Patent Publication Number: US-11022942-B2

Title: Isochronous pivot for timepiece resonators

Description:
This application claims priority from European Patent Application No. 17188264.0 filed on Aug. 29, 2017, the entire disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The invention concerns an isochronous pivot for timepiece resonators, with a flexure bearing, comprising at least one pair including two flexible strips each joining a first point of attachment of a first element to a second point of attachment of a second element, said first attachment points defining, with said respective second attachment points, two main strip directions, said first element and said second element each being stiffer than each of said flexible strips, and each able to form a movable inertial element inside a said resonator, and said two main strip directions defining a theoretical pivot axis at their intersection when said two flexible strips are coplanar, or at the intersection of their projections onto a reference plane parallel to said two flexible strips when said two flexible strips extend on two levels parallel to said reference plane but are not coplanar, each said flexible strip having a free length between its two attachment points, and having an axial distance between said theoretical pivot axis and whichever of its said two attachment points is the farthest therefrom, and wherein, for each said flexible strip, the main attachment point ratio X=D/L between said axial distance and said free length is greater than one, and wherein said two main strip directions define with said theoretical pivot axis a first apex angle α whose value in degrees satisfies the relation f1(X)&lt;α&lt;f2(X), with f1(X)=108+67/(10X−6), and f2(X)=113+67/(10X−6). 
     The invention also concerns a resonator comprising a first element and a second element, joined by at least one pair including two flexible strips comprised in such a pivot. 
     The invention also concerns a timepiece movement including at least one such resonator. 
     The invention also concerns a watch including at least one such movement. 
     BACKGROUND OF THE INVENTION 
     The use of flexure bearings, particularly having flexible strips, in mechanical timepiece oscillators, is made possible by processes, such as MEMS, LIGA or similar, for developing micromachinable materials, such as silicon and silicon oxides, which allow for very reproducible fabrication of components which have constant elastic characteristics over time and high insensitivity to external agents such as temperature and moisture. Flexure pivots, such as those disclosed in European Patent Applications No. 14199039 or No. EP16155039 by the same Applicant, make it possible to replace a conventional balance pivot, and the balance spring usually associated therewith. Removing pivot friction also substantially increases the quality factor of an oscillator. 
     However, flexure pivot bearings are known to have a non-linear return force, and undesired movement of the instantaneous axis of rotation. 
     The work by Wittrick in 1951 allowed a reduction in undesired displacement by positioning the crossing point of the strips at seven eighths of their length. European Patent Application No. 14199039 by the same Applicant proposes, in combination with this arrangement, the choice of a specific apex angle between the strips, to optimise the linearity of the elastic return force in order to make the resonator isochronous. 
     Such a pivot comprising two strips which cross in projection cannot, however, be etched in two dimensions in a single etch, which complicates manufacture. 
     European Patent Application No. 16155039 by the same Applicant, SWATCH GROUP RESEARCH &amp; DEVELOPMENT Ltd, proposes a 2D geometry achievable on a single-level etch, and which has the desired advantages of linearity and low undesired displacement. It describes a timepiece resonator mechanism comprising a first support with a first anchor and a second anchor to which is attached a flexure pivot bearing mechanism, which defines a virtual pivot axis about which rotatably pivots a pivoting weight, and which includes at least one front RCC flexure pivot and one back RCC flexure pivot, mounted in series and head-to-tail relative to each other about said virtual pivot axis, said front RCC flexure pivot comprising, between said first support and an intermediate rotary support, two straight flexible front strips of the same front length LA between their attachment points, defining two linear front directions which intersect at said virtual pivot axis and which define a front angle with said virtual pivot axis, and wherein the respective anchors of said two straight flexible front strips farthest from said virtual pivot axis are both at a same front distance DA from said virtual pivot axis, and said back RCC flexure pivot including, between said intermediate rotary support, which includes a third anchor and a fourth anchor, and said pivoting weight, two straight flexible back strips of the same back length LP between their attachment points, defining two linear back directions which intersect at said virtual pivot axis and which define a back angle with said virtual pivot axis, and wherein the respective anchors of said two straight flexible back strips farthest from said virtual pivot axis are both at a same back distance DP from said virtual pivot axis. This flexure pivot bearing mechanism is planar, the centre of inertia of the assembly formed by said pivoting weight and any added inertial weight carried by said pivoting weight is on said virtual pivot axis or in immediate proximity thereto, and said front angle expressed in degrees is comprised between:
 
109.5+5/[(DA/LA)−(⅔)] and 114.5+5/[(DA/LA)−(⅔)]
 
and said back angle expressed in degrees is comprised between:
 
109.5+5/[(DP/LP)−(⅔)] and 114.5+5/[(DP/LP)−(⅔)].
 
     SUMMARY OF THE INVENTION 
     The invention proposes to define a resonator with a flexure pivot bearing, which is isochronous, simple to manufacture and which withstands shocks without thereby degrading a high quality factor. 
     It more particularly concerns optimising the use of an RCC (Remote Compliance Centre) pivot, with strips forming a V-shape, and an offset centre of rotation, which is easy to manufacture and robust, to satisfy this specification. 
     To this end, the invention concerns an isochronous pivot for a timepiece resonator with a flexure bearing. 
     The invention also concerns a resonator comprising a first element and a second element, joined by at least one pair including two flexible strips comprised in such a pivot. 
     The invention also concerns a timepiece movement including at least one such resonator. 
     The invention also concerns a watch including at least one such movement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the invention will appear upon reading the following detailed description, with reference to the annexed drawings, in which: 
         FIG. 1A  represents a schematic plan view of the attachment points of an RCC pivot with two strips forming a V-shape, between a fixed base and a solid inertial element. 
         FIG. 1B  represents, in a similar manner, the free lengths L and axial distances D of these strips, their apex angle α, and their crossing point. 
         FIG. 2  is a graph which represents the evolution of the optimum apex angle on the ordinate, as a function of the ratio X=D/L on the abscissa, with an area of improved isochronism between two curves, lower curve f1 and upper curve f2, on either side of a mean curve fm. 
         FIG. 3  represents, in a similar manner to  FIG. 1 , a similar RCC pivot including an eye around the crossing point of the strips. 
         FIG. 4  represents, in cross-section, the area of an eye like that of  FIG. 3 , which surrounds a core provided with two stop weights forming an axial shock absorber on either side of this eye. 
         FIG. 5  represents, in a similar manner to  FIG. 4 , the reverse configuration with the eye provided with a movable arbor between two upper and lower counterbores of a fixed element. 
         FIG. 6  represents, in a similar manner to  FIG. 1 , the series arrangement of two RCC pivots, each with an optimum angle, with the inertial element including a stud provided with an eye as in  FIG. 3 , and having a peripheral surface partially of revolution delimited by a chamber of an intermediate element suspended to the fixed base by two strips in the opposite direction to those connecting the inertial element to the intermediate element. 
         FIG. 7  represents, in a similar manner to  FIG. 6 , the series arrangement of two RCC pivots, each with an optimum angle, with the inertial element including an eye as in  FIG. 3 , and including a peripheral surface partially of revolution delimited by a chamber of the fixed base, to which is suspended by two strips in opposite directions, an intermediate frame element to which is suspended the inertial element. 
         FIG. 8  represents, in a similar manner to  FIG. 6 , the series arrangement of two RCC pivots, each with an optimum angle, with the strips disposed in the same direction, but with two different apex angles, chosen above and below the middle of the area of improved isochronism. 
         FIG. 9 , which is similar to  FIG. 2 , shows the positioning of the two angles of  FIG. 8 . 
         FIG. 10  is a block diagram which represents a watch including a movement incorporating a resonator provided with such a pivot. 
         FIG. 11 , taken from European Patent Application No. EP14199039 by the same Applicant, illustrates a resonator pivot configuration with non-coplanar crossed strips. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention concerns an isochronous pivot  1  for a timepiece resonator  100 , with a flexure bearing, comprising at least one pair, generically denoted  2 , or more particularly a first pair  21  or a second pair  22 , according to the variants illustrated by the Figures. This pair  2  includes two flexible strips  3 :  31 ,  32 , each joining a first attachment point  41 ,  42  of a first element  4  to a second attachment point  51 ,  52  of a second element  5 . These first attachment points  41 ,  42  define, with the respective second attachment points  51 ,  52 , two main strip directions DL 1  and DL 2 . The first element  4  and second element  5  are each stiffer than each of flexible strips  3 , and each is able to form a movable inertial element inside a resonator  100 . 
     The two strip directions DL 1 , DL 2  define a theoretical pivot axis A, at their crossing point when the two flexible strips  31 ,  32  are coplanar, or at the crossing point of their projections onto a reference plane parallel to the two flexible strips  31 ,  32 , when the two flexible strips  31 ,  32  extend on two levels parallel to the reference plane but are not coplanar. 
     Each flexible strip  31 ,  32  has a ‘free’ length L 1 , L 2  between its two attachment points  41 ,  51 ,  42 ,  52 , and having an axial distance D 1 , D 2  between theoretical pivot axis A and whichever of its two attachment points  41 ,  51 ,  42 ,  52  is farthest therefrom. 
     For each flexible strip  31 ,  32 , the main attachment point ratio D 1 /L 1 , D 2 /L 2  between the axial distance and the free length is greater than one. 
     The invention also concerns a resonator  100  including a first element  4  and a second element  5 , joined by at least one such pair  2  which includes two flexible strips  31 ,  32  comprised in such a pivot  1 . 
     According to the invention, and as seen in  FIG. 2 , the two main strip directions DL 1 , DL 2  define, with theoretical pivot axis A, the first apex angle α, whose value depends on the main attachment ratio D 1 /L 1 , D 2 /L 2 , between a first lower function f1 such that α=f1(D/L), and a first higher function f2, such that α=f2(D/L), 
     More particularly, f1(X)=108+67/(10X−6), with X=D/L. 
     More particularly, f2(X)=113+67/(10X−6), with X=D/L. 
     In particular, D 1 /L 1 =D 2 /L 2 =D/L=X, and is greater than 1. The value in degrees of first apex angle α then satisfies the relation f1(D/L)&lt;α&lt;f2(D/L), with f1(D/L)=108+67/(10X−6), and f2(D/L)=113+67/(10X−6), with X=D/L. 
     More particularly, the two main strip directions DL 1  and DL 2  define, with theoretical pivot axis A, a first apex angle α, which is comprised between 115° and 130° inclusive. This is equivalent to confining the value X=D/L between the values X=1 and X=1.55. 
     More particularly, theoretical pivot axis A is geometrically located in first element  4  or in second element  5 . 
       FIG. 3  illustrates the advantageous case where theoretical pivot axis A is geometrically located in second element  5 . 
     More particularly, this theoretical pivot axis A is geometrically located in an opening called an ‘eye’  40 ,  50 , comprised in first element  4  or second element  5 . 
       FIG. 4  represents, in cross-section, the area of an eye  50  in second inertial element  5 , which surrounds a core  40 , integral here, in a non-limiting manner, with first element  4  which forms a fixed weight, and said core  40  is provided with two stop weights  6  and  7  forming an axial shock absorber on either side of eye  50  and of inertial element  5 , whose lower surface  56  and upper surface  57  are arranged to cooperate as end stops with stop weights  6  and  7 .  FIG. 5  illustrates the reverse configuration with eye  50  provided with a movable arbor  59  between two upper  46  and lower  47  counterbores of a fixed element  4 , the ends  56  and  57  of this arbor  59  being arranged to cooperate as end stops with counterbores  46  and  47 . 
     More particularly, pivot  1  includes at least one pair  2  including two flexible strips  31 ,  32 , which are either identical in symmetry with respect to a plane of symmetry passing through theoretical pivot axis A when the two flexible strips  31 ,  32  are coplanar, or identical in symmetry in projection onto a reference plane parallel to the two flexible strips  31 ,  32  when the two flexible strips  31 ,  32  extend on two levels parallel to the reference plane but are not coplanar, with respect to a plane of symmetry passing through theoretical pivot axis A. 
     More particularly, each pair  2  comprised in pivot  1 , includes two flexible strips  31 ,  32 , which are either identical in symmetry with respect to a plane passing through theoretical pivot axis A when the two flexible strips  31 ,  32  are coplanar, or identical in symmetry in projection onto a reference plane parallel to the two flexible strips  31 ,  32  when the two flexible strips  31 ,  32  extend on two levels parallel to the reference plane but are not coplanar. 
     More particularly, pivot  1  includes at least one pair  2  including two flexible strips  31 ,  32 , which are coplanar. 
     More particularly, according to the invention, pivot  1  includes at least one pair  2  including two flexible strips  31 ,  32 , which extend on two levels parallel to the reference plane but are not coplanar. 
     More particularly, pivot  1  is symmetrical, at least in projection onto the reference plane, with respect to a plane of symmetry passing through theoretical pivot axis A, and the centre of mass of resonator  100  in its rest position is geometrically located on the plane of symmetry. 
     Advantageously, the centre of mass of the resonator, in its rest position, is positioned on the axis of symmetry of the pivot, and at a short distance from the axis of rotation A defined by the crossing point of the extensions of the strips, which has the effect of offsetting the undesired displacement of the pivot. For a single pivot, the optimum distance depends on the relation X=D/L. Thus, the centre of mass of resonator  100  in its rest position is separated from theoretical pivot axis A by a distance ε which depends on the free lengths L 1 , L 2 , and on attachment point ratios D 1 /L 1 , D 2 /L 2 . More particularly, D 1 /L 1 =D 2 /L 2 =D/L=X. More particularly still, D 1 =D 2 =D and L 1 =L 2 =L, and this distance ε is substantially equal to the value (2D 2 /L−1.6. D−0.1 L). More particularly, this distance ε is comprised between 0.8 times and 1.2 times the value (2D 2 /L−1.6. D−0.1 L). 
     The RCC pivots described above can be combined in various ways. In particular,  FIGS. 6 to 8  illustrate non-limiting variants, resulting from different combinations of pairs of strips. 
     More particularly, pivot  1  thus includes, in addition to a first pair  21  including the two flexible strips  31 ,  32 , each joining a first attachment point  41 ,  42  of a first element  4  to a second attachment point  51 ,  52  of a second element  5 , at least a second pair  22  including two other flexible strips  33 ,  34 , each joining, on the one hand, a primary attachment point  43 ,  44 , of first element  4  or a secondary attachment point  53 ,  54  of second element  5 , to, on the other hand, a tertiary attachment point  63 ,  64 , comprised in a third element  6  arranged to be fixedly secured to a fixed structure of resonator  100 . 
     More particularly, first element  4  or second element  5  is an inertial element, and theoretical pivot axis A is geometrically located in an eye  40 ,  50  of said inertial element.  FIGS. 6 to 8  include such an eye  50  in inertial element  5 . 
     More particularly, as seen in  FIGS. 6 and 8 , first element  4  or second element  5  includes a concave surface  74  arranged to surround at a distance, over at least 180°, a convex surface  75  comprised in second element  5  or respectively first element  4 , with, in a rest position of pivot  1 , a clearance which, at every point, is greater than or equal to a safety clearance J between convex surface  75  and concave surface  74 . 
       FIG. 6  illustrates the series arrangement of two RCC pivots, each with an optimum angle, with inertial element  5  including a stud provided with an eye  50 , and including a peripheral surface partially of revolution  75 , delimited by a chamber  74  of an intermediate element  4  suspended to fixed base  6  by two strips  33 ,  34 , in the opposite direction to those  31 ,  32  connecting inertial element  5  to intermediate element  4 . 
     More particularly, as seen in  FIG. 7 , first element  4  or second element  5  is an inertial element, and third element  6 , particularly the fixed base, includes an internal surface  76  arranged to surround at a distance, over at least 180°, a convex surface  75  comprised in the inertial element, with, in a rest position of pivot  1 , a clearance which, at every point, is greater than or equal to a safety clearance J between convex surface  75  and internal surface  76 . 
       FIG. 7  concerns the series arrangement of two RCC pivots, each with an optimum angle, with inertial element  5  including an eye  50 , and including a peripheral surface partially of revolution  75  delimited by a chamber  76  of fixed base  6 , to which is suspended by two strips  33 ,  34  in opposite directions, an intermediate element  4  forming a frame and to which is suspended inertial element  5 . 
     More particularly, as seen in  FIG. 8 , which concerns the series arrangement of two RCC pivots, each with a different optimum angle α, β with the strips disposed in the same direction, but with two different apex angles, chosen above and below the middle of the area of improved isochronism: on the one hand, the primary attachment points  43 ,  44  of first element  4  or secondary attachment points  53 ,  54  of second element  5 , and on the other hand, tertiary attachment points  63 ,  64  defining two secondary strip directions DL 3 , DL 4 , which form together a second apex angle β at a secondary axis B defined by the intersection, in plane or in projection, of the secondary strip directions DL 3 , DL 4 . This second apex angle β, like first apex angle α, is comprised between 115° and 130° inclusive, or its value depends on a secondary attachment point ratio D 3 /L 3 , D 4 /L 4 , between a first lower function f1 such that β=f1(D/L), and a first upper function f2, such that β=f2(D/L), and, more particularly and as seen in  FIG. 9 , a median function fm(X)=110.5+67/(10X−6) defines an intermediate ratio between the first lower function f1(X)=108+67/(10X−6), and the first upper function f2(X)=113+67/(10X−6), with X=D/L, and the first apex angle α and the second apex angle β are positioned between the first lower function f1 and the first upper function f2, on either side of the median function fm. 
     More particularly, theoretical pivot axis A is coincident with secondary axis B. 
     The invention also concerns a timepiece movement  1000  including at least one such resonator  100 . 
     The invention also concerns a watch  2000  including such a timepiece movement  1000 .