ELECTROMAGNETIC TRANSDUCER FOR HARVESTING VIBRATORY ENERGY

Abstract

An electromagnetic transducer for harvesting vibratory energy is provided. In particular, an electromagnetic transducer comprising a support, a central mass, and at least one spring linking the central mass to the support, the spring allowing the displacement of the central mass with respect to the support on a first axis. A set of electromagnetic transducers is also provided.

Claims

1. An electromagnetic transducer comprising a support, a central mass, and at least one spring linking the central mass to the support, the spring allowing the displacement of the central mass with respect to the support on 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 on the first axis by the first magnet and flanked on a second side, opposite the first side on the first axis, by the second magnet, the first magnet and the second magnet being each flanked on 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 outer ferromagnetic element, the electromagnetic transducer being configured in such a way that the magnetic flux from the first magnet and the magnetic flux from the second magnet each follow one path out 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 via the outer ferromagnetic element, the displacement of the central mass on the first axis driving the modification of the path of at least one of the magnetic fluxes from the first path to the second path or vice versa.

2. The electromagnetic transducer according to claim 1, the spring comprising a first spring fixed onto an outer face of one of the additional ferromagnetic elements and a second spring fixed onto an outer face of the other additional ferromagnetic element.

3. The electromagnetic transducer according to claim 1, the spring comprising at least one flat spring extending primarily in a plane at right angles to the first axis, preferably being a three-branch spiral spring.

4. The electromagnetic transducer according to claim 1, the coil being entirely embedded in the outer ferromagnetic element.

5. The electromagnetic transducer according to claim 1, the central mass being flanked on at least one side on the first axis by a third magnet and an additional ferromagnetic element.

6. The electromagnetic transducer according to claim 5, the support comprising several coils.

7. The electromagnetic transducer according to claim 1, the axes of the poles of the first magnet and of the second magnet being reversed on the first axis.

8. The electromagnetic transducer according to claim 1, the central mass being cylindrical.

9. The electromagnetic transducer according to claim 1, further comprising a protection of the coil.

10. A set of electromagnetic transducers according to claim 1, the coils of the electromagnetic transducers being linked to one another in parallel and/or in series.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Other features, details and advantages of the invention will emerge on reading the description given in reference to the attached drawing, which is given by way of example and in which:

[0033] FIG. 1 represents a model describing the generic model behaviour of electromagnetic transduction harvesters assuming small displacements, associated with an impedance matching circuit.

[0034] FIG. 2 schematically represents an electromagnetic transducer whose central mass is in a position of equilibrium.

[0035] FIG. 3 schematically represents an electromagnetic transducer whose central mass is displaced with respect to the position of equilibrium and on a first axis.

[0036] FIG. 4 schematically represents an electromagnetic transducer whose central mass is displaced with respect to the position of equilibrium, on a first axis and the reverse of the displacement of the central mass in FIG. 3.

[0037] FIG. 5a represents a flat spiral spring with three branches 0.5 mm thick.

[0038] FIG. 5b represents a flat spiral spring with three branches 1 mm thick.

DETAILED DESCRIPTION

[0039] Preferentially, the electromagnetic transducer of the present invention has an essentially axisymmetrical structure that can be contained in a simple volume such as a cylinder. This notably makes it possible to minimize the edge effects of the magnetic field, to minimize the volume lost by its packaging and to facilitate its incorporation in a generic context. Furthermore, the essentially axisymmetrical structure of the electromagnetic transducer of the present invention makes it possible to speed up the convergence of the EMF simulations necessary to its dimensioning.

[0040] FIG. 2 represents an electromagnetic transducer 20 according to the invention, it being notably in a position of equilibrium, of rest, that is to say the position at which the central mass is situated when no vibration affects the electromagnetic transducer. The electromagnetic transducer 20 comprises a central ferromagnetic element 25 flanked on a first side on the first axis 27 by a first magnet 21 having a face 21a opposite the central ferromagnetic element 25 and an opposite face 21b. This first magnet 21 is flanked on the first axis 27 and opposite its opposite face 21b by an additional ferromagnetic element 23. The ferromagnetic element 25 is flanked on a second side on the first axis 27 and opposite the first side by a second magnet 22 having a face 22a opposite the central ferromagnetic element 25 and an opposite face 22b. This second magnet 22 is flanked on the first axis 27 and opposite its opposite face 22b by an additional ferromagnetic element 24. The first magnet and the second magnet are disposed in such a way that the opposite faces on 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, and likewise for the faces 22a and 22b.

[0041] Preferentially, the elements forming the central mass, that is to say 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.

[0042] 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 outer ferromagnetic element 32. The outer ferromagnetic element 32 is notably made of a single block. Indeed, when the outer ferromagnetic element 32 is made in several parts, the presence of joins between its parts can disturb the magnetic fluxes 28, 29. When the coil 31 is closed at least partially in the outer ferromagnetic element 32, that is to say that each of its faces is at least partially covered by a part of the outer ferromagnetic element 32, the latter must then be in several parts. The outer ferromagnetic element 32 can notably comprise an open recess on the central mass 21 into which the coil 31 is inserted, as represented. The coil 31 can notably be flush with the surface of the outer ferromagnetic element 32 or else set back from the surface thereof. In particular, the coil is dimensioned to be set back from the surface thereof, which limits the functional plays. In particular, the recess is configured for the coil 31 to be disposed opposite only the central ferromagnetic element 25 in the position of equilibrium of the electromagnetic transducer 20, as represented. Thus, in an advantageous embodiment, the height of the coil 31 is less than or equal to that of the central ferromagnetic element 25. The path of the magnetic fluxes 28, 29 from the magnets 21, 22 is not therefore influenced by the coil 31 in the position of equilibrium of the electromagnetic transducer 20. It is not then necessary to cover the coil 31 partially on the face opposite the central mass 21 by a ferromagnetic element, this covering being produced in order to avoid a modification of the magnetic flux 28, 29 from the first and second magnets 21, 22 in the position of equilibrium. Thus, advantageously, since the coil 31 is not covered on its face opposite the central mass 21, and the air gap separating these two elements is consequently smaller than with a cover, the electromagnetic transducer 20 is more compact. Because of this smaller air gap, as is described in detail hereinbelow, the transition of the magnetic fluxes 28, 29 from a short path (in which they do not pass through the coil 31) to a long path (in which they pass through the coil 31) is easier and therefore the electromagnetic transducer 20 is more efficient. In the particular embodiment of FIG. 2, the coil 31 is situated radially closest to the central mass and the outer ferromagnetic element surrounds the coil radially and on 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 includes the coil.

[0043] In the invention, the coil 31 advantageously acts both as current generator, through the modification of the path of the magnetic fluxes 28, 29, and as insulator, which makes it possible to reduce the air gap separating the central mass 21 from the coil 31 and therefore obtain a compact electromagnetic transducer 20.

[0044] The magnetic flux 29 from 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 guided by the central ferromagnetic element 25, the outer ferromagnetic element 32 and the additional ferromagnetic element 23. The magnetic flux 29 does not pass through the coil 31. The magnetic flux 28 from 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 guided by the central ferromagnetic element 25, the outer ferromagnetic element 32 and the additional ferromagnetic element 24. The magnetic flux 28 does not pass through the coil 31.

[0045] FIG. 3 represents an electromagnetic transducer 20 similar to the electromagnetic transducer of FIG. 2 but whose central mass is not in a position of rest. Following a vibration, the spring was able to be deformed and the central mass was displaced with respect to the support on the first axis 27 and in the direction 33. Upon this vibration, the displacement of the central mass drives a modification of at least one of the paths followed by the magnetic flux from one of the magnets with respect to the coil 31. Thus, the coil 31 is at least partially opposite the first magnet 21 or the second magnet 22 which influences the path of the corresponding magnetic flux 28, 29. In FIG. 3, the coil 31 is partially opposite the second magnet 22 and the magnetic flux 28 from the second magnet 22 is modified and passes through the coil 31, continuing around the coil 31, guided by the central ferromagnetic element 25, the outer ferromagnetic element 32 and the additional ferromagnetic element 24. The modification of the path of the magnetic flux 28 drives an increase in the electromagnetic coupling and therefore increases the effective bandwidth of the electromagnetic transducer, provided that the latter is associated with an impedance matching circuit.

[0046] Following this displacement of the central mass on the first axis 27 with respect to the support 30 and in a direction 33, the central mass will be displaced in the direction of the first axis 27 by an oscillation movement about its position of equilibrium, which will once again cause modification of the path of the magnetic flux 28 to the short path in as much as the coil 31 will no longer be opposite the second magnet 22, and, if the displacement of the central mass is sufficient, will drive the coil 31 opposite the first magnet 21 and modify the path of the magnetic flux 29 from the first magnet in such a way that it passes through the coil 31.

[0047] FIG. 4 represents an electromagnetic transducer 50 comprising a coil 51, an outer ferromagnetic element 52, a first magnet 53, an additional ferromagnetic element 54 and a central ferromagnetic element 55. The electromagnetic transducer 50 comprises in particular a protection 56 made of non-ferromagnetic material which limits the risk of bonding between the central mass and the outer ferromagnetic element 52. The electromagnetic transducer 50 comprises a flat spring 62 with three branches which links the support to the central mass via an attachment 64 and a nut 65. The attachment 64 is linked to the central mass by a bonding of an end of the attachment in disc form to the central mass. The other opposite end of the attachment on the axis can comprise a threaded rod making it possible to compress the spring between the attachment 64 and the nut 62. The support comprises in particular a ring 63c and a ring 63e which hold the outer ferromagnetic element 52 by means of a screw 63d. The spring 62 is linked to the support by being compressed between a ring 63a and the ring 63c by means of a screw 63b.

[0048] The electromagnetic transducer 50 according to FIG. 4 comprises a coil support 57 facilitating the fabrication and the linking of the coil 51 with the outer 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. Notably, the coil support 57 is optionally linked to the protection 56.

[0049] The electromagnetic transducer 50 according to FIG. 4 is an example of electromagnetic transducer according to the present invention and does not limit the invention to this example. Notably, other linking means between the central mass and the support can be used. Also, the support can be linked to the support or to the central mass by other means, for example by welding, bonding or snap-fitting.

[0050] The dimensions indicated in FIG. 4, notably a total height of 44 mm, a total width of 46 mm and a width of the central mass of 20 mm are given as an indication and an electromagnetic transducer according to the invention can have other dimensions.

[0051] FIGS. 5a and 5b each represent a flat spiral spring with three branches, forming a disc extending radially about an axis passing through the middle of each spring. The flat spring 70 of FIG. 5a comprises three branches 71, the branches forming spirals towards the middle 72 of the spring 70. The branches 71 of the spring 70 describe approximately one turn about the axis passing through the middle 72 between the outside and the inside of the spring with respect to the axis. According to EMF simulations, the flat spiral spring with three branches 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 describe approximately one and a half turns about the axis passing through the middle 82 between the outside and the inside of the spring with respect to the axis. The branches 81 of the spring 80 are thinner radially at the axis. The thickness of the spring 70 is less than the thickness of the spring 80. According to EMF simulations, the flat spring with three branches of FIG. 5b with a thickness of 1 mm would have an axial stiffness of approximately 16.8 N/mm and a radial stiffness of 57 N/mm.

[0052] The springs 70, 80 of FIGS. 5a and 5b are examples of spring that the electromagnetic transducer of the invention can include. However, other springs, and particularly other flat springs, can be used to implement the invention. The person skilled in the art would be able to adapt the thickness of the flat spring, the number of branches, the number of turns that the branches describe between the outside and the inside of the spring with respect to the axis and the material or materials of the springs according to the environment in which the electromagnetic transducer is to be used and according to the dimensional constraints.

[0053] The flat spiral springs with three branches that are present can be dimensioned so as to have an axial stiffness that is low enough for the natural resonance frequency to be close to 50 Hz. On the other hand, the radial stiffness of such flat springs is strong enough to guide the central mass along the first axis without the central mass and the support coming into physical contact, that is to say without bonding.

[0054] Assuming a quality factor of an electromagnetic transducer according to the present invention of 100, a resonance frequency of 50 Hz and that this electromagnetic transducer is subjected to a vibration whose acceleration amplitude is equivalent to 0.5 m/s.sup.2 while the frequency is defined over a range centred around 50 Hz, an estimation of the harvestable power can be obtained. By taking the example of a volume of the smallest cylinder that can contain the electromagnetic transducer that is 73 cm.sup.2, the maximum normalized power density would be of the order of 28 kg.Math.s/m.sup.3. According to this example, the effective bandwidth at −3 dB would be equivalent to 4.2 Hz.

[0055] Different electromagnetic transducers presented here can be used in groups rather than individually, so as to form a set. It is not therefore necessary for these electromagnetic transducers to be identical and electromagnetic transducers according to different embodiments presented here can be used together without limitation. They can notably be disposed in series or in parallel. In particular, the coils of the electromagnetic transducers of one and the same set can be linked in series and/or in parallel.

[0056] The different embodiments presented in this description are not limiting and can be combined with one another. 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