EP3638429B1

EP3638429B1 - High frequency wideband wide beam ultrasound emitter transducer for underwater communications

Info

Publication number: EP3638429B1
Application number: EP18746282.5A
Authority: EP
Inventors: Marcos SILVA MARTINS; Luís Miguel VALENTE GONÇALVES; António João FREITAS GOMES DA SILVA; Sergio Manuel MACHADO JESUS; José Manuel TAVARES VIEIRA CABRAL
Original assignee: Universidade do Minho; Universidade do Algarve
Current assignee: Universidade do Minho; Universidade do Algarve
Priority date: 2017-06-16
Filing date: 2018-06-18
Publication date: 2021-08-04
Anticipated expiration: 2038-06-18
Also published as: EP3638429A1; WO2018229735A1; PT3638429T

Description

Technical field

The present disclosure relates to a high frequency, MHz range, ultrasound underwater emitter transducer with nearly omnidirectional radiation and a broadband frequency response using a multilayer structure of PVDF films for underwater communications. The transducer has the geometric shape of a toroid outer surface, using curved surfaces to control the acoustic wave's divergence angle and PVDF thin films in multilayer structures for a broadband frequency response.

Background

Electroacoustic transducers are a technology with more than 200 years, but the technology underwent a huge development during the First World War. The electroacoustic transducers are divided into 6 different types of technologies: Piezoelectric, Electro-Restrictive, Magnetostrictive, Electrostatic, Variable Reluctance Transducers and Dynamic Coil Transducers. However, the most widely used technology today is Piezoelectric.
The applications have been increasing exponentially, from the SONARES, acoustic image, to the welding by ultrasound, among others. Each application requires a very distinct transducer, with well-defined characteristics. The size and shape of the transducer influences the operation mode. The operating frequency definition, for a transmitter or, for a receiver, has different meanings. Transmitters generally operate at frequencies close to the resonant frequencies, where they allow higher output performance. The receivers, on the other hand, are usually used below their resonant frequencies with a much higher bandwidth.
The beam pattern can be defined as the relative amplitude of the acoustic pressure as a function of the angle. Different patterns are achieved by using particular shapes and/or arrays of transducers, by amplitude shading, beam steering and phase shading. The width of the main lobe, in degrees, is defined as the beam spread angle. Usually, there are additional lobes around the main lobe, which are called lateral lobes. The maximum response axis or the acoustic axis of a transducer is defined as the direction in which the acoustic response admits its maximum value. The levels of the secondary lobes can be reduced at the expense of the path extension by applying different stresses to the elements of a matrix. This is called amplitude fading.
The electroacoustic efficiency of an emitter is defined as the ratio of the acoustic power generated to the total electrical power input and varies with frequency and it is expressed in percentage.
The transmitting voltage response is defined as the acoustic output of a projector referenced to 1 m, for an input of 1 Vrms. The actual drive voltage of a transducer may be much higher than 1 Vrms and result in a higher acoustic output than its transmitting voltage response. This level is called sound pressure level.
The sound pressure level and the associated power input of a transducer are limited by various factors including:

Voltage breakdown;
Electrical/mechanical stress;
Thermal effects (heat dissipation);
Cavitation;
Acoustic interaction between projectors.

Some of these limitations are related to the pulse length and the duty cycle which is defined as the ratio of the pulse length to the period.
The receiving performance of a receiver is expressed as the open circuit voltage receiving sensitivity. The depth capability of transducers is limited basically by the failure of its pressure release material, degradation of the piezoelectric elements under hydrostatic pressure, and by the stress limits of the material and the structure. In addition to the acoustical and electrical parameters described above, the following ones should also be considered:

Cable type, length and connector;
Mounting scheme of the transducer;
Environmental conditions (temperature, pressure, vibration, sea-state, exposure to sun);
Weight and size limits;
Service life in the water.

Through the projection and the proper selection of piezoelectric materials, good acoustic, electronic and mechanical performance can be achieved, but the high cost could limit their applications. In this way it is necessary to find a balance between these parameters and their accessibility.
There are several piezoelectric materials available for ultrasound transducers. The most common are the Lead Zirconate Titanate (PZT), Lead Titanate (PT), Lead Magnesium Niobate (PMN) and Lead Zinc Niobate (PZN) ceramics and Polyvinylidenefluoride(PVDF) and P(VDF-TrFE) polymers. Single crystals of PZT, PMN and PZN can also be used. In the present disclosure polyvinylidenefluoride (PVDF) and P (VDF-TrFE) polymers will be used as examples because of their mechanical properties, namely low acoustic impedance. Equivalent polymers with equivalent mechanical and piezoelectric properties may be used.
Piezoelectric ultrasound transducers at high frequencies usually operate in the 33 mode, that is, the deformation along the polarization axis and the excitation electric field point into the same direction. The free displacement of the material in direction 3, without restraining force and assuming uniform strain over the surface, is given by: $ξ = d_{33} v$

where ξ is the free displacement, V is the applied voltage and d33 is the coupling coefficient in the thickness direction.
The maximum force the piezoelectric element can apply to a medium is obtained by: $F = d_{33} Y_{3}^{E} \frac{A_{p}}{t_{p}} v, s_{33}^{E} = \frac{1}{Y^{E}},$

where SE33 is the elastic compliance coefficient, Ap is the area of the piezoelectric element and the tp is the thickness. The deformation creates a pressure wave in the medium, whose amplitude can be obtained by: $F = {pA}_{p}, p = cρ 2 πfξ,$

where c is sound speed, ρ is the material density, f is the signal wave frequency and ξ is the piezoelectric material displacement.
The fundamental resonance frequency can be calculated from: $f_{r} = \frac{c}{2 t_{p}}$
The acoustic beam has a pattern characterized by its divergence angle, which depends on the transducer diameter and on the wavelength. The value of half beam divergence angle for a sound speed of 1500 m/s (in water) is given by: $δ = \arcsin (\frac{λ}{D})$
To design ultrasound transducer it's very important to consider physical characteristics of the medium relating to the propagation of sound such acoustic impedance. The acoustical impedance is defined as the ratio between the density of the material and the speed of sound: $Z = ρc,$
The conjugation of the acoustic impedance of the transducer material and medium will dictate if the wave is reflected or transmitted. The ratio of the acoustic amplitude that passes from one medium to another is called the transmission coefficient T (7) and the ratio of the reflected amplitude to the incident amplitude is called the reflection coefficient R (8). $T = \frac{2 Z_{1}}{Z_{1} + Z_{2}},$
$R = \frac{Z_{2} - Z_{1}}{Z_{1} + Z_{2}}$
Thus, to optimize the energy transfer from one medium to another it is necessary to use a piezoelectric material with acoustic impedance closer to the water acoustic impedance as possible. Another advantage of improving the energy transfer between the transducer and the medium is the reduction of accumulated energy inside the transducer, it reduces the resonant effect, leading to an increase of the operating frequency range.
The main application identified for the present disclosure is wireless underwater communications.
Underwater wireless communications represent today a technological challenge that has not yet found a solution to the current requirements. The underwater environment proves to be quite adverse with respect to wireless communications. Electromagnetic waves with radio frequency and optical can not propagate long distances from below water.
The technology aimed at solving this problem is the acoustic waves. However, there are still many problems associated with this technology. Such as: low sound speed (+ -1500 m / s), echoes, multipath, noise, exponential attenuation with frequency and low performance of acoustic transducers in digital communications.
Thus one of the main problems that the present disclosure seeks to intervene with is the application of acoustic transducers to be used as ultrasound emitters in underwater wireless communications. For good performance the transducer needs to have the following characteristics of acoustic wave control (centering the acoustic or omnidirectional beam), broadband frequencies (from kHz to MHz) and low power consumption (tens of watts).
A broadband transducer is presented by Hans and Wash in U.S. Pat. 3,833,825 . A matrix structure composed of parallelepiped with the same surface area elements but with different heights allows generate acoustic waves in a wide range of frequencies. The resonance frequency of each element is defined by the height of the parallelepiped, while the beam angle is defined by the transducer surface area. The problem with this solution is that for frequencies in the MHz range the transducer surface area for an omnidirectional response is greatly reduced which leads to also reduced acoustic power.
Another solution for a broadband transducer was presented by Ray Merewether in U.S. Pat. 5,343,443 , which is summarized to a transducer composed of 4 piezoelectric elements in the format of resonant discs operating in mode 33. Each disc has a resonance frequency (150, 300, 600, 1200 kHz). The limitations of this solution are obvious, in fact the transducer has only 4 operating frequencies and does not allow to control the opening angle of the acoustic beam.
In the U.S.Pat. 5.321.332 Minoru Toda presents a solution for a broadband transducer consisting of a PVDF coiled film attached to a disc, the film is excited with an electric potential along the thickness but the displacement is effected along the coil which resulting in the disc vibrating. However, the transducer only has a broadband response for frequencies below 100 kHz and does not allow the control of the beam angle.
A solution presented in U.S. Patent 8,027,224 B2 by Brown, Aronov and Bachand claiming a broadband and almost omnidirectional transducer, however the transducer is similar to U.S. Pat. 5,343,443 because it is constituted by 3 piezoelectric elements of different dimensions, and each element has a resonance frequency and a quasi-omnidirectional acoustic propagation angle aperture.
DE102011121006 discloses an electroacoustic converter, in particular an emitter converter, for underwater operation. DE102011121006 discloses that a hollow cylindrical ring body is structured from a plurality of ring segments, which can be positioned next to each other in a ring shape. Each ring segment has at least one ceramic segment which is embedded in the ring segment by means of a gel filling enclosing the ceramic segment. The ring body has a wrapping contacting the ceramic segments on the outside for fastening the ceramic segments.
WO2016/071961 relates to a spherical ultrasonic wave transducer that can be formed at low cost and with ease. An energy converting part of a spherical ultrasonic wave transducer includes a plurality of piezoelectric elements and a spherical base. Each piezoelectric element is mainly composed of a piezoelectric ceramic element piece that has a tapered end portion the width of which becomes smaller toward at least one side in the longitudinal direction, that has a convexly curved outer surface and a concavely curved inner surface, and that is arc-shaped on the whole when seen from the side; and electrodes are respectively provided on the outer surface and the inner surface. The spherical base is arranged such that the tapered end portions of the piezoelectric elements face one location on the spherical base and support each piezoelectric element from the inner surface side thereof so as to form a spherical shell shape on the whole. Each piezoelectric element vibrates in a vibration mode of a length direction natural frequency which corresponds to the length of the piezoelectric element, so as to expand and contract in the length direction.
CN101604020 relates to the field of underwater acoustic navigation, in particular to a method for realizing a high-frequency wideband omnidirectional cylindrical array. The method comprises the steps of: utilizing a matched layer to excite an even mode to broaden the bandwidth of a transducer first; then utilizing the matched layer to construct a cylindrical radiation surface which increases the uniformity of wave beams; and finally also utilizing a plurality of matched-layer annular transducers to concentrically stack up and down to achieve the aim of vertical-dimensional beam control.
M. Martins et all "Optimization of piezoelectric ultrasound emitter transducers for underwater communications" relates to a study of ultrasound transducers for underwater communications focusing on a piston type emitter transducer operating in thickness mode.
However, to date, it is not possible to identify in the literature a transducer that has in particular the following combined characteristics:

High frequencies (MHz)
Broadband in the frequency spectrum (from kHz to some MHZ).
Omnidirectional propagation.
Low power consumption.

These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.

General Description

The invention is defined by the transducer defined in claim 1. An underwater ultrasound transducer for underwater communications according to the present invention is characterised by the features recited in the characterising portion of claim 1.
The disclosure relates to a high frequency wideband wide beam ultrasound emitter/transducer.
It is disclosed an underwater ultrasound transducer for underwater communications comprising an assembly of a backing structure and multi-layer piezoelectric polymer films attached to a surface of said backing structure, wherein said surface has a shape of a torus or of part of a torus and each said film has a shape of part of a cylindrical segment, wherein each said film is arranged in and around the outer half-surface of the torus shape.
The invention comprises a plurality of multi-layer piezoelectric polymer films attached to the surface of said backing structure, wherein each said film has a shape of part of a cylindrical segment and said films are arranged around the outer half-surface of the torus shape. The outer surface is the surface of the torus facing it surroundings.
In an embodiment, said films are arranged such that the axis of each cylindrical segment is collinear with the torus central line in the region of the respective film. This can be understood such that the cylinder of the cylindrical segment and the torus arm of the torus, or part-torus, are substantially parallel/coincident in said region.
In an embodiment, said films have a flattened shape of a rectangle wherein said rectangle is arranged on said surface lengthwise in respect of a meridian line of said torus.
The films having a shape of a strip with the same width along a meridian line of said torus provides a transducer with improved signal quality, important for underwater communication. This thus provides an improved transducer, even if same torus area is not actively used, especially in the wider torus part, in particular around the equator region of the torus.
In an embodiment, said surface has a shape of a torus cut along its equator plane. This cut can be understood as a 'bagel'-cut.
In an embodiment, said surface has a shape of a torus sector. This can be understood as a slice-of-cake kind of cut.
In an embodiment, said surface has a shape of a torus 180° sector.
In an embodiment, said surface has a shape of a torus sector cut along the torus equator plane.
In an embodiment, said surface has a shape of a torus 180° sector cut along the torus equator plane.
In an embodiment, said films are Polyvinylidenefluoride(PVDF) and P(VDF-TrFE) polymer thin films.
In an embodiment, said films are glued to said surface of the backing structure.
In an embodiment, said glue is cured under pressure or under vacuum.
In an embodiment, said glue is silicone based or polyurethane based.
An embodiment comprises electrical connections to said films.
In an embodiment, said connections are made of aluminium, gold, silver, silver ink or copper.
In an embodiment, the backing structure has an acoustic impedance superior to the acoustic impedance of the film such that the majority of acoustic energy is sent towards the exterior of the transducer.
An embodiment comprises a waterproofing layer.
In an embodiment, the waterproofing layer comprises silicone or polyurethane.
In a preferred embodiment, the transducer is an emitter.
The most basic and common transducer shape is the piston-type transducer, which is, basically, a piezoelectric with a disk shape. Most often those transducers are manufactured with ceramic piezoelectric materials such as: lead zirconate titanate (PZT), lead titanate (PT), lead magnesium niobate (PMN) and lead zinc niobate (PZN). These ceramics are commonly used as resonators since they show a high piezoelectric coefficient and high acoustic impedance. Besides ceramics, some other materials can be used, such as: polymers (polyvinylidenefluoride (PVDF) and P(VDF-TrFE)) and single crystals (PZT, PMN and PZN). The polymeric based solutions have the lowest acoustic impedance among all materials used in underwater acoustic transducers. One of the major advantages of using low acoustic impedance is related to the high transfer of energy between the transducer and the medium, decreasing significantly the resonance effect. The resonance effect reduction has two major consequences: First, it reduces the sound pressure level output; second, it increases the transducer usable bandwidth which is desirable for broadband digital communications.
PVDF has a low Piezoelectric Coefficient, almost 20 times lower than common piezo ceramics. Nevertheless, it is possible to overcome this limitation by suitable transducer design. Using a laminated transducer by gluing several layers of PVDF films, as presented in Fig 1, it is possible to significantly increase the transducer performance. Another possible solution is to increase the transducer surface area but, for the piston transducer, this will reduce the beam divergence angle, according to equation (5). One solution to implement a wide beam transducer without compromising the acoustic power level is to use a curved surface, where the area is practically unlimited, once the transducer surface is proportional to the circumference radius, as shown in Fig. 2.

Brief Description of the Drawings

The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of invention.

Figure 1 : Schematic representation of a multilayer transducer, electrical scheme for a PVDF thin films 4-layer structure.
Figure 2 : Schematic representation of an embodiment of a cylinder shape transducer with radius r, where the transducer length (in red) is equal to the arc length in the circular sector defined by the central angle θ.
Figure 3 : Schematic representation of a front view of an embodiment of the omnidirectional transducer composed of several multilayer structures of PVDF, where S represents thin film multi-layered PVDF structures.
Figure 4 : Schematic representation of a side view of an embodiment of the omnidirectional transducer composed of several multilayer structures of PVDF, where S represents thin film multi-layered PVDF structures.
Figure 5 : Schematic representation of a view of the lateral inner cut of an embodiment of the omnidirectional transducer composed of several multilayer structures of PVDF, where S represents thin film multi-layered PVDF structures.
Figure 6 : Schematic representation of results obtained from the FEM simulation of sound pressure level for 750 kHz and 1.25 MHz.
Figure 7 : Schematic representation of results obtained for unnormalized radiation diagram for various frequencies between 250 kHz and 1.5 MHz.
Figure 9 : Photographic illustration of an embodiment of the disclosed transducer.

Detailed Description

The transducer, according to an embodiment, is composed of 3 different parts: backing layer, active element and waterproofing layer.
The backing layer, according to an embodiment, is composed by a material with an acoustic impedance far superior to the acoustic impedance of the active element to ensure that the acoustic energy is sent in its entirety in the desired direction. The support, or backing, layer is also responsible for securing and forming the active element, which in this case, according to an embodiment, corresponds to a stainless-steel structure in the geometric form equivalent to the outer surface of a toroid. The toroid geometric form is composed of two circles, in which one all turns into ringshaped. Where R is the ring radius and r is the radius of the tube. Its area can be calculated by A = 4π2Rr. In this way it is possible to increase the surface area of the transducer without compromising the opening angle of the acoustic beam. The multilayer structures are placed on the outer side surface in the ring as shown, according to an embodiment, in Figure 3 and 4.
The active element, according to an embodiment, consists of a PVDF thin films multilayer structure with electrical connections in parallel. The active element can be divided into smaller transducers in order to control the opening angle of the acoustic beam, where the minimum angle is equal to the opening of a single structure and the maximum angle corresponds to the use of all structures. PVDF thin films with electrodes can range from 5 to 200 10^-6 meters. Before the layers are processed it is necessary to create the electrical connections using, according to an embodiment, aluminium or copper tape. Each layer, according to an embodiment, is glued using low density silicone or polyurethane and the cured in a hydraulic press. This process allows to remove the glue excesses and to guarantee a thin thickness and homogeneity throughout the whole structure. After curing, according to an embodiment, all layers are wired between layers.
The waterproofing, according to an embodiment, consists in a layer of silicone or polyurethane with low densities to ensure a good acoustic conduction between the active element and the medium. This layer has as main objective to prevent that water or other liquids infiltrate in the electrical connections. This layer is the last procedure performed during the transducer manufacture.

For the purposes of simulation and testing the transducer was selected according to an embodiment of the disclosure with the following characteristics:

Characteristics	Value
Frequency	Up to 1 MHz
Cylinder Radius	7.5 cm
Ring radius	11.5 cm
Beam Angle on XY axis	70°
Beam Angle on XX axis	10°
Number of layers	2
Thickness	110x10^-6 m
PVDF film width	1.7 cm
PVDF film length	9.2 cm

In order to obtain an omnidirectional transducer it would be necessary to use a structure of 64 transducers with the mentioned characteristics.
The design model prototype was implemented in a Finite Element Method (FEM) simulation platform COMSOL Multiphysics in a 2D symmetric plane with the models Piezo Strain Plane for the active element actuation and the model Pressure Acoustic for the pressure waves. The selected mesh has particles with triangular shape and with 300 µm size in a half-sphere shaped environment with 30 cm radius. The simulations were performed with the following settings: fresh water as propagation medium, 20 °C of temperature.
The simulation results are presented in Figure 6 for the sound pressure level (dB re 1 µPa) at 750 kHz and 1.25 MHz in a symmetrical axis.
The results show that the transducer exceeded the expected 70 degrees, for both cases. For the 750 kHz the beam spread reaches the 80 degrees with an average pressure level of 155 dB. Considering (1) when the frequency increases, the beam spreading angle should reduce, however this is not verified on these results. In the 1.25 MHz simulation the beam spread overcome the 90 degrees with a higher-pressure level around 175 dB at 20 degrees and an average of 165 dB.
Considering the transducer dimensions and the frequencies used, a piston transducer under these conditions should show a beam pattern purely directional.
In conclusion, the simulations results are in accordance with the expected, making this transducer design suitable for implementation.
Figure 7 shows the measured transmitting voltage response (TVR) at 1 meter as a function of the beam spreading angle for 250, 500, 750, 1000, 1250 and 1500 kHz. The graph shows the response to a symmetric axis in XY plane according Figure 2.
In terms of angle response, the transducer has a beam wider than the expected 70 degrees and in terms of bandwidth at 1 meter showed a quality factor of 1.5 centered in 755 kHz, demonstrating high bandwidth properties.
Through the simulation it was possible to predict the transducer beam angle for all frequencies and also the increase of pressure levels in the lateral lobes (at 20 degrees) for frequencies above 1 MHz.
Before comparing the simulation results with the experimental ones, is important to remind that the hydrophone only displays a linear behavior (±3 dB) up to 1MHz, therefore to results above 1MHz will be highly affected by the hydrophone sensibility. Another aspect that may cause some differences in the results was the distance, the simulation was performed with 30 cm, while the tests were performed at 1 m. Considering this aspects we can conclude that the experimental results are in agreement with those obtained in simulations.
Figure 8 shows the transducer response between 200 kHz and 1.5 MHz for 1, 5 and 10 meters distances.
The transducer, according to this embodiment, was designed for frequencies up to 1 MHz, but the results show that the transducer is able to operate with frequencies up to 1.5 MHz at short distances. However, it was not possible to do proper measurements at higher distances since, as previously mentioned, the hydrophone sensibility strongly decrease after 1 MHz and at 5 and 10 m it was not enough to capture the acoustic signal.
The results obtained were as expected, and demonstrate a high potential for applications in short-range (up to 10 m) broadband underwater communications since the transducer presents a high bandwidth and beam-width, up to 500 kHz and 70 degrees respectively.

Claims

Underwater ultrasound transducer for underwater communications comprising an assembly of a backing structure, characterised in that it further comprises a plurality of multi-layer piezoelectric polymer films (S) attached to a surface of said backing structure, wherein said surface has a shape of a torus or of part of a torus and each said film has a shape of part of a cylindrical segment, wherein said films are arranged in and around the outer half-surface of the torus shape.
Underwater ultrasound transducer according to the previous claim wherein said films are arranged such that the axis of each cylindrical segment is collinear with the torus central line in the region of the respective film.
Underwater ultrasound transducer according to any of the previous claims wherein said films have a flattened shape of a rectangle wherein said rectangle is arranged on said surface lengthwise in respect of a meridian line of said torus.
Underwater ultrasound transducer according to any of the previous claims wherein said surface has a shape of a torus cut along its equator plane.
Underwater ultrasound transducer according to any of the previous claims wherein said surface has a shape of a torus sector.
Underwater ultrasound transducer according to the previous claim wherein said surface has a shape of a torus 180° sector.
Underwater ultrasound transducer according to claims 4 and 5 wherein said surface has a shape of a torus sector cut along the torus equator plane.
Underwater ultrasound transducer according to the previous claim wherein said surface has a shape of a torus 180° sector cut along the torus equator plane.
Underwater ultrasound transducer according to any of the previous claims wherein said film is Polyvinylidenefluoride, PVDF, or Polyvinylidene fluoride/trifluoroethylene, P(VDF-TrFE), polymer thin film.
Underwater ultrasound transducer according to any of the previous claims wherein said film or films are glued to said surface of the backing structure.
Underwater ultrasound transducer according to any of the previous claims comprising electrical connections to said films and the connections are made of aluminium, gold, silver, silver ink, or copper.
Underwater ultrasound transducer according to any of the previous claims wherein the backing structure has an acoustic impedance superior to the acoustic impedance of the films such that the majority of acoustic energy is sent towards the exterior of the transducer.
Underwater ultrasound transducer according to any of the previous claims comprising a waterproofing layer comprising silicone or polyurethane.
Underwater ultrasound transducer according to any of the previous claims wherein said transducer is an emitter.
Underwater ultrasound transducer set comprising a transducer according to any of the previous claims as an emitter and a transducer according to any of the previous claims as a receiver.