Abstract:
A method for converting mechanical energy into electrical energy by at least one piezoelectric element and at least one variable capacitor. The method: a) mechanically deforms the piezoelectric element; b) recovers charges produced by the deformation of the piezoelectric element; c) transfers the charges from the piezoelectric element to the capacitor; d) modifies the capacitance of the capacitor; and e) recovers at least some of the electrical energy. A device for converting mechanical energy into electrical energy includes a piezoelectric element and a variable capacitor. The piezoelectric element is capable of transferring charges to the capacitor.

Description:
TECHNICAL FIELD AND PRIOR ART 
     This invention mainly relates to a method and a device for converting mechanical energy, especially vibrations, into electrical energy with improved efficiency. 
     The prior art discloses, for example in the document “ Vibration - to - Electric Energy Conversion” , Meninger, S.; Mure-Miranda, J. O.; Amirtharajah, R.; Chandrakasan, A.; Lang, J H; Very Scale Integration (VLSI)  Systems, IEEE Transactions on , Volume 9, Issue 1, Feb. 2001 Page(s): 64-76, a device for recovering mechanical energy in the form of vibrations into electrical energy, comprising a plurality of capacitors of variable capacity, formed by two opposite elements respectively carried by a fixed comb and a moving comb, wherein the moving comb is moved by the effect of the external vibrations and modifies the gap between the two elements and thus the capacity of the capacitors. 
     Consequently, by generating electrostatic forces between the armatures of the capacitor, it is possible to recover electrical energy. 
     The prior art also discloses the use of a piezo-electrical material to transform mechanical energy into electrical energy, wherein the mechanical energy causes the deformation of the piezo-electric material and the appearance of electrical charges in the material, which may be recovered. 
     This recovery by electrostatic or piezo-electric means is not optimal. 
     Consequently this is the purpose of this invention, by proposing a method for recovering electrical energy from mechanical energy with improved efficiency with respect to the methods of the prior art. 
     It is also a purpose of this invention to propose a device to convert optimally mechanical energy into electrical energy. 
     DESCRIPTION OF THE INVENTION 
     The purposes described above are achieved by an energy conversion device featuring piezo-electric means capable of converting mechanical energy into electrical energy, associated to electrostatic means which amplify the electrical energy produced by the piezo-electric means. 
     In other terms, the conversion device associates a piezo-electric element and a capacitor of variable capacity, wherein the charges generated by the piezo-electric element during its deformation are transmitted to the capacitor, which by varying its capacity will amplify the electrical energy produced by the piezo-electric element. 
     The variation of the capacity is obtained by the relative movement of the armatures of the capacitor. When the capacity of the capacitor drops, the electrical charge creates electrostatic forces which tend to brake the movement of the armatures of the capacitor, thus transforming part of the mechanical energy into electrical energy. Consequently the electrical energy produced by the piezo-electric element during its deformation is amplified during the drop in capacity of the capacitor. 
     Advantageously, the structure for converting mechanical energy into electrical energy combines the principles of electrostatic and piezo-electric conversion in a synchronised manner. Indeed, if during the deformation of the piezo-electric element, there is a variation of capacity, it is possible to amplify by means of the electrostatic structure the energy acquired by the piezo-electric element, then possibly to re-inject it into the piezo-electric element for successive amplification. At each amplification phase, a braking force (electrostatic or piezo-electric) appears; it is stronger each time and thus permits more mechanical energy to be converted into electrical energy. 
     The variation of mechanical capacity used by the invention is not just a simple variation of capacity as may be the case with switched capacities, but a transduction of mechanical energy into electrical energy. Indeed, the switching of capacity disclosed in the prior art permits the electrical potential of an electrical energy source to be raised, but in no way its energy level. Indeed, the increase in the voltage induces a drop in the output current which maintains a mean output power equal to the mean input power, wherein the voltage-current product remains constant, this is an electrical/electrical energy conversion and in no way a mechanical/electrical conversion. On the other hand, thanks to this invention, with a drop in capacity due to mechanical deformation, not only is the electrical potential increased, but also the electrical energy stored; the mechanical energy provided to deform the capacity is in fact transformed into electrical energy thus amplifying the electrical energy initially stored in the capacity. 
     With a drop in capacity by mechanical deformation, the electrical energy is amplified, whereas with a switched capacity system such as that described in the document EP 1 050 955, only the voltage is amplified. In the case of this invention, when the mechanical capacity is deformed and drops in value whereas the charge stored Q on its electrodes remains constant (circuit temporarily open), the voltage U, which is equal to the charge stored Q divided by the value of the capacity C, increases, and in the same way, the electrical energy stored 
               E   capa     =       1   2     ⁢   QU           
increases when U increases, which is to say when C drops.
 
     
       
         
           
             
               E 
               capa 
             
             = 
             
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   CU 
                   2 
                 
               
               = 
               
                 
                   
                     1 
                     2 
                   
                   ⁢ 
                   QU 
                 
                 = 
                 
                   
                     Q 
                     2 
                   
                   
                     2 
                     ⁢ 
                     C 
                   
                 
               
             
           
         
       
     
     Therefore when Q is constant and the capacity C drops, E capa  increases. 
     A switched capacity system, which for example passes capacities in parallel to in series behaves differently than a variation of capacity by mechanical variation. Indeed, when the capacities are in parallel, the charge is spread over the capacities, and placing them in series does not modify this distribution, the equivalent capacity indeed drops but only part of the charge is found at the external terminals. 
     Placing ten identical pre-charged capacities in series provides a voltage multiplied by 10, but also to an equivalent charge divided by 10 and unchanged stored energy ½QU. This is natural because no external energy has been added to the system. 
     Within the scope of this invention, the energy converted by the mechanical variation of the capacity is therefore significant with respect to the energy converted by the piezo-electric element. 
     Preferably, an extreme capacity of the electrostatic element corresponds to an extreme voltage at the terminals of the piezo-electric element. Using simple electronic means, the energy converted by the piezo-electric element may be efficiently transferred to the electrostatic element when the capacity value is at maximum. This energy is then amplified during the phase when the capacity drops. 
     Indeed, if in addition to the synchronisation between the electrostatic conversion and the piezo-electric conversion, at each extreme value of capacity on the electrostatic structure an extreme deformation of the piezo-electric element occurs, then these amplification phases may be optimised. When the deformation is at maximum, the potential electrical energy stored in the piezo-electric element is simply transferred to the electrostatic structure which has a maximum capacitive value. Then, when the electrostatic structure passes from its maximum capacitive value to its minimum value, there is an increase in the potential electrical energy within this structure. When this potential energy is at maximum, it is either transferred to a storage circuit (battery) or it is re-injected into the piezo-electric element so that it is once again amplified. 
     The system may be used to recover mechanical vibration energy in everyday environments (automobile, machine tools, etc.), of the order of a few microwatts per gram of moving mass with an efficiency of around 20%. 
     The device according to the invention furthermore permits advantageously to start without the need for an initial source of electrical energy, while conserving the advantages of electrostatic structures in terms of small mechanical/electrical coupling. 
     The main purpose of the invention is therefore a method of converting mechanical energy into electrical energy, by means of at least one piezo-electric element and at least one capacitor of variable capacity, wherein said method comprises the steps: 
     a) of mechanical deformation of the piezo-electric element, 
     b) of recovery of the charges produced by the deformation of the piezo-electric element, 
     c) of a transfer of charge from the piezo-electric element to the capacitor, 
     d) of modification of the capacity of the capacitor, 
     e) of recovery of at least part of the electrical energy. 
     Advantageously, the deformation of the piezo-electric element and the modification of the capacity of the capacitor are simultaneous. 
     Furthermore, also very advantageously, the deformation of the piezo-electric element is at maximum when the capacity of the capacitor is at maximum. 
     The capacity is, for example, modified by mechanical deformation of the capacitor. The piezo-electric element and the capacitor may thus be deformed by a same mechanical constraint. 
     It may be provided that part of the electrical energy converted by the capacitor is transmitted to the piezo-electric element. 
     The charges may also be transferred to a first or a second capacitor depending on a direction of deformation of the piezo-electric element. 
     It may also be provided that, when a voltage at the terminals of the piezo-electric element is at maximum, its sign is inverted to create a mechanical braking force permitting the energy converted to be increased in a following conversion cycle. 
     Furthermore, the method according to the invention may comprise the additional step of transferring the electrical energy produced by a capacitor to a second capacitor and inversely until a predetermined level of amplification of the electrical energy is reached. 
     A quantity of excess energy with respect to said amplification is then transferred to a storage or operational unit. 
     Very advantageously, the transfer of charges of step c) is controlled by a diode or a switch fitted with a control circuit. 
     Another purpose of this invention is a device for converting mechanical energy into electrical energy, comprising at least one piezo-electric element and at least one capacitor of variable capacity and means of controlling the transfer of charges between the piezo-electric element and the capacitor, such that the charge produced during a deformation of the piezo-electric element is transmitted to the capacitor to generate electrical energy amplified by variation of its capacity. 
     The means of controlling the transfer of charges are advantageously formed by at least one diode or at least one switch fitted with a control circuit. 
     The capacity of the capacitor is, for example, modified by moving towards one another the armatures of the capacitor. 
     The piezo-electric element and the armatures of the capacitor are advantageously substantially parallel so that they are subjected to the same mechanical constraint. 
     The device according to the invention may comprise two capacitors whose capacities vary inversely. 
     In one embodiment, the device according to the invention comprises a part that moves by the application of a mechanical constraint, fitted with teeth on either side of its axis of movement, wherein two fixed parts also equipped with teeth move opposite the teeth of the moving part, wherein the piezo-electric elements are perpendicular to the movement of the moving part and are electrically connected to the moving part, wherein the pairs of opposite teeth form two sets of capacitors in series whose capacities vary inversely. 
     In another embodiment of the device according to the invention, the piezo-electric element is in the form of a flexible membrane suspended at a distance from an electrostatic electrode, so that the electrostatic electrode and a second electrode of the piezo-electric element form a capacitor of variable capacity by deformation of the membrane. 
     The device according to the invention may also comprise a stratified structure in which the capacitor is formed by an alternation of first electrodes with a first potential, layers of electrically isolating material and second electrodes with a second potential, wherein the electrically isolating material is elastically deformable and in which the piezo-electric element forms a layer of the stratified structure. 
     The piezo-electric element advantageously comprises a layer of piezo-electric material and a first and a second electrode on either side of the layer of piezo-electric material. 
     The piezo-electric material may be chosen from the ceramics: PZT (PbZrtiO 3 ) or PLZT (with lanthane) or BaTiO 3 , from the nano-crystals (PZN-PT or PMN-PT), from the polymers (PVDF) or AFCs (Active Fibre Composites). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other characteristics and advantages of the invention will be more clearly understood upon reading the following description, made in reference to the appended drawings, in which: 
         FIG. 1A  is a top view of a diagrammatical representation of one embodiment of a device according to the invention, 
         FIG. 1B  is an example of an electrical diagram corresponding to the device of  FIG. 1A , 
         FIG. 1C  shows the variations of capacities in picofarads of the capacitors of the device of  FIGS. 1A and 1B  in function of time in milliseconds when the piezo-electric element is deformed, as well as the deformation in micrometers of the piezo-electric element, 
         FIG. 1D  shows the variations of the potential at the terminals of the capacitors of the device of  FIGS. 1A and 1B  in function of time when the piezo-electric element is deformed, 
         FIG. 1E  is a model of the operation electrical of a piezo-electric element, 
         FIG. 2A  is a diagrammatical representation in perspective of another embodiment of an energy recovery device according to the invention, 
         FIG. 2B  is a basic principle electrical diagram of a device according to the invention, 
         FIG. 3  is a diagrammatical representation in perspective of another embodiment of this invention, 
         FIG. 4  is a diagrammatical representation of another embodiment of this invention, 
         FIG. 5A  is a diagrammatical representation of another embodiment of this invention, 
         FIGS. 5B and 5C  are detailed views of the device of  FIG. 5A , 
         FIG. 5D  is an example of an electrical diagram corresponding to the device of  FIG. 5A , 
         FIG. 6  is an example of an electrical diagram corresponding to another embodiment of this invention, 
         FIG. 7  is an example of an electrical diagram corresponding to another embodiment of this invention, 
         FIG. 8  is an example of an electrical diagram corresponding to another embodiment of this invention, 
         FIG. 9  is an example of an electrical diagram corresponding to another embodiment of this invention. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Throughout the description, by “deformable” it is meant an element that is capable of elastically deforming under a given charge, wherein the material of the element is chosen to suit the constraints to which the recovery device is likely to be subjected. 
       FIGS. 1A and 1B  show an embodiment of the invention comprising a moving part  202  and two fixed parts  204 ,  206  surrounding the moving part  202 , wherein the moving and fixed parts are intermeshed. 
     The fixed parts and the moving part are made from conductive material. 
     The moving part  202  comprises a body  208  with a Z axis fitted with a plurality of teeth  210  extending on either side of the Z axis and perpendicular to it. The fixed parts  204 ,  206  are parallel to the Z axis and comprise teeth  212 ,  214  perpendicular to the Z axis. The teeth  212  of the fixed part  204  are opposite the faces  210 . 1  located on the right of the teeth  210  in the drawing and the teeth  214  are opposite the faces  210 . 2  located on the left of the teeth  210  in the drawing. 
     The moving part  202  may move along the Z axis towards the right and towards the left. Thus, when the moving part  202  moves towards the right, the teeth  210  move away from the teeth  212  and move closer to the faces  214 . When the moving part  202  moves towards the left, the teeth  210  move closer to the teeth  212  and away from the teeth  214 . 
     The moving part  202  is connected to a carrier  216  by deformable arms  218 , so as to authorise the movements along the Z axis. As concerns the fixed parts  204 ,  206 , they are connected rigidly to the carrier  216 . 
     Piezo-electric elements P are located on the arms  218  so that they may also be deformed when the moving part  202  moves; electrically, they may be in series or in parallel. The piezo-electric elements P comprise a piezo-electric material and two electrodes on either side of the piezo-electric material. 
     All of the teeth  212  of the fixed part  204  and the teeth  210  of the moving part  202  form a first series of capacitors, whose capacity increases when the moving part  202  moves towards the right. 
     All of the teeth  214  of the fixed part  206  and all of the teeth  210  of the moving part  202 , opposite, form a second series of capacitors whose capacity increases when the moving part  202  moves towards the left. 
     In the rest of the description, we will consider that when the moving part  202  moves towards the right, the voltage V p  at the terminals of the piezo-electric element increases and when the moving part moves towards the left, the voltage V p  at the terminals of the piezo-electric element drops. 
       FIG. 1B  shows an example of an electrical circuit that may be associated to the device of  FIG. 1A . The electrical circuit is composed of two sub-circuits C 1 , C 2 . The first sub-circuit C 1  comprises the piezo-electric element P, mounted in series with a first diode  224  and a first capacitor  226  of variable capacity C 226 . The capacitor  226  represents the capacitor equivalent to the succession of capacitors formed by the teeth  210  and  212 . The second sub-circuit C 2  comprises the same piezo-electric element  22  also mounted in series with a second diode  228  and a second capacitor  230  of variable capacity C 230 . The second capacitor represents the capacitor equivalent to the capacitors formed by the teeth  210  and  214 . The first  224  and second  228  diodes are mounted opposite one another to allow each one to have the passage of a current in the opposite direction. 
     Consequently, when the voltage V P  at the terminals of the piezo-electric element P increases, the diode  224  allows current to pass, and when voltage V P  at the terminals of the piezo-electric element P drops, it is the diode  228  which allows current to pass. 
     In the example shown, the circuits C 1  and C 2  are each connected to a storage unit C 5  or to a distinct application, but they may be connected to the same unit or the same application. 
     We will now describe the operation of this device. 
     When the carrier  216  is subjected to mechanical vibrations, the moving part moves along the Z axis from left to right. The movement of the moving part causes the deformation of the piezo-electric element P. A voltage then appears at its terminals, and the charges produced are then transmitted to one of the capacitors of variable capacity, which amplifies the electrical energy recovered due to the variation of the capacity and the variation of the potential at its terminals. 
     Consequently, in each direction of movement, a piezo-electric element P is deformed, causing the mechanical energy to be converted into electrical energy. 
     In the example shown, the means of controlling the charge transfer are formed by the diodes  224 ,  228 , however switches equipped with a control circuit could be provided for example. 
     In the case of a diode, this transfer is carried out continuously from a minimum charge value. Whereas in the case of a control switch, this transfer is carried out in a single operation when the potential between the electrodes of the piezo-electric element is at maximum. The switch must be controlled inversely to the diodes, which switch automatically to allow current to pass. 
       FIG. 1E  shows an electrical diagram which shows a simplified model of the electrical operation of a piezo-electric element. 
     This circuit comprises a capacitor  222 . 1  of capacity C P  and an alternative voltage generator  222 . 2  formed by the deformed piezo-electric material. 
       FIG. 1C  shows the variations of the capacities at the terminals of the capacitors  226  (curve III),  230  (curve IV), and the deformation of the piezo-electric element (curve V) in function of time. 
       FIG. 1D  shows the variation of the potential at the terminals of capacitors  226  (curve I, II) and  230  (curve I′, II′). 
     When the moving part  202  moves towards the right, the piezo-electric element P is deformed and the voltage Vp at the terminals of the piezo-electric element P increases and renders the diode  304  conductive (part I,  FIG. 1D ). The voltage V 226  at the terminals of the capacitor  226  therefore follows the voltage Vp by a progressive transfer of the electrical charges stored in the capacitor P. 1  of the piezo-electric element of  FIG. 1B  to the capacitor  226 . Then, when the deformation of the piezo-electric element reaches a maximum value, the voltage Vp at the terminals of this element starts to drop whereas the voltage V 226  on the contrary increases (part II of  FIG. 1D ), which causes the diode  224  to block. This increase of V 226  is due to the fact that when the moving part  202  moves towards the left, the capacity C 226  drops (part II′,  FIG. 1D ), and the capacitor  226  is then electrically isolated from the rest, due to the blocking of the diode  224 , wherein its charge Q 226 =C 226 V 226  remains constant. If this charge Q 226  is constant and the capacity C 226  drops, then the voltage V 226  increases. When the voltage V 226  reaches a maximum value, a classic discharge circuit (Flyback type structure or other) may be used to transfer the energy of this charge to the storage unit C 5  or directly to the application to be powered. 
     The electrical energy E 226  acquired during this phase where the capacity C 226  decreases is equal to ½Q 226  (V 226max −V 226min ) or even ½Q 226  (1/C 226min −1/C 226max ). The charge Q 226  is the charge transferred by the piezo-electric element P to the capacitor  226  up until the voltage at the terminals of the piezo-electric element starts to drop (start of part II,  FIG. 1D ); the voltage V 226max  is the voltage at the terminals of the capacitor  226  at the end of part II ( FIG. 1D ); the voltage V 226min  is the voltage at the terminals of the capacitor  226  at the start of part II ( FIG. 1D ); the capacity C 226min  is the capacity of the capacitor  226  at the end of part II ( FIG. 1D ); the capacity C 226max  is the capacity of the capacitor  226  at the start of part II ( FIG. 1D ). 
     To detect the extreme voltage values at the piezo-electric element P or the electrostatic structure, it suffices for example to detect the passage of the drift of this voltage from a positive value to a negative value. 
     Furthermore, the device advantageously offers symmetry, thus a similar operation intervenes in the circuit C 2  when the voltage Vp becomes negative, which is to say when the moving part  202  moves towards the left. This construction symmetry thus permits energy to be recovered in both directions of movement. It is possible to provide a single circuit if desired. 
     Advantageously, it is provided that the voltage V P  at the terminals of the piezo-electric element is at maximum when the capacity of the capacitor  226 ,  230  is at maximum. The conversion is thus optimised, as the values of the voltages V 226  to calculate the electrical energy converted are the extreme values at the terminals of the capacitor  226 . 
     Indeed, if the voltage Vp of the piezo-electric element P was at maximum before or after the capacity C 226  of the capacitor  226 , the values of the voltages V 226  at the terminals of the capacitor  226  would not be the extreme values, but values in part II ( FIG. 1D ) in the interval ]V 226min ; V 226max [. 
     Again advantageously, the conversion carried out by the piezo-electric element and the conversion carried out by the capacitors  226 ,  230  are synchronised, as the deformation of the piezo-electric element P and the capacitors  226 ,  230  result from the same external mechanical efforts, wherein the latter cause the appearance of a difference in potential at the terminals of the piezo-electric element, and the variation of capacity of the capacitors  226 ,  230 . 
     The device according to the invention furthermore has the advantage, with respect to an electrostatic system of the prior art, of being able to start without the need for an initial electrical energy source, as this is provided by the piezo-electric element when it is deformed by the moving part  202 . 
     Furthermore, the active charge cycle of the prior art is a source of electrical losses due to the consumption of the electrical circuit, which permits the energy to be taken from a storage unit that is already charged, to detect the charge instant of the capacity and to inject this energy into the device. According to the invention, the active charge cycle is advantageously replaced by a totally passive and naturally synchronised charge cycle. As the energy conversion is mainly carried out via the electrostatic structure, the piezo-electric element, whose main purpose is to inject a small initial charge into the electrostatic structure, does not need to be very efficient. 
       FIG. 2A  shows another embodiment of a device according to the invention, comprising a piezo-electric element P and an electrostatic electrode  4 . The piezo-electric element P and the electrode  4  are substantially parallel and separated by a dielectric material, for example air. The piezo-electric element P and the fixed electrode  4  are attached by their longitudinal ends to a carrier  6 . The piezo-electric element P and the fixed electrode  4  are electrically isolated. 
     The piezo-electric element P comprises a piezo-electric material  9 , for example a membrane made of piezo-electric PVDF type polymer, a first  8  and a second  10  electrode on either side of the membrane. The second electrode  10  is opposite the fixed electrode  4  so as to form a capacitor  12 . 
     The piezo-electric element P is capable of deforming when a mechanical charge is applied in the Z direction, wherein this deformation causes a movement of the second electrode  10  and thus a variation of the distance d between the fixed electrode  4  and the moving electrode  10 . The capacitor  12  therefore has a capacity which varies with the application of a mechanical effort on the membrane  9 . 
       FIG. 2B  shows a basic principle electrical diagram of a device according to the invention, for example the device of  FIG. 1A . Means of controlling the charge transfer  20  are fitted to the circuit. These means are, for example of the diode or switch type and are fitted with a control circuit. These means  20  authorise the charge transfer of the piezo-electric element P to the capacitor  12 . 
     When an effort is exerted on the piezo-electric element P in the Z axis, the piezo-electric membrane  9  is deformed, causing the appearance of an electrical potential between the first  8  and second  10  electrodes. This deformation of the piezo-electric element P causes the second electrode  10  and the fixed electrode  4  to move towards one another, and an increase in the capacity of the capacitor  12 . 
     The operation is identical to that described in relation to  FIGS. 1A to 1E . 
     The charge generated by the deformation of the piezo-electric element P is transferred to the capacitor  10 . Then, when the effort is no longer exerted on the piezo-electric element P, the capacitor regains its rest position, and the electrodes  4  and  10  move away from one another causing a drop in the capacity. 
     This capacity variation associated to a variation of the potential at the terminals of the capacitor  12 , permits electrical energy to be recovered that may be stored or directly used by a determined application. 
     The deformation of the piezo-electric element P is synchronised with the capacity variation of an electrostatic element, as it is the same external mechanical energy which causes the variation in capacity and the deformation of the piezo-electric element. 
     This device may, for example be used to recover the energy from pressure or impacts that may be exerted on a membrane, such as that resulting from pressing a switch . . . , the pressure variation of a gas, the pressure variation of a blood vessel, or even a drop of rain falling onto the piezo-electric element. 
       FIG. 3  shows another embodiment of a device according to the invention comprising a carrier  104  with an elongated U shape, whose inside ends  106 ,  108  of the branches carry electrostatic electrodes  110 ,  112  opposite one another. The device also comprises a piezo-electric element P fixed to the bottom of the U. The piezo-electric element P comprises two parts  114 ,  116  made of piezo-electric material respectively comprising on their external face an electrode  118 ,  120  attached by their internal face to a first end of a beam  122  made of conductive material extending in parallel to the branches of the U. The beam comprises at its second end a moving mass  124  positioned between the electrostatic electrodes  110 ,  112 . The mass  124  comprises conductive faces  126 ,  128  respectively opposite electrodes  110 ,  112 , and separated from them by a dielectric material, for example air. 
     The faces  126 ,  128  and the electrodes  110 ,  112  respectively form capacitors, whose capacity may vary when the mass  124  moves between the branches of the U. 
     Consequently, the device may be used, thanks to the inertia of the moving mass  124 , to recover electrical energy if the carrier  104  is subjected to mechanical vibrations. When the carrier  104  is subjected to mechanical vibrations, the mass  124  oscillates between the branches of the U, causing a deformation of the piezo-electrics parts  114 ,  116  and therefore the appearance of an electrical potential between the electrodes  118 , 120  and the common electrode  122  and simultaneously a variation of the capacity of the capacitors. 
     As the beam is conductive, the potential of the electrode  122  is in the electrodes  1126 ,  128 . 
     It is therefore possible to recover electrical energy by associating an electrical circuit, for example that of  FIG. 1B . 
     The operation is identical to that of the device of  FIG. 1A . 
       FIG. 4  shows another embodiment of a device for converting a mechanical constraint into electrical energy, with a sandwich structure formed by a plurality of layers. 
     The device comprises a piezo-electric element P equipped, on either side of a piezo-electric material  404 , with electrodes  406 , 408 . The device also comprises a multi-layer structure comprising first conductive layers  410  with a same potential U 410  alternated with second conductive layers  412  with a same potential U 412 , separated by deformable isolating layers  414 . This structure is deposited on the piezo-electric element. 
     The potential U 410  is different from the potential U 412 . 
     The first  410  and second  412  conductive layers form with the isolating layers  414 , a succession of capacitors of variable capacity, that are electrically in parallel. 
     The electrode  406  is in electrical contact with the electrode  410 . 
     One side opposite that of the piezo-electric element is designed to receive a variable mechanical constraint in the Z direction. 
     When a constraint is applied to the sandwich, there is an increase in the potential electrical energy at the terminals of the piezo-electric element P and an increase of the electrostatic capacity by the layers  410 ,  412  moving towards one another. These layers  410 ,  412  can move towards one another due to the choice of a very flexible electrical insulator, for example rubber, which under the force of the constraint will allow the layers  410 ,  412  to move towards one another. 
     As for the device of  FIG. 1A , the electrical energy produced by the piezo-electric element P is transferred to the capacitor  416  when it is at maximum. The potential electrical energy is then amplified when the constraint applied to the device is released. 
     During the compression, the piezo-electric element converts part of the mechanical energy into electrical energy and when the constraint is released, it is the capacitor or the electrostatic part which amplifies this conversion. 
     Electrostatic structures may also be mentioned. Indeed, when the conductive layers  410 ,  412  are brought closer to one another, they receive an electrical charge from the piezo-electric element P. This electrical charge then creates electrostatic forces which tend to prevent the conductive layers from moving away from one another, thus transforming part of the potential mechanical energy acquired during the compression into potential electrical energy. This acquired potential electrical energy may be used to power an electronic circuit, an actuator or an energy storage unit. 
       FIGS. 5A ,  5 B and  5 C show a representation of a practical embodiment of a device according to the invention with a similar form to that of the device of  FIG. 1A . 
     For reasons of simplification, we will use the same references as those used in  FIG. 1A . 
     The arms  218  are formed by the piezo-electric elements P extending perpendicularly to the axis of movement Z across the entire the width of the carrier. The piezo-electric elements P are for example embedded by their ends between the carrier and the fixed parts  204 ,  206 , as may be seen in the detailed view of  FIG. 5B . A layer  223  of electrically isolating material is interposed between the piezo-electric elements P and the fixed parts  204 ,  206 . The piezo-electric elements comprise an electrode  234  positioned on one side of a piezo-electric material  232 , wherein this electrode is electrically connected to the moving part  202 , and two electrodes  236 ,  238  positioned on the other side of the piezo-electric material  232 . 
     The electrodes  236 ,  238  are connected together to a same electrical potential. 
     The device advantageously comprises stops  239  to avoid a short circuit by contact between the teeth at the end of their travel. 
     It may be noted that the energy conversion device may comprise a greater number of teeth than shown. 
     The dimensions shown in  FIGS. 5A to 5C  are provided solely by way of example and are in no way restrictive. 
     The moving part  202  has a width of 300 μm and a length of 7000 μm. The teeth are spaced at a distance of 150 μm and have a height of 1200 μm. The device may have a thickness of 400 μm, comprise 46 teeth on each side of the moving part  202  and weight approximately 1 g. The gap between the teeth may vary between 1 μm and 50 μm. 
     When the moving part  202  moves towards the right of the drawing in the Z direction with respect to the fixed parts  204 ,  206 , this causes an increase in the capacity between the moving part  202  and the fixed part  204 , and a drop in the capacity between the moving part  202  and the fixed part  206 . There is also at the same time a deformation, especially elongation, of the piezo-electric elements P. 
     We consider that the electrodes  234  of the piezo-electrical elements have the same electrical potential as the moving part  202 . 
     When the piezo-electric elements P are elongated, we consider that by symmetry the potentials of the electrodes  236 ,  238  increase in the same way and may be connected together to an electrical potential V. 
     The system may be made partially or totally using microelectronic technologies or at a larger scale using standard manufacturing techniques (machining, casting or other). 
       FIG. 5D  shows an electrical circuit adapted to the mechanical structure of the circuit of  FIG. 5A . 
     In addition to the circuit shown in  FIG. 1B , it comprises an example of a circuit C 3  for discharging the electrical energy recovered by conversion of the mechanical energy to an electrical energy storage unit C 5 , and advantageously a circuit of a secondary electrical source C 4  to actuate the start of the discharge circuit C 3 . 
     The discharge circuit C 3  is connected in parallel to the circuits C 1  and C 2 . The discharge circuit comprises a switch K 1 , K 2  in series with a coil L 1 , L 2 , respectively in parallel with the circuit C 1 , C 2 . It also comprises, connected to the switch K 1 , K 2  a diode D S  and a coil L S  in series with the storage unit C 5 . The terminals L 1 , L 2  and L S  also have the same magnetic circuit. 
     A diode D P  is also mounted in parallel with the piezo-electric element P. 
     The circuit of the secondary source C 4  comprises, for example two diodes  224 ′ and  228 ′ connected to the diodes  224  and  228  and to a source E′ that may be recharged by the diodes  224 ′ and  228 ′. 
     Every time that the capacity C 226  or C 230  reaches a maximum value C 226max  or C 230max , the piezo-electric element P also reaches maximum deformation and produces a positive voltage Vp capable of recharging the capacitor  226 ,  230  whose capacity has reached its maximum value. 
     The energy of the injected charge is then amplified by the drop in the value of the capacity of the pre-charged capacitor  226 ,  230  or the increase in the voltage at the terminals of the capacitor  226 ,  230 , in the same way as for the device of  FIG. 1A . 
     When the voltage at the terminals of the capacitor  226 ,  230  reaches a maximum value, its electrical energy is then transferred to the storage unit C 5  by means of the discharge circuit C 3 , by closing, for one quarter of a resonance period (L 1 C 226  or L 2 C 230 ), the corresponding switch K 1 , K 2 . The potential electrical energy acquired by the capacitor  226 ,  230  is then transferred to the discharge circuit C 3  via the switch K 1  or K 2 , then from the discharge circuit C 3  to the storage unit C 5  via the diode D S . 
     If when started the storage unit C 5  is empty, the energy required to control the switches K 1  and K 2  may be provided by the secondary source E′ which charges naturally with the diodes  224 ′ and  228 ′. Next, as soon as the storage unit C 5  has reached a sufficient voltage, it may power the control of these switches. 
     The role of the diode Dp is to re-inject into the electrode  222 . 3  of the piezo-electric element P, the charges previously transferred to the capacitor  226  or  230 . 
     To minimise the losses, the diodes are preferably chosen with a low threshold, low resistance when they allow current to pass and high impedance in inverted voltage, for example they may be Schottky type diodes. The transistors are preferably chosen with low resistance when they allow current to pass, very high impedance in the open state and low parasite capacity, for example they may be transistors of the MOSFET or JFET type. 
     As concerns the piezo-electric element, the material used, its thickness and its length are chosen so that they are capable of providing sufficient voltage and current to pre-charge the electrostatic structure comprising the circuits C 1  and C 2 , and that they preferably provide a resonance of the electrostatic structure within the frequency range of mechanical vibrations or relative movements that are sought to be recovered. 
     The piezo-electric material may be chosen among the ceramics: PZT (PbZrtiO 3 ) or PLZT (with lanthane) or BaTiO 3 , among the nano-crystals (PZN-PT or PMN-PT), among the polymers (PVDF) or the AFCs (Active Fibre Composites). 
     The magnetic circuit of the inductive transformer is chosen so that it can store the energy acquired by the capacitor  226  or  230  during a cycle. The number of windings of the coil L 1 , L 2  and Ls fitted to this magnetic circuit is such that the charge/discharge times of the magnetic circuit are negligible with respect to the mechanical period of the relative movement. This number of windings moreover depends on the value of C 226min , C 230min  and the voltage at the terminals of the storage unit C 5 . 
     It is possible for example that the resonance period of the circuits L 1 C 226min  and L 2 C 230min  is of the order of a microsecond, which is to say very low with respect to that of mechanical vibrations which have a period of several milliseconds. 
     Based on the dimensions of the device provided above, we will determine the energy that may be recovered by the device according to the invention. 
     The capacitive surface area of the capacitor  226  or  230  is equal to:
 
 S= 46*0.4*1=18.4 mm 2  
 
 C   226min   =C   230min =∈ O   S/d   max =∈ O *18.4 mm 2 /50 μm=3.3 pF
 
 C   226max   =C   230max =∈ O   S/d   min =∈ O *18.4 mm 2 /1 μm=162.7 pF
 
     d max  and d min  are respectively the maximum and minimum gaps between two teeth of the C 226  and C 230  capacitors during movement. 
     If the resonance frequency is also considered as being equal to f r =50 Hz, then the global stiffness k of the piezo-electric elements with respect to a movement according to the Z axis of the moving part  202  is equal to:
 
 k=M (2 πf   r ) P =10 −3 (2π50) P =98.7 N/m.
 
     The energy that is mechanically available in a cycle where A is the amplitude of the relative movement is equal to E dispo ≈½ kA 2 . 
     In the most favourable case (that which generates the most energy), A is equal to (d max −d min )/P=24.5 μm. 
     Therefore: E dispo ≈½ kA 2 =½*98.7*(24.85*10 −6 ) 2 =29.6 nJ. 
     The energy that is electrically convertible in a cycle by the electrostatic structure is equal to: 
     
       
         
           
             
               E 
               C 
             
             = 
             
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   
                     C 
                     max 
                   
                   
                     C 
                     min 
                   
                 
                 ⁢ 
                 
                   ( 
                   
                     
                       C 
                       max 
                     
                     - 
                     
                       C 
                       min 
                     
                   
                   ) 
                 
                 ⁢ 
                 
                   V 
                   charge 
                   2 
                 
               
               = 
               
                 
                   4.10 
                   
                     - 
                     9 
                   
                 
                 ⁢ 
                 
                   V 
                   charge 
                   2 
                 
               
             
           
         
       
     
     If we want the electrically convertible energy to be equal to the energy available mechanically, which correspond to damping close to 1: E=E dispo , therefore V 2   charge =2.7 V. 
     The energy to be injected into the electrostatic structure (C 226  or C 230 ) to carry out the pre-charge is equal to: 
     
       
         
           
             
               E 
               
                 pre 
                 ⁢ 
                 
                   - 
                 
                 ⁢ 
                 charge 
               
             
             = 
             
               
                 
                   1 
                   2 
                 
                 ⁢ 
                 
                   C 
                   max 
                 
                 ⁢ 
                 
                   V 
                   charge 
                   2 
                 
               
               = 
               
                 0.6 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 n 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 
                   J 
                   . 
                 
               
             
           
         
       
     
     It is therefore sufficient for the piezo-electric elements to be capable, for a relative amplitude movement of 24.5 μm to produce, during each cycle, energy of 0.6 nJ with 2.7 V, which corresponds to energy that is 49 times less than that which will be converted by the electrostatic structure. It therefore appears that in this case the only role of the piezo-electric beams is to start the operation of the electrostatic conversion structure. 
     As the electrical energy converted by the piezo-electric elements only represents 2% of the mechanical energy available, the choice of the piezo-electric materials may thus be made by mainly taking account of their compatibility with the microelectronic manufacturing technologies used. 
     The power that may be recovered with such a system at 50 Hz and full amplitude is: 
     P recoverable50Hz =29.6.10 −9 *2*50=2.96 μW, as there are two cycles per mechanical period, one cycle with the capacitor C 226  and one cycle with the capacitor C 230 . 
     This invention permits a gain of 20% in efficiency with respect to a purely electrostatic system. 
       FIG. 6  shows a diagram of an electrical circuit that may be used to use the association of synchronised piezo-electric and electrostatic elements according to one advantageous embodiment of the invention. 
     The circuit comprises a piezo-electric element P, a diode  504 , a capacitor  506  of variable capacity C 506 , a switch  508  and a coil  510  in parallel with the piezo-electric element P. The voltage at the terminals of the piezo-electric element P is V P  and the voltage at the terminals of capacitor  506  is V 506 . 
     In this case we consider that the capacity of the capacitor  506  increases when Vp drops inversely. 
     This embodiment provides for the actuation of the switch  508  to inverse the voltage at the terminals of the piezo-electric element and to improve the efficiency of the device. 
     When the voltage Vp is at maximum, the voltage at the terminals of the piezo-electric element P may be inverted by closing the switch  508  for a half-resonance period of the circuit formed by the coil  510  and the capacity of the piezo-electric element P. This inversion of the sign of the charges stored in the piezo-electric element P causes the appearance of a mechanical braking force which permits the energy converted in the next cycle to be increased. This cycle corresponds to the phase in which the voltage Vp drops. Then, when the voltage Vp is at minimum, the switch  508  is again closed for a half-resonance period causing an inversion of the polarity of the voltage Vp and, by means of the diode  504 , charging of the capacitor  506  when it is at maximum. Then, when the capacity C 506  drops, its potential electrical energy increases up to a maximum value before it is transferred totally or partially to the application to be powered in a very short space of time with respect to the variation period of the capacity C 506 . Residual energy may be left in the capacitor  506 , so that the voltage Vp starts the next cycle with a value that is not zero. 
       FIG. 7  shows a diagram of an electrical circuit of another advantageous embodiment of the invention, permitting the conversion device to be started without supplying energy. 
     The circuit comprises a piezo-electric element P mounted, in parallel with a diode  604  and a capacitor  606 , and also in parallel with a diode  608 . 
     The piezo-electric element P and the capacitor  606  are connected by one of their terminals to a mass potential  610 . The capacitor  606  is connected by its other terminal to a discharge circuit. 
     It is supposed that the voltage Vp at the terminals of the piezo-electric element increases, when the capacity C 606  of the capacitor  606  increases and vice versa. 
     When the voltage Vp increases, the diode  604  allows current to pass and charges the capacitor  606  until the voltage Vp reaches a maximum value. Then, when the capacity C 606  drops, the charge that has been transferred to the capacitor  606  is amplified. When this potential electrical energy is at maximum, it is transferred, via a discharge circuit, to an application to be powered or a storage unit. 
     The introduction of the diode  608  permits, when the voltage Vp is normally negative, to recharge the capacity Cp of the piezo-electric element P in order to increase the maximum value of the voltage Vp during its positive cycle. 
       FIG. 8  shows an electrical diagram of one advantageous embodiment of the invention, which allows the piezo-electric element to be made active for the recovery of energy. 
     Rendering the piezo-electric element active during the energy recovery means, within the scope of this application, similarly to the electrostatic conversion, that an electrical charge is injected into the piezo-electric element so as to create a piezo-electric force which opposes its deformation. By thus creating a force which opposes the movement, the energy converted in a cycle by the piezo-electric element increases significantly with respect to a totally passive operation. 
     The circuit of  FIG. 8  comprises a first circuit C 6  and a second circuit C 7 . The first circuit C 6  comprises a piezo-electric element P, a diode  704 , a capacitor  706 , a switch  708  and a coil  710 . The switch  708  and the coil  710  are mounted in parallel with the diode  704 . 
     The second circuit C 7  comprises the piezo-electric element P, a diode  712 , a capacitor  714 , a switch  716  and a coil  718  mounted in parallel with the diode  712 . The switch  708  and the coil  710  are mounted in parallel with the diode  704 . 
     The piezo-electric element P is connected to a storage unit C 5 . The first circuit C 6  between the switch  708  and the coil  710  and the second circuit C 7  between the switch  716  and the coil  718  are connected to the storage unit C 5  by a switch  720 ,  722  respectively. 
     When all of the switches  708 ,  716 ,  720 ,  722  are open, the circuit operates like that of  FIG. 1B . This configuration permits the system to be started, which is to say that, when the voltage Vp at the terminals of the piezo-electric element increases, the capacitor  706  is charged, then, when the capacity of the capacitor  706  drops, the voltage V 706  at the terminals of the capacitor increases. Then, when the voltage V 706  is at maximum, instead of completely discharging the electrostatic structure to the storage unit C 5 , part of the electrical energy is transferred to the piezo-electric element P. For this purpose, the switch  708  is closed, which has the effect of establishing a current between the capacitor  706  and the piezo-electric element P. This circulation of current permits the voltage Vp to be inverted and thus create a mechanical braking force at the piezo-electric element P. Finally, part of the total electrical energy (stored in the piezo-electric element, the capacitor  706  and the coil  710 ) may be sent back to the storage unit by opening the switch  708  and by closing the switch  720 , until the current circulating between the circuit C 6  and the storage unit C 5  is cancelled out. 
     It may be noted that, due to the presence of the diode  704 , the capacitor  706  may not be completely discharged if the capacity of the piezo-electric element Cp is not to be completely discharged, in order to start the following cycle with a polarisation not equal to zero. This is not a problem as long as this voltage Vp remains low compared to the maximum value of the voltage V 706 . The closing time of the switch  708  may be set according to the polarisation value desired at the piezo-electric element P and the energy that is to be transferred to the storage unit C 5  at each cycle. Due to the symmetry of the structure, a similar operation exists via the C 7  circuit, when the piezo-electric element moves in the other direction similarly to the device of  FIG. 1A . 
       FIG. 9  shows a diagram of an electrical circuit of another advantageous variant of the invention in which the piezo-electric element is used solely as an initial source of energy, wherein the energy is then amplified in several steps via the variable capacities of the electrostatic structure. 
     The circuit comprises a first circuit C 9  and a second circuit C 10 . The circuit C 9  comprises a piezo-electric element P, a diode  804  and a capacitor of variable capacity  806 . 
     The second circuit C 10  comprises a piezo-electric element P, a diode  808  and a capacitor of variable capacity  810 . 
     The capacitor  806  is mounted in parallel with a switch  812  and a coil  814 . 
     The capacitor  810  is mounted in parallel with a switch  816  and the coil  814 . 
     The coil  814  is also connected to a storage unit C 5  by means of a switch  818 . 
     Initially, the voltage V 806  at the terminals of the capacitor  806  is zero. When the voltage at the terminals of the piezo-electric element Vp becomes positive, there is an initial charge of the capacitor  806 , then the energy associated to this charge is amplified by the drop in the capacity of the capacitor  806 . When the capacity of the capacitor  806  reaches its minimum value, which is to say when the voltage V 806  is at maximum, the potential electrical energy of the capacitor  806  is transferred, first of all to the inductance  814 , by closing the switch  812 , then from the inductance  814  to the capacitor  810 , by closing the switch  816 . Furthermore, given that when the capacity of the capacitor  806  is at minimum, the capacity of the capacitor  810  is maximum, there is another increase of the potential electrical energy during the drop in the capacity of the capacitor  810 . 
     The time to transfer the energy from one capacity to the other during the mechanical period are considered as negligible, which is to say that the resonance frequencies of the circuit formed by the coil  814  and the capacitor  806  and the circuit formed by the coil  814  and the capacitor  810  are much higher than the mechanical oscillation frequency of the moving part. Then, when the potential energy of the capacitor  810  reaches its maximum value, its energy is transferred to the capacitor  806 . 
     The transfers between the two capacitors  806 ,  810  continue until the amplification level becomes sufficient. In each mechanical period, the excess energy is transferred to a storage unit C 5 . This may be carried out using the energy temporarily stored in the coil  816 , then transferred partially or totally to the storage unit C 5  by closing the switch  818 . 
     Due to the symmetry of the layout, the initial charge may be made by the second circuit, with a negative voltage Vp at the terminals of the piezo-electric element. 
     This device has the advantage of starting with a very low voltage in the piezo-electric element, of the order of a few tenths of volts (voltage slightly higher than the threshold voltages of the diodes  806  and  810 ). 
     The operation of the devices, especially that of  FIG. 2A , has been described considering a compression force, but the conversion of energy applied in the form of a traction force designed to move the layers  410 ,  412  apart is also possible. 
     This invention is not restricted to micro-systems, it also applies to metric sized systems and nano-metric sized systems.