Abstract:
The invention concerns an element to be driven, a driving element designed to be urged into engagement with the element to be driven and an actuating element adapted to move the driving element so that it drives the element to be driven in step-by-step displacement, the driving element and the actuating element being formed by etching in a semiconductor material wafer. The invention is characterized in that it comprises elastic prestressing means for maintaining the driving element in contact with the element to be driven.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to the field of microelectromechanical systems (MEMS). 
     Such microelectromechanical systems can be formed by etching in blocks or wafers made of semiconductor material, generally silicon. 
     2. Discussion of Related Art 
     The document FR 2 852 111 (published on 10 Sep. 2004) describes a clock-making device comprising a toothed wheel, a driving element for meshing sequentially with the toothed wheel and an actuator adapted to move the driving element according to hysteresis movement such that the driving element meshes with successive teeth of the wheel. In such a device, all the elements (toothed wheel, driving element, and actuator) are formed by microetching in the same block made of semiconductor material. The precision of the relative positioning of the elements is determined by the precision with which the block is etched. 
     It is preferred to connect a driving device formed by microetching in a wafer and a driven element made by means of any alternating or alternative technology (clock-making, micro-molding, machining by electroerosion or other technology). 
     This hybrid approach would use a standard driving device and link it to a driven element adapted to a particular application, for example an entry wheel for a reduction mechanism for a watch or clock, a cogged discoid rotor of a step-by-step rotary micromotor, or a rack of a linear engine. 
     This would also simultaneously create a large number of drive devices in the same block of semiconductor material (wafer). 
     However, when the driving device and the driven element are being connected, the relative positioning of the driving device and of the driven element is delicate. In fact, the uncertainty of positioning due for example to the manufacturing precision of the driven element and the mechanical clearances can in certain cases be greater than the amplitude of mechanical oscillations of the movement of the driving element. The result is that the driving element does not mesh with the element to be driven and the device does not function. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a device allowing the driving of the element to be driven, in spite of the positioning uncertainties relative of the driven element vis-à-vis the driving device. 
     This object is attained by the present invention by a device comprising an element to be driven, a driving element to be engaged with the element to be driven and an actuating element adapted to move the driving element so that it drives the element to be driven according to a step-by-step movement, the driving element and the actuating element being formed by etching in a wafer made of semiconductor material, characterized in that it comprises elastic prestressing device or means for maintaining the driving element in contact with the element to be driven. 
     The elastic device or means keep the driving element in contact with the element to be driven and thus compensate for defects in positioning of the wafer relative to the element to be driven. 
     The device of the invention can also have the following characteristics: 
     the elastic device or means extend between the actuating element and the driving element, 
     the elastic device or means comprise a flexible blade connecting the driving element to the actuating element, 
     the device comprises a support on which the wafer and the element to be driven are arranged, 
     the device comprises positioning pins fixed on the support allowing positioning of the wafer on the support, 
     the wafer is arranged in support on positioning pins, 
     the wafer has at least one notch formed on an edge of the wafer, the notch being able to receive a positioning pin for positioning the wafer on the support, 
     the actuating element comprises a first actuating module adapted to move the driving element according to a first direction to drive the element to be driven and a second actuating module adapted to move the driving element in a second direction to move the driving element away from the element to be driven, the actuating modules being able to be controlled simultaneously to generate a combine hysteresis movement of the driving element. 
     the second actuating module comprises an electrode and a flexible rod, the electrode being able to be controlled to deform the flexible rod so as to shift the driving element in a second direction to move the driving element away from the element to be driven, 
     the electrode has a convex lateral surface, preferably parabolic, extending opposite a portion of the flexible blade, 
     the second actuating module comprises a series of stops arranged along a lateral surface of the electrode, the stops being able to prevent contact between the blade and the electrode. 
     The invention also proposes a process for installing a device comprising or consisting of arranging on the same support: 
     an element to be driven, 
     a wafer made of semiconductor material in which are formed by etching a driving element to be engaged with the element to be driven and an actuating element adapted to move the driving element so that it drives the element to be driven according to a step-by-step movement relative to the support, 
     characterized in that it comprises maintaining the driving element in contact with the element to be driven by way of elastic means or an elastic device. 
     The process can have the following characteristics: 
     the device comprising positioning pins fixed on the support, the process comprises arranging the wafer in support on positioning pins, 
     the wafer having at least one notch formed on an edge of the wafer, the process comprises positioning a pin in the notch for positioning the wafer on the support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages will emerge from the following description, which is purely illustrative and non-limiting and must be considered with respect to the attached figures, in which: 
         FIG. 1  schematically illustrates, in perspective, a device according to a first embodiment of the invention, 
         FIG. 2  schematically illustrates, in plan view, a device according to the first embodiment of the invention, 
         FIG. 3  schematically illustrates, in plan view, a device according to a second embodiment of the invention, when the wafer is not yet positioned relative to the element to be driven, 
         FIG. 4  schematically illustrates, in plan view, a device according to the second embodiment of the invention, once the wafer has been positioned relative to the element to be driven, 
         FIG. 5  schematically illustrates an electrode to be utilized in the device shown in  FIGS. 3 and 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIGS. 1 and 2 , the device, according to a first embodiment of the invention, comprises a driven element  10 , a driving device  20  and a support  30 . 
     The support  30  comprises a planar reception surface  31  on which are placed the element to be driven  10  and the driving device  20 . 
     The element to be driven  10  comprises a toothed wheel of general cylindrical shape. The toothed wheel comprises a denture composed of asymmetrical teeth  1 ,  2 ,  3 ,  4 ,  5  and a substantially cylindrical shaft  11 . 
     The support  30  comprises orifices  34 ,  35  of a general cylindrical shape, for receiving the shaft  11 . The orifices  34  and  35  form bearings for guiding the shaft  11  in rotation. 
     The driving device  20  comprises a wafer  21  made of semiconductor material, such as silicon. The driving device  20  comprises an actuating element  200 , an indexing element  50  (not shown in  FIG. 1 ) and a driving element  250 , formed by microetching in the wafer  21 . 
     The driving element  250  is in the form of a tooth having a triangular shape. The tooth extends in the vicinity of the wheel  10  with the point directed towards the wheel  10 , in a radial direction relative to the wheel. The driving element  250  is thus able to mesh with the teeth  1 ,  2 ,  3 ,  4 , and  5  of the wheel  10 . 
     The actuating element  200  is mainly made up of a tangential elementary actuating module  202  and an indexing module  50 . 
     The tangential actuating module  202  comprises an interdigitized comb structure  222  (known as a “comb drive”) and a flexible blade  212  extending in a general tangential direction relative to the wheel  10 . The driving element  250  is connected by the tangential blade  212  to the interdigitized comb structure  222 . 
     When the tangential actuating module  202  is controlled by an alternating addressing or control signal, the tangential actuating module  202  generates an alternative movement in a tangential direction (arrow I). 
     The indexing module  50  comprises a flexible blade  511  extending in a tangential direction relative to the wheel  10  and an indexing element  550 . The flexible blade  511  extends in overhang from the substrate and is flexible in a radial direction relative to the wheel  10 . The flexible blade  511  supports at its free end the indexing element  550 . The indexing element  550  is in the form of a tooth having a triangular shape. The tooth extends near the wheel  10  with the point directed towards the wheel  10 , in a radial direction relative to the wheel. The indexing element  550  is thus able to engage with the teeth  1 ,  2 ,  3 ,  4 , and  5  of the wheel  10 . 
     The device, according to the first embodiment of the invention, also comprises two positioning pins  32  and  33  fixed on the support  30 . The positioning pins  32  and  33  have a general cylindrical form and extend in a direction perpendicular to the receiving surface  31  of the support  30 . 
     The wafer  21  has two notches  22  and  23  formed on an external edge  24  of the wafer. The notches  22  and  23  are arranged on either side of the driving element  250 . The notch  22  is intended to receive the first positioning pin  32  and the notch  23  is intended to receive the second positioning pin  33  for positioning the wafer  21  on the support  30 . The positioning pins  32  and  33  define a single position of the wafer  21  on the receiving surface  31  of the support  30 . 
     The first notch  22  has a general V shape. The notch  22  has two support faces forming between them an angle of 120°. Each support face of the first notch  22  is intended to be supported on the cylindrical surface of the pin  32 . 
     The second notch  23  has a single support face, parallel to the edge  24  of the wafer  21 , intended to be supported on the cylindrical surface of the pin  33 . 
     The positioning pins  32  and  33  cooperate with the notches  22  and  23  to define a position of the wafer  20  on the receiving surface  31  of the support  30 . The positioning pins  32  and  33  thus form reference pins having the function to wedge the driving device parallel to the receiving surface  31 . 
     Installing the device comprises the following steps. 
     According to a first installation step, the wheel  10  is mounted to rotate on the support  30 . To this end, the shaft  11  is mounted on the bearings  34  and  35  such that the wheel  10  solid with the shaft  11  is free to revolve about an axis of rotation perpendicular to the receiving surface  31 . 
     According to a second installation step, the wafer  21  is placed in support on the first and second pins  32  and  33 . 
     The pins  32  and  33  are arranged on the support  30  such that the driving element  250  comes into contact with the wheel  10 . 
     The driving element  250  is kept in contact with the wheel  10  by means of the tangential blade  212 . The blade  212  extends in overhang from the interdigitized comb structure  222  and is flexible in a radial direction relative to the wheel  10 . Due to the elasticity of the blade  212 , the driving element  250  is biased toward and held meshed with the wheel  10 . 
     The flexible blade  212  absorbs the positioning defects of the wafer  21  relative to the element to be driven  10 . 
     In particular, as is evident in  FIG. 1 , the wheel  10  is mounted on a shaft  11  guided by the bearings  34 ,  35 . The position of the wheel relative to the support  30  is subject to uncertainties associated with the machining tolerances of all the pieces of the device, especially: 
     the geometric defects of the wheel  10  and of the shaft  11  (e.g., defects in cylindricity and concentricity of the wheel, defects in rectitude of the shaft  11 ), 
     positioning defects of the bores receiving the guide bearings  34 ,  35  of the shaft  11 , 
     mechanical installation clearances of the wheel  10  on the shaft  11  and guide clearances between the bearings  34 ,  35  and the shaft  11 . 
     Also, the uncertainties in positioning of the wafer  21  relative to the wheel  10  also result from positioning defects of the positioning pins  32  and  33  on the support  30 . 
     The device functions as follows. 
     The tangential actuating module  202  is controlled by an alternating addressing or control signal. 
     During a first alternation of the movement generated by the tangential actuating module  202 , the driving element  250  meshes with the wheel  10  and drags the wheel  10 . The indexing element  550  crosses a tooth of the wheel  10 . 
     During a second alternation in the opposite direction generated by the tangential actuating module  202 , the indexing element  550  blocks the wheel  10  and the driving element  250  slips on the wheel  10 . 
     The wheel  10  is thus driven according to a step-by-step rotation movement (arrow III) by the driving element  250 . The indexing element  50  forms an anti-return mechanism which prevents rotation of the wheel  10  in the inverse direction. 
       FIGS. 3 and 4  show a device according to a second embodiment of the invention. The installation is identical to the installation of the first embodiment, except that the actuating element  200  comprises an actuating module radial  203 . 
     The device comprises a driven element  10 , a driving device  20  and a support  30 . 
     The element to be driven  10  comprises a wheel, optionally toothed. 
     The driving device  20  comprises a wafer  21  made of semiconductor material, such as silicon. The driving device  20  comprises an actuating element  200  and a driving element  250  formed by microetching in the wafer  21 . 
     In this second embodiment, the actuating element  200  is mainly composed of a tangential elementary actuating module  202  and an elementary radial actuating module  203 . 
     In a variant (not shown), it is also feasible to form an indexing element in the wafer  21 . 
     The tangential actuating module  202  comprises an interdigitized comb structure  222  and a flexible blade  212  extending in a general tangential direction relative to the wheel  10 . The driving element  250  is connected by the tangential blade  212  to the interdigitized comb structure  222 . 
     When the tangential actuating module  202  is controlled by an alternating addressing or control signal, the tangential actuating module  202  generates an alternative movement in a tangential direction (arrow I). 
     The actuating module radial  203  comprises an electrode  223 , a flexible blade  210  and stops  243 . 
     The flexible blade  210  has a general L shape and comprises a first branch  213  and a second branch  214 . 
     The first branch  213  extends in a tangential direction relative to the wheel  10 . The first branch  213  extends in overhang from the substrate and is flexible in a radial direction relative to the wheel  10 . 
     The second branch  214  extends in a general radial direction relative to the wheel and connects the free end of the first branch  213  to the driving element  250 . 
     The electrode  223  is illustrated in greater detail in  FIG. 5 . The electrode  223  has a lateral surface  233  of general convex shape, preferably parabolic. The stops  243  are arranged at regular intervals along the lateral surface  233 . The stops  243  are formed by pins etched in the wafer  21 . The pins are electrically insulated from the electrode  223 . 
     When voltage is applied to the electrode  223 , this voltage creates a difference in potential between the electrode  223  and the blade  210 . An electric field is established between the electrode  223  and the blade  210 . This electric field generates an electrostatic force which tends to unite the branch  213  of the surface  233  of the electrode  223 . This electrostatic force causes deformation of the branch  213  and consequently translation of the driving tooth  250  in a radial direction relative to the wheel  10 . 
     The stops  243  limit the amplitude of the movement of the blade  210  for maintaining the blade  210  at a distance from the electrode  223  and prevent the first branch  213  from coming into contact with the lateral surface  233  of the electrode  223 . In fact, contact by the blade  210  and the electrode  223  fed by different voltages would cause a short-circuit likely to cause the device to breakdown. 
     The convex form of the surface  233  of the electrode controls the movement of the rod  210 , irrespective of the initial deformation of the branch  213  due to positioning of the driving tooth  250  relative to the wheel  10 . 
     The branch  213  of the blade  210  thus compensates for uncertainties in positioning of the wafer relative to the wheel  10 . 
     When the tangential actuating module  202  is controlled by an alternating addressing or control signal, the tangential actuating module  202  generates an alternative movement in a tangential direction (arrow I) relative to the wheel  10 . 
     When the electrode  223  of the actuating module radial  203  is controlled by an alternating addressing or control signal, the radial actuating module  203  generates an alternating movement in a radial direction (arrow II) relative to the wheel  10 . 
     The device functions as follows. 
     The tangential actuating module  202  and the radial actuating module  203  are controlled by alternating addressing or control signals. The addressing signals are dephased such that the driving element  250  is displaced according to a hysteresis movement. The hysteresis movement of the driving tooth  250  alternates the driving (arrow I) and actuating (arrow II) phases. The driving element  250  meshes with the successive teeth of the wheel  10  and drives the latter according to a step-by-step rotation movement. 
     It is evident that the lateral flexibility of each of the blades  212  and  210  permits deformation of the latter under the action of the other blade. The two flexible radial and tangential  212  and  214  blades ensure mechanical decoupling of the modules  202  and  203 . In fact, the flexibility of the blades allows displacement of the driving element  250  independently according to at least two elementary degrees of freedom, specifically: in the two directions of radial and tangential translation.