Patent Description:
Flextensional transducers are used to produce a high power low frequency sound in a compact package for underwater use. For instance, they may be incorporated into water based sonar systems, e.g. a sonobuoy. However, current designs can be too large and heavy rendering them unsuitable.

The size of the device impacts the resonant frequency of the device. Reducing the size of the transducer will increase the resonant frequency of the device, for example. To produce a smaller device with a comparatively low resonant frequency, the stave material can be made less stiff either through choice of material or by increasing a degree of porosity in the stave structure by using a lattice.

The use of a lattice increases the amount of damping in the transducer stave design. This damping increase reduces the sound power level created by the device.

The aim of the subject-matter of the present disclosure is to improve on the prior art.

<CIT> describes a leaky-wave antenna for fluid environments including a waveguide cavity defined by a waveguide wall. The waveguide cavity is filled with a waveguide fluid. The waveguide walls are made of either an anisotropic material that utilize one of orthotropic stiffness of the anisotropic material to control mode conversion, a band gap material to approximate an acoustically rigid boundary, and a combination of the two materials.

<NPL>, describes a review of hydrophones, their design considerations, physical aspects, and structures.

<CIT> describes a flextensional transducer for underwater operation including a driving element and a stave. The stave is made from a material with elastic properties and has a porous structure.

According to an aspect of the present disclosure, there is provided a flextensional transducer as recited in claim <NUM>. The stave being made from an auxetic lattice results in radial deflection that is larger per unit of lengthwise deflection compared to a structure that is not auxetic. As a result, sensitivity of the transducer is increased. The increase in sensitivity occurs whether the transducer is used as a transmitter or a receiver.

The auxetic lattice may be an isotropic auxetic lattice. An isotropic lattice exhibits increased sensitivity in all directions.

The auxetic lattice may be an anisotropic auxetic lattice. An intended axis of movement of the stave in-use may be configured to have a Poisson's ratio having a highest magnitude compared to the other two axes. The stave will exhibit the highest sensitivity in the direction in which the axis of the lattice having the highest Poisson's ratio extends. In this way, employing an auxetic lattice provides a means to tune the sensitivity of the stave.

The auxetic lattice may be an orthotropic auxetic lattice.

The dimensions of the orthotropic auxetic lattice's structure may be different in each Cartesian axis.

The stave may have a non-uniform cross-section. Non-uniformity may improve the sensitivity of the stave in a particular direction.

The stave may have an elliptical cross-section.

The auxetic lattice may be constructed using one or more lattice types selected from a list including: S-shape hinges, re-entrant honeycomb, chiral truss, square matrix, triangular matrix, and bucklicrystals.

The stave may extend in a first direction and wherein a volume fraction of the stave may be graduated along the first direction. Graduating the volume fraction enables a reduction in fatigue at certain points of the stave.

Graduating the volume fraction enables a reduction in fatigue at certain points of the stave. This inventive concept is independent of the auxetic nature of the lattices referenced above. This is because the graduated porosity of the stave enables reduced fatigue at certain points of the stave for both auxetic and non-auxetic lattices.

The volume fraction may increase toward a centre of the stave, or the volume fraction may decrease toward a centre of the stave. The centre of a stave may exhibit more or less deflection than respective ends of the stave, depending upon the overall configuration and shape of the flextensional transducer.

The stave may be made by additive manufacturing.

The stave may be made by laser powder bed fusion.

The flextensional transducer may be for use as a sonar transmitter.

The flextensional transducer may be for use as a sonar receiver.

The flextensional transducer may be an inverted barrel-type transducer.

According to a further aspect of the present disclosure, there is provided a water based sonar device comprising the aforementioned flextensional transducer.

The subject-matter of the present disclosure is best understood with reference to the accompanying figures, in which:.

With reference to <FIG>, according to embodiments of the present disclosure, there is shown a flextensional transducer <NUM>. The flextensional transducer <NUM> is suitable for use underwater. More specifically, the flextensional transducer <NUM> is suitable for use as part of a sonobuoy.

The flextensional transducer <NUM> is an inverted barrel type transducer. In other words, the flextensional transducer <NUM> has a first end <NUM> and a second end <NUM>, which are both substantially cylindrical. The flextensional transducer <NUM> includes a stave <NUM> extending between the first end <NUM> and the second end <NUM>. The stave <NUM> has a substantially concave cross section (see <FIG>). The concave cross section means that the stave <NUM> bends inwards towards a centre axis of the flextensional transducer <NUM> at a central portion of the stave <NUM>.

With reference to <FIG>, the flextensional transducer <NUM> includes a centre bolt <NUM>, a ceramic stack <NUM>, electric insulators <NUM>, a first end cap <NUM>, a second end cap <NUM>, a first end nut <NUM>, a second end nut <NUM>, and a rubber coating (not shown).

The centre bolt <NUM> is made from an elongate threaded cylinder made from a metallic material. The metallic material may include steel and may be pre-stressed. The centre bolt <NUM> forms the central structure to which the other parts are mounted.

The ceramic stack <NUM> is mounted to the bolt <NUM>. The ceramic stack <NUM> may be substantially cylindrical and have a central bore. The ceramic stack includes a plurality of piezoelectric discs. The discs may be ring shaped in view of the central bore. The centre bolt <NUM> passes through the central bore of the ceramic stack <NUM>.

Whilst not shown in the figures for brevity, the ceramic stack <NUM> includes electrical insulators. A electrical insulator may be provided between the ceramic stack <NUM> and the centre bolt <NUM>. There is also an insulator <NUM> at each end of the ceramic stack <NUM> between the ceramic stack <NUM> and the first and second end caps <NUM>, <NUM>. Although not shown in the figures, there is an insulator between each set of adjacent ceramic discs. For instance, the number of ceramic discs is one less than the number of insulators <NUM>.

Whilst not shown in the figures for brevity, the flextensional transducer <NUM> includes electrodes connecting the piezoelectric discs together, and a wire to connect the electrodes to outside the device for sensing the piezoelectric charge or for receiving the signal to charge the piezoelectric discs.

The first and second end caps <NUM>, <NUM>, are at either end of the ceramic stack <NUM>. The first and second end caps <NUM>, <NUM>, each have a substantially T-shaped cross-section with a bore extending through them. The T-shaped cross-section is formed from a head <NUM> of the caps <NUM>, <NUM>, substantially in the shape of a hexagonal prism (see <FIG> and <FIG>), and an elongate shaft <NUM>. The bore is dimensioned and configured to receive the centre bolt <NUM> therethrough. The head <NUM> includes a plurality of circumferentially arranged bores <NUM> including interior threads extending radially from a central axis. In the embodiment shown in <FIG> there may be six bores spaced equally from one another, e.g. about <NUM> degrees separating each bore <NUM>. In other embodiments, the number of bores may be more or less.

As shown in <FIG>, the stave <NUM> may be constructed from a plurality of stave segments. Each stave segment may be substantially the same as the other stave segments. In the embodiment shown in <FIG>, there may be six stave segments, though the number may be more or less. The stave segments include a bore <NUM> at each end. The stave segments are attached to the end caps <NUM>, <NUM>, using screws <NUM>. The screws <NUM> are dimensioned and configured to pass through the bores <NUM> of the stave segments and threadingly engage the bores <NUM> of the end caps <NUM>, <NUM> shown in <FIG>. The screws <NUM> may be made from a metallic material. The screws <NUM> are shown as cross-head type screws though in other embodiments they may be flat-heat type or another type of head. The ends of the stave segments may include a flat interior face to engage one of the faces of the hexagonal end caps <NUM>, <NUM>, and may have a substantially arc shaped exterior face. The exterior face may include a counter hole to counter-sink the screw <NUM> into the bores <NUM>.

In the embodiment shown in <FIG> and <FIG>, the structure of the stave <NUM> is perforated between the first and second ends. In other words, the stave <NUM> may be perforated in the concave section and monolithic at the end sections. The holes of the perforated stave <NUM> may be substantially uniformly distributed.

The end caps <NUM>, <NUM> may be secured to the centre bolt <NUM> by fastening the first and second end nuts <NUM>, <NUM> to the centre bolt <NUM>. O-rings <NUM>, <NUM>, may be provided between the first end cap <NUM> and the first end nut <NUM>, and between the second end cap <NUM> and the second end nut <NUM>.

A rubber coating (not shown) may be applied to the exterior of the assembled flextensional transducer <NUM> to improve water resistance.

With reference to <FIG> and <FIG>, when the flextensional transducer <NUM> is used as an active device, i.e. a transmitter, the ceramic stack receives an electrical signal through the wiring and electrodes. A charge is formed in each of the piezoelectric discs. The piezoelectric discs are configured to move in response to the charge by converting the electrical signal to kinetic energy. The movement of the discs causes a force to be exerted on the end caps <NUM>, <NUM>. In turn, the end caps <NUM>, <NUM>, move outwardly. The outward movement may be followed by inwards movement and the end caps may oscillate. The movement of the end caps <NUM>, <NUM>, induces movement into the stave <NUM>. The movement of the stave <NUM> follows the movement of the end caps <NUM>, <NUM>. For instance, when the end caps <NUM>, <NUM>, move away from one another, the stave <NUM> are tensioned. The tension induced in the stave <NUM> causes the stave <NUM> to flex radially outwards. Conversely, when the end caps <NUM>, <NUM>, move towards one another, the stave <NUM> is compressed. The compression causes the stave <NUM> to flex radially inwards. The transducer may also be used as a passive device. In other words, the flextensional transducer <NUM> may be configured as a receiver. When used as a receiver, the flextensional transducer <NUM> may receive vibrations in the surrounding water and deflect the stave <NUM>. When the stave deflects, the end caps may move apart and together in-sync with the outwards movement of the stave <NUM> (see <FIG>). The movement of the end caps <NUM>, <NUM>, is converted to an electrical signal by the piezoelectric discs.

The movement pattern of the flextensional transducer results from its inverted barrel shape with the concave stave. In this way, the flextensional transducer may be understood as being an auxetic assembly. An auxetic is a structure exhibiting a negative Poisson's ratio.

With reference to <FIG>, the material of the stave of a conventional transducer may be made from a substantially monolithic structure. In other words, the conventional stave may itself have a positive Poisson's ratio at bulk level. As can be seen from <FIG>, the bulk stave at rest has certain dimensions for height and width. As can be seen from <FIG>, the bulk stave material may have a larger height and a smaller width when experiencing a forcing in the height direction stretching the bulk material, as would be the case when the transducer is transmitting the signal.

Therefore, a conventional stave at bulk level may impair the sound power or the sensitivity of the device when used as either a transmitter or receiver.

With reference to <FIG>, a stave <NUM> according to the present disclosure may itself be made from an auxetic material. The auxetic material may be an auxetic lattice, which at unit cell level exhibits auxetic behaviour. In this embodiment, the stave <NUM> may be made from a re-entrant honeycomb. The re-entrant honeycomb is configured to expand in a width ways direction in response to being expanded in a lengthways direction. As can be seen from <FIG>, by making the stave <NUM> from an auxetic material, e.g. in the form of a re-entrant honeycomb, the amount of radial expansion of the stave <NUM> in response to separation of the end caps <NUM>, <NUM>, is larger than when the stave <NUM> is made from a non-auxetic material (e.g. the monolithic structure) shown in <FIG>. This increase in radial deflection acts to amplify the sound power when transmitting sound. In this way, when the flextensional transducer <NUM> is used as a transmitter, the sound waves produced will exhibit increased amplitude. Conversely, when the flextensional transducer <NUM> is used as a receiver, the receiver will benefit from increased sensitivity.

The auxetic material of the stave <NUM> may be an isotropic auxetic material. In other words, the auxetic material may have substantially equivalent Poisson's ratios in each axis of a Cartesian co-ordinate system.

In other embodiments, the auxetic material of the stave <NUM> may be anisotropic. In other words, the auxetic material may have dissimilar Poisson's ratios in one axis of a Cartesian co-ordinate system compared to the other axes. The anisotropic auxetic material may be an orthotropic auxetic material having dissimilar Poisson's ratios in each axis of the Cartesian co-ordinate system. In such embodiments, it is possible to tune the auxetic properties of the material in a particular direction. In the case of a flextensional transducer <NUM>, the Poisson's ratio may be configured to have highest magnitude in a direction in which the stave <NUM> extends radially. In this way, the sensitivity/power may be increased or even maximised. For instance, where the bulk material may have ratios of <NUM>:<NUM>:<NUM>, an anisotropic lattice may have a ratio of <NUM>:<NUM>:-<NUM>, for example.

Another reason to tune the response of the stave <NUM> using an anisotropic auxetic material is to change the resonant frequency. Changing the resonant frequency can be used for providing a frequency filter. For instance, the frequency filter may be configured as a band pass filter. The band pass filter may be configured to permit waves between pre-determined frequencies to be transmitted/received, but preventing others below and above the pre-determined frequencies by structuring the material to damp out those frequencies outside of the desired band.

In order to adjust the response in each axis, it is possible to change the geometry of the material cells. In addition, it may be possible to adjust the geometry of the flextensional transducer <NUM> to tune the response in a particular direction, e.g. by making the flextensional transducer <NUM> have a non-circular cross-section. For example, the flextensional transducer <NUM> may be configured to have an elliptical cross-section.

The auxetic material may include a lattice structure. The lattice structure can be understood as having a degree of porosity, where the pores are the voids between the lattice structure. The pores modify the effective Young's Modulus of the stave <NUM> by decreasing the volume fraction, i.e. decreasing the volume of solid material by increasing a void volume. The volume fraction is the volume of the solid material divided by the sum of the solid material volume and the void volume. The relationship between the degree of porosity may be seen the formula below <MAT> where E is the effective Young's Modulus in the presence of a porous structure, E<NUM> is the Young's Modulus of the stave <NUM> without holes, p is the degree of porosity (measured between zero and <NUM>), pc is the degree of porosity at which the effective Young's Modulus E becomes zero (usually found to be approximately equal to <NUM>), f is the parameter dependent on the grain morphology and pore geometry of the porous structure.

For example, in a situation where the f parameter is found to be equal to <NUM>, and a volume of bulk steel is made <NUM>% porous (p = <NUM>), the effective Young's Modulus of that volume of porous bulk steel is reduced from an approximate Young's Modulus of 200GPa to 108GPa. This also has the effect of the overall effective density of the stave <NUM> being reduced from <NUM>/m<NUM> to <NUM>/m<NUM>.

The degree of porosity can therefore be varied to adjust the effective Young's Modulus and overall effective density as required in order to achieve the target resonant frequency of the flextensional transducer <NUM> for a given size, shape, and material. This allows materials to be employed with properties that would not conventionally be appropriate for such devices, such as those that fall outside the normally acceptable areas shown on an Ashby material selection chart.

According to an embodiment of the present disclosure, the porosity of the stave <NUM> material can be varied along the stave <NUM>. In this way, the stave <NUM> may be said to have graduated porosity. Graduating the porosity enables density changes locally at regions of the stave <NUM> that experience the largest displacement, in-use. Graduating the porosity is an inventive concept that is independent of the auxetic nature of the lattices described herein. This is because the graduated porosity of the stave will enable reduced fatigue at certain points of the stave for both auxetic and non-auxetic lattices. As an example, a non-auxetic lattice may be a lattice, e.g. a honeycomb, having a positive Poisson's ratio.

The graduated density may be configured such that the stave <NUM> exhibits a highest volume fraction toward a centre of the stave <NUM>, where the displacement may be highest. Alternatively, the stave may exhibit a highest volume fraction toward the monolithic ends at a route of the concave section.

As described above, the stave <NUM> has two types of pores. Firstly, the stave <NUM> includes voids within the lattice of the stave <NUM>. Secondly, the stave <NUM> includes perforation holes. The lattice voids and/or the perforation holes may be considered as empty space voids for the volume fraction calculation.

A rubber boot may be placed around the lattice to provide water proofing. Alternatively, to provide waterproofing, the lattice voids may be filled with a filler. The filler may be made from an elastic material. The filler may be made from a comparatively low density material. The filler may be made from a material with comparatively high flexibility. An example material may be silicone.

<FIG> show various alternative embodiments for auxetic materials that may be used with the present disclosure. They will each be described in turn below. For the avoidance of doubt, any features not mentioned with reference to <FIG> are common with the embodiments described above.

<FIG> shows a stave <NUM> where the auxetic material is in the form of an S-hinge lattice.

<FIG> shows a stave <NUM> where the auxetic material is in the form of a chiral truss lattice.

<FIG> shows a stave <NUM> where the auxetic material is in the form of a square lattice with no external forces applied. <FIG> shows a stave <NUM> where the auxetic material is in the form of a square lattice with lateral forces applied. It can be seen from <FIG> that by applying lateral force to stretch the stave <NUM>, the lattice expands also in the vertical direction.

<FIG> shows a stave <NUM> where the auxetic material is in the form of a triangular lattice with no external forces applied. <FIG> shows a stave <NUM> where the auxetic material is in the form of a triangular lattice with lateral forces applied. It can be seen from <FIG> that by applying a lateral force to stretch the stave <NUM>, the lattice expands also in the vertical direction.

<FIG> shows a stave <NUM> where the auxetic material is made using Bucklicrystals and is shown in a perspective view. The Bucklicrystals are shown include <NUM> holes in this embodiment. However, in other embodiments, there may be a different number of holes. For example, there may be <NUM> holes per crystal or there may be <NUM> holes per crystal. <FIG> shows the Bucklicrystals from <FIG> from a side view.

There are various ways to manufacture the staves <NUM> to <NUM> described above. For instance, the staves may be made by additive manufacturing. In addition, the staves <NUM> to <NUM> may be made via laser powder bed fusion using an aluminium alloy. In this case, a base centred cubic structure with single axis reinforcements is preferred. The single axis may be z-axis, such that the material is a base centred cubic structure with z-axis reinforcements, BCCZ.

The foregoing embodiments is not an exhaustive set of options for the flextensional transducer. In addition, the flextensional transducer described herein is an inverted barrel type transducer. However, the inventive concepts defined herein may be applied to other flextensional transducer types. <FIG> shows some other types of flextensional transducer that the inventive concepts may be applied to. As can be seen from <FIG>, the inverted barrel type is Class III. There are seven class (I to VII) shown in <FIG>.

Claim 1:
A flextensional transducer (<NUM>) for use underwater, wherein the flextensional transducer (<NUM>) comprises a stave (<NUM>) made from a material having a lattice structure, wherein the stave (<NUM>) bends inwards towards a centre axis of the flextensional transducer (<NUM>) at a central portion of the stave (<NUM>), characterized in that the material is an auxetic lattice material.