Classifications

• • G—PHYSICS
  • G04—HOROLOGY
  • G04D—APPARATUS OR TOOLS SPECIALLY DESIGNED FOR MAKING OR MAINTAINING CLOCKS OR WATCHES
  • G04D7/00—Measuring, counting, calibrating, testing or regulating apparatus
  • G04D7/10—Measuring, counting, calibrating, testing or regulating apparatus for hairsprings of balances
• • G—PHYSICS
  • G04—HOROLOGY
  • G04D—APPARATUS OR TOOLS SPECIALLY DESIGNED FOR MAKING OR MAINTAINING CLOCKS OR WATCHES
  • G04D7/00—Measuring, counting, calibrating, testing or regulating apparatus
  • G04D7/12—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard
  • G04D7/1207—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard only for measuring
  • G04D7/1235—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard only for measuring for the control mechanism only (found from outside the clockwork)
  • G04D7/125—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard only for measuring for the control mechanism only (found from outside the clockwork) for measuring frequency
• • G—PHYSICS
  • G04—HOROLOGY
  • G04D—APPARATUS OR TOOLS SPECIALLY DESIGNED FOR MAKING OR MAINTAINING CLOCKS OR WATCHES
  • G04D7/00—Measuring, counting, calibrating, testing or regulating apparatus
  • G04D7/12—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard
  • G04D7/1257—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard wherein further adjustment devices are present
  • G04D7/1271—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard wherein further adjustment devices are present for the control mechanism only (from outside the clockwork)
  • G04D7/1285—Timing devices for clocks or watches for comparing the rate of the oscillating member with a standard wherein further adjustment devices are present for the control mechanism only (from outside the clockwork) whereby the adjustment device works on the mainspring

Definitions

• the present invention relates to the field of control and manufacture of parts for watchmaking.
• the invention relates more particularly to a method for controlling and manufacturing clockwork spiral springs, otherwise known as resonators.
• the movements of mechanical watches are regulated by means of a mechanical regulator comprising a resonator, that is to say an elastically deformable component whose oscillations determine the rate of the watch.
• a mechanical regulator comprising a resonator, that is to say an elastically deformable component whose oscillations determine the rate of the watch.
• Many watches include, for example, a regulator comprising a hairspring as a resonator, mounted on the axis of a balance wheel and set in oscillation thanks to an escapement.
• the natural frequency of the balance-spring couple makes it possible to regulate the watch and depends in particular on the stiffness of the balance-spring.
• the stiffness of the hairspring also defines its intrinsic vibratory characteristics, such as the natural frequency and the resonant frequencies.
• the natural frequency of an elastic system (a single resonator or a resonator-pendulum couple) is the frequency at which this system oscillates when it is in free evolution, that is to say without exciting force.
• a resonance frequency of an elastic system subjected to an exciting force is a frequency at which one can measure a local maximum of displacement amplitude for a given point. of the elastic system.
• the displacement amplitude follows an upward slope before this resonant frequency, and follows a downward slope after, in all point that does not correspond to a vibration node.
• the recording of the displacement amplitude as a function of the excitation frequency shows a displacement amplitude peak or resonance peak which is associated with or which characterizes the resonance frequency.
• the stiffness of a spiral-type resonator typically depends on the characteristics of the material, as well as its dimensions and in particular the thickness (that is to say the width) of its turns along its bar.
• the stiffness is given more specifically by: [equation 2] with :
• M the restoring torque of the spiral spring, where M, for a bar of constant section made of a specific material, is given by:
• the natural frequency of the regulator member formed by the balance spring of stiffness R coupled to a balance wheel of inertia I is in particular proportional to the square root of the stiffness of the balance spring.
• the main specification of a spiral spring is its stiffness, which must be within a well-defined range in order to be paired with a pendulum, which forms the inertial element of the oscillator. This pairing operation is essential to precisely adjust the frequency of a mechanical oscillator.
• silicon hairsprings can be manufactured on a single wafer using micro-fabrication technologies. It is in particular known to produce a plurality of silicon resonators with very high precision using photolithography and machining/etching processes in a silicon wafer.
• the methods for producing these mechanical resonators generally use monocrystalline silicon wafers, but wafers made of other materials can also be used, for example polycrystalline or amorphous silicon, other semiconductor materials, glass, ceramic , carbon, carbon nanotubes or a composite comprising these materials.
• monocrystalline silicon belongs to the cubic crystalline class m3m whose coefficient of thermal expansion (alpha) is isotropic.
• Silicon has a very negative value of the first thermoelastic coefficient, and consequently the stiffness of a silicon resonator, and therefore its natural frequency, varies greatly depending on the temperature.
• documents EP1422436, EP2215531 and WO2016128694 describe a spiral-type mechanical resonator made from a core (or two cores in the case of WO2016128694) of monocrystalline silicon and whose temperature variations Young's modulus are compensated by a layer of amorphous silicon oxide (SiO2) surrounding the core (or cores), the latter being one of the rare materials with a positive thermoelastic coefficient.
• SiO2 amorphous silicon oxide
• the final functional yield will be given by the number of hairsprings whose stiffness corresponds to the pairing interval, divided by the total number of hairsprings on the wafer.
• the micro-manufacturing and more particularly etching steps used in the manufacture of hairsprings on a wafer typically result in a significant geometric dispersion between the dimensions of the hairsprings of the same wafer, and therefore a dispersion between their stiffnesses, notwithstanding that the engraving pattern is the same for each hairspring.
• the measured stiffness dispersion normally follows a Gaussian distribution. In order to optimize the manufacturing yield, we are therefore interested in centering the average of the Gaussian distribution on a value of nominal stiffness and also in reducing the standard deviation of this Gaussian.
• the documents WO2015113973 and EP3181938 propose to remedy this problem by forming a hairspring with dimensions greater than the dimensions necessary to obtain a hairspring of a predetermined stiffness, by measuring the stiffness of this hairspring formed in the coupling with a balance equipped with a predetermined inertia, by calculating the thickness of material to be removed in order to obtain the dimensions necessary for obtaining the hairspring with the predetermined stiffness, and by removing this thickness from the hairspring.
• document EP3181939 proposes to remedy this same problem by forming a hairspring with dimensions smaller than the dimensions necessary to obtain a hairspring of a predetermined stiffness, by determining the stiffness of this hairspring formed by coupling it with a pendulum with a predetermined inertia, by calculating the thickness of material to be added to obtain the dimensions necessary for obtaining the hairspring with the predetermined stiffness, and by adding this thickness of material to the hairspring.
• the present invention aims to provide an approach free from the above drawbacks, which allows a faster production flow and / or with less risk of pollution (s), and / or greater sampling, and /or a more precise measurement, and therefore a more individualized correction of the hairsprings of the wafer.
• the invention relates to a method for controlling a hairspring or a hairspring blank arranged to form a hairspring, the hairspring having to have at least one predetermined resonant frequency, comprising the following steps: has. applying to the hairspring or to the hairspring blank a vibratory excitation that varies over time to cover a predetermined frequency range, b. identifying at least one characteristic of a resonant frequency, such as a resonance peak, of the hairspring or of the hairspring blank during vibratory excitation over the predetermined frequency range, c. subjecting the resonant frequency characteristic identified in step b to a prediction machine. to determine a stiffness of the hairspring or of the hairspring blank and/or to determine whether a dimensional correction of the hairspring or of the hairspring blank is necessary to obtain the predetermined resonant frequency.
• the method according to the implementation above comprises a step of vibratory excitation of the hairspring or of the hairspring blank and the measurement of a characteristic of a resonant frequency, to then deduce therefrom by prediction a stiffness and/or if a dimensional correction is necessary.
• the measurement is carried out on the hairsprings or the blanks alone, which limits the errors induced by other components or their assembly, as well as any pollution. Measurement accuracy is improved because there are fewer sources of variability due to other components or pollution. In other words, the hairspring or the hairspring blank is tested alone.
• the vibratory excitation is applied to the part or to the unit blank, not coupled to any pendulum, weight or oscillating system.
• the process makes it possible to control the unitary and free parts (that is to say with at least one free end, not attached to any mechanism or pendulum), which at least brings advantages of productivity gains (no assembly with an oscillating system), quality gains (no pollution of parts, no breakage, and more parts can be tested within the same budget), precision gain (no error related to other components of an oscillating system).
• the vibratory excitation is applied to the hairspring or to the hairspring blank having a free end (typically the central shroud) and another end fixed to the wafer or to a clamp.
• a free end typically the central shroud
• another end fixed to the wafer or to a clamp.
• the vibratory excitation is applied to a mass (located at the balance spring's center of gravity) connected to a frame of reference (a gripper for a single balance spring, or the rest of a substrate or a plate for a blank, for example made of silicon and not detached) by a spring (the elastic part of the hairspring).
• the vibratory excitation sets the suspended mass in motion.
• a dimensional correction must be made to the part tested (or to all the unit parts attached to the same wafer, or even to the unit parts attached to a zone of a wafer, whether or not including the part tested), this can be done on the unit part(s) without dismantling anything (for example, it can be provided to apply directly at the test output oxidation on a silicon part). It is therefore possible to add or remove material from the unit part(s) to vary its intrinsic stiffness. In other words, the dimensional correction is performed on the unit part(s), by changing its dimensions (typically the width and/or the thickness of the bar forming the elastic part of the hairspring).
• the method according to the implementation above therefore makes it possible to test spiral blanks during manufacture while limiting the risks of pollution or assembly errors. A dimensional correction (of section, height and/or thickness) is then possible.
• the method according to the implementation above makes it possible just as well to test finished hairsprings in order for example to carry out a classification by increments of stiffness, in order to provide pairing with a particular balance wheel.
• the frequency range of the spectrum obtained does not only depend on the source of vibratory excitation but also on the sensor of the measuring instrument used.
• the frequency range is linked both to the excitation frequency range and to the frequency range over which the instrument for measuring the amplitude of oscillation (vibrometer or other) is sensitive.
• the excitation frequency range will be chosen so as to include at least one resonance frequency of the hairspring or of the tested blank.
• the predetermined resonance frequency that the hairspring must have once finished can be a target natural frequency or a target resonance frequency, or a target natural frequency range, or a target resonant frequency range defined by a tolerance around a target value.
• the dimensional correction predicted by the prediction machine can typically be a correction of the section of the flexible bar forming the hairspring or the hairspring blank, that is to say a correction either of the height or of the thickness, or both.
• the characteristic of a resonance frequency is a characteristic of the oscillatory response measured over a predetermined frequency range, comprising at least one resonance frequency.
• a characteristic is typically identified after processing a raw measurement signal (for example measuring the amplitudes or speeds or displacement accelerations of certain points of the hairspring or of the hairspring blank), the processing possibly including for example a transform of Fourier to identify resonance peaks and therefore resonance frequencies.
• the method can determine a stiffness to perform a classification of the part, and/or to then calculate/deduce a level of dimensional correction to be applied to obtain a target stiffness.
• the identified resonance frequency can be taken into account to directly calculate/deduce a level of dimensional correction to be applied to obtain a target stiffness.
• step a the frequency range is applied simultaneously to a plurality of balance-springs or balance-spring blanks.
• the speed is improved, because the vibratory excitation can typically be imposed on a wafer supporting several hundred balance-spring blanks, which would for example still be attached to the wafer.
• the frequency range is predetermined to encompass at least one frequency range:
• the hairspring has at least two predetermined resonant frequencies, and the frequency range is predetermined to cover at least the two predetermined resonant frequencies. By covering or sweeping a wide range of frequencies, one can measure multiple resonant peaks (or resonant frequencies), which can provide better accuracy.
• step a comprises the use of a source, such as a piezoelectric source, making it possible to induce or impose an acoustic excitation on a slice of a wafer supporting the blank of hairspring, or preferably on or even under the hairspring or the hairspring blank to be specifically excited.
• a source such as a piezoelectric source
• the acoustic source can be coupled to an excitation cone chosen to excite at least one hairspring or a hairspring blank.
• the acoustic source can be coupled to an excitation cone chosen to excite at least some and preferably all of the balance-spring blanks.
• the acoustic source can be chosen and/or adjusted to generate the variable vibratory excitation over time to cover the predetermined frequency range:
• step b comprises the use of an optical measuring means, such as a laser Doppler effect vibrometer.
• step b is based on a measurement over time of an amplitude or a speed, or even an acceleration of displacement of at least one point of the hairspring or of the balance-spring blank, preferably performed at least partially during step a.
• step b comprises:
• a step of identifying a resonance frequency of the hairspring or of the hairspring blank as a function of an operational or modal deformation of at least one point of the hairspring or of the hairspring blank is typically defined by an amplitude or speed of displacement or else by an acceleration and a direction of oscillation (outside or in a particular plane) as a function of the excitation frequency.
• the hairspring or the hairspring blank is contained in a base plane, and step b comprises:
• step b for measuring an amplitude or a speed or an acceleration of displacement of at least one point of the hairspring or of the hairspring blank in a direction contained in the base plane.
• step b' of measuring a displacement or a speed of at least one point of the hairspring or of the hairspring blank in a direction normal to the base plane is performed, and/or
• step b” of measuring a displacement or a speed of at least one point of the hairspring or of the hairspring blank in a direction contained in the base plane is done.
• the mode of vibration in response to vibrational excitation may vary.
• step b comprises a step of processing the measurement signal with, for example, a Fourier transform, to identify resonance peaks of displacement amplitude or speed or acceleration, and /or phase, depending on the excitation frequency.
• step b comprises:
• - a step of identifying a resonance peak of the hairspring or of the hairspring blank as a function of an amplitude or of a displacement speed of at least one point of the hairspring or of the hairspring blank.
• the resonance frequency is identified based on the width of the resonance or amplitude peak, halfway up the maximum value of the amplitude resonance peak.
• step c comprises a step of calculating a stiffness of the hairspring or of the hairspring blank.
• the calculation of the stiffness makes it possible to determine with improved precision whether a dimensional correction is necessary, and of what value this correction must be. In addition, this also makes it possible to pre-dimension or choose a balance wheel to couple the hairspring once it is finished manufacturing.
• the method comprises a step: d. calculate, with the prediction machine, the dimensional modification (modification of section, height and/or thickness) to be applied from the resonance frequency characteristic identified in step b.
• the prediction machine implements a polynomial formula to predict whether a dimensional correction is necessary.
• the prediction machine implements a classification performed for example by a neural network to predict whether a dimensional correction is necessary.
• the prediction machine implements a classification based on partitioning into k-means or into k-medians to predict whether a dimensional correction is necessary.
• the hairspring blank being formed on a wafer comprising a plurality of hairspring blanks distributed over several sectors of the wafer
• step b comprises a step consisting in identifying at least one characteristic of a resonant frequency of at least one balance-spring blank for each sector
• step c comprises a step consisting in determining a stiffness of the balance-spring blank and/or in determining for the balance-spring blanks of each sector if a dimensional correction is necessary.
• the precision of the dimensional correction (section, height and/or thickness) is improved by refining the analysis by sectors of the wafer.
• control method comprises a step of calculating, with the prediction machine, the dimensional modification to be applied for the balance-spring blanks of each sector.
• step a comprises a step consisting in modifying a direction of vibratory excitation over time, preferably in a direction pointing to the hairspring or the hairspring blank whose frequency characteristic resonance is identified in step b.
• control method comprises a preliminary step consisting in taking into account the material of the hairspring or of the hairspring blank, and in adjusting a maximum amplitude of the vibratory excitation and/or a range frequency of the predetermined frequency range depending on the material of the hairspring or of the hairspring blank.
• the frequency range obtained extends over a frequency range ranging from 0 Hz to 100 kHz, preferably from 0 Hz to 50 kHz, more preferably from 0 Hz to 40 kHz, and very preferably from 10kHz to 35kHz.
• step a and step b are repeated at least several times for the same measurement point of the hairspring or of the hairspring blank.
• step a and step b are synchronized.
• Such synchronization provides the possibility of detecting a phase shift, or an attenuation, or a coupling, the consideration of which can improve the precision of the prediction, or make it possible to adjust or recalibrate the vibratory excitation source.
• a second aspect of the invention relates to a process for manufacturing a hairspring having at least one predetermined resonant frequency comprising the steps consisting of:
• the manufacturing method comprises a step consisting of:
• the dimensions can be corrected by removing or adding material.
• the hairspring or the hairspring blank is formed from silicon, or glass, or ceramic, or metal, or carbon nanotubes.
• conventional hairsprings made of metal can be tested.
• the metal hairspring is pinched or referenced by a tool which positions it opposite the emission source and the displacement measuring device.
• the hairspring blank is formed on a wafer, with a plurality of other hairspring blanks.
• a third aspect of the invention relates to a method for learning a prediction machine to implement step c of the control method of the first aspect, comprising the steps consisting in: i- forming hairsprings or drafts of hairsprings, ii- applying to each of the hairsprings or to each of the drafts of a hairspring a vibratory excitation that varies over time to cover a predetermined frequency range, iii- identifying at least one characteristic of a resonance frequency of each hairspring or each hairspring blank during vibratory excitation over the predetermined frequency range, iv'- mounting a plurality of hairsprings or hairspring blanks in an oscillating mechanism having a predetermined inertia so as to measure for each hairspring or each balance spring a free oscillation frequency or a stiffness and/or iv”- modeling in a simulation tool a plurality of balance springs or balance spring blanks in an osc mechanism illant having a predetermined inertia so as to calculate for each hairspring or each hairspring
• step iii- comprises a preliminary phase of identifying reference measurement points with:
• Such a step of identifying the reference points makes it possible to eliminate the points or the zones which are nodes (that is to say immobile points) at one or more resonance frequencies.
• the hairspring or the hairspring blank has a radius Ra defined between a free central end and a recessed peripheral end, and at least two reference points, and preferably four reference points are chosen. and located:
• the shell has large dimensions compared to the turns (a turn typically has a width of 20 ⁇ m to 40 ⁇ m, the shell may have dimensions of at least 110 ⁇ m) which makes aiming the measuring tool easier, and on the other hand, the ferrule can be considered non-deformable during vibratory excitation and all the points of the ferrule present displacements / movements / similar vibrations. Consequently, aiming the measurement point (with a size of 4 ⁇ m for a laser sensor for example) on the ferrule will be easier, and/or a small error in locating the measurement point on the ferrule will have little consequence on the end result. Furthermore, having chosen a particular measurement point on the part, it is possible to identify and choose a particular frequency range to conduct the stiffness prediction.
• an image analysis step to, for example, recognize each type of part, and/or the position and/or the orientation of each part
• a step of positioning the substrate or the tool supporting the parts to be tested in a vibration excitation and measurement device According to this implementation, it is possible to automate the excitation and the measurement in the case of a wafer which still carries the blanks of the hairsprings:
• an automatic image analysis is carried out to know at least the X-Y position of each part (we can also do a recognition of the type or model of part),
• each balance-spring blank is successively placed automatically next to the excitation source and the measuring device to be tested by aiming at the correct measurement point and applying the correct excitation specification.
• an autofocus step can be performed, i.e. an adjustment of the relative position along z of the position of the head of the vibrometer, making it possible to obtain the sharpest possible image of the part observed.
• the laser beam is thus focused exactly on the surface of the part, provided that the focal planes of the laser beam and of the observation camera coincide, or that their offset is known and systematically compensated.
• a step consisting in giving a particular orientation to the direction of excitation and/or to the direction of measurement.
• an excitation direction or an axial direction of the excitation source
• an excitation direction or an axial direction of the excitation source
• an excitation direction or an axial direction of the excitation source
• reception sensor adapted to receive the reflected signal, depending on the roughness of the parts: for slightly rough "mirror” parts, it is possible to provide a reception sensor with a large collection cone (which preferably covers at least twice the angle of inclination), or offset, while for "rough” parts, the reception sensor can be combined with the emission source of light.
• a substrate such as a wafer
• this preliminary sampling makes it possible to test single parts in good conditions (measurement errors and interference are limited) to choose the best test conditions for the parts that have remained attached to the substrate.
• the excitation of parts attached to a substrate provision can be made to excite and measure the response of the substrate, in order to identify and subsequently exclude the spectral ranges on which the latter vibrates.
• FIGS. 3A-3F are a simplified representation of a process for manufacturing a mechanical resonator, here a hairspring, on a wafer,
• FIG. 4 represents a device allowing the evaluation of the torque of a hairspring
• FIG. 5 schematically represents the implementation of the evaluation of the stiffness of a hairspring by vibration analysis
• FIG. 6 shows an example of frequencies applied to a silicon wafer supporting balance-spring blanks, to impose vibratory excitation
• - Figure 7 shows an example of measuring the displacement amplitudes of a point of a balance-spring blank, in response to the imposed frequency range of Figure 6,
• figure 9 represents the resonance peaks measured and superimposed for the particular frequency of figure 8
• figure 10 represents an example prediction model constructed from data extracted from figure 9.
• Figures 3A-3F are a simplified representation of a method of manufacturing a mechanical resonator 100 on a plate 10.
• the resonator is intended in particular to equip a regulating member of a timepiece and, according to this example, is in the form of a silicon spiral spring 100 which is intended to equip a balance wheel of a mechanical clockwork movement.
• the wafer 10 is illustrated in FIG. 3A as an SOI (“silicon on insulator”) wafer and comprises a substrate or “handler” 20 bearing a sacrificial layer of silicon oxide (SiO2) 30 and a layer of monocrystalline silicon 40.
• the substrate 20 can have a thickness of 500 ⁇ m
• the sacrificial layer 30 can have a thickness of 2 ⁇ m
• the silicon layer 40 can have a thickness of 120 ⁇ m.
• the monocrystalline silicon layer 40 can have any crystalline orientation.
• a lithography step is shown in Figures 3B and 3C.
• lithography is meant all the operations making it possible to transfer an image or pattern on or above the wafer 10 to the latter.
• layer 40 is covered with a protective layer 50, for example of a polymerizable resin.
• This layer 50 is structured, typically by a photolithography step using an ultraviolet light source as well as, for example, a photo-mask (or another type of exposure mask) or a stepper and reticle system. This structuring by lithography forms the patterns for the plurality of resonators in layer 50, as shown in Figure 3C.
• the patterns are machined, in particular etched, to form the plurality of resonators 100 in the layer 40.
• the etching can be performed by a deep reactive ion etching technique (also known by the acronym DRIE for "Deep Reactive Ion Etching"). After etching, the remaining part of the protective layer 50 is subsequently removed.
• DRIE deep reactive ion etching technique
• the resonators are released from the substrate 20 by locally removing the sacrificial layer 30 or even by etching all or part of the silicon of the substrate or handler 20. Smoothing (not shown) of the etched surfaces can also take place before the release step, for example by a thermal oxidation step followed by a deoxidation step, consisting for example of wet etching based on hydrofluoric acid (HF).
• HF hydrofluoric acid
• the turns 110 of the silicon resonator 100 are covered with a layer 120 of silicon oxide (SiO2), typically by a thermal oxidation step to produce a thermo-compensated resonator.
• This layer 120 which generally has a thickness of 2-5 ⁇ m, also affects the final stiffness of the resonator and therefore must be taken into account during the previous steps to obtain the vibratory characteristics of the hairspring leading to obtaining a particular natural frequency. of the balance-spring couple in a given watch mechanism.
• the various resonators formed in the wafer generally have a significant geometric dispersion between them and therefore a significant dispersion between their stiffnesses, notwithstanding that the steps Pattern formation and machining/etching through these patterns are the same for all resonators.
• this dispersion of stiffness is even greater between the hairsprings of two wafers engraved at different times even if the same process specifications are used.
• the above description relates to resonators 100 made of silicon, but it is possible to envisage making the resonators out of glass, ceramic, carbon nanotubes, or even metal.
• the resonators obtained in step 3E on the wafer 10 in question can be deliberately formed with dimensions d which are different from the dimensions necessary (for example greater) to obtain a nominal or target stiffness.
• d the dimensions necessary (for example greater) to obtain a nominal or target stiffness.
• the present invention proposes to determine from at least one characteristic of a resonant frequency of a sample of resonators 100 on the wafer in step 3E and whether a geometric correction of the resonators is necessary. If so, the present invention proposes to precisely calculate the thickness of material to be modified (to be removed or added), around each turn, to obtain the dimensions leading to obtain the vibratory characteristics of the resonators (natural frequency and/or resonance frequencies, and/or stiffness) corresponding to target values, according to a more efficient method than the methods of the prior art.
• the invention proposes to determine at least one characteristic of a resonant frequency of a sample of resonators by vibration measurement and to apply a predictive method (for example a numerical model or a method of classification or categorization) to relate the result of said vibration measurement to the necessary geometric correction.
• a predictive method for example a numerical model or a method of classification or categorization
• the measurement of the vibratory response of the resonators makes it possible to deduce at least one characteristic of a resonance frequency, such as for example a value of a resonance frequency.
• a vibratory excitation on the wafer.
• a piezoelectric source or any other source making it possible to induce or impose an acoustic excitation
• the edge of the wafer, on, or under the blank of the hairspring 200 to be excited specifically which excites at a particular frequency fo (continuous mono-frequency excitation).
• the excitement is maintained.
• the piezoelectric source or any other source making it possible to induce or impose an acoustic excitation
• the balance-spring blank 200 to be excited specifically (preferred) which excites at a variable frequency over time to cover a predetermined frequency range, ranging for example from 0 to 100 kHz, preferably from 0 to 75 kHz, preferably from 0 to 50 kHz, preferably from 5 kHz to 50 kHz, and preferably 10 to 35 kHz.
• the entire frequency range can be swept or covered in a time interval that can range from a fraction of a second to a few seconds.
• the excitation frequency changes continuously.
• Measurements in the time domain use an excitation hammer (or any other source making it possible to induce an acoustic impulse) on the edge of the wafer, on, or under the hairspring to be excited specifically (preferred) which gives an acoustic impulse as short as possible (multi-frequency pulse excitation).
• the excitation is punctual and not maintained.
• the measurements can be performed by following a particular sampling, for example according to a sampling range of 4, 2 or 1 Hz.
• a sampling range of 4, 2 or 1 Hz the resolution for processing the acquisition data according to for example a transform of Fourier depends directly on the duration of this acquisition.
• Optical reflectometry a. Analysis of vibration by beam deflection on a multi-dial detector or camera, b. Analysis by TCSPC type temporal analysis,
• FIG. 5 schematically represents a silicon wafer 25 on which are formed a plurality of spiral blanks 200.
• a vibratory excitation source 400 is coupled to the wafer 25, so as to be able to impose a vibratory excitation . Consequently, each hairspring blank 200 will begin to vibrate, and a laser vibrometer 300, here focused on a point of the hairspring blank 200 on the right, will be able to measure the vibration amplitudes of the measurement point over time. Provision can be made to measure the displacements in a direction normal to the plane of the wafer 25, but can just as well measure the displacements in one or more directions contained in the plane of the wafer 25.
• the laser vibrometer 300 can be moved to another measurement point of the hairspring blank 200, or move on to another hairspring blank 200 of the wafer 25. Of course, one can alternately moving the spiral blank 200 relative to the laser vibrometer.
• Figure 6 shows an example of vibratory excitation over time.
• the excitation frequency varies over time, between 0 Hz and 50 kHz, and a succession of rising edges can be imposed, each spaced by a period of rest without excitation.
• a plurality of rising edges can be imposed (between 2 rising edges and 60 rising edges), each lasting between 0.5 s and 2 s for example.
• a step can be provided consisting in identifying points of the resonator for which the vibratory response is significant. Indeed, in the case of a hairspring on which a vibration is imposed, especially if the frequency varies over time, the vibratory response will cause nodes to appear on the hairspring, that is to say particular points of the hairspring whose displacement amplitude is low or zero. If a displacement measurement is made on a point of the hairspring which turns out to be a node at one or more particular frequency(ies), the identification of resonant frequency characteristics will be negatively affected.
• a preliminary step of measuring displacement on a plurality of predetermined points of the hairspring for example at least ten predetermined points, preferably at least twenty predetermined points, and very preferably at least least thirty predetermined points. Provision can be made to select the predetermined points arranged on an orthonormal reference XY in the plane of the hairspring. At the end of this preliminary step of amplitude measurement on the predetermined points, provision can be made to identify resonance frequencies for each measurement point, and then a step of selecting reference points for which the Displacement amplitude measurement during excitation shows that they are not nodes at these resonant frequencies.
• the identified nodes have, at at least one resonance frequency, a zero displacement amplitude or less than a first threshold peak value, and these points forming nodes are moved away from the reference points to be considered for subsequent measurements. It can also be noted that the reference points are different depending on the position of the spiral blank 200 on the wafer 25.
• At least two reference points will be selected, and preferably at least four reference points will be selected.
• the resonator has a radius Ra and is anchored or embedded on the plate by its outer end of the stud, it is possible to preferably select four chosen and located reference points:
• the reference points are far from the part anchored on the wafer and naturally have a high capacity for oscillatory displacement, which ensures better precision of the displacement measurement.
• 1 - variant with sustained excitation i. Temporally integrate the amplitude and the phase of oscillation long enough to have a good spectral resolution at the excitation frequency fo, ii. Shift the oscillation frequency from delta f to excite at frequency fo + A f and repeat integration step i, iii. Reconstruct oscillation amplitude and phase spectra as a function of excitation frequency (possibly with multiple peaks at multiple frequencies).
• the area of the curve between 25% and 75% of the maximum amplitude value of the resonance peak has better accuracy than the part above 75% (typically the peak), which offers better accuracy on the exact frequency of determined resonance.
• FIG. 7 represents an example of a vibration spectrum for a point of a balance-spring blank 200 of FIG. 5, reconstructed from the displacement amplitude measurements of the measurement point considered in response to the vibratory excitation of Figure 6, between 10 kHz and 15 kHz.
• Each amplitude peak has a resonant frequency, and the peak amplitudes vary greatly.
• FIG. 8 represents in detail the processing that can be done on an amplitude peak, that at 11 kHz for example.
• the goal is to find the resonant frequency and give it as accurate a value as possible.
• the applicant noticed that better precision could be achieved by determining the length of the segment connecting the rising part and the falling part of the curve, at mid-height of the peak.
• the resonance frequency being typically the value in the middle of this segment.
• FIG. 9 represents, for the example of an amplitude peak at around 10 kHz, the amplitude peaks constructed for around ten balance spring blanks 200 tested. It can be noted that from one balance-spring blank to another, the frequency position of the amplitude peak varies (from approximately 9.8 kHz to 10.02 kHz), and that the maximum displacement amplitude varies in a ratio of 1 to 5 approx. Since the tops of amplitude peaks are not really symmetrical, it seems judicious to determine the resonant frequency on the basis of the width of the peak at mid-height.
• the stiffness can also be deduced from a reaction torque measurement at the ferrule using a rheometer.
• the acquired signal represents the evolution of the torque as a function of the amplitude.
• the analysis of the slope of this curve for low amplitudes (linear part) makes it possible to deduce the stiffness, and then the dimensions of the bar of the resonator. The dimensions of the hairspring bar can then be determined.
• a high-resolution 3D X-ray tomography approach would make it possible to extract point clouds giving the 3D material density of the balance-springs, and, subject to appropriate image reconstruction, a cartography of the section of the balance-spring.
• point clouds giving the 3D material density of the balance-springs
• image reconstruction a cartography of the section of the balance-spring.
• Another approach consists in analyzing the forced oscillations of a hairspring on a reference balance wheel with an escapement.
• An alternative can be envisaged from an acoustic acquisition (Witschi type microphone) which records the shocks of the different operating phases of the escapement/anchor system. The data measured are either scatter plots of the passage times of the arms of the balance wheel, or the temporal evolution of the sound pressure level.
• the oscillation amplitude measurements are performed on physical resonators, and resonance frequencies are identified.
• a correlation phase must be provided during which a predictive model is constructed.
• This database can also be supplemented by experimental measurements by measuring vibration spectra, oscillation periods and the positions of hairsprings on the wafer as well as their associated stiffnesses.
• One of the advantages of this approach lies in the fact that the learning database is enriched as the trials progress. This can make it possible to have an adaptive model according to the pads and the hairsprings and contributes to the reduction of the standard deviation in stiffness on the pads.
• This database can be used to build a prediction model, and several solutions are offered.
• a digital model for example polynomial, can be constructed to calculate, as a function of a resonance frequency value, a real thickness, a dimensional correction or a real stiffness.
• a neural network for example a perceptron
• the learning phase includes a test phase (excitation of resonators with measurement of the vibration characteristics to reconstruct a vibration spectrum and identify resonance frequencies).
• a phase of measuring the stiffnesses and/or the dimensions of the bar of the resonators is also carried out.
• the construction phase of the prediction model can be carried out.
• stiffness can therefore be predicted and compared with the actual measured stiffness as shown in the table below, with for the first six lines the data used to build or train the linear regression, and for the last four lines, a prediction only:
• figure 10 represents the linear regression line for the values of the first six lines.
• the established prediction model has good sensitivity, that is to say that for two different input values, the model gives two distinct output values.
• the leading coefficient is 0.0015 10' 7 N.mm/Hz.
• the resonance modes in particular the modes of deformation and/or displacement of the resonators
• the resonance modes could differ significantly, which can also affect the sensitivity of stiffness and/or dimensional correction prediction. It is advantageous to provide, during the learning phase, a step of comparing the sensitivity of the prediction to choose to consider later such or such resonance frequency and not another to predict as accurately as possible a stiffness and /or a dimensional correction depending on the vibration response.
• the learning phase makes it possible to choose either resonance peaks at high frequencies and/or resonance peaks which correspond to particular resonance modes making it possible to predict precise and reliable values, and the range frequency will be predetermined to include at least one resonance peak and preferably several, to be able to make either a single prediction as precise as possible, or several predictions (one per resonance peak deemed interesting) to then carry out cross-checks, averages or even readjustments of the predicted values.
• the learning phase is complete, it is possible to move on to a prediction phase, for example during a resonator control method.
• the control process can typically be carried out on hairspring blanks made on a wafer and still attached to this wafer, so as to estimate the stiffness and/or the dimensions of the bar of the hairsprings of the sample, in order to determine whether a correction dimension is to be brought.
• control procedure to be deployed is as follows: 1 ) Identification of the position of the hairspring on the wafer, vibration measurement of the spectra or period of oscillation (as described above),
• the manufacturing process can include, in addition to the control above:
• step 1) and step 2) of the checking process to check the stiffness/dimensions of the hairspring and confirm that the target values are reached, within a tolerance threshold, or repeat these steps and the dimensional correction until until the stiffness/dimension predicted by the model reaches the target values.
• the method consisting in identifying resonance frequencies by imposing a vibratory excitation on the balance-spring blanks alone, makes it possible to quickly obtain measurement data, without having for example to carry out operations to mount a balance wheel, while limiting measurement errors because only the balance-spring blank is tested (there is no error that can be linked to the balance wheel, such as its mass, its mounting position, etc.).

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• 2021
  • 2021-01-18 EP EP21152144.8A patent/EP4030243A1/de active Pending
• 2022
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