Electromagnetic transducer for vibrational energy harvesting

Abstract

The present invention relates to an electromagnetic transducer for harvesting vibrational energy. In particular, the invention relates to an electromagnetic transducer comprising a support, a central mass, and at least one spring connecting the central mass to the support, the spring allowing the central mass to move relative to the support along a first axis. The present invention also relates to an assembly of electromagnetic transducers. Figure for the abstract: Fig. 2

Description

Title of the invention: Electromagnetic transducer for the recovery of vibrational energy Field of invention

[0001] The present invention relates to an electromagnetic transducer for the recovery of vibrational energy. In particular, the invention relates to an electromagnetic transducer comprising a support, a central mass, and at least one spring connecting the central mass to the support, the spring allowing the central mass to move relative to the support along a first axis. The present invention also relates to a set of electromagnetic transducers. Prior art

[0002] Vibration energy harvesters are used to electrically power electronic systems from the vibrations present in their environment. They require less maintenance and generate less pollution than batteries and are particularly interesting when the system to be powered is located in an inaccessible place, deprived of light and sufficient thermal gradient. However, the industrialization and marketing of these vibration energy harvesters is blocked by their still insufficient robustness. Indeed, although many harvesters are theoretically capable of providing the power required by their application (power supply of very low consumption electronic systems), they are often incapable of adapting to the frequency fluctuations of real vibration sources.Furthermore, these vibration energy harvesters are used in environments subject to temperature changes, whereas these harvesters are unsuitable for such variations. Also, vibration energy harvesters have difficulty resisting the aging of their constituent elements. To correct these problems without significantly reducing their power density (as is often the case with strategies based on mechanical adjustment, multimode systems, or even non-resonant systems), one of the most promising avenues is electrical adjustment. This method consists of maximizing the power transfer between the vibration source and the electrical load, not only at resonance (when the vibration frequency coincides with the natural resonant frequency of the mechanical resonator) but also off-resonance, by means of an impedance matching circuit located at the electrical load.This results in an increase in the effective bandwidth of the collector, making it more robust under real-life usage conditions.

[0003] The impact of electrical adjustment on bandwidth depends firstly on the type of transducer performing the energy conversion, the type of transducer determining the physical equations governing the behavior of the recuperator. In particular, there are piezoelectric transducers and electromagnetic transducers. Secondly, the electrical adjustment on the bandwidth depends on the characteristic parameters of the collector such as the mobile mass related to the total volume (effective density of the collector), the mechanical quality factor, the mechanical resonance frequency, and the coupling. We speak of piezoelectric coupling in the case of a piezoelectric transducer, and electromagnetic coupling in the case of an electromagnetic transducer.

[0004] In the case of piezoelectric transducers, excellent performances have already been achieved (-3dB bandwidth greater than 10%) by means of strongly coupled recuperators associated with very efficient extraction circuits. In the case of electromagnetic transducers, such performances are more difficult to achieve because of their generally too weak electromagnetic coupling. Under conditions of small displacement of the moving mass around an operating point, the derivative of the magnetic flux in the winding with respect to the displacement of the mobile is called the electromagnetic transduction coefficient, and the coupling of an electromagnetic recuperator can be defined as the square of this coefficient divided by the internal resistance of the coil and by the total volume of the recuperator.In other words, the performance of electromagnetic harvesters is limited by the fact that, for a given displacement of the mobile, the variation in magnetic flux is often too small for the electrical adjustment method to significantly increase the effective bandwidth of the harvester. The performance of the electrical adjustment strategy is therefore better for piezoelectric harvesters.

[0005] However, electromagnetic recuperators have decisive advantages in other areas such as the technological maturity of the manufacturing processes, cost, mechanical resistance to shocks and long-term use, etc. This is why the development of electromagnetic type recuperators remains relevant. Brief description of the invention

[0006] The invention aims to overcome all or part of the problems mentioned above by proposing an electromagnetic transducer allowing strong coupling, significantly higher than known electromagnetic recuperators, as well as a high effective density. Thus, the invention aims to achieve better performance in terms of bandwidth and power density with the electrical adjustment method.

[0007] The electrical adjustment method is based on the theorem of maximizing the power transfer between any source and load. According to this theorem, the transferred power is maximum when the impedance of the load is equal to the complex conjugate of the source impedance. [Fig.l] represents a generic model 10 describing the behavior of electromagnetic transduction harvesters 11 under the assumption of small displacements, associated with an impedance matching circuit 12. The harvester, delivering the voltage v and the current i, is assimilated to the source while the matching circuit constitutes the load. By adequately controlling the values ​​of capacitor Cioad and resistor Rioad according to the input frequency, it is possible to place oneself in the conditions of the theorem at the natural resonance of the harvester but also off resonance.

[0008] Some known devices are based on the relative displacement of a magnetic field source with respect to a winding. The displacement, e.g. translation or rotation, of a magnetic field created by a magnet causes a variation of the magnetic flux in a winding. To obtain the strongest possible derivative of the magnetic flux with this type of device, it is necessary to use a ferromagnetic guide integral with the magnet in order to channel the lines of the magnetic field at the point through which the coil passes, while reducing the air gap of the magnetic circuit to its strict minimum. This minimum air gap is the thickness of the coil (simply because if the air gap were less thick than the thickness of the coil, the latter could no longer pass through the assembly consisting of the ferromagnetic guide and the magnet). The coil, for its part, must be adequately sized to maximize the derivative of the magnetic flux within it.In particular, a coil that is too thin leads to a magnetic flux derivative that is too low even with the ferromagnetic guide. Therefore, a compromise must be found between the thickness of the air gap and that of the coil, and this compromise leads to an electromagnetic coupling significantly lower than the electromagnetic coupling achievable with transducers based on a variation of the magnetic field (transducers for which the thickness of the air gap is independent of the thickness of the coil). On the other hand, collectors based on the relative displacement of a magnetic source with respect to a winding have the advantage of not generating a magnetic force between the mobile and the base of the collector, this force being the source of modeling difficulties (complex analytical calculation, non-linearity), design and manufacturing.In particular, there are possibilities of sticking during the assembly or use phase, or the generation of dry friction, etc.

[0009] Some recuperators are transducers based on a variation in the shape or intensity of the magnetic field created by the magnet. These are transducers for which the variation in magnetic flux in the winding is caused by a modification of the magnetic field created by the magnet by means of ferromagnetic elements moving relative to it. The movement of a mobile causes a variation in the thickness of the air gaps, i.e. the distance separating the mobile from the rest of the ferromagnetic guide containing the magnets. This variation in the air gap causes a variation in the reluctance of the magnetic circuit powered by the magnets, so that the flux captured by the coil is maximum when the mobile is in a position such that the air gap is small, and minimum in a position such that the air gap is large. However, such devices require special attention regarding the magnetic force because it can cause sticking or dry friction if no provision is made during the design of the recuperator.

[0010] The invention aims to overcome the problems mentioned above by proposing an electromagnetic transducer significantly less subject to sticking or dry friction and which is particularly compact.

[0011] For this purpose, the invention relates to an electromagnetic transducer comprising a support, a central mass, and at least one spring connecting the central mass to the support, the spring allowing the central mass to move relative to the support along a first axis; the central mass comprising a central ferromagnetic element, a first magnet, a second magnet, and two additional ferromagnetic elements, the ferromagnetic element being flanked on a first side along the first axis by the first magnet and flanked on a second side, opposite the first side along the first axis, by the second magnet, the first magnet and the second magnet each being flanked along the first axis by one of the additional ferromagnetic elements;the support surrounding the central mass radially to the first axis and an air gap separating the support from the central mass, the support comprising at least one coil wound around the first axis and secured to an external ferromagnetic element; the electromagnetic transducer being configured so that the magnetic flux of the first magnet and the magnetic flux of the second magnet each follow a path among a first path and a second path, the first path not passing through the coil, the second path passing through the coil and continuing around the coil via the external ferromagnetic element: the movement of the central mass along the first axis causing the modification of the path of at least one of the magnetic fluxes from the first path to the second path or vice versa.;

[0012] By "the spring allowing the displacement of the central mass relative to the support along a first axis", it is understood that the spring is capable of deforming in response to a vibration, its deformation causing a displacement of the central mass along a first axis.

[0013] By "flanked on a first side along the first axis and flanked on a second side, opposite the first side along the first axis, by a second magnet, the first magnet and the second magnet each being flanked along the first axis by an additional ferromagnetic element", it is understood that the central ferromagnetic element, the first magnet, the second magnet and the additional ferromagnetic elements are stacked axially.

[0014] By "the support surrounding the central mass radially to the first axis and an air gap separating the support from the central mass", it is understood that the support is arranged around and at a distance from the central mass. This distance is sufficiently large so that vibrations which would cause the movement of the central mass do not cause friction between the central mass and the support. This air gap or this distance depends essentially on the difference in stiffness between the radial magnetic force tending to move the mass away from its translation axis, and the radial mechanical force linked to the springs tending to compensate for the magnetic stiffness and bring the mass back onto its translation axis. If this difference is positive then it is necessary either to increase the radial stiffness of the springs if this is possible or to increase the air gap in question in order to reduce the magnetic stiffness.The air gap is a function of the dimensions of the central mass and must be assessed according to the technical tolerance margin of its manufacturing method and according to a thermal expansion tolerance depending on the environment of use.

[0015] By "a coil wound around the axis" it is understood that the coil is arranged radially to the first axis.

[0016] By "the magnetic flux of the first magnet and the magnetic flux of the second magnet each follow a path", it is understood that the spatial distribution of the magnetic field and in particular the majority of the lines of the magnetic field are guided along a specific path.

[0017] In "the first path not passing through the coil" and "the second path passing through the coil and continuing around the coil", "passing through the coil" means that the magnetic field lines are guided so as to pass through the radial plane in which the coil is arranged relative to the first axis. "Around the coil" means that the magnetic field lines are guided to form a loop going from one pole to the other of the same magnet, the loop winding around the coil.

[0018] The electromagnetic transducer of the present invention therefore combines (i) the relative displacement of a magnetic source, here the central mass comprising a first magnet and a second magnet, with respect to a winding, here the support comprising the wound coil and (ii) a variation of the magnetic field created by the magnets of the central mass according to the ferromagnetic elements which are in motion (ferromagnetic elements included in the central mass) or not (ferromagnetic elements included in the support). In addition, the electromagnetic transducer of the present invention comprises a minimized air gap, the air gap being only large enough to separate the support from the central mass and allow the movement of the central mass relative to the support. These Advantages allow a considerable improvement in the effective density of the electromagnetic transducer as well as the mechanical quality factor and coupling. Also, ferromagnetic elements allow to increase the derivative of the magnetic flux with respect to the displacement of the central mass as well as the transduction coefficient.

[0019] According to the present invention, the coil is located on the support which, during use of the electromagnetic transducer, is fixed. Thus, the coil does not undergo the movement of the central mass and the risk of breakage by repeated movement is reduced. Also, the electromagnetic transducer of the present invention is adapted so that the central mass occupies a maximum volume in the electromagnetic transducer, which makes it possible to maximize the effective density and the electromagnetic coupling.

[0020] Additionally, the electromagnetic transducer of the present invention can be made using magnets of standard shape and magnetization, reducing manufacturing or assembly costs as well as potential maintenance costs.

[0021] Furthermore, the electromagnetic transducer allows a displacement of the central mass which is limited only along the first axis and by the spring, no other element included in the electromagnetic transducer limiting the travel of the central mass. Thus, the dimensioning of the electromagnetic transducer of the present invention is simple and independent of the displacement of the central mass.

[0022] Advantageously, the spring of the electromagnetic transducer of the invention comprises a first spring fixed on an external face of one of the additional ferromagnetic elements and a second spring fixed on an external face of the other additional ferromagnetic element.

[0023] Advantageously, the spring of the electromagnetic transducer of the invention comprises at least one flat spring extending mainly in a plane perpendicular to the first axis, preferably being a three-branch spiral spring.

[0024] Advantageously, the coil of the electromagnetic transducer of the invention is entirely embedded in the external ferromagnetic element.

[0025] Advantageously, the central mass of the electromagnetic transducer of the invention is flanked on at least one side along the first axis by a third magnet and an additional ferromagnetic element.

[0026] Advantageously, the support of the electromagnetic transducer of the invention comprises several coils.

[0027] Advantageously, the axes of the poles of the first and second magnets of the electromagnetic transducer of the invention are reversed along the first axis.

[0028] Advantageously, the central mass of the electromagnetic transducer of the invention is cylindrical.

[0029] Advantageously, the electromagnetic transducer of the invention further comprises protection for the coil.

[0030] The invention also relates to a set of electromagnetic transducers as described above, the coils of the electromagnetic transducers being connected to each other in parallel and / or in series. Brief description of the figures

[0031] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawing given by way of example and which represents:

[0032] [Fig. 1] [Fig. 1] represents a model describing the behavior of a generic model of electromagnetic transduction recuperators under the assumption of small displacements, associated with an impedance matching circuit.

[0033] [Fig.2] [Fig.2] schematically represents an electromagnetic transducer whose central mass is in an equilibrium position.

[0034] [Fig.3] [Fig.3] schematically represents an electromagnetic transducer whose central mass is displaced relative to the equilibrium position and along a first axis.

[0035] [Fig.4] [Fig.4] schematically represents an electromagnetic transducer whose central mass is displaced relative to the equilibrium position, along a first axis and inversely to the displacement of the central mass in [Fig.3].

[0036] [Fig.5a] [Fig.5a] represents a flat spiral spring with three branches of thickness 0.5 mm.

[0037] [Fig.5b] [Fig.5b] represents a flat spiral spring with three branches of thickness 1 mm. Detailed description of the invention

[0038] Preferably, the electromagnetic transducer of the present invention has an essentially axisymmetric structure that can be contained in a simple volume such as a cylinder. This makes it possible in particular to minimize the edge effects of the magnetic field, to minimize the volume lost by its packaging and to facilitate its integration in a generic context. In addition, the essentially axisymmetric structure of the electromagnetic transducer of the present invention makes it possible to accelerate the convergence of the FEM simulations necessary for its dimensioning.

[0039] [Fig. 2] represents an electromagnetic transducer 20 according to the invention, being in particular in an equilibrium position, that is to say the position in which the central mass is located when no vibration affects the electromagnetic transducer. The electromagnetic transducer 20 comprises a central ferromagnetic element 25 flanked on a first side along the first axis 27 by a first magnet 21 having a face 21a facing the central ferromagnetic element 25 and an opposite face 21b. This first magnet 21 is flanked along the first axis 27 and facing its opposite face 21b by an additional ferromagnetic element 23. The ferromagnetic element 25 is flanked on a second side along the first axis 27 and opposite the first side by a second magnet 22 having a face 22a facing the central ferromagnetic element 25 and an opposite face 22b. This second magnet 22 is flanked along the first axis 27 and opposite its opposite face 22b by an additional ferromagnetic element 24. The first magnet and the second magnet are arranged in such a way that the faces opposite along the first axis each constitute a pole, that is to say that the face 21a corresponds to the north or south pole while the face 21b corresponds to the opposite pole, likewise for the faces 22a and 22b.

[0040] Preferably, the elements forming the central mass, i.e. the central ferromagnetic element 25, the first magnet 21, the second magnet 22 and the additional ferromagnetic elements 23, 24 are cylinders, preferably cylinders of revolution.

[0041] The electromagnetic transducer 20 also comprises a support 30 surrounding the central mass radially to the first axis and an air gap separating the support 30 from the central mass. The support 30 comprises a coil 31 wound around the first axis and secured to an external ferromagnetic element 32. In the particular embodiment of [Fig. 2], the coil 31 is located radially closest to the central mass and the external ferromagnetic element surrounds the coil radially and along the first axis, that is to say that only a central part of the face of the support 30 radially closest to the central mass comprises the coil.

[0042] The magnetic flux 29 of the first magnet 21 describes a loop going from one pole thereof to another, that is to say from a face 21a to a face 21b or vice versa, and being guided by the central ferromagnetic element 25, the external ferromagnetic element 32 and the additional ferromagnetic element 23. The magnetic flux 29 does not pass through the coil 31. The magnetic flux 28 of the second magnet 22 describes a loop going from one pole thereof to another, that is to say from a face 22a to a face 22b or vice versa, and being guided by the central ferromagnetic element 25, the external ferromagnetic element 32 and the additional ferromagnetic element 24. The magnetic flux 28 does not pass through the coil 31.

[0043] [Fig. 3] represents an electromagnetic transducer 20 similar to the electromagnetic transducer of [Fig. 2] but whose central mass is not in a rest position. Following a vibration, the spring may have been deformed and the central mass moved relative to the support along the first axis 27 and in the direction 33. During this vibration, the movement of the central mass causes a modification of at least least one of the paths followed by the magnetic flux of one of the magnets relative to the coil 31. In [Fig. 3], the magnetic flux 28 of the second magnet 22 is modified and passes through the coil, continuing around the coil 31, guided by the central ferromagnetic element 25, the external ferromagnetic element 32 and the additional ferromagnetic element 24. The modification of the path of the magnetic flux 28 results in an increase in the electromagnetic coupling and therefore increases the effective bandwidth of the electromagnetic transducer, provided that it is associated with an impedance matching circuit.

[0044] Following this movement of the central mass along the first axis 27 relative to the support 30 and along a direction 33, the central mass will move in the direction of the first axis 27 in an oscillating movement around its equilibrium position, which will again cause the path of the magnetic flux 28 to be modified and, if the movement of the central mass is sufficient, will cause the path of the magnetic flux 29 of the first magnet to be modified so that it passes through the coil 31.

[0045] [Fig. 4] shows an electromagnetic transducer 50 comprising a coil 51, an external ferromagnetic element 52, a first magnet 53, an additional ferromagnetic element 54 and a central ferromagnetic element 55. The electromagnetic transducer 50 particularly comprises a protection 56 made of a non-ferromagnetic material which limits the risk of sticking between the central mass and the external ferromagnetic element 52. The electromagnetic transducer 50 comprises a flat spring 62 with three branches which connects the support to the central mass by means of a clip 64 and a nut 65. The clip 64 is connected to the central mass by gluing one end of the disc-shaped clip to the central mass. The other opposite end of the clip along the axis may comprise a threaded rod making it possible to compress the spring between the clip 64 and the nut 62.The holder particularly comprises a ring 63c and a ring 63e which hold the external ferromagnetic element 52 by means of a screw 63d. The spring 62 is connected to the holder by being compressed between a ring 63a and the ring 63c by means of a screw 63b.

[0046] The electromagnetic transducer 50 according to [Fig. 4] comprises a coil support 57 facilitating the manufacture and connection of the coil 51 with the external ferromagnetic element 52. Such a coil support 57 can be used in other embodiments of the invention and is not particularly linked to the embodiment of [Fig. 4]. In particular, the coil support 57 is optionally linked to the protection 56.

[0047] The electromagnetic transducer 50 according to [Fig. 4] is an example of an electromagnetic transducer according to the present invention and does not limit the invention to this example. In particular, other means of connection between the central mass and the support can be used. Also, the spring can be connected to the support or to the mass central by other means, for example by welding, gluing or snap-fitting.

[0048] The dimensions indicated in [Fig.4], in particular a total height of 44 mm of height, a total width of 46mm and a central mass width of 20mm are given for information purposes and an electromagnetic transducer according to the invention may have other dimensions.

[0049] Figures 5a and 5b each show a three-legged spiral flat spring, forming a disc extending radially around an axis passing through the middle of each spring. The flat spring 70 of [Fig.5a] comprises three legs 71, the legs forming spirals towards the middle 72 of the spring 70. The legs 71 of the spring 70 make approximately one turn around the axis passing through the middle 72 between the outside and the inside of the spring relative to the axis. According to FEM simulations, the three-legged spiral flat spring of [Fig.5a] with a thickness of 0.5 mm would have an axial stiffness of approximately 16.8 N / mm and a radial stiffness of 803 N / mm. The flat spring 80 of [Fig.5b] comprises three branches 81, the branches forming spirals towards the middle 82 of the spring 80. The branches 81 of the spring 80 make approximately one and a half turns around the axis passing through the middle 82 between the outside and the inside of the spring relative to the axis.The legs 81 of the spring 80 are thinner radially to the axis. The thickness of the spring 70 is less than the thickness of the spring 80. According to FEM simulations, the three-legged flat spring of [Fig.5b] with a thickness of 1 mm would have an axial stiffness of about 16.8 N / mm and a radial stiffness of 57 N / mm.

[0050] The springs 70, 80 of Figures 5a and 5b are examples of springs that the electromagnetic transducer of the invention may comprise. However, other springs, and particularly other flat springs, may be used to implement the invention. A person skilled in the art would know how to adapt the thickness of the flat spring, the number of branches, the number of turns that the branches make between the outside and the inside of the spring relative to the axis and the material(s) of the springs depending on the environment in which the electromagnetic transducer must be used and depending on the dimensional constraints.

[0051] The present three-legged spiral flat springs may be dimensioned so as to have a sufficiently low axial stiffness so that the natural resonant frequency is close to 50 Hz. Conversely, the radial stiffness of such flat springs is sufficiently high to guide the central mass along the first axis without the central mass and the support coming into physical contact, i.e. without sticking.

[0052] Assuming a quality factor of an electromagnetic transducer according to the present invention at 100, a resonance frequency of 50 Hz and that this electromagnetic transducer is subjected to a vibration whose amplitude in acceleration is equivalent at 0.5 m / s2 while the frequency is set to a range centered around 50 Hz, an estimate of the recoverable power can be obtained. Taking the example of a volume of the smallest cylinder that can contain the electromagnetic transducer being 73 cm2, the maximum normalized power density would be of the order of 28 kg.s / m3. According to this example the effective bandwidth at -3 dB is equivalent to 4.2 Hz

[0053] Different electromagnetic transducers presented herein may be used in groups rather than individually, so as to form an assembly. It is not necessary that these electromagnetic transducers be identical and electromagnetic transducers according to different embodiments presented herein may be used together without limitations. In particular, they may be arranged in series or in parallel. In particular, the coils of the electromagnetic transducers of the same assembly may be connected in series and / or in parallel..

[0054] The various embodiments presented in this description are not limiting and can be combined with each other. Furthermore, the present invention is not limited to the embodiments previously described but extends to any embodiment falling within the scope of the claims.

Claims

Claims

1. An electromagnetic transducer (20) comprising a support (30), a central mass (21, 22, 23, 24, 25), and at least one spring connecting the central mass to the support, the spring allowing the central mass to move relative to the support along a first axis (27), the central mass comprising a central ferromagnetic element (25), a first magnet (21), a second magnet (22), and two additional ferromagnetic elements (23, 24), the ferromagnetic element being flanked on a first side along the first axis by the first magnet (21) and flanked on a second side, opposite the first side along the first axis, by the second magnet (22), the first magnet and the second magnet each being flanked along the first axis by one of the additional ferromagnetic elements (23, 24), the support (30) surrounding the central mass radially to the first axis and an air gap separating the support from the central mass,the support comprising at least one coil (31) wound around the first axis and secured to an external ferromagnetic element (32), the electromagnetic transducer being configured so that the magnetic flux (29) of the first magnet (21) and the magnetic flux (28) of the second magnet (22) each follow one of a first path and a second path, the first path not passing through the coil, the second path passing through the coil and continuing around the coil (31) via the external ferromagnetic element, the movement of the central mass along the first axis causing the path of at least one of the magnetic fluxes (28, 29) to change from the first path to the second path or vice versa, the coil (31) is entirely embedded in the external ferromagnetic element (32).,

2. Electromagnetic transducer (20) according to claim 1, characterized in that the spring comprises a first spring fixed on an external face of one of the additional ferromagnetic elements (23, 24) and a second spring fixed on an external face of the other additional ferromagnetic element (23, 24).

3. Electromagnetic transducer (20) according to one of the preceding claims, characterized in that the spring comprises at least one flat spring (70, 80) extending mainly in a plane perpendicular to the first axis (27), preferably being a three-coil spiral spring branches (71, 81).

4. Electromagnetic transducer (20) according to one of the preceding claims, characterized in that the central mass is flanked on at least one side along the first axis (27) by a third magnet and an additional ferromagnetic element.

5. Electromagnetic transducer (20) according to claim 4, characterized in that the support (30) comprises several coils (31).

6. Electromagnetic transducer (20) according to one of the preceding claims, characterized in that the axes of the poles of the first and second magnets are reversed along the first axis (27).

7. Electromagnetic transducer (20) according to one of the preceding claims, characterized in that the central mass is cylindrical.

8. Electromagnetic transducer (50) according to one of the preceding claims, characterized in that it further comprises a protection (57) of the coil (1).

9. Set of electromagnetic transducers according to one of the preceding claims, characterized in that the coils of the electromagnetic transducers are connected to each other in parallel and / or in series.

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