Patent Description:
Recent advances in material technology have resulted in new piezoelectric crystal materials becoming available. Examples of such materials include relaxor ferroelectric single crystal (RFSC) materials. Such materials can be used in vibration energy harvesting and ultrasonic projection devices, but such materials do not last long if placed under tension due to internal flaws creating stress concentrations and/or crack propagation. Patent document <CIT> discloses a piezoelectric actuator (fluid jet apparatus) comprising a piezo element compressed by a permanent magnet and a magnetic member that is disposed around the piezo element.

It is desired to address or ameliorate one or more shortcomings or disadvantages associated with prior vibration energy transduction devices, or to at least provide a useful alternative thereto.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims.

Some embodiments relate to an energy transduction apparatus, including:.

In various embodiments, the magnet arrangement and the second magnet are configured to exert between about <NUM> Newtons (N) and about <NUM> Newtons of substantially static compressive force to the piezoelectric transducer. In some embodiments, the static compressive force is between about <NUM> N and about <NUM> N. In some embodiments, the static compressive force is between about <NUM> N and about <NUM> N.

The magnet arrangement may be disposed at least partly around the piezoelectric transducer. The vibratable mass may comprise a resonant mass.

The piezoelectric transducer may include a relaxor ferroelectric single crystal (RFSC). The piezoelectric transducer may include a binary or ternary piezoelectric single crystal. The piezoelectric transducer may be a PMN-PT or PZN-PT crystal. The piezoelectric transducer may be a PIN-PMN-PT or PMN-PZT crystal. The piezoelectric transducer may be a Mn-PIN-PMN-PT crystal or a Mn-PMN-PZT crystal. The piezoelectric transducer may alternatively be or include a piezoceramic material (e.g. PZT) or a piezopolymer material (e.g. PVDF - polyvinylidene fluoride or polyvinylidene difluoride).

A piezoelectric crystal of the piezoelectric transducer may be poled [<NUM>] and arranged to operate in transverse extension (<NUM>-<NUM>) mode, with a <NUM> axis of the piezoelectric crystal being substantially coaxial with the first magnet, the magnet arrangement, the spacer, the second magnet and the vibratable mass.

The first magnet, the magnet arrangement and the second magnet may be rare earth magnets. The apparatus may further comprise an impedance matching layer and/or an acoustic lens disposed at an outer end of the second magnet.

The apparatus may include a first thin shim disposed between the first magnet and a first end of the piezoelectric transducer and a second thin shim disposed between the second magnet and an opposite second end of the piezoelectric transducer. The first thin shim and the second thin shim may be formed of a machinable glass ceramic material.

The piezoelectric transducer may include a spacer positioned between the first magnet and the second magnet. The spacer may be significantly more compressible than the magnet arrangement and the piezoelectric transducer. The spacer may define an aperture to receive the piezoelectric transducer therethrough. The spacer may have an axial thickness of between about <NUM> and about <NUM> when the vibratable mass is at rest. The spacer may have an axial thickness of between about <NUM> and about <NUM> when the vibratable mass is at rest.

The magnet arrangement may define a passage through which the piezoelectric transducer extends, and the magnet arrangement and the piezoelectric transducer may not contact each other in the passage. The magnet arrangement may be symmetrical about multiple axes. The magnet arrangement may comprise a unitary magnet body. The magnet arrangement may comprise multiple magnet bodies fixed in position relative to each other. An axial spacer may be disposed between two of the multiple magnet bodies. The apparatus may further include an alignment disc disposed between and coaxial with two of the multiple magnet bodies, the alignment disc defining an alignment aperture in a centre of the alignment disc to receive and axially align the piezoelectric transducer. The alignment disc may be formed of a magnetically inert material. The magnet arrangement may be substantially cylindrical.

The spacer may comprise a plurality of compressible ligaments arranged to separate the magnet arrangement and the second magnet, wherein an axial length of the piezoelectric transducer is substantially the same as a combined axial length of the spacer and the magnet arrangement.

In alternative embodiments, the apparatus may be configured to convert vibration energy of the vibratable mass into current in the electrical conductors and to thereby act as an energy harvesting apparatus.

Some embodiments relate to an aircraft or watercraft comprising the apparatus installed and/or mounted on or within the aircraft or watercraft so as to project vibration energy from a component, such as a mounting body, of the aircraft or watercraft. Some embodiments relate to a watercraft comprising the apparatus mounted on the watercraft to project vibration energy from the watercraft during use of the watercraft.

The magnet arrangement may be disposed concentrically with the piezoelectric transducer.

The piezoelectric transducer may at least partially surround the magnet arrangement. The piezoelectric transducer may comprise multiple stacked piezoelectric transducer elements. The magnet arrangement may comprise a cylindrical magnet that is one of:.

The combination of the first magnet, the magnet arrangement and the second magnet may be configured to exert between about <NUM> Newtons and about <NUM> Newtons of substantially static compressive force to the piezoelectric transducer. In some embodiments, the static compressive force is between about <NUM> N and about <NUM> N.

The static compressive force and the movement of the piezoelectric transducer may be aligned in a same axial direction.

The apparatus may be configured to convert vibration energy of the second magnet into current in the electrical conductors and to thereby act as an energy harvesting apparatus.

The apparatus may be configured to convert current in the electrical conductors into vibration of the second magnet in a frequency range of about <NUM> to about <NUM> to thereby act as an acoustic projector.

Some embodiments relate to an energy transduction device, including:.

In some embodiments, the static compressive force is between about <NUM> Newtons and about <NUM> Newtons. In alternative embodiments, the static compressive force is between about <NUM> N and about <NUM> N, optionally between about <NUM> N and about <NUM> N.

The gap may define an axial separation between axially adjacent parts of the magnet assembly of between about <NUM> and about <NUM>.

Some embodiments relate to an acoustic projection system, comprising multiple ones of the apparatus or the device described herein mounted to one or more mounting bodies to project vibration energy away from the one or more mounting bodies.

Some embodiments relate to an acoustic detection system, comprising multiple ones of the apparatus or the device described herein mounted to one or more mounting bodies and configured to detect vibration energy when the respective ones of the apparatus or device are not being used for acoustic projection.

In various embodiments of the acoustic projection or detection system, ones of the apparatus or device may be positioned at spaced locations on the one or more mounting bodies.

In various embodiments of the acoustic projection or detection system, multiple ones of the apparatus or device are arranged in an array or bank on the one or mounting bodies.

In various embodiments of the acoustic projection or detection system, multiple ones of the apparatus or device are arranged to face a same direction.

In various embodiments of the acoustic projection or detection system, multiple ones of the apparatus or device are arranged to face a different direction.

Embodiments are described in further detail below, by way of example, with reference to the accompanying drawings, in which:.

Embodiments relate generally to high frequency vibration energy transduction devices and systems. Embodiments include a vibratable mass as part of the device. In particular, embodiments relate generally to a novel arrangement of magnets and a piezoelectric transducer that is aimed at keeping the piezoelectric transducer in compression. For example, some embodiments may apply a static compressive force to the piezoelectric transducer in the range of about <NUM> Newtons (N) to about <NUM> Newtons or about <NUM> Newtons to about <NUM> Newtons.

Some vibration energy transduction embodiments are optimised for vibration energy harvesting by transducing vibration energy into electrical energy, while other embodiments are optimised for pressure wave generation by transducing electrical energy into (kinetic) vibration energy. Embodiments that are optimised for pressure wave generation by transducing electrical energy into (kinetic) vibration energy may be described as electro-acoustic transduction (or "acoustic projection") devices. Many of the same energy transduction principles and device design considerations apply to both forms of transduction.

Embodiments of vibration energy transduction devices and techniques that are optimised for vibration energy harvesting, but also suitable or modifiable for pressure wave generation, are described first, with reference to <FIG>. <FIG> and <FIG> show in detail the arrangement of components of a vibration energy transducer optimised as a vibration energy harvesting device <NUM> according to some embodiments. <FIG> is a schematic cross-sectional illustration of a vibration energy harvesting device <NUM> having the same components and configuration as device <NUM> but with an alternative magnet arrangement. Embodiments of vibration energy harvesting device <NUM>, <NUM> are generally arranged as an axial stack of coaxial components. This means that vibration that is only along an axis orthogonal to the axis of the energy harvesting device <NUM>, <NUM> will negligibly or not at all excite the energy harvesting device <NUM>, <NUM>.

Vibration energy harvesting device <NUM> includes a base <NUM> that is anchored, coupled or otherwise connected to a vibrating host structure <NUM> via a mount <NUM> (see <FIG>). As used herein, the term proximal indicates a direction toward the base <NUM> and the term distal indicates a direction away from the base <NUM>. The vibrating host structure <NUM> may form part of a plant, equipment, vehicle or craft <NUM>, for example. The craft <NUM> may include an aircraft, such as a helicopter, and the vibrating host structure <NUM> may include a gearbox, for example. In some embodiments, not forming part of the present invention, the base <NUM> may be omitted.

Some vibration energy harvesting applications benefit from matching the resonant frequency of the vibration energy harvesting device to that of the host structure <NUM>. For other applications, it may be preferable that the resonant frequency of the vibration energy harvesting device is not matched to that of the host structure <NUM>.

The vibration energy harvesting device <NUM> further includes a first magnet <NUM>, a second magnet <NUM> and a magnet arrangement <NUM> in between the first and second magnets <NUM>, <NUM>. Also interposed between the first and second magnets <NUM>, <NUM> are first and second shims 115a, 115b, a spacer <NUM>, thin adhesive layers <NUM>, <NUM>, <NUM> and optionally an alignment disc <NUM>. Also disposed in between the first magnet <NUM> and the second magnet <NUM> and at least partially surrounded by the magnet arrangement <NUM> is a piezoelectric transducer <NUM>. These components of vibration energy harvesting device <NUM> are housed within a housing <NUM>. The components of vibration energy harvesting device <NUM> are generally aligned along a single central axis <NUM>. Axis <NUM> also corresponds to the proximal-distal direction as described herein. The magnet arrangement <NUM> defines a passage through which the piezoelectric transducer <NUM> extends, and the magnet arrangement <NUM> and the piezoelectric transducer <NUM> do not contact each other in the passage.

As used herein, the term magnet arrangement is intended to describe an arrangement involving one magnet or a combination of magnets that cooperate to achieve a described function. A magnet assembly as described herein is intended to describe a combination of multiple magnets that are physically and/or magnetically coupled together. A magnet arrangement may include a magnet assembly and a magnet assembly may include a magnet arrangement. However, specific embodiments described herein contemplate a magnet assembly that includes a magnet arrangement. For example, vibration energy harvesting devices <NUM>, <NUM> include a magnet assembly that comprises magnetic head and tail masses and a magnet arrangement between the head and tail masses.

First and second magnets <NUM>, <NUM> are preferably rare earth magnets and have strong attraction to magnetic materials. First magnet <NUM> is magnetically and/or mechanically coupled to base <NUM> on a proximal side of the first magnet <NUM>. The first shim 115a is disposed on a distal side of the first magnet <NUM> and adhered thereto by an adhesive layer <NUM>. The adhesive layers <NUM>, <NUM> and <NUM> may include suitable epoxy adhesives, for example such as CB359. The first and second shims 115a, 115b may be machinable glass ceramic shims, for example. An example material for such a shim is a machinable glass ceramic material marketed under the Macor brand sold by Corning, Inc. Such shims may be suitable for their ability to allow a slight amount of surface deformation, thereby providing a slightly softer surface than most rare earth magnets and reducing the likelihood of fractures forming in the piezoelectric transducer (when formed as a crystal), for example due to the relatively high static compression forces applied to the piezoelectric transducer by the magnet assemblies described herein. However, in some embodiments, other analogously deformable materials may be used in place of shims 115a, 115b. Such analogous materials may be provided as a coating, layer, layer with a material compositional gradient, or thin sheet, disposed on a distal side of first magnet <NUM> or a proximal side of second magnet <NUM>, for example. Such analogous materials may, for example, include a magnet-glass composite material with a higher glass concentration at a surface at which it is intended to contact the piezoelectric transducer <NUM>.

Magnet arrangement <NUM> is positioned on a distal side of the first shim 115a and adhered thereto by adhesive layer <NUM>. Thus, a proximal end of the magnet arrangement <NUM> is bonded to a distal face of the first shim 115a. The magnet arrangement <NUM> is generally cylindrical with an annular cross-section that defines an internal bore or hollow to receive the piezoelectric transducer <NUM> therein. The magnet arrangement <NUM> shown in <FIG> and <FIG> is made up of a plurality of axially stacked concentric magnetic annuli <NUM>. Each of the annuli <NUM> may be adhered to adjacent distal and proximal axial structures by an adhesive layer <NUM>. Each of the magnetic annuli <NUM> is preferably a rare earth magnet.

The purpose of the magnet arrangement <NUM> is to surround or at least partially surround the piezoelectric transducer <NUM> while exerting a strong magnetic attraction on axially adjacent components, in order to result in a compressive force being applied to opposite ends of the piezoelectric transducer <NUM>. This strong magnetic attraction is due to the small separation between adjacent magnets and the choice of material for the magnets, such as are used in rare earth magnets. Because of the strong magnetic attraction forces between the components of the magnet arrangement <NUM> and/or first and second magnets <NUM>, <NUM>, the piezoelectric transducer <NUM> is kept under compression. For some piezoelectric materials, such as piezoceramics or piezoelectric crystals, which are a preferred form of piezoelectric transducer <NUM>, allowing such materials to go into tension can rapidly result in structural breakdown of the material.

The presence of the spacer <NUM> within the axial stacks of magnets making up the magnet arrangement <NUM> and the first and second magnets <NUM>, <NUM> aligned along axis <NUM> ensures that there is a small axial gap (occupied by the relatively compressible spacer) to allow for some relative axial movement of the tip mass <NUM> relative to the base <NUM> or at least the first magnet <NUM>. This relative axial movement is permitted by the spacer <NUM> having an effective spring constant that is at least one or two orders of magnitude lower than the spring constant of the first and second magnets <NUM>, <NUM> and magnet arrangement <NUM> and at least one or two orders of magnitude lower than the spring constant of the piezoelectric transducer <NUM>. This allows the spacer <NUM> to experience small axial compression and extension when the host structure <NUM> (to which the base <NUM> is coupled) vibrates. Although the mount <NUM> that couples the vibration energy harvesting device <NUM> to the host structure <NUM> may have its own spring constant that factors into the transmission of vibration, it is generally intended that the spring constant of the mount <NUM> be one or two orders of magnitude higher than the spring constant of the spacer <NUM>. Because of the inertia of the tip mass <NUM>, vibrations transmitted through the base <NUM> tend to result in repeated compressions of the spacer <NUM> and simultaneous repeated compressions of the piezoelectric transducer <NUM>.

Spacer <NUM> has a plate-like base <NUM> that defines a central aperture <NUM> sized to allow the piezoelectric transducer <NUM> to pass therethrough. Central aperture <NUM> may be slightly larger than <NUM> by <NUM>, for example. The spacer base <NUM> has a series of radially oriented ligaments <NUM> projecting upwardly therefrom. Each of the ligaments <NUM> has the same height so that the spacer <NUM> can keep an even distance from its proximal side to its distal side during vibration. The spacer <NUM> is formed of a material, such as a suitable plastic material, having a spring constant that is in the order of <NUM>/<NUM>th or less of the spring constant of the piezoelectric transducer <NUM>. The gap maintained by the spacer <NUM> may be in the order of about <NUM>, for example, when the resonant tip mass <NUM> is at rest. The described arrangement allows around <NUM> microns of axial movement and compression of the piezoelectric transducer <NUM> during vibration.

The gap maintained by the presence of the spacer <NUM> is selected to allow compressive force due to magnetic attraction to be between about <NUM> Newtons and about <NUM> Newtons (N), for example. The spacer <NUM> may have an axial thickness of between about <NUM> and about <NUM> when the resonant tip mass <NUM> is at rest. In some embodiments, the spacer may have an axial thickness of between about <NUM> and about <NUM> when the resonant tip mass <NUM> is at rest.

In some embodiments, spacer <NUM> is positioned axially between two annuli <NUM> of the magnet arrangement <NUM>. In other embodiments, the spacer <NUM> may be positioned distally of the magnet arrangement <NUM>, so that the spacer <NUM> is positioned axially between the magnet arrangement <NUM> on its proximal side and the second magnet150 (and second shim 115b) on its distal side. The second magnet <NUM> is magnetically or mechanically coupled to the resonant tip mass <NUM> on the distal side of the second magnet <NUM>.

The resonant tip mass <NUM> is coupled only to the second magnet <NUM>, or possibly an intervening structure, on the proximal side of the tip mass <NUM>. The distal end of the resonant tip mass <NUM> is not fixed in position and is free to move axially. In some embodiments in which the vibration energy harvesting device <NUM> is positioned within a housing, that housing allows some freedom of movement, for example in the order of <NUM> to <NUM>, of lateral and/or axial movement before the resonant tip mass <NUM> will contact a wall of the housing. The resonant tip mass <NUM> may be formed primarily of tungsten carbide, for example.

The first and second shims 115a, 115b are selected to have a relatively low Young's Modulus so that they are soft enough to allow for the piezoelectric transducer <NUM> (when formed as a crystal) to not quite be perfectly axially aligned during manufacture or use of the vibration energy harvesting device <NUM>, <NUM>. Shims 115a, 115b may have a thickness of less than <NUM>, for example. Otherwise, the crystal structure of the piezoelectric transducer <NUM> can tend to fracture and/or wear too much during vibration under compression. For such reasons, shims having a similarly low Young's Modulus are also employed in other energy transducer device embodiments described herein, such as electro-acoustic transduction devices, example of which include acoustic projectors <NUM>, <NUM>, <NUM>.

In some embodiments, a thin alignment disc <NUM> may be present in the middle of the axial stack of annuluses <NUM> of the magnet arrangement <NUM>. The alignment disc <NUM> defines a central aperture <NUM> sized to be just larger than an outer perimeter of the piezoelectric transducer <NUM>. The alignment disc <NUM> serves to assist the piezoelectric transducer <NUM> to be positioned in axial alignment with the magnet arrangement <NUM>. The aperture <NUM> of spacer <NUM> also serves a similar alignment function to the alignment disc <NUM>. Both the spacer <NUM> and the alignment disc <NUM> may be formed of a suitable polycarbonate material, for example.

<FIG> is an example plot of power spectral density (PSD) versus frequency up to <NUM>. <FIG> shows a power spectral density of acceleration measurements made at a top of the resonant mass <NUM> with the base <NUM> driven by a wide band vibration in the range of <NUM>-<NUM>. The peak shown in the region of <NUM>-<NUM> is the resonant region, and shows that the response is relatively wideband, which in this case is over a band of around <NUM>.

<FIG> is an illustration of an example piezoelectric transducer <NUM> in the form of a piezoelectric crystal. Piezoelectric transducer <NUM> may be formed of other suitable materials, such as piezoceramics like lead zirconate titanate (PZT) or a piezopolymer material such as PVDF (polyvinylidene fluoride or polyvinylidene difluoride), for example. Certain piezoelectric crystals may be more effective as transduction devices than others. For example, piezoelectric crystals that are anisotropic with a mechanically soft axis may be preferred for allowing the vibration energy harvesting device <NUM>, <NUM> to resonate at lower frequencies. Preferred materials for the piezoelectric transducer <NUM> include single crystal ferroelectric materials. Ferroelectric single crystals convert mechanical energy to electrical energy or vice versa. This makes them a candidate as the active material in energy harvesting devices. By utilizing the direct piezoelectric effect when mechanical is available from the environment, the mechanical can be converted to electric charge polarization in relaxor ferroelectric single crystal material and useful amounts of energy can be obtained.

A current promising class of materials for energy harvesting is relaxor-ferroelectric single crystals (RFSC). These materials are single crystals of ferroelectric materials (for example, lead zinc niobate-lead titanate, known as PZN-PT). These materials have been shown to deliver a relatively high output Voltage at greater efficiency when the crystal is subjected to stress. In some ferroelectric crystal material compositions, for example certain compositions of ternary lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT), the crystal material undergoes a phase transformation when subjected to a critical amount of stress from an external source.

Relaxor single crystals display both a linear piezoelectric effect and a nonlinear electromechanically coupled phase transition. The linear piezoelectric effect in relaxor single crystals has been observed to be approximately a factor of six times that of the ceramic lead zirconate titanate (PZT). Reversible stress- and temperature-induced phase transformations are associated with spontaneous charge generation in the relaxor single crystals. These phase transformations can provide more than an order of magnitude increase in energy density per cycle for mechanical energy harvesting. Utilizing this phase transformation behavior allows a stress-biased energy harvester to take maximum advantage of the phase transformation in the relaxor single crystal material.

Lead zirconate titanate (Pb[ZrxTi<NUM>-x]O<NUM> , or PZT) materials typically exhibit a piezoelectric charge constant, d, in the range of <NUM>-<NUM> pC/N and an electromechanical coupling factor, k, of ~<NUM> to <NUM>. RFSC materials can exhibit significantly larger charge constant d and coupling factor k parameters. For example, first generation RFSC materials, Pb(Zn<NUM>/<NUM>Nb<NUM>/<NUM>)O<NUM> (PZM-PT) and Pb(Mg<NUM>/<NUM>Nb<NUM>/<NUM>)O<NUM> (PMN-PT), have piezoelectric charge constants that can be an order of magnitude greater than that of PZT, with an electromechanical coupling factor ><NUM>. These improved coefficients are present in relaxor ferroelectric compositions that are close to the morphotropic phase boundary (MPB). The first generation materials are not without their drawbacks. For example, the coercive field of PMN-PT is small (Ec ~<NUM> kV cm-<NUM>) compared with that of PZT (Ec ~<NUM>-<NUM> kV cm-<NUM>). The rhombohedral-tetragonal phase transition temperature PMN-PT is low (TRT ~<NUM>), which means such materials may be incompatible with applications that experience elevated temperatures. Second generation RFSC materials, such as Pb(In<NUM>/<NUM>Nb<NUM>/<NUM>)O<NUM>-Pb(Mg<NUM>/<NUM>Nb<NUM>/<NUM>)O<NUM>-PbTiO<NUM> (or PIN-PMN-PT) have shown promise for use in energy harvester devices due to material properties such as a relatively high transition temperature of (TRT ~<NUM>-<NUM>) and around three times the coercive field (Ec ~<NUM> kV/cm) of first generation PMNPT single crystals. They also have piezoelectric charge constants that can be more than an order of magnitude higher than for PZT (e.g. d~<NUM>-<NUM> pC/N).

More recently, it has been reported that the power density of third generation manganese modified Pb(Mg<NUM>/<NUM>Nb<NUM>/<NUM>)O<NUM>-Pb(Zr,Ti)O<NUM> (Mn-PMN-PZT) [<NUM>] poled single crystals is over <NUM> times higher than that of PZT4 ceramic. Third generation relaxor ferroelectric single crystals have shown improved fracture toughness and coercive field and also higher Curie and phase transition temperatures.

An additional benefit of RFSC materials for energy harvesting is their anisotropic material parameters, with electro-mechanical compliances s and charge constants d that can vary significantly with crystal direction. The d<NUM> transverse extension mode (or '<NUM>-<NUM> mode') is particularly useful for energy harvesting applications. When used with a [<NUM>] poled RFSC, the '<NUM>-<NUM> mode' permits the design of a harvester to exploit: (i) the large d<NUM> charge constant for improved electromechanical transduction compared with PZT; (ii) the large coupling factor k~ <NUM> also for improved transduction efficiency; and (iii) the mechanically soft axis '<NUM>' axis of [<NUM>] PIN-PMN-PT. The '<NUM>' axis of [<NUM>] PIN-PMN-PT has a compliance significantly greater than its '<NUM>' axis and also much greater than that of PZT, permitting the harvester's resonant frequency to be <NUM> to <NUM> times lower for an identically sized transducer element. In addition to these three advantages of '<NUM>-<NUM> mode' transduction, there is evidence that [<NUM>] poled RFSC materials are more resistant to large-cycle induced degradation than [<NUM>] poled materials. These benefits mean that [<NUM>] poled PIN-PMN-PT single crystals, in '<NUM>-<NUM> mode', can be suitable for use as a piezoelectric transducer in a vibration energy harvester.

<FIG> is a perspective view of the spacer <NUM> according to some embodiments. Although various configurations can be selected to achieve similar functions, the embodiments of spacer <NUM> shown have the effect of providing a compressible gap between axially adjacent magnetic components in the axial stack to thereby allow varying compression of the piezoelectric transducer <NUM> (positioned and aligned with an aperture <NUM>) in response to vibrations experienced by the vibration energy harvesting device <NUM>. Upper faces <NUM> of the ligaments <NUM> are made to be generally coplanar and may be adhered to the proximal surface of an annulus <NUM> or the second shim 115b, for example. Advantageously, the spaced radially directed array of ligaments <NUM> around the distal surface of the spacer base <NUM> allows electrical conductors to pass through spaces <NUM> between the ligaments <NUM>, so that those conductors (e.g. 811a, 811b, <FIG>) can be coupled to opposite conductive faces of the piezoelectric transducer <NUM>. The piezoelectric transducer <NUM> may have conductive epoxy <NUM> on opposite faces thereof to readily enable conductors 811a, 811b to be electrically coupled to the piezoelectric transducer <NUM>.

The ligaments <NUM> are formed so that their combined mechanical stiffness is much less than the piezoelectric transducer <NUM> so as to not interfere with the main resonance of the harvesting device <NUM>, <NUM>, which is ostensibly determined by the effective spring constant of the piezoelectric transducer <NUM> and the size of the resonant tip mass <NUM>. The spacer <NUM> should have an effective mechanical stiffness less than ten times that of the piezoelectric transducer <NUM> so that any secondary resonances due to the spacer/resonant-mass interactions will be well away from the main frequency of interest for the energy harvesting device <NUM>, <NUM>. In addition, an approximate <NUM>:<NUM> ratio of transducer-to-spacer stiffness will ensure that the magnetic compressive force acts mainly through the piezoelectric transducer <NUM>, maximizing the magnetic compression on the piezoelectric transducer <NUM> and minimising the possibility that it will go into tension during resonant motion.

The spacer <NUM> may be formed of a magnetically passive material, such as aluminium, polycarbonate, or similar materials, for example. Some embodiments may employ a spacer <NUM> that comprises ferromagnetic material, provided that such material has a suitably low spring constant and does not have the effect of reducing magnetic attraction between magnetic components on adjacent opposite sides of the spacer <NUM>. The spacer <NUM> needs to be durable under dynamic loading to ensure a long operational life for the harvesting device <NUM>, <NUM>. In addition, the mechanical and material characteristics of the spacer <NUM> should not vary greatly at elevated temperatures. The spacer <NUM> may be designed so as to protect the piezoelectric transducer <NUM> against mechanical bending stresses if the harvesting device <NUM>, <NUM> is oriented horizontally or if there is a lateral component in the host vibration. Due to the low aspect ratio of the spacer <NUM> (τ=length/outer diameter), it is estimated that a spacer <NUM> manufactured from three-dimensional printed polycarbonate will only allow a small static deflection in the order of a few µm, which is unlikely to be enough to damage the piezoelectric transducer <NUM>.

<FIG> illustrates a hollow cylindrical magnet as an example of a magnet configuration <NUM> that can be used in place of magnet configuration <NUM>. Magnet arrangement <NUM> may be a single continuous body. An outside diameter (OD) of the magnet arrangement <NUM> may be generally similar to the outside diameter of the magnet arrangement <NUM> and also about the same as the outside diameter of first and second magnets <NUM>, <NUM>. Magnet arrangement <NUM> also has an inside diameter (ID) that defines an inner axial bore <NUM> that is large enough to accommodate the maximum dimensions of piezoelectric transducer <NUM> without contacting the piezoelectric transducer <NUM> during normal operation. Magnet arrangement <NUM> has an axial direction of magnetisation M that is aligned with the interior bore of the magnet body. A length (L) of the magnetic arrangement <NUM> is about the same as the length of the piezoelectric transducer <NUM>, minus the axial length of the spacer <NUM>. In other words, the axial length of the magnet arrangement <NUM> (and <NUM>) plus the axial length of the spacer <NUM> and any alignment discs <NUM> (if present) is approximately equal to the length of the piezoelectric transducer <NUM>. The magnet arrangement <NUM> defines a passage through which the piezoelectric transducer <NUM> extends, and the magnet arrangement <NUM> and the piezoelectric transducer <NUM> do not contact each other in the passage.

In some embodiments, the piezoelectric transducer <NUM> may be a RFSC with dimensions approximately <NUM> by <NUM> (width and depth) and approximately <NUM> in length. For piezoelectric transducers <NUM> in the form of a RFSC, the crystal may have a cross section between about <NUM>×<NUM><NUM> and about <NUM>×<NUM><NUM>. Lengths of such crystals may be between about <NUM> and about <NUM> or <NUM>, and may be up to about <NUM> or <NUM> or even <NUM>, for example.

<FIG> is a schematic cross-sectional view of a vibration energy harvesting device <NUM> that is the same configuration as vibration energy harvesting device <NUM>, except that device <NUM> uses magnet arrangement <NUM> and has the spacer <NUM> positioned between magnet arrangement <NUM> and second magnet <NUM>. To avoid unnecessarily obscuring <FIG>, adhesive layers <NUM>, <NUM>, <NUM> are not shown, although they are present as needed. Similarly, shims 115a, 115b are present between the first magnet <NUM> and the piezoelectric transducer <NUM> and between the second magnet <NUM> and the piezo electric transducer <NUM>, although they are not shown in <FIG> shows electrical conductors 811a, 811b, which may in the form of thin wires, for example, extending through gaps between ligaments <NUM> of spacer <NUM> to contact and electrically couple to opposite sides of piezoelectric transducer <NUM>.

<FIG> illustrate alternative magnet configurations. <FIG> shows a top view, looking proximally from the second magnet <NUM>, of the continuous cylindrical magnet arrangement <NUM> as shown in <FIG> and <FIG>. Magnet arrangement <NUM> may have an outer diameter that is substantially the same as an outer diameter of the first magnet <NUM> and/or the second magnet <NUM>.

<FIG> shows an alternative configuration in a top view, looking proximally from the second magnet <NUM>, in which the magnet arrangement partially surrounds the piezoelectric transducer <NUM>, but does not fully surround it. For example, the magnet arrangement that partially surrounds piezoelectric transducer <NUM> may include at least two magnetic bodies <NUM>, <NUM> that are disposed on opposite (e.g. lateral, not axial) sides of the piezoelectric transducer <NUM>. More than two magnetic bodies may be arrayed or arranged to at least partially surround piezoelectric transducer <NUM>, such as three, four, five, six or more bodies. The at least two magnetic bodies <NUM>, <NUM> may be arranged so that there is at least one axis of symmetry when viewed from the top as shown in <FIG>. The configuration of the magnet arrangements in the vibration energy harvesting devices <NUM>, <NUM> can be varied, as long as a suitable spacer can be accommodated to separate axially adjacent magnetic components by a small distance so that a strong magnetic attraction between those axially adjacent magnetic components can be used to provide a relatively strong magnetic compression force to a compressible (soft) axis of a piezoelectric transducer <NUM>.

Conversely, in other alternative configurations, the vibration energy transducer may include at least two piezoelectric transducer bodies that are disposed on opposite sides of a central or intermediate magnetic body. More than two piezoelectric transducer bodies may be arrayed or arranged to at least partially surround the central or intermediate magnetic body, such as three, four, five, six or more piezoelectric transducer bodies. The at least two piezoelectric transducer bodies may be arranged so that there is at least one axis of symmetry when viewed from the top.

In embodiments of the vibration energy harvesting device <NUM> and <NUM>, the magnetic components <NUM>, <NUM>, <NUM>, <NUM>, the spacer(s) <NUM> and <NUM>, the tip mass <NUM> and the piezoelectric transducer <NUM> are coaxial along a single axis <NUM> that extends between proximal and distal ends of the device <NUM>, <NUM>. Preferably, the magnetic components <NUM>, <NUM>, <NUM>, <NUM>, the spacer(s) <NUM> and <NUM>, the tip mass <NUM> and the piezoelectric transducer <NUM> are concentric. In a rest (non-vibrating) position of the vibration energy harvesting device <NUM> and <NUM>, each component is in contact with another axially adjacent component. The vibration energy harvesting device <NUM> and <NUM> have no air gaps other than in the spacer <NUM> and the hollow bore of the magnet arrangement <NUM>, <NUM>.

The compressive force applied to the piezoelectric transducer <NUM> (in the form of a suitable crystal as described herein) by the combination of the second magnet <NUM> and the magnet arrangement <NUM> has been estimated using the process described below.

A set of typical expected geometries for the magnet arrangement <NUM> are detailed in Table <NUM>.

For an ideal permanent magnet, the magnetization M is independent of magnetic field H, which leads to a linear magnetic flux B-H demagnetisation curve in the second quadrant. The calculation of magnetic force requires the magnetisation M of the cylindrical magnet arrangements, rather than magnetic flux B. M is independent of H for an ideal permanent magnet, and so M in the cylinder would be equal to the remanent magnetisation Mr if the magnet were ideal (i.e. for a N38H magnet, Mr = <NUM>. 26T/µ<NUM> ~ <NUM> MA/m, where µ<NUM> is the permeability of free space (~ 4π × <NUM>-<NUM> H/m)). We do not assume an ideal magnet and use the actual B-H/M-H characteristics of the material, hence yielding somewhat lower values for M.

The shape demagnetisation factor of a cylinder Nz was taken into account when determining the magnetic operating point through the permeance coefficient PC=[<NUM>-Nz]/Nz, which determines the gradient of the load line. Table <NUM> provides examples of the permeance coefficient for various cylindrical geometries.

For the purposes of calculating permeance, the operating temperature was assumed to be <NUM>. A conservative estimate of the magnetic permeance of the cylindrical magnet arrangement was made, taking into consideration the tubular nature of the magnet arrangement <NUM>, <NUM>, and also the additional spacing due to the machinable glass shims 115a, 115b. The second magnet <NUM> was not included in the estimate of magnetic permeance, adding to the conservative nature of the estimate. <FIG> shows a plot <NUM> of the predicted magnetic permeance as a function of geometry, indicating a minimum permeance Pc ~ <NUM>.

Using this conservative estimate of magnetic permeance, a magnetic load line (i.e. with a gradient equal to the permeance of <NUM>) was mapped onto the data sheet for a typical high temperature Neodymium-Boron-Iron magnet (N38H). This allowed the B-H operating point to be determined, from which the magnetic polarization J could be estimated (see plot <NUM> in <FIG>, which is adapted from a plot in a document sourced from
https://www. eclipsemagnetics. com/media/wysiwyG/brochures/neodymium_grades_data. pdf as of <NUM> March <NUM> or earlier). Knowing the magnetic polarization J, the magnetization M can be calculated, which can then be used to find the magnetostatic energy constant Kd. This Kd is conservative and reasonable for all of the magnetic geometries and temperatures (<= <NUM>) considered.

The calculated Kd is then substituted into the equation below (taken from equation <NUM> of <NPL>), allowing estimates of the magnetic force to be calculated, <MAT> where Fz is the magnetic force, R is the radius of the cylinder, τi =ti/(<NUM> R), i=<NUM>,<NUM>, are the aspect ratios of the lower cylinder (consisting of the magnet arrangement <NUM> and the first magnet <NUM>) and the upper cylinder (consisting of the second magnet <NUM>), with Kd = µ<NUM> M<NUM>/<NUM> and M being the magnetisation, permeability of free space µ<NUM> = <NUM>π × <NUM>-<NUM> N/A<NUM>, ε = Z/R is the reduced distance between the two cylinders, and J<NUM>(q) is the Bessel Function of the first kind.

<FIG> show plots <NUM> and <NUM> of the magnetic compressive force as a function of Gap (dictated by the axial thickness of the spacer <NUM>) and outer diameter OD, respectively. For larger magnets, forces in the range of <NUM>'s of N are predicted, with <NUM> N of compression readily achievable. It is estimated that forces in the range of about <NUM> N to about <NUM> N can be achieved with the described axial arrangement of the vibration energy harvesting device <NUM>, <NUM>. A compressive force of <NUM> N is the force needed to generate a mechanical compressive stress in the range of <NUM> MPa, which is needed to utilise the phase transition mechanism for a RFSC within a resonant harvesting device, such as vibration energy harvesting device <NUM>, <NUM>, for example. Although operation of an energy harvesting device as described herein around the phase transition can lead to higher energy harvesting efficiency, such operation is not necessary and energy harvesting efficiency at other piezoelectric states can still be acceptable.

Exemplary design features for the magnetic pre-loading approach are shown in <FIG>, <FIG> and <FIG>. A magnetic support structure, including magnet arrangement <NUM> or <NUM>, is depicted schematically as a series of ring magnets around a rectangular piezoelectric plate-element, such as piezoelectric transducer <NUM>, according to some embodiments. <FIG> and <FIG> show estimated or plotted relationships of device parameters (including magnet outer diameter, gap and head mass length) based on example device dimensions and configurations as described immediately below. In examples of such embodiments, the ring magnets may have an outer diameter rm-OD and inner diameter rm-ID, with each ring magnet having a height of approximately <NUM> and the total height of the ring magnets rm-H. The same rm-OD may be applied even where a single cylindrical magnet <NUM> is used instead of stacked ring magnets. The (distal) head magnet <NUM> is attracted to the magnetic support structure <NUM> or <NUM>, applying a compression to both the piezoceramic element <NUM> and the spacer <NUM>, and has an outer diameter similar to r-OD, and height hm-H ~ rm-OD/<NUM>. An additional head mass may be included (e.g. <NUM> in <FIG> or <FIG>) which has diameter similar to rm-OD, with height selected to tune the device resonant frequency as needed. The tail magnet <NUM> completes the magnetic circuit, has a diameter similar to rm-OD, and a height similar to the height of the head mass <NUM> (hm-H).

An example spacer <NUM> is shown in <FIG>, with vertical ligaments that are designed to have a mechanical stiffness approximately ten times less than the (crystal) piezoelectric transducer <NUM>. The spacer <NUM> creates a gap in the magnetic circuit with height gap-H. Thin disks <NUM> of machinable glass (e.g. formed of Macor™) with height M-H=<NUM> that protect the piezoelectric transducer <NUM> from damage during mechanical loading, can be located either end of the piezoelectric element <NUM> and optionally also between the ring magnets <NUM>.

A total height of the ring magnets rm-H = (length of piezoelectric transducer)+ (<NUM>*height of glass disks) - gap-H. Minor height variations can be addressed with very thin disk-layers of polymer, such as polycarbonate, between adjacent ring magnets, if required. Example embodiments employ a <NUM> × <NUM> × <NUM> RFSC element as piezoelectric transducer <NUM>. The proof mass may consists of the head magnet and a single ring magnet, and the shims <NUM> and other trivial masses do not significantly impact device resonant frequency.

According to one vibration energy harvester embodiment of device <NUM>, <NUM>, <NUM>, an approximate static compressive magnetic force of 50N can be applied to the piezoelectric transducer <NUM> under vibration of maximum frequency ~<NUM>, with rm-OD=<NUM>, rm-ID=<NUM>, gap-H=<NUM>, rm-H=(<NUM> + <NUM>*<NUM>)-<NUM> =<NUM>, hm-H=<NUM>.

According to another vibration energy harvester embodiment of device <NUM>, <NUM>, <NUM>, an approximate static compressive magnetic force of 500N can be applied to the piezoelectric transducer <NUM> under vibration of maximum frequency ~<NUM> (i.e. only head magnet as proof mass), with rm-OD=<NUM>, rm-ID=<NUM>, gap-H=<NUM>, rm-H=(<NUM> + <NUM>*<NUM>)-<NUM> =<NUM>, hm-H=<NUM>.

In an acoustic projector embodiment of device <NUM>, <NUM>, <NUM> with maximum drive frequency of <NUM> (i.e. only head magnet <NUM> as proof mass), an approximate static compressive magnetic force of 45N can be produced, with rm-OD=<NUM>, rm-ID=<NUM>, gap-H=<NUM>, rm-H=(<NUM> + <NUM>*<NUM>)-<NUM> =<NUM>, hm-H=<NUM>. Further example modelled acoustic projector embodiments (modelled on a single12×<NUM>×<NUM><NUM> RFSC that produces <NUM> microns of DC displacement under 90N compression) indicate that: for a vibration (drive) frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 5N is needed; for a vibration frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 8N is needed; for a vibration frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 5N is needed; for a vibration frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 1N is needed; for a vibration frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 3N is needed; for a vibration frequency of <NUM> at <NUM> V drive voltage, a static compressive force of around <NUM>. 3N is needed. Such modelled embodiments indicate that for vibration frequencies at or somewhat above about <NUM> at <NUM> V drive voltage, a static compressive force of around 5N is needed. Further, such modelled embodiments indicate that a static compressive force in the vicinity of around 50N is feasible for vibration frequencies between about <NUM> and about <NUM> at <NUM>-<NUM> V drive voltage, is needed.

Such example embodiments illustrate some example device configurations and are presented to illustrate how different device configurations can lead to different static compressive forces and operate under different vibration or drive frequencies. Various other device configurations are possible based on the principles described herein and illustrated in the Figures, without departing from the described embodiments.

Referring further to <FIG>, some embodiments are directed to a movable craft or fixed plant <NUM> that has a vibrating host structure <NUM> with the vibration energy harvesting device <NUM>, <NUM> mounted in fixed relation thereto via mount <NUM>. The craft or plant <NUM> may have multiple such vibration energy harvesting devices <NUM>, <NUM> mounted to the same or different host structures <NUM>. The craft or plant <NUM> may have one or more sensors <NUM> to monitor machinery conditions, for example. The energy output of the one or more vibration energy harvesting devices <NUM>, <NUM> may be provided to one or more batteries <NUM> (or other electrical energy storage devices) that are electrically coupled to the one or more sensors <NUM>. The one or more sensors <NUM> may then use the electrical energy from the batteries to provide output to a monitoring system <NUM>, for example.

Some embodiments of vibration energy harvesting device <NUM>, <NUM> are designed to be able to operate effectively at somewhat elevated temperatures to allow them to function properly in conditions normally experienced in operating plant or crafts <NUM>. For example, vibration energy harvesting device <NUM>, <NUM> may be designed to be able to operate with increased effectiveness at temperatures of <NUM>-<NUM> degrees Celsius.

Referring now to <FIG>, embodiments of transduction devices optimised for electro-acoustic transduction (including acoustic projection), but also suitable or modifiable for energy harvesting, will now be described. <FIG> is a schematic illustration of a vibration energy transducer optimised as an acoustic projector device <NUM> that has similar components and principles of design to the vibration energy harvester <NUM>, <NUM>, except that acoustic projector <NUM> is designed to convert electric energy from a current source <NUM> to vibration energy to generate output pressure waves <NUM>. Embodiments of acoustic projector <NUM> can also be used as a sensor device when it is not actively generating pressure waves. When acting as a sensor device, acoustic projector <NUM> can transduce vibrations into electrical signals in the manner described above for vibration energy harvesting and the electrical signals can be processed by a separate processing device or circuitry associated with an acoustic projection system. Thus, acoustic projector <NUM> is an example of an electro-acoustic transduction device that can emit pressure waves and receive (detect) pressure waves, for example at different times.

Acoustic projector <NUM> includes a base <NUM>, a magnetic tail mass analogous to first magnet <NUM>, thin shims 115a, 115b, a piezoelectric element <NUM> at least partially surrounded by a magnetic support structure (such as magnet arrangement <NUM>, <NUM>), a spacer <NUM> and a head mass <NUM>. An alignment disc <NUM> may also be included in the axial stack of projector components in a similar manner to vibration energy harvesting device <NUM> where the geometry of the magnetic support structure allows for it. Such components are housed in a housing <NUM>.

Housing <NUM> may include a case to enclose and hold the acoustic projector <NUM> components together. The housing <NUM> may also include a decoupling material between the case and the head mass <NUM>. In some embodiments, a soft sealing outer encasement, such as a rubber casing, surrounds part or all of the housing <NUM>. The housing <NUM> may be a ferromagnetic material, such as steel, mu-metal or iron, for example, in order to complete a magnetic circuit with the components of the magnetic assembly including the magnetic tail mass <NUM>, magnet arrangement <NUM>, <NUM> and head mass <NUM>.

In some embodiments, head mass <NUM> acts as both a distal magnet and the head mass. However, in some embodiments, the head mass <NUM> includes a magnet as shown in <FIG> and an additional non-magnet mass, such as a tungsten carbide mass, for example. Head mass <NUM> may also have an impedance matching layer <NUM> and/or acoustic lens positioned on or adjacent its outer distal surface <NUM>. Alternatively, the impedance matching layer <NUM> and/or acoustic lens may be used in place of the tungsten carbide mass.

The spacer <NUM> employed in the acoustic projector <NUM> is substantially the same as the spacer <NUM> used in the vibration energy harvester <NUM>, <NUM>, although it may be positioned more proximally. For example, spacer <NUM> and acoustic projector <NUM> may be disposed axially between the magnet arrangement <NUM>, <NUM> and the magnetic tail mass <NUM>. The shim 115a may be disposed between the spacer <NUM> and the magnetic tail mass <NUM>. As with the vibration energy harvester <NUM>, <NUM>, the piezoelectric element <NUM> passes through an aperture in the spacer <NUM> and contacts the shim 115a, against which the piezoelectric element <NUM> is compressed at a proximal end by axial forces due to magnetic compression. At its distal end, piezoelectric element <NUM> abuts the distally positioned shim 115b, which is adhered to head mass <NUM>.

Acoustic projector <NUM> applies an alternating current source <NUM> to the piezoelectric element <NUM> (which may be formed of any of the piezoelectric crystal materials described above) to cause axial expansion and contraction at frequencies ranging from around <NUM> hertz to around <NUM>. This small axial expansion and contraction is due to the selected orientation of the soft axis of the piezoelectric crystal chosen for the piezoelectric transducer <NUM>. Resultant displacement of the piezoelectric transducer <NUM> may be in the order of <NUM> or <NUM> microns to around <NUM> microns, for example, depending on the mass of magnets in the magnet assembly, plus any additional head mass. Any of the piezoelectric materials discussed above in relation to vibration energy harvesting device <NUM> can be used for the piezoelectric element <NUM> in acoustic projector <NUM>.

The movement of the piezoelectric transducer <NUM> under the influence of current from AC source <NUM> causes axial displacement of magnetic head mass <NUM> at a frequency dictated by the frequency of the alternating current. Since the magnetic head mass <NUM> is a free end of the acoustic projector <NUM> (in contrast to the magnetic tail mass <NUM> and base <NUM> that are coupled to the housing <NUM> and a host structure), vibrational axial displacement of an outer distal surface <NUM> of the magnetic head mass <NUM> causes pressure waves <NUM> to propagate in a distal direction away from the acoustic projector <NUM>. Depending on the medium, substance or material at the distal end of the magnetic head mass <NUM>, a radiation impedance <NUM> of the pressure waves <NUM> may vary at the distal end of the acoustic projector <NUM>. In some embodiments, an impedance matching layer <NUM> may be positioned on or adjacent outer distal surface <NUM> to maximise the amplitude of the pressure wave in the target propagating medium. In such embodiments, the impedance matching layer <NUM> may have variable properties, or may include a lens or lens system to align or focus the acoustic energy.

As with the vibration energy harvester <NUM>, <NUM>, the acoustic projector <NUM> relies on magnetic compression of a piezoelectric transducer to apply a static compression load (e.g. between about 5N and about 500N, between about 5N and about 50N, between about 50N and about 500N or between about 90N and about 400N) and thereby operate the piezoelectric transducer <NUM> in a mode that provides effective electrical to vibration energy conversion. The spacer <NUM> serves to slightly separate the magnet arrangement <NUM>, <NUM> that at least partially surrounds the piezoelectric transducer <NUM> from the magnetic tail mass <NUM> (in other embodiments, the head mass <NUM>) so that the magnets are separated by a small gap that yields strong magnetic attraction in order to result in relatively high compression forces on the piezoelectric transducer <NUM>.

<FIG> show schematic illustrations of alternative energy transduction device embodiments in the form of acoustic projectors <NUM>, <NUM>. Acoustic projectors <NUM>, <NUM> are examples of electro-acoustic transduction devices. Acoustic projectors <NUM>, <NUM> may resemble vibration energy harvesting device embodiments described herein but are optimised as acoustic projectors. Acoustic projectors <NUM>, <NUM> are also suitable or modifiable for energy harvesting and/or detection purposes. In particular, acoustic projector devices described herein, such as acoustic projectors <NUM>, <NUM>, may be used as vibration detector devices. In such a context, acoustic projectors <NUM>, <NUM> may be used to detect large or small pressure waves impinging on the distal projection/detection surface of the device by observing current fluctuations on electrodes that are electrically coupled to the piezoelectric transducer of such devices.

Referring first to <FIG>, acoustic projector <NUM> is similar in general design to the acoustic projector <NUM>, except that it has a magnet <NUM> that is disposed at an axial centre, while a piezoelectric transducer <NUM> is disposed coaxially and concentrically around the magnet <NUM>. Acoustic projector <NUM> includes a magnetic tail mass <NUM>, shims 1515a, 1515b and a magnetic head mass <NUM> in a similar configuration to acoustic projector <NUM>. In acoustic projector <NUM>, no physical spacer is interposed between the central magnet <NUM> and the tail mass <NUM>. However, there is still an axial gap <NUM> (of between about <NUM> and about <NUM>, for example) defined between the central magnet <NUM> and the tail mass <NUM> in order to induce a static compressive force due to magnetic attraction.

The components of acoustic projector <NUM> may be wholly or at least in part housed in a housing <NUM>. The acoustic projector <NUM> may also have an outer casing <NUM> to at least cover its distal projecting surface, and optionally to cover most or all of the housing <NUM>. The outer casing may include a thin rubber or silicone sheet material, for example.

Magnet <NUM> may include multiple magnet elements coupled together in a magnet arrangement or may comprise a unitary magnet body, for example. Electrical conductors (not shown) are coupled to the piezoelectric transducer <NUM> in order to apply an excitation current from a varying current source, such as AC source <NUM>. Magnet <NUM> may be affixed to the head mass <NUM>, for example by a suitable adhesive, in addition to being coupled to head mass <NUM> by magnetic attraction.

Magnetic head mass <NUM> may have an additional head mass <NUM> coupled thereto on a distal face of the magnetic head mass <NUM> in order to provide additional resonant mass for frequency tuning or impedance matching. In some embodiments, the additional head mass <NUM> may have a distal outer surface <NUM> that is greater in surface area than an axial cross-section of the magnetic head mass <NUM> or shaped as an acoustic lens in order to generate larger acoustic wavefronts than would be possible with the magnetic head mass <NUM> alone. In other embodiments, the magnetic head mass <NUM> may define the distal outer surface <NUM> and may be configured to have an increasing cross-sectional area in the distal direction in order to generate larger acoustic wavefronts.

The piezoelectric transducer <NUM> may include multiple transducer elements <NUM>. Transducer <NUM> may be arranged on opposite sides of, or at least partly around, the magnet arrangement. In some embodiments, piezoelectric transducer <NUM> includes a series of axially stacked piezoelectric transducer elements <NUM> that are generally of an annular or approximately annular form to extend fully or partway around the magnet <NUM>. In other embodiments, piezoelectric transducer <NUM> may include a ringed or circumferentially spaced or positioned array of axially aligned single crystal transducer elements. The transducer elements of such an array may be wedge-shaped, for example, to allow them to fit together easily. Such a ringed array may resemble the array of ligaments <NUM> of spacer <NUM> shown in <FIG>, for example. The ringed or circumferential array may have the piezoelectric transducer elements tightly or loosely packed in a generally circular or near-circular surrounding fashion about the magnet <NUM>. The ringed or circumferential array is preferably symmetric about at least two axes that are orthogonal to the proximal-to-distal (alignment) axis of the acoustic projector <NUM>. The piezoelectric transducer <NUM> is arranged to have a central axis that is axially aligned with the rest of the acoustic projector <NUM>, including the magnet <NUM>, tail mass <NUM> and head mass <NUM>.

The material of piezoelectric transducer <NUM> and its constituent piezoelectric elements <NUM> may be formed of or comprise one or more RFSC transducer elements as described above in relation to vibration energy harvesting embodiments or it may include more conventional piezoceramics, such as Navy Type lead zirconate titanate (PZT) compositions. Where relaxor ferroelectric single crystals are used for piezoelectric transducer <NUM> or piezoelectric elements <NUM>, the crystals may be arranged in a d32-mode cylinder using [<NUM>] poled material. In described piezoelectric transducer arrangements for acoustic projectors <NUM>, <NUM> using an array of multiple RFSCs, the high applied excitation voltage may be applied via electrical conductors (not shown) to the radially inner piezoelectric faces, with ground on the outer faces, the <NUM> direction oriented from the inside of the ring radially outwards, and the <NUM> direction aligned with the axial direction of the acoustic projector <NUM>, <NUM>.

Shims 1515a, 1515b are or may be formed of a thin (relatively soft) machinable ceramic material, such as Macor. Shims 1515a, 1515b are different from shims 115a, 115b in that they are annular and define central apertures through which the magnet <NUM> passes.

Shim 1515a is proximally positioned and adhered to the tail mass <NUM>. Shim 1515a has a proximal end of the piezoelectric transducer <NUM> abutting it. The material thickness and the central aperture of shim 1515a are sized to allow the magnet <NUM> to pass partly into and out of the aperture as the piezoelectric transducer <NUM> undergoes axial expansion or contraction in response to varying current from AC current source <NUM>. Shim 1515a thus acts as a spacer and may have a thickness slightly greater than the expected axial deflection of the piezoelectric transducer <NUM>. For example, if the maximum expected axial deflection is <NUM>, then the thickness of the shim 1515a may be about <NUM>.

Shim 1515b is distally positioned and adhered to the head mass <NUM>. Shim 1515b has a distal end of the piezoelectric transducer <NUM> abutting it. The material thickness of the shim 1515b does not need to be the same as for shim 1515a and the central aperture defined by the annulus of shim 1515b need only be sized to allow the magnet <NUM> to pass through it. Both shims 1515a, 1515b should at least provide sufficient flat surface area for contacting end faces of the piezoelectric transducer <NUM>.

Referring also to <FIG>, a further acoustic projector embodiment is shown as acoustic projector <NUM>. Acoustic projector <NUM> is the same as acoustic projector <NUM>, except that the magnet <NUM> is coupled to the tail mass <NUM> instead of the head mass <NUM> and the functions and configurations of the shims 1515a, 1515b are reversed. In other words, the distal shim 1515b is the one that has a material thickness and central aperture sized to allow the magnet <NUM> to pass partly into and out of the aperture as the piezoelectric transducer <NUM> undergoes axial expansion or contraction in response to varying current from AC current source <NUM>. In acoustic projector <NUM>, shim 1515b thus acts as a spacer and may have a thickness slightly greater than the expected axial deflection of the piezoelectric transducer <NUM>. For example, if the maximum expected axial deflection is <NUM>, then the thickness of the shim 1515b may be about <NUM>.

Shims 1515a, 1515b may be suitable for their ability to allow a slight amount of surface deformation, thereby providing a slightly softer surface than most rare earth magnets and reducing the likelihood of fractures forming in the piezoelectric transducer <NUM> (when formed as a crystal or including a series of crystals). However, in some embodiments, other analogously deformable materials may be used in place of shims 1515a, 1515b. Such analogous materials may be provided as a coating, layer, layer with material compositional gradient, or thin sheet, disposed on a distal side of the tail (first) magnet <NUM> or a proximal side of the head (second) magnet <NUM>, for example. Such analogous materials may, for example, include a magnet-glass composite material with a higher glass concentration at a surface at which it is intended to contact the piezoelectric transducer <NUM>.

For acoustic projector <NUM>, the magnet <NUM> may make direct contact with the head mass <NUM> or may be separated therefrom by an adhesive bonding layer that is sufficiently thin that the magnetic attraction between magnet <NUM> and head mass <NUM> is negligibly affected. Similarly with acoustic projector <NUM>, the magnet <NUM> may make direct contact with the tail mass <NUM> or may be separated therefrom by an adhesive bonding layer that is sufficiently thin that the magnetic attraction between magnet <NUM> and tail mass <NUM> is negligibly affected.

In acoustic projector <NUM>, no physical spacer is interposed between the central magnet <NUM> and the head mass <NUM>. However, there is still an axial gap <NUM> (of between about <NUM> and about <NUM>, for example) defined between the central magnet <NUM> and the head mass <NUM> in order to induce a high (e.g. <NUM>-500N or <NUM>-500N) static compressive force due to magnetic attraction. For acoustic projectors <NUM>, <NUM>, no spacer is needed because mechanical loads, such as compression and bending, are taken up by the piezoelectric transducer <NUM>.

Other than as noted above, acoustic projector <NUM> is the same as acoustic projector <NUM>. For example, the components, such as housing <NUM>, piezoelectric transducer <NUM>, tail mass <NUM>, head mass <NUM>, additional head mass <NUM> and outer casing <NUM> are shown by the same reference numerals in <FIG>.

Acoustic projectors <NUM>, <NUM>, <NUM> and other energy transduction device embodiments described herein employ an arrangement in which the piezoelectric transducer <NUM>, <NUM> is coaxial with a magnet arrangement <NUM>, <NUM> (or <NUM>, <NUM> in <FIG>) or a magnet <NUM>. In some embodiments, the magnet arrangement <NUM>, <NUM> or <NUM>/<NUM> extends in a direction parallel to an axis, such as axis <NUM>, along which centres of the components are aligned and are positioned at one or more positions radially spaced (outward) from the central axis, with a piezoelectric transducer, such as piezoelectric transducer <NUM>, being aligned with and extending along the central axis. In other embodiments, the radial positions of the magnet arrangement or magnet and the piezoelectric transducer are swapped. In these other embodiments, a piezoelectric transducer, such as piezoelectric transducer <NUM>, is positioned to have one or more constituent parts disposed radially outside a magnet, such as magnet <NUM>, which is aligned with and extends along the central axis. In this context, the term coaxial is intended to describe an arrangement where the centre of mass of the magnet arrangement or magnet is generally axially aligned with the centre of mass of the piezoelectric transducer, irrespective of exactly what form each magnet or piezoelectric component takes and how many constituent parts make up each magnet or piezoelectric component. The term coaxial also describes the axial alignment of the piezoelectric transducer <NUM>, <NUM> and magnet arrangement <NUM>, <NUM> or magnet <NUM> with the other device components, such as tail mass <NUM>, <NUM>, head mass <NUM>, <NUM>, shims 115a, 115b, 1515a, 1515b, alignment disc <NUM> (if present) and spacer <NUM> (if present).

In some embodiments, the magnet or piezoelectric components will have a generally circular or circular array configuration. In such embodiments, but also in non-circular embodiments (such as is depicted in <FIG>), the magnet and piezoelectric components may also be described as concentric, with the magnet component disposed radially inside the piezoelectric component or the piezoelectric component disposed radially inside the magnet component.

Acoustic projector devices described herein, such as acoustic projector <NUM>, <NUM>, <NUM>, may form part of an acoustic projection system including multiple such devices in combination. Such multiple acoustic projector devices may be located adjacent each other in an array or bank of such projectors, or they may be arranged at spaced locations. In such a system, multiple ones of the acoustic projectors may be directed in a substantially same direction and/or multiple ones of the acoustic projectors may be directed towards different directions.

As described herein, various embodiments apply a compressive mechanical pre-load to the piezoelectric transducer element of an acoustic projector. Some prior acoustic ultrasonic projector designs utilise an axial bolt/nut (otherwise known as a tie rod and sometimes called a stress rod) to provide a static compressive stress to the piezoelectric element. The greater the pre-stress, the larger the amplitude of operation permitted before the transducer is driven into tension, where it will typically fail (due to it being a ceramic).

The arrangement of magnets in combination in an axial magnet assembly as described herein provides an alternative source of static compressive stress to a tie rod. The described arrangement has an advantage of lower damping and a greater range of unhampered resonant motion, since no plate spring is required at the end of the tie rod. The magnetic arrangement is not limited to a single cylinder surrounding the piezoelectric element, but can be configured to have various numbers of magnets and spacers with varying geometries, examples of which are described above.

The results of magnetic calculations shown in Table <NUM> indicate the significant compressive force that can be produced using a magnetic pre-stress arrangement. This has multiple potential advantages for acoustic projection, such as:.

The results of magnetic calculations shown in Table <NUM> indicate the significant compressive force that can be produced using a magnetic pre-stress arrangement, and lowest resonant frequencies that can be achieved.

The key objective of an acoustic projector is to produce a relatively large mechanical displacement, which in turn radiates acoustic energy into the adjacent medium. For example, the dynamic strain of a vibrating piezoelectric bar can be approximated by: <MAT>.

where S is the dynamic strain, Qm is the mechanical quality factor, dij is the piezoelectric coefficient and E is the applied electric field. For the proposed magnetic compression arrangements, d<NUM> is chosen due to the compliant <NUM>-axis and the benefits which stem from it, including lower operational frequencies and greater power density which is beneficial for a more efficient and/or portable design. As an example, a <NUM>rd generation RFSC has a large Qm (typically <NUM>), a large, dij (typically <NUM> pC/N), and a large Ec (<NUM> kV/cm); so is capable of producing large dynamic strains.

There are benefits to using <NUM>st generation RFSCs over more conventionally utilised piezoceramics. Such benefits include the lower modulus/higher compliance <NUM>-axis, the higher coupling constant, and the higher piezoelectric strain constant; <NUM>rd generation RFSCs have these benefits as well as an exceptional Qm, making it an ideal choice.

Predictions of dynamic strain are presented in Table <NUM>. The crystal transducer geometry is assumed to be <NUM> × <NUM> × <NUM><NUM> and the value of the maximum voltage is assumed to be <NUM> EC (for a distance of <NUM> in the <NUM>-direction). For demonstration purposes only the transducer is considered, the effect of the surrounding projector structure is ignored. The potential effects of crystallographic phase change are also ignored.

Table <NUM> indicates that Mn-PMN-PZT is the most appropriate choice for transmission/projection, at least in air and at shallow water depths. Using Mn-PMN-PZ-PT as the electrical-to-mechanical transducer is beneficial for projection due to its high Qm, large piezoelectric constant d, low elastic modulus sE, and high coupling k. PIN-PMN-PT may be practical for sensory applications due to the sensitivity provided by its greater piezoelectric constant d<NUM> and coupling k. The acoustic projector devices <NUM>, <NUM>, <NUM> described herein include both magnets and support structure that provides additional mass, and may include additional mass or stiffness since the magnets act as a spring in parallel with the crystal transducer. This effects the overall mechanical quality of the device through the following equation: <MAT>
where M is the mass, k is the spring constant and D is the damping coefficient.

The proposed method of magnetic compressive pre-stress, when coupled with <NUM>rd generation piezoelectric elements, will allow the benefits described below.

The compliant <NUM>-axis of the crystal transducer allows for lower operating frequency than devices manufactured using traditional piezoceramic transducers. Scattering losses typically increase with the <NUM>th power of frequency, however the size of the projector is inversely proportional to the working frequency when resonance conditions are required.

With sinusoidal tone burst excitations (as exemplified in <FIG>) instead of a single rectangular, or "spike" pulse, the piezoelectric transducer <NUM>, <NUM> will be capable of a resonant response, increasing the signal projection efficiency and signal reception sensitivity of the device. After four interfaces (from the probe to the couplant(s), to the desired location and back again), there will only be a small percentage of the original acoustic energy left. This issue is partially mitigated by resonant operation.

A lower transmission frequency is helpful for sonotrodes (i.e. for ultrasonic machining, welding, and mixing), for example as a compact acoustic excitation device for sonic thermography with the benefits for acoustic propagation provided by low operational frequency, particularly, <=<NUM>.

With a significant static pre-load provided by the magnetic arrangement, the transducer will be compact and able to endure a large electrical excitation voltage without being driven into tension. This increases the operation capability and general durability of the system.

For acoustic projection devices, the matching of acoustic impedances in the head mass to both air and water is considered. When sound waves pass through an interface between two materials, only a portion of the energy is transmitted; the rest is reflected and otherwise lost. The proportion of the energy transmitted depends on how closely the acoustic impedance of the two materials matches. The frequency attenuation of air exponentially increases with frequency, therefore air-coupled ultrasound devices operate below <NUM>.

The reflection and transmission coefficients are given by the following formulas (for a wave excitation perpendicular to the test plane):
<MAT>
<MAT>
where R is the reflection coefficient and T is the transmission coefficient, with the wave travelling from a medium with acoustic impedance Z<NUM> to a medium with acoustic impedance Z<NUM>, and Zi = ρi × vi, i=<NUM>,<NUM>, with ρi the density and vi the velocity of sound in the medium. It is apparent that the closer the two values are to each other, the greater the transmission and lesser the reflection, hence the need to match acoustic impedances as closely as possible for a better signal-noise ratio. Typical acoustic impedances are shown for various commonly used materials in Table <NUM>.

The energy transmission coefficient from one medium to another is calculated by the following formula (multipliable by <NUM> for percentage of energy transmission):
<MAT>.

Table <NUM> shows the parameters for a range of materials useful in acoustic projection models. The Table <NUM> parameters can be used for estimating acoustic transmission (for examples, see Tables <NUM> and <NUM> below).

Tables <NUM> & <NUM> show the benefit of acoustic impedance matching the projector to the medium. One way of accomplishing impedance matching is through optimising the interface materials shown in Table <NUM>.

An inherent weakness of air-coupled ultrasound is the low acoustic impedance of air, which is typically <NUM><NUM> times lower than other materials (compare Table <NUM> for air with Table <NUM> for water). This leads to small values of acoustic energy transmission, diminishing but not eliminating the effects of acoustic impedance matching. However, the unhampered resonant motion of a device using magnetic compressive pre-loading (as opposed to using a tie rod) may counteract this by providing greater electrical to mechanical efficiencies.

A magnetically pre-stressed air-coupled acoustic projector can be used to produce Lamb/Plate waves which can travel significant distance in suitable materials, especially at a low frequency. An example through-transmission arrangement that uses a separate transmitter <NUM> and receiver <NUM> is shown in <FIG>. It is possible to arrange the measurement such that the air-coupled transmitter <NUM> and receiver <NUM> are on opposite sides of the test piece, as shown in <FIG>, or on the same side of the test piece, such as shown in <FIG>. The air-coupled transmitter <NUM> may measure line segments instead of single points, greatly increasing testing speeds in applications where precise imaging is not required. It is contemplated that acoustic projector embodiments <NUM>, <NUM>, <NUM> may be used for the transmitter <NUM> in the arrangements shown in <FIG>. It is also contemplated that such acoustic projector embodiments <NUM>, <NUM>, <NUM> may be used in an acoustic detection mode for the receiver <NUM> in the arrangements shown in <FIG>. In some embodiments, the receiver <NUM> can be or include a sensor (other than vibration energy transduction devices described herein) that is configured to directly or indirectly sense an output or effect from or induced by the transmitter <NUM>. Examples of such a sensor include a thermal camera or a scanning laser vibrometer. In such embodiments, acoustic energy from the transmitter may excite radiation, damage or another thermally or optically detectable effect in the test piece or other interposed medium that can be detected by the receiver <NUM>.

Underwater acoustic technology may be used for industrial and scientific purposes. Active sonar transmits and receives echoes returning from the target, while passive sonar only intercepts noise radiated by an external target source. Examples of industrial and scientific applications include but are not limited to:.

For oceanography, high powered low frequency projectors are desirable. The low power output of the design can be compensated for by the use of multiple electro-acoustic transduction devices, such as multiple individual acoustic projector devices <NUM>, <NUM>, <NUM>, in an array. An example oceanographic application is illustrated in <FIG> by a craft <NUM>, such as a water craft, that has an electro-acoustic transduction device, which may be an acoustic projector <NUM>, <NUM>, <NUM>, for example, mounted to amounting body, such as an underside of the hull <NUM> of the craft <NUM>. The watercraft may include a boat, ship or submarine, for example. In some embodiments, multiple ones of the electro-acoustic transduction device (e.g. in the form of acoustic projector <NUM>, <NUM>, <NUM>) may be mounted to one or more mounting bodies to project vibration (acoustic) energy away from the one or more mounting bodies. The one or more mounting bodies may include the hull <NUM> and/or mounting structures that are in turn mounted to the hull <NUM>. In other examples, the one or more mounting bodies may include a movable craft other than a watercraft or one or more static mounting bodies, such as pylons, walls or fixed surfaces that face toward a fluid volume, such as water or air. The multiple electro-acoustic transduction devices may be directed to emit or detect pressure waves to or from the same or multiple directions. Such multiple transduction apparatus/device embodiments may form part of an acoustic projection system or an acoustic detection system (e.g. including or as part of craft <NUM>) that includes mounting structure/mounting bodies and suitable control systems and power supplies for operation of such systems.

The Figure of Merit (FoM) for piezoelectric transducers in underwater applications is dijQm which is associated with initial acoustic velocity and/or k<NUM>Qm which is associated with electroacoustic efficiency. Given the already mentioned properties of the proposed magnetic arrangement, in particular a high Qm, resonant motion and the potential for crystallographic phase change, it can be inferred that:.

The low-power and low-directivity of individual low-frequency projectors can be overcome by assembling several of them in a close-packed array with suitable control by a local controller that controls excitation currents to each of the acoustic projectors. This can result in a larger source level and increased directivity when compared to a single acoustic projector.

An array of acoustic projectors utilising magnetic pre-stress may be configured such that it does not require individual housing cases for each acoustic projector. Potentially, the magnetically active individual acoustic projector may be arranged in an appropriate magnetic circuit to optimise the pre-stress on an individual acoustic projector.

Tonpilz acoustic projectors can be used as hull-mounted underwater electro-acoustic transducers, for example in the manner illustrated in <FIG>. They utilise a stack of ring-shaped piezoelectric material with an axial tie rod, with the rings polarised along the length of the stack with alternating polarity, interspersed with electrodes, bonded together and electrically connected in parallel. Tonpilz projectors are mounted within sturdy, water-tight housings, with the front radiation surface covered with an acoustically transparent rubberised "boot".

The resonant frequencies of Tonpilz transducers are greater than that of barrel-stave flextensional transducers, examples of which have been found to resonate at ><NUM>. The Tonpilz example shown by <NPL>) ("Inoue et al") has a resonant frequency of ~<NUM>. A <NUM> paper describing the design, optimization, manufacture and characterization of a Tonpilz-type transducer for low frequency applications had a resonant frequency of ~<NUM>.

The device shown by Inoue et al is optimised for a low operational frequency. The Inoue et al paper shows that the volume of their Tonpilz transducer (including everything minus the housing case) is approximately <NUM><NUM>, whereas the acoustic projector device <NUM> (<FIG>) with a <NUM> long tungsten carbide tip mass has a volume of approximately <NUM><NUM> (<NUM>% of volume of the device in Inoue et al). The mass of the prototype of acoustic projector device <NUM> is ~<NUM>% of the design in Inoue et al, and the volume of the piezoelectric element is ~<NUM>% of the design in Inoue et al. This demonstrates the compact form that the proposed magnetic pre-stress arrangement allows.

Vibration energy transduction devices according to embodiments of the present disclosure advantageously do not employ non-magnetic mechanical compression mechanisms to exert the static compressive force. For example, embodiments do not use (are free of) a tie rod for exerting the static compressive force on the piezoelectric transducer <NUM>.

Claim 1:
An energy transduction apparatus, including:
a base (<NUM>);
a first magnet (<NUM>) coupled to or comprising the base (<NUM>);
a piezoelectric transducer (<NUM>) disposed adjacent the first magnet (<NUM>);
a magnet arrangement (<NUM>) co-axial with the piezoelectric transducer (<NUM>), wherein the magnet arrangement is disposed on opposite sides of or at least partly around the piezoelectric transducer, or the piezoelectric transducer is disposed on opposite sides of or at least partly around the magnet arrangement, wherein the magnet arrangement is poled to have a first end of the magnet arrangement attracted to the first magnet;
a second magnet (<NUM>) poled to be attracted to a second end of the magnet arrangement that is opposite the first end;
a vibratable mass (<NUM>) coupled to or comprised by the second magnet;
electrical conductors electrically connected to the piezoelectric transducer to conduct current between the piezoelectric transducer and external circuitry; and
wherein the first magnet, the piezoelectric transducer, the magnet arrangement, and the second magnet are substantially coaxial;
wherein the first magnet, the second magnet and the magnet arrangement cooperate to keep the piezoelectric transducer in compression;
wherein vibrational movement of the second magnet is directly related to compression of the piezoelectric transducer and current flow in the electrical conductors; and
wherein the apparatus is configured to convert current in the electrical conductors into vibration of the vibratable mass in a frequency range of about <NUM> to about <NUM> to thereby act as an acoustic projector.