Abstract:
The invention relates to a composite balance ( 45, 45 ′) formed in a layer of silicon-based material ( 21 ) and including a hub ( 39, 39 ′) connected to a felloe ( 37, 37 ′) by at least one arm ( 40, 41, 42, 43 ). According to the invention, the felloe ( 37, 37 ′) includes at least one additional part approximately in the shape of a notched ring ( 23, 23 ′) of higher the than the silicon-based material, which increases the inertia of the balance. The invention also relates to a method  1  of manufacturing this type of balance. The invention concerns the field of timepiece movements.

Description:
This is a National Phase Application in the United States of America of International Patent Application PCT/EP2009/053001 filed Mar. 13, 2009, which claims priority on European Patent Application No. 08153093.3 of Mar. 20, 2008. The entire disclosures of the above patent applications are hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a balance and the method of manufacturing the same and, more specifically, a composite balance. 
     BACKGROUND OF THE INVENTION 
     The regulating member of a timepiece generally includes an inertia wheel, called a balance, and a resonator called a hairspring. These parts have a determining role as regards the working quality of the timepiece. Indeed, they regulate the movement, i.e. they control the frequency of the movement. 
     The balance and the hairspring are different in nature, which makes it extremely complex to manufacture the regulating member, said manufacturing including the manufacture of the balance, the balance spring and the resonant assembly of the two parts. 
     The balance has thus been manufactured in various materials, but without resolving the isochronism difficulties that are linked to a temperature change in the regulating member on which the balance depends. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome all or part of the aforecited drawbacks by proposing a composite balance, whose features as a function of temperature can be more easily adjusted and which is obtained via a manufacturing method that comprises fewer steps. 
     The invention thus relates to a composite balance formed in a layer of silicon-based material and including a hub connected to a felloe by at least one arm, characterized in that the felloe includes at least one additional part, approximately in the shape of a notched ring of larger density than said silicon-based material, for increasing the inertia of said balance. 
     According to other advantageous feature of the invention:
         said at least one additional part is mounted on one of the main faces of the felloe, which amplifies the inertia adjustment;   said at least one additional part is mounted in a recess made in one of the main faces of the felloe;   said at least one additional part projects from one of the main faces of the felloe;   said at least one additional part includes a series of studs spaced at regular intervals to compensate for any thermal expansion in said at least one additional part;   said at least one additional part is formed from a metallic material, such as gold, which has a much higher density than silicon;   the hub includes at least a second additional part for receiving the balance staff, which is driven therein;   said at least one second additional part is mounted on one of the main faces of the hub;   said at least one second additional part is mounted in a recess made in one of the main faces of the hub;   said at least one second additional part projects from one of the main faces of the hub;   said at least one second additional part is approximately cylinder-shaped;   said at least one second additional part is formed from a metallic material;   said at least one arm is slim so as to allow it to deform axially and/or radially in the event of any shocked transmitted to the balance.       

     The invention also relates to a timepiece, characterized in that it includes a balance according to any of the preceding variants. 
     Finally, the invention relates to a method of manufacturing a balance including the following steps:
     a) providing a substrate made of silicon-based material;
 
characterized in that it further includes the following steps:
   b) selectively depositing at least one metal layer on the substrate to define the pattern of at least one metal part of said balance;   c) selectively etching at least one cavity in the substrate to define the pattern of the balance including said at least one metal layer;   d) releasing the balance from the substrate.   

     In accordance with other advantageous features of the invention:
         step b) includes step e): growing said deposition by successive metallic layers at least partially on the surface of the substrate so as to form a metallic part for increasing the mass of the balance made of silicon-based material and/or a metallic part for receiving an arbour that is driven therein;   step b) includes step f): selectively etching at least one cavity in the substrate for receiving said at least one metal part and step g): growing said deposition by successive metal layers at least partially in said at least one cavity so as to form a metal part for increasing the mass of said third part of silicon-based material and/or a metal part into which an arbour will be driven,   step b) includes the last step h): polishing the metal deposition,   several composite balances are made on the same substrate, which allows batch manufacture.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages will appear clearly from the following description, which is given by way of non-limiting illustration, with reference to the annexed drawings, in which: 
         FIGS. 1 and 2  show views of successive steps of the manufacturing method according to a first embodiment; 
         FIGS. 3 to 5  show views of successive steps of the method according to a second embodiment; 
         FIGS. 6 and 7  are perspective diagrams of a composite balance according to a first embodiment; 
         FIGS. 8 and 9  are perspective diagrams of a composite balance according to a second embodiment; 
         FIG. 10  is a flow chart of the method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention relates to a method, generally designated  1 , for fabricating a balance  45 ,  45 ′ for a timepiece movement. As illustrated in  FIGS. 1 to 5  and  10 , method  1  includes successive steps for forming at least one type of composite balance, i.e. which is preferably formed of two different materials, such as silicon and metal. 
     With reference to  FIGS. 1 ,  3  and  10 , the first step  3  consists in taking a substrate  21  that includes a silicon layer. Preferably in this step  3 , substrate  21  is selected such that, as seen in  FIGS. 1 and 3 , its thickness approximately matches the desired thickness of the silicon part of balance  45 ,  45 ′. Thus, the thickness of substrate  21  may be, for example, comprised between 100 and 400 μm. 
     Advantageously according to the invention, after the first step  3 , method  1  can comprise two embodiments  19 ,  20  as illustrated in  FIG. 10 . 
     According to a first embodiment  19 , in a second step  5 , shown in  FIG. 1 , method  1  includes implementation of a LIGA process (also known by the German name “röntgenLIthographie, Galanoformung &amp; Abformung”) including a series of steps for electroplating, in a particular shape, a metal on substrate  21  using a selectively photostructured resin. As this LIGA process is very well known, it will not be described in more detail here. However, the deposited metal may be, for example, gold or nickel or an alloy of these metals. 
     In the example illustrated in  FIG. 1 , step  5  can consist in depositing a notched ring  23  and/or a cylinder  25 . In the example illustrated in  FIG. 1 , ring  23  includes a series of studs  22  shaped approximately in the arc of a circle and it is advantageously used for increasing the mass of the future balance  45 . Indeed, one of the advantages of silicon is its insensitivity to temperature variations. However, it has the drawback of having low density. 
     Consequently, a first feature of the invention thus consists in increasing the mass of balance  45  using metal obtained by electroplating in order to increase the inertia of the future balance  45 . However, in order to keep the advantages of silicon, the metal deposited on substrate  21  includes a space between each stud  22  that can compensate for any thermal expansion of ring  23 , while avoiding transmitting any stress linked to such expansion to the silicon. 
     In the example illustrated in  FIG. 1 , cylinder  25  is for receiving a balance staff, which is advantageously driven therein. In fact, another drawback of silicon is that it has very small elastic and plastic zones, which means that it is very brittle. Another feature of the invention thus consists in tightening the balance staff, not against silicon, but on the inner diameter  24  of metal cylinder  25 , electroplated during step  5 . 
     Advantageously, according to method  1 , the cylinder  25  obtained by electroplating allows complete freedom as regards its geometry. Thus, in particular, the inner diameter  24  is not necessarily circular, but for example polygonal, which could improve the transmission of forces in rotation with an arbour of matching shape. 
     In a third step  7 , shown in  FIG. 2 , cavities  26  to  34  are selectively etched, for example by a DRIE method, in silicon substrate  21 . 
     Cavities  26  to  34  form preferably form the pattern  35  of the future balance  45 . As illustrated in the example of  FIG. 2 , the pattern  35  obtained includes a felloe  37  connected to hub  39  by four arms  40  to  43 . However, advantageously according to method  1 , the etch over substrate  21  allows complete freedom as to the geometry of pattern  35 . Thus, in particular, the number and geometry of the arms may be different, and the rim is not necessarily circular but, may be, for example, elliptical. Moreover, the arms may be slimmer to allow them to deform axially and/or radially in the event of any shock transmitted to balance  45 . 
     It should also be noted that, with inner diameter  24  of metal cylinder  25 , cavity  34  made in hub  39  forms a hollow space that can receive an arbour. It will be noted finally that bridges of material  36  are formed to hold pattern  35  to substrate  21 . 
     According to embodiment  19 , method  1  ends with final step  9 , which consists in releasing the manufactured balance  45  from substrate  21 . Advantageously, step  9  is achieved simply by applying sufficient force to balance  45  to break its bridges of material  36 . This force can, for example, be generated by machining or manually by an operator. 
     After final step  9 , as illustrated in the example of  FIGS. 6 and 7 , a balance  45  is thus obtained, mainly formed of silicon with one or two metal parts  23 ,  25 . It is thus clear that balance  45  is of the composite type and that it has at least two types of material and is made in one-piece, in that element  35  and elements  23  and/or  25  cannot be separated without being destroyed. Balance  45  includes a hub  39  radially connected to felloe  37  by four arms  40 ,  41 ,  42  and  43 . Hub  39  is advantageously also axially connected to metal cylinder  25  and felloe  37  includes notched ring  23 , over one part of its main faces. 
     According to a second embodiment  20 , method  1  includes a second step  11 , shown in  FIG. 3 , in which cavities  38  and/or  44  are selectively etched, for example, by a DRIE method, in one part of the thickness of silicon substrate  21 . These cavities  38 ,  44  form recesses that can be used as a container for at lest one metal part  23 ′,  25 ′. As in the example illustrated in  FIG. 3 , the obtained cavities  38  and  44  may respectively take the form of a ring or disc. 
     Advantageously, according to method  1 , cavities  38  and/or  44  obtained by etching leave complete freedom as to their geometry. Thus, in particular, cavities  38  and/or  44  are not necessarily circular but may be, for example, polygonal. 
     In a third step  13 , as illustrated in  FIG. 4 , method  1  includes implementation of a galvanic growth or LIGA process for filling cavities  38  and/or  44  in a particular metal shape. Preferably, the deposited metal may be, for example, gold or nickel or an alloy of these metals. 
     In the example illustrated in  FIG. 4 , step  13  may consist in depositing a notched ring  23 ′ in cavity  38  and/or a cylinder  25 ′ in cavity  44 . Moreover, in the example illustrated in  FIG. 4 , ring  23 ′ has a series of studs  22 ′ approximately in the arc of a circle and it is advantageously used for increasing the mass of the future balance  45 ′. In fact, as already explained above, one drawback of silicon is that it has low density. 
     Thus, as for embodiment  19 , one feature of the invention thus consists in increasing the mass of balance  45 ′ using metal obtained by electroplating, which increases the inertia of the future balance  45 ′. However, in order to keep the advantages of silicon, the metal electroplated on substrate  21  has a space between each stud  22 ′ that can compensate for any thermal expansion in ring  23 ′, while preventing transmitting any stress linked to such expansion to the silicon. 
     In the example illustrated in  FIG. 4 , cylinder  25 ′ is for receiving a balance staff, which is advantageously driven therein. In fact, as already explained above, one advantageous feature of the invention consists in tightening the balance staff not against the silicon, but on the inner diameter  24 ′ of metal cylinder  25 ′, which is electroplated during step  13 . Advantageously according to method  1 , the electroplated cylinder  25 ′ allows complete freedom as to its geometry. Thus, in particular, the inner diameter  24 ′ is not necessarily circular but may be, for example, polygonal, which could improve the transmission of forces in rotation with an arbour of matching shape. 
     Preferably, method  1  can include an fourth step  15 , illustrated by dotted lines in  FIG. 10 , consisting in polishing the metal deposition(s)  23 ′,  25 ′ made during step  13 , in order to make them flat. 
     In a fifth step  17 , shown in  FIG. 5 , cavities  26 ′ to  34 ′ are selectively etched, for example, by a DRIE process, in silicon substrate  21 . 
     These cavities  26 ′ to  34 ′ preferably form the pattern  35 ′ of the future balance  45 ′. As illustrated in the example of  FIG. 5 , the pattern  35 ′ obtained includes a felloe  37 ′ connected to hub  39 ′ by four arms  40 ′ to  43 ′. However, advantageously according to method  1 , the etch on the substrate  21  leaves complete freedom as to the geometry of pattern  35 ′. Thus, in particular, the number and geometry of the arms may be different, and the rim is not necessarily circular, but may be elliptical, for example. Moreover, the arms may be slimmer to allow them to deform axially and/or radially in the event of any shock transmitted to the regulating member  45 ′. 
     It should also be noted that cavity  34 ′ made in hub  39 ′ forms, with inner diameter  24 ′ of metal cylinder  25 ′, a hollow space that can receive an arbour. It should be noted finally that bridges of material  36 ′ are formed to hold pattern  35 ′ on substrate  21 . 
     Embodiment  20  ends like embodiment  19 , i.e. in final step  9  which consists in releasing the manufactured balance  45 ′ from substrate  21 . Advantageously, step  9  is achieved simply by applying sufficient force to balance  45 ′ to break its bridges of material  36 ′. This force can, for example, be generated by machining or manually by an operator. 
     After final step  9 , as illustrated in the example of  FIGS. 8 and 9 , a balance  45 ′ formed mainly of silicon is obtained, with one or two metal parts  23 ′,  25 ′. It is thus clear that balance  45 ′ is composite in that it includes at least two types of material and one-piece in that element  35 ′ and elements  23 ′ and/or  25 ′ cannot be separated without being destroyed. The balance  45 ′, includes a hub  39 ′ radially connected to the felloe  37 ′ by four arms  40 ′,  41 ′,  42 ′ and  43 ′. Hub  39 ′ advantageously also includes metal cylinder  25 ′. Finally, felloe  37 ′ includes notched ring  23 ′. 
     Advantageously, according to method  1  of the invention explained above, it is clear that it is possible for several balances  45 ,  45 ′ to be made on the same substrate  21 , which allows batch manufacture. 
     Of course, the present invention is not limited to the example illustrated, but is capable of various variants and alterations, which will be clear to those skilled in the art. In particular, the hub  39 ,  39 ′ according to embodiment  19 ,  20  might not include a metal driving cylinder  25 ,  25 ′. Cylinder  25 ,  25 ′ could then, for example, be replaced by resilient means etched in the silicon hub  39 ,  39 ′ and could take the form of those disclosed in FIGS. 10A to 10E of EP Patent No. 1 655 642 or those disclosed in FIGS. 1, 3 and 5 of EP Patent No. 1 584 994, which are incorporated herein by reference. 
     It is also possible for the electroplated metal parts  25 ,  25 ′ in embodiments  19  and  20  to be inverted, i.e. projecting part  25  of embodiment  19  could be replaced by integrated part  25 ′ of embodiment  20  or vice versa (which only requires minimum adaptation of method  1 ), or even for part  25 ′ integrated in the hub to project from substrate  21 . 
     In accordance with similar reasoning, it is also possible for metal parts  23 ,  23 ′ electroplated in embodiments  19  and  20  to be inverted, i.e. projecting part  23  of embodiment  19  could be replaced by integrated part  23 ′ of embodiment  20  or vice versa, or part  23 ′ integrated in the felloe could project from substrate  21 . 
     Moreover, method  1  may advantageously also provide, after release step  9 , a step of adapting the inertia of balance  45 ,  45 ′. This step could then consist in etching, for example by laser, recesses made in the peripheral wall of felloe  37 ,  37 ′ and/or on electroplated metal parts  23 ,  23 ′. Conversely, inertia-block regulating structures could also be envisaged for increasing the inertia of balance  45 ,  45 ′. 
     Finally, a polishing step like step  15  may also be performed between step  5  and step  7 .