EP3598242A1 - Cam timepiece mechanism - Google Patents

Abstract

The clock mechanism (1) according to the invention comprises a cam (4) intended to be driven in rotation, a cam follower (2) and a return spring (9) arranged to hold the cam follower (2) in support. against the cam (4), the return spring (9) being arranged to work within a predetermined range of winding angles during each rotation of the cam (4). The stiffness of the return spring (9) is zero or negative in at least part of the predetermined range.

Description

The present invention relates to a cam clock mechanism.

Mechanisms are known in watchmaking for instantaneous driving of an indicator comprising a spiral cam known as a snail cam or snail cam against which a rocker rests under the action of a return spring applied against the rocker. The return spring is a V-shaped, U-shaped or spiral blade. At each rotation of the cam, the rocker slides from the lower part towards the upper part of the cam, which gradually arms the return spring, then the rocker drops from said upper part to said lower part, this sudden movement, considered instantaneous, being used to actuate an indicator such as a needle associated with a scale or a disc bearing indications and cooperating with a window. Patent applications CH 702137 and EP 2241944 , for example, describe such mechanisms for a minute counter.

These mechanisms have the disadvantage that the torque to be produced to rotate the cam varies as a function of time. Indeed, the resistant torque exerted by the rocker and its return spring increases during the movement of the rocker from the lower part to the upper part of the cam, this due to the force of the return spring which increases linearly with its degree of armament and also because of the spiral shape of the cam which increases the lever arm of the force applied to the cam by the rocker.

This variation in torque increases the energy consumption and affects the regularity of the oscillations of the regulating body of the watch and therefore the accuracy of the measurement.

Similar problems arise with other types of watchmaking cam mechanisms, instantaneous or not, for example retrograde display mechanisms comprising a snail cam cooperating with a rake, the instantaneous date display mechanism described in request patent EP 1746470 where the periphery of the cam has a main spiral part, a convex part and a concave part, or the mechanisms with a patatoid cam such as the mechanisms of equation of time or display of sunrise and sunset times.

The present invention aims to alleviate these problems and proposes for this purpose a timepiece mechanism according to claim 1, namely a timepiece mechanism comprising a cam intended to be driven in rotation, a cam follower and a return spring arranged to hold the follower cam resting against the cam, the return spring being arranged to work within a predetermined range of winding angles during each rotation of the cam, characterized in that the stiffness of the return spring is zero or negative in at least part of the predetermined range.

Particular embodiments of the timepiece mechanism according to the invention are defined in the dependent claims.

The invention further provides a timepiece, such as a wristwatch or a pocket watch, comprising this timepiece mechanism.

• la figure 1 est une vue plane de dessous d'un mécanisme horloger à came selon un exemple de réalisation de l'invention ;
• la figure 2 est une vue plane de dessus d'une pièce du mécanisme horloger selon l'invention comprenant un ressort de rappel ;
• la figure 3 est une vue plane de dessus d'une variante de ladite pièce ;
• la figure 4 est une représentation graphique schématique du moment de rappel élastique exercé dans la pièce illustrée à la figure 2 ;
• la figure 5 représente les coordonnées de points définissant une forme particulière d'un bras élastique constituant le ressort de rappel ;
• la figure 6 est une représentation graphique du moment de rappel élastique exercé dans la pièce illustrée à la figure 2 par le ressort de rappel ayant la forme telle que représentée à la figure 5 ;
• la figure 7 est une représentation graphique d'un moment de rappel élastique normalisé exercé dans la pièce illustrée à la figure 2 par un bras élastique ayant la forme telle que représentée à la figure 5 selon différentes variantes du bras élastique, à savoir un tel bras à section constante (courbe C1) et un tel bras à section variable (autres courbes), la section variant selon un premier mode de variation ;
• la figure 8 est une représentation graphique d'un moment de rappel élastique normalisé exercé dans la pièce illustrée à la figure 2 par un bras élastique ayant la forme telle que représentée à la figure 5 selon différentes variantes du bras élastique, à savoir un tel bras à section constante (courbe C1) et un tel bras à section variable (autres courbes), la section variant selon un deuxième mode de variation.

Other characteristics and advantages of the present invention will appear on reading the following detailed description made with reference to the accompanying drawings in which:

• the figure 1 is a plan view from below of a cam clock mechanism according to an exemplary embodiment of the invention;
• the figure 2 is a plan view from above of a part of the timepiece mechanism according to the invention comprising a return spring;
• the figure 3 is a plan view from above of a variant of said part;
• the figure 4 is a schematic graphic representation of the elastic return moment exerted in the part illustrated in the figure 2 ;
• the figure 5 represents the coordinates of points defining a particular shape of an elastic arm constituting the return spring;
• the figure 6 is a graphic representation of the elastic return moment exerted in the part illustrated in the figure 2 by the return spring having the shape as shown in the figure 5 ;
• the figure 7 is a graphic representation of a normalized elastic recall moment exerted in the part illustrated in the figure 2 by an elastic arm having the shape as shown in the figure 5 according to different variants of the elastic arm, namely such a constant section arm (curve C1) and such a variable section arm (other curves), the section varying according to a first mode of variation;
• the figure 8 is a graphic representation of a normalized elastic recall moment exerted in the part illustrated in the figure 2 by an elastic arm having the shape as shown in the figure 5 according to different variants of the elastic arm, namely such a constant section arm (curve C1) and such a variable section arm (other curves), the section varying according to a second mode of variation.

To the figure 1 a clockwork mechanism 1 is shown according to an exemplary embodiment of the invention, mounted on a frame 1a. In this example, the mechanism 1 is an instantaneous minute counter mechanism of a chronograph. It includes a rocker 2 pivoted in O and having a feeler 3 cooperating with a snail cam 4 mounted on, and driven by, the chronograph axis 5. This chronograph axis 5 carries at its upper end the hand indicating the seconds of chronograph 6 and is integral in rotation with the chronograph wheel 7 and the chronograph seconds reset heart 8. The lever 2 is held in abutment against the periphery of the snail cam 4 by a lever return spring 9 acting on the axis 10 of a finger 11, this finger 11 acting itself on the lever 2. The cooperation between the finger 11 and the lever 2 is of the rolling type. The finger 11 indeed interacts with the wall of a recess 12 of the rocker 2 in the manner of an engagement, almost without friction. The rocker 2 and the finger 11 thus rotate in opposite directions.

A hook 13 is pivoted at P on the free end of the lever 2 and is subjected to the action of a hook return spring 14, mounted on the lever 2, tending to apply the spout 15 of the hook 13 against the wolf teeth teeth of a minute counter wheel 16. The axis 17 of the minute counter wheel 16 carries a chronograph minute indicator 18, such as a hand (as shown) or a disc, displaying the chronograph minutes in cooperation with the chronograph dial. A heart for resetting the chronograph minutes 19 to zero is integral in rotation with the minute counter wheel 16. The minute counter wheel 16 is held in angular positions determined between its successive actuations by a jumper 20 on which acts a jumper return spring 21.

In the example illustrated, the snail cam 4 has a slot 22 in its terminal part, in accordance with the teaching of the patent application EP 2241944 , but it could have a more classic shape, without this slot 22.

At each revolution of the snail cam 4, the rocker 2 and its return spring 9 are armed as the probe 3 slides from the bottom part B towards the top part H of the cam 4. Each minute, the probe 3 and with it all the rocker 2 falls from the upper part H to the lower part B of the snail cam 4 under the action of the rocker return spring 9. During this fall, the hook 13 advances the wheel one step of the minute counter 16 to instantly change the value indicated by the chronograph minute indicator 18. Then, during the progressive resetting of the lever 2 by the snail cam 4, the hook 13 passes from the the minute counter wheel 16 in which it was located during the fall in the previous crank against the action of its return spring 14, to again advance the minute counter wheel 16 during one step the next fall of rocker 2.

According to the invention, the rocker return spring 9 is specially shaped to improve the constancy of the torque or moment of force which it exerts (indirectly) on the cam 4 and thus, on the one hand, improve the regularity of the oscillations of the chronograph regulator and therefore the accuracy of the measurement and, on the other hand, reduce energy consumption.

As shown to figures 1 and 2 , the rocker return spring 9 is in the form of an elastic arm or blade forming part of a part 23 further comprising a base 24 and a rotary element 25, the elastic arm 9 connecting the base 24 to the element rotary 25, only the elastic arm 9 deforms during the operation of the mechanism 1. The base 24 is fixed, for example by means of pins 26, to the frame 1a. The rotary element 25, intended to rotate on itself, is eccentric relative to the base 24. The rotary element 25 is mounted on the axis 10 of the finger 11 and is integral in rotation with this finger 11. In a variant of the invention, shown in figure 3 , the rotary element 25 is the finger 11 itself, in other words the base 24, the elastic arm 9 and the finger 11 form the part 23.

The part 23 is typically monobloc. It is for example made of metal, alloy, silicon, plastic, mineral glass or metallic glass. It can be produced by machining or by the LIGA technique, in particular in the case where it is made of a metal or alloy, by deep reactive ion etching known as DRIE, in particular in the case where it is made of silicon, by molding, in particular in the case where it is made of plastic or metallic glass, or by laser cutting, in particular in the case where it is made of mineral glass.

For the understanding of the invention, the behavior of the part 23 considered in isolation, that is to say free of any interaction with the rest of the mechanism 1, is described below. The figure 2 represents this isolated piece 23.

Due to the shape of its elastic arm 9, the part 23 has a preferred direction of rotation of its rotary element 25 relative to its base 24, this direction being defined as that which allows, from a state of rest, the isolated piece 23 in which its elastic arm 9 is at rest, the greatest relative angular displacement of the rotary element 25 relative to the base 24. This preferred direction of rotation is the counterclockwise direction to the figure 1 and clockwise at the figure 2 .

Let θ be the angular position of the rotary element 25 of the isolated part 23 relative to the base 24, θ being equal to zero when the isolated part 23 is at rest, that is to say when its elastic arm 9 is at rest, and increasing with the relative angular displacement of the rotary element 25 relative to the base 24 in the preferred direction of rotation of the isolated piece 23; the figure 4 illustrates the evolution M (θ) of the elastic return moment exerted by the elastic arm 9 in the piece 23 isolated as a function of the angular position θ of the rotary element 25 relative to the base 24.

Generally, when the rotary element 25 is in the angular position in which θ = x °, it is said that the part 23 is armed with x °.

• pour un angle θ compris entre 0 et une première valeur θ₁, le moment de rappel élastique augmente rapidement avec la position angulaire θ ;
• au-delà de cette première valeur θ₁, la pièce 23 est dans une phase sensiblement stable. En effet, entre cette première valeur θ₁ et une seconde valeur θ₂, le moment de rappel élastique est sensiblement constant par rapport à la position angulaire θ.
  On entend par moment « sensiblement constant » un moment ne variant pas de plus de 10%, de préférence 5%, de préférence encore 3%, étant entendu que ce pourcentage peut être diminué davantage. Plus précisément, soient respectivement M_min et M_max les valeurs des moments minimum et maximum exercés dans la pièce 23 isolée sur une plage [θ₁, θ₂] donnée de positions angulaires de l'élément rotatif 25 par rapport à la base 24, le moment exercé dans cette pièce 23 isolée est sensiblement constant dès lors que l'inéquation « (M_max-M_min)/((M_max+M_min)/2) ≤ 0,1 » est vérifiée, plus précisément, dès lors que l'inéquation « (M_max-M_min)/((M_max+M_min)/2) ≤ y% », avec y=10, de préférence y=5, de préférence encore y=3, est vérifiée.
  Dans cette phase sensiblement stable, le moment de rappel élastique exercé par le bras élastique 9 dans la pièce 23 isolée atteint toutefois localement un maximum pour une position angulaire θ_a, puis est décroissant dans l'intervalle de positions angulaires compris entre les valeurs θ_a et θ_b, où θ_a et θ_b sont compris entre θ₁ et θ₂ ;
• au-delà de la valeur θ₂, le moment de rappel élastique augmente à nouveau jusqu'à atteindre une valeur limite M_limite, pour un déplacement angulaire θ=θ₃. Cette valeur M_limite dépend des propriétés du matériau dans lequel la pièce 23 est réalisée et correspond à la contrainte maximale que peut subir cette pièce.

As can be seen on the curve M (θ) of the figure 4 , this moment of elastic recall follows an evolution in three phases:

• for an angle θ between 0 and a first value θ ₁ , the elastic return moment increases rapidly with the angular position θ;
• beyond this first value θ ₁ , the part 23 is in a substantially stable phase. Indeed, between this first value θ ₁ and a second value θ ₂ , the elastic return moment is substantially constant with respect to the angular position θ.
  By "substantially constant" moment is meant a moment not varying by more than 10%, preferably 5%, more preferably 3%, it being understood that this percentage can be further reduced. More precisely, let M _min and M _{max respectively be} the values of the minimum and maximum moments exerted in the isolated part 23 over a given range [θ ₁ , θ ₂ ] of angular positions of the rotary element 25 relative to the base 24, the moment exerted in this isolated part 23 is substantially constant as soon as the inequality "(M _max -M _min ) / ((M _max + M _min ) / 2) ≤ 0.1" is verified, more precisely, from then on that the inequality "(M _max -M _min ) / ((M _max + M _min ) / 2) ≤ y%", with y = 10, preferably y = 5, more preferably y = 3, is verified.
  In this substantially stable phase, the elastic return moment exerted by the elastic arm 9 in the isolated piece 23, however, locally reaches a maximum for an angular position θ _a , then decreases in the range of angular positions between the values θ _a and θ _b , where θ _a and θ _b are between θ ₁ and θ ₂ ;
• beyond the value θ ₂ , the elastic return moment increases again until reaching a limit value M _limit , for an angular displacement θ = θ ₃ . This _limit value M depends on the properties of the material from which the part 23 is made and corresponds to the maximum stress that this part can undergo.

The isolated piece 23 having a curve M (θ) of the type of that shown in the figure 4 differs from conventional elastic structures. Its properties are based on a sinuous shape of its elastic arm 9 which deforms so as to generate a substantially constant elastic return moment (the curve M (θ) has a plateau between θ ₁ and θ ₂ ) over a predetermined range of angular positions of its rotary element 25 relative to its base 24. Obtaining such an elastic arm 9 requires a specific and parameterized design. It can for example be obtained by topological optimization by applying the teaching of the publication "Design of adjustable constant-force forceps for robot-assisted surgical manipulation", Chao-Chieh Lan et al., 2011 IEEE International Conférence on Robotics and Automation, Shanghai International Conférence Center, May 9-13, 2011, China .

The topological optimization discussed in the aforementioned article uses parametric polynomial curves such as the Bézier curves to determine the geometric shape of the elastic arm.

Bézier curves are defined, together with a series of m = (n + 1) control points (Q ₀ , Q ₁ , ... Q _n ), by a set of points whose coordinates are given by sums of Bernstein polynomials weighted by the coordinates of said control points.

The geometric shape of the elastic arm 9 is a Bézier curve whose control points have been optimized to take into account, in particular, the dimensions of the part 23 to be designed as well as a constraint "(M _max -M _min ) / ( (M _max + M _min ) / 2) ≤ 0.05 ". The inequality "(M _max -M _min ) / ((M _max + M _min ) / 2) ≤ 0.05" corresponds to a constancy of the elastic return moment of 5% over an angular range.

In general, the elastic arm or rocker return spring 9 is designed, in particular by its shape, to exert, in the part 23, a substantially constant elastic return moment (constancy of 5%) over a range of angular positions of the rotary element 25 relative to the base 24 of at least 10 °, preferably at least 15 °, more preferably at least 20 °.

avec t ∈ [0, 1],
où les $B_i^n$

sont les polynômes de Bernstein donnés par la fonction $$B_i\left(t\right)=\frac{\left(m-1\right)!}{i!\left(m-1-i\right)!}{t}^i{\left(1-t\right)}^{m-i-1}$$

avec t ∈ [0, 1],
et où les Q_i sont les points de contrôle Q₀ à Q_n. Elle correspond à la représentation graphique dans un repère orthonormé de l'ensemble des points définis par les couples de coordonnées (x ; y) définis respectivement par les fonctions x(t) et y(t), t ∈ [0, 1], ci-dessous : $$x\left(t\right)=\sum _{i=0}^{m-1}{Q}_{\mathit{ix}}{B}_i\left(t\right)$$

$$y\left(t\right)=\sum _{i=0}^{m-1}{Q}_{\mathit{iy}}{B}_i\left(t\right)$$

dans lesquelles Q_ix et Q_iy sont respectivement les coordonnées x et y des points de contrôle Q_i.More precisely, the geometric shape of the elastic arm 9 is defined by the set of points $$\underset{i=0}{\overset{\mathrm{not}}{\Sigma }}{B}_i^{\mathrm{not}}\left(t\right)\mathrm{.}{Q}_i,$$

with t ∈ [0, 1],
where the $B_i^{\mathrm{not}}$

are the Bernstein polynomials given by the function $$B_i\left(t\right)=\frac{\left(m-1\right)!}{i!\left(m-1-i\right)!}{t}^i{\left(1-t\right)}^{m-i-1}$$

with t ∈ [0, 1],
and where the Q _i are the control points Q ₀ to Q _n . It corresponds to the graphic representation in an orthonormal coordinate system of all the points defined by the pairs of coordinates (x; y) defined respectively by the functions x (t) and y (t), t ∈ [0, 1], below : $$x\left(t\right)=\underset{i=0}{\overset{m-1}{\Sigma }}{Q}_{\mathit{ix}}{B}_i\left(t\right)$$

$$\mathrm{there}\left(t\right)=\underset{i=0}{\overset{m-1}{\Sigma }}{Q}_{\mathit{iy}}{B}_i\left(t\right)$$

in which Q _ix and Q _iy are respectively the x and y coordinates of the control points Q _i .

The formulas indicated above give the coordinates of a Bézier curve of order m, that is to say a Bézier curve based on m control points. For practical reasons, such a Bézier curve can be broken down into a succession of Bézier curves of order less than m, in which case the geometric shape of the elastic arm is a succession of Bézier curves.

• Distance entre le centre de rotation de l'élément rotatif 25 et le point de jonction du bras élastique 9 à l'élément rotatif 25 : 0,5 mm ;
• Distance entre le centre de rotation de l'élément rotatif 25 et le point de jonction du bras élastique 9 à la base 24 : 2,5 mm ;
• Distance entre les deux extrémités du bras élastique 9 : 2 mm
• Longueur curviligne du bras élastique 9 : 2,4 mm ;
• Epaisseur (largeur) du bras élastique 9 : 25,6 µm ;
• Hauteur de la pièce 23 : 0,3 mm.

Using this principle, the applicant has designed a particular part 23 having the following dimensions:

• Distance between the center of rotation of the rotary element 25 and the junction point of the elastic arm 9 with the rotary element 25: 0.5 mm;
• Distance between the center of rotation of the rotary member 25 and the junction point of the elastic arm 9 at the base 24: 2.5 mm;
• Distance between the two ends of the elastic arm 9: 2 mm
• Curvilinear length of elastic arm 9: 2.4 mm;
• Thickness (width) of the elastic arm 9: 25.6 μm;
• Height of the workpiece 23: 0.3 mm.

As part of this design, seven control points Q ₀ , Q ₁ , Q ₂ , Q ₃ , Q ₄ , Q ₅ , Q ₆ were used. The coordinates of these control points are given in table 1 below. <b> Table 1 </b>: Coordinates of control points Q ₀ to Q ₆. variables Coordinates x [mm] Coordinates y [mm] Q ₀ 0.756625 0.653875 Q ₁ 1.87325 1,619 Q ₂ 2.8125 -0.59125 Q ₃ 3.4375 .4535 Q ₄ 3.75 1.032875 Q ₅ 4.375 0 Q ₆ 5 0

With these seven control points it would have been possible to produce a Bézier curve of order seven. However, according to the principle indicated above, the Bézier curve has been broken down into two segments, a first segment corresponding to a curve of Bézier of order 4 based on control points Q ₀ to Q ₃ and a second segment corresponding to a Bézier curve of order 4 based on control points Q ₃ to Q ₆ .

Using the coordinates of the control points Q ₀ to Q ₆ above in the above-mentioned functions x (t) and y (t), the applicant obtained the coordinates of the points defining the geometric shape of the elastic arm 9. A certain number of these pairs of coordinates are given in table 2 below. <b> Table 2 </b>: Coordinates of crossing points of the optimized elastic arm. X [mm] Y [mm] 0.756625 0.653875 1.0861324 0.8545816 1.404044 0.903348 1.7094066 0.8387564 2.001267 0.699389 2.2786719 0.5238281 2.540668 0.350656 2.7863021 0.2184549 3.014621 0.165807 3.2246714 0.2312946 3.4155 .4535 3.4155 .4535 3.5242745 0.5815901 3.648736 0.628816 3.7871415 0.6110484 3.937748 0.544158 4.0988125 0.4440156 4.268592 0.326492 4.4453435 0.2074579 4.627324 0.102784 4.8127905 0.0283411 5 0

The graph of the figure 5 shows the external surface of the rotary element 25, the internal surface of the base 24 and the elastic arm 9 of the particular part 23 that the applicant has designed, the geometry of the arm 9 being defined by a curve passing through the assembly coordinates of points defined in table 2 above. This graph is made in an orthonormal coordinate system.

The figure 6 represents the results of a simulation of the evolution of the elastic return moment of the particular part 23 thus produced as a function of the angular position θ of its rotary element 25 relative to its base 24.

The simulation carried out considers a part 23 produced in an amorphous alloy based on zirconium, titanium, nickel, copper and beryllium, more precisely in a metallic glass of the Vitreloy 1b type, but any suitable material can be used. For example materials such as other metallic glasses, other alloys such as Nivaflex® 45/18 (alloy based on cobalt, nickel and chromium), nickel-phosphorus or CK101 (non-alloy structural steel ), silicon, typically coated with silicon oxide, or plastic are also suitable. It is important to take into account the relationship between the elastic limit and the Young's modulus of the material to choose the material constituting the elastic arm 9.

The analysis of the results presented in the figure 6 that an elastic return moment locally maximum then decreasing and finally locally minimum is obtained during a displacement of the rotary element 25 of the particular isolated part 23 studied with respect to its base 24 from an angular position θ _a = 17 ° at an angular position θ _b = 28 °, i.e. over a range of 11 °.

The stiffness of the part 23, more precisely of its elastic arm 9, is the derivative of the function M (θ) defined above.

On the range of angular positions [θ _a , θ _b ] the stiffness is zero at the angular positions θ _a and θ _b and negative between these positions θ _a and θ _b . In the present invention, one places oneself in this range [θ _a , θ _b ] or at least partially in this range.

Within the mechanism 1, the part 23 is therefore arranged so that, at each rotation of the snail cam 4 against the return action of the elastic arm or rocker return spring 9, the rotary element 25 moves in a predetermined range of angular positions relative to the base 24, this range being included in the range of positions [θ ₁ , θ ₂ ] associated with the part 23 and comprising at least part of the range of positions [θ _a , θ _b ] in which the stiffness of the elastic arm 9 is zero or negative. Preferably, said predetermined range is included in the range [θ _a , θ _b ] or constituted by the latter. More preferably, said predetermined range is included in the range] θ _a , θ _b [where the stiffness is negative at each point.

To obtain such an arrangement, the rotary element 25 is angularly positioned when it is mounted on the axis 10 of the finger 11 so that the rocker return spring 9 is armed with θ _arm degrees when the probe 3 of the flip-flop 2 is located on the lower part B of the snail cam 4, this value θ _arm being the lower limit of the aforementioned predetermined range. In order to facilitate this positioning operation, the rotary element 25 may include a mark 27 to be aligned for example with the finger 11. At the figure 1 the part 23 is shown in its rest position, before its pre-winding. The length of the predetermined range is defined by the difference in radius between the upper part H and the lower part B of the cam 4, the position of the lever 2 and that of the finger 11. In the example illustrated, it is 3 °.

Thanks to the at least partly zero or negative stiffness of the rocker return spring 9 in the predetermined range of angular positions that the rotary element 25 can take during the operation of the mechanism 1, the average intensity of the force applied to the snail cam 4 by the rocker return spring 9 via the finger 11 and the rocker 2 on a rotation of the snail cam 4 can be reduced compared to a traditional rocker return spring, for the same force applied to the cam 4 when the probe 3 is on the lower part B, thereby reducing the energy required to rotate the snail cam 4. The springs recall of traditional rockers, V, U or spiral, indeed all have a linear behavior, their stiffness is positive and constant over their entire working range.

The negative stiffness of the rocker return spring 9 also makes it possible to at least partially compensate for the variation of the lever arm of the force applied to the cam 4 by the rocker 2 on a rotation of this cam, more precisely the increase in the lever arm of the force applied to the cam 4 during the movement of the rocker 2 from the lower part B to the upper part H. A smaller variation in the torque required to rotate the cam 4 and therefore better timing can thus be obtained.

It is possible to adjust the negative stiffness value by designing the rocker return spring or elastic arm 9 with a variable section. The figure 7 shows different curves representative of a normalized moment of force M (θ) exerted by the elastic arm 9 in the isolated part 23 for different variations in section of the elastic arm 9. The highest curve, designated by C1, corresponds to an arm elastic 9 of constant section and thickness (width) 30 μm. The curves located below the curve C1 correspond to an elastic arm 9 whose thickness increases linearly from the rotary element 25 to the base 24, the thickness at the point of junction with the base 24 being 30 μm for each curve, the thickness at the junction point with the rotary element 25 being 29 μm for the first curve C2 under the curve C1, of 28 μm for the second curve C3 under the curve C1, of 27 μm for the third curve C4 under curve C1, and so on by decrementing by 1 µm. It is noted that, for at least the first curves, the stiffness decreases (the force moment decreases more) in the range of winding angles of interest where the stiffness is negative when the variation in section is increased. It should also be noted that the length of the range of reinforcement angles where the stiffness is negative increases. We can therefore choose a negative stiffness which completely or almost completely compensates for the effect of the increase in the lever arm of the force applied to the snail cam 4 by the rocker 2 during its displacement from the lower part B to the upper part H. Such compensation makes the energy consumed for the rotation of the cam 4 substantially constant over time and therefore allows the timing to be less disturbed.

Other modes of variation of the section of the elastic arm 9 can be envisaged. The figure 8 shows different curves representative of a normalized moment of force M (θ) exerted by the elastic arm 9 in the isolated part 23. The highest curve, designated by C1, corresponds to an elastic arm 9 of constant section and 30 μm thick. The curves located below the curve C1 correspond to an elastic arm 9 whose thickness increases linearly from the rotary element 25 in the middle of the elastic arm 9 and decreases linearly from the middle of the elastic arm 9 to the base 24, l ' thickness in the middle of the elastic arm 9 being 30 μm for each curve, the thickness at the junction point with the rotary element 25 and at the junction point with the base 24 being 29 μm for the first curve C2 ′ under the curve C1, 28 µm for the second curve C3 'under the curve C1, 27 µm for the third curve C4' under the curve C1, and so on by decrementing by 1 µm. It is noted that this mode of variation of the section of the elastic arm 9 also makes it possible to adjust the negative stiffness in order for example to completely or almost completely compensate for the effect of the increase in the lever arm of the force applied to the snail cam 4 by rocker 2 during its movement from the lower part B to the upper part H.

In general, in cases where the elastic arm 19 has a variable section, this typically varies strictly monotonously (it increases or decreases without interruption but not necessarily linearly) over at least one continuous portion of the elastic arm representing 10% , preferably 20%, preferably 30%, preferably 40%, of the length (curvilinear) of the elastic arm. The variation of the section is also chosen to make the stiffness of the elastic arm 19 more negative over the range [θ _a , θ _b ] or at least over the part of the predetermined range which overlaps with the range [θ _a , θ _b ], relative to an elastic arm of the same shape as the arm 19 but of constant section.

In variants, the rocker return spring or elastic arm 9 may have a shape different from that illustrated in figures 1 and 2 . It can in particular take a form as described in the article "Functional joint mechanisms with constant-torque outputs", Mechanism and Machine Theory 62 (2013) 166-181, Chia-Wen Hou et al.

It will be clear to a person skilled in the art that instead of being made up of a single elastic arm, the rocker return spring 9 could comprise several elastic arms connecting the base 24 to the rotary element 25, to the like the devices described in the two articles "Design of adjustable constant-force forceps for robot-assisted surgical manipulation" and "Functional joint mechanisms with constant-torque outputs" mentioned above. In the embodiment illustrated in figure 1 , a single elastic arm 9 is sufficient since the latter has no guiding function - the rotary element 25 is guided by the axis 10 of the finger 11 - but only fulfills an elastic return function. It will also be noted that producing the rocker return spring 9 in the form of a single elastic arm has the advantage of greater compactness. In general, the choice of the number of elastic arms (s), their length and their thickness determines the intensity of the force produced. One can also play on the inclination of the elastic arm (s) relative to the rotary element 25 (in the plane of the part 23) to modify the intensity of the force produced.

The present invention is not limited to a minute counter mechanism or a snail cam. It is also not limited to an instantaneous action mechanism, causing a jump movement of an indicator or other movable member. It can be applied to any watch mechanism comprising a cam which successively, one or more times per revolution, arms and disarms (partially) a rocker, a rake or other cam follower. In the context of the present invention, the term "cam follower" means a member which cooperates with the periphery of a cam, typically for reading information, without having any function of maintaining the cam in positions determined by normal operation of the mechanism, unlike for example a jumper or a pawl cooperating with a toothed wheel to position it.

The use of the intermediate finger 11 between the rocker return spring 9 and the rocker 2 allows, by playing on the lever arms, to reduce the size of the mechanism 1 for a given return torque applied to the rocker 2. However , this finger 11 could be removed and the rocker return spring 9 could act more directly on the rocker 2, for example the rotary element 25 could be mounted on the axis of the rocker 2. The return spring of lever 9 could also form a single piece with lever 2, or even be an integral part of the lever and guide in rotation relative to a base a rigid end acting as a cam follower.

Claims (19)

Clock mechanism (1) comprising a cam (4) intended to be driven in rotation, a cam follower (2) and a return spring (9) arranged to hold the cam follower (2) in abutment against the cam (4 ), the return spring (9) being arranged to work within a predetermined range of winding angles during each rotation of the cam (4), characterized in that the stiffness of the return spring (9) is zero or negative in at least part of the predetermined range. Clock mechanism (1) according to claim 1, characterized in that the stiffness of the return spring (9) is zero or negative over substantially the entire predetermined range. Clock mechanism (1) according to claim 1 or 2, characterized in that the stiffness of the return spring (9) is negative over substantially the entire predetermined range. Clock mechanism (1) according to any one of claims 1 to 3, characterized in that the return spring (9) comprises at least one elastic arm. Clock mechanism (1) according to any one of claims 1 to 4, characterized in that the return spring (9) comprises a single elastic arm. Clock mechanism (1) according to claim 4 or 5, characterized in that the or each elastic arm is of sinuous shape. Clock mechanism (1) according to any one of claims 4 to 6, characterized in that the geometric shape of the or each elastic arm is a Bézier curve or a succession of Bézier curves. Clock mechanism (1) according to any one of Claims 4 to 7, characterized in that the or each elastic arm has a variable section whose variation is chosen to make the stiffness of the return spring (9) in said au more negative. at least part of the predetermined range, preferably over substantially the entire predetermined range, relative to an elastic arm of the same shape but of constant section. Clock mechanism (1) according to any one of claims 1 to 8, characterized in that the return spring (9) is part of a single piece (23) further comprising a base (24) fixed to a frame ( 1a) of the clock mechanism (1) and a rotary element (25), the return spring (9) connecting the base (24) to the rotary element (25). Clock mechanism (1) according to claim 9, characterized in that the rotary element (25) is mounted on the axis of rotation (10) of a finger (11) arranged to cooperate with the cam follower (2) . Clock mechanism (1) according to claim 9, characterized in that the rotary element (25) comprises a finger (11) arranged to cooperate with the cam follower (2). Clock mechanism (1) according to claim 9, characterized in that the rotary element (25) is mounted on an axis of rotation (O) of the cam follower (2). Clock mechanism (1) according to any one of Claims 1 to 12, characterized in that it further comprises a movable member (16, 17, 18) arranged to be driven by the cam follower (2). Clock mechanism (1) according to claim 13, characterized in that the movable member (16, 17, 18) comprises an indicator (18). Clock mechanism (1) according to claim 13 or 14, characterized in that the cam (4) and the cam follower (2) are arranged to allow a displacement by jumps of the movable member (16, 17, 18). Clock mechanism (1) according to any one of claims 1 to 15, characterized in that the cam (4) has a general shape of a spiral or comprises a main part having a general shape of a spiral. Clock mechanism (1) according to any one of claims 1 to 16, characterized in that the cam follower (2) comprises a rocker or a rake. Clock mechanism (1) according to any one of Claims 1 to 16, characterized in that the cam follower (2) comprises a lever and in that the clock mechanism (1) comprises a hook (13) pivoted on the lever and a wolf tooth wheel (16) arranged to be driven by the hook (13). Timepiece comprising a timepiece mechanism (1) according to any one of claims 1 to 18.

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