The invention relates to a vibrational energy harvester. Preferably the invention relates to a vibrational energy harvester for harvesting or collecting energy from a source of vibration, such as ambient or environmental vibration.

In a conventional energy harvester, a direct resonator, or in some cases a plurality of direct resonators, is responsive to an input vibration, such as vibration of a structure to which the energy harvester is attached In the art, direct resonators may also be termed linear, or ordinary, resonators. The vibration excites the resonator(s) and the resonator(s) are electrically damped, for example by means of a permanent magnet carried by a resonator so that it oscillates in the proximity of a conducting coil, to extract an electrical power output. Such energy harvesters can be used to charge a battery or to operate an electronic device such as a sensor and/or a wireless transmitter in a self-contained device, in a known manner.

Such conventional energy harvesters suffer from several problems which limit their efficacy for converting vibration energy into electrical energy. One particular problem which limits the performance of the resonator(s) is that the resonator in a conventional energy harvester has a specific resonant frequency and can only be effectively excited by vibration frequencies close to that resonant frequency. Away from the resonant frequency the power output drops significantly. Natural or ambient vibrations available for driving an energy harvester tend to contain a variety or spectrum of vibration frequencies and a direct resonator may only be excited by a narrow band of the available vibration frequencies close to the resonant frequency of the resonator.

One approach which has been used to address this is to incorporate into an energy harvester a plurality of direct resonators of different resonant frequencies, but this adds to the complexity of the energy harvester. The prior art harvester disclosed in US Patent No. US 6,858,970, for example, comprises multiple cantilever beam resonators, each resonator comprising a piezoelectric layer. As the cantilever beam vibrates, mechanical strain is generated with the piezoelectric material, which is converted to an electrical potential difference which may be harvested. Providing multiple resonators with different resonant frequencies allows electrical energy to be generated at a range of frequencies. Attempts have been made in the prior art to design vibrational energy harvesters that are capable of harvesting power at two separate resonant frequencies, for example in GB2446685A and JP2018182941 A. Due to the increased complexity of a dual-band resonator compared to a single-band harvester, however, prior art designs have been susceptible to problems relating to mechanical complexity, tolerancing issues within the design, and undesired vibration modes causing a loss of power within the harvester.

It would be desirable to provide a reliable and compact vibrational energy harvester that has a significant power output over a wide band of vibration frequencies. Furthermore, it would be desirable for such a harvester to have a simpler construction, and greater reliability than the dual-band harvesters known in the art.

Statement of Invention

The invention provides a vibrational energy harvester, as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the invention are set out in dependent sub-claims.

In a first aspect the invention may provide a vibrational energy harvester comprising: a first flexure assembly and a conductive coil, the coil being arranged in a first plane and fixed to the first flexure assembly; a second flexure assembly and a magnet, the magnet being fixed to the second flexure assembly; in which the first flexure assembly has a first resonant frequency in which the coil moves relative to the magnet in a first direction; and in which the second flexure assembly has a second resonant frequency different from the first resonant frequency in which the magnet moves relative to the coil in the first direction, in which the flexure assemblies are configured so that the coil oscillates in the first plane, and the magnet is positioned adjacent the coil and oscillates in a plane parallel to the first plane, so that relative movement of the coil and the magnet induces an electrical current in the coil.

In use, ambient vibrational energy experienced by the harvester may excite a vibrational mode of one or both of the first and second flexure assemblies, so that the flexure assemblies flex and the coil and/or magnet oscillates back and forth along the first direction. In a vibrational mode in which the coil moves relative to the magnet in the first direction, the first flexure has the first resonant frequency and the second flexure assembly has the second resonant frequency. Oscillation of the coil and/or magnet relative to one another advantageously induces an electrical current in the coil, which may be harvested as electrical energy.

The first flexure assembly may function as a direct resonator. The second flexure assembly may function as a direct resonator.

As the coil is fixed to the first flexure assembly and the magnet is fixed to the second flexure assembly, the magnet and the coil are advantageously both free to oscillate in response to input vibration to the energy harvester. This advantageously increases the range of vibrational frequencies over which the harvester creates relative movement between the magnet and the coil, and over which the relative motion between the two flexure assemblies generates electrical energy.

Input vibration to the energy harvester is preferably provided by a machine or engine which vibrates during use. For example, the harvester may be mounted on a machine or engine, so that vibration of the machine or engine excites vibrational modes of the harvester. The present harvester is advantageously particularly suitable for harvesting energy from a source of input vibrations that operates with a variable-speed, such as a variable speed motor, machine or engine. As the frequency of the input vibrations will vary depending on the running speed of the motor, for example, the wide frequency band of the present harvester allows usable power to be generated over a larger range of motor speeds than would be possible with a single direct resonator.

The first flexure assembly is configured to oscillate at a first resonant frequency, and the second flexure assembly is configured to oscillate at a second resonant frequency different from the first resonant frequency. Thus the magnet and the coil may advantageously oscillate to different extents at different input frequencies of vibration. This may advantageously allow the harvester to generate electrical energy over a wider range of vibrational frequencies. The different resonant frequencies of the first and second flexure assemblies therefore give rise to a broader band of response for the harvester.

When input vibrational energy has a frequency near the first resonant frequency but not near the second resonant frequency, the first flexure assembly may be excited into oscillation while the second flexure assembly is not significantly excited. In this mode, the coil fixed to the first flexure assembly oscillates in proximity to the magnet, inducing an electrical current in the coil. Alternatively, when input vibrational energy has a frequency near the second resonant frequency but not near the first resonant frequency, the second flexure assembly may be excited into oscillation while the first flexure assembly is not significantly excited. In this mode, the magnet fixed to the second flexure assembly oscillates in proximity to the conductive coil, inducing an electrical current in the coil. At frequencies between the first resonant frequency and the second resonant frequency, vibrational modes of both the first and the second flexure assemblies may be excited, so that the coil and the magnet oscillate relative to one another, generating electrical current in the coil.

Therefore at least one of the flexure assemblies of the harvester may be excited into a vibrational mode over a wider range of frequencies than is possible with a single direct resonator. This means that the harvester can output electrical energy over a wider frequency bandwidth. This is particularly beneficial when the frequency of ambient vibrations is variable, for example if the harvester is mounted on a machine or engine which may operate at different frequencies.

The vibrational energy harvester may comprise a frame, which may be fixed relative to a source of input vibration. The first and second flexure assemblies may be fixed to the frame or configured to be fixed relative to a frame or housing, so that in use the coil and the magnet oscillate in the first direction relative to the frame. In a preferred embodiment, a frame portion of the first flexure assembly is fixed relative to a frame portion of the second flexure assembly.

In the present harvester the coil and the magnet are fixed to first and second flexure assemblies, respectively. In a preferred embodiment, the first and second flexure assemblies comprise first and second cantilever flexures, respectively. The present inventors have found that by mounting the coil and magnet on flexures rather than coil springs, the separate assemblies can be aligned with greater consistency, and the parallel alignment can be better maintained during oscillation, as flexures are less prone to out-of- plane movement than coil springs. Coil springs are typically susceptible to out-of-plane vibrational modes that could lead to undesired collisions between the first and second flexure assemblies.

In the present harvester, the coil is arranged in the first plane, so that the circumference of the coil lies in the first plane. The central axis of the coil, around which the coil is wound, extends orthogonally through the first plane. In this arrangement, the “first direction” along which the coil oscillates is contained in the first plane, and lies parallel to a diameter of the coil. This differs from prior art arrangements in which the coil is configured to oscillate along its own axis.

The harvester has a planar construction, in which the coil is arranged in the first plane, and the magnet is positioned adjacent to the coil and arranged to oscillate in a second plane parallel to the first plane. The planes of oscillation are parallel and adjacent to one another. In use, the coil oscillates back and forth along the first direction in the first plane, and the magnet oscillates back and forth along the first direction in a plane adjacent to the coil. The present parallel-planes-of-oscillation harvester construction has advantageous constructional simplicity, and allows more consistent and reliable alignment between the first and second flexure assemblies.

The first flexure assembly is preferably configured to constrain movement of the coil to the first plane, and the second flexure assembly is preferably configured to constrain movement of the magnet to the plane parallel to the first plane. The plane in which the magnet oscillates may be termed the second plane, while in embodiments comprising magnets on either side of the coil, the magnets may be considered to oscillate in second and third parallel planes positioned on opposite sides of the first plane. The flexure assemblies may thus advantageously maintain the magnet(s) and coil in proximity to allow the generation of electrical current in the coil. By constraining movement to parallel planes, the flexure assemblies may also avoid out-of-plane vibrational modes that could lead to undesired collisions between the first and second flexure assemblies.

The first flexure assembly may comprise one or more first flexures, which are preferably cantilever flexures. The first flexures preferably have a fixed end that is coupled or couplable to a frame or frame portion, and the coil is preferably fixed to the first flexures at a position spaced from the fixed end, for example at a free end of the first flexure opposite the fixed end.

The second flexure assembly may comprise one or more second flexures, which are preferably cantilever flexures. The second flexures preferably have a fixed end that is coupled or couplable to a frame or frame portion, and the magnet is preferably fixed to the second flexures at a position spaced from the fixed end, for example at a free end of the second flexure opposite the fixed end. The first flexure assembly preferably comprises a pair of first flexures arranged parallel to one another, and connected to one another by the coil, or by a crossbar.

The second flexure assembly preferably comprises a pair of second flexures arranged parallel to one another, and connected to one another by the magnet, or by a crossbar.

The use of pairs of parallel joined flexures in each flexure assembly advantageously increases out-of-plane stiffness and increases the resonant frequency of torsional vibrational modes. This reduces the susceptibility of each flexure assembly to undesirable out-of-plane bending or torsional modes, which may cause the first and second flexure assemblies to collide. This flexure arrangement also desirably stabilises the oscillation of the flexure assemblies in the desired “vertical” bending modes within the first plane (for the coil on the first flexure assembly) and parallel planes (for the magnet(s) on the second flexure assembly).

The flexures of the first and/or second flexure assemblies may be, for example: single-end clamped flexures which are fixed to a frame or frame portion at one end; or clamped - clamped flexures which are fixed to a frame at both ends. The first and second flexure assemblies may comprise the same or different types of flexures.

Preferably the first and second flexure assemblies comprise first and second cantilever flexures, respectively.

The first and second flexures may have different lengths. As flexure length is inversely related to spring constant, the length of the flexures affects their spring constant, and therefore their resonant frequency. The first flexure may preferably be longer than the second flexure. This may advantageously allow the harvester to be made more compact.

As the flexures may be folded, the “length” of a flexure typically relates to the distance from the fixed end of the flexure to its centre of mass, measured along the flexure from its fixed end.

The first and second flexure assemblies are arranged so that, in a resting configuration, the first and second flexures are adjacent to one another in a plane orthogonal to the first direction. The resting configuration of a cantilever flexure refers to the flexure in an unstrained state, for example when the entire energy harvester is at rest and all parts of the harvester are stationary relative to one another. In the present harvester, the coil is preferably configured so that the axis of the coil is orthogonal to the first plane, and the magnet is configured so that its magnetic N-S axis is parallel with the axis of the coil. In this arrangement, the magnetic flux of the magnet is cut by the coil oscillating adjacent to the magnet.

In a preferred embodiment, the harvester comprises a pair of magnets fixed to the second flexure assembly. The second flexure assembly is preferably configured so that the pair of magnets are arranged one on either side of the coil with their poles aligned N-S-N-S, and so that each magnet oscillates in a plane parallel to the first plane. This may advantageously provide a well contained magnetic field across the coil. In this configuration, the first plane containing the coil is sandwiched by two parallel planes of oscillation each of which contains one magnet. In this arrangement, magnetic flux is directed across the coil from one magnet to another in a direction orthogonal to the first plane. When the coil oscillates in the first direction within the first plane, the coil cuts the lines of magnetic flux so that the relative movement of the coil and the magnet induces an electrical current in the coil.

Preferably a second pair of magnets is fixed to the second flexure assembly and arranged one on either side of the coil with their poles aligned S-N-S-N. Mounting two pairs of magnets on the second flexure assembly, with each pair aligned across the first plane, doubles the magnetic flux that is cut by the oscillating coil, and therefore increases the generated electrical current relative to a single magnet or pair of magnets. Aligning one pair of magnets N-S-N-S across the first plane, and the other pair S-N-S-N across the first plane may advantageously mean that the flux generated forms a closed loop which is cut by the coil during relative movement.

The magnets are preferably bar magnets.

In a preferred embodiment, the magnets are shaped to correspond to the shape of the coil. In a particularly preferred embodiment, for example, the second flexure assembly comprises four semi-circular, or semi-elliptical bar magnets, two magnets being arranged on each side of the coil.

The coil may be a circular coil. Alternatively, the coil may be a square or trapezoidal coil.

In a particularly preferred embodiment, the coil is oblong, or elliptical. The coil is preferably elongated in one dimension relative to a circular coil, so that the coil is oval or elliptical in shape. The coil may be defined by its major axis (the distance across its longest dimension) and its minor axis (the distance across its shortest dimension, which is perpendicular to the major axis). The oblong coil is preferably fixed to the first flexure assembly so that its minor axis is aligned parallel with the first direction, in which the coil oscillates. The inventors have realised that, rather than using a conventional circular coil, lengthening the dimension of the coil that is perpendicular to the first direction of oscillation, means that a greater proportion of the coil generates electrical current during oscillation, so more current is harvested.

The absolute dimensions of the coil may vary depending on the size of harvester required. However, in order to increase the generated current relative to a circular coil, preferably the oblong coil has an aspect ratio of at least 1 .25:1 , or 1 .5:1 , or 2:1 , or 2.5:1 , or 5:1 .

In a preferred embodiment, the harvester comprises an oblong coil positioned in the first plane between two pairs of semi-oblong magnets, the perimeters of which have been shaped to correspond to the oblong shape of the coil. This arrangement advantageously allows the coil to cut the maximum amount of magnetic flux as the coil and magnets move relative to one another. If the coil were rectangular with its major axis in the magnetic field, then as the coil moves up and down the short sides (the minor axis) of the coil would generate no power because they are parallel to the direction of travel (Fleming’s right hand rule). By curving the outside surfaces of the magnets to match the oblong (quasi-elliptical) shape of the coil, the flux cut (the resolved component of wire direction, wire movement and magnetic flux ) is maximised, so that more electrical current is generated.

There may be a design trade-off between the strength of the magnetic flux, the amount of power generated and the bandwidth of the harvester. For example, more magnetic flux may mean lower generated power but a wider frequency bandwidth.

The first resonant frequency and the second resonant frequency, and the frequency difference between them (which may be termed a frequency bandgap), may be controlled by controlling the spring constants and the masses of the first and second flexure assemblies relative to one another. For example, where the harvester is intended to harvest vibrational energy from a particular source of vibrations, the first and second resonant frequencies may be chosen to correspond to characteristic vibrational frequencies of the source. The frequency bandgap may advantageously be controlled to maximise the usable power bandwidth of the harvester, by maximising the range of frequencies at which the generated electrical energy is above a predetermined threshold. The first resonant frequency and the second resonant frequency are preferably not harmonics of one another.

When the harvester is connected to an electrical system configured to consume the power generated by the harvester, in order to be providing “usable power” the harvester must be generating at least the minimum electrical power to operate the electrical system. For example, usable power may be defined as power above a threshold power level necessary to operate the electrical system. This must be greater, for example, than the leakage power of the electrical system.

The vibrational energy harvester is preferably configured to generate a power greater than the “usable power” threshold across the widest possible frequency range. The harvester may generate usable power across a frequency range wider than the frequency difference between the first and second resonant frequencies.

ISO10816 and ISO20816 are well known standards that specify different levels of vibration for machines of various sizes. In these ISO standard the amount of vibration is broadly classified into four bands (Good, Satisfactory, Unsatisfactory and Unacceptable), which occupy vibration levels of between 0.28mm/s and 45 mm/s. The harvester of the present invention may for example be configured so that for input vibrations in the “Good” band, the harvester generates sufficient power to power, for example, a sensor.

The first and second resonant frequencies are preferably selected so that for input vibrations of a predetermined magnitude (such as input vibrations in the IS010816/IS020816 “Good” band), a predetermined power is generated at input frequencies between the first resonant frequency and the second resonant frequency. The first and second resonant frequencies may be selected so that for input vibrations of a predetermined magnitude, in a graph of usable power vs input vibration frequency, the power between the first and second resonant frequencies is maintained above a predetermined desired power level.

While the flexure assemblies may have their own resonant frequencies at which the amplitude of oscillation, and therefore the collected power, is at its maximum, the oscillation response of the flexure assemblies naturally extends over a range of frequencies either side of the resonant frequency. The 3dB bandwidth is the range of frequency over which the collected power is above half of its maximum amplitude, and may otherwise be termed the full width half maximum (FWHM) of the flexure assembly’s oscillation response. The output power of the harvester may approximate to and be graphically depicted on a graph of output power vs frequency as two adjacent bell curves, one of which is centred on the first resonant frequency and the other of which is centred on the first resonant frequency. Preferably the harvester is configured so that the two curves overlap one another in the frequency range between the first and second resonant frequencies. Preferably the first and second resonant frequencies are selected to maximise the width of the frequency range over which the total output power of the harvester is above a predetermined threshold power.

The first and second resonant frequencies may be selected so that oscillation modes are excited, and therefore the harvester generates power, in the expected frequency range of a given vibration source. So if the harvester is intended for use with a source of input vibrations such as, for example, a variable-speed motor which vibrates in use at frequencies between 40 Hz and 50 Hz, then the first or second resonant frequencies may be chosen so that the resonant peak’s lower 3 dB point is around 40 Hz, while the other resonant frequency peak’s upper 3 dB point is around 50 Hz. The harvester may then advantageously generate electrical power at all frequencies between 40 Hz and 50 Hz, so that when the vibrational frequency of the motor changes during use, the harvester still generates electrical power.

The frequency difference between the first resonant frequency and the second resonant frequency may be selected to suit the source of input vibration, so that the harvester generates power across the operating frequency range of the source. The frequency difference may therefore differ depending on the type of asset from which the harvester is intended to harvest energy. In certain preferred embodiments, for example, the frequency difference between the first resonant frequency and the second resonant frequency may be between 10 and 100 Hz, or between 20 and 80 Hz, or between 30 Hz and 50 Hz, depending on the vibrational frequency range of the input vibration source.

The coil may form all or part of a first mass fixed to the first flexure assembly, and the magnet may form all or part of a second mass fixed to the second flexure assembly. The resonator characteristics of the flexure assemblies may thus be calculated based on the first mass and second mass, respectively.

The resonant frequency w _re s of a mass on a flexure may be calculated as:

COres = (k/m) where w _re s is the resonant frequency of the flexure assembly, k is the spring constant of the flexure assembly, and m is the mass fixed to the flexure assembly. The spring constant k is inversely proportional to the length of the flexure cubed, so longer flexures lead to lower spring constants, and thus lower resonant frequencies.

The first mass and/or the second mass may optionally comprise a counterweight in addition to the mass of the coil and magnet respectively. The addition of a counterweight to one or both of the flexure assemblies may advantageously allow the resonant frequency of the flexure assembly to be tuned. A counterweight may also advantageously increase the power output of the harvester assembly, as the power produced is proportional to the mass of the flexure assembly. Increased mass of a flexure assembly may be compensated for by stiffer flexures, which may be advantageous at low frequencies as stiffer springs are affected less by gravity.

An electrical circuit is preferably connected to the coil to harvest the electrical power generated by the harvester. This electrical circuit provides an electrical load, the magnitude of which is preferably variable.

The first flexure assembly and the second flexure assembly may be electromagnetically coupled to one another during oscillation, as movement of the first flexure assembly may affect the movement of the second flexure assembly, and vice versa, due to electromagnetic interaction between the magnet and the coil. This coupling may consist of damping of the movement of each flexure assembly, which may be termed electrical damping.

Electrical damping is caused by electromagnetic interaction between the magnet and the coil when the flexure assemblies move relative to one another. Interactions in the electrical domain of the magnet and coil circuitry manifest as an apparent damping force in the mechanical domain (Lorentz force) that opposes the relative motion between the magnet and coil. Part of this damping force may be attributed to Eddy currents, but the majority of this damping force may be attributed to the current flowing through the coil connected to a circuit.

Electrical damping depends on a load impedance (such as electrical resistance) in the circuit connected to the coil. If the circuit to which the coil is connected is arranged to be an open circuit (in which electrical resistance = infinity), then there will be no electrical damping applied to resist the relative motion of magnet and coil. There will be an open circuit voltage, but no current flow. As the electrical load resistance is connected, and its value decreased from infinity, current flow increases and electrical damping increases. Electrical damping would be maximum if load resistance were zero, i.e. the electromagnetic generator is short circuited.

As the magnet and coil components of the present harvester are located on separate movable flexure assemblies, this electrical damping may mutually damp the movement of both the first and second flexure assemblies. Thus the motion of the first and second flexure assemblies may affect one another, and the flexure assemblies may be “coupled” together by electrical damping. This coupling between separate oscillating flexure assemblies would not be experienced in alternative harvester designs, for example, if piezoelectric transducers were used instead of the magnet and coil transducer of the present invention.

Positioning the magnet and the coil on separate movable flexure assemblies with different resonant frequencies may advantageously provide a vibrational energy harvester that harvests more power than two individual direct resonators. In particular, the electrical coupling between the motion of the two flexure assemblies, and the phase difference between the oscillating coil and magnet, may advantageously broaden the bandwidth over which usable power is collected by the harvester.

The vibrational energy harvester may further comprise electric circuitry connected to the coil. The electric circuitry may comprise, or be connected to, an energy storage device.

The coil may be termed a conductive coil, or an electrically-conductive coil.

The harvester may comprise a flexible cable connected to the coil. Unlike conventional harvesters known in the art, by mounting the coil on a flexure, the coil in the present harvester moves during operation. In order to harvest the electric current induced in the coil, a flexible cable is coupled to the coil and configured to flex with the coil when the coil is oscillating.

Displacement Limiter

The harvester preferably comprises means for limiting or restricting the displacement of the coil relative to the magnet in the first direction, and vice versa. This may be termed a displacement limiter. The displacement limiter may advantageously restrict how far the coil and the magnet(s) can move relative to one another when oscillating. This advantageously contains the first and second flexure assemblies to the range of relative displacements at which the largest quantity of usable power is generated. For example, the displacement limiter advantageously prevents the flexure assemblies from reaching relative positions (at large amplitudes of oscillation, for example) where the coil is no longer cutting magnetic flux, and therefore no power is generated. The displacement limiter may also enable the harvester to be provided in a more compact device than would otherwise be possible.

The displacement limiter is preferably configured to restrict the relative displacement of the coil and the magnet in the first direction to ± 5 mm or less, or ± 2.5 mm or less, or ± 1 .5 mm or less, or ± 1 mm or less.

The displacement limiter may be configured to restrict the relative displacement of the coil and the magnet to a different extent depending on the operating frequency range of the harvester.

Limiting the relative displacement of the first and second flexure assemblies may also advantageously limit the maximum strain experienced by the flexure assemblies during oscillation. Limiting the strain on the flexures during oscillation may advantageously increase the lifetime of the harvester.

The displacement limiter may comprise one or more cushioning members and one or more stop surfaces, configured so that a cushioning member contacts a stop surface when the relative displacement of the magnet and the coil reaches a predetermined maximum displacement. Contact between the cushioning member and the stop surface preferably imparts a damping force to damp the oscillation of one or both flexure assemblies.

The cushioning member advantageously cushions impacts between the flexure assemblies to prolong the lifetime of the harvester.

As the flexure assemblies are configured to oscillate back and forth along the first direction, two stop surfaces are preferably provided, to limit relative displacement in both directions. Thus whichever way the cushioning member is travelling, when it reaches the predetermined relative displacement, it will contact one of the stop surfaces, or vice versa.

Preferably, one of the cushioning member and the stop surfaces is provided on the first flexure assembly, and the other of the cushioning member and the stop surfaces is provided on the second flexure assembly. The displacement limiter may therefore function when the cushioning member on one flexure assembly contacts the stop surface on the other flexure assembly, so as to prevent further relative displacement.

The displacement limiter may advantageously allow the harvester to generate even more power at high frequencies because contact between the cushioning member and the stop surface excites resonances in both flexure assemblies when the limiting occurs.

The cushioning member may be configured to absorb and dissipate kinetic energy from the flexure assemblies when the flexure assemblies reach the predetermined maximum displacement, for example when the stop surface comes into contact with the cushioning member.

Alternatively, the cushioning member may be configured to absorb and re-release kinetic energy when the flexure assemblies reach the predetermined maximum displacement. This may advantageously help to conserve kinetic energy of the harvester, while preventing the flexure assemblies from oscillating at too high an amplitude, and potentially breaking from too much strain. For example, the cushioning member may comprise a spring configured to be compressed by the stop surface when the flexure assemblies reach the predetermined maximum displacement. The cushioning member may alternatively comprise a resilient material, for example silicone rubber, which stores and releases energy.

The cushioning member may comprise a resilient material. In a preferred embodiment, the cushioning member is an O-ring. O-rings may advantageously be available in a variety of sizes and thicknesses, so that the cushioning member is straightforward to replace during the lifetime of the harvester.

The cushioning member may be, for example, an O-ring arranged around the coil mount for cushioning contact on either side of the coil mount.

In preferred embodiments of the harvester, the second flexure assembly comprises magnets on either side of the coil. In these embodiments, the second flexure assembly preferably comprises a connector which connects the portions of the assembly on opposite sides of the coil so that both portions of the second flexure assembly always oscillate in phase. In order to connect the portions on opposite sides of the coil, the connector must extend through the first plane in which the coil oscillates. In this embodiment, either the cushioning member or the stop surfaces may be provided on the connector at a position in the first plane, with the other of the cushioning member or the stop surfaces provided on the first flexure assembly, also in the first plane. When the relative displacements of the connector and the first flexure assembly in the first plane reaches the predetermined maximum displacement, the cushioning member and a stop surface will come into contact with one another to prevent further relative displacement.

The displacement limiter preferably comprises a first cushioning member and a first stop surface that are configured to contact one another when the first flexure assembly reaches the predetermined maximum displacement moving one way along the axis of oscillation, and a second cushioning member and a second stop surface which are configured to contact one another when the first flexure assembly reaches the predetermined maximum displacement moving the other way along the axis of oscillation.

Preferably one of the first cushioning member and the first stop surface is provided on the first flexure assembly and the other of the first cushioning member and a first stop surface is provided on the second flexure assembly. Likewise, preferably one of the second cushioning member and second stop surface is provided on the first flexure assembly and the other of the second cushioning member and second stop surface is provided on the second flexure assembly.

One of the first or second flexure assemblies may comprise the first cushioning member and the second cushioning member spaced from the first cushioning member in the first direction. The other of the first or second flexure assemblies preferably comprises the first stop surface, and the second stop surface spaced from the first stop surface in the first direction.

Alternatively, one of the first or second flexure assemblies may comprise the first cushioning means and the second stop surface spaced from the first cushioning means in the first direction, while the other of the first or second flexure assemblies comprises the second cushioning means and the first stop surface spaced from the second cushioning means in the first direction.

In addition to a displacement limiter that limits relative displacement of the magnet and the coil, the harvester may additionally comprise a stop surface to restrict the amplitude of oscillation of the first and/or second flexure assemblies to a predetermined maximum amplitude. Limiting the relative displacement of the first and second flexure assemblies may advantageously limit the maximum strain experienced by the flexure assemblies during oscillation, to increase the lifetime of the harvester. The stop surface may advantageously be configured for shock loading.

Simplified Harvester Construction

The present design solves a variety of the problems associated with prior art designs for dual-band harvesters, provides significantly simplified construction, more reliable alignment, and also enables the addition of more than two resonant frequencies to the harvester.

In a particularly preferred embodiment, the present harvester provides a modular design, in which the first flexure assembly is provided as a first module, and the second flexure assembly is provided as a second and preferably third module which can be easily mounted adjacent to the first module.

First Flexure Assembly

In a preferred embodiment, the first flexure assembly comprises a pair of parallel first flexures spaced from one another in the first direction. The flexures each comprise a fixed end connected to a frame portion, and a free end, the free ends of the pair of flexures being connected to one another by a crossbar.

The first flexure assembly is preferably provided as a first-flexure-module comprising the pair of parallel first flexures, the crossbar, and a frame portion. Providing these components as a unitary module means that they can be quickly and straightforwardly fastened to other modules to construct the harvester without having to individually attach each component, for example using screws or bolts. The components within the module are advantageously always consistently aligned, and this modular construction removes a significant source of error and inconsistency, as fewer components can be misaligned, and there are fewer individual fastenings (screws, bolts, etc) which might come loose during use.

Particularly preferably, the first flexure module, including the pair of parallel first flexures, the crossbar, and the frame portion, is formed from a single block of material.

The pair of parallel first flexures, the crossbar, and the frame portion may be formed from the same block of material by laser cutting, milling, or electrical discharge machining (EDM). Forming the first flexure module from a single block of material advantageously eliminates the need to fasten components together using fasteners such as screws or bolts. As the harvester is subject to consistent vibration during its lifetime, this reduces the chance of fasteners becoming loose and causing the harvester to fail. This also eliminates the need to align the first flexures, the crossbar and the frame portion with one another, and prevents them from becoming misaligned during use. Forming the unitary module from a single block of material using EDM, for example, also means that the shape and thickness of each part of the module can be carefully controlled. In particular, the thickness of the flexures can be selected and machined to a high degree of precision.

Preferably a coil mount is provided on the crossbar, and the coil is mounted on the coil mount between the parallel first flexures. The coil mount may be fastened to the crossbar by one or more fasteners, for example one or more bolts.

In order to allow precise tuning of the first resonant frequency of the first flexure assembly, additional weights are mountable on the crossbar. In a preferred embodiment, the crossbar is configured to receive additional weights. For example, the crossbar may comprise a threaded bore for receiving a bolt, so that additional weights may be bolted onto the crossbar. This may advantageously allow additional precise tuning of the first resonant frequency, so that the desired resonant frequency can be obtained regardless of the tolerances of the machining process used to form the first-flexure-module.

Second Flexure Assembly

In a preferred embodiment, the second flexure assembly comprises a pair of parallel second flexures spaced from one another in the first direction, in which the flexures each comprise a fixed end connected to a frame portion, and a free end, the free ends of the pair of second flexures being connected by a magnet assembly.

The frame portion of the second flexure assembly is preferably releasably connectable to the frame portion of the first flexure assembly to form the frame of the harvester.

The second flexure assembly preferably comprises a second-flexure-module comprising the pair of parallel second flexures, the magnet assembly, and the frame portion.

Particularly preferably, the second flexure assembly comprises two second flexure modules, which are mountable to the frame on either side of the first flexure assembly. The two second flexure modules are preferably connectable to one another to form the second flexure assembly.

In a preferred embodiment, each second flexure module is provided as a unitary module in which the pair of parallel second flexures, the magnet assembly, and the frame portion are formed from the same block of material. As described above in relation to the first-flexure- module, this enables precise control of component size, shape and thickness, and advantageously reduces the number of separate components that must be manually fastened together during construction.

Similarly to the first-flexure-module, the second-flexure-module may be formed from the same block of material by laser cutting, milling, or electrical discharge machining (EDM).

Each magnet assembly preferably comprises one or more magnets. Particularly preferably, each magnet assembly, and therefore each second-flexure-module, comprises two magnets. The magnets may be mounted in recesses in the magnet assemblies.

In order to allow precise tuning of the second resonant frequency of the second flexure assembly, additional weights are mountable on the magnet assembly. In a preferred embodiment, the magnet assembly is configured to receive additional weights. For example, the magnet assemblies may comprise one or more threaded bores for receiving a bolt, so that additional weights may be bolted onto the magnet assembly. This may advantageously allow additional precise tuning of the second resonant frequency, so that the desired resonant frequency can be obtained regardless of the tolerances of the machining process used to form the second-flexure-module.

The free ends of each second-flexure-module are preferably connected to one another by a connecting rod. Connecting the two second-flexure-modules together ensures that the magnets mounted on each second-flexure-module oscillate together, at the second resonant frequency.

The connecting rod may extend through an aperture in the coil mount to connect the free ends of two modular second flexure units positioned on either side of the coil mount. This advantageously provides a particularly compact harvester.

With this modular construction, the harvester may be constructed simply by fastening the frame portions of two second-flexure-modules onto either side of the frame portion of a first-flexure-module, and then connecting the two magnet assemblies together. Thus the entire harvester may be constructed using only two fasteners to hold the frame portions of the three modules together, and one connector to link the second-flexure-modules. This advantageously enables much quicker and simpler construction of the harvester than prior designs, in which individual flexures, crossbars, frames etc. all had to be separately bolted together while taking care to align the components correctly and apply the correct compression to each fastener. This also reduces the chance of device failure, as there are far fewer fastenings to loosen during use.

In order to obtain first and flexure assemblies oscillating in parallel planes adjacent to one another, it is necessary to precisely align the flexures of the first and second flexure assemblies with one another. Where parallel beam flexure assemblies are used with pairs of flexures separated in the first direction, these flexures must also be aligned precisely in order to constrain oscillation to the plane containing the first direction. In order to generate a stable electric current in the coil, the separation of the magnet assembly and the coil assembly must be precisely controlled, as the magnetic field of the magnet assembly must be kept within range of the coil in order to induce a current.

Where separate individual flexures are used, these flexures must be painstakingly aligned with one another, as any misalignment may introduce unwanted vibrational modes, and may lead to unwanted contact between the first and second flexure assemblies during oscillation.

The modular construction of the present harvester advantageously overcomes these problems with alignment. Preferably each module is parallel-sided, so that adjacent modules are automatically aligned when their frame portions are connected. Spacers may be used in between adjacent modules to ensure correct alignment, or preferably the frame portions of each module may be shaped widthways to automatically space the flexures correctly from the flexures of the adjacent module(s).

Instead of coupling the frame portions of adjacent modules together, the modules may alternatively be mounted adjacent to one another, with the frame portions of each module separately fixed onto a frame. In this embodiment, the modular design would still allow simplified construction, and the modules could be located on the frame in positions which provide the correct spacing and alignment. Preferably one of a cushioning member or a stop surface is provided on the connecting rod, and the other of the cushioning member and the stop surface is provided on the coil mount, so that the cushioning member and the stop surface are configured to come into contact with one another when the relative movement of the first and second flexure assemblies increases beyond a threshold relative displacement, so that contact between the stop surface and the cushioning member prevents further relative displacement of the two flexure assemblies.

Additional Resonators

A particular benefit of the modular construction of the harvester is that additional modules may be added to the harvester by connecting the frame portions of the additional modules to the frame formed by the combined frame portions of the existing modules. The parallel- plane-oscillation arrangement of the present harvester means that additional flexure assemblies can be added in a “stacked” side-by-side arrangement, and the modular design allows these adjacent modules to be consistently aligned with one another with minimal effort required during construction.

Further modules having the same resonant frequencies may be added, to increase the electrical power being harvested across the existing band of frequencies. Or alternatively, further modules having different resonant frequencies may be added, to increase the bandwidth over which the harvester generates power.

In a preferred embodiment, for example, the vibrational energy harvester comprises a third flexure assembly and a second coil, the second coil being fixed to the third flexure assembly. The third flexure assembly may be positioned adjacent to the second flexure assembly and configured so that the second coil oscillates in a plane parallel to the first plane, so that relative movement of the coil and the magnet induces an electrical current in the second coil.

The third flexure assembly may be configured to oscillate at the first resonant frequency, in order to increase the power generated by the harvester at and around the first resonant frequency.

In a preferred embodiment, the third flexure assembly is configured to oscillate at a third resonant frequency which is different from the first and second resonant frequencies. This may advantageously allow the harvester to generate usable power across a wider frequency band than is possible with only a dual-band harvester.

The third flexure assembly is preferably constructed as a first-flexure-module as described above. The module for the third flexure assembly is preferably mountable adjacent to one of the second-flexure-modules by fastening the frame portion of the third flexure assembly to the frame portion of the adjacent second-flexure-module.

In order to increase power generation, a further second-flexure-module may be added outside the third flexure assembly module. This second-flexure-module may then be connected to the two existing second-flexure-modules to form an extended second flexure assembly, so that all three second-flexure modules oscillate together at the second resonant frequency. In this embodiment, the second flexure assembly comprises three second-flexure-modules, each comprising one or more magnets, which are mounted to the frame between and on either side of the first flexure assembly and the third flexure assembly, the second-flexure-modules being connected to one another to form the second flexure assembly.

Further modules may be added to the harvester as desired, by fastening the frame portions of additional modules to the frame portion of the outermost module on the harvester.

Where additional second flexure modules are added, the second flexure modules may be connected together using connector rods.

Preferably, the frame portion of each module may be connected to the frame portion of the adjacent module using only a single fastener, such as a single bolt or screw, or optionally two fasteners.

The modular construction of the harvester therefore allows the generated power to be increased beyond what is possible for a dual-band harvester, and allows the harvester’s frequency band broadened by adding additional modules with different resonant frequencies.

Tuning Means

A variety of mechanisms for tuning flexures in vibrational energy harvesters are known in the prior art. The inventors of the present invention, however, have arrived at implementations of tuning means that are particularly advantageous for use with multi-band harvesters comprising more than one flexure assembly. The following tuning means may be applied to any vibrational energy harvester within the scope of the claims, but are particularly suitable for use with the simplified harvester construction described above.

The harvester may comprise a tuning means, for tuning the resonant frequencies of the first and/or second flexure assemblies. The following description of flexure tuning is applicable to both the first and second flexure assemblies, and any other flexure assemblies present in the harvester.

The tuning means preferably comprises a tuning member which is configured to apply a force to oppose the movement of a flexure assembly, when the flexure assembly moves in the first direction. The force from the tuning member may advantageously serve to effectively alter the stiffness of the flexures in the flexure assembly, so that the resonant frequency of the flexure assembly can be altered. This may be used in addition to weights which are fixable to the flexure assemblies to alter their mass.

The tuning member may oppose the movement of the flexure assembly by providing a repulsive force, which opposes the movement of the flexure assembly as it moves towards the tuning member, or by providing an attractive force which opposes movement of the flexure assembly as it moves away from the tuning member.

This may be particularly suitable for use with the simplified “modular” harvester construction of the present invention, as the integrated flexures of the flexure assemblies mean that the thickness of the flexures cannot be varied post-production. The conventional prior art approach of adjusting flexure stiffness by using multiple stacked flexure plates is therefore not typically suitable for preferred embodiments of this harvester construction. As the modules of the simplified construction may be manufactured as unitary modules, for example by EDM, the manufacturing process may result in a range of natural resonant frequencies because the machining will typically produce flexures of slightly different thicknesses. By providing the tuning means it is possible to tune the as-manufactured flexure assemblies over a range of frequencies, so that the desired final resonant frequency can be achieved.

The tuning means may preferably be configured to tune the resonant frequencies of the flexure assemblies by at least +/- 3 Hz, or +/- 4Hz. Preferably the tuning member is arranged in the plane of oscillation of the flexure assembly, so that movement of the flexure assembly in the first direction moves the flexure assembly towards the tuning member.

The tuning member may comprise a spring, which is configured to apply a repulsive force to repel the flexure assembly when the flexure moves towards the spring in the first direction. Alternatively the tuning member may comprise a magnet, which is configured to apply an attractive force to attract the flexure assembly towards the magnet, and to oppose the movement of the flexure assembly away from the magnet.

Particularly preferably, the tuning member is movable along the length of a cantilever flexure of the flexure assembly, to vary the position on the flexure at which the opposing force is applied. By varying the position of the tuning member relative to the length of the flexure, the effect of the opposing force on the flexure’s resonance may be varied, and the resonant frequency of the flexure may therefore be tuned.

The tuning means may comprise a mount or frame for the tuning member. Preferably the mount is adjustable so that the tuning member may be fixed in a range of positions relative to the length of the flexure being tuned. Particularly preferably, the mount is configured to hold a first tuning member for tuning the first resonant frequency of the first flexure assembly, and a second tuning member for tuning the second resonant frequency of the second flexure assembly. Both the first and second tuning members are preferably positionable in a variety of positions relative to the flexures, for example by fixing the tuning members to different fixings on the mount. One or more slots may conveniently be provided in the mount, along which the tuning members may be moved and fixed in the desired position, for example by one or more screws or bolts.

In a first preferred embodiment, the tuning means is a magnet, and the flexures in the flexure assembly are formed from magnetic material. The magnet is positioned in the plane of oscillation and separated from the flexure assembly in the first direction. Thus, as the flexure assembly oscillates and moves in the first direction, the flexure assembly gets closer to the magnet, and the magnet applies a repelling magnetic force which opposes the movement of the flexure assembly.

In an alternative preferred embodiment, the tuning means is a coil spring. One end of the coil spring is fixed relative to the mount, while the other is arranged to abut a flexure of the flexure assembly. Preferably a resilient portion, for example a rubber tip, is provided between the flexure and the end of the coil spring, to reduce the strain on the flexure at the point of contact. Particularly preferably a semi-spherical rubber ball plunger is provided on the end of the coil spring, so that the ball plunger is in contact with the flexure. The coil spring is positioned in the plane of oscillation and separated from the flexure assembly in the first direction. Thus, as the flexure assembly oscillates and moves in the first direction, the flexure assembly compresses the coil spring, and the spring applies a repelling spring force which opposes the movement of the flexure assembly. The stiffness of the coil spring affects the frequency of the flexure assembly, so a stiffer coil spring may be used to increase the frequency of the cantilever flexure in the flexure assembly, for example.

In a particularly preferred embodiment, the harvester may comprise a tuning means which comprises one or more magnets, in which the one or more magnets are configured to apply a repulsive force to repel a flexure assembly when the flexure assembly moves towards the spring in the first direction, or to apply an attractive force to attract the flexure assembly when the flexure assembly moves away from the spring in the first direction.

By applying a repulsive force to repel the flexure assembly when the flexure moves towards the spring in the first direction, the magnetic tuning means may effectively reduce the stiffness of the flexures to lower the resonant frequency of the flexure assembly. By applying an attractive force to attract the flexure assembly when the flexure moves towards the spring in the first direction, the magnetic tuning means may effectively increase the stiffness of the flexures to increase the resonant frequency of the flexure assembly. Either of these options may therefore be used to tune the resonant frequencies of the flexure assemblies to a desired frequency.

Particularly preferably, the tuning member may be one or more frame tuning magnets which are mounted on the frame, so that the tuning means comprises one or more tuning magnets. The tuning means may also comprise one or more flexure tuning magnets, which are mounted on the flexure assemblies. Preferably there may be a first flexure tuning magnet mounted on the first flexure assembly, and a second flexure tuning magnet mounted on the second flexure assembly.

In use, when the first flexure assembly oscillates back and forth along the first direction, the first flexure tuning magnet oscillates in the first direction relative to the one or more frame tuning magnets, and the magnets repel or attract one another to alter the effective stiffness of the first flexure. Likewise, when the second flexure assembly oscillates back and forth along the first direction, the second flexure tuning magnet oscillates in the first direction relative to the one or more frame tuning magnets, and the magnets repel or attract one another to alter the effective stiffness of the second flexure.

Having separate first and second flexure tuning magnets advantageously means that the resonant frequencies of the two flexure assemblies are separately tunable.

The stationary positions (when the apparatus is not oscillating) of the frame tuning magnets and the flexure tuning magnets are preferably adjustable relative to one another, to tune the resonant frequency of the flexure by varying the separation between the magnets, and therefore varying the magnitude of the forces between the magnets. For example, in a first embodiment the relative separation of the frame tuning magnets and the flexure tuning magnets may be adjustable in the first direction. In another embodiment, the relative separation of the fixed tuning magnets and the flexure tuning magnets may be adjustable in a second direction, for example a second direction orthogonal to the first direction.

The frame tuning magnet(s) may be mounted directly onto the frame, or the apparatus may comprise a mount for mounting the frame tuning magnet(s) on the frame. The mount may be adjustable so that the position of the fixed tuning magnet(s) is adjustable relative to the flexure tuning magnets. Particularly preferably, the mount is configured to hold a first frame tuning magnet for tuning the first resonant frequency of the first flexure assembly, and a second frame tuning magnet for tuning the second resonant frequency of the second flexure assembly. The frame tuning magnets may be fixed in position relative to the frame. Alternatively, the frame tuning magnets may be positionable in a variety of positions on the frame, for example by fixing the frame tuning magnets to different fixings on the mount. The mount may comprise a slot, along which the tuning magnets may be moved and fixed in the desired position, or a thread along which the frame tuning magnets may be screwed.

Adjusting the relative separation of the frame tuning magnet(s) and the flexure tuning magnet(s) advantageously allows the resonant frequencies of the flexure modules to be adjusted, for example over a 2-5 Hz range, or a 1-3 Hz range. The tuning means may allow the first and/or second resonant frequencies to be lowered by 1 -5 Hz, or 2-3 Hz from the initial resonant frequency of the as-manufactured flexure assemblies. The tuning means therefore advantageously enables the harvester to be tuned to the desired resonant frequencies and to eliminate any departures from the intended resonant frequencies caused by manufacturing inconsistencies. The opposing force applied by the tuning member may increase relative to increasing amplitude of oscillation of the flexure assembly. Thus as well as tuning the resonant frequency, the tuning member may act as an absolute stop to prevent the flexure assembly vibrating at too high an amplitude. Restricting the amplitude of oscillation in this way may advantageously reduce the strain on the flexures and increase the lifetime of the harvester.

Instead of, or as well as, a tuning member that provides a force to oppose the movement of the flexure assembly, the tuning means may comprise a means of varying the separation of the two flexures in a given flexure assembly. By varying the separation of the fixed ends of the two first flexures, for example, the shape of the first flexure assembly is altered so that the pair of first flexures are no longer parallel. This changes the resonant frequency of the flexure assembly, and thus the resonant frequencies of the flexure assemblies may be tuned.

In the preferred simplified construction of the present harvester, frame portions of the first and second flexure assemblies are fixed to one another by bolts. By providing cam surfaces on the bolts, and providing a frame portion which is expandable in the first direction, the frame portion may be expanded by rotating the bolts to a particular angle, to vary the separation of the flexures. This may provide an advantageously compact tuning means without requiring an additional mount for a tuning member.

Brief Description of the Drawings

Specific embodiments of the invention will now be described with reference to the figures, in which:

Figure 1 is a schematic illustration of a single-resonator vibrational energy harvester known in the prior art;

Figure 2 is a schematic illustration of a vibrational energy harvester comprising two resonators, according to the first aspect of the present invention;

Figures 3a - 3c show the simulated frequency response of a vibrational energy harvester as shown in Figure 2;

Figure 4a shows a perspective view of an assembled dual-resonator vibrational energy harvester according to a first preferred embodiment of the present invention; Figure 4b is a cross-sectional view of the dual-resonator vibrational energy harvester of Figure 4a;

Figure 4c is an exploded view of the dual-resonator vibrational energy harvester of Figures 4a and 4b;

Figures 5a and 5b are graphs of the open circuit voltage vs frequency of vibration for the two resonators of the vibrational energy harvester of Figures 4a-4c;

Figure 5c is a graph of power generation vs frequency of vibration for the dual resonator vibrational energy harvester of Figures 4a-4c.

Figure 6 shows a perspective view of an assembled dual-resonator vibrational energy harvester according to a second preferred embodiment of the present invention;

Figure 7 is an exploded view of the dual-resonator vibrational energy harvester of Figure 6;

Figure 8 is a cutaway view of a dual-resonator vibrational energy harvester according to an embodiment of the present invention;

Figure 9 is a cutaway view of an alternative dual-resonator vibrational energy harvester according to a third embodiment of the present invention;

Figure 10 is an exploded view of a multiple-band vibrational energy harvester according to a preferred embodiment of the present invention;

Figure 11 is a side view of a second flexure assembly with a magnetic tuning member and tuning member mount according to a preferred embodiment of the present invention;

Figure 12 is a perspective view of the second flexure assembly, magnetic tuning member and mount shown in Figure 11 ;

Figure 13 is a side view of a second flexure assembly with a coil spring tuning member and tuning member mount according to a preferred embodiment of the present invention;

Figure 14 is an exploded perspective view of the coil spring tuning member shown in Figure 13; Figure 15 is a perspective view of the second flexure assembly, magnetic tuning member and mount shown in Figure 13;

Figure 16 is a graph of power generation vs frequency of input vibration for the dual resonator vibrational energy harvester of Figure 6, both with and without a coil spring tuning member acting on one of the flexure assemblies;

Figure 17 is a graph of power generation vs frequency of input vibration for the dual resonator vibrational energy harvester of Figure 6, both with and without a magnetic spring tuning member acting on one of the flexure assemblies;

Figure 18 is an exploded view of a vibrational energy harvester according to a preferred embodiment of the invention, in which a tuning means is provided by cam surfaces on the fixing bolts; and

Figure 19 is an assembled view of the vibrational energy harvester of Figure 18;

Figure 20 is a perspective view of a vibrational energy harvester according to a preferred embodiment of the invention, in which a tuning means is provided by a plurality of tuning magnets;

Figure 21 is an exploded view of the vibrational energy harvester of Figure 20;

Figure 22 is a sectional view of the vibrational energy harvester of Figures 20 and 21 .

In the known vibrational energy harvester 1 shown in Figure 1 , a cantilever flexure 10 is fixed at one end to a vibrating support structure 20. A proof mass 30 is suspended from the other, free end of the cantilever flexure. A magnet 70 is fixed relative to the proof mass, and an electrically-conductive coil 40 is fixed relative to the support structure or frame of the harvester. Vibration of the support structure 20 excites a vibrational mode of the flexure so that the proof mass oscillates relative to the support structure. This creates relative movement of the magnet 70 and conducting coil 40, inducing an electrical current in the coil which may be extracted as an electrical power output.

This prior art harvester 1 may be termed a direct resonator.

The resonator has a resonant frequency which depends on the spring constant of the flexure and the mass of the proof mass as described above in the summary of invention. The flexure is excited into a vibrational mode only when ambient vibrations are at or near the resonant frequency of the resonator. The amplitude of oscillation, and therefore the output power of the harvester, is at a maximum when ambient vibrations are at the resonant frequency of the resonator. As the frequency of the ambient vibrations gets further away from the resonant frequency of the resonator however, the oscillation response of the resonator falls away relatively quickly, with the amplitude of oscillation decreasing quickly at frequencies further from the resonant frequency. This means that the output power from the coil is only high enough to be useful when the resonator is vibrating at or close to its resonant frequency.

As shown in Figure 2, a vibrational energy harvester 100 according to a preferred embodiment of the present disclosure comprises a first flexure assembly 110 fixed to a frame 120. In the illustrated embodiment the first flexure assembly comprises a first cantilever flexure 130, one end of which is fixed to the frame 120, and the other end of which is free to flex relative to the frame, and a coil 140 of electrically-conductive wire mounted with a first proof mass 135 on the free end of the first flexure 130. Movement of the free end of the first cantilever flexure 130 causes the coil 140 to move relative to the frame 120.

A second flexure assembly 150 is also fixed to the frame 120, and comprises a second cantilever flexure 160 on which a magnet 170 and a second proof mass 175 are mounted. The first and second flexure assemblies are arranged on the frame so that, when the harvester 100 is stationary and the cantilever flexures are in their unstrained rest positions as shown in Figure 1 , the coil 140 and the magnet 170 are adjacent to one another. The coil 140 and the magnet 170 are close enough so that the coil experiences the magnetic field of the magnet.

An electrical circuit 180 is connected to the coil 140 by flexible wires so that the electrical load of the circuit is applied across the coil.

In use, vibrational energy is applied to the harvester 100, which causes the frame 120 to vibrate. These vibrations in turn cause the first and second cantilever flexures to flex, so that the flexure assemblies 110, 150 oscillate relative to the frame along a first direction (indicated by double ended arrows in Figure 1 ). The first flexure 130 and the coil 140 are arranged to oscillate back and forth in a first plane which doubles as the plane of the coil, while the second flexure 150 and the magnet 170 are arranged to oscillate in a plane adjacent and parallel to the first plane. The oscillation behaviour of the flexure assemblies depends on their resonant frequencies cores, which in turn depends on the spring constants k of the first and second flexures, and the masses of the flexure assemblies.

The first and second flexure assemblies 1 10, 150 have different resonant frequencies, so for a given frequency of input vibrations Winput, the oscillation response of the first flexure assembly 110 differs from the oscillation response of the second flexure assembly 150. Vibration of the harvester 100 at Winput therefore causes the coil 140 to move relative to the magnet 170. Relative movement of the coil 140 and the magnet 170 causes the coil to experience a changing magnetic field, which induces an electrical current in the coil. The induced current flows to the electrical circuit 180 and is harvested as electrical energy.

In use, the vibrational energy harvester 100 may be mounted on a source of ambient vibrations, for example a motor, which vibrates during regular operation.

In prior art vibrational energy harvesters with a single direct resonator, the frequency response of the flexure typically means that usable electrical power is only generated in a narrow range of frequencies close to the resonant frequency of the flexure. If the input vibrations, which may be termed ambient vibrations, have a frequency not near the resonant frequency of the resonator, then no power is generated.

The dual-resonator harvester of the present invention, however, allows usable electrical power to be generated over a much broader frequency range. Both the first flexure assembly 110 and the second flexure assembly 150 have their own resonant frequencies at which their respective oscillations are at their greatest amplitude, and the resonant frequencies may be selected so that usable power is generated across a broad range of frequencies containing the two resonant frequencies. By covering a broader frequency bandwidth, at least one of the flexure assemblies of the harvester may be excited into a vibrational mode over a wider range of input frequencies than is possible with a single direct resonator. This means that the harvester can output electrical energy over a wider frequency bandwidth. This is particularly beneficial when the frequency of ambient vibrations is variable, for example if the source of ambient vibrations is a motor that operates across a range of frequencies.

The first and second flexure assemblies 1 10, 150 do not touch one another, so the first flexure is free to oscillate independently of the second and vice versa. In use, however, the oscillations of the first and second flexure assemblies may be coupled together by electrical damping caused by electromagnetic interactions between the magnet and the coil with its electrical load.

Figures 3a to 3c show simulated frequency responses for a double-resonator vibrational energy harvester. For these simulations, the first flexure assembly (response shown in Figure 3a) was modelled with a resonant frequency of 70.62 Hz, while the second flexure assembly (response shown in Figure 3b) was modelled with a resonant frequency of 81 .53 Hz.

In the example diagrams of Figures 3a to 3c, a swept frequency, from low frequency (60Hz) to high frequency (90Hz), was applied to a Scilab simulation of the vibrational energy harvester 100 over a period of 100 seconds. The resulting graphs are as follows:

Figure 3a: Time domain response of the first flexure assembly 110 plotted on a linear scale. As expected, the first flexure assembly only responds by oscillating near its natural resonant frequency of 71 Hz.

Figure 3b: Time domain response of the second flexure assembly 150 plotted on a linear scale. As expected, the second flexure assembly only responds near its natural resonance frequency of 82 Hz, which in this case is higher than that of the first flexure assembly 110 as determined by its spring stiffness and mass.

Figure 3c: Figure 3c shows the combined frequency response of the dual-resonator harvester plotted on a logarithm scale. This shows that useful power is generated across a wide range, for this case from 60Hz through to 90Hz. The 3dB bandwidth is narrower than the usable power bandwidth, but for many applications the usable power is a more meaningful measure. For example if sufficient (useful) power is generated across the range of a variable frequency drive (VFD) motor then a harvester mounted on that motor can extract energy to power sensors across the normal operating range of the motor. A conventional single cantilever harvester, however, would have a much narrower useful power bandwidth and hence would not be applicable in a VFD environment.

Dual-Flexure Harvester

Figures 4a to 4c illustrate a first preferred embodiment of a dual-resonator vibrational energy harvester 500 embodying the harvester shown schematically in Figure 2. The harvester 500 comprises two dual flexure elements 400, in which an internal flexure 430 and a U-shaped external flexure 440 are both formed from a single sheet of metal, such as spring steel. A slot 410 is formed through the sheet of spring steel so that the slot approximately defines three sides of a rectangle, with an open end adjacent a first end 420 of the flexure element. The section of the flexure element inside the slot 410 forms an internal flexure 430, while the section of the flexure element between the slot and the perimeter of the flexure element forms a U-shaped external flexure 440. The internal flexure and the external flexure are connected to one another at the first end 420 of the flexure element where the slot 410 ends. As the slot 410 separates the two flexure elements at their other ends, however, the internal flexure has a second end 450 that is free to flex independently of a second end 460 of the external flexure.

The use of the dual-flexure element 400 advantageously means that the oscillating parts of the harvester 500 are automatically aligned in parallel planes when they are attached to the dual flexure element. This helps to eliminate alignment problems and problematic out-of- plane oscillations experienced by prior art designs.

In the harvester 500 of Figures 4a to 4c, the first ends 420 of two identical dual flexure elements 400 are attached to opposite sides of a frame 510 by screws 520, so that the width of the frame 510 separates the two flexure elements 400, and the flexure elements extend from the frame so that they are parallel to one another.

A crossbar 530 is positioned between the two flexure elements 400, and screwed to the second ends 460 of both of the U-shaped external flexures 440 to rigidly connect the two flexures. A coil mount 540 is connected to the crossbar 530 between the two parallel flexures, and extends out of the crossbar in the direction of the frame.

A round coil 550 of electrically-conductive wire is mounted on the coil mount, so that the coil is positioned mid-way between the parallel flexure elements 400 and roughly mid-way between the crossbar 530 and the frame 510. The circumference of the coil 550 is arranged in plane with the two flexure elements, the frame and the crossbar.

A magnet assembly 560 is screwed to the second ends 450 of the internal flexures 430, so that the magnet assembly is suspended between the parallel internal flexures. The magnet assembly has two opposing halves, with two magnets 570 mounted on each half. The magnets 570 are arranged in pairs with the magnetic poles of one half opposing the magnetic poles of the magnets on the other half of the assembly. A space between the sides of the magnet assembly is configured to receive the coil 550, and the magnets are oriented so that their magnetic poles are orthogonal to the plane of the coil.

A silicone O-ring 580 is arranged around the coil mount 540, so that when the harvester 500 is assembled the O-ring is aligned with a stop surface 590 on the magnet assembly.

When assembled, as shown in Figure 4a, the coil 550 is suspended between the second ends 460 of the external flexures 440, and the magnet assembly is suspended between the second ends of the internal flexures 430, with the coil 550 positioned in the space between the two sides of the magnet assembly. In this arrangement, the coil 550 experiences the magnetic flux of the magnet assembly, so that relative movement between the coil and the magnets induces an electrical current in the coil.

The parallel orientation of the two flexure elements 400 allows the coil and the magnet assembly each to oscillate relative to the frame along a first direction, or a first axis. The coil oscillates in a first plane, while the magnets oscillate in parallel planes on either side of the first plane.

As the internal flexures have a different length from the external flexures, and as different masses are fixed to the internal and external flexures, the resonant frequency of the internal flexures and magnet assembly is different from the resonant frequency of the external flexures and coil.

In use, when the frame vibrates as a result of ambient vibrations, the flexure assemblies (the internal flexures plus magnet assembly, and the external flexures plus coil, respectively) are excited and oscillate relative to the frame. As the flexure assemblies have different resonant frequencies, however, the oscillation response of the two flexure assemblies differs according to the frequency of the input vibration and its proximity to the resonant frequencies of the flexure assemblies.

If the relative displacement of the flexure assemblies becomes too great, the O-ring 580 comes into contact with the stop surface 590 to limit the travel of the over-excited flexure assembly. During a collision, the silicone O-ring cushions the impact by absorbing some kinetic energy from the oscillating assemblies, and re-releases the energy to both flexure assemblies as the flexure assemblies bounce apart. This may advantageously retain kinetic energy in the harvester to maximise the power generated, while preventing damaging collisions between the two oscillating flexure assemblies, and limiting the strain experienced by the flexures due to high-amplitude oscillations. By restricting the relative displacement of the coil and the magnet, this also contains the flexure assemblies to the range of travel over which the greatest quantity of power is generated.

An electrical circuit (not shown) is connected to the coil 550 by flexible cables, so that the electrical load of the circuit is applied across the coil. Electrical power generated in the coil can then be used by the electrical circuit.

Figure 5a shows the open circuit voltage produced across the coil over a range of input vibration frequencies, when only the internal flexures 430 and the magnet assembly 560 is oscillating. Figure 5b shows the open circuit voltage produced across the coil over a range of input vibration frequencies, when only the external flexures 440 and the coil 550 is oscillating. The different amplitude responses at different frequencies illustrate the different resonant frequencies of the two flexure assemblies.

Figure 5c shows the power produced by the vibrational energy harvester of Figures 4a to 4c over a range of input vibration frequencies, when both the internal and external flexure assemblies are free to oscillate relative to the frame. The acceleration of the input vibrations was varied from 0.1 g (g = gravitational acceleration = 9.8 m/s ² ) to 0.5 g in 0.1 g increments, and the output power of the harvester 500 was measured.

At low accelerations of 0.1 g, the power output of the harvester 500 shows two distinct peaks at the resonant frequencies of the internal and external flexure assemblies, respectively. As the acceleration increases, however, the power generated at frequencies between the two resonant frequencies increases significantly, and a greater magnitude of power is generated over a wider frequency bandwidth.

The inventors have found that the electrical coupling effect achieved by allowing both the coil and the magnet assembly to oscillate relative to the frame and relative to one another, advantageously leads to this increased power generation, which is significantly greater than that of two individual direct resonators with equivalent resonant frequencies.

Simplified Harvester Construction

Particularly preferred embodiments of a dual-band vibrational energy harvester 600 are shown in Figures 6 to 9. These embodiments provide all of the aforementioned benefits of a dual-band vibrational energy harvester, but also have a significantly improved construction that allows for simpler assembly and more reliable performance of the harvester throughout its lifetime.

The harvester 600 is constructed from three flexure modules. Each module has a frame portion, and the three modules are connected to one another by fastening the frame portions of each module together using only two screws or bolts. The modules are parallelsided, so that once the frame portions of the modules are bolted together, the modules are automatically aligned to oscillate in parallel planes adjacent to one another.

A first-flexure-module 610 includes a pair of parallel first flexures 620, a crossbar 630 that connects the first flexures, and a frame portion 640. The frame portion 640 is mountable on a frame 650, as shown in Figure 6. Additional weights 660 are fastened to the crossbar 630 to control the mass of the first-flexure-module resonator, and therefore tune its resonant frequency.

The pair of parallel first flexures 620 are cantilever flexures positioned one above the other, to confine the oscillation of the first-flexure module to a first plane.

As shown in the partial cutaway image of Figure 8, a coil mount 670 is fastened to the inside of the crossbar 630, and an oblong coil 680 is held in the coil mount 670 between the first flexures 620. The plane of the coil 680 is aligned with the plane containing the first flexures, the crossbar and the frame portion, so that when the flexures oscillate, the coil oscillates back and forth in-plane.

The harvester contains two second-flexure-modules 690, positioned one on either side of the first-flexure-module 610. Each second-flexure-module 690 includes a pair of parallel second flexures 700 which are connected to a frame portion 710 at a first “fixed” end, and connected to a magnet assembly 720 at their other “free” end. The magnet assembly 720 is positioned between the pair of parallel second flexures 700 for compactness, but is free to oscillate relative to the frame portion 710. Each magnet assembly 720 contains a pair of recesses in which are held two bar magnets 730. The embodiment of Figure 7 contains semi-oblong bar magnets 730, while in Figure 8 rectangular bar magnets 730 are shown without the rest of the magnet assembly. Additional weights 740 are fastened to the magnet assembly 720 to control the mass of the second-flexure-module resonator, and therefore tune its resonant frequency. The frame portions 710 of the second-flexure-modules contain holes through which the two second-flexure modules are fastenable to the frame portion 640 of the first-flexure module 610. In the embodiment shown, the three modules are fastenable together using only two bolts 750.

Spacer portions 760, which have a width greater than the first flexures 620, are provided on the frame portion 640 of the first-flexure-module 610. These spacer portions ensure that, when the second-flexure-modules 690 are fastened to the frame portion 640 of the first- flexure-module, the second flexures 700 are correctly spaced and aligned with the first flexures 620.

A connecting rod 770 connects the free ends of both of the second-flexure-modules, so that the two modules oscillate in phase with one another. The connecting rod 770 is received in holes 780 in the free ends of each magnet assembly 720, and extends laterally through an opening in the coil mount 670. A rubber o-ring is positioned around the connecting rod, so that when the relative displacement of the coil and the magnet in the first direction becomes too great, the o-ring cushions collisions between the connecting rod and the coil mount.

The harvester 600 is assembled by bolting the frame portions 710 of the two second- flexure-modules 690 to the frame portion 640 of the first-flexure-module 610. Once assembled, the first-flexure-module acts as a first resonator, which oscillates in a first plane at a first resonant frequency, and the two second-flexure-modules act as a second resonator, which oscillates in planes parallel to the first plane at a second resonant frequency. If tuning of the resonant frequencies is desired, additional weights can be added to the crossbar and/or magnet assemblies as required.

The individual modules are formed from a single block of metal, preferably by electrical discharge machining (EDM). This method allows the frame portions, the flexures and the connecting crossbar or magnet assembly to be machined precisely to the desired thickness. In particular, the thicknesses of the flexures can be selected, so that there is no need to use multiple flexures as is typical in the prior art.

This harvester construction is exceptionally simple to assemble, as the frame portions of the first and second flexure modules simply need to be fastened together using two bolts or screws 750. The modular construction also ensures immediate alignment and spacing without having to use additional components. As the additional weights 660, 740 are mountable to the outside of the device, it is also quick and straightforward to tune the resonant frequencies as desired.

Figures 8 and 9 show variants of this harvester construction, containing bar magnets having different shapes.

The bar magnets 730 in Figure 8 are conventional rectangular bar magnets.

In Figure 9, each magnet assembly contains two semi-oblong, or semi-elliptical, bar magnets 830, which are arranged to align with the oblong shape of the coil 680. By curving the outside surfaces of the magnets to match the oblong (quasi-elliptical) shape of the coil, the flux cut (the resolved component of wire direction, wire movement and magnetic flux) is maximised, so that more electrical current is generated.

Additional Resonators

The modular harvester construction also allows the addition of further flexure modules.

By adding a further module with the same resonant frequency, the power harvested across the bandwidth of the harvester can be increased.

Alternatively, a further module having a third resonant frequency can be added to the harvester, to change the characteristics of the frequency bandwidth across which usable power is generated. For example, if the third resonant frequency is higher or lower than the first and second resonant frequencies, the bandwidth of the harvester can be widened.

Figure 10 shows a vibrational energy harvester 900 in which a first-flexure-module 610 and a third-flexure-module 940 also containing a coil 680 are sandwiched by three second- flexure-modules 690. In this embodiment, the second-flexure-module in the centre of the harvester has a slightly different structure from the two outermost second-flexure-modules. In the magnet assembly 910 of the central second-flexure-module, both sides of the central magnets 920 are exposed, while in the outermost modules the magnets are received in recesses with closed ends. This arrangement allows magnetic flux from the central magnets 920 to flow in both directions, through both coils.

All three second-flexure-modules 690 are connected to one another by a connecting rod 930, so that the three second-flexure-modules oscillate in phase with one another and form a single resonator with the second resonant frequency. The three second-flexure-modules 690, the first-flexure-module 610 and the third-flexure- module 940 are fixed together by fastening the frame portions of all modules to one another, preferably using only two bolts. The simplified assembly of the dual-resonator embodiment is therefore also achieved in harvesters with additional modules.

The first flexure module 610 and the third-flexure-module 940 may both be configured (by attaching the same number of additional weights 660, for example) to oscillate at the same first resonant frequency. Having two coils 680 oscillating in between six magnets then allows a greater amount of electrical power to be harvested, across the same frequency bandwidth as, for example, the harvester 600 in Figure 6.

Alternatively, the first flexure module 610 and the third-flexure-module 940 may be configured (by attaching a different number of additional weights 660, for example) to oscillate at different resonant frequencies. Having three resonators oscillating at different resonant frequencies may therefore allow power to be generated across a wider frequency bandwidth than, for example, the harvester 600 in Figure 6.

The modular construction of the harvester advantageously means that as well as simplified assembly, it is also easy to add additional flexure modules to increase the number of resonators in the vibrational energy harvester. This has not been possible with prior art designs.

Tuning Means

While the tuning means according to the present invention may be applied to multi-band harvesters of different constructions, they are illustrated in the Figures with reference to the simplified modular construction described above, as the tuning means are particularly effective for this harvester design.

Figures 11 and 12 illustrate a second-flexure-module 690 with a magnetic tuning means.

The tuning means includes a tuning member mount 1102 which is fixable to a housing or frame (not shown) of the harvester, and does not touch the second-flexure-module. The mount 1102 includes a plate 1104 which is positioned above and parallel to one of the second flexures 700. The plate 1104 comprises a first slot 1106 which extends lengthways above the position where a first flexure 620 of the first-flexure-module 610 will be when the harvester is fully assembled. The plate 1104 has a second slot 1108 which extends above the second flexure 700. A tuning member 1110 is made up of a magnet 1112 held in a magnet holder 1114. The tuning member 1110 is positioned below the plate 1104, between the plate 1104 and the second flexure 700, and is fixed to the plate 1104 by a pair of bolts which extend through the second slot 1108. The longitudinal position of the magnet 1112 relative to the second flexure 700 can be varied by loosening the bolts and sliding the magnet holder 1114 to a different position along the second slot.

In use, as the second-flexure-module 690 oscillates and the second flexure 700 moves towards the magnet 1112, the magnet will provide an opposing force that will act against the motion of the second flexure. The magnet 1112 therefore gives the effect of “stiffening” the second flexure, which affects the second resonant frequency of the module. The effect of the magnet 1112 on the resonant frequency depends on its position along the second flexure, so the resonant frequency of the second-flexure-module 690 can be tuned by sliding the magnet holder 1114 to different positions along the second slot 1108.

Where another second-flexure-module 690 is present in the harvester, a further tuning member may be provided to tune the resonant frequency of the other second-flexure- module. However, as both second-flexure modules are rigidly connected by a connecting rod when the harvester is assembled, one tuning member 1110 may also suffice to tune the second resonant frequency.

Another tuning member (not shown) may be fixed to the first slot 1106 to tune the resonant frequency of the first-flexure-module 610.

Figures 13, 14 and 15 show a second flexure assembly with an alternative tuning means.

In the tuning means of Figures 13-15, the tuning member mount 1102 is the same as in the magnetic embodiment, but the tuning member 1210 uses a coil spring 1212 rather than a magnet. The tuning member 1210 is made up of a coil spring 1212, one end of which is held in a spring holder 1214, and the other of which is held in a plastic plunger 1216. The plastic plunger 1216 is biased away from the spring holder 1214 by the spring, but its travel is restricted by a pair of arms 1218 of the spring holder. Like the tuning member in Figures 11 and 12, the tuning member 1210 is positioned below the plate 1104, between the plate 1104 and the second flexure 700, and is fixed to the plate 1104 by a pair of bolts which extend through the second slot 1108. The longitudinal position of the tuning member 1210 relative to the second flexure 700 can be varied by loosening the bolts and sliding the spring holder 1214 to a different position along the second slot. When the tuning member 1210 is fixed to the plate 1104 of the mount, the coil spring 1204 forces the plastic plunger 1216 into contact with the second flexure 700. As the second- flexure-module 690 oscillates and the second flexure moves towards the plate 1104, the plastic plunger compresses the coil spring 1212 against the spring holder 1214, such that the coil spring applies a spring force to oppose its compression. This force acts against the movement of the second flexure 700, and effectively “stiffens” the spring force of the second flexure itself.

Like the magnetic tuning member 1110, the position of the tuning member 1210 can be varied in order to tune the resonant frequency of the flexure module, and additional tuning members 1210 can be fixed to the plate 1104 to tune other flexure modules when the harvester is assembled.

Figure 16 is a graph of power generation vs frequency of input vibration for the dual resonator vibrational energy harvester of Figure 6, both with and without a coil spring tuning member acting on one of the flexure assemblies. In this example, a sprung tuning member 1210 was applied to the higher-frequency of the two flexure assemblies, causing an increase of approximately 3 Hz in the resonant frequency of that flexure assembly.

Figure 17 is a graph of power generation vs frequency of input vibration for the dual resonator vibrational energy harvester of Figure 6, both with and without a magnetic spring tuning member acting on one of the flexure assemblies. In this example, a magnetic tuning member 1110 was applied to the second-flexure-module 690, which was the lower- frequency of the two flexure assemblies, causing a decrease of approximately 2 Hz in the resonant frequency of that flexure assembly.

Figures 18 and 19 show a vibrational energy harvester similar in construction to that of Figure 6. In the harvester of Figures 18 and 19, however, the bolts 1810 used to assemble the harvester comprise an asymmetrical cam portion 1820, and the frame portions 710 of the second-flexure-modules 690 are split into two parts 710a, 710b, each of which receives one bolt.

The bolts 1810 are designed to have a cylindrical threaded portion at the distal end of the bolt, a cylindrical central section mid-way along the bolt, and the cam portion on an upper section of the bolt nearest the head. The lengths of the threaded, central and cam bolt portions are such that, when the bolts 1810 are inserted through the frame portions 710 of both second-flexure-modules 690 and the first-flexure-module 610, the threaded portion of the bolt is received in the frame portion 710 of one second-flexure-module, the cylindrical central section of the bolt is received in the frame portion 640 of the first-flexure-module, and the cam portion 1820 is received in the frame portion 710 of the other second-flexure module.

The collars (not shown) for receiving the bolts 1810 in the frame portion 710 of the second- flexure-modules are shaped so that rotation of the cam portion 1820 applies a force to the collars and alters the separation of the two parts 710a, 710b of the frame portion 710. This alters the separation of the fixed ends of the second flexures and in turn affects thei resonant frequency of the second flexure assembly 690, so the bolts 1810 can be rotated to tune the second resonant frequency of the harvester.

Likewise, the bolts 1810 which attach the weights 660 to the crossbar 630 of the first- flexure-module 610 also comprise a cam portion which interacts with the weights 660 to alter their position. Altering the position of the weights alters the centre of mass of the first- flexure-module 610 and therefore tunes the first resonant frequency.

Magnetic Tuning Means

While Figures 11 -15 illustrate two possible embodiments of tuning means according to the present invention, another preferred embodiment of a tuning means is illustrated in Figures 20-22, in which a tuning means is provided by a plurality of tuning magnets. While the tuning means may be applied to multi-band harvesters of different constructions, they are illustrated in the Figures with reference to the simplified modular construction described above, as the tuning means are particularly effective for this harvester design.

Figures 20-22 show a vibrational energy harvester 2600 similar in construction to the harvester 600 shown in Figures 6-8. In the harvester of Figures 20-22, the harvester components are adapted for an alternative tuning means comprising a plurality of tuning magnets.

The harvester 2600 is constructed from three flexure modules. Each module has a frame portion, and the three modules are connected to one another by fastening the frame portions of each module together using only two screws or bolts. The modules are parallelsided, so that once the frame portions of the modules are bolted together, the modules are automatically aligned to oscillate in parallel planes adjacent to one another. A first-flexure-module 2610 includes a pair of parallel first flexures, a crossbar 2630 that connects the first flexures, and a frame portion 2640. The frame portion 2640 is mountable on a frame (not shown).

The pair of parallel first flexures are cantilever flexures positioned one above the other, to confine the oscillation of the first-flexure module to a first plane.

As shown in the cross-sectional view of Figure 22, a coil mount 2670 is fastened to the inside of the crossbar 2630, and an oblong coil 2680 is held in the coil mount 2670 between the first flexures. The plane of the coil 2680 is aligned with the plane containing the first flexures, the crossbar and the frame portion, so that when the flexures oscillate, the coil oscillates back and forth in-plane.

The harvester 2600 contains two second-flexure-modules 2690, positioned one on either side of the first-flexure-module 2610. Each second-flexure-module 2690 includes a pair of parallel second flexures which are connected to a frame portion 2710 at a first “fixed” end, and connected to a magnet assembly 2720 at their other “free” end. The magnet assembly 2720 is positioned between the pair of parallel second flexures for compactness, but is free to oscillate relative to the frame portion 2710. Each magnet assembly 2720 contains a pair of recesses in which are held two bar magnets (not shown).

The frame portions 2710 of the second-flexure-modules are fastened to the frame portion 2640 of the first-flexure module 2610 by two bolts 2750.

A connecting rod 2770 connects the free ends of both of the second-flexure-modules, so that the two modules oscillate in phase with one another. The connecting rod 2770 is received in holes 2780 in the free ends of each magnet assembly, and extends laterally through an opening in the coil mount 2670.

The harvester 2600 is assembled by bolting the frame portions 2710 of the two second- flexure-modules 2690 to the frame portion 2640 of the first-flexure-module 2610. Once assembled, the first-flexure-module acts as a first resonator, which oscillates in a first plane at a first resonant frequency, and the two second-flexure-modules act as a second resonator, which oscillates in planes parallel to the first plane at a second resonant frequency.

The individual modules are formed from a single block of metal, preferably by electrical discharge machining (EDM). This method allows the frame portions, the flexures and the connecting crossbar or magnet assembly to be machined precisely to the desired thickness. In particular, the thicknesses of the flexures can be selected, so that there is no need to use multiple flexures as is typical in the prior art.

This harvester construction is exceptionally simple to assemble, as the frame portions of the first and second flexure modules simply need to be fastened together using two bolts or screws 2750. The modular construction also ensures immediate alignment and spacing without having to use additional components.

Additional weights 2660 are fastened to the crossbar 2630 to control the mass of the first- flexure-module resonator. Rubber shock-absorbers 2000 are attached to the upper and lower ends of the weights 2660 to act as displacement limiters which will impact the frame (not shown) and a tuning magnet mount 2102 if the oscillation of the flexure assembly reaches too high an amplitude.

In order to allow fine tuning of the resonant frequencies of each flexure assembly, a tuning means is formed by a plurality of magnets. The tuning means advantageously enables the harvester 2600 to be tuned to the desired resonant frequencies regardless of any manufacturing inconsistencies.

The tuning means includes a tuning magnet mount 2102 which is fixable to a housing or frame (not shown) of the harvester, and does not touch either of the flexure modules. The tuning magnet mount 2102 comprises a magnet channel 2106 which extends lengthways above the first flexure and the first-flexure-module 2610. Inside the magnet channel 2106 two button magnets are positioned: a first frame tuning magnet 2108 and a second frame tuning magnet 2109.

The frame portion 2640 of the first-flexure-module 2610 is connected to a first magnet mount 21 12 adapted to hold a first flexure tuning magnet 2110, and to position the first flexure tuning magnet 2110 adjacent to the first frame tuning magnet 2108 when the harvester is stationary.

The first flexure tuning magnet 21 10 is positioned in a threaded channel 2115 in the first magnet mount 2112, and the position of the magnet along the length of the channel is adjustable by turning a first fine pitched screw 21 16.

The magnet assemblies 2720 of the second-flexure-modules 2690 are connected by a second magnet mount 21 14 which is adapted to hold a second flexure tuning magnet 21 11 , and to position the second flexure tuning magnet 2111 adjacent to the second frame tuning magnet 2109 when the harvester is stationary.

The second flexure tuning magnet 2111 is positioned in a threaded channel 2125 in the second magnet mount 2114, and the position of the magnet along the length of the channel is adjustable by turning a second fine pitched screw 2117.

Adjusting the positions of the first and second flexure tuning magnets 2110, 2111 in their respective threaded channels 2115, 2125, adjusts the magnitude of the magnetic force between the frame tuning magnets and the flexure tuning magnets.

In use, as the second-flexure-module 2690 oscillates, the second flexure tuning magnet 2111 moves in the first direction relative to the second frame tuning magnet 2109, the magnets will provide an opposing force that will act against the motion of the second flexure assembly. The magnets therefore give the effect of “stiffening” the second flexure, which affects the second resonant frequency of the module. The effect of the magnets on the resonant frequency depends on the separation between the frame tuning magnet and the flexure tuning magnet, so the second resonant frequency of the two connected second- flexure-modules 2690 can be tuned by adjusting the position of the second flexure tuning magnet 2111 along the threaded channel 2125.

Likewise, the first resonant frequency of the first-flexure-module 2610 can be tuned by adjusting the position of the first flexure tuning magnet 2110 along the threaded channel 2115.

Adjusting the fine pitch screws allows the resonant frequencies of the flexure modules to be adjusted over a 2-5 Hz range. The absolute adjustment depends upon the resonant frequency of the flexure modules.

In the illustrated embodiment, the two frame tuning magnets 2108, 2109 are fixed in position relative to the frame, while the positions of the two flexure tuning magnets 2110, 2111 are adjustable to vary the separation between the frame tuning magnets and the flexure tuning magnets when the harvester is stationary. In an alternative embodiment, however, the position of the frame tuning magnets may be adjustable.