between 100 µm and 400 µm, and a length total arc between 50 mm and 200 mm.
    [4]
    4. Element exerting a return torque according to one of the preceding claims, having a number of turns between 5 and 20.
    [5]
    5. A return torque member according to one of the preceding claims, wherein said at least one peripheral coating (190a) of the balance spring (100,100a, 100b) is formed of silicon dioxide.
    [6]
    6. The element exerting a return torque according to claim 5, wherein said at least one peripheral coating (190a) has a thickness between 3 µm and 6 µm.
    [7]
    7. Element exerting a return torque according to one of the preceding claims, wherein the orientation angle of the monocrystalline silicon wafer <110> produces slight variations in the overall rigidity of the balance spring (100, 100a, 100b ) which can be used for fine adjustment of stiffness.
    [8]
    8. Element exerting a return torque according to one of the preceding claims, the body of the spiral spring (110, 110a, 110b) having a width (140, 140a, 140b) varying cyclically along at least a portion of the total arc length based on the turn to compensate for variations in the section stiffness of the balance spring (100, 100a, 100b) due to Young's modulus anisotropy in the wafer plane.
    [9]
    9. Element exerting a return torque according to claim 8, having a width (140, 140a, 140b) for the wafer in monocrystalline silicon <110> which varies according to the equation:
    [10]
    10. The element exerting a return torque according to one of the preceding claims, wherein the body of the spiral spring (110, 110a, 110b) is formed by a manufacturing technique comprising deep reactive ion etching (DRIE).
    [11]
    11. Element exerting a return torque according to one of the preceding claims, wherein the longitudinal ridges formed at the intersection of the faces of the height of the spiral spring body and the faces of the width of the spiral spring body have a chamfer. (185b) which extends at least over a portion of the total arc length.
    [12]
    12. A torque-exerting member according to claim 11, said chamfer (185b) providing a reduction in the structural stress concentration at said ridges during elastic deformation of the body of the spiral spring (110, 110a, 110b) at the same. use.
    [13]
    13. Element exerting a return torque according to claim 11 or 12, said chamfers (185b) being formed by wet etching.
    [14]
    14. Oscillator for a timepiece, said oscillator comprising an element exerting a return torque according to one of the preceding claims, and a balance fixed to the inner end (115) of the element exerting a return torque.
    [15]
    15. A method of forming an element exerting a return torque according to one of claims 1 to 13, the spiral spring body (110, 110a, 110b) being formed by an engraving manufacturing technique including a manufacturing technique. by deep reactive ion etching (DRIE).
    [16]
    16. A method of forming an element exerting a return torque according to one of claims 1 to 13, wherein the width for the monocrystalline silicon wafer <110> varies according to the equation:
    [17]
    17. Mechanical oscillator for timepieces, the mechanical oscillator comprising:an element exerting a return torque comprising a spiral spring body (110, 110a, 110b) having an N number of turns with an inner end (115) intended to be engaged in a rotating inertial element by means of a ferrule (117 ) for rotation about an axis, and an outer end intended to be engaged in a stationary rooster, and has a width (140, 140a, 140b), a height (150a, 150b) and a total arc length;a rotary inertial element in which is engaged the inner end (115) of the spiral spring (100, 100a, 100b), rotating around this axis,the spiral spring body (110, 110a, 110b) comprising a core (180a, 180b) formed of a monocrystalline silicon wafer oriented along the <110> crystallographic axis; andthe body of the spiral spring (110, 110a, 110b) comprising at least one peripheral coating (190a) of a material having a thermoelastic constant different from that of the core (180a, 180b) of the body of the spiral spring (110, 110a, 110b ) in order to maintain the oscillation frequency of the oscillator including the element exerting a return torque substantially insensitive to variations in ambient temperature.
    [18]
    18. A mechanical oscillator according to claim 17, wherein said element exerting a restoring torque is an element exerting a restoring torque according to one of claims 2 to 13.
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    同族专利:
    公开号 | 公开日
    HK1222462A1|2017-06-30|
    US20160238994A1|2016-08-18|
    EP3056948A1|2016-08-17|
    TWI685592B|2020-02-21|
    EP3056948B1|2019-02-20|
    WO2016131377A1|2016-08-25|
    HK1224386A1|2017-08-18|
    TW201702439A|2017-01-16|
    CN105892259A|2016-08-24|
    US9903049B2|2018-02-27|
    EP3056948A8|2016-11-23|
    HK1209578A2|2016-04-01|
    CN105892259B|2020-04-17|
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    法律状态:
    优先权:
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    HK15101776.1A|HK1209578A2|2015-02-17|2015-02-17|Silicon hairspring|
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