AU692960B2 - Hydrophone - Google Patents

Description

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a a a A USTRALIA Patents Act 1990 MARSCHALL ACOUSTICS PTY LTD

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Hydrophone The following statement is a full description of -this invention including the best method of performing it known to us:-

HYDROPHONE

Field of the Invention The present invention relates generally to piezoelectric transducers for use in seismic exploration and like applications. In particular, the invention relates to hydrophones for use in hydrophone arrays.

Background Art Piezoelectric transducers have a wide range of applications including, for example, being placed in arrays for use in towed sensing devices.

Piezoelectric transducers can employ ceramic materials such as barium titanate or lead zirconate titanate (PZT) or plastic materials such as polyvinylidene fluoride (PVDF) or piezo-rubber composites, A well known form of transducer is the flextensional transducer which is a device, as is indicated by the name, that utilises flexure motion to improve transducer characteristics. One basic form of this type of 15 transducer can comprise a cylindrical transducer consisting of an oval shell bearing on or at the edges of a bar of piezoelectric active material.

Disclosure of the Invention According to one aspect, the present invention consists in an acoustic transducer comprising: S" 20 a tube comprised at least in part of piezoelectric material and having an inner and outer surface; a first shell member, having a first edge and a second edge, disposed outside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the outer surface of the tube; and a second shell member, having a first and second edge, disposed inside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the inside surface of the tube.

In one embodiment, the tube can be comprised solely of piezoelectric material. In another embodiment, the tube can be comprised of a combination of piezoelectric and non-piezoelectric material.

The tube is preferably substantially cylindrical, however, other shapes could be employed.

In a preferred embodiment of the invention, the first and second shell members each extend the full longitudinal length of the piezoelectric tube.

They also preferably respectively encase the outer and inner surfaces of the tube.

The transducer can have any shape. In one preferred embodiment, when viewed in perspective, the transducer can appear substantially toroidal, while when viewed in cross-section, the piezoelectric tube can appear linear with the first and second shell members appearing arcuate. In another embodiment, the first and second shell members when viewed in 15 cross-section can appear rectangular. The shell members may also be stepped outwardly from the tube. In a preferred embodiment, the shell members proximate the first and second edges are preferably arranged so as to lie substantially normal to the respective surfaces of the tube thereby providing enhanced stress relief.

20 The piezoelectric material preferably comprises an active polarised ceramic material such as barium titanate or lead zirconate titanate (PZT) or a piezoelectric plastics material. In one embodiment, it could comprise one or a plurality of polarised piezoelectric polymer films such as polyvinylidene fluoride (PVDF) or piezo-rubber composites.

Both the inner and outer surface of the piezoelectric material are preferably coated at least in part with an electrically conductive material thereby providing an electrode for each surface. The total thickness of the piezoelectric material will depend on the material employed but would preferably lie in the range 0.1mm 3mm, In one arrangement, the tube of piezoelectric material is comprised of at least one wrap of piezoelectric s polymer around a supporting element. More preferably, the piezoelectric polymer is wrapped a plurality of times around the supporting element.

Preferably, each electrode is a thin coating of metal such as silver, gold or aluminium. Where wraps of polymer are employed, to prevent electrical short circuiting between the respective wraps, a non-conductive coating such as Mylar, can be applied over the conductive electrodes. Electrical connection to each electrode of the piezoelectric material can be facilitated by conductive leads attached to the electrodes by conductive epoxy or adhesive metallic tape.

The first and second shell members are preferably fabricated from an aluminium alloy or bronze alloy with a wall thickness of approximately 1mm.

The transducer is preferably arranged to be mounted in any appropriate chamber. The chamber can be filled, as applications dictate, with a solid, or 15 fluid material such as liquid or gel. The space between the shell members and the tube can be filled with any compressible material. The space between the first shell member and the tube can be filled with a different material than that between the second shell member and the tube. In those applications where the chamber is connected to a cable, such as a towing cable, the transducer can be mounted symmetrically on the cable. A strain shielding element may also be provided on the cable which substantially shields the transducer from stresses generated in the cable. The strain S "shielding element preferably consists of a ry ndrical member which fits around the cable. The cylindrical membei is preferably a stiff material such as a metal, alloy or carbon fibre.

Also preferably provided between the strain shielding element and the transducer is at least two suspension spidprs which act as a further buffer between the cable and the transducer. Preferably, a suspension spider is located proximate both the first edge and the second edge of the tube. Each suspension spider is preferably comprised of a plurality of radial members -L IL~ which are arranged to allow axial movement of the transducer while substantially preventing radial movement.

One or the plurality of the acoustic transducers according to the present invention can be used in any towed, vertical or seismic downhole array, As one example of its application, it can be mounted in the towed sonar array arrangement described in US Patent No 4958329, the description of which is incorporated herein by reference. The transducer is particularly suitable for use in a towed sensor arrangement behind a geophysical survey vessel. The cable in such an application must provide towing forces for the acoustic transducers and provide the electrical and other signal connections between each acoustic transducer and associated processing and data analysis equipment which is preferably located in the vessel towing the array. Signal transmission through the cable may be through standard electrical bearers or through the use of optical fibres.

15 The above defined arrangement can be employed as an acoustic receiver or transmitter as needs dictate. When used as a receiver, the acoustic signal impinges as a pressure wave on the shell members causing the shell members to flex inwardly and outwardly in response to the signal. The inward and outward flexing of the shell members acts as a stress amplifier and leads to a corresponding amplification in the change in the longitudinal extension of the piezoelectric tube which in turn produces a corresponding electrical signal. In a preferred embodiment, the shell members provide a 6- 12 times increase in the stress experienced by the piezoelectric tube than would be achieved with a typical transducer where the pressures impinge directly on the piezoelectric element. The electrical signal is available at the electrodes for transmission by leads to the data analysis equipment described herein.

When used as a transmitter an electrical signal of appropriate magnitude and frequency is connected to the electrodes of the piezoelectric material in such a way that the tube undergoes variation in longitudinal -L ~s extension which in turn causes the shell walls to flex and produce the desired output, Brief Description of the Drawings Hereinafter by way of example only, preferred embodiments of the invention will be described with reference to the accompanying drawings, in which.- Fig, 1 is a perspective view of one embodiment of the transducer according to the present invention with internal features represented in phantom; Fig, 2 is a perspective view of a quarter section of the acoustic transducer of Fig, 1; Fig. 3 is a cross-sectional view of the transducer of Fig. 2; Fig. 4 is a perspective view of the transducer of Fig. 1 mounted on a towing cable; Fig, 5 is a simplified vertical cross-sectional view of the arrangement of Fig. 4; Fig. 6 is a sectional view along line VI-VI of Fig. 5 with the external chamber removed; and Fig. 7 is a cross-sectional view of another embodiment of the 20 transducer according to the present invention.

Preferred Mode of Carrying out the Invention An acoustic transducer suitable for use in seismic exploration and like applications is generally shown as 10 in Figs. 1 to 6 and 30 in Fig, 7.

As is illustrated in Fig. 1, in one embodiment the acoustic transducer 10 can in essence comprise a substantially toroidal ring comprising a cylindrical tube 11 of piezoelectric material and two shells 12 and 13. As is depicted in Fig, 7, where like parts have the same reference number, in an alternative embodiment the shells 12 and 13 of the transducer 30 are stepped outwardly from the tube 11 and are rectangular in cross-section. The piezoelectric material can comprise a polarised ceramic material such as lead zirconate titanate (PZT) or a polarised plastics material such as polyvinylidene fluoride (PVDF). In the embodiments of the invention depicted in the drawings, the tube 11 is comprised entirely of piezoelectric material. In other embodiments, not depicted, the tube 11 could be comprised of a combination of segments of piezoelectric material with nonpiezoelectric material.

The piezoelectric tube 11 has an outer surface 14 and inner surface each surface being coated with an appropriate thin metal electrode to allow electrical connection of the tube 11 to standard electrical equipment known in the art for processing transducer outputs.

The first shell 12 is disposed above the outer surface 14 of the piezoelectric member 11, while the second shell 13 is disposed above the inner surface of the piezoelectrical member 11. The volumes 26 and 27 between these surfaces can be filled with materials of different compressibility. As is clearly depicted in Figure 3, when viewed in crosssection the piezoelectric member 11 appears linear while the first and second shells 12 and 13 appear arcuate.

Proximate the ends 16 and 17 of the piezoelectric member 11, each shell 12 and 13 bears against the respective surfaces 14 and 15. The shells 20 12 and 13 are preferably an aluminium or bronze alloy and are glued to their respective surfaces of the piezoelectric member 11 where they bear against the respective surfaces.

In operation, acoustic transducers 10 or 30 could be employed alone or 444** as part of a hydrophone array in a towed active sensing device such as is described in US4958329 or in any other hydrophone array. An example of such an array is now described using acoustic transducer The transducer 10 can be mounted on an appropriate support within a towed device, such as is depicted in Figs. 4, 5 and 6. In the arrangement depicted in Figs. 4, 5 and 6, the transducer 10 having the cylindrical piezoelectric tube 11, inner shell 13 and outer shell 12 is mounted I -B ~ILI symmetrically around a cable 18. The cable 18 in this embodiment is a towing cable which provides towing forces for each hydrophone and maintains the electrical and signal connections between each hydrophone and associated data analysis equipment which is located on a vessel towing the cable.

Surrounding the cable 18 is a strain shielding element 19 for the transducer 10. This element 19 is made of a stiff material and substantially shields the transducer 10 from residual stresses generated in the cable 18.

The element 19 would typically be fabricated from a metal, alloy or carbon fibre. Between the strain shielding element 19 and the transducer 10 there is provided two suspension spiders 21 and 22. The suspension spiders 21 and 22 (as is best depicted in Fig. 6) are each comprised of a plurality of radial members 23 which extend between the strain shielding element 19 and the o edge of the transducer, Spider 21 is located at end 16 of the transducer 15 while spider 22 is located at end 17. The suspension spiders 21 and 22 are arranged so as to substantially prevent radial relative movement between the cable 18 and the transducer 10 while allowing ready axial movement of the transducer 10 along the cable 18, The entire arrangement is mounted in a chamber 24 (as depicted in Fig. 5) which protects the transducer 10 from the elements and is preferably i. comprised of soft material such as polyvinyl chloride (PVC) or polyurethane, The chamber 24 and the space between the shells and the tube can be filled with a gel, fluid or solid 25 as is applicable for the transducer's application.

The maximum diameter of the ring comprising the acoustic transducer 10 would be approximately 4cm, with the inner diameter being approximately 3cm. Total length of each transducer is envisaged at approximately In operation as an acoustic receiver, the acoustic signal impinges as a pressure wave on the shells 12 and 13 causing the shells to flex inwardly and outwardly. This flexing of the shells 12 and 13 acts as a stress amplifier and leads to a corresponding variation in the longitudinal extension of the piezoelectric member 11 which in turn produces an appropriate electrical signal. The signal can then be fed via the electrodes to the appropriate electronics (not depicted) which process the received signal as needs dictate.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

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Claims (16)

1. An acoustic transducer comprising: a tube comprised at least in part of piezoelectric material and having an inner and outer surface; a first shell member, having a first edge and a second edge, disposed outside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the outer surface of the tube; and a second shell member, having a first and second edge disposed inside the tube and extending longitudinally of the tube, the first and second edges being connected to or bearing against the inside surface of the tube.

2. The acoustic transducer of claim 1 wherein the tube is substantially cylindrical.

3. The acoustic transducer of claim 1 or 2 wherein the first and second shell members each extend the full longitudinal length of the tube. 4, The acoustic 'l"ansducer of claim 3 wherein the first and second shell members respectively encase the outer and inner surfaces of the tube. The acoustic transducer of any one of the preceding claims wherein :0 the first shell member encloses a first space outside the tube and the second 20 shell member encloses a second space inside the tube, the first and second spaces being filled respectively with materials of the same or different compressibilities.

6. The acoustic transducer of any one of the preceding claims wherein the transducer has a shape which when viewed in perspective is 25 substantially toroidal, 7 The acoustic transducer of claim 1 wherein a portion of each of the first and second shell members is stepped outwardly a distance from the tube greater than the remainder of the respective shell members.

8. The acoustic transducer of any one of the preceding claims wherein 30 proximate the first and second edges the shell members are respectively arranged to be substantially normal to the respective surfaces of the tube.

9. The acoustic transducer of any one of the preceding claims wherein the piezoelectric material comprises an active polarised ceramic material. The acoustic transducer of claim 9 wherein the ceramic material is barium titanate or lead zirconate titanate (PZT). ~s 11

11. The acoustic transducer of any one of claims 1-8 wherein the piezoelectric material comprises a plastic material.

12. The acoustic transducer of claim 11 wherein the plastic material comprises one or a plurality of piezoelectric polymer films.

13. The acoustic transducer of claim 12 wherein the piezoelectric polymer film is polyvinylidene fluoride (PVDF).

14. The acoustic transducer of any one of claims 1-8 wherein the piezoelectric material comprises a piezo-rubl- material. 'he acoustic transducer of any one Lhe preceding claims wherein the first and second shell members are fabricated from an aluminium alloy or a bronze alloy.

16. The acoustic transducer of any one of the preceding claims wherein the transducer is mounted in a chamber.

17. The acoustic transducer of any one of the preceding claims wherein the transducer is connected to a towing cable,

18. The acoustic transducer of claim 17 wherein a strain shielding element is provided between the cable and the transducer which substantially shields the transducer from stresses generated in the cable,

19. The acoustic tran iducer of claim 18 wherein at least two suspension 20 spiders are mounted between the cable and the strain shielding element which are arranged to allow axial movement of the transducer while substantially preventing radial movement. A hydrophone array comprising a towing cable having a plurality of acoustic transducers according to any one of the preceding claims connected thereto.

21. An acoustic transducer according to the present invention substantially as described with reference to the drawings. C I- a0

22. A hydrophone array according to the present invention substantially as described with reference to the drawings. DATED this twenty-eighth day of April 1998, MARSCIIALL ACOUSTICS PTY LTD Patent Attorneys for the Applicant: F.B. RICE CO, 0 S. S S S S 0 S 55 S S S 5* S 5* S. S S S~ *5 S5 S ABSTRACT An acoustic transducer comprising a tube of piezoelectric material having on each surface a shell member arranged to act as a amplifier of the stress experienced by the piezoelectric material. A hydrophone array having a plurality of acoustic transducers according to the invention connected thereto. 0 6 I