Balance spring for a timepiece resonator

Abstract

The present invention relates to a balance spring for a timepiece resonator, comprising a wound blade having a variable stiffness and / or a variable pitch. The variation in stiffness and / or pitch is such that the rate due to the eccentric development of the balance spring in the horizontal position decreases across the whole range of oscillation amplitudes between 130° and 220°.

Description

[0001]Spiral for watch resonator The present invention relates to a spiral for a watch resonator. A watch resonator generally consists of a balance wheel and a balance spring mounted on the balance staff and serving as a return spring for the balance wheel. The oscillations of the balance wheel are maintained by an escapement. The resonator and the escapement together form an oscillator. The balance wheels generally oscillate between amplitudes of 240 and 300°. In this range, the average rate of the balance wheel (average of the rates in its different horizontal and vertical positions) is decreasing so that it can compensate for the delay due to the escapement, as described in patent EP 2917787 of the present applicant and mentioned in patent EP 2299336 in the name of Rolex SA. It may nevertheless be desired to oscillate the balance wheel at lower amplitudes, less than 220°,in order to reduce the power dissipated by the balance wheel and thus increase the power reserve of the watch movement that the oscillator equips. However, the average rate of known hairsprings – whether they have the classic shape of an Archimedean spiral, with or without a Breguet terminal curve, or whether they have variable rigidity and / or a variable pitch along their blade – is initially increasing for small amplitudes, then has a maximum around or beyond an amplitude of 150°, then decreases up to large amplitudes. Known hairsprings therefore do not allow the rate delay due to the escapement to be compensated for for small oscillation amplitudes. The present invention aims to remedy this drawback and for this purpose proposes a hairspring according to claim 1. In the state of the art, variations in rigidity and / or pitch are generally used to make the development of the hairspring concentric,cf. for example patent EP 1473604 of the present applicant and the aforementioned patent EP 2299336 in the name of Rolex SA. These rigidities and / or variable pitches do not change, however, or only very little,the shape of the rate curves for small amplitudes. The present applicant has discovered that variations in rigidity and / or pitch along the blade forming the balance spring can be found which allow the rate due to the eccentric development of the balance spring in the horizontal position (corresponding to the average rate) to decrease throughout the range of oscillation amplitudes between 130° and 220° and thus partially or totally compensate for the rate due to the escapement throughout this range. The oscillator can therefore be operated at small amplitudes without degrading its isochronism. The present invention will be better understood upon reading the following detailed description made with reference to the appended drawings in which: - Figure 1 is an isochronism (rate) diagram, obtained by analytical modeling,of a traditional spiral in the form of an Archimedean spiral positioned horizontally; - figures 2 and 3 are reproductions of figures 4 and 8 of patent EP 2299336, which represent isochronism curves of spirals whose thickness and pitch vary in a non-monotonous manner to make the development of the spiral substantially concentric; - figure 4 is a plan view of a spiral according to a first exemplary embodiment of the invention; the numbers 3, 6, 9 and 12 around the spiral indicate the angular position of the spiral relative to the dial of the timepiece that it is intended to equip; - figure 5 is a diagram of the thickness of the blade of the spiral of figure 4 as a function of the number of turns measured from the inner end of the spiral; - figure 6 is a diagram of the pitch of the spiral of figure 4 as a function of the number of turns measured from the inner end of the spiral; - figure 7 is a diagram, obtained by numerical simulation,showing the rate due to the eccentric development of the balance spring of figure 4 in the horizontal position in the oscillation range from 130° to 220°; - figure 8 is a diagram, obtained by numerical simulation, of the rates due to the weight of the balance spring of figure 4 in the four vertical reference positions 3H, 6H, 9H and 12H; - figure 9 is a plan view of a balance spring according to a second exemplary embodiment of the invention; - figure 10 is a diagram of the thickness of the blade of the balance spring of figure 9 as a function of the number of turns measured from the inner end of the balance spring; - figure 11 is a diagram of the pitch of the balance spring of figure 9 as a function of the number of turns measured from the inner end of the balance spring; - figure 12 is a diagram, obtained by numerical simulation, showing the rate due to the eccentric development of the balance spring of figure 9 in the horizontal position in the oscillation range from 70° to 220°; - figure 13 is a diagram,obtained by numerical simulation, of the steps due to the weight of the balance spring of figure 9 in the four vertical reference positions 3H, 6H, 9H and 12H; - figure 14 is a plan view of a balance spring according to a third exemplary embodiment of the invention; - figure 15 is a diagram of the thickness of the blade of the balance spring of figure 14 as a function of the number of turns measured from the inner end of the balance spring; - figure 16 is a diagram of the pitch of the balance spring of figure 14 as a function of the number of turns measured from the inner end of the balance spring; - figure 17 is a diagram, obtained by numerical simulation, showing the step due to the eccentric development of the balance spring of figure 14 in the horizontal position in the oscillation range from 70° to 220°. In figure 1 is represented the theoretical isochronism curve of a traditional balance spring in the form of an Archimedean spiral and with constant blade thickness,in horizontal position. We can see that between an amplitude of 100°, or even 130°, and an amplitude of 220° the rate is not monotonous but is increasing then decreasing. This prevents the delay caused by the escapement, which is monotonous (increasing rate), from being correctly compensated. The aforementioned isochronism curve is the rate curve due to the eccentric development of the balance spring in horizontal position, that is to say the rate curve due to the reactions of the balance staff pivots to the decentering of the balance spring. Indeed, the non-concentric development of a balance spring during the oscillation of the balance with which it is associated generates lateral forces from the pivots on the bearings in which they rotate,lateral forces that vary depending on the oscillation amplitude and decrease or increase the frequency of the balance-spring resonator. This disturbance is the same in all horizontal or vertical positions of the resonator. It can be expressed by a rate μ1, in seconds per day (s / d), as a function of the amplitude θ0 of the balance: ^^ 86400 ^^ ^, ^ (^ ^ ) = 2^^ ^ ^ ^^(^)^^(^)^^ ^ ^^ ^ where: θ is the elongation of the pendulum relative to its equilibrium position: θ(φ) = θ0cos φ σ being the radius of gyration of the balance spring and Δ(θ) being the displacement of the geometric center of the balance spring (located on the balance wheel rotation axis) during contractions and expansions of the balance spring in a theoretical situation where this geometric center is free, the balance wheel axis not being held by any bearing, the outer end of the balance spring being taken as a fixed reference point. To this disturbance due to the reactions of the pivots is added, in vertical positions, a disturbance due to the weight of the balance spring, linked to the displacements of the center of mass of the balance spring caused by the eccentric development of the latter. This additional disturbance can be expressed by a rate μ2, in seconds per day (s / d), as a function of the amplitude θ0 of the balance: where Ms is the mass of the hairspring, L is the length of the hairspring, E is the Young's modulus of the hairspring, I is the quadratic moment of the hairspring, g is the constant of gravity, θ is the elongation of the balance wheel relative to its equilibrium position (θ = θ0 cos ^) and yg is the ordinate of the center of mass of the hairspring in a frame (O, x, y) where O is the geometric center of the hairspring and the y axis is opposite to the force of gravity. For more explanations on the two aforementioned disturbances, please refer to the book "Traité de construction horlogère" by M. Vermot, P. Bovay, D. Prongué and S. Dordor, published in 2011 by Presses polytechniques et universitaires romandes. As already indicated, the rate μ1 is identical in all positions, horizontal and vertical, of the oscillator. The same is true of the rate due to the escapement. On the other hand, the rate μ2 due to the weight of the hairspring differs depending on the vertical position.The average of the steps μ2 for the four vertical reference positions (3H, 6H, 9H and 12H) spaced 90° apart is zero. We can therefore consider the step μ1 as representing the average of the steps of the balance spring in all the positions of said balance spring. Figures 2 and 3 are reproductions of Figures 4 and 8 of patent EP 2299336. The FH curve of each of these figures corresponds to the step μ1 described above, namely to the step due to the eccentric development of the balance spring in the horizontal position or step due to the reactions of the pivots. If we draw the average curve of the steps in the four vertical positions (average of the curves 3H, 6H, 9H and 12H), we see that it is confused with the FH curve, which is consistent with the above statement. The CH curve of each of Figures 2 and 3 does not represent only the step due to the reactions of the pivots. An additional disturbance, not indicated in the text of patent EP 2299336, has clearly been added.The balance springs according to patent EP 2299336 are formed from a blade of variable thickness (more generally of variable rigidity) wound with a pitch that also varies. The variations in thickness and pitch are chosen so that the development of the balance spring is substantially concentric and so that the radial forces of the pivots in their bearings are substantially zero. The FH curve shows that the rate due to the reactions of the pivots only decreases from an amplitude of approximately 200° (figure 2) or even does not decrease (figure 3), which makes it impossible to compensate for the rate delay due to the escapement for small amplitudes. In the present invention, the balance spring is designed with a variable blade thickness and / or a variable pitch so as to obtain a decreasing rate μ1 in a range of amplitudes including small amplitudes.In all that follows, the thickness of the hairspring blade and the pitch of the hairspring are measured radially relative to the geometric center of the hairspring. In addition, the pitch, i.e. the distance between two consecutive coils, is measured between the neutral fibers of the coils. Examples of hairsprings according to the invention are illustrated in Figures 4, 9 and 14.These hairsprings are made of a silicon-based material, more precisely silicon covered with a thermal compensation layer of silicon oxide, and have a coil height of 120 μm, a distance between their inner end and their geometric center (distance measured between the neutral fiber and the geometric center) of 0.565 mm, a distance between their outer end and their geometric center (distance measured between the neutral fiber and the geometric center) of 2.35 mm and a number of turns which varies from one hairspring to another, namely respectively 9.62 turns, 8.44 turns and 7.37 turns for the hairsprings of figures 4, 9 and 14. As can be seen in figures 5, 10 and 15, the thickness of the blade forming the hairspring decreases continuously and over more than one turn from the inner end of the hairspring (abscissa 0 on the diagram) until reaching a minimum.Then, from this minimum, the thickness of the blade increases continuously and over more than one turn to the outer end of the hairspring. The pitch of these examples of hairsprings is illustrated in Figures 6, 11 and 16. It increases continuously over more than half of its length, in number of turns, and can increase continuously from the inner end to the outer end. More generally, it is advantageous in the present invention for the thickness of the blade forming the hairspring to decrease continuously and over more than one turn in the direction of winding of the blade from a first point located on the inner coil and to decrease continuously and over more than one turn in the direction opposite to the direction of winding of the blade from a second point located upstream of the first point in said direction opposite to the direction of winding of the blade.Preferably, the second point is located on the outer coil or on the coil immediately following the outer coil in said direction opposite to the winding direction of the blade. The first point may be the inner end of the hairspring or a point distinct from the inner end. The second point may be the outer end of the hairspring or a point distinct from the outer end. Preferably, in the winding direction of the blade, the thickness of the blade decreases continuously from the first point until it reaches a minimum (which may be the thickness of a single point of the hairspring or of a portion of the hairspring), then increases continuously from this minimum to the second point. Preferably, the point or portion of minimum thickness is closer to the first point than to the second point in number of turns.Preferably, the thickness at the first point is greater, and even greater by a factor greater than 2, than the thickness at the second point. Figures 7, 12 and 17 show the rate μ1 due to the eccentric development of the balance spring in the horizontal position for the balance springs of Figures 4, 9 and 14 respectively. It can be seen that, in the entire range of amplitudes from 130° to 220°, or even from 120° to 220°, or even from 110° to 220°, or even from 100° to 220°, or even from 90° to 220°, or even from 80° to 220°, or even from 70° to 220°, the rate μ1 is decreasing, which makes it possible to compensate for the increasing rate of an escapement, in particular a Swiss lever escapement. The balance springs of figures 4, 9 and 14 produce a rate deviation μ1 between the amplitudes of 130° and 220° greater than 1.2 s / d, 1.5 s / d and 5 s / d respectively, making it possible to compensate for a corresponding rate delay due to the escapement in this amplitude range.In general, the balance springs according to the invention preferably produce a rate deviation μ1 between the amplitudes of 130° and 220° of at least 1 s / d, or even at least 1.5 s / d, or even at least 2 s / d, or even at least 3 s / d, or even at least 4 s / d. These examples of balance springs are further designed such that the minimum distance between the turns, at the most unfavorable point in contraction, distance measured between the two facing faces, is 25 μm. Figures 8 and 13 show the rate μ2 due to the weight of the balance spring for the balance springs of Figures 4 and 9 respectively. The rate μ2 is shown for the four vertical reference positions 3H, 6H, 9H and 12H spaced 90° apart. We can see that for each of these four vertical positions, the step μ2 is cancelled at an oscillation amplitude between 200° and 240°, or even between 210° and 230°.This passage of the curves through zero at approximately 220°, rather than at amplitudes of 163.5° and 330.5° as in the case of an Archimedean spiral, makes it possible to apply the teachings of patent EP 3433680 and patent application EP 3913441 of the present applicant, that is to say in particular to compensate the rate μ2 due to the weight of the balance spring by the rate due to the balance wheel imbalance, the balance wheel imbalance (unbalance and angular position of the center of gravity) being chosen for this purpose. In this way, the rate differences between the different vertical positions are minimized. It is therefore possible in the present invention to optimize both the isochronism of the oscillator, by minimizing the variations in the rate or frequency as a function of the oscillation amplitude, and the rates at the positions, and this while the spirals have an eccentric, or even very eccentric, development.The present invention is not limited to the examples of hairsprings described above. Other combinations of variations in thickness and pitch may be found. It is also possible to vary only the thickness or only the pitch. Furthermore, an alternative to varying the thickness would be to vary the stiffness of the blade in another way, for example by heat treatment or doping the silicon. The present invention is also not limited to silicon. Other materials, in particular metallic materials, may be used.

Claims

CLAIMS 1. Spiral for a watch resonator comprising a wound blade having variable stiffness and / or variable pitch, characterized in that the variation in stiffness and / or pitch is such that the rate of change due to the eccentric development of the spiral in the horizontal position decreases throughout the entire range of oscillation amplitudes between 130° and 220°.

2. Spiral according to claim 1, characterized in that the variation in stiffness and / or pitch is such that the rate of change due to the eccentric development of the spiral in the horizontal position decreases throughout the entire range of oscillation amplitudes between 120°, preferably 110°, preferably 100°, preferably 90°, preferably 80°, preferably 70°, and 220°. 3.A spiral according to claim 1 or 2, characterized in that the difference in said step between the amplitude of 130° and the amplitude of 220° is at least 1 second per day, preferably at least 1.5 seconds per day, preferably at least 2 seconds per day, preferably at least 3 seconds per day, preferably at least 4 seconds per day.

4. A spiral according to any one of claims 1 to 3, characterized in that the stiffness of the blade decreases continuously over more than one turn in the direction of the blade winding from a first point located on the inner coil of the spiral and decreases continuously over more than one turn in the opposite direction to the blade winding from a second point located upstream of the first point in said opposite direction to the blade winding.

5. Spiral according to claim 4, characterized in that the second point is located on the outermost turn of the spiral or on the turn immediately following the outermost turn in said direction opposite to the direction of winding of the blade.

6. Spiral according to claim 4 or 5, characterized in that the first point is the inner end of the spiral and / or the second point is the outer end of the spiral.

7. Spiral according to any one of claims 4 to 6, characterized in that, in the direction of winding of the blade, the stiffness of the blade decreases continuously from the first point to a minimum, and then increases continuously from this minimum to the second point.

8. Spiral according to any one of claims 4 to 7, characterized in that the stiffness of the blade at the first point is greater than the stiffness of the blade at the second point. 9.A spiral according to any one of claims 1 to 8, characterized in that the stiffness of the blade at the inner end of the spiral is greater than the stiffness of the blade at the outer end of the spiral.

10. A spiral according to any one of claims 1 to 9, characterized in that its pitch increases continuously over more than half the length of the blade, in number of turns.

11. A spiral according to any one of claims 1 to 10, characterized in that the variation in stiffness and / or pitch is such that each of the respective steps due to the weight of the spiral in four vertical positions. spaced 90° apart cancel each other out at an oscillation amplitude between 200° and 240°, preferably between 210° and 230°.

12. A clockwork oscillator comprising a balance spring according to any one of claims 1 to 11, a balance wheel associated with the balance spring, and an escapement for maintaining the oscillations of the balance wheel, said escapement partially compensating for at least a rate delay due to the escapement throughout the entire range of oscillation amplitudes between 130°, preferably 120°, preferably 110°, preferably 100°, preferably 90°, preferably 80°, preferably 70°, and 220°.

13. A clockwork oscillator according to claim 12, characterized in that the escapement is a Swiss lever escapement.

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