Converter for converting a variation in energy to be recovered into a potential difference

A converter including a first transducer that can lengthen inside at least a first deformation zone and simultaneously shrink inside at least a different second deformation zone. Inner faces of second and third electromechanical transducers are secured respectively, with no degree of freedom, substantially to the first and second deformation zones of the layer of the first transducer. The third transducer differs from the second transducer by the polarization of its layer of piezoelectric material in a different direction to polarization of the layer of piezoelectric material of the second transducer and/or by existence of a separation that mechanically and electrically insulates a first electrode of the third transducer from a first electrode of the second transducer located on a same side of the layer of piezoelectric material.

The invention relates to a converter of a variation in energy to be harvested into a potential difference and to a generator of electricity incorporating this converter.

Here, by potential difference, what is meant is a voltage between two electrical terminals, one of these electrical terminals comprising an excess of electrical charges relative to the other of these terminals.

Such known converters include:a first transducer including a layer extending essentially in a plane, called a reference plane, and behaving mechanically as a single block of material, this layer being able to lengthen in the interior of at least one first zone of deformation of the reference plane and, simultaneously, to shrink in the interior of at least one second zone of deformation of the reference plane in response to a given variation in energy to be harvested, the second zone being distinct from the first zone; andelectromechanical second and third transducers, each electromechanical transducer including:a layer made of piezoelectric material extending essentially parallelly to the reference plane and having exterior and interior faces extending parallelly to this plane, the interior face being fixed with no degree of freedom to the layer of the first transducer in order to undergo a mechanical stress exerted by one of the zones of this layer; andfirst and second electrodes made of electrically conductive material extending essentially parallelly to the reference plane along at least one of the faces of the layer made of piezoelectric material of the second transducer in order to cause an excess of electrical charges to appear on one of these electrodes in response to the mechanical stress undergone by the layer made of piezoelectric material.

The invention aims to increase the efficacy of this type of converter. Efficacy is defined here as being the amount of electrical energy produced for a given variation in the energy to be harvested.

The subject of the invention is therefore such a converter according to claim1.

By “polarization of the layer made of piezoelectric material” what is meant is the polarization of this layer that was created during its manufacture and that exists even in the absence of application of a mechanical stress to this layer. Because of this, this polarization is also called “pre-polarization”. It especially determines on which electrode of the electromechanical transducer will appear an excess of electrical charges in response to a tensile stress applied to the layer made of piezoelectric material.

In known converters, the electromechanical transducer typically comprises:a layer made of piezoelectric material that extends both over the first and second zones of deformation; anda single electrode that covers the entire exterior face of the layer made of piezoelectric material.

Thus, in known converters, the layer made of piezoelectric material generates, above first zones of deformation, electrical charges of a first sign and simultaneously, above second zones of deformation, electrical charges of a second opposite sign. In known converters, the same electrode covers the entire exterior face of the piezoelectric layer. Thus, the electrical charges generated above the second zones cancel out charges generated above the first zones. This decreases proportionally the potential difference generated by known converters in response to a variation in the energy to be harvested and therefore the efficacy of such converters.

The Applicants have discovered this effect and have exploited it to, in contrast, increase the efficacy of the converter. Thus, the claimed converter includes not one but at least two distinct electromechanical transducers. These electromechanical transducers are placed relative to the first transducer in order to exploit separately its first and second zones of deformation. Therefore, the electrical charges generated by the third transducer are no longer subtracted from the charges generated by the second transducer. This makes it possible to increase the efficacy of the converter. In addition, it is possible to electrically interconnect the electrodes of the electromechanical transducers so as to increase the energy harvested, and thus further increase the efficacy of the converter.

Embodiments of this converter may include one or more of the features of the dependent claims.

These embodiments of the converter furthermore have the following advantages:using electromechanical transducers the layers made of piezoelectric material of which have opposite polarization directions, said transducers being placed on the same face of the first transducer, allows the electrical connection of this converter to be simplified because all the charges generated on a given side of the converter are of the same sign;using a common electrode for the electromechanical first and second transducers allows the manufacture of the converter to be simplified;stacking a fourth transducer similar to the first transducer on the exterior face of the layers made of piezoelectric material of the second and third transducers allows the efficacy of the converter to be increased;producing thin partitions between the various layers of piezoelectric material, i.e. partitions the thickness of which is essentially smaller than the thickness of the layer made of piezoelectric material, allows the efficacy of the converter to be increased especially when the latter is very small in size;employing second and third transducers that together cover almost the entirety of the surface of the first and second zones of deformation allows the efficacy of the converter to be increased;placing specimens of the second and third transducers symmetrically relative to a median plane of the first transducer allows the deflection of the converter in directions perpendicular to this plane of symmetry to be limited;using a disk-shaped first transducer makes it possible to produce electrical energy efficiently from a rotary movement of a permanent magnet relative to the converter.

Another subject of the invention is a generator of electricity including the above converter.

Embodiments of this generator may include one or more of the features of the dependent claims.

The arrangement of the permanent magnets so that their orthogonal projections in the reference plane are orthogonal increases the efficacy of the converter. Specifically, this makes it possible to increase the shrinkage in the interior of the second zone of deformation relative to the case where the second permanent magnet is absent.

FIG. 1shows a generator2of electricity. This generator2includes:a source4of energy to be harvested; andan energy harvester6suitable for converting a variation in the energy to be harvested into electricity.

Here, the generator2is described in the particular case where the energy to be harvested is a variable magnetic field.

More precisely, in this embodiment, the variable magnetic field is a rotating magnetic field the magnetic moment of which giving rise thereto turns about itself. To this end, the source4comprises:an apparatus10equipped with a shaft8driven to rotate about itself about a vertical axis9; anda permanent magnet12fixed with no degree of freedom to a stationary mounting14of this apparatus10in proximity to the distal end of the shaft8.

InFIG. 1and the following figures, the vertical direction is identified by a Z direction of an orthogonal XYZ coordinate system. The X and Y directions of this XYZ coordinate system are therefore horizontal. In the following, the terms “above”, “below”, “lower” and “upper”, “top” and “bottom”, “up and down” and “underneath” are defined relative to the Z direction.

For example, the apparatus10is a liquid or gas meter and the shaft8is the shaft of this apparatus10driven to rotate by the liquid or the gas consumed. To simplify the figure, only one portion of the apparatus10has been shown.

The permanent magnet12is a uniaxial permanent magnet having a magnetic moment parallel to the X direction. A uniaxial permanent magnet is a permanent magnet that has a single magnetic moment in a single direction.

The harvester6includes a converter20that converts the variable magnetic field into a corresponding excess of electrical charges on a connection terminal22or24. The excess is here measured relative to the other of the terminals22or24. This converter20is fixed with no degree of freedom to the distal end of the shaft8. Thus, in this embodiment, the rotating magnetic field is obtained by making the converter20and not the magnet12turn about itself. The converter20is electrically connected to the terminals22,24by way of a conventional device for electrical connection of a rotating part to fixed electrical terminals.

The harvester6also includes:a circuit30for collecting the excess of electrical charges on the terminal22or24and for transferring these collected electrical charges to a piece of electrical equipment32; anda circuit34for controlling the circuit30.

For example, the piece of equipment32is a capacitor that stores the harvested energy. The collecting circuit30and the controlling circuit34are for example identical to those described in patent application WO 2007/063194 and, preferably, identical to one of those described in the patent application filed under the number FR 1260047, on 22 Oct. 2012 by the Commissariat à l'énergie atomique et aux énergies alternatives. Therefore, these circuits30and34are not described here in more detail.

The converter20is capable of converting into electricity very slow variations in the energy to be harvested. By “slow variation”, what is meant is variations the fundamental frequency of which is lower than 10 Hz. To this end, the converter20converts a variation in the amplitude of the variable magnetic field in a given direction into a generation of an excess of charges on the terminal22and, in alternation, on the terminal24. This conversion takes place almost instantaneously so that the voltage between the terminals22and24varies as and at the same time as the amplitude of the variable magnetic field varies in a given direction.

The converter20includes a magnetic transducer associated with electromechanical transducers. The magnetic transducer is a layer40of magnetostrictive material. The layer40is produced from a magnetostrictive material the absolute value, at saturation, of the magnetostrictive coefficient λsof which is higher than 10 ppm (parts per million) and preferably higher than 100 or 1000 ppm. The coefficient λsis defined by the following relationship: λs=ΔL/L, where:ΔL is the degree of elongation of the magnetostrictive material, for example, along its preferential axis of deformation; andL is the length of this material along this axis in the absence of exterior magnetic field.

The preferential axis of deformation is the axis along which the amplitude of the deformation of the layer made of magnetostrictive material is maximal when said layer is passed through by field lines parallel to this axis.

Here, the coefficient λsis positive. For example, the magnetostrictive material is Terfenol-D or FeSiB or an FeCo alloy or a derivative compound.

Here, the layer40is formed from a single block of material. This layer40has the shape of a horizontal disk. This disk is centered on the axis9and has horizontal planar faces on each side, which faces are, in the following, referred to as the “top” and “bottom” faces, respectively. InFIGS. 2A and 2B, the direction of the field lines generated by the magnet12in the interior of the layer40are represented by arrows. The field lines in the interior of the layer40extend parallel to the X direction.

The layer40has four distinct zones36to39(FIG. 2B) of deformation. In the position shown inFIG. 2B, in the interior of the zones36and38, the layer40lengthens along an axis52parallel to the X direction much more than it shrinks in the Y direction. Simultaneously, in the interior of the zones37and39, the layer40shrinks along an axis53parallel to the Y direction much more than it lengthens in the X direction. By “much more” what is meant is the fact that the amplitude of the deformation in one direction is at least two or five or ten times larger than the amplitude of the deformation in the orthogonal direction. These axes52,53intersect at a point located on the axis9of the shaft8. The shape of the zones36to39especially depends on the shape of the layer40. InFIG. 2B, they are separated from one another by dashed lines. For example, along these dashed lines, the length of the layer40is the same as in the absence of magnetic field. In the case described here, in which the layer40has a disk shape, each zone36to39is a respective angular sector of the disk the apex angle of which is located on the axis9. The value of the apex angle is equal to 90°. More precisely, in the position shown inFIG. 2B, the axis52forms the bisector of the zones36and38and the axis53forms the bisector of the two other zones37and39.

In this embodiment, the converter20also includes for each zone of deformation and on each face of the layer40, an electromechanical transducer. Here, the converter20includes:four electromechanical transducers42to45(FIG. 3) fixed with no degree of freedom to the top face of the layer40; andfour electromechanical transducers fixed with no degree of freedom to the bottom face of the layer40.

InFIG. 2A, only the top transducers42and44and two bottom transducers48and50may be seen. The “bottom” transducers, i.e. the transducers fixed to the bottom face of the layer40, are symmetric with the “top” transducers, i.e. the transducers42to45, relative to a median horizontal plane cutting the layer40in half heightwise. Therefore, in the following, the bottom transducers are not described in detail.

The transducers42to45are arranged relative to one another so that there exists at least one angular position of the converter20, relative to the magnet12, in which the transducers42to45are placed solely above the zones36to39, respectively. One example of such a particular angular position is shown inFIG. 3. To this end, in the position shown inFIG. 3, the transducers42and44are fixed one after the other along the axis52and on either side of the axis53. The transducers43and45are arranged one after the other along the axis53and on either side of the axis52. The vertical plane passing through the axis52forms a plane of symmetry for all the top transducers. Likewise, the vertical plane passing through the axis53forms a plane of symmetry for all the top transducers.

In this embodiment, the transducers42to45are structurally identical and differ from one another solely by their position along the axes52and53. Thus, in the following, only the transducer42will now be described in more detail.

The layer54includes a horizontal interior face fixed directly with no degree of freedom to the top face of the layer40and a horizontal exterior face located on the opposite side. The thickness of this layer54measured along the Z direction is in the following denoted “e”.

For example, the coupling coefficient k of the piezoelectric material of the layer54is higher than 5% or 10%. This coupling coefficient is defined in the ANSI/IEEE standard 176-1987 “ANSI/IEEE standard on piezoelectricity” or in the standards of the family EN50324. Preferably, the g31coefficient of this piezoelectric material is higher than 5×10−3Vm/N or 10×10−3Vm/N and, advantageously, higher than 100×10−3Vm/N or 200×10−3Vm/N at 25° C. For example, the piezoelectric material of the layer54is PZT (lead zirconate titanate) or PMN-PT (lead magnesium niobate-lead titanate).

Here, during its manufacture, each piezoelectric material is polarized by a strong electric field so as to have a polarization perpendicular to its interior and exterior faces. Here the polarization directions of the piezoelectric layers of the transducers42and44are identical. In contrast, the polarization directions of the layers made of piezoelectric material of the transducers43and45are parallel but of opposite sign to that of the layer54.

The layer54is mechanically separated from the layers made of piezoelectric material of the other immediately adjacent transducers43to45by partitions58and60, respectively (FIG. 3).

The partitions58and60are shaped so that the layer54extends essentially over the zone36of the layer40in the position shown inFIG. 3. In the case described here, the layer54extends solely over the zone36. Under these conditions, in the position shown inFIG. 3, the layer40mainly exerts a tensile stress on the transducer42. Here, the partition58extends rectilinearly along a horizontal axis62intersecting the axis9and inclined by 45° to the axis52. The partition60is symmetric with the partition58relative to the vertical plane passing through the axis52. This partition60is therefore not described in more detail.

The partition58is devoid of piezoelectric material. For example, it is formed by a trench filled with an electrically insulating material. Here, a material is considered to be electrically insulating if its resistivity is higher than 102Ω·m and, preferably, higher than 105Ω·m or 1015Ω·m at 25° C.

Here, the width of the partition58measured along a horizontal direction orthogonal to the axis62is constant. For example, this width is smaller than 2e and advantageously smaller than e or than e/2 or than e/10.

Thus, in this embodiment, the layer54occupies more than 80% or 90% of the area of the zone36of the layer40located underneath in the position shown inFIG. 3.

Partitions61,63delimit the layer made of piezoelectric material of the transducer44. These partitions61,63are, respectively, symmetric with the partitions58,60relative to the vertical plane passing through the axis53. These partitions61,63are shaped so that the layer made of piezoelectric material of the transducer44extends solely over the zone38in the position shown inFIG. 3. The partitions58,61thus delimit the piezoelectric material of the transducer43and the partitions60,63thus delimit the piezoelectric material of the transducer45. Thus, the piezoelectric material of the transducers43,45extends solely over the zones37and39, respectively, in the position shown inFIG. 3.

The electrode56is made from an electrically conductive material. Here, a material is considered to be electrically conductive if its resistivity at 25° C. is lower than 10−5Ω·m and advantageously lower than 10−6Ω·m or 10−7Ω·m.

The electrode56is deposited directly on the exterior face of the layer54. Typically, it covers most of this exterior face and, preferably, more than 70% or 80% of this exterior face.

In this embodiment, the electrode56is common to all the transducers42to45. It therefore also covers more than 70% or 80% of the exterior faces of the layers made of piezoelectric material of the transducers43to45in order to also form the electrodes of these transducers. For example, the electrode56is made from a one-piece layer made of an electrically conductive material that covers most of the exterior faces of the transducers42to45. InFIG. 3, the electrode56has not been shown above the partitions58,60,61,63in order to allow these partitions to be better seen.

In the particular case described here, the layer40is also electrically conductive. Under these conditions, the layer40also plays the role of interior electrode for each of the transducers42to45.

The layer54is used in an operating mode in which the excess of electrical charges is created between the horizontal interior and exterior faces of this layer. Specifically, when the converter20is used to generate electricity, it is preferable to maximize the capacitance of each of the transducers42to45. Therefore here the d31mode of the layer54and not its d33mode is used.

During operation of the converter20, since the magnet12is uniaxial, in the first position shown inFIG. 3, the lengthening of the layer40is maximal along the axis52and, simultaneously, the shrinkage of this layer40is maximal along the axis53. In this first position, the zones36and38mainly exert a tensile stress in the X direction on the transducers42and44and the zones37,39mainly exert a compressive stress on the transducers43and45. Therefore, because of the inversion of the sign of the polarization of the layers made of piezoelectric material of the transducers42,44relative to that of the transducers43and45, in the first position, the layers made of piezoelectric material of the transducers42to45all generate charges of the same sign on the electrode56. In this first position a maximum in the potential difference between the terminals22and24is obtained. When the converter20turns by 90° relative to the position shown inFIG. 3, the transducers42and44are entirely located above the zones37and39and the transducers43and45are entirely located above the zones36and38. In this second position, the transducers42to45simultaneously generate a maximum of electrical charges of the same but opposite sign to that corresponding to the position shown inFIG. 3. Thus a minimum in the potential difference between the terminals22and24is obtained.

The structure of the converter20therefore makes it possible to simultaneously exploit the zones of the layer40exerting a tensile stress and the zones of the layer40exerting a compressive stress in order to increase the generated excess of electrical charges. This makes it possible to increase the efficacy of the converter20by at least 20% or 30% relative to a converter which is identical but in which the layers made of piezoelectric material of the transducers42to45all have the same polarization sign.

It will also be noted that since the converter20turns relative to the permanent magnet12, each transducer generates, in alternation, an excess of positive charge then an excess of negative charge as the converter20rotates relative to the magnet12.

FIG. 4shows a converter70capable of being used in place of the converter20in the energy harvester6.

This converter70is identical to the converter20except that the transducers42to45are replaced by transducers72to75, respectively.

As in the preceding embodiment, the bottom transducers may be deduced from the top transducers72to75by symmetry relative to a median horizontal plane located halfway up the layer40.

In this embodiment, the transducers72to75are placed relative to one another so that there exists at least one angular position of this converter relative to the magnet12in which the transducers72to75are placed solely over the zones36to39of the layer40, respectively. One example of such a position is shown inFIG. 4. To this end, the two vertical planes passing through the axes52and54form planes of symmetry for all these transducers72to75. Since the transducers72to75are structurally identical, only the transducer72will now be described in more detail.

The transducer72includes a layer78made of piezoelectric material identical to the layer54except that it extends uniformly right over the top face of the layer40. Therefore, the layer78also forms the layer made of piezoelectric material of the transducers73to75. Thus, in this embodiment, the layers made of piezoelectric material of the various transducers72to75are not mechanically insulated from one another by partitions. In addition, in this embodiment, the signs of the polarizations of the layers made of piezoelectric material of the transducers72to75are all identical.

The transducer72also includes an exterior electrode80deposited directly on the exterior face of the layer78. This electrode80is identical to the electrode56except that it is mechanically separated from the electrodes of the other transducers73to75by separations82to85. These separations82to85are devoid of electrically conductive material. Thus, these separations electrically insulate the electrode80from the electrodes of the other transducers73to75. For example, these separations82and83are formed by trenches filled with an electrically insulating material in gaseous form, such as air, or in solid form.

The separations82and83are shaped so that the electrode80extends solely above the zone36of the layer40in the position shown inFIG. 4. To this end, the separation82extends along the axis62and the separation83is symmetric with the separation82relative to a vertical plane passing through the axis52.

As above, preferably, the width of the separation82measured in a direction perpendicular to the axis62is smaller than 2e and, preferably, smaller than e or than e/2 or than e/10.

The separations84,85are symmetric with the separations82,83relative to the vertical plane passing through the axis53. In this way, in the position shown inFIG. 4, the transducers73and75are solely subjected to a compressive stress when the transducers72,74are solely subjected to a tensile stress and vice versa.

The layer40forms an interior electrode common to the transducers72to75.

In this embodiment, given that the polarization direction is the same for each transducer72to75, when the layer40lengthens along the axis52, a tensile stress is exerted on those sections of the layer78that are located over the zones36and38and, simultaneously, a compressive stress is exerted on those sections of the layer78that are located over the zones37and39. Thus, an excess of electrical charges, for example positive charges, appears on the exterior electrodes of the transducers72and74and, simultaneously, an excess of negative electrical charges appears on the exterior electrodes of the transducers73and75. However, this production of negative charges does not decrease the production of positive charges because the exterior electrodes of the transducers72and74are electrically insulated from the exterior electrodes of the transducers73and75. On the contrary, this excess of positive charges on one side, and negative charges on the other side, is here exploited by electrically and permanently connecting the exterior electrodes of the transducers72and74solely to the terminal22and the exterior electrodes of the transducers73and75solely to the terminal24. This makes it possible to obtain, in the position shown inFIG. 4, a larger potential difference than if the exterior electrodes of the transducers72to75were replaced by a single common exterior electrode. This therefore makes it possible to increase the efficacy of the converter70.

FIG. 5shows a generator90of electricity. This generator90is identical to the generator2except that:the permanent magnet12is replaced by an assembly92of permanent magnets; andthe converter20is replaced by a converter94.

InFIG. 5, for the sake of simplicity, only those elements of the generator90that are different from those of the generator2have been shown.

The assembly92has four horizontal magnetic moments uniformly distributed about the axis9. InFIG. 5, two of these magnetic moments are aligned with the X direction and of opposite sign. The two other magnetic moments are aligned with the Y direction and of opposite sign. For example, the assembly92is obtained by assembling four uniaxial permanent magnets.

The converter94is identical to the converter20except that in the layer40there is a larger number of zones of deformation able to exert mainly either a tensile stress or a compressive stress. There is therefore a larger number of electromechanical transducers on each of the faces of this layer40. These zones of deformation and the location of the transducers on the top face of the layer40will now be described with reference toFIG. 6.

The periphery of the layer40, as seen from above, in the absence of the magnetic field generated by the assembly92, is shown by the dashed line inFIG. 6. Here, it is the shape of a circle centered on the axis9.

The layer40, in the presence of the magnetic field generated by the assembly92when it is in the position shown inFIG. 5, is shown by the solid line inFIG. 6. In this figure, the deformation of the layer40has been exaggerated. It may in fact be smaller than 100 μm or 10 μm.

In the configuration shown inFIG. 6, the layer40lengthens simultaneously along the axes52and53and, at the same time, shrinks along the horizontal axes96and98. These axes96and98are orthogonal to each other and shifted angularly by 45° relative to the axes52and53. There are therefore eight distinct zones of deformation in this embodiment. In the position shown inFIG. 5, the zones that mainly exert a tensile stress on the electromechanical transducers are located along the axes52and53and the zones that exert mainly a compressive stress are located along the axes96and98. InFIG. 6, the eight zones of deformation are delimited by dashed lines. In addition, the zones that exert a tensile stress are identified by the “+” symbol and those that exert a compressive stress are identified by a “−” symbol.

On each of the zones of deformation is formed a transducer according to the teaching given with reference toFIGS. 2A, 2B and 3or according to the teaching given with reference toFIG. 4.

In the case where the teaching ofFIGS. 2A, 2B and 3is applied, the partitions between the piezoelectric layers of the various top transducers are formed, typically, along the dotted lines delimiting the various zones of deformation of the layer40. In the case where the teaching given with reference toFIG. 4is applied, the separations between the exterior electrodes of the various transducers are formed, typically, along the dotted lines delimiting these various zones of deformation.

As the converter94rotates, each electromechanical transducer undergoes in alternation mainly a tensile stress then mainly a compressive stress.

FIG. 7shows an assembly100of permanent magnets capable of being used in place of the assembly92in the generator90. This assembly100is the shape of a horizontal disk centered on the axis9. This disk is divided into eight successive magnetized angular sectors104to111in the counterclockwise direction. The apex angle of each angular sector is located on the axis9. These sectors104to111are uniformly distributed about the axis9. They each have an apex angle of 60° located on the axis9. In the position shown inFIG. 7, the vertical planes passing through the axes52,53,96and98are planes of symmetry of the assembly100. Each magnetized sector104to111comprises at least one uniaxial permanent magnet. InFIG. 7, the magnetic moments are represented by double arrows. Specifically, in this application, the sign of the magnetic moment is of little importance because a magnetostrictive material is sensitive solely to the direction of the magnetic field but not to its sign. In the angular position shown inFIG. 7, the magnetic moments of the permanent magnets of sectors104and108are parallel to the axis52. Similarly, the magnetic moments of the permanent magnets of sectors106and110are parallel to the axis53. The magnetic moments of the permanent magnets of sectors105and109are perpendicular to the axis96. Lastly, the magnetic moments of the permanent magnets of sectors107and111are perpendicular to the axis98. A generator equipped with the assembly100operates as the generator90except that the presence of magnets the magnetic moment of which is perpendicular to the axes96,98increases the contractive stress exerted on the electromechanical transducers located above the zones of the layer40identified by the “−” symbol inFIG. 6. This makes it possible to further increase the efficacy of the converter.

FIG. 8shows an assembly120of permanent magnets, which assembly is identical to the assembly100except that it has only four magnetized angular sectors124to127uniformly distributed about the axis9. In the position shown, the magnetic moments of the sectors124and126are parallel to the axis52and the magnetic moments of the sectors125and127are perpendicular to the axis53. This assembly120functions as the assembly100but is intended to replace the magnet12of the generator2of electricity. Thus, in the position shown in this figure, the presence of the magnets125and127above the zones37and39allows the compressive stress exerted by the layer40on the transducers43and45to be increased. Specifically, as in the preceding embodiment, the field lines generated by the magnets125and127tend to shrink the layer40along the axis53in the zones37and39.

The source132comprises an assembly136of permanent magnets fixed to one another with no degree of freedom and a device138that moves the assembly136translationally parallel to the X direction. Here, the device138moves the assembly136with a reciprocal movement back and forth along a horizontal axis140. Thus, in this embodiment, the variable magnetic field is not a rotating magnetic field.

The assembly136comprises a succession of uniaxial permanent magnets that are aligned one after another along the axis140. In this example, only four uniaxial permanent magnets are shown, numbered142to145from left to right. Each of these permanent magnets has a magnetic moment represented by a double arrow inFIG. 9. These permanent magnets are arranged relative to one another so that, from left to right along the axis140, the directions of the magnetic moments alternate between directions parallel to the X and Y directions. Thus, inFIG. 9, the magnetic moment of the magnets142and144is parallel to the Y direction whereas the magnetic moment of the magnets143and145is parallel to the X direction. Here, the width of the magnets142to145along the axis140is the same whatever the magnet in question.

The harvester134is identical to the harvester6except that the converter20is replaced by a converter150. As in the preceding embodiments, the converter150comprises a horizontal layer152made of magnetostrictive material, above and below which are fixed electromechanical transducers. To simplifyFIG. 9, only the layer152of the converter150has been shown. In addition, inFIG. 9, so that the layer152may be seen, it has been represented to the side of the assembly136. However, in fact, the converter150and the layer152are located in vertical alignment with the assembly136in the position shown in this figure.

Here, the converter150remains stationary in the XYZ coordinate system.

The layer152extends mainly along a horizontal axis154parallel to the axis140. For example, the layer152is rectangular. It is located directly opposite the assembly136of permanent magnets so that the magnetic field lines of each of these permanent magnets passes therethrough essentially horizontally. Under these conditions, each magnet142to145creates a respective zone of deformation in the layer152directly opposite. The zones of deformation created by the magnets142to145are numbered156to159, respectively. In the position shown inFIG. 9, the zones156and158tend to lengthen in the Y direction under the effect of the magnetic field of the magnets142and144. The lengthening in the Y direction corresponds to a shrinkage in the X direction and therefore to a compressive stress, along the axis154, exerted on the electromechanical transducer located in this zone. This compressive stress is represented by the “−” symbol inFIG. 9. At the same time, the zones157and159tend to lengthen in the X direction under the effect of the magnetic field of the magnets143and145. The zones157and159therefore exert a tensile stress, along the axis154, on the electromechanical transducers located in these zones. This tensile stress is represented by the “+” symbol. To simplifyFIG. 9, the separations between the zones are represented by dashed lines parallel to the Y direction. However, in fact, the shape of these lines may be more complex.

Distinct electromechanical transducers are fixed with no degree of freedom to each of the zones156to159in order to convert both the simultaneous lengthening and shrinkage of the layer152in the X direction into a generation of excess of electrical charges. These electromechanical transducers are produced as described in the preceding embodiments and are not described in more detail here.

When the assembly136is moved by a step equal to the width of the magnets142to145to the right or to the left, along the axis140, the mechanical stresses exerted on the electromechanical transducers placed over the zones156to159change sign. Thus, after a movement of one step, the electromechanical transducers that mainly underwent a tensile stress mainly undergo a compressive stress and vice versa. The operating mode of the generator130may therefore be deduced from that described in the case of the assembly100of permanent magnets.

FIG. 10shows a generator170of electricity. The generator170is identical to the generator2except that:the source4of energy to be harvested is replaced by a source172of energy to be harvested; andthe energy harvester6is replaced by an energy harvester174.

The source172is here a heat source the temperature of which varies over time. This heat source heats substantially uniformly all of the converter of the harvester174.

The harvester174is identical to the harvester6except that the converter20is replaced by a converter176. The converter176is identical to the converter20except that the layer40is replaced by a layer178made of shape-memory material. To simplifyFIG. 10, only the converter176has been shown. Here, it is a question of a shape-memory material that lengthens, along its preferential axis of deformation, by at least more than 0.5% or 1% in response to a temperature variation of 10° or 20° C. The preferential axis of deformation of the layer178is the axis along which the amplitude of its deformation is maximal. This layer178is then configured so that in response to an increase in the exterior temperature, it contracts along an axis such as the axis52and, simultaneously, lengthens along another axis such as the axis53. To achieve this, for example, the preferential axis of deformation of the layer178is aligned with the axis52in the first position. For more information on the shape-memory materials usable in such a converter, the reader may refer to articles A2 and A3.

The operating mode of the generator170may be deduced from that of the generator2.

FIG. 11shows an assembly190of permanent magnets, which assembly is usable in place of the assembly92inFIG. 5. In this embodiment, the assembly190is the shape of a horizontal ring centered on the axis9. The assembly190includes four permanent magnets192to195uniformly distributed about the axis9. Each magnet192to195occupies a respective angular sector of the ring. The angular sectors occupied by the magnets192to195are the same as the sectors occupied by the magnets104,106,108and110inFIG. 7. The angular sectors between these magnets192to195are for example devoid of permanent magnets. These angular sectors devoid of permanent magnets are hatched inFIG. 11. Preferably, when the assembly190is used, the layer40is also in the shape of a ring with the same inside and outside diameters as the assembly190.

FIG. 12shows a converter200capable of being used in place of the converter20. The converter200results from the assembly, on top of one another, in the vertical direction, of a plurality of specimens of the converter20. To simplifyFIG. 12, only two specimens202and204are shown. More precisely, these specimens are stacked on top of one another so that the transducers42of each specimen are located on top of one another in the vertical direction. Under these conditions, each transducer42is fixed above and below a respective zone39in the position shown inFIG. 2B. Preferably, the exterior electrode56of the specimen202also plays the role of the electrode for the layer48of the specimen204.

During use of the converter200, all the electrodes56and their symmetric (relative to a median plane of the layer40) equivalents generate at the same time electrical charges of the same sign. Thus, preferably, all these electrodes are electrically connected directly to the same terminal22. Likewise, all of the layers40are electrically connected directly to the terminal24. The converter200then has a better efficacy relative to the converter20.

Up to now, only the d31mode of the layers made of piezoelectric material has been used.FIG. 13shows a converter220capable of being used in place of the converter20but in which it is the d33mode of the layers made of piezoelectric material that is used. The converter220is identical to the converter20except that the electromechanical transducers are replaced by other electromechanical transducers of the same shape and arranged relative to one another as described with reference toFIGS. 2A, 2B and 3. Thus, here, only one transducer222replacing the transducer42is described in more detail.

The transducer222is produced as described with reference toFIG. 3of article A1cited above. In other words, the transducer222includes a layer224made of piezoelectric material polarized parallelly to the X direction in the position shown inFIG. 13. This layer224is for example formed by juxtaposing, in a horizontal plane, parallelly to one another fibers made of PZT extending in the X direction and polarized in this direction. Each electrode226,228of the transducer222is formed from one or more combs. Each comb is composed of a plurality of fingers made of electrically conductive material, each of which extends parallelly to the Y direction over one face of the layer224. Each comb of the electrode226is interlaced or “interdigitated” with a corresponding comb of the electrode228in order to form what is known as an interdigitated comb configuration. Here, the electrode226includes two combs230,232placed on the exterior and interior faces of the layer224, respectively. Likewise, the electrode228includes two combs234,236placed on the exterior and interior faces of the layer224, respectively. The comb230is interlaced with the comb234and the comb232is interlaced with the comb236.

The other transducers placed over the zones37to39may be deduced from the transducer222by a rotation about the axis9of 90°, 180° and 270°, respectively. Thus, in this embodiment, the directions of polarization of these transducers are aligned along the axes52and53in the first position. In addition, the electrodes of the transducers aligned with the axis52in the first position are electrically insulated from the electrodes of the transducers aligned with the axis53in the same position.

The operating mode of the generator2equipped with the converter220may be deduced from what was explained above when the generator is equipped with the converter20.

Many other embodiments are possible. For example, the electromechanical transducers are formed only on one portion of the zones of deformation of the layer40or152. By way of illustration, it is possible therefore to form an electromechanical transducer solely on the half of the zones of deformation marked by the “+” symbol inFIG. 6and on the half of the zones of deformation marked by the “−” symbol in the sameFIG. 6.

Piezoelectric materials other than PZT or PMN-PT may be used to produce the electromechanical transducers. In practice, by piezoelectric material what is meant is any material capable of converting a mechanical stress into an excess of electrical charges on one of its horizontal faces. For example, the material PZT may be replaced by PVDF (polyvinylidene fluoride). Preferably, the thickness “e” of the layer54when made of PVDF is smaller than 300 μm and advantageously smaller than 30 μm or 40 μm. Generally, the thickness “e” is larger than 10 μm. Specifically, the choice of PVDF as piezoelectric material with a thickness smaller than 300 μm, in the arrangement described here of the converter20, allows the efficacy of the converter to be increased. The material PZT may also be replaced by a piezoelectric foam such as one of those described in the following article:

Some of the transducers may be fixed to the top face of the layer40, others of the transducers being fixed to the bottom face. For example, only the transducers72,74are fixed to the top face of the layer40and only the symmetric equivalents of the transducers73and75are fixed to the bottom face of the layer40. This makes it possible to have on each face of the layer40solely electromechanical transducers that generate charges of the same sign.

In another variant, the electromechanical transducers are fixed only to a single side of the layer40.

Preferably, the layer made of piezoelectric material extends over more than 80% or 90% of the zone of deformation in the first position. In the first position, it may also extend a little beyond this zone of deformation. However, in any case, most of the layer made of piezoelectric material is fixed to this zone of deformation in the first position.

The embodiments ofFIGS. 2A and 4may be combined. In this case, partitions mechanically insulate the piezoelectric layers and separations mechanically insulate the electrodes of each transducer. In the latter case, the piezoelectric layers of the transducers43and45may be polarized in the same direction as the layers made of piezoelectric material of the transducers42and44. In the latter embodiment, the electrical connection to the terminals22and24is achieved as described for the embodiment inFIG. 4.

In the embodiment inFIGS. 1 to 3, the partitions58,60,61and63may be omitted. Specifically it is possible to polarize various sectors of a given layer made of piezoelectric material in opposite directions without however needing to use partitions electrically and mechanically separating these sectors.

The electrodes of transducers that systematically and simultaneously generate electrical charges of the same sign do not need to be mechanically insulated from one another by separations. For example, the electrode80may also extend above the layer made of piezoelectric material of the transducer74in order to form in addition the electrode of the latter transducer. This does not cause problems because the transducers72and74are aligned along the same axis of deformation and have the same polarization direction so that they generate charges of the same sign in response to given deformations of the layer40.

As a variant, an interior electrode made of electrically conductive material is interposed between the layers40and54. This proves to be necessary in any embodiment in which the layer40or178is produced from a material that is not electrically conductive.

The exterior electrodes are not necessarily fixed directly to the layer made of piezoelectric material. For example, they may be separated from this layer by a thin intermediate layer. Typically, the thickness of this thin intermediate layer is smaller than 1/100 or than 1/1000 of the thickness of the piezoelectric layer. They may also be able to move slightly relative to the layer made of piezoelectric material.

As a variant, all the electrodes are placed on the same face of the layer made of piezoelectric material. For example, the combs232,236are omitted and, alternatively, the combs230,234are omitted. Also as a variant, it is possible to preserve only the combs232and234for example.

The layer40or178is not necessarily formed from a single block of material. As a variant, the layer40or178is formed by juxtaposing small blocks of magnetostrictive or shape-memory material assembled with no degree of freedom to one another to form a layer that behaves mechanically as the layer made of a single block of material.

The magnetostrictive material may also be replaced by a magnetic shape-memory alloy such as NiMnGa, i.e. a material that works as described for the preceding shape-memory materials except that the deformation is triggered by a variation in magnetic field and not by a temperature variation.

The shape of the transducer capable of converting the variation in the energy to be harvested into a variation in mechanical stress may be different from a disk. For example, in another advantageous embodiment, the layer40has the shape of an ellipse or a ring. In other variants, the layer40may be a square, a rectangle or a polygon having more than four corners.

The converter20may be mounted at the end of the shaft without it being transpierced at its center by the shaft8.

The rotation of the magnetic field relative to the converter may also be obtained by making the assembly of magnets turn in the XYZ coordinate system rather than the converter. This may simplify connection.

What was described for the case of a rotary movement of the converter relative to a permanent magnet also applies to the case where the variable magnetic field is generated by the translation of a permanent magnet. The variable magnetic field may also be generated by a coil through which a current of variable magnitude flows.

Other directions of the magnetic moments of the assemblies of permanent magnets are possible. For example, in the assembly136the direction of the magnetic moments of the magnets142and144may be inclined, in the horizontal plane, by +45° to the axis140whereas the direction of the magnetic moments of the magnets143and145may be inclined by −45°, in the horizontal plane, to the same axis140.

The assembly of permanent magnets may include many more permanent magnets than were shown. For example, this number of permanent magnets may be higher than ten or forty, especially in the embodiments inFIGS. 9 and 11.

Depending on the nature of the energy to be harvested, transducers other than a layer made of magnetostrictive material may be used. For example, if the energy to be harvested is a temperature variation, the layer made of magnetostrictive material is replaced by a layer made of shape-memory material. In this case, in order to make the temperature of the layer made of shape-memory material vary, it is not necessary for the heat source to move. It may be stationary relative to the layer made of shape-memory material and only its temperature varies. In the case where the layer152made of magnetostrictive material is replaced by a layer made of shape-memory material the preferential axis of deformation of which is coincident with the axis154, to obtain the same harvester operating mode, the assembly136of permanent magnets is replaced by an anisotropic heat source. The anisotropic heat source is for example obtained by replacing each magnet of the assembly136with a thin tube or filament aligned with the magnetic moment of the magnet that it replaces. Thus, in the position shown inFIG. 9, the tube or the filament facing the zone156is perpendicular to the axis154so that this zone lengthens little in the X direction. In contrast, the tube or filament facing the zone157is parallel to the axis154so that this zone lengthens much more in the X direction.

The apparatus10may be a mechanical bearing. In this case, the axis8is the axis of this bearing and the mounting14is the mounting of the bearing.