The present invention relates to a device for converting mechanical vibrations into electrical energy operating over an extended range of vibration frequencies.
It is more and more attempted to recover lost mechanical energy, especially vibrations, to produce electricity. For example, they can be vibrations of an airplane or car engine when it operates or vibrations generated during a movement.
For this, it is known to use devices including a piezoelectric material which, when it is deformed for example under the effect of vibrations, generates electricity.
Such a device can include a vibrating structure comprising a beam embedded at a longitudinal end into a support and a mass fastened to the other longitudinal end, two layers of piezoelectric material on both faces of the beam and electrodes to collect electric charges generated. When the environment undergoes vibrations, the mass oscillates and the layers of piezoelectric material are deformed, generating electric charges which are collected.
Such a vibrating structure has a resonant frequency defined by its mechanical characteristics. This structure has the drawback of being frequency-selective, i.e. it ensures conversion of mechanical vibrations at frequencies close to the resonant frequency of the structure. It is therefore not adapted to an application to systems vibrating over an extended frequency range.
One purpose of the present invention is therefore to offer a device for converting mechanical energy into electrical energy the operation of which over a wide frequency range is improved.
The purpose set forth above is achieved by a device for converting mechanical vibrations into electricity including a support, a suspended structure including at least one beam embedded into the support by an end, and a mass fastened to the other end of the beam, a piezoelectric material on at least one of the faces of the beam so as to undergo a flexure upon deforming the beam, and at least one transverse element extending substantially transversely relative to the direction of the beam over at least half the width of the piezoelectric material.
The at least one transverse element therefore limits the transverse deformation of the structure while enabling a flexural deformation of the structure, electromechanical coupling of the structure is then increased, which raises the capacity to adapt the resonant frequency of the structure to adapt to modifications of the vibration frequency of the environment. Thus, a frequency adjustable device is made, enabling the frequency range of recoverable vibrations to be extended.
By virtue of the invention, by means of a control circuit, it is possible to control the resonant frequency of the structure so that it is close to the vibration frequency of the support, which enables energy recovery to be optimised.
Preferably, the at least one transverse element is rigid and has a reduced dimension in the longitudinal direction of the structure, in order to limit at best transverse deformation of the structure while reducing impact on flexural deformation of the structure.
For example, the device includes transverse elements on either side of the neutral axis of the beam.
Advantageously, the transverse elements are distributed along the structure.
The transverse elements preferably have a high rigidity. Moreover, they preferably have a low dimension in the length direction of the structure to limit their effect on longitudinal deformation.
One subject-matter of the present application is thereby a device for converting mechanical energy into electrical energy including a support, a structure extending along a longitudinal direction, said structure being suspended to the support through embedding by a first longitudinal end and including a mass fastened to a second longitudinal end, at least one layer of piezoelectric material at least partly extending between the first longitudinal end and second longitudinal end of the structure and disposed so that, when the mass moves in a direction orthogonal to the longitudinal direction, the layer is flexurally deformed, electrodes on either side of the layer of piezoelectric material. The structure includes at least one transverse element integral with the layer of piezoelectric material extending transversely relative to the longitudinal direction over a length at least equal to half the transverse dimension of the layer of piezoelectric material.
Embedding the structure into the support, i.e. the embedding connection between the structure and the support, can be achieved by manufacturing as a single piece the support and the structure or by intermediate means connecting the structure to the support through embedding.
The present invention will be better understood based on the following description and accompanying drawings in which:
In
The structure S1 extends along the longitudinal direction X. It includes a longitudinal end 4.1 embedded into the support and a mass M fastened to its other longitudinal end 4.2. The structure S1 is to vibrate along the direction Z orthogonal to the direction X. In this example, the mass M extends on either side of the neutral axis of the structure. As a variant, it extends above or under the same.
The structure includes a beam 6 which is embedded into the support and which carries the mass M. The beam has a low thickness e, width L and length l. Preferably, l/e>5.
The beam 6 then includes two opposite faces 8, 10 orthogonal to the direction Z.
The structure also includes layers of piezoelectric material 12, 14 fastened to both opposite faces 8, 10 of the beam 6.
Electrodes E1, E2, represented in
Preferably, the transverse elements are located as close as possible to the piezoelectric layers, advantageously in contact with the same, or directly in contact with the electrodes. In
For example, the beam can be made of a metal material or alloy, such as steel, brass, aluminium, silicon, of a polymeric material such as epoxy.
The mass can be made of a metal material or alloy, such as steel, brass, aluminium, tungsten or silicon,
The piezoelectric layer is for example made of PZT (Lead zirconate titanium), PMN-PT (Lead-Magnesium-Niobate-Lead-Titanium), PZN-PT (Lead-Zinc-Niobate-Lead-Titanium), AlN, PVDF (polyvinylidene fluoride).
The electrodes are for example of silver, gold or copper.
In the example represented, the mass is directly fastened to the beam and the piezoelectric layers only cover the free zones of the faces 8 and 10.
Preferably, the layers 12 and 14 entirely cover the free zones of the faces 8 and 10, maximising the amount of piezoelectric material and therefore the amount of electric charges which can be generated. A device in which the piezoelectric layer(s) does/do not entirely cover the free zone(s) of the face(s) of the beam does not depart from the scope of the present invention.
Thus, when vibrations are applied to the support, the suspended mass moves along the direction Z orthogonal to the direction X, flexurally deforming the beam and the layers 12, 14 generating electric charges.
In the example represented, the structure includes transverse elements 16 fastened to the electrodes located on the outermost side of the stack. Dimensions and/or rigidity of the transverse elements are selected to limit transverse deformation of the structure, especially of the piezoelectric material.
The transverse elements 16, 18 extend over at least half the width of the layers 12, 14. In the example represented and preferably, without being limited thereto, the layers 12, 14 have the same width as the beam. A structure in which the piezoelectric layer(s) are wider or less wide than the beam does not depart from the scope of the present invention.
Preferably, the transverse elements have a length equal to the width of the layers 12, 14.
The transverse elements 12, 14 are preferably parallel to each other and orthogonal to the direction X reducing their effect on flexural deformation of the structure.
The material of the transverse elements is a material having some rigidity. By way of example, the transverse elements are made of cobalt or manganese for the use of deposition techniques. Steel or brass enables several millimetre long bars to be made in a simplified manner.
The transverse elements have a dimension in the direction Z, referred to as the height h (
The width La (
Preferably, materials of the beam and transverse elements are selected so that:
with E bar: Young's modulus of the transverse elements,
E piezo: Young's modulus of the piezoelectric layer,
Lp: beam length,
Ltot: sum of the lengths of the transverse elements.
Moreover, heights of the transverse elements and thickness of the piezoelectric layer can be advantageously selected so that:
with hbar=thickness of the transverse elements hpiezo: thickness of the piezoelectric layer.
The thickness of the beam can also be advantageously selected so that:
with
Emoy=(thickness of the piezoelectric layer×Epiezo+beam thickness×Ebeam)/Eptotale
Ebeam being the Young's modulus of the beam,
Eptotale being the total thickness of the beam which is equal to the sum: thickness of the piezoelectric layer+thickness of the beam.
The ratio h/La is at least equal to 10−2, preferably at least equal to 2·10−1 and more preferably greater than 1.
The transverse element has a length Lo.
Furthermore, the number of transverse elements and their distribution along the direction X also enable their effect in limiting the transverse deformation of the structure to be adjusted.
In the example represented, the transverse elements 16, 18 on the layers 12, 14 on either side of the beam are distributed in staggered rows. This distribution is particularly advantageous when the number of transverse elements is reduced. Indeed, the transverse elements 16 thus distributed can have an effect on the layer 14 even if the same is reduced relative to the effect they have on the layer 12. Limiting the transverse deformation is then distributed at best along the length of the beam.
In this example, the transverse elements are evenly distributed over the whole length of the layers 12, 14.
Preferably, the distance between two parallel faces facing two adjacent transverse elements is greater than La, the width of the transverse elements.
The transverse elements can have dimensions different from each other, for example depending on their disposition relative to the embedding zone.
Implementing layers of piezoelectric material on either side of the neutral axis of the beam enables energy recovery to be optimised. It will be understood that a structure including a single layer of piezoelectric material does not depart from the scope of the present invention.
The vibrating structure has a resonant frequency Fr1, which is set by dimensions of the different elements of the structure, their dimensions and mechanical properties. It is the frequency at which the structure has the most recovered energy.
Preferably, the device includes a circuit for controlling CC the resonant frequency of the structure enabling the mechanical resonant frequency of the structure to be adjusted.
For example, the control circuit includes an adjustable impedance electric charge. Depending on the desired mechanical resonant frequency, the control circuit CC matches the impedance connected across the electrodes. Indeed, by modifying the electric conditions of the piezoelectric material, the latter stiffens or softens; which modifies the resonant frequency of the structure. Advantageously, the control circuit sets the mechanical resonant frequency of the structure such that it is close or equal to the vibration frequency of the system.
Preferably, the mechanical resonant frequency is selected so that:
|Resonant frequency−Vibration Frequency|/(Vibration Frequency)<5%.
For example, an automated frequency tracking system can be implemented to manage the mechanical resonant frequency. This capacity to adjust the resonant frequency is all the greater that the electromechanical coupling coefficient is great. Thus, by virtue of the invention, the device can be adapted to environments vibrating at various frequencies while keeping a high recovered power.
By way of illustration of the effect of the invention, longitudinal and transverse deformations of different structures and coupling gain have been estimated by finite element simulations, for example using the COMSOL® software.
The structure considered is that of
The layers 12 and 14 are of a piezoelectric material: [001]-oriented PMN-PT. Since the electrodes are very thin, for example of a thickness of at least 10 times lower than the thickness of the piezoelectric material, their effect is insignificant.
The piezoelectric layers 12, 14 have a length of 45 mm, a width of 10 mm and a thickness of 0.5 mm.
The steel mass has a length of 45 mm, a width of 10 mm and a thickness of 5 mm.
The steel beam has a length of 45 mm, a width of 10 mm and a thickness of 0.5 mm.
14 steel transverse elements having length l=10 mm, height h=1 mm and width L=1 mm, distributed on either side of the neutral axis of the beam.
In
By way of comparison, in
It is noticed that the structure according to the invention has a substantially reduced transverse deformation relative to that of the structure of the state of the art while having a substantially identical longitudinal deformation. Furthermore, the electromechanical coupling coefficient is multiplied by 1.75 by virtue of the invention, whereas the closed loop resonant frequency is not much modified.
In the example represented, several transverse elements are implemented. However, a structure with one transverse element on each layer 12, 14 has an electromechanical coupling coefficient multiplied by 1.25 relative to that of a structure of the state of the art.
For example, the transverse elements are joined to the structure. For example, they are adhered to the structure.
Alternatively, the transverse elements are made according to techniques of microelectronics, i.e. by layer deposition and structuring, for example by etching.
The operation of the recovery device D1 will be described.
When the system to which the support of the device is fastened undergoes vibrations, the suspended mass M moves along the direction Z, flexurally deforming the beam and piezoelectric layers which is vibrated. This deformation causes electric charges which are collected and transmitted to the electric circuit C to be generated. If vibrations undergone by the system are close to the resonant frequency Fr1 of the structure, recovery is optimum. Generating electric charges is all the greater that the longitudinal deformation of the structure is great.
Preferably, the control circuit CC adjusts the mechanical resonant frequency of the structure beforehand so that it is close to the frequency(ies) of vibrations to be recovered.
In
In
In
For example, the transverse elements are made by structuring a substrate by techniques of microelectronics. The piezoelectric layers and electrodes are made by deposition.
Alternatively, the transverse elements are joined one by one to the central plate 107, which makes it possible to use a more rigid material to manufacture the transverse elements relative to the material of the central plate which is to be flexurally deformed.
In
According to one alternative of
In the above-described examples, the transverse elements have a rectangular or square cross-section. Other shapes are also contemplatable. They can have a trapezoidal cross-section, for example oriented so that the small base is on the side of the beam.
The beam 6 can have any shape mainly extending in the longitudinal direction, for example a corrugated or zigzag shape.
In
In
In
In a non-shown variant, elements 516, 518, 520, 522 are distributed in staggered rows with respect to each other.
The number of piezoelectric layers can be higher than 2 and the number of transverse elements sets ca be higher than 4.
The device includes at least n layer of piezoelectric material(s), n being at least equal to 1 and at least one transverse element integral with at least one of the layers. Indeed a device with three layers of piezoelectric material and transverse elements associated with one layer falls within the scope of the present invention.
By virtue of the invention, a device which can be controlled to enable an optimised energy recovery over a wide frequency range is made.
The device is relatively easily made, especially by microelectronic methods.
A system including several recovery devices comprising structures with different resonant frequencies can be contemplated, which enables a still wider frequency range to be covered.
Number | Date | Country | Kind |
---|---|---|---|
19 10240 | Sep 2019 | FR | national |