GB2439411A

GB2439411A - A generator for converting mechanical vibrational energy into electrical energy

Info

Publication number: GB2439411A
Application number: GB0708217A
Authority: GB
Inventors: Stephen Roberts
Original assignee: Perpetuum Ltd
Current assignee: Perpetuum Ltd
Priority date: 2007-04-27
Filing date: 2007-04-27
Publication date: 2007-12-27
Anticipated expiration: 2027-04-27
Also published as: JP5248598B2; EP2149195B1; US20080265692A1; WO2008132423A1; US7586220B2; CN101682248B; CN101682248A; JP2010525779A; GB0708217D0; EP2149195A1; GB2439411B

Abstract

The electromechanical generator comprising a housing 4, an electrically conductive coil assembly 22 (40,42) fixedly mounted in the housing, the coil assembly having radially inner and outer sides, and upper and lower edges, thereof, a mount 24 for the coil assembly extending inwardly of the radially inner side for fixing the coil assembly in a fixed position in the housing, a magnetic core assembly 50 movably mounted in the housing for linear vibrational motion along an axis A-A, and biasing devices 82,84 mounted between the housing and the magnetic core assembly to bias the magnetic core assembly in opposed directions along the axis towards a central position, wherein the magnetic core assembly encloses the electrically conductive coil assembly on the radially outer side and on the upper and lower edges, and on a part of the radially inner side, the magnetic core assembly having a gap on a radially inner portion thereof through which the mount extends, and the radially inner portion including two opposed magnets 53,54 spaced along the axis. The biasing deice may comprise spiral arms 108 extending between radial inner 90,92 and radial outer 82,84 mountings which are threaded 94,96,102,104 to the core and central tube 28 at opposing ends of the tube. Alternatively the bias means may comprise a stepped yoke configuration of springs (fig 3). The interior 16 of the housing may be gas filled and may be hermetically sealed.

Description

AN ELECTROMECHANICAL GENERATOR FOR CONVERTING

MECHANICAL VIBRATIONAL ENERGY INTO ELECTRICAL ENERGY

Background to the Invention

The present invention relates to an electromechanical generator for converting mechanical vibrational energy into electrical energy. In particular, the present invention relates to such a device which is a miniature generator capable of converting ambient vibration energy into electrical energy for use, for example, in powering intelligent sensor systems. Such a system can be used in many areas where there is an economical or operational advantage in the elimination of power cables or batteries.

Description of the Prior Art

It is known to use an electromechanical generator for harvesting useful electrical power from ambient vibrations, e.g. for powering wireless sensors. A typical magnet-coil generator consists of a spring-mass combination attached to a magnet or coil in such a manner that when the system vibrates, a coil cuts through the flux formed by a magnetic core.

It is generally known in the art that as a rule the greater the mass of the spring-mass combination of the magnetic core generator, the greater the output electrical power. An energy harvester needs to produce high power over a wide bandwidth because the vibration frequency is not known before deployment, or could change. High power over a wide bandwidth for a resonant vibration energy harvester requires a high mass, a high Q and a high magnetic coupling factor.

Summary of the Invention

The present invention aims to provide a device that maximizes all three of these parameters in a practical manner.

The present invention accordingly provides an electromechanical generator for converting mechanical vibrational energy into electrical energy having the features of claim 1, claim 11 or claim 12.

Preferred features are defined in the dependent claims.

In the electromechanical generator of the preferred embodiment of the present invention a high moving mass can be achieved by filling almost all of the internal space with a metallic magnetic core assembly. This can be achieved at least partly because flat springs at opposed ends of the magnetic core assembly are volume efficient. In addition, a high Q comes from the fact that the "enclosed" structure of the magnetic core assembly leaks very little flux, and so there is very little eddy current in the surrounding material of the stationary housing. Accordingly, little clearance needs to be kept between the moving magnetic core assembly and the housing, allowing more moving mass. A high magnetic coupling comes also from the enclosed nature of the magnetic core assembly where very little flux leaks out -almost all the magnetic flux gets channeled through the coil.

Brief Description of the Drawings

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a schematic side section through an electromechanical generator for converting mechanical vibrational energy into electrical energy in accordance with an embodiment of the present invention; Figure 2 is a schematic plan view of a first spring in the electromechanical generator of Figure 1; and Figure 3 is a schematic perspective view of a second spring in the electromechanical generator of Figure 1.

Detailed Description of the Preferred Embodiments

The electromechanical generator of the present invention is a resonant generator known in the art as "velocity-damped" where all of the work done by the movement of the inertial mass relative to the housing is proportional to the instantaneous velocity of that movement. Inevitably, a portion of that work is absorbed overcoming unwanted mechanical or electrical losses, but the remainder of the work may be used to generate an electrical current via a suitable transduction mechanism, such as the electrical coil/magnetic assembly described below.

Figures 1 to 3 show an electromechanical generator 2 for converting mechanical vibrational energy into electrical energy in accordance with an embodiment of the present invention. 1. The electromechanical generator 2 comprises a housing 4. The housing 4 comprises an annular outer peripheral wall 6, an integral circular lid 8 and a circular base 10. The base 10 is securely fitted at its circular edge 12 to a lower edge 14 of the outer peripheral wall 6, for example by means of adhesive or a threaded coupling (not shown). The outer peripheral wall 6 defines a cylindrical cross-section interior volume 16, having an axis of rotation A-A. A circular opening 18 is formed through the lid 8, which opening 18 is coaxial with the cylindrical cross-section interior volume 16.

The base 10 is provided with a fitting 20 in its outer surface for securely mounting the electromechanical generator 2 to a support (not shown).

An electrically conductive coil 22 is fixedly mounted in the housing 4. The coil 22 is circular and is coaxial with the housing 4, and has radially inner and outer sides 21, 23, the sides 21, 23 extending parallel to the axis of rotation A-A. The coil 22 has upper and lower edges 27. 29. The coil 22 is mounted within an annular coil support 24 which is located substantially midway in a radial direction between the axis A-A and the outer peripheral waIl 6, and also substantially midway in an axial direction between the lid 8 and the base 10. The coil support 24 has an integral annular central mounting portion 26 that extends radially inwardly from a central part of the coil 22 and is mounted on a central tubular body 28 that is securely fitted between the lid 8 and the base 10. This assembly mounts the coil 22 in a fixed position within the housing 4. Preferably the coil support 24 is made from a very low-conductivity material, such as glass-loaded plastic.

Preferably the central tubular body 28 is made from a low-permeability, low-conductivity, but high-elastic-modulus material such as 316 stainless steel.

The mounting portion 26 defines an annular recess 30 in which is received circuitry 32 for electrically conditioning the electrical output of the coil 20, for example by voltage regulation. The circuitry 32 is encapsulated within the annular recess 30 by a plastic or rubber sealing material 34, which seals and protects the circuitry 32 against undesired enviroimiental influences, such as humidity, liquids, etc.. The coil 22 is connected the circuitry 32 by first wires 36 and in turn the circuitry 32 has second wires 38 extending therefrom through the opening 18 in the lid 8 for connecting to external circuitry (not shown).

The coil 20 has first and second coil portions 40, 42 thereof respectively located above and below the mounting portion 26.

A magnetic core assembly 50 is movably mounted in the housing 4 for linear vibrational motion along the axis A-A. The magnetic core assembly 50 is rotationally symmetric and includes a pair of axially aligned annular magnets 52, 54, each typically a rare earth permanent magnet having a high magnetic field strength The magnets 52, 54 are mounted on opposite sides, above and below, of the mounting portion 26 and radially inwardly of the coil 20. The magnets 52, 54 are each axially spaced from the mounting portion 26, and define a gap 55 through which the mounting portion 26 extends. As shown in Figure 1, the magnets 52, 54 are aligned so that their identical poles 56, 58 (e.g. the north (N) poles as shown in Figure 1) face each other on opposite sides of the mounting portion 26.

The magnetic core assembly 50 also includes a common ferromagnetic body 64. The magnets 52, 54 are mounted between opposed annular arms 60, 62 of the common ferromagnetic body 64. The poles 66, 68 (e.g. the south (S) poles) of the magnets 52, 54 that face away from each other in an axially outward direction are each mounted on a respective annular arm 60, 62. The common ferromagnetic body 64 also includes a tubular portion 70 comprised of two mutually interlocking tubular members 72, 74, each integral with a respective annular arm 60, 62. In this way, each of the first and second coil portions 40, 42 is respectively at least partly located between tubular portion 70 of the common ferromagnetic body 64 and one of the magnets 52, 54.

This magnetic core assembly 50 of the radially outer common ferromagnetic body 64 coupled to the radially inner magnets 52, 54 defines therebetween an annular enclosed cavity 43 in which the coil 22 is received. The magnets 52, 54 are in the vicinity of the inner side 21 of the coil 22 and the common ferromagnetic body 64 is in the vicinity the outer side 23 of the coil 22. The magnets 52, 54 and the common ferromagnetic body 64 are slightly spaced from the coils 22 to permit relative translational movement therebetween. The magnetic core assembly 50 encloses the coil 22 on the radially outer side 23 and on the upper and lower edges 27, 29, and on a part of the radially inner side 21, the magnetic core assembly having the gap 55 on a radially inner portion thereof, comprised of the magnets 52, 54, through which the mounting portion 26 extends. The common ferromagnetic body 64 comprises the radially outer and upper and lower portions of the magnetic core assembly 50. The magnetic core assembly 50 therefore has a substantially C-shaped cross-section and is rotationally symmetric.

The cavity 43 has respective cavity portions 44, 46 between each of the first and second coil portions 40, 42 and the central tubular body 28, and above or below, respectively, the mounting portion 26 The common ferromagnetic body 64 is composed of a ferromagnetic material having a high magnetic permeability, and a high mass, such as soft iron. The assembly of the common ferromagnetic body 64 and the magnets 52, 54 therefore forms two axially spaced magnetic circuits 76, 78 of the magnetic core assembly 50, the magnetic flux being shown by the dashed lines in Figure 1, one for each magnet 52, 54. The limits of the lines of magnetic flux each magnetic circuit 76, 78 are defined by the respective annular arm 60, 62 and tubular member 72, 74, which substantially prevents magnetic flux from each magnet 52, 54 extending axially or radially outwardly from the common ferromagnetic body 64. Since the opposed magnets 52, 54 face each other with common poles 56, 58 (e.g. N poles), at the central region 80 of the magnetic core assembly 50 the magnetic flux of the opposed magnetic circuits 76, 78 are in opposition thereby directing the magnetic flux radially outwardly towards the common ferromagnetic body 64.

The resultant effect is that a single magnetic core assembly 50 comprises two separate magnets 52, 54 and each has a respective magnetic circuit 76, 78 in which a very high proportion of the magnetic flux is constrained to pass through the respective coil portion 40, 42. This in turn provides a very high degree of magnetic coupling between the magnets 52, 54 and the coil 22. Consequently, any relative movement between the magnets 52, 54 and the coil 22, in particular as described below by linear axial resonant movement of the magnetic core assembly 50 relative to the fixed coil 22, produces a very high electrical power output at the coil 22.

The common ferromagnetic body 64 is movably mounted to the central tubular body 28 by a pair of opposed plate springs 82, 84. One spring 82, 84 is located between each respective upper or lower end 83, 85 of the common ferromagnetic body 64 and a respective upper or lower end 86, 88 of the central tubular body 28. A radially inner annular edge 90, 92 of each spring 82, 84 is securely fitted, e.g. by a screw thread 94, 96, to the respective upper or lower end 86, 88. A radially outer annular edge 98, 100 of each spring 82, 84 is securely fitted, e.g. by a screw thread 102, 104 to the respective upper or lower end 83, 85 of the common ferromagnetic body 64.

As shown in Figure 2, in one alternative arrangement each spring 82, 84 has a spiral configuration, with plural spiral arms 108 extending between the radially inner annular edge 90, 92 and the radially outer annular edge 98, 100.

As shown in Figure 3, in another alternative arrangement each spring 82, 84 has a stepped yoke configuration, with plural eccentric yokes 110, 112 extending in a cascading stepwise manner between the radially inner annular edge 90, 92 and the radially outer annular edge 98, 100.

The two springs 82, 84 each apply the same mechanical biasing force against the magnet assembly 50 when the magnetic core assembly 50 is moved away from a central equilibrium position. The two springs 82, 84 preferably have the same spring constant.

The provision of a pair of plate springs 82, 84 at opposed axial ends of the movable magnetic core assembly 50 provides a structure that can not only provide a sufficient spring biased restoring force on the magnetic core assembly 50 to bias it towards an axially central position with respect to the coil 22 but also takes up substantially minimum volume within the housing 4. In particular, the location of the springs 82, 84 at opposed axial ends of the movable magnetic core assembly 50 enables the magnetic core assembly 50 to extend radially outwardly substantially as far as the interior radial limits of the housing 4. This maximizes the size of the magnetic core assembly SO for a given interior volume 16, which not only maximizes the magnetic coupling, but also importantly permits the mass of the movable magnetic core assembly to be correspondingly maximized. As known in the art, there is a desire to maximize the mass of the movable magnetic core assembly in a resonant vibrational electromagnetic energy harvester because this increases the output electrical power.

The provision of a pair of plate springs 82, 84 also avoids the need for expensive and cumbersome helical springs surrounding the movable magnetic core assembly. This decreases the manufacturing cost by reducing the component cost.

The high degree of magnetic coupling between the movable magnetic core assembly and the coil, and the high mass of the movable magnetic core assembly, enables the resonant frequency readily to be tuned accurately to a desired value, and also permits a high self-restoring force to be applied to the movable magnetic core assembly during its resonant oscillation to minimize the amplitude of the oscillation. Since the amplitude is limited, the springs 82, 84 are only ever deformed by a very small degree, well within their linear spring characteristics. Typically, the annular gap 114, 116 between annular fitting 102, 104 and the lid 8 or base 10 respectively is about 1mm, and the maximum amplitude is accordingly less than this distance. Again, this maximizes the useful volume 16 of the housing 4 in an axial direction.

The springs 82, 84 bias, back towards the central position, the magnetic core assembly which can move axially along the axis A-A when the electromechanical generator 2 is subjected to an applied mechanical force, in particular a mechanical vibration, having at least a component along the axis A-A. The springs 82, 84 have a high stiffness in the lateral, i.e. radial, direction so as substantially to prevent non-axial movement of the magnetic core assembly 50.

The interior volume 16 of the housing 4 may include a gas. The housing 4 may hermetically seal the interior volume 16 of the housing 4.

The electromechanical generator 2 uses a resonant mass-spring arrangement mounted within the housing 4. If the electromechanical generator 2 is subject to a source of external vibration that causes it to move along the direction A-A, then the magnetic core assembly 50 comprises an inertial mass which may move relative to the housing 4, also along the direction A-A. In doing so, the springs 82, 84 are deformed axially, and work is done against a damper comprising the static electrical coil and the movable magnetic core assembly that generates a region of magnetic flux within which the electrical coil is disposed. Movement of the electrical coil within the magnetic flux causes an electrical current to be induced in the electrical coil which can be used as a source of electrical power for driving an external device (not shown).

Also, although in this embodiment the springs are plate springs, other biasing elements may be employed.

The mass of the magnetic core assembly can be made to be very high relative to the size of the device, thereby to increase the overall mass density of the device as compared, for example, to a cantilever device. For a given volume to be occupied by the device, a greater moving mass can be provided. This also maximizes the electrical power output, for the reasons stated above.

By increasing the electrical output, as a result of increased magnetic coupling, the operating band width of the device can be greatly increased. This in turn greatly enhances the ability of the device to be used in many new energy harvesting applications.

Simple plate springs can be employed in the electromechanical generator. This provides a reliable and simple structure to prevent lateral movement on the magnetic core assembly, with low friction and avoiding complicated, intricate and/or expensive

S

manufacturing techniques. The resultant structure is robust and compact. Since the plate springs are subjected to a very low amplitude of deformation, their mechanical properties are not especially critical, because they are never deformed anywhere near their mechanical limits of linear elastic movement, and so they can accordingly be of relatively conventional quality, and consequently have a low component cost.

Other modifications and embodiments of the present invention will be apparent to those skilled in the art.

Claims

WHAT IS CLAIMED IS: 1. An electromechanical generator for converting

mechanical vibrational energy into electrical energy, the electromechanical generator comprising a housing, an electrically conductive coil assembly fixedly mounted in the housing, the coil assembly having radially inner and outer sides, and upper and lower edges, thereof, a mount for the coil assembly extending inwardly of the radially inner side for fixing the coil assembly in a fixed position in the housing, a magnetic core assembly movably mounted in the housing for linear vibrational motion along an axis, and a biasing device mounted between the housing and the magnetic core assembly to bias the magnetic core assembly in opposed directions along the axis towards a central position, wherein the magnetic core assembly encloses the electrically conductive coil assembly on the radially outer side and on the upper and lower edges, and on a part of the radially inner side, the magnetic core assembly having a gap on a radially inner portion thereof through which the mount extends, and the radially inner portion including two opposed magnets spaced along the axis.

2. An electromechanical generator according to claim 1 wherein the biasing device comprises a pair of springs, each being located at a respective end of the magnetic core assembly.

3. An electromechanical generator according to claim 2 wherein the springs comprise plate springs.

4. An electromechanical generator according to any foregoing claim wherein the magnetic core assembly comprises two opposed magnetic circuits spaced along the axis.

5. An electromechanical generator according to any foregoing claim wherein the magnetic core assembly comprises a pair of magnets spaced along the axis, poles of the magnets having a first common polarity facing towards each other, and poles of the magnets facing away from each other being of a second common polarity and being coupled to a common ferromagnetic body located radially outwardly of the magnets relative to the axis.

6. An electromechanical generator according to claim 5 wherein the common ferromagnetic body is tubular and has radially inwardly extending arms at each end thereof, each arm mounting a respective magnet thereon.

7. An electromechanical generator according to claim 5 or claim 6 wherein the common ferromagnetic body comprises a radially outer and upper and lower portions of the magnetic core assembly and the magnets comprise the radially inner portion.

8. An electromechanical generator according to any foregoing claim wherein the mount for the coil assembly comprises an annular coil support which has a central mounting portion that extends radially inwardly from a central part of the coil assembly and is mounted on a central body that is fixed along the axis.

9. An electromechanical generator according to claim 8 wherein the mounting portion defines an annular recess in which is received circuitry for electrically conditioning the electrical output of the coil assembly.

An electromechanical generator according to claim 9 wherein the circuitry is encapsulated within the annular recess by a sealing material, which seals and protects the circuitry against undesired environmental influences.

11. An electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising a fixed electrically conductive coil assembly and a magnet assembly movably mounted for linear vibrational motion along an axis, and a biasing device for biasing the magnet assembly in opposed directions along the axis towards a central position, wherein the magnet assembly comprises two opposed magnetic circuits spaced along the axis, each magnetic circuit being associated with a respective portion of the coil assembly.

12. An electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising a fixed electrically conductive coil assembly and a magnet assembly movably mounted for linear vibrational motion along an axis, and a biasing device for biasing the magnet assembly in opposed directions along the axis towards a central position, wherein the magnet assembly is rotationally symmetric and has a substantially C-shaped cross-section enclosing an annular cavity, having a gap on the inner radius thereof, in which cavity the coil * assembly is disposed.

13. An electromechanical generator for converting mechanical vibrational energy into electrical energy substantially as hereinbefore described with reference to Figures 1 to3.

Amendments to the claims have been filed as follows WHAT IS CLAIMED IS: 1. An electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising a housing, an electrically conductive coil assembly fixedly mounted in the housing, the coil assembly having radially inner and outer sides, and upper and lower edges, thereof, a mount for the coil assembly extending inwardly of the radially inner side for fixing the coil assembly in a fixed position in the housing, a magnetic core assembly movably mounted in the housing for linear vibrational motion along an axis, and a biasing device mounted between the housing and the magnetic core assembly to bias the magnetic core assembly in opposed directions along the axis towards a central position, wherein the magnetic core assembly encloses the electrically conductive coil assembly. on the radially outer side and on the upper and lower edges, and on a part of the radially inner side, the magnetic core assembly having a gap on a radially inner portion thereof through which the mount extends, and the radially inner portion including two opposed magnets spaced along the axis.

2. An electromechanical generator according to claim 1 wherein the biasing device comprises a pair of springs, each being located at a respective end of the magnetic core assembly.

3. An electromechanical generator according to claim 2 wherein the springs comprise plate springs.

4. An electromechanical generator according to any foregoing claim wherein the magnetic core assembly comprises two opposed magnetic circuits spaced along the axis.

S. An electromechanical generator according to any, foregoing claim wherein the magnetic core assembly comprises a pair of magnets spaced along the axis, poles of the magnets having a first common polarity facing towards each other, and poles of the magnets facing away from each other being of a second common polarity and being coupled to a common ferromagnetic body located radially outwardly of the magnets relative to the axis, p

V

6. An electromechanical generator according to claim 5 wherein the common ferromagnetic body is tubular and has radially inwardly extending arms at each end thereof, each arm mounting a respective magnet thereon.

7. An electromechanical generator according to claim 5 or claim 6 wherein the common ferromagnetic body comprises a radially outer and upper and lower portions of the magnetic core assembly and the magnets comprise the radially inner portion.

8. An electromechanical generator according to any foregoing claim wherein the mount for the coil assembly comprises an annular coil support which has a central mounting portion that extends radially inwardly from a central part of the coil assembly and is mounted on a central body that is fixed along the axis.

9. An electromechanical generator according to claim 8 wherein the mounting portion defines an annular recess in which is received circuitry for electrically conditioning the electrical output of the coil assembly.

An electromechanical generator according to claim 9 wherein the circuitry is encapsulated within the annular recess by a sealing material, which seals and protects the circuitry against undesired environmental influences. * S..

*.** 11. An electromechanical generator for converting mechanical vibrational energy s... into electrical energy, the electromechanical generator comprising a fixed electrically conductive coil assembly and a magnet assembly movably mounted for linear vibrational motion along an axis, wherein the magnet assembly comprises a pair of magnets spaced * *.* * * .. along the axis, poles of the magnets having a first common polarity facing towards each 5S *S * * other and separated by a gap, and poles of the magnets facing away from each other being of a second common polarity and being coupled to a common ferromagnetic body located radially outwardly of the magnets relative to the axis, and a biasing device for biasing the magnet assembly, comprising the pair of magnets and the ferromagnetic body coupled thereto, in opposed directions along the axis towards a central position, wherein the magnet assembly comprises two opposed magnetic circuits spaced along the axis, each magnetic circuit being associated with a respective portion of the coil assembly.

12. An electromechanical generator for converting mechanical vibrational energy into electrical energy, the electromechanical generator comprising a fixed electrically conductive coil assembly and a magnet assembly movably mounted for linear vibrational motion along an axis, and a biasing device for biasing the magnet assembly in opposed directions along the axis towards a central position, wherein the magnet assembly is rotationally symmetric and has a substantially C-shaped cross-section enclosing an annular cavity, having a gap on the inner radius thereof, in which cavity the coil assembly is disposed, the magnet assembly comprising two opposed magnetic circuits spaced along the axis.

13. An electromechanical generator for converting mechanical vibrational energy into electrical energy substantially as hereinbefore described with reference to Figures 1 to3.

SISS

S *S** *5**

S 5*5*

S

. **S* * * **** S... S. *S * S * S *

IS