Vibrational energy harvester

A vibrational energy harvester (100, 200, 300, 400) comprises a first mass (101, 201, 301) comprising a first internal cavity (102, 202, 302) and a second mass (103, 203) disposed within and configured to move within the first internal cavity. Movement of the second mass relative to the first mass induces an electrical current in one of the first mass and the second mass. The vibrational energy harvester also comprises a housing (104, 204, 404) comprising a second internal cavity (105, 405). The first mass is disposed within and configured to move within the second internal cavity. An adjustment mechanism (419) is also provided, configured to adjust a size of the second internal cavity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage of International Patent Application No. PCT/EP2020/068269, filed 29 Jun. 2020, which claims priority to Great Britain Patent Application No: 1909277.4, filed on 27 Jun. 2019. The disclosures of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The invention relates generally to a vibrational energy harvester. In particular, the invention relates to a dual-mass vibrational energy harvester.

BACKGROUND

Vibrational energy harvesters are typically based on linear oscillators which have a narrow frequency bandwidth (i.e., a resonant frequency) that is a characteristic given by the vibrational energy harvester's mass, spring constants and dimensions. Such vibrational energy harvesters can therefore only harvest energy efficiently if the resonant frequency of the vibrational energy harvester is close to the main frequency of ambient vibrations from which energy is to be harvested.

Different methods have been suggested to overcome the problem of narrow bandwidth vibrational energy harvesters, for example using coupled harmonic oscillators. However, increasing the output power and broadening the bandwidth of vibrational energy harvesters simultaneously and maintaining a frequency response below 100 Hz for small-scale harvesters are among the main challenges. In general, and particularly for linear resonators, resonant frequency increases as scale decreases, which makes it difficult to realise small-scale energy harvesters in practice (most sources feature frequencies below 100 Hz).

Aspects and embodiments of the present invention have been devised with the foregoing in mind.

SUMMARY

According to an aspect, there is provided a vibrational energy harvester. The vibrational energy harvester may comprise a first mass. The first mass may comprise a first internal cavity. The vibrational energy harvester may comprise a second mass. The second mass may be disposed within the first internal cavity. The second mass may be configured to move within the first internal cavity. Movement of the second mass relative to the first mass may induce an electrical current in one of the first mass and the second mass. The vibrational energy harvester may comprise a housing. The housing may comprise a second internal cavity. The first mass may be disposed within the second internal cavity. The first mass may be configured to move within the second internal cavity. The vibrational energy harvester may further comprise an adjustment mechanism. The adjustment mechanism may be configured to adjust a size of the second internal cavity.

Common vibrational energy harvesters have a resonant frequency that is a characteristic of the device itself (given by the harvester's mass, spring constants and dimensions). Each device can therefore work only in specific applications targeted at harvesting particular specific vibration frequencies, since its resonant frequency cannot be changed or altered. By implementing an adjustment mechanism allowing for a size of the second internal cavity to be adjusted, a resonant frequency of the vibrational energy harvester can be adjusted. A single vibrational energy harvester may therefore be utilised in or tailored for a number of different applications each harvesting vibrations of a different frequency. Use of the vibrational energy harvester in multiple applications may be further enhanced if a resonant frequency of the vibrational energy harvester may be adjusted without adjusting an external size of the vibrational energy harvester.

Movement of the second mass within the first internal cavity defined by the first mass may allow impacts between the first mass and the second mass. Impacts between the first mass and the second mass may enable momentum transfer between the two masses, and may provide velocity amplification of the first mass and/or the second mass (which may increase a relative velocity between the two masses and therefore increase a magnitude of induced electrical current). Impacts between the two masses may also introduce non-linear mechanical effects, which may enable a frequency response or bandwidth of the vibrational energy harvester to be improved. Similar considerations may apply to impacts of the first mass with the housing of the vibrational energy harvester. Disposing the second mass inside the first mass may also reduce a volume of the vibrational energy harvester whilst providing advantages associated with a dual-mass vibrational energy harvester.

The adjustment mechanism may be configured to adjust the size of the second internal cavity without adjusting an external size of the vibrational energy harvester.

The housing may comprise a plurality of portions. At least one of the portions of the housing may be movable relative to one or more of the other portions of the housing. The adjustment mechanism may be or comprise at least one of the movable portions of the housing.

The vibrational energy harvester may further comprise a cam. The cam may be located within the housing. The cam may be coupled to the at least one movable portion of the housing. The second internal cavity may be defined between the cam and an internal surface of the housing. The internal surface of the housing may be or comprise an end surface of the housing.

An axial position of the cam relative to the one or more other portions of the housing may be adjustable in response to movement of the at least one movable portion of the housing relative to the one or more other portions of the housing. An axial position of the cam within the housing may be adjustable in response to movement of the at least one movable portion of the housing relative to the one or more other portions of the housing. Movement of the at least one movable portion of the housing may be rotational movement or axial movement (for example, in a longitudinal direction of the vibrational energy harvester) relative to the one or more other portions of the housing. A cam configured to move axially within the housing as a result of linear or axial force may be referred to as a slider cam or a slider. For example, a cam for which an axial position of the cam relative to the one or more other portions of the housing is adjustable in response to axial movement of the at least one moveable portion of the housing relative to the one or more other portions of the housing may be referred to as a slider cam or a slider.

The cam and the at least one movable portion of the housing may be coupled to one another. The coupling may allow or enable relative movement between the cam and the at least one movable portion of the housing. The cam and the at least one movable portion of the housing may be coupled via corresponding male and female connectors (for example, one or more female recesses or grooves configured to engage with corresponding male protrusions, flanges or tongues). Movement of the male connector within the female connector may enable relative movement between the cam and the at least one movable portion of the housing. The cam may comprise either of the male or female connectors, and the at least one movable portion may comprise the other of the male or female connectors. The female recess or groove may be one of a helical groove and a linear groove.

Alternatively, the coupling may not allow or enable relative movement between the cam and the at least one movable portion of the housing. The cam and the at least one movable portion of the housing may be coupled together and configured to move together (for example, as a single entity). The cam and the at least one movable portion of the housing may be coupled together and configured to move together relative to the one or more other portions of the housing. The coupled cam and the at least one movable portion of the housing may additionally be coupled to the one or more other portions of the housing, for example via one or more channels in the one or more other portions of the housing and one or more connecting elements (for example, one or more rods or beams). Each of the one or more connecting elements may engage with (for example, extend through) at least one of the one or more channels. The cam may be coupled to the at least one movable portion by the one or more connecting elements.

The second mass may be configured to move along a first fixed axis within the first internal cavity. The first mass may be configured to move along a second fixed axis within the second internal cavity. The first fixed axis and the second fixed axis may be coaxial or may be or comprise the same axis. Efficiency of energy harvesting may be improved by ensuring movement of each of the first mass and the second mass is directed along a respective fixed axis, and further improved or maximised by directing movement of each of the first mass and the second mass to be along the same axis or coaxial axes.

The vibrational energy harvester may further comprise one or more elements disposed at each end of the first internal cavity. The elements are configured to control oscillation of the second mass in the first internal cavity. The one or more elements may be or comprise one of magnetic springs and mechanical springs. The vibrational energy harvester may further comprise one or more elements disposed at each end of the second internal cavity. The elements may be configured to control oscillation of the first mass in the second internal cavity. The one or more elements may be or comprise one of magnetic springs and mechanical springs.

Elements for controlling oscillation of the first mass and/or the second mass may improve or maximise efficiency of energy harvesting by enabling kinetic energy of the first mass and the second mass to be recovered. For example, kinetic energy of the first mass may be transformed into and stored as potential energy on impact with the one or more elements configured to control oscillation, rather than dissipated as heat energy on impact with a surface of the housing. Stored potential energy may then in turn be transformed into kinetic energy of the first mass as the one or more elements propel the first mass in a different direction (for example, an opposite direction to its direction of travel before impact with the one or more elements for controlling oscillation). The same principle may be applied with respect to the elements controlling oscillation of the second mass. In this way, relative movement between the first mass and the second mass to induce an electrical current in one of the first mass and the second mass may be improved or maximised, in turn increasing or maximising efficiency of energy harvesting.

Magnetic springs may reduce or limit mechanical losses typically associated with mechanical springs. Magnetic springs may also reduce a resonant frequency of the vibrational energy harvester, as magnetic springs usually require or take up less physical space than mechanical springs. Magnetic springs may also introduce non-linear contributions to dynamics of the vibrational energy harvester, which may increase or improve a bandwidth or frequency response of the vibrational energy harvester. Mechanical springs may reduce the number of magnetic components which could interfere with magnetic material of the first mass or the second mass (which can potentially reduce a power output of the vibrational energy harvester).

The first mass may have a greater mass than the second mass. The second mass may have a greater mass than the first mass. The first mass may have substantially the same mass as the second mass. A mass ratio of the first mass to the second mass may be between substantially 1:1 and substantially 10:1, for example between substantially 1:1 and substantially 5:1, or between substantially 6:1 and substantially 10:1.

The first mass may comprise an electrically conductive material and the second mass may comprise a magnetic material. Alternatively, the second mass may comprise an electrically conductive material and the first mass may comprise a magnetic material. A mass of the first mass and/or the second mass, or a mass ratio of the first mass to the second mass may be altered or controlled to adjust a resonant frequency of the vibrational energy harvester. Different masses or mass ratios may be used to more efficiently harvest vibrational energy at different vibrational frequencies.

The first mass may be or comprise a plurality of electrically conductive coils. The plurality of coils may be wound using a single wire or using multiple wires. The coils may be wound such that adjacent coils alternate between being wound clockwise and being wound anti-clockwise. Alternatively, adjacent coils may be connected in series and anti-series. In this way, current cancellation may be minimised.

The second mass may be or comprise a stack of magnets. The stack of magnets may be arranged to maximise a magnetic field intensity along a length of the second mass. The stack of magnets may comprise one or more of a Halbach stack, a stack of oppositely axially polarised magnets and a stack of axially polarised magnets.

The vibrational energy harvester may comprise an electrical output portion. The electrical output portion may be electrically connected to the one of the first mass or the second mass in which an electrical is induced. The electrical output portion may be connected to the one of the first mass or the second mass in which an electrical current is induced via one or more conductive springs. Conductive springs may increase the operational lifetime of electrical contacts between the electrical output portion and the conductive material (compared to, for example, soldered electrical contacts). An elastic constant of the one or more conductive springs may be lower than an elastic constant of elements controlling oscillation of the first mass or the second mass in the second internal cavity or the first internal cavity respectively. Conductive springs having a lower elastic constant than elements controlling oscillation of the first mass or the second mass may provide a robust electrical connection without affecting the dynamic oscillatory behaviour of the first mass or the second mass.

A size of the second internal cavity may be adjustable in increments of between substantially 0.25 mm and 1.0 mm (for example in increments of substantially 0.5 mm). A size of the second internal cavity may be adjustable between a length of substantially 80 mm and a length of substantially 40 mm, or between a length of substantially 70 mm and a length of substantially 50 mm, or between a length of substantially 64 mm and a length of substantially 53 mm. Fine adjustment of the size of the second internal cavity may enable fine control of a resonant frequency of the vibrational energy harvester, allowing the vibrational energy harvester to maximise efficiency or energy harvesting across a number of applications at different resonant frequencies.

A resonant frequency of the vibrational energy harvester may be adjustable by substantially 40 Hz, or by substantially 30 Hz, or by substantially 20 Hz. A resonant frequency of the vibrational energy harvester may be adjustable by adjusting a size of the second internal cavity.

A resonant frequency of the vibrational energy harvester may be adjustable between substantially 5 Hz and substantially 45 Hz, or between substantially 10 Hz and substantially 40 Hz, or substantially 15 Hz and substantially 35 Hz. A resonant frequency of the vibrational energy harvester may be adjustable by adjusting a size of the second internal cavity.

Features which are described in the context of separate aspects and embodiments of the invention may be used together and/or be interchangeable, and these embodiments are specifically envisaged. Similarly, where features are, for brevity, described in the context of a single embodiment, these may also be provided separately or in any suitable sub-combination.

It should be noted that the figures are diagrammatic and may not be drawn to scale. Relative dimensions and proportions of parts of these figures may have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings.

The same or like reference signs are generally used to refer to corresponding or similar features in modified and/or different embodiments. Features which are described in the context of separate aspects and embodiments of the invention may be used together and/or be interchangeable wherever possible. Similarly, where features are, for brevity, described in the context of a single embodiment, these may also be provided separately or in any suitable sub-combination.

DETAILED DESCRIPTION

FIG.1shows an embodiment of a vibrational energy harvester100according to the invention. The vibrational energy harvester100comprises a first mass101. The first mass101comprises or defines a first closed internal cavity102. The first mass101also comprises or is manufactured from an electrically conductive material (e.g., copper). The vibrational energy harvester100also comprises a second mass103. The second mass103comprises or is manufactured from a magnetic material (e.g., the second mass103may be a magnet). The magnetic material may be or comprise at least one of Neodymium Iron Boron (NdFeB), AlNiCo magnetic alloy, Samarium Cobalt, Strontium Ferrite and Barium Ferrite. The second mass103is disposed within the first closed internal cavity102. The second mass103does not fill the first closed internal cavity102, and is configured to move within the first closed internal cavity102.

Movement of the magnetic material of the second mass103(in the first closed internal cavity102) relative to the electrically conductive material of first mass101induces an electrical current in the conductive material of the first mass101. Alternatively, the first mass101may comprise or be manufactured from a magnetic material, and the second mass103may comprise or be manufactured from an electrically conductive material. The first mass101may have a greater mass than the second mass103, or vice versa. Alternatively, the first mass101and the second mass103may have the same mass. In some embodiments, the mass ratio of the first mass to the second mass is substantially 5:1. Alternatively, the mass ratio of the first mass to the second mass may be greater than substantially 5:1, for example, between substantially 6:1 and substantially 10:1.

The vibrational energy harvester100also comprises a housing104. The housing104comprises a second internal cavity105. The first mass101is disposed within the second closed internal cavity105. The first mass101does not fill the second closed internal cavity105, and is configured to move within the second closed internal cavity105. In the embodiment shown, the first mass101is arranged between resilient elements106a,106b. In the embodiment shown, the resilient elements106a,106bare disposed at (e.g., fixed to) opposite ends of the first mass101. The resilient elements106a,106bmay be springs. The resilient elements106a,106bare configured to control movement (e.g., oscillation) of the first mass101within the second closed internal cavity105. Alternatively, the resilient elements106a,106bmay be disposed on (e.g., fixed to) each end of the second closed internal cavity105and opposite ends of the first mass101. In another alternative embodiment, the resilient elements106a,106bare disposed on (e.g., fixed to) each end of the second closed internal cavity105only, such that first mass101is able to move independently of the resilient elements106a,106b. In such an embodiment, resilient elements106a,106bare still configured to control oscillation of the first mass101within the second closed internal cavity105.

The first closed internal cavity102and the second closed internal cavity105need not be strictly ‘closed’ e.g., air-tight or fluid-tight. It is sufficient that the first closed internal cavity102and the second closed internal cavity105each provide a cavity surface through which the second mass103and the first mass101respectively cannot pass through on impact. For example, such a cavity surface may be provided by providing a first closed internal cavity102defined by one or more walls comprising a plurality of wall regions separated from one another by a distance less than a minimum dimension of the second mass103(e.g., the one or more walls may comprise a grid, grill or grating structure).

In the embodiment shown inFIG.1, the first closed internal cavity102comprises an elongated portion (e.g., in an axial or longitudinal direction as shown by arrow A inFIG.1) having a constant or uniform cross-sectional shape (e.g., in a transverse direction as shown by arrow B inFIG.1). The cross-sectional shape may comprise a substantially circular shape, a substantially square shape, a substantially polygonal shape or any other suitable shape (e.g., the first closed internal cavity102may be substantially cylindrical, substantially cuboidal or substantially prismatic). In the embodiment shown, the second mass103comprises a cross-sectional shape (e.g., in the transverse direction) substantially similar to the cross-sectional shape of the elongated portion of the first closed internal cavity102. The first closed internal cavity102of the first mass101is therefore configured to ensure movement of the second mass103in a fixed direction (e.g., along a longitudinal axis of the elongated portion of the first closed internal cavity102). Directing movement of the second mass103along a fixed axis may allow energy to be harvested most efficiently by ensuring the second mass103is oriented relative to the first mass in order to maximise the induced electrical current (e.g., such that movement of the second mass103causes a maximal magnetic flux of the magnetic material of the second mass103to be cut by the conductive material of the first mass101). Alternatively, the first closed internal cavity102and the second mass103may be or comprise different shapes or configurations. The first closed internal cavity102may not be configured to limit movement of the second mass103to be along a fixed axis or in a fixed direction. The second mass103may be able to move in any direction within the first closed internal cavity102.

Similarly, in the embodiment shown inFIG.1, the second closed internal cavity105comprises an elongated portion (e.g., in an axial or longitudinal direction as shown by arrow A inFIG.1) having a constant or uniform cross-sectional shape (e.g., in a transverse direction as shown by arrow B inFIG.1). The cross-sectional shape may comprise a substantially circular shape, a substantially square shape, a substantially polygonal shape or any other suitable shape (e.g., the second closed internal cavity105may be substantially cylindrical, substantially cuboidal or substantially prismatic). In the embodiment shown, the first mass101comprises a cross-sectional shape substantially similar to the cross-sectional shape of the elongated portion of the second closed internal cavity102. The second closed internal cavity105of the housing104is therefore configured to ensure movement of the first mass101is in a fixed direction (e.g., along a longitudinal axis of the elongated portion of the second closed internal cavity105). In the embodiment shown, the longitudinal axis of the elongated portion of the first closed internal cavity102is coaxial with the longitudinal axis of the elongated portion of the second closed internal cavity105. Directing movement of the second mass103may allow energy to be harvested most efficiently by concentrating elastic force produced by the first mass101oscillating between the resilient members106a,106balong a single direction in order to maximise relative movement between the first mass101and the second mass103.

FIG.2shows an embodiment of a vibrational energy harvester200according to the invention. The vibrational energy harvester200comprises corresponding features to the vibrational energy harvester100as shown inFIG.1. In addition, the vibrational energy harvester200comprises magnets or magnetic portions207a,207blocated at (e.g., provided at or on) opposite ends of a first closed internal cavity202. In the embodiment shown, the magnetic portions207a,207bextend from an internal surface of a first closed internal cavity202. Alternatively, the magnetic portions207a,207bmay be embedded in an internal surface of the first closed internal cavity202(e.g., embedded in a recess in the first mass101), or integrally formed with the first mass201. The magnetic portions207a,207bact as magnetic springs between which a second mass203comprising or manufactured from a magnetic material (e.g., the second mass203may be a magnet) can oscillate. The magnetic material may be or comprise at least one of Neodymium Iron Boron (NdFeB), AlNiCo magnetic alloy, Samarium Cobalt, Strontium Ferrite and Barium Ferrite. Using magnetic springs in conjunction with the second mass203may help to reduce or limit mechanical losses that are typically associated with mechanical springs (e.g., compression springs). Magnetic springs may also reduce the resonant frequency of the vibrational energy harvester200, enabling energy from vibration frequencies typically associated with human motion (e.g., below 100 Hz, such as between 2 Hz and 20 Hz) to be harvested. Low resonant frequencies are generally achieved by increasing both a mass and a size of vibrational energy harvesters. Magnetic springs enable energy to be harvested from low frequencies (e.g., below 100 Hz, such as between 2 Hz and 20 Hz) without increasing a mass or a size of the vibrational energy harvester200, as magnetic springs can require less space than mechanical springs. Magnetic springs also introduce non-linear contributions to the dynamics of the vibrational energy harvester200that can increase the bandwidth of the vibrational energy harvester200. In contrast, resilient elements206a,206bare mechanical elements such as compression springs which exhibit mechanical elastic behaviour to govern oscillation of a first mass201. Resilient elements206a,206bdiffer from magnetic springs in that resilient elements206a,206butilise elastic force generated in response to mechanical deformation (hence, mechanical elements) of the resilient elements206a,206bto govern oscillation of the first mass201, rather than a magnetic interaction as utilised in magnetic springs. Using mechanical elements such as compression springs in place of magnetic springs to control oscillation of the first mass201may reduce the number of magnetic components which can interfere with magnetic material of the second mass203and potentially reduce a power output of the vibrational energy harvester200.

In alternative embodiments, the second mass203may oscillate between mechanical springs similar to resilient elements206a,206b. In alternative embodiments, magnetic springs are used in place of resilient elements206a,206b(it will be appreciated this also applies with respect resilient elements106a,106bof the vibrational energy harvester100shown inFIG.1).

In the embodiment shown inFIG.1(and similarly inFIG.2), the first mass101and the second mass103are configured to impact each other whilst moving within the first closed internal cavity102and the second closed internal cavity105respectively. For example, whilst the first mass101is moving within the second closed internal cavity105, an internal surface of the first closed internal cavity105may impact the second mass103. It will be appreciated that the same is true for the first mass201and the second mass203of the vibrational energy harvester200as shown inFIG.2.

Impact between the first mass101and the second mass103allows momentum transfer between the two masses and can provide velocity amplification of the first mass101and/or the second mass103, depending on the relative masses of the first mass101and the second mass103. For example, the second mass103may be moving at a first velocity within the first mass101, whilst the first mass101may be moving at a second velocity. A collision or impact between the second mass103and the first mass101may transfer momentum from the first mass101to the second mass103such that the second mass103travels at a third velocity having a greater amplitude (i.e., speed) than the first velocity of the second mass103. Velocity amplification may therefore result in an increased speed of movement of the second mass103relative to the first mass101, or vice versa. An increased speed of movement of the second mass103relative to the first mass101increases a magnitude of the induced electrical voltage in the conductive material of the first mass101, which results in a greater induced electrical current in the conductive material of the first mass101. In turn, an output power of the vibrational energy harvester100is increased. Locating the second mass103inside the first mass101can reduce an overall volume of the vibrational energy harvester100. Such a configuration also enables simple implementation of velocity amplification to improve an output of the vibrational energy harvester100. Impacts between the first mass101and the second mass103can also introduce non-linear mechanical effects which can enhance the frequency response of the vibrational energy harvester100. The non-linear effects introduced by impacts between the first mass101and the second mass103can increase the bandwidth relative to simple linear spring-mass resonator systems. The overall power spectrum of the vibrational energy harvester100is the result of the superposition of all of its vibrational modes. Similarly, impacts between the first mass101and the housing104can also provide velocity amplification. It will be appreciated that the same is true for the first mass201, the second mass203and the housing204of the vibrational energy harvester200as shown inFIG.2.

In the embodiment shown, the second mass103,203or comprises one or more magnets. In some embodiments, the second mass103,203is or comprises a stack of magnets. In some embodiments, magnets in the stack of magnets forming the second mass103,203are arranged to maximise the gradients in the magnetic field. In some embodiments, magnets in the stack of magnets forming the second mass103,203are arranged to maximise the magnetic field intensity along a length of the second mass103,203(rather than concentrating magnetic field intensity at opposing poles or ends of the second mass103,203). The stack of magnets forming the second mass103,203may comprise one or more of a Halbach stack, a stack of opposing axially polarised magnets and a stack of axially polarised magnets.

In typical vibrational energy harvesters, a conductive material is disposed in a fixed location on or relative to a housing, rather than on a moving, oscillating or vibrating part of the vibrational energy harvester. Durability of electrical contacts between the conductive material and an electrical output portion, e.g., connector (usually located on the housing), which can be used to deliver the electrical power generated by the vibrational energy harvester to an external device, is therefore not an issue. In contrast, during operation of the vibrational energy harvester100the conductive material of the first mass101is moving relative to the housing104to maximise the relative velocity between the conductive material of the first mass101and the magnetic material of the second mass103. In the embodiment shown inFIG.1(and similarly inFIG.2), the housing104comprises a connector104awhich is configured to deliver the electrical power generated by the vibrational energy harvester to an external device such as wireless sensor nodes and low power electronic devices. In the embodiment shown, the conductive material of the first mass101is electrically connected to the connector104avia one or more mechanical springs108. The springs108may be connected directly to the conductive material of the first mass101. Alternatively an intermediate conductor (e.g., a wire) may connect the conductive material of the first mass101to the springs108(e.g., if the conductive material of the first mass101is not located at least in part near an end of the first mass101). In some embodiments, the elastic constant of the springs108is lower than the elastic constant of the resilient elements106a,106bso as not to affect the oscillatory behaviour of the first mass101within the housing104(e.g., the dynamics of the first mass101are not affected). Connecting the conductive material of the first mass101to the connector104ausing the springs108may increase the operational lifetime of the electrical contacts between the conductive material of the first mass101and the connector104a. The springs108may be more robust and be less susceptible to vibrational motion over numerous operational cycles when compared to, for example, typical soldered contacts. In some embodiments, the springs108comprise or are manufactured from stainless steel. It will be appreciated that the above is true of springs208of the vibrational energy harvester200as shown inFIG.2. Alternatively, the conductive material of the first mass101,201may be connected to the connector104a,204avia soldered contacts and a length of conductive wire.

FIG.3Ashows an embodiment of a first mass301configured to be used in a vibrational energy harvester such as the vibrational energy harvesters100,200described above. In the embodiment shown, the first mass301comprises one or more conductive coils or windings309forming the conductive material of the first mass301. In some embodiments, the coils or windings309are or comprise copper. The first mass301comprises a hollow core310. The hollow core310may be open at one or both ends, or may be closed at both ends. In the embodiment shown, a plurality of arms or projections311extend outwards from the hollow core310, as shown more clearly inFIG.3B. In the embodiment shown, the core310comprises a substantially cylindrical shape, but the core310may alternatively be or comprise any suitable shape (e.g., the core310may be substantially cuboidal or substantially prismatic). The plurality of arms311is disposed on the core310equidistant from one another along an axial length of the core310. Alternatively, the arms311may be disposed at varying intervals along an axial length of the core310. A space is formed between each adjacent pair of arms311in which each of the coils or windings309is disposed. The arms311may help to locate the coils309accurately and prevent movement of the coils309relative to each other during vibration of the vibrational energy harvester100,200. In the embodiment shown, the coils309are wound from a single length of wire. Each of the coils309may be wound clockwise or anticlockwise. Adjacent coils309may alternate between being wound clockwise and being wound anti-clockwise to minimise current cancellation. In some embodiments, the coils309are wound from multiple lengths of wire that are electrically connected to each other. Adjacent coils309are connected in series and anti-series in order to minimise current cancellation. In the embodiment shown, the first mass301comprises seven coils309, but may alternatively comprise any number of coils (e.g., 1, 2, 3, 4, 5, 6, 8, 9, 10 and so on). In the embodiment shown, each of the arms311comprises one or more arm portions311aseparated by one or more channels or gaps312. Adjacent coils309may be connected to one another via one or more of the channels or gaps312. In some embodiments, the arms311are not used or present. In some embodiments, each of the coils309may not be disposed between adjacent arms311extending from the core310, but may be disposed on an external surface of the core310of the first mass301(e.g., wrapped around the core310). In some embodiments, the core310is manufactured from polyoxymethylene (POM). Alternatively, the core310may be manufactured from at least one another suitable material such as polycarbonate, acetal, Teflon®, a nonmagnetic material such as aluminium etc. In some embodiments, the core310is manufactured using three-dimensional (3D) printing.

FIGS.3C and3Dshow a cap313configured, together with the core310as shown inFIGS.3A and3B, to form a first closed internal cavity302of the first mass301. As noted above, in some embodiments, the core310may be open at one or both ends. A cap313fits over one or both open ends of the core310, forming the first closed internal cavity302in which a second mass103may be disposed and move within. In the embodiment shown inFIG.3C, an external (e.g. top) surface of the cap313comprises a recess314for receiving a resilient element106a,106b. Walls enclose and define the recess314. The walls have a height h sufficient to prevent over-compression of (and therefore damage to) the resilient element106a,106bduring oscillation of the first mass301. An end surface of the walls of the recess314also functions as a mechanical stopper for impact between the first mass301and the second mass103,203. The same is true with respect to resilient elements206a,206bas shown inFIG.2. Alternatively, the recess314may be used to locate magnetic material to form one or more magnetic springs configured to control oscillation of the first mass301.

In the embodiment shown inFIGS.3A and3B, the coils309are disposed on the first mass301which is configured to move within a housing104(see discussion ofFIG.1above). As discussed above, that arrangement is such that relative velocity between the coils309and the second mass103can be maximised. As also discussed above, the conductive material of the first mass301may be connected to a connector104avia one or more springs108. In the embodiment shown inFIG.3C, a hook or aperture315is provided on or in the top surface of the cap313. The hook315is configured to connect to a spring108(not shown) to form an electrical contact between the coils309and the connector104a(not shown, but shown inFIG.1). The springs108may be connected to only one of the caps313. Alternatively, the springs108may be connected to the conductive material of the first mass301(e.g., coils309), either directly or via the cap313, using, for example, a conductive adhesive or another conductive connection mechanism. A cap313in accordance with the above may be used with the springs208and connector204aof the vibrational energy harvester200as shown inFIG.2.

FIG.3Dshows an internal (e.g., bottom) view of the cap313. In the embodiment shown, an underside of the cap313comprises a recess316configured to contain magnetic portions207a,207b(discussed with respect toFIG.2above). A depth of the cavity may be equal to or greater than a thickness of the magnetic portions207a,207b. By retaining the magnetic portions207a,207bwithin the recess316of the cap313, the magnetic portions207a,207bare prevented from colliding directly with the second mass203during vibration of the vibrational energy harvester100. An inner surface of the cap313(level with an open end of the recess316) acts as a mechanical stopper for impacts between the first mass301and the second mass103,203. Alternatively, the underside of the cap313may not comprise a recess, and the magnetic portions207a,207bmay extend from the underside of the cap313into the first closed internal cavity302defined by the cap313and the core310. In the embodiment shown, the underside of the cap313also comprises one or more grooves317. In some embodiments, the one or more grooves317align with one or more corresponding mating flanges317a. The mating flanges371aare located on one or both ends of the core310. In some embodiments, interaction of the grooves317with the mating flanges317alocates and retains or locks in place the cap313on the core310. Additionally, one or more grooves318are located on an internal surface of the core310(the core310at least partially defining the first internal cavity302, as shown inFIG.3B). The grooves318are configured to receive corresponding flanges on the second mass103(not shown, but discussed above with respect toFIG.1) to minimise contact points (and therefore friction) between the first mass301and the second mass103whilst simultaneously controlling the movement of the second mass103within the first mass101. The interaction of the grooves318and the corresponding flanges on the second mass103may also prevent rotation of the second mass103within the first closed internal cavity302whilst the second mass103is oscillating or moving within the first closed internal cavity302. It will be appreciated that the above may be utilised in respect of the second mass203of the vibrational energy harvester200as shown inFIG.2. Alternatively, the core310may not comprise respective grooves318, and the second mass103,203may not comprise corresponding flanges.

FIG.3Eshows the embodiment of the first mass301ofFIGS.3A to3Din an assembled state together with springs108configured to connect conductive material of the first mass301to the connector104a,204a(not shown) of the housing104,204of the vibrational energy harvester100,200as shown inFIGS.1and2.

FIG.4shows an embodiment of a vibrational energy harvester400in accordance with the invention. The vibrational energy harvester400comprises an adjuster419configured to adjust a size of a second closed internal cavity405comprised by a housing404of the vibrational energy harvester400, without altering an overall size of the vibrational energy harvester400. In the embodiment shown, a length of the second closed internal cavity405(e.g., in an axial or longitudinal direction, as indicated by the arrow A inFIGS.4A and4B) is adjustable by the mechanism419. In the embodiment shown, the vibrational energy harvester400comprises a housing404having a first (e.g., upper) portion420aand a second (e.g., lower) portion420b. A cam421is disposed within the housing404. The second closed internal cavity405is formed by the housing404and the cam421. The adjuster419is disposed between the upper portion420aand the lower portion420bof the housing404. In the embodiment shown, the adjuster419forms a part of the housing404. The adjuster419is rotatably coupled to both the upper portion420aand the lower portion420bof the housing404. The housing404therefore comprises two fixed portions420a,420band a moveable portion419, one of the fixed portions disposed either side of the moveable portion (e.g., adjuster419). Alternatively, the housing404may comprise one fixed portion and one moveable portion. For example, the adjuster419may form one of the upper portion420aand the lower portion420bof the housing404. The adjuster419may be rotatably coupled to the other portion420a,420bof the housing404. The adjuster419may be connected or coupled to the other portion(s)420a,420bof the housing404using an O-ring seal to prevent water penetration into the vibrational energy harvester400. Alternatively, the housing404may comprise a plurality of portions that are each moveable or rotatable with respect to one another.

The adjuster419is coupled to the cam421such that rotation of the adjuster419relative to the upper portion420aand the lower portion420bof the housing404causes axial displacement of the cam421within the housing404. Axial displacement of the cam421within the housing404causes the position of the cam421within the housing to change, thereby adjusting a size (e.g., length in an axial or longitudinal direction) of the second closed internal cavity405. The size of the second closed internal cavity405is therefore adjustable using the adjuster419. This is illustrated inFIGS.4A and4B, wherein a length L of the second closed internal cavity405is greater inFIG.4Athan inFIG.4B. This is also illustrated inFIGS.4C and4D(corresponding toFIGS.4A and4Brespectively), which shows an isometric view of the embodiment ofFIGS.4A and4Bwithout the upper portion420aof the housing404. The cam421is situated higher within the housing404inFIGS.4A and4Cthen inFIGS.4B and4D, illustrating how the size of the second closed internal cavity405may be adjusted.

By adjusting the length L of the second closed internal cavity405(i.e., by adjusting a single geometrical parameter of the vibrational energy harvester400), a resonant frequency of the vibrational energy harvester400can be tuned. For example, an outer diameter D of the vibrational energy harvester400may be approximately 40 mm, and an overall external length l of the vibrational energy harvester400may be approximately 74 mm (e.g., a size comparable with a D battery). By changing a length L of the second closed internal cavity405from, for example, approximately 64 mm to approximately 53 mm, a resonant frequency of the vibrational energy harvester can be adjusted by up to substantially 20 Hz (e.g., between substantially 15 Hz and 35 Hz). For a second internal cavity405having a length adjustable between substantially 80 mm and substantially 40 mm or between substantially 70 mm and substantially 50 mm, a resonant frequency of the vibrational energy harvester can be adjusted by up to substantially 40 Hz (e.g., between substantially 5 Hz and substantially 45 Hz) or substantially 30 Hz (e.g., between substantially 10 Hz and substantially 40 Hz) respectively. The frequency response of the vibrational energy harvester can also be altered by modifying an elastic constant of elements configured to control oscillation of the first mass101,201,301and/or the second mass103,203(e.g., resilient elements such as springs, or magnetic springs). Additionally, the frequency response of the vibrational energy harvester400may be altered by modifying a mass of the first mass101,201,301and/or the second mass103,203. This concept can be extended to vibrational energy harvesters400of different sizes, but is particularly suitable for small, low mass devices intended to harvest energy from lower frequencies, e.g., below 100 Hz, or frequencies typically associated with human motion, industrial machinery (such as pumps and compressors) and various types of transport (such as automobiles, aircraft, rail vehicles). An adjustable vibrational energy harvester400for which the resonant frequency can be tuned may be used to harvest energy efficiently in a variety of operating environments or applications, each exhibiting (a range of) different primary vibration frequencies. In conjunction with velocity amplification as discussed above, a small sized, low mass device such as vibrational energy harvester400can be used to provide improved output power at a variety of lower frequencies typically associated with human motion. Alternatively, an overall length l of the vibrational energy harvester may be adjusted to adjust the length L of the second closed internal cavity405in an axial or longitudinal direction A. It will be appreciated that the housing404may be implemented with the vibrational energy harvesters100,200as shown inFIGS.1and2.

The adjuster419and the cam421are shown in greater detail inFIGS.5A and5Brespectively. In the embodiment shown, the adjuster419is configured to be coupled to the cam421via one or a plurality of helical channels422disposed on an outer surface of the cam421, the or each of the helical channels422configured to engage with a or one of a plurality of corresponding protrusions423on an internal surface of the adjuster419. In the embodiment shown, three helical channels422and three protrusions423are included (one protrusion422to locate and move in each channel423), but it will be appreciated that any number of corresponding helical channels422and protrusions423could be used. When assembled, and when the adjuster419is rotated relative to the other portion(s)420a,420bof the housing404, the protrusions423move within and follow the helical channels422, resulting in axial displacement of the cam421within the housing404. It will be appreciated that other arrangements could be implemented to couple the adjuster419to the cam421such that rotation of the adjuster419relative to other portion(s)420a,420bof the housing404results in axial displacement of the cam421within the housing404. For example, the adjuster419may be coupled to the cam421via corresponding screw threads on an internal surface of the adjuster419and an external surface of the cam421respectively.

In the embodiment shown inFIGS.4A to4D and5A, the cam421has or comprises a hollow cylindrical shape. In the embodiment shown, the cam421has one closed end and one open end. A first mass101(not shown, for example as described above) is configured to be located within and move (e.g., oscillate) within the second closed internal cavity405defined by the cam421and the housing404. A resilient element106a,106b(not shown) is configured to be located at the closed end of the hollow cylindrical shape of the cam421. In the embodiment shown, the first mass101is configured to be at least partially disposed within the hollow cylindrical shape of the cam421at any point of oscillation of the first mass101within the second closed internal cavity405. In the embodiment shown, one or more grooves424on an internal surface of the cam421are configured to engage with one or more corresponding flanges (not shown) on the first mass101to prevent rotation of the first mass101during oscillation within the second closed internal cavity405, and to reduce friction by minimising contact points between the first mass101and the cam421.FIG.6shows an embodiment of a cap513(forming a part of the first mass101) comprising flanges525on an external surface of the cap513configured to engage with the grooves424of the cam421. In some embodiments, the cam421may be open at both ends, but may comprise at one end an annular ring or flange on which a resilient element106a,106bmay be disposed and configured to interact with the first mass101. Alternatively, the first mass101may not comprise flanges, and the internal surface of the cam421may not comprise corresponding grooves. The first mass101may be free to rotate as it oscillates within the second closed internal cavity405. It will be appreciated it that the cam421and the housing404may also be implemented with the first masses201,301as shown inFIGS.2and3.

Alternatively, the cam421may be or comprise a solid cylindrical shape. A resilient element106a,106bmay be disposed between (e.g., in contact with both) a flat external (e.g., top) surface of the solid cylindrical cam421and the first mass101. The cam421may not comprise a hollow cylindrical portion configured to at least partially enclose the first mass101during oscillation of the first mass101within the second closed internal cavity405. The first mass101may comprise one or more flanges (not shown) configured to engage with one or more corresponding grooves on an internal surface of the housing404to prevent rotation of the first mass101during oscillation. Alternatively, the first mass101may not comprise flanges, and an internal surface of the housing404may not comprise corresponding grooves. The first mass101may be free to rotate as it oscillates within the second closed internal cavity405. An outer surface of the solid cylindrical cam421may comprise one or more helical channels422each configured to engage with one of a plurality of corresponding protrusions423on an internal surface of the adjuster419, as described above (or alternatively, an outer surface of the cam421and an internal surface of the adjuster419may comprise corresponding screw threads, as described above).

The cam421may be provided with a detent to hold the cam421in a particular axial position with respect to the housing404. The detent may be or comprise a plunger, catch or other releasable fixing mechanism. For example, returning toFIG.5A, in the embodiment shown, the cam421comprises a first plurality of apertures, holes or indentations426located equidistant from one another along an axial length of the cam421. The apertures, holes or indentations426are configured to interact with a corresponding spring-loaded plunger (not shown) located on an internal surface of the housing404. The spring-loaded plunger is configured to engage with one of the holes426to prevent unintended axial displacement of the cam421relative to the housing404during vibration of the vibrational energy harvester400. However, the spring-loaded plunger is also configured to allow axial displacement of the cam421if sufficient force is applied to overcome the elastic resistance of the spring-loaded plunger during rotation of the adjuster419. The plurality of holes426are spaced apart from one another according to a desired incremental change in size (e.g., length) of the second closed internal cavity405. For example, the plurality of holes may be spaced between substantially 0.25 mm and 1 mm apart from one another, for example substantially 0.5 mm apart from one another (e.g., measuring from the centre of adjacent holes), allowing fine adjustment of the size of the second closed internal cavity405. Increments may be indicated to a user of the vibrational energy harvester400using indicia on an outer surface of one or both of the adjuster419and the housing404, to indicate an axial position of the cam421within the housing404. In some embodiments, further pluralities of holes426may be disposed at different radial locations around the cam421, each configured to interact with a separate spring-loaded plunger. Alternatively, a frictional force between the protrusions423and the helical channels422(or alternatively, for example, corresponding screw threads) on the cam421and the adjuster419respectively may be large enough to resist unintended axial displacement of the cam421relative to the housing during vibration of the vibrational energy harvester400.

In other embodiments, a substantially similar approach for adjusting a size of a second internal cavity of the vibrational energy harvester may be used for other device geometries in accordance with the present disclosure. For example, a vibrational energy harvester in accordance with the present disclosure may be or comprise a substantially non-cylindrical shape. In some embodiments, a vibrational energy harvester is or comprises a substantially parallelepiped shape.FIG.5Cshows a cam or slider (for example, a cam configured to move axially as a result of linear, axial force rather than on application of rotational force)421′ having a substantially rectangular parallelepiped shape. The function of the cam or slider421′ is substantially similar to the function of the cam421described with respect toFIGS.4A to4D,5A and5B. In the embodiment shown, the slider421′ is hollow. The slider421′ comprises one closed end. A first mass101is configured to oscillate in a second closed internal cavity defined between an internal surface of a housing of the vibrational energy harvester and the closed end of the slider421′, similar to the cam421described above. Alternatively, the slider421′ may be solid, such that the first mass101is configured to oscillate in a second closed internal cavity defined between an internal surface of a housing the vibrational energy harvester and an outer surface of the cam421′. In the embodiment shown, the hollow recess defined by the slider421′ is substantially the same shape as the slider421′ (e.g., a substantially rectangular parallelepiped). Alternatively, the hollow recess defined by the slider421′ may be or comprise a different shape to the slider421′. The hollow recess defined by the slider421′ is configured to at least partially accommodate a first mass101. In some embodiments, the first mass101has a substantially identical shape to the shape of the hollow recess of the slider421′. For example, the hollow recess of the slider421′ may be or comprise a substantially cylindrical shape, for example to accommodate a substantially cylindrical first mass101(such as that described above and shown inFIGS.3A to3E).

One or more of the first mass101, an adjuster419′ and one or more portions of a housing of the vibrational energy harvester having a substantially rectangular parallelepiped shape may be or comprise a substantially rectangular parallelepiped shape. As shown inFIG.5C, the adjuster419′ is configured to be coupled to the slider421′ via one or a plurality of channels422′ disposed on an outer surface of the slider421′. The channels422′ may extend in a direction substantially parallel to a longitudinal axis of the vibrational energy harvester400′. The channels422′ may have a depth less than a wall thickness of a hollow slider421′. Each of the channels422′ is configured to engage with a or one of a plurality of corresponding protrusions423′ on an internal surface of the adjuster419′. In the embodiment shown, two channels422′ and two protrusions423′ are shown, but it will be appreciated that any number of corresponding channels422′ and protrusions423′ could be used. In the embodiment shown, the channels422′ are substantially linear. When the vibrational energy harvester is assembled, the adjuster419′ may be displaced axially relative to the cam421, following a path of the protrusions423′ within the channels422′. This is illustrated in more detail inFIGS.5D and5Erespectively.FIG.5Dshows a vibrational energy harvester400′ comprising a housing404′, an adjuster419′ and a slider421′. In the embodiment shown, the adjuster419′ forms a part of the housing404′. The housing404′ comprises an upper portion420a′ and a lower portion420b′. In the embodiment shown, the adjuster419′ is affixed to the upper portion420a′ of the housing404′. When the adjuster419′ is axially displaced, for example by applying an axial force to the adjuster419′, (with protrusions423′ of the adjuster419′ travelling within channels422′ of the slider421′), a size (e.g., a length in an axial or longitudinal direction) of a second closed internal cavity405′ of the vibrational energy harvester400′ (in the embodiment shown, defined between a closed end of the hollow slider421′ and an internal surface of the upper portion420a′ of the housing404) may be increased or decreased. This is illustrated by the difference in a length L inFIGS.5D and5Erespectively, with the adjuster419′ positioned at different axial displacements relative to the slider421′ (L indicating a size of the second closed internal cavity405′). In so doing, a resonant frequency of the vibrational energy harvester400′ may be adjusted in a similar manner as that described above. To retain the slider421′ in a desired axial position relative to the housing404′, a detent may be provided. For example, the slider421′ may comprise a plurality of apertures, holes or indentations in an outer surface of the slider421′ The apertures, holes or indentations may be located equidistant from one another along an axial length of the slider421′. The apertures, holes or indentations may be configured to engage with a corresponding spring-loaded plunger located on an internal surface of the adjuster419′ or the upper portion420a′ of the housing404′, substantially as described in respect of the vibrational energy harvester400. The skilled person will be aware that such an arrangement could be implemented in a vibrational energy harvester having a different device geometry other than substantially parallelepiped (e.g., substantially rectangular parallelepiped). For example, the arrangement described with respect toFIGS.5C to5Ecould be implemented in a vibrational energy harvester having a cylindrical device geometry, or having a polygonal prism device geometry.

In the embodiment shown inFIGS.5D and5E, an external size of the vibrational energy harvester400′ increases as the size of the second closed internal cavity405′ increases. In alternative embodiments of a vibrational energy harvester400″, as shown inFIGS.5F and5G, an adjuster419″ may be affixed to a slider421″ rather than to an upper portion420a″ of a housing404″. One or more sides of the housing404″ (e.g., one or more sides of the upper portion420a″ of the housing404″) may comprise a channel422″ extending through a thickness of the housing404″. The channels422″ may extend in a direction substantially parallel to an axial direction (e.g., a longitudinal direction) of the vibrational energy harvester400″. The adjuster419″ may form a part of the housing404″ (e.g., it may form a part of an external surface of the vibrational energy harvester400″). The adjuster419″ may be affixed to the slider421″ (located within the housing404″) via one or more protrusions423″. Each protrusion423″ may extend through a channel422″ in a side of the housing404″ and be attached to both the adjuster419″ and the slider421″ (as indicated by the hatching inFIGS.5F and5G). A width of the adjuster419″ may be larger than a width of a channel422″ to prevent the adjuster from falling through a channel422″. Accordingly, axial movement or displacement (e.g., in a longitudinal direction of the vibrational energy harvester400″) of the adjuster419″ relative to the housing404″ results in axial movement or displacement of the slider421″ within the housing404″. Axial movement of the adjuster419″ and the slider421″ is guided by the engagement or interaction of the protrusions423″ with the channels422″. A size of a second closed internal cavity405″ in the housing404″ (in the embodiment shown, defined between an internal surface of the housing404″ and a closed end of the hollow slider421″) of the vibrational energy harvester may be increased or decreased. This is illustrated by the difference in a length L inFIGS.5F and5Grespectively, with the adjuster419″ (and therefore the slider421″) positioned at different axial displacements relative to the housing404″ (L indicating a size of the second closed internal cavity405″). In so doing, a resonant frequency of the vibrational energy harvester400″ may be adjusted in a similar manner as that described above. To retain the slider421″ in a desired axial position relative to the housing404″, a detent may be provided. For example, the slider421″ may comprise a plurality of apertures, holes or indentations in an outer surface of the slider421″. The apertures, holes or indentations may be located equidistant from one another along an axial length of the slider421″. The apertures, holes or indentations may be configured to engage with a corresponding spring-loaded plunger located on an internal surface of the housing404′, substantially as described in respect of the vibrational energy harvester400. Alternatively, a spring-loaded plunger may be provided on an external surface of the slider421″ and configured to interact with apertures, holes or indentations in an inner surface of the housing404″. Alternatively, a spring-loaded plunger may be provided on an internal surface of the adjuster419″ and configured to interact with apertures, holes or indentations in an outer surface of the housing404″. The embodiment shown inFIGS.5F and5Genables a size of the second closed internal cavity405″ to be adjusted without altering an external size of the vibrational energy harvester400″. The skilled person will be aware that such an arrangement could be implemented in a vibrational energy harvester having a different device geometry other than substantially parallelepiped (e.g., substantially rectangular parallelepiped). For example, the arrangement described with respect toFIGS.5F to5Gcould be implemented in a vibrational energy harvester having a cylindrical device geometry, or having a polygonal prism device geometry.

FIG.7shows a spring mount627which may be implemented in a vibrational energy harvester such as the vibrational energy harvesters100,200,400. The spring mount627is configured to be located over an end of a first mass101(or for example over a cap313of a first mass301) within a housing104of a vibrational energy harvester100. The spring mount627is configured to provide a connection point to electrically connect one or more springs108(not shown), which in turn are connected to conductive material of the first mass101, to a connector104a(not shown) configured to deliver the electrical power generated by the vibrational energy harvester to an external device. In the embodiment shown, the spring mount627comprises one or more apertures or hooks628configured to attach to or receive an end of each of the one or more springs108. The spring mount627also comprises an aperture629configured to receive the connector104a. In the embodiment shown, the spring mount627comprises one or more channels630configured to receive wires connecting the springs108(or the hooks628in contact with the springs108) to the connector104a. Alternatively, the spring mount627may not comprise the channels630. In the embodiment shown, the spring mount627comprises a plurality of holes631. The holes631are each configured to allow, for example, a screw to pass through and connect, for example, an upper portion420aand a lower portion420bof the housing104together. The spring mount627also comprises a recess632configured to receive a resilient element106a,106b, such that the resilient element106a,106bis disposed between the first mass101and the spring mount627.

Part of an assembled vibrational energy harvester700in which the spring mount627is implemented is shown inFIG.8. A first mass701comprising a cap713defines a first closed internal cavity702in which a second mass703is disposed. A resilient member706a,706bis located at either end of the first mass701. The spring mount627receives one of the resilient members706a. Springs708are configured to form an electrical connection between the conductive material of the first mass701and a connector104a(not shown) via the spring mount627.

FIG.9Ashows an embodiment of a vibrational energy harvester800in accordance with the invention. The vibrational energy harvester800comprises a housing804. The housing804comprises a first (e.g., upper) portion820a, a second (e.g., lower) portion820b, and an adjuster819. The adjuster819is rotatably couplable to the upper portion820aand the lower portion820bof the housing804via respective O-ring seals834a,834b. A cam821is disposed within the housing804. The cam821is coupled to the adjuster819(via one or more helical channels822on the cam and one or more corresponding protrusions823on the adjuster819) such that rotation of the adjuster relative to the upper portion820aand the lower portion820bof the housing804causes axial displacement of the cam821within the housing804. Spring-loaded plungers833(shown in more detail inFIG.9B) are configured to interact and engage with one of a plurality of holes426on the cam821. In the embodiment shown inFIG.9B, the spring-loaded plungers833comprise a thread configured to engage with a corresponding thread through a thickness of the housing804, securing the spring-loaded plungers833to the housing804. Alternatively, the spring-loaded plungers833may slot or clip into the housing804using corresponding male and female components located on the spring-loaded plungers833and the housing804respectively. A first mass801is disposed within and configured to move within a second closed internal cavity105formed by the cam821and the housing804. A spring mount827is disposed between a first end of the first mass801and the upper portion820aof the housing804. The spring mount827comprises a recess (e.g., similar to recess632as shown inFIG.7) configured to receive a resilient element806a. The resilient element806ais also in contact with the first end of the first mass801. The spring mount827is also configured to provide a connecting point between springs108(not shown), which in turn are connected to coils309of the first mass801, and a connector804aconfigured to deliver electrical power generated by the vibrational energy harvester800to an external device. In the embodiment shown, the connector804apasses through an aperture in the upper portion820aof the housing804. In the embodiment shown, screws are configured to pass through apertures in the upper portion820aof the housing804(and apertures in the spring mount827) and engage with screw threads in the lower portion820bof the housing804to secure the various portions of the housing together, and enclose various components of the vibrational energy harvester800within the housing804.

Vibrational energy harvesters such as those described above may be utilised as an alternative power source to single use or even rechargeable batteries, in particular for uses such as powering wireless sensor nodes and small, portable electronic devices. The vibrational energy harvesters100,200,400,800described above may provide usable electrical power, wide frequency response for applications which exhibit vibration frequencies typically associated with human motion, industrial machineries, railways and automotive, aeronautical, and agricultural applications (e.g., below 100 Hz, such as between 5 Hz and 20 Hz).

Individual features pertaining to each of the separate embodiments described above may be combined with any or all of the features described with respect to any of the other embodiments described. For example, the first mass301described with respect toFIG.3may be utilised in any of the vibrational energy harvesters100,200,400,700,800. The housing404described with respect toFIGS.4and5may be implemented in any of the vibrational energy harvesters100,200,400,700,800. The cap513as shown inFIG.6may be utilised with the first masses101,201,301,701,801in any of the vibrational energy harvesters100,200,400,700800. The spring mount627described with regard toFIG.7may be implemented in any of the vibrational energy harvesters100,200,400,800.

For the sake of completeness it is also stated that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, and any reference signs in the claims shall not be construed as limiting the scope of the claims.