Abstract:
The energy harvester module of the capsule comprises: a primary oscillating structure subjected to an external low-frequency stress; a secondary oscillating structure comprising an elastic element and able to vibrate in high-frequency resonance; and an electrostatic structure with a first electrode coupled to the primary structure and a second electrode coupled to the secondary structure. The electrodes exert a mutual attraction between them driving the secondary structure away from its stable equilibrium position with tensioning of the elastic element, up to a limit beyond which the secondary structure is released by relaxation effect to vibrate at a resonance frequency. A transducer coupled to the secondary structure converts these high frequency vibration movements into electrical energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of and priority to French Patent Application No. 1359466, filed Oct. 1, 2013. French Patent Application No. 1359466 is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     The invention is related to the field of the “medical devices” as defined by the directive 93/42/CE of Jun. 14, 1993 of the European Communities, and notably the “active implantable medical devices” as defined by the directive 90/385/CEE of Jun. 20, 1990 of the European Communities. This definition in particular includes the implants that continuously monitor the cardiac rhythm and deliver if necessary to the heart electrical pulses of stimulation, cardiac resynchronization, cardioversion and/or defibrillation in case of a rhythm disorder detected by the device. It also includes neurological devices, cochlear implants, etc., as well as devices for pH measurement or devices for intracorporeal impedance measurement (such as the measure of the transpulmonary impedance or of the intracardiac impedance). 
     The invention relates more particularly to those of these devices that implement autonomous implanted capsules and are free from any physical connection to a main implanted (such as the can of a stimulation pulse generator). 
     These autonomous capsules are called for this reason “leadless capsules” to distinguish them from the electrodes or sensors placed at the distal end of a lead, this lead being traversed throughout its length by one or more conductors connecting by galvanic liaison the electrode or the sensor to a generator connected at the opposite, proximal end, of the lead. Such leadless capsules are, for example, described in U.S. 2007/0088397 A1 and WO 2007/047681 A2 (Nanostim, Inc.) or in U.S. 2006/0136004 A1 (EBR Systems, Inc.). 
     These leadless capsules can be epicardial capsules, fixed to the outer wall of the heart, or endocardial capsules, fixed to the inside wall of a ventricular or atrial cavity, by a protruding anchoring helical screw, axially extending the body of the capsule and designed to penetrate the heart tissue by screwing to the implantation site. The invention is nevertheless not limited to a particular type of capsule, and is equally applicable to any type of leadless capsule, regardless of its functional purpose. 
     A leadless capsule includes various electronic circuits, sensors, etc., and a transmitter/receiver for wireless communication for remote data exchange. The signal processing inside the capsule and its remote transmission requires a non-negligible energy compared to the energy resources this capsule can store. However, due to its autonomous nature, the capsule can only use its own resources, such as an energy harvester circuit (by the movement of the capsule), associated with an integrated small buffer battery. 
     A first type of energy harvester uses a transducer coupled to an inertial mechanism including a mobile mass, called “seismic mass”, oscillating in the capsule according to the movements of the latter, which is subject to forces due to movements of the wall the organ of the patient and to fluid forces from the surrounding medium. The recovered power mainly depends on the excitation frequency of the seismic mass, of the amplitude of the movement and of the value of mass. However, in the case of the environment of the human body, the excitations from the acceleration of the body or organs do not have stable specific frequencies for which the harvesting may be optimized to produce a mechanism resonance. Thus, it is not possible to benefit from a mechanical amplification which would increase the amplitude and allow harvesting of a maximum of inertia energy. Furthermore, the excitation frequencies involved are very low, of the order of 0.5 to 10 Hz for typical pulse frequency of blood flow and 15 to 40 Hz for the movements of the heart walls, which limits performance of the harvester. Finally, the mass value of the seismic mass must remain very low, for fulfilling miniaturization requirements. 
     Another, non-inertial, type of energy harvester uses variations of the pressure of the fluid surrounding capsule (typically blood medium) to cyclically deform or move a flexible membrane or a bellows coupled to a transducer. The energy that can be harvested depends mainly on the magnitude of the cyclic movement of the diaphragm or bellows operated by the surrounding fluid (which amplitude is necessarily limited for reasons of mechanical reliability), on the frequency of the cyclic movement and on the area of the moving surface (necessarily limited for obvious reasons of miniaturization of the capsule). Again, the pressure variations occur at the heart rate, of the order of 1 to 3 Hz, and therefore only allow applying low frequency to the transducer, thus imposing a limitation of the performance of the energy harvester. 
     One aspect of the present invention may overcome these limitations by proposing a new type of energy harvester provided with a mechanism increasing the excitation frequency of the transducer, so as to benefit, for a single cycle of external stress, from a plurality of transduction cycles for converting the procured mechanical energy. 
     On this aspect, U.S. 2011/0140577 A1 describes a ciliated energy harvesting device including two suspended magnets, mounted face-to-face and in opposite poles and each carried by an elastic membrane, together with an inertial mass bearing a third intermediate magnet. The comings and goings of the inertial mass causes successive coupling/decoupling of the suspended magnets at a frequency higher than that of the oscillation of the inertial mass. The oscillation energy of each suspended magnet is harvested by a fixed coil within which the magnet oscillates. While this structure improves the efficiency of energy harvesting, nevertheless it retains the disadvantages described above relating to the dual presence, by nature essential, of a seismic mass and of magnetic means. 
     Another aspect of the invention may provide such a mechanism that can be used both with a seismic mass inertial harvester, biased by external vibrations and movements of the surrounding environment and with a non-inertial harvester with a membrane or bellows biased by cyclical variations in fluid pressure that surrounds the capsule. 
     Yet another aspect of the invention may provide such a mechanism that does not implement any magnetic element that would create a risk during MRI or any repetitive shock or mechanical contact which would result in the long term mechanical reliability problems. 
     SUMMARY 
     The present invention may implement two moving elements, namely: 
     A first structure, hereinafter “primary oscillating structure” submitted to a low frequency external stress that can be a force directly applied e.g. from pressure changes cyclically moving a membrane or a bellows, or indirectly by a seismic mass integral with this structure, this structure moving at the same low frequency; and 
     A second structure, hereinafter “secondary oscillating structure”, vibrant at a higher frequency, typically with a resonance effect and coupled to the transducer of the energy harvester. 
     To achieve this frequency conversion, both primary and secondary structures are coupled together by a mechanism achieving a “pull-release” function, creating a relaxation phenomenon. This coupling between the two oscillating structures is made, typically, by a coupling structure operating by electrostatic interaction, and therefore without any rubbing or impact magnetic or mechanical element. 
     More specifically, the invention proposes an autonomous intracorporeal capsule having a body and, inside said body, electronic circuits and a module for energy harvesting for the power supply these electronic circuits. The energy harvesting module includes a transducer adapted to convert into electrical energy a cyclic external physical stress applied to the body of the capsule, and resulting from pressure variations in the environment surrounding the capsule and/or from movements of a wall in which the capsule is anchored. 
     According to an embodiment of the invention, the energy harvesting module includes: 
     A primary oscillating structure subjected to external cyclic stress, this primary oscillating structure being adapted to be moved alternately in one direction and in the other at the frequency of the external cyclic stress; 
     A secondary oscillating structure not subject to the cyclic external stress, this oscillating structure including a secondary deformable elastic member and being adapted to freely vibrate at a resonant eigenfrequency higher than the frequency of the external cyclic stress; and 
     An electrostatic structure including a first capacitor electrode coupled to the primary oscillating structure or to the body of the capsule, and a second capacitor electrode coupled to the secondary oscillating structure. 
     The first and second electrodes are configured so that, under the effect of the external cyclic stress applied to the primary oscillating structure, they operate together under the effect of electrostatic interactions by a mutual attraction force driving the secondary oscillating structure away from its stable equilibrium position and with a tensioning of the deformable elastic element. The secondary oscillating structure is thus driven to a limit so that the tension exerted by the deformable resilient element exceeds the mutual attraction force of the electrodes, such that the secondary oscillating structure is then released by relaxation effect to vibrate freely to said resonance eigenfrequency. Finally, the transducer is coupled to the secondary oscillating structure, to convert into electrical energy the vibration movements thereof at said resonance eigenfrequency. 
     The transducer can be of the electrostatic type. It then includes conversion electrodes forming a capacitor, one of these conversion electrodes being cyclically driven by the secondary oscillating structure, and the transducer operating by cyclic variations of the surfaces vis-à-vis and/or of the dielectric interval of the conversion electrodes with correlative variation in the capacitance of the capacitor. The conversion electrodes may include the first and second electrodes of the electrostatic structure. The transducer can also be of the piezoelectric type, including a deformable piezoelectric element configured to be cyclically constrained by the secondary oscillating structure. The transducer may also be of the electromagnetic type, including a mobile magnetic element or a mobile coil configured to be cyclically driven by the secondary oscillating structure. 
     In a first embodiment, the primary oscillating structure includes an inertial mass coupled to the body of the capsule by a resilient suspension adapted to be cyclically stressed under the effect of the movement of the capsule into the surrounding medium. The primary oscillating structure can in particular be coupled to a deformable surface outer of the capsule body, adapted to be cyclically stressed under the effect of pressure changes in the medium surrounding the capsule. 
     In a second embodiment, the primary and secondary oscillating structures are coupled together by the electrostatic structure. The secondary oscillating structure can be coupled to the body of the capsule by the resilient deformable element, and the first electrode of the electrostatic structure can be connected to the primary oscillating structure, the second electrode of the electrostatic structure being connected to the secondary oscillating structure. 
     In a third embodiment, the primary and secondary oscillating structures are coupled together by the deformable elastic element. The first electrode of the electrostatic structure can be connected to the capsule body, the second electrode of the electrostatic structure being connected to the secondary oscillating structure. 
     In a fourth embodiment, the first and second electrodes of the electrostatic structure are formed as respective fingers of nested comb(s) and counter-comb(s). The fingers of one of the at least first and second electrodes of the electrostatic structure are then resiliently deformable fingers, together forming the deformable elastic element of the secondary oscillating structure. The distal ends of these fingers can be joined together by a rigid coupling. 
     According to another embodiment, there is a method for harvesting energy in an implantable device. The method includes the steps of receiving an external force that causes a first oscillating structure to move in the direction of a second oscillating structure and engaging the second oscillating structure with the first oscillating structure via the movement of the first oscillating structure towards the second oscillating structure. When engaged, the second oscillating structure is pulled away from an equilibrium position to create tension in a deformable elastic element coupled to the second oscillating structure. Release between the second oscillating structure and the first oscillating structure occurs when the tension in the deformable elastic element exceeds a tension limit. Energy is then harvested from the oscillations of the second oscillating structure (e.g., after it is released from the first oscillating structure and vibrates at a resonant frequency). 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       Further features, characteristics and advantages of the present invention will become apparent to a person of ordinary skill in the art from the following detailed description of preferred embodiments of the present invention, made with reference to the drawings annexed, in which like reference characters refer to like elements and in which: 
         FIG. 1  schematically illustrates a set of medical devices including leadless capsules, implanted within the body of a patient. 
         FIG. 2  is a functional block diagram showing the various stages constituting a leadless capsule. 
         FIGS. 3 and 4  illustrate two possible embodiments of a leadless capsule body with harvesting methods of the pressure variations of the surrounding fluid. 
         FIGS. 5-7  show variants of the internal structure of a known leadless capsule with harvesting methods of the pressure variations and an electrostatic transducer. 
         FIG. 8  illustrates the principle of frequency conversion according to the invention, by implementing a dual oscillating structure and a relaxation electrostatic coupling. 
         FIG. 9  illustrates the respective movements of the two oscillating structures of the configuration of  FIG. 8 , during a cycle of mechanical stress in the primary structure. 
         FIG. 10  is the counterpart of  FIG. 8 , for an alternative embodiment for a set of fixed electrodes integral with the capsule body. 
         FIGS. 11-13  illustrate various examples of possible electrostatic configurations between two oscillating structures. 
         FIGS. 14-16  illustrate various possible examples of transducers to associate to the secondary oscillating structure of the mechanism of the invention. 
         FIG. 17  illustrates a possible implementation of the system of frequency conversion according to the invention, using the rotation of rigid electrostatic fingers with respect to a central structure forming the primary oscillating structure. 
         FIG. 18  is a graph showing variations in capacity over time of a structure such as that of  FIG. 17 . 
         FIG. 19  is a graph showing variations of the output power generated by the structure of  FIG. 17 , and in comparison the energy generated with a conventional harvesting device. 
         FIG. 20  illustrates another possible implementation of the system of frequency conversion according to the invention, using the deformation of flexible electrostatic fingers under the effect of electrostatic attraction forces. 
         FIG. 21  illustrates a variant of the system of  FIG. 20  wherein the ends of the flexible fingers are connected together in order to ensure synchronization of their high-frequency movements. 
     
    
    
     DETAILED DESCRIPTION 
     It will first be described with reference to  FIGS. 1 to 7 , the basic structure of a capsule and a leadless embodiment of an energy harvester for such a capsule, according to prior art. 
     In  FIG. 1  a set of medical devices implanted within the body of a patient is illustrated. The patient is implanted for example with an implant  10  such as an implantable defibrillator/pacemaker/resynchronizer or a subcutaneous defibrillator or a long-term event recorder. This implantable device  10  is the master device of a network including a plurality of slave devices  12  to  18 , which may include intracardiac ( 12 ) or epicardial ( 14 ) capsules located directly on the patient&#39;s heart, other devices such as myopotential sensors or neurological stimulation devices, and optionally an external device  18  disposed on an armband and provided with electrodes in contact with the skin. The device  10  can also be used as a gateway with the external environment to communicate with an external peripheral device  20  such as a programmer or a data remote transmission device with which it communicates by telemetry. 
       FIG. 2  schematically illustrates the different internal autonomous circuits of the implanted capsules  12 - 16  shown in  FIG. 1 . 
     The capsule includes for example a pair of electrodes  22 ,  24  connected to a stimulation pulse generator circuit  26  (for an active capsule incorporating this feature) and/or a detection circuit  28  for the collection of depolarization potential collected between electrodes  22  and  24 . A central circuit  30  includes all of the electronics for controlling the various functions of the capsule, for storing the collected signals, etc. It includes a microcontroller and an oscillator generating the clock signals required for the operation of the microcontroller and for the communication. It may also contain an analog/digital converter and a digital storage memory. The capsule may also be provided with a sensor  32  such as an acceleration sensor, a pressure sensor, an hemodynamic sensor, a temperature sensor, an oxygen saturation sensor, etc. The capsule includes an energy harvesting module  34  powering all the circuits via an energy power management stage  36 . Electrodes  22  and  24  are also connected to a transmission/reception circuit of pulses  38  used for wireless communication with the master device or other capsules. 
     The present invention relates particularly to the energy harvesting module  34 . The purpose is to harvest the energy contained in the mechanical forces to which the capsule is subjected. 
     Two special cases of external stresses can be considered: 
     Harvesting of vibration energy, also referred to as inertial energy, by a seismic mass subjected to the acceleration efforts and which produces a force equal to the product of this acceleration by the moving mass; or 
     Harvesting of cardiac pressure variations by providing a body part of the capsule deformable under the influence of these pressure fluctuations. The force generated is proportional to the product of the amplitude of the pressure variations by the surface of the deformable element. 
     In either case, the frequency of the cyclic stresses is, in the cardiac environment, in ranges of the order of 1 to 3 Hz (frequency of the heartbeat) and also around 15 to 20 Hz, for the vibrations of a seismic mass, and in the range 1-3 Hz of the heartbeat for the changes in blood pressure. 
       FIGS. 3-7  illustrate various embodiments of existing devices suitable for the harvesting of pressure forces by a capsule, such as those disclosed in, for example, EP 2520333 A1 (Sorin CRM). However, the aspects of the present invention discussed herein are not limited to use in devices such as those shown in the figures or described in EP 2520333, and may also apply to, for example, energy harvesting systems implementing a seismic mass. 
     To take into account pressure variations, the capsule is formed, as shown in  FIGS. 3 and 4 , with a body  40  provided with one or more deformable elements  42  biased at the rhythm of the pressure variations of the fluid that surrounds the capsule, typically changes in blood pressure. The deformable element  42  includes a rigid surface  44  on which the pressure variations are exerted, and is connected to the body  40  by a deformable bellows  46 . In the example of  FIG. 3 , this surface/bellows set  44 / 46  is disposed on an axial end side of the capsule  40 , while in the example of  FIG. 4  there are provided two surface/bellows sets  44 / 46  disposed on lateral sides of the body  40  of the capsule, the rigid surfaces  44  being parallel to each other and to the main axis of the capsule. 
       FIGS. 5-7  illustrate various possible structures of an electrostatic transducer for energy harvesting of the pressure variations by the deformable element  42 . This transduction mode is, however, not limiting the present invention which, as will be seen below, may also implement a transducer operating according to another, for example an electromagnetic or a piezoelectric, principle. 
     In  FIGS. 5-7 , the mobile surface  44  is coupled to the body  40  by a resilient connection element  48  ( FIGS. 5 and 6 ) or by a bellows  46  ( FIG. 7 ). This mobile surface  44  is connected to a series of first conversion electrodes  50  via a coupling element  52 . These electrodes are preferably configured in the form of combs, for example, made by conventional photoetching techniques. The device further includes second conversion electrodes  54  provided in the form of counter-combs interdigitated with the electrodes combs  50  and connected to the body  40  by a peripheral support  56 . The assembly formed by these electrodes  50 ,  54  is enclosed in the sealed volume  58  formed by the body  40  enclosed by the deformable element  42 . 
     This provides a transducer which can be modelled by a variable capacitor including: 
     A first, suspended, conversion electrode constituted by the combs  50  which are mechanically and electrically connected together and to the mobile surface via the element  52 ; 
     A second, fixed, conversion electrode constituted of the counter-combs  54  which are mechanically and electrically connected together and to the body  40  via the support  56 ; and 
     A dielectric gap, defined between the two electrodes. 
     In the configuration of  FIG. 5 , in case of depression of the mobile surface  44  the gap and the overlap in the plane of the combs remain constant, but the vertical overlap changes during movement. In the case of  FIG. 6 , the electrodes are configured with a variable dielectric interval, while in the case of  FIG. 7 , the electrodes are configured with overlapping in the plane, the rest of the structure being identical to what has been shown in  FIG. 5 . But in all cases, there is an element whose capacitance cyclically varies at the pace of movement of the mobile surface  44  and therefore the variations of the blood pressure of the surrounding medium. 
     If the capacitor has been charged in advance, a diminution of capacity produces excess energy that can be discharged through appropriate circuitry to a storage module, and vice versa. For each cardiac cycle an amount of energy can be recovered and that will be sufficient in the long run to ensure continuous operation of the electronic circuits of the capsule, without additional energy input. 
       FIGS. 8-21  depict various aspects and embodiments of the invention to substantially increase the conversion efficiency of devices such as those described above. 
       FIG. 8  shows the conversion principle according to the invention. Essentially, it involves using electrostatic forces involved in the application of a voltage of an electrical charge between two parts of a variable capacitor to cause a phenomenon of “pull-release” of a structure for transduction of a mechanical energy into electrical energy. Specifically, this structure includes two mobile parts, hereinafter referred to as “primary oscillating structure” (designated  60  in the figures) and “secondary oscillating structure” (designated  70  in the figures, the numerals being maintained in all figures to designate in different embodiments, functionally similar structures). 
     The primary structure  60  is subjected to low frequency external stress such as a direct force or, as in the embodiment of  FIG. 8 , the vibration of seismic mass  62  mounted on a resilient suspension  64  connected to the body  40  of the capsule. The secondary structure  70  includes a mobile element  72  and an elastically deformable element  74 , which is in the embodiment of  FIG. 8  a suspension connecting the mobile part  70  to the body  40  of the capsule. The mass of the mobile element  72  and the elasticity of the suspension  74  define for the secondary structure  70  a self-resonant eigenfrequency, which is typically substantially higher than the frequency at which the primary structure  60  is biased. 
     The two oscillating structures  60 ,  70  are coupled together by an electrostatic structure referenced  80 , including two respective series of electrodes  82 ,  84  whose relative displacement has the effect of varying the capacitance of the capacitor formed by the respective electrodes. The following various possible configurations of the electrodes  82 ,  84  of the electrostatic structure  80  are exhibited hereafter, the same numeral references  80 ,  82  and  84  being retained in the different figures to denote similarly functioning elements. 
     In the embodiment of  FIG. 8 , the electrodes  82  are integral with the primary oscillating structure  60 , and the electrodes  84  are integral with the secondary oscillating structure  70 . The dynamic behaviour of this structure can be explained with reference to  FIGS. 8 and 9 . 
     The primary structure  60 , during its movement (at the cyclic external stress frequency, corresponding to curve A in  FIG. 9 ) will end up facing the secondary structure  70  with progressive overlapping of electrodes  82 ,  84  (passage from state (a) to state (b) in  FIG. 8 ). If a voltage or an electrical charge is applied between the electrodes  82 ,  84  facing each other, an electrostatic interaction, typically a pull, takes place between these electrodes. This interaction will effectively attract the secondary structure  70  to the primary structure  60 , as shown by curve B in  FIG. 9 , which represents the displacement of the structure  70 . The resulting attraction of the electrostatic interaction corresponds to the displacement in the area (b) of  FIG. 9 . 
     The electrostatic attraction force between the two structures  60  and  70  will pass through a maximum, strong enough for the secondary structure  70  driven by the primary structure  60  (“pull” step). But the displacement of the secondary structure  70  is more limited. Once a boundary L is reached, the stiffness of the suspension  74  will separate the two structures (“release” step). No longer attracted by the primary structure  60  which moves away, the secondary structure  70 , released from a stretched state of the suspension  74 , will enter into free vibration at its natural resonance eigenfrequency, as shown in (c) in  FIGS. 8 and 9 . 
     The vibrations of the secondary structure  70 , being the one that provides the transduction, will cause many transduction cycles before the primary structure returns in the action area of the secondary structure, to perform a new pull-release cycle-to-cycle. In order to achieve the desired effect of “pull-release”, the electrostatic attraction should result in a displacement of the secondary structure of the same order of magnitude as the characteristic dimensions of the structure. 
     Hereinafter, the expression “of the same order of magnitude” should be understood in its specific meaning conventionally used in physics, that is to say in a ratio of 0.1 to 10. More specifically, the electrostatic attraction force f e  (expressed in N) between the fingers forming the electrodes is:
 
 f   e   =∈*L*V   2   /g,  
 
∈ being the permittivity of the gas (typically air or vacuum) between the fingers forming the capacitor electrodes, L being the length of the face-to-face surface of the electrodes in the dimension perpendicular to the coupling movement, V being the voltage applied between the electrodes, and g being the gap between these electrodes.
 
     If the deformable element supporting the secondary structure has a stiffness k (N/m), then the spring force f r  under deformation x is (N):
 
 f   r   =k*x.  
 
Thus, in quasi-static equilibrium during pull movement, the displacement is:
 
 x=∈*L*V   2 ( g*k ).
 
     If h is a characteristic dimension of the fingers, such as for example their thickness, then for a displacement x of the same order of magnitude as h, a stiffness k of at least the same order of magnitude as ∈*L*V 2 /(g*h) is required. 
     In contrast, the deformable element should not be too soft, that is to say, with k too low, otherwise there will never be a “release”, the stiffness being never significant enough to leave the structure of the electrostatic attraction and for producing relaxation around the equilibrium position. That is why it is necessary that the stiffness k of the system is equal (to an order of magnitude) to ∈*L*V 2 /(g*h). 
     From the mechanical point of view, if each finger is approximated to a beam of length L, of thickness h (in the direction of the bending direction) and of width w (transverse to the direction of bending), the value of the electrostatic force can be expressed as a linear force (expressed in N/m) of a value f e =∈*V 2 /g, acting along the fingers. 
     Using the laws of mechanics, the finger rigidity f r  can be expressed by unit length (N/m), when the latter undergoes bending of an amplitude x, at the fingertip, by:
 
 f   r =⅔ *E*x*h   3   *w/L   4 ,
 
wherein E is the Young&#39;s modulus of the material of the finger. The phenomenon of bending of the fingers, and therefore of frequency conversion, becomes significant when f e  becomes substantially greater than f r , for a deflection x of the order of magnitude of h.
 
     Thus, to produce the characteristic effects of the present invention, the fingers should be dimensioned such that the value of ∈*V 2 /g is equal to that of E*x*h 3 *w/L 4 , to an order of magnitude. 
     In addition, to avoid instabilities of the fingers in the plane, namely the fact that they can deflect in the plane by reducing the gap (pull-in effect), the fingers preferably have a width w greater than their thickness h to make them rigid in the plane, compared with their desired off-plane flexibility. 
     The multiplication of the operating frequency of the transducer (at the natural resonant eigenfrequency of the secondary oscillating structure  70  and not at the low frequency of the cyclic external stresses) has the effect of substantially increasing the conversion efficiency of the transducers transforming mechanical energy of forces applied to the capsule into electrical energy. Indeed, if at each transduction cycle the transducer converts a given fraction of mechanical energy into electrical energy, and if the transduction frequency is a multiple of the excitation frequency, then the transduction efficiency by excitation cycle is multiplied accordingly. 
     The system described above may be implemented in various possible embodiments. In the case of  FIG. 10 , the deformable elastic element that defines the resonance frequency of the secondary structure  70  is no longer, as in  FIG. 8 , a suspension coupling this structure to the body of the capsule, but a spring  76  connecting the seismic mass  62  of the primary structure  60  to the mobile portion  72  of the secondary structure  70 . Electrodes  84  are always carried by the mobile portion  72  of the secondary structure  70 ; the electrodes  82  on the other hand are fixed electrodes integral to the body  40  of the capsule. 
     As illustrated in (a) in  FIG. 10 , the system combines the primary structure  60  and the secondary structure  70  and initially has a uniform motion induced by low-frequency excitation of the primary structure  60  by the external stress. When this structure is within the field of electrostatic attraction of the fixed structure constituted by the electrodes  82 , as shown in step (b) of  FIG. 10 , then the secondary structure  70  will experience, in addition to the restoring force connecting it to the primary structure  60  (due to the spring  76 ), an electrostatic attraction force which will cause a separation of the movement of the two primary and secondary structures  60  and  70  (“pull” step). As with the embodiment of  FIG. 8 , when the primary structure  60 , in its slow oscillation movement, begins to move away, the rigidity between the two structures takes precedence (“release” step) and the secondary structure  70  suddenly goes out of the attraction field of the fixed electrostatic structure  82 , causing the high-frequency vibration with multiplication of transduction cycles as shown in (c) in  FIG. 10 . 
       FIGS. 11, 12 and 13  show three possible examples of electrostatic structure, wherein the configuration of the facing surfaces advantageously includes surfaces wide separated by a small gap, so as to create a high capacity and therefore a large variation in the capacity during the relative movement of the electrodes. It is thus possible in particular to have interdigitated combs to maximize the facing surfaces, with a variable overlap, outside the plane ( FIG. 11 ) or in the plane ( FIG. 12 ). The capacitance change may also result from a variation of air gap between the facing surfaces of the electrodes ( FIG. 13 ). 
     Transduction in electrical energy of the movement of the secondary structure  70  may be made according to various methods, as schematically illustrated in  FIGS. 14-16 . 
       FIG. 14  illustrates the case of a piezoelectric transducer, for example by providing a part of the suspension of the secondary structure  70  carrying the electrodes  84  with a piezoelectric material  92 , which is subjected to stresses during the movement and therefore will generate charges on these electrodes. These electrodes are situated on either side of the piezoelectric material  92  and are connected to an electronic control circuit, typically a current rectifier and a filtering capacitor. 
       FIG. 15  illustrates the case of an electrostatic transducer, which in the illustrated embodiment includes electrodes  50 ,  54  (functionally similar to those described above in connection with  FIGS. 5-7 ) forming a capacitor  94  whose capacity varies at high frequency. The terminals of the capacitor  94  are connected to an electronic circuit performing at each capacity variation cycle a charging and discharging of the capacitor, so as to generate a positive electrical power. Note that in  FIG. 15  a transducer  94  is described including a set of electrodes  50 ,  54  separate from the electrodes  82 ,  84  of the electrostatic structure  80  performing the frequency conversion according to the invention. It is however possible, and even advantageous, to implement the electric power transduction through the variable capacitor formed by the electrodes  82 ,  84  of the electrostatic structure  80  in the various embodiments described herein, in other words, using these electrodes  82 ,  84  not only for the coupling of the oscillating structures and the conversion of the oscillation frequency (mechanical function) but also to ensure the conversion of the movement into electrical energy (mechanic-electric transduction function). 
       FIG. 16  illustrates the case of an electromagnetic transducer, wherein the mobile part of the secondary structure  70  supports a mobile permanent magnet  96  within a stationary coil  98  secured to the body  40  (or vice versa) so as to generate a high frequency variable magnetic field. 
     These various types of transducers are not limiting, and other techniques may also be used, for example based on magnetostrictive transducers, using electroactive polymers, etc. 
       FIG. 17  illustrates another possible embodiment of the frequency conversion system according to the invention, by implementing a rotation of the electrodes  84  relative to the electrodes  82 , which are fixed and rigid. 
     The states (a) to (d) illustrate the successive phases of deformation and of relative displacements of the different elements of the set. At rest (condition (a)), combs and combs-against of respective electrodes  82 ,  84  are still and face to face. The central element  60  of the assembly makes up the primary structure, which is subjected to external forces, via a seismic mass in the case of an inertial harvesting, or via a mobile surface with a bellows in the case of pressure change harvesting, and therefore its movement is dictated by this low-frequency cyclic external stress. At the start of movement (state (b) in  FIG. 17 ), the central portion  60  begins to shift; however, the combs  84  are subject to electrostatic attraction of the counter-combs  82 , which opposes the movement. In this way, the flexible element  76  connecting the central element  60  forces the combs and counter-combs  82 ,  84  to remain facing each other to the maximum (“pull” step). 
     As the movement grows, the stiffness of the flexible element  76  outweighs the electrostatic attraction, and combs will be suddenly be released (“release” step corresponding to the state (c) in  FIG. 17 ). These combs  84  will then enter a phase of high frequency free vibration, this frequency which can reach hundreds or thousands of hertz being defined by the very low mass of combs  84  and the stiffness of the flexible element  76 . During this vibration, the capacitance between the combs  84  and the counter-combs  82  will also vary at high frequency, and several charge/discharge cycles of the capacitor will be performed, greatly increasing the energy extracted per time unit. 
       FIG. 18  illustrates changes over time of the capacitance formed by the combs and counter-combs  82  and  84  of the assembly of  FIG. 17 .  FIG. 19  shows the energy harvested at the output through this capacity variation. 
     These  FIGS. 18 and 19  illustrate the recorded curves for the system of  FIG. 17 , but these variations of capacity and energy would be similarly obtained with the other structures described in this description. 
     In  FIG. 18 , it is seen that the capacity variation includes a sinusoid with decreasing amplitude O superimposed with high frequency variations V. Thanks to the phenomenon of frequency conversion, the variable capacitor will provide a very large number of maxima and minima per unit of time which, for the same stress cycle, will allow, as can be seen in  FIG. 19 , to harvest a much higher electrical energy E 2  than that E 1  that would be obtained with a conventional device without frequency conversion. 
       FIG. 20  discloses an alternative embodiment wherein the electrostatic fingers  82 ,  84  of the electrostatic structure  80  constitute the resilient deformable element defining the resonant eigenfrequency of the secondary oscillating structure  70 . Specifically, the fingers  82 ,  84  of the combs and counter-combs have geometry such that electrostatic instability phenomena at these fingers occur during movement of the mobile part, thus creating variations in the capacity at high frequencies. The fingers have for this purpose a high aspect ratio, with a thin and elongated shape, thus high flexibility out of the plane allowing vertical electrostatic forces to be significant. 
     As illustrated by the different states (a) to (e) in  FIG. 20 , during the vertical displacement of the mobile part  60  the fingers  84  of combs will bend so as to remain facing the fingers  82  of the counter-combs, the combs/counter-combs electrostatic attraction force being strong compared to the mechanical stiffness of the combs. However, when the movement of combs grows, the combs stiffness takes over and the combs are spontaneously released from their electrostatic attraction vis-à-vis the counter-combs. If we superimpose several layers of combs/counter-combs such as those illustrated in  FIG. 20 , when the combs are released from the attraction of counter-combs of the initial layer, they will enter the field of attraction of counter-combs of the upper layer. This jump from one layer to the other will cause a vibration of high frequency of the combs, causing a change in capacity of the transducer at the same frequency and thereby increasing the number of cycles of charging and discharging of the capacitor, and finally increasing the harvested energy. 
     As shown in  FIG. 21 , it may be advantageous to couple the ends of the fingers  84  so as to allow a perfect synchronization of their movement on the different layers, and thus of the capacity change collected from each finger.