Patent Publication Number: US-10317843-B2

Title: Mechanical oscillator for a horological movement

Description:
FIELD 
     The present invention concerns a mechanical oscillator for a horological movement that has a very low isochronism error and that is insensitive to the direction of gravity. The present invention also concerns a horological movement comprising the mechanical oscillator. 
     DESCRIPTION OF RELATED ART 
     A regulating device is the heart of a mechanical watch. It generates oscillations which separate the time into equal units and is responsible for the accuracy of the watch. In a conventional mechanical watch, the regulating device comprises a balance, a spiral spring and an pallet anchor escapement. 
     In a conventional regulating device, energy losses can be significant due to friction at the pivot of the balance and pallet anchor and of the different interfaces. The accuracy of the spiral spring can also be affected by its orientation of in space. Problems due to flat-hanging difference affect the isochronism of the watch and increase dry friction. 
     Patent EP2090941 to the present applicant describes an oscillatory system constituted of a balance and a return spring. A frequency correction device has flexible elastic straps that are supported on a T-shaped connection member or stop. The straps have ends connected to a fixation and adjusting interface via pins using locking screws, respectively. The interface is secured to a frame by a screw, and the member or stop is directly fixed to the balance. The member or stop is pressed against free ends of the straps during a part of oscillation period. The oscillatory system can significantly increase the power reserve of the watch. 
     However, the oscillatory system described in this document is sensitive to the direction of gravity. Indeed, the displacement of the center of mass effect create a “pendulum” effect that affects the stiffness of the blade, changing slightly the frequency of the pendulum. 
     SUMMARY 
     The present disclosure concerns a mechanical oscillator for a horological movement, the oscillator comprising: a central fixed part being configured to be fixed to a frame of the horological movement; an inertial rim coaxial with a pivoting axis of the mechanical oscillator; at least two rigid links extending radially between the central fixed part and the inertial rim and supporting the inertial rim; and at least two flexible links extending radially from the central fixed part; each flexible link comprising a first flexible element and a second flexible element substantially coplanar to the first element, the first flexible element and the second flexible element being rigidly connected at their distal extremity; the proximal extremity of the first flexible element being fixed to the fixed part and the proximal extremity of the second flexible element being fixed to one of said at least two rigid links, such that the inertial rim can oscillate around the pivoting axis; the first flexible element comprising two first blades and the second flexible element comprises one second blade coplanar with said first blades, the second blade being between the two first blades. 
     The mechanical oscillator provides a very low isochronism error and has a low sensitivity to the direction of gravity. The stiffness of the flexible elements during the oscillation of the mechanical oscillator is constant. Deficiencies in the isochronism can be cancelled by a proper design of the mechanical oscillator, in particular by adjusting a ratio of a distance between the proximal extremity of the second flexible element and the pivoting axis, over the length of the flexible elements. The pivoting axis does not shift during the oscillation such that the mechanical oscillator has a low energy consumption. Moreover, the movable parts of the oscillator are not subjected to any friction, except with the surrounding air. The mechanical oscillator can be made of non-magnetic materials such as silicon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood with the aid of the description of an embodiment given by way of example and illustrated by the figures, in which: 
         FIG. 1  shows a perspective view of a mechanical oscillator, according to an embodiment; 
         FIGS. 2 a  and 2 b    show a top view of parts of the mechanical oscillator of  FIG. 1 ; 
         FIG. 3  shows a perspective view of the mechanical oscillator, according to another embodiment; 
         FIGS. 4 a  and 4 b    show a top view of parts of the mechanical oscillator of  FIG. 3 ; 
         FIG. 5  represents a perspective view of the mechanical oscillator according to yet another embodiment; 
         FIGS. 6 a  and 6 b    illustrate a top view of parts of the mechanical oscillator of  FIG. 5 ; 
         FIG. 7  shows the variation in the stiffness as a function of the amplitude of the angular movement of the inertial rim; 
         FIG. 8  illustrates an example of the angular movement of the inertial rim; 
         FIG. 9  reports variation of stiffness as a function geometrical features of the mechanical oscillator; 
         FIG. 10  represents a central part of the mechanical oscillator, according to another embodiment; and 
         FIG. 11  is a schematic representation of the flexible link. 
     
    
    
     DETAILED DESCRIPTION OF POSSIBLE EMBODIMENTS 
       FIG. 1  shows a perspective view of a mechanical oscillator  10  according to an embodiment. The mechanical oscillator  10  comprises a central fixed part  1 , an inertial rim  4  coaxial with a pivoting axis  11  of the mechanical oscillator, four rigid links  3  extending radially between the central fixed part  1  and the inertial rim  4  and supporting the inertial rim  4 . The central fixed part  1  is configured to be fixed to a frame, or any fixed part, of a timepiece movement. 
     The mechanical oscillator  10  further comprises four flexible links  2  extending radially from the central fixed part  1 . The four flexible links  2  and the four rigid links  3  are angularly equally spaced. However, other arrangements are also possible. Each flexible link  2  comprises a first flexible element  5  and a second flexible element  7  substantially coplanar to the first element  5 . Each of the first flexible element  5  and the second flexible element  7  is rigidly connected at their distal extremity. The proximal extremity of the first flexible element  5  is fixed to the fixed part  1  and the proximal extremity of the second flexible element  7  being fixed to one of the four rigid links  3 , such that the inertial rim  4  can oscillate around the pivoting axis  11 . 
     The oscillation movement of the mechanical oscillator  10  can be transmitted to an escapement (not shown) of a regulator in a horological instrument. 
     The first flexible element  5  and the second flexible element  7  are configured to bend substantially perpendicular to their radial extension. When the inertial rim  4  is pivoted around the pivoting axis  11  for a given angle, the first flexible element  5  and the second flexible element  7  bend such to exert a return force opposed to the pivoting direction. The inertial rim  4  can thus oscillate around an equilibrium angular position around the pivoting axis  11 . 
     As shown in  FIG. 1 , the first flexible element  5  comprises a two first blades  5   a ,  5   b  and the second flexible element  7  comprises a single second blade  7 . The two first blades  5   a ,  5   b  and the second blade  7  are arranged coplanar in a plane passing through the pivoting axis  11 . In the special arrangement of  FIG. 1 , the central fixed part  1  comprised a first fixed part  1   a  and a second fixed part  1   b  coaxial with the first fixed part  1   a . One of the first blades  5   a  is fixed to the first fixed part  1   a  while the other first blade  5   b  is fixed to the second fixed part  1   b . The distal extremity of the two first blades  5   a ,  5   b  is fixed to the second blade  7 . In the example of  FIG. 1 , the distal extremity of the two first blades  5   a ,  5   b  is connected to the second blade  7  through a distal connecting element  9 . The second blade  7  can have a width that is substantially twice the width of the two first blades  5   a ,  5   b.    
     The configuration of the first flexible element  5  and the second flexible element  7  allows for guiding the movement of the inertial rim  4  in a way that only a rotation movement around the pivoting axis  11  is possible. 
     The mechanical oscillator  10  is geometrically symmetric with the ring-shaped inertial rim  4  and disc-shaped first and second fixed parts  1   a ,  1   b , and the center of mass does not move when the inertial rim  4  is pivoted. The distal extremity of the first and second flexible element  5 ,  7  are not fixed and can move freely radially. The mechanical oscillator  10  thus has a constant stiffness (flexibility) and a high degree of isochronism. The symmetry of the mechanical oscillator  10  further allows for limiting a possible twisting effect on the distal connecting element  9 . 
     In an embodiment, a middle stiffening element  8  is comprised in a middle portion of the first and second flexible elements  5 ,  7 . The middle stiffening element  8  increases the stiffness of the first and second flexible elements  5 ,  7 , out of the plane of the flexible elements  5 ,  7 , and thus increases the resistance to shocks and perturbations of the mechanical oscillator  10 . In that case, each of the first blades  5   a ,  5   b  and the second blade  7  have a middle stiffening element  8 , independent from the middle stiffening element  8  of the other blades  5   a ,  5   b ,  7  such that each blade  5   a ,  5   b ,  7  can bend independently from each other. 
     Moreover, the distal connecting element  9  can play the role of a stiffening element or can comprise a distal stiffening element  15  (see  FIG. 3 ) The distal stiffening element  15  can be used for assembling and positioning the first and second flexible elements  5 ,  7 . 
       FIGS. 2 a  and 2 b    show a top view of parts of the mechanical oscillator  10  of  FIG. 1 , according to an embodiment. In particular,  FIG. 2 a    shows a central part  13  of the mechanical oscillator  10  comprising the four rigid links  3 , the inertial rim  4  and the four second blades  7 , each having a middle stiffening element  8 . Each of the four second blades  7  is fixed at their proximal extremity to a respective rigid link  3  and comprises a distal connecting element  9  at their distal extremity. The second blades  7  extend radially from proximal end of the rigid link  3 .  FIG. 2 b    shows a upper part  14  of the mechanical oscillator  10  comprising the four first blades  5   a  connected to the first fixed part  1   a  at their proximal extremity. Each of the four first blades  5   a  are also provided with a middle stiffening element  8  and a distal connecting element  9  at their distal extremity. 
     The complete mechanical oscillator  10  can then be formed by assembling the central part  13  with the upper part  14  on top of the central part  13  and a lower part  14 ′, identical to the upper part  14  and represented by the same  FIG. 2 b   , beneath the central part  13 . During the assembly, the connecting elements  9  of the second blade  7  can be connected to the connecting elements  9  of the first blades  5   a ,  5   b.    
     The first blades  5   a  of the upper part  14  and the first blades  5   b  of the lower part can have the same width, such that the stiffness (flexibility) of the first blades  5   a ,  5   b  is the same for the upper part  14  and the lower part. 
       FIG. 3  shows a perspective view of the mechanical oscillator  10  according to another embodiment. In this embodiment, the first flexible element  5  comprises two first blades  5   a ,  5   b  and the second flexible element comprise a single blade  7  as in the example of  FIG. 1 . However, the first and second first flexible elements  5 ,  7  do not comprise a middle stiffening element  8 . The second blade  7  can have a width that is substantially twice the width of the two first blades  5   a ,  5   b.    
       FIGS. 4 a  and 4 b    show a top view of parts of the mechanical oscillator  10  of  FIG. 3 , according to an embodiment. In particular,  FIG. 4 a    shows a central part  13  of the mechanical oscillator  10  comprising the four rigid links  3 , the inertial rim  4  and the four second blades  7 . Each of the four second blades  7  is fixed at their proximal extremity to the rigid links  3  via a rigid ring  16  and comprises a distal connecting element  9  at their distal extremity. In this specific embodiment, the rigid links  3  extend radially from the rigid ring  16  and support a rigid external ring  17  to which the inertial rim  4  is rigidly connected.  FIG. 4 b    shows a upper part  14  of the mechanical oscillator  10  comprising the four first blades  5   a  connected to the first fixed part  1   a  at their proximal extremity. Each of the four first blades  5   a  are also provided with a distal connecting element  9  at their distal extremity. 
     The complete mechanical oscillator  10  of  FIG. 3  can then be formed by assembling the central part  13  with the upper part  14  on top of the central part  13  and a lower part  14 ′, identical to the upper part  14  and represented by the same  FIG. 4 b   , beneath the central part  13 . During the assembly, the connecting elements  9  of the second blade  7  can be connected to the connecting elements  9  of the first blades  5   a ,  5   b.    
     As shown in the  FIGS. 3 and 4   b , the first fixed part  1   a  and the second fixed part  1   b  comprise four protruding portions  19  extending radially from the pivoting axis  11 . The four protruding portions  19  are angularly distributed such as to extend between the first blades  5   a ,  5   b  and be aligned with the four rigid links  3  when the upper part  14 , lower part  14 ′ and the central part  13  are assembled. Each of the protruding portions  19  can comprise two abutments  18 . The abutments  18  can be used for limiting the amplitude of the pivoting movement of the inertial rim  4 , for example by abutting on the rigid links  3  when the inertial rim  4  oscillates. 
     A length L of the flexible link  2  can be defined as a distance between the proximal extremity of the flexible link  2  fixed to the central fixed part  1 , and the distal extremity of the flexible link  2  fixed to the distal connecting element  9 . A radius R can be defined as a distance between the fixation point of the second flexible element  7  (or proximal extremity of the second flexible element  7 ) of the flexible link  2  to one of the rigid links  3  and the pivoting axis  11 . 
     In the configuration of  FIGS. 3 and 4   a , the length L is the distance between the proximal extremity of the flexible link  2  fixed to the rigid ring  16  and its distal extremity fixed to the distal connecting element  9 . The radius R corresponds to the radius of the rigid ring  16 . In the configuration of  FIGS. 1 and 2   a , the radius R can be defined as the distance between the pivoting axis  11  and the point where the second flexible element  7  is attached to the rigid link  3 . In  FIG. 2 a   , this point is represented by the dotted circle of radius R. 
     In an embodiment, the ratio of the radius R of the rigid ring  16  over the length L corresponds to about 0.6. 
       FIG. 5  shows a perspective view of the mechanical oscillator  10  according to yet another embodiment.  FIGS. 6 a  and 6 b    illustrate a top view of the central part  13  and of the upper and lower parts  14 ,  14 ′ of the mechanical oscillator  10  of  FIG. 5 . The configuration of the mechanical oscillator  10  shown in  FIGS. 5, 6   a  and  6   b  is substantially the same as the one shown in  FIG. 3 . However, here, the first and second first flexible elements  5 ,  7  comprise a middle stiffening element  8 . Moreover, the second blades  7  are fixed at their proximal extremity to the rigid links  3  via a rigid hub  20  having a radius that is smaller than the radius of the ring  16  shown in  FIG. 4 b   . In other words, the central part  13  does not comprise the ring  16  and the rigid links  3  are directly connected to the rigid hub  20 . In this configuration, the radius R corresponds to the radius of the rigid hub  20 . 
     In an embodiment, the ratio R/L, of the length L over the radius R of the rigid hub  20  corresponds to about 0.2. 
     An optimal value of the ratio R/L, i.e. to obtain a good isochronism of the mechanical oscillator  10 , depends on the dimensions of the flexible links  2 , and thus on the dimensions of the first flexible element  5  (such as the first blades  5   a ,  5   b ) and the second flexible element  7  (such as the second blades  7 ), and on the Poisson&#39;s ratio of the material used to make the flexible links  2 . 
     The optimal value of the ratio R/L can be determined by using a finite element method, for example, by using elements that can model an out-of-plane stress gradient, possibly taking into account large displacement hypothesis. Successive simulations can then be run such as to determine the ratio that corresponds to the specific configuration of the mechanical oscillator  10  and to a specific application. 
     An optimal value of the ratio R/L can further be determined by running by using an approximate empiric formula, when using silicon material with a Poisson modulus of about 0.28. 
     An optimal value of the ratio R/L can further be determined by adjusting the length of the flexible links  2  and/or the displacement (dimensions) of the fixation means  16 ,  20  of the flexible links  2 . To this end, an adjusting device (not shown) can be included to the mechanical oscillator  10 . By performing such adjustment and by measuring the oscillating frequency function of the amplitude a good isochronism of the mechanical oscillator  10  can be achieved. 
     According to an embodiment, an optimal value of the ratio R/L is determined by using the empirical equation 1:
 
ρ 0 ( R   el   ,R   es )=6.38·10 −4   R   el   2 −0.393 ·R   el   ·R   es +3.26·10 −2   ·R   el +5.408 ·R   es −0.108
 
where R el  is the slenderness ratio of the flexible link  2  and with R el =L/b, where b is the width of the flexible link  2 ; R es  is the slenderness ratio of the flexible link  2  cross-section, with R es =h/b where h is the thickness of the flexible link  2 .  FIG. 11  is a schematic representation of the flexible link  2  showing the width b, the thickness b and the length L of the flexible link  2 . The domain of validity of equation 1 is given by:
 
 R   el ∈[0,10]
 
and
 
 R   es ∈[0,0.25]
 
     Determining an optimal value of the ratio R/L allows for achieving a constant stiffness of the flexible links  2  and thus, an isochronous mechanical oscillator  10 . 
     Isochronism deficiency can originate from a deformation of the flexible links  2  according to a non-natural axis implying a stiffening of the flexible links  2 . This effect can be cancelled by using a ratio R/L being equal to about 0.6. Isochronism deficiency can further originate from the bending of the first flexible element  5  and the second flexible element  7  during the oscillation of the inertia rim  4 . The bending depends on the dimensions of the first and second flexible elements  5 ,  7 , in particular the bending amplitude increases with decreasing the thickness of the first and second flexible elements  5 ,  7  and with increasing their length. Here, the isochronism deficiency can be cancelled by decreasing the ratio R/L. 
       FIG. 7  shows the variation in the stiffness in Nm/rad calculated as a function of the amplitude θ z  of the angular movement of the inertial rim  4  (see  FIG. 8 ) around the pivoting axis  11  of the mechanical oscillator  10  for several combinations of widths and lengths of the first and second flexible elements  5 ,  7 . Depending on the combination of width and length of the first and second flexible elements  5 ,  7 , the stiffness can increase or decrease with increasing amplitude θ z , from the unsolicited angular position θ z =0. 
       FIG. 9  reports the ratios (max(k)−min(k))/min(k) where max(k) is the calculated maximum stiffness and min(k) is the calculated minimum stiffness taken from  FIG. 7  as a function of the ratio R/L, for the several combinations of widths and lengths of the first and second flexible elements  5 ,  7 .  FIG. 9  shows that for a ratio R/L of 0.6, max(k)=min(k), resulting in a constant stiffness of the first and second flexible elements  5 ,  7  and thus, an isochronous mechanical oscillator  10 , when neglecting the Poisson modulus. 
     In an embodiment, the ratio R/L, is between 0.1 and 0.6, depending on the Poisson modulus. 
     The isochronism of the mechanical oscillator  10  can be influenced by external effects such as the maintenance of the oscillations of the mechanical oscillator  10  by an escapement or a variation in the inertia of the mechanical oscillator  10  when the latter oscillates. In that case, the ratio R/L, can be such that the external effects are compensated, i.e., the isochronism deficiency originating from a deformation of the flexible links  2  compensates the one due to the external effects. In other words, the ratio R/L can be selected such that the isochronism deficiency of the mechanical oscillator  10  is substantially null. 
     More particularly, a ratio R/L between 0.2 and 0.6 allows for obtaining an isochronism deficiency of the mechanical oscillator  10  as low as ±1.5 second per day for an amplitude θ z  of the angular movement between 10° and 15° (corresponding to phi0, ⅔*phi0) of the mechanical oscillator  10  around the pivoting axis  11 . The ratio R/L can be between 0.05 and 0.6. Using a wider range of ratio R/L may result in a non-null isochronism deficiency. For instance, obtaining a negative isochronism deficiency may be useful for compensating a positive isochronism deficiency originating from an external perturbation (such as an escapement). 
     The material used to make the mechanical oscillator  10  disclosed herein is preferably silicon but can also include any other suitable materials such as quartz, glass, metallic glass, metal, polymer or any combination of these materials. 
     The mechanical oscillator  10  can be fabricated by using an suitable machining process including for example Deep Reaction Ion Etching (DRIE), Wire-Electro-Discharge Machine (w-EDM), femto-second laser structuring, LIGA, molding or classical machining of monolithic parts or assembled parts. 
     In the case silicon is used as material forming the mechanical oscillator  10 , a correction of the thermal drift can be performed by adding a silicon oxide layer of an appropriate thickness. This correction can be made to cover a temperature range comprised between 8° C. and 38° C. The thickness of the oxide layer is usually comprised between 0 and 3 micrometers. 
     The inertia rim  4  provide the inertia of the mechanical oscillator  10 . In the configurations of  FIGS. 3 and 5 , the inertia rim  4  can be formed integral with the external ring  17 . Alternatively, the external ring  17  can be used as the inertia rim  4 . In that case, the inertia is provided by the material used for machining the mechanical oscillator  10 , made integral (the flexible elements  2 ,  5 ,  7  being made on the same material as the rigid elements  3 ,  4 ). 
     The oscillation frequency of the mechanical oscillator  10  can be adjusted by adjusting the inertia of the mechanical oscillator  10 . This can be achieved, for example by adding, or removing, small quantities of material on the inertia rim  4 . For instance, a material such as gold or any other adapted material can be deposited on the inertia rim  4 . The added material has preferably a high density and can adhere well enough on the surface of the inertia rim  4 . Other method than deposition can be used for adding and/or removing material, such as adding to the inertia rim  4  or cutting out from the inertia rim  4  pieces of material. 
     The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. 
     For example, the distal extremity of the first flexible elements  5  and the second flexible elements  7  can be linked by a coupling ring  12 . Such coupling ring  12  is represented in  FIG. 10  showing the central part  13  of the mechanical oscillator  10 , wherein the coupling ring  12  is coupling the distal extremity of the second flexible elements  7 . The coupling ring  12  allows for couplings the different vibration modes of the first and second flexible elements  5 ,  7 . The coupling ring  12  is preferably made more compliant such that it becomes flexible, in order to avoid impeding a movement of the first and second flexible elements  5 ,  7  in the radial direction. 
     Moreover, other configurations of the mechanical oscillator  10  are possible. For example, the mechanical oscillator  10  can comprise at least two flexible links  2 , for instance, three, four, five, six or eight flexible links  2 . The mechanical oscillator  10  can comprise at least two rigid links  3 , for instance, three, four, five, six or eight rigid links  3 . The number of flexible links  2  need not to be equal to the number of rigid links  3 . 
     The first flexible element  5  can comprise one or a plurality of coplanar first blades  5   a ,  5   b , for example, more than two. Similarly, the second flexible element  7  can comprise a plurality of coplanar second blades. 
     REFERENCE NUMERAL USED IN THE FIGURES 
     
         
           1  central fixed part 
           1   a  first fixed part 
           1   b  second fixed part 
           2  flexible link 
           3  rigid link 
           4  inertia rim 
           5  first flexible element 
           5   a  first blade 
           5   b  first blade 
           6  rigid part 
           7  second flexible element, second blade 
           8  middle stiffening element 
           9  distal connecting element 
           10  mechanical oscillator 
           11  pivoting axis of the mechanical oscillator 
           12  coupling ring 
           13  central part 
           14  upper part 
           15  distal stiffening element 
           16  rigid ring 
           17  external ring 
           18  abutment 
           19  protruding portion 
           20  hub 
         θ z  amplitude of the angular movement