Abstract:
A device comprising an element to be driven and a driving element designed to be urged into engagement with the element to be driven and an actuating element adapted to generate a reciprocating movement to move the driving element, the driving element and the actuating element being formed by etching in a semiconductor material block, are provided. During a first alternation (a) of the movement generated by the actuating element, the driving element is urged into engagement with the element to be driven to pull the element to be driven. During a second alternation (b) in the opposite direction generated by the actuating element, the driving element slides on the element to be driven, such that the element to be driven is displaced in a step-by-step movement by the driving element.

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 in a semiconductor material, generally in silicon. 
   2. Discussion of Related Art 
   Document FR 2 852 111 (published on Sep. 10, 2004) describes a clock device comprising a toothed wheel, a driving element capable of sequentially meshing with the toothed wheel and an actuator capable of displacing the driving element according to a hysteresis movement so that the driving element meshes with the successive teeth of the wheel. In such a device, the actuator comprises two electrostatic driving modules. One of the modules is capable of displacing the driving element along a radial direction relatively to the wheel and the other module is capable of displacing the driving element along a tangential direction relatively to the wheel. 
   This device requires the use of an addressing system so that the actuating electrostatic modules are controlled by out-of-phase signals to produce the hysteresis movement of the driving element. The hysteresis movement of the driving element generates rotation of the toothed wheel. 
   SUMMARY OF THE INVENTION 
   An object of the invention is to provide a simpler device for displacing an element to be driven. 
   For this purpose, the invention proposes a device comprising an element to be driven, a driving element intended to engage with the element to be driven, and an actuating element capable of generating a reciprocating movement in order to displace the driving element, the driving element and the actuating element being formed by etching in a block of semiconductor material, characterized in that it is arranged such that: 
   during a first alternation of the movement generated by the actuating element, the driving element engages with the element to be driven to pull the element to be driven, 
   during a second alternation in the opposite direction generated by the actuating element, the driving element slides on the element to be driven, 
   so that the element to be driven is displaced in a step-by-step movement by the driving element. 
   Such a device only requires a single actuating element and may operate by means of a single control signal. 
   Consequently the control or addressing of the device is simplified. 
   Further, the invention only requires a power supply for a single control or addressing route and thereby the power consumed by the device may be reduced as compared with a device with two modules. 
   The device of the invention may further have the following features: 
   the driving element has a driving face intended to come into contact with the element to be driven in order to pull the element to be driven during the first alternation and a guiding face intended to slide on the element to be driven during the second alternation, 
   the driving element comprises a supporting face intended to limit the engagement of the driving element with the element to be driven, 
   the element to be driven is a toothed element, 
   the teeth of the element to be driven are asymmetrical, 
   each tooth of the element to be driven has a driving face on which the driving element will come into contact in order to pull the element to be driven during a first alternation and a guiding face on which the driving element slides during a second alternation, 
   the device is arranged so that: 
   during the first alternation, the driving element is engaged between two teeth of the element to be driven, 
   during the second alternation, the driving element crosses one of the two teeth and will engage between two other adjacent teeth of the element to be driven, 
   the driving element comprises at least one flexible leaf arranged so that during the first alternation, the driving element is displaced in a first direction, which causes deformation of the flexible leaf, and during the second alternation, the driving element is displaced in a second direction, opposite to the first direction, by elastic return of the flexible leaf, 
   the driving element crosses one of the two teeth by restitution of residual elastic energy stored in the flexible leaf, 
   the device comprises a non-return pawl capable of blocking the element to be driven during the second alternation, 
   the non-return pawl comprises an indexing element capable of: 
   during the first alternation of the movement generated by the actuating element, sliding on the element to be driven, 
   during the second alternation, engaging with the element to be driven in order to block the element to be driven, 
   the pawl has a guiding face intended to slide on the element to be driven during the first alternation and a blocking face on which the element to be driven will come into contact in order to block the element to be driven during the second alternation, 
   the element to be driven is a toothed element having a step, the driving element comprises a guiding face intended to come into contact with the element to be driven at a contact point during the sliding of the driving element on the element to be driven, and wherein at rest: 
                   ⁢           {               d   2     ⁢   ⁢   p     -   e   +     k   ·   p                   d   2     ⁢   ⁢     k   ·   p                       
wherein:
 
   p is the step of the element to be driven, 
   e is the distance between a contact point of the guiding face and the driving face of the driving element, 
   d 2  is the distance between the contact point of the guiding face of the driving element and the blocking face of the pawl, 
   and k is a positive integer, 
   the device comprises an elastic pre-stressing component to maintain the driving element against the element to be driven. 
   The invention further proposes a method for operating the device as previously defined. The device comprising an element to be driven, a driving element and an actuating element, the driving element and the actuating element being formed by etching in a block of semiconductor material, characterized in that it comprises the steps according to which: 
   the actuating element is controlled so that it generates a reciprocating movement for displacing the driving element, 
   during a first alternation of the movement generated by the actuating element, the driving element engages with the element to be driven to pull the element to be driven, 
   during a second alternation in the opposite direction, generated by the actuating element, the driving element slides on the element to be driven, 
   so that the element to be driven is displaced in a step-by-step movement by the driving element. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other features and advantages will become further apparent from the description which follows, which is purely illustrative and non-limiting, and should be read with reference to the appended drawings, wherein: 
       FIG. 1  schematically illustrates a device according to an embodiment of the invention. 
       FIG. 2  schematically illustrates an actuating element. 
       FIG. 3  schematically illustrates the relative positions of a driving element, of a non-return pawl and an element to be driven during the different operating steps of the device. 
       FIG. 4  schematically illustrates the shape of the teeth of the element to be driven and the shape of the driving element. 
       FIG. 5  schematically illustrates the positioning of the driving element and of the non-return pawl. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , the device comprises an element to be driven  10 , a driving device  20  and an indexing device  50 . 
   The driving device  20  and the indexing device  50  are simultaneously formed by micro-etching in a single block of semiconductor material, for example in silicon. The element to be driven  10  may also be formed in the same block or be added and positioned facing the driving device  20 . The block thus forms a substrate on which the etched elements are arranged. 
   In the following text, the expression “fixed” describes any element fitted onto the substrate, and the expression “mobile” describes any element maintained at an altitude of a few microns from the substrate, by elastic suspensions also fitted onto the substrate. 
   The element to be driven  10  comprises a rotationally mobile toothed wheel. 
   The expression “radial” describes any element which extends along a radial direction relatively to the wheel, and the expression “tangential” describes any element extending along a tangential direction relatively to the wheel, the radial and tangential directions being considered at the point of the wheel where the driving device engages with the wheel. 
   The driving device  20  comprises an actuating element  202  capable of generating a reciprocating movement in a general tangential direction, a tangential flexible leaf  212  and a driving element  250  intended to engage with the toothed wheel  10 . The driving element  250  is connected to the actuating element  202  via the tangential flexible leaf  212 . The driving element  250  has the shape of a tooth intended to be engaged between two successive teeth of the toothed wheel. 
   The indexing device  50  comprises a tangential flexible leaf  511  and an indexing element  550  intended to engage with the toothed wheel  10 . The indexing element  550  is connected to the substrate via a tangential flexible leaf  511 . The indexing element  550  has the shape of a tooth intended to be engaged between two successive teeth of the toothed wheel. 
   As may be seen in  FIG. 2 , the actuating element  202  is an electrostatic actuating element having a comb structure (known as a comb drive). This type of structure comprises pairs of interdigitated combs. 
   The actuating module  202  is formed with a fixed portion  222  and a mobile portion  232  to which the tangential leaf  212  is connected. 
   The fixed portion  222  comprises a tangential electrode  224  from which a set of parallel fixed combs  226  extend along a radial direction. Each comb is formed with a main leaf and a series of parallel fingers or cilia connected to the main leaf and extended perpendicularly to the latter. 
   The mobile portion  232  comprises a mobile frame  234  including two flexible leaves  233  and  235  connected to the substrate by fitting blocks  238 ,  240 . The flexible leaves  233  and  235  form elastic suspensions providing mobility of the frame  234  relatively to the substrate. 
   The mobile portion  232  comprises a set of combs  236  extending from the mobile frame  234  in a general radial direction. Each comb is formed with a main leaf and a series of parallel fingers or cilia connected to the main leaf and extending perpendicularly to the latter. 
   The combs  226  of the fixed portion  222  and the combs  236  of the mobile portion  232  are positioned parallel to each other and intercalated with each other. Further, each mobile comb  236  is positioned facing a fixed comb  226  so that the fingers are intercalated between each other thereby forming a pair of interdigitated combs. 
   The intercalated fingers of the interdigitated combs are similar to planar capacitors, one of the plates of which is connected to the electrode  224  and the other plate is connected to ground via the fitting connections  238 ,  240 . 
   When a voltage is applied to the electrode  224 , this voltage generates a difference in potentials between the fixed portion  222  and the mobile portion  232  of the actuating element  222 . An electric field is established between the plates of the capacitors formed by the fingers of the fixed combs  226  and the fingers of the mobile combs  236 . This electric field generates an electrostatic force which tends to attract the mobile combs  236  towards the fixed combs  226  in the direction of the fingers. This electrostatic force causes the displacement of the frame  234  in the tangential direction and therefore the translation of the driving tooth  250  by action of the teeth  212  in a tangential direction relatively to the wheel  10  in a first direction (arrow a). 
   The actuating module  202  also comprises a fixed mechanical stop  241 . The mechanical stop  241  limits the deformation of the mobile portion  232  of the actuating element  202  and therefore the stroke of the driving element  250 . 
   When the voltage applied to the electrode  224  becomes zero, the frame  234  returns to its rest position by returning the elastic energy stored in the flexible leaves  233 ,  235 . This elastic return phase has the effect of causing translation of the tooth  250  in a tangential direction relatively to the wheel  10  in a second direction (arrow b), opposite to the first direction. 
   In other words, in a first direction (arrow a), the movement of the actuating element  202  is generated by the electrostatic forces generated between the fixed  226  and mobile  236  combs, whereas in a second direction (arrow b), the movement of the actuating element  202  is generated by the elastic return forces due to the energy stored by the flexible leaves  233 ,  235  of the frame  234 . 
   Thus, when an alternating signal V t  is applied to the electrode  224  for addressing the actuating element  202 , the actuating element generates an reciprocating movement in a tangential direction relatively to the wheel  10 . 
   The stop  241  is arranged so that the driving element  250  is displaced in order to drive the wheel  10  by one step during an alternation (arrow a). 
   The method for operating the device will now be described. 
     FIG. 3  schematically illustrates the relative positions of the driving element  250 , the indexing element  550  and the wheel  10  during the displacement of the wheel when the actuating element generates a reciprocating movement. The movement generated by the actuator involves a first alternation or alternating movement, during which the driving element  250  is displaced in the direction a, and of a second alternation or alternating movement, during which the driving  250  is displaced in the direction b, opposite to the direction a. 
   In  FIG. 3 , the wheel  10  has at its circumference successive teeth referenced  1 ,  2 ,  3  and  4 , respectively. The teeth  1 ,  2 ,  3  and  4  of the wheel  10  are asymmetrical. The configuration illustrated in  FIG. 3  favors the driving of the wheel  10  in the clockwise direction. 
   Initially (in A), the indexing element  550  is engaged between two teeth  2  and  3  of the wheel  10 . The driving element  250  is engaged between two teeth  3  and  4  of the wheel  10 . 
   During the first alternation (an active alternation during which the movement of the driving element  250  is generated by electrostatic forces), the device operates according to the following steps: 
   According to a first step (B), the driving element  250  is driven into translation by the actuating element in a tangential direction relatively to the wheel  10  (arrow a). The driving element  250  comes into contact with the tooth  4 . 
   According to a second step (C), the driving element  250  pulls the wheel  10  so that the wheel  10  is displaced in rotation in the clockwise direction. 
   Rotation of the wheel  10  has the effect that the indexing element  550  will come into contact with the tooth  2  of the wheel  10 . 
   According to a third step (D), the rotation of the wheel  10  has the effect that the indexing element  550  slides on the tooth  2  of the wheel  10 . This is possible because the leaf  511  is flexible and allows a movement of the indexing element  550  in a radial direction relatively to the wheel  10 . 
   According to a fourth step (E), the indexing element  550  crosses the tooth  2 . Because of its elasticity, the leaf  511  brings the indexing element back towards the wheel  10  so that the indexing element  550  is now found engaged between the teeth  1  and  2 . The driving tooth  250  is found at its end-of-travel position. 
   During the second alternation (a passive alternation during which the movement of the driving element  250  is generated by the elastic return forces due to the energy stored in the elastic leaves  233 ,  235 ), the device operates according to the following steps: 
   According to a fifth step (F), the driving element  250  is driven into translation by the actuating element in a tangential direction relatively to the wheel (arrow b). The driving element  250  will come into contact with the tooth  3 . 
   According to a sixth step (G), the driving element  250  pulls the wheel  10  so that the wheel  10  is displaced in rotation in the anti-clockwise direction. Rotation of the wheel  10  has the effect that the indexing element  550  will come into contact with the tooth  2  of the wheel  10 . 
   According to a seventh step (H), the indexing element  550  blocks the rotation of the wheel  10 . Blocking the wheel  10  has the effect that the driving element  250  slides on the tooth  3 . 
   According to an eighth step (I), the driving element  250  crosses the tooth  3 . Because of its elasticity, the leaf  212  brings the driving element  250  back towards the wheel  10  so that the driving element  250  is now found engaged between the teeth  2  and  3 . 
   According to a ninth step (J), the driving element  250  has returned to its initial rest position (in A). 
   From the first to the ninth step, the wheel  10  was driven into rotation in the clockwise direction and was displaced by an angle corresponding to one tooth of the wheel. 
   Thus, the foregoing steps are repeated and the driving element  250  displaces the wheel  10  in a step-by-step movement, each step corresponding to one tooth of the wheel. 
   The indexing device  50  blocks the wheel in rotation when the driving element  250  returns to its rest position. The indexing device is a pawl which allows rotation of the wheel in a single direction. 
     FIG. 4  schematically illustrates the shape of a tooth  1  of the wheel  10  and the shape of the driving tooth  250 . 
   As this may be seen in this figure, the tooth  1  of the wheel  10  comprises a driving face  11  onto which the driving tooth  250  comes into contact in order to pull the wheel  10  during the first alternation (a) and a guiding face  12  on which the driving tooth  250  slides during the second alternation (b) of the driving cycle. 
   The driving face  11  is oriented to form a clearance angle D 1  relatively to the radial direction of the wheel  10 , the clearance angle D 1  being preferably comprised between about 5 and 15 degrees. 
   The guiding face  12  of the tooth  1  is oriented to form an angle α as small as possible, relatively to the tangential direction of the wheel. The angle α is preferably comprised between about 20 and 60 degrees. 
   The driving tooth  250  comprises a driving face  251  intended to come into contact with the wheel  10  in order to pull the wheel  10  during the first alternation (a), a guiding face  252  intended to slide on the wheel  10  during the second alternation (b) and a supporting face  253  intended to press onto the wheel in order to limit engagement of the driving tooth  250  between the teeth of the wheel  10 . 
   The driving face  251  is oriented to form a zero angle relatively to the radial direction of the wheel  10 . 
   The guiding face  252  is oriented to form an angle D 2  relatively to the tangential direction of the wheel  10 , the angle D 2  being preferably comprised between about 0 and 30 degrees. 
   The supporting face  253  is oriented to form a zero angle relatively to the tangential direction of the wheel  10 . The supporting face  253  and the guiding face  251  thereby form a shoulder which limits the engagement of the driving tooth  250  between the teeth of the wheel  10 . 
   The driving faces  11  and  251  are not parallel to each other but they form an angle D 1 . This feature has the effect of limiting the contact surface between the driving faces  11  and  251  which may lead to irreversible adhesion between the driving tooth  250  and the tooth  1  of the wheel when the driving tooth  250  pulls the wheel  10 . 
   Also, the guiding faces  12  and  252  are not parallel to each other but form an angle. This feature has the effect of limiting the contact surface between the guiding faces  12  and  252  which may lead to irreversible adhesion between the driving tooth  250  and the tooth  1  of the wheel  10  when the driving tooth  250  slides on the wheel  10 . Thus, the guiding faces  12  and  252  are in contact at a point  254  of the guiding face  252  of the driving element  250 . 
   Moreover, the shoulder formed by the supporting face  253  and the guiding face  251  of the driving tooth  250  prevents the driving tooth  250  from being blocked between the teeth of the wheel  10 . Indeed, penetration of the driving tooth  250  into the bottom of the teeth of the wheel  10  may possibly lead to irreversible blocking of the driving device. 
     FIG. 5  schematically illustrates the positioning of the driving element  250  and of the indexing element  550  in the rest position (i.e., when the actuating element  202  is not powered). 
   The driving element  250  and the indexing element  550  have similar shapes. 
   The indexing element  550  comprises a blocking face  551  intended to come into contact with the wheel  10  in order to block the wheel  10  during the second alternation (b), a guiding face  552  intended to slide on the wheel  10  during the first alternation (a) and a supporting face  553  intended to come and press on the wheel  10  in order to limit the engagement of the indexing tooth  550  between the teeth of the wheel  10 . 
   It is considered that the actuating element  202  generates a reciprocating movement, the amplitude of which is Δ. In other words, Δ is the stroke of the actuating element. 
   In order that the driving element  250  crosses a tooth of the wheel  10  during the second alternation (alternation b), the residual elastic deformation of the actuating element  202  needs to be sufficient so as to allow the driving element  250  to slide on the wheel  10  when the driving element  250  is at the top of the tooth to be crossed (as illustrated in  FIG. 3 , in configuration I). 
   When the driving element  250  is at the top of the tooth to be crossed, the residual elastic deformation is geometrically defined as follows:
 
Δ residual   =p−e−d   2   +k·p   [1]
 
wherein:
 
   Δ residual  is the residual elastic deformation of the actuating element, 
   p is the step of the wheel  10 , 
   e is the distance between the contact point  254  of the guiding face  252  and the driving face  251  of the driving element  250 , 
   d 2  is the distance between the contact point  254  and the blocking face  551  of the indexing element  550 , 
   and k is a positive integer. 
   In the operation of the device, the residual elastic deformation defined by the relationship [1] should be positive and in the relative positioning of the driving element  250  and of the indexing element  550 , as illustrated in  FIG. 5 , the distance d 2  should be positive. In other words, the distance d 2  should generally be larger than a k multiple of the step of the wheel  10  if k+1 teeth of the wheel  10  are intercalated between the driving element  250  and the indexing element  550 , i.e.: 
   
     
       
         
           
             
               
                 { 
                 
                   
                     
                       
                         p 
                         - 
                         e 
                         - 
                         
                           d 
                           2 
                         
                         + 
                         
                           
                             k 
                             · 
                             p 
                           
                           ⁢ 
                           ⁢ 
                           0 
                         
                       
                     
                   
                   
                     
                       
                         
                           d 
                           2 
                         
                         ⁢ 
                         ⁢ 
                         
                           k 
                           · 
                           p 
                         
                       
                     
                   
                 
               
             
             
               
                 [ 
                 2 
                 ] 
               
             
           
         
       
     
   
   wherefrom:
 
k·p           d 2           p−e+k·p  [3]

   If for example k=0 (as illustrated in  FIG. 5 ) is selected,
 
0           d 2           p−e  [4]

   is obtained. 
   For example, if the following parameters are considered: 
   p=15 μm 
   e=5 μm 
   One should have:
 
0           d 2           10 μm  [5]

   If k=1 is selected, one should have:
 
15 μm           d 2           25 μm  [6]

   If k=2 is selected, one should have:
 
30 μm           d 2           40 μm  [7]