FR2855009A1 - Towed floating hydrophone acoustic element having piezo electric polymer leaf buried following pull axis with pull axis along tubular length - Google Patents

Abstract

The acoustic element is a piezo electric polymer leaf buried along the pulley axis. The configuration is tubular (12) with the pulling axis oriented along the tubular axis.

Description

The present invention relates to elements

  acoustic, and in particular, but not exclusively, acoustic elements intended for use in hydrophone assemblies.

  Hydr (-) phones sets can include a large number (of acoustic elements linked so as to form a rem system (Orc. In towed non-optical systems, the means of detecting acoustic elements is generally constituted by disks of piezo-ceramic material held in support members, normally of plastic material. In order to obtain the desired sensitivity, the ceramic disks must have a certain diameter and furthermore, because of their disk shape, the large number of electrical conductors 15 necessary to couple the active elements of the hydrophone system must be housed around the elements support members. Consequently the systems, when properly applied, have a diameter of about 90 mm When deployed to form towed systems, hydrophonries must be able to detect level acoustic signals very weak, the source of which can be located at a distance of about 350 m from the system. The systems are therefore generally very long and each elementary hydrophone of the system must also be. as long as possible in order to effectively integrate the noise due to the alu current along the system. When not in use, the systems should be stored on the ship from which they were deployed, usually on large drums. It will therefore be understood that the] .e storage of large systems 1 actrle actlle] ls, relatively cumbersome, presents difficult problems taking into account the real physical dimension of the systems. There is therefore a need for a tailor-made system which has the required sensitivity, but which is of a design which allows the components of the system to be housed in (2 sets of substantially reduced diameter. the present invention is to provide an acoustic element which makes it possible to produce a compact towed system before the required sensitivity, and in which the aforementioned storage problems are substantially overcome.

  Thus, an acoustic element is proposed comprising a sheet of piezoelectric polymeric material stretched along a stretching axis and configured in a substantially tubular shape, the stretching axis being oriented substantially along the circumference of the tubular shape. .

  Preferably, the tubular shape is cylindrical or polygonal.

  Alternatively, the tubular shape can be realized by a sheet material configured in a spiral.

  Advantageously, the sheet material includes metallic layers, for example copper or ductile nickel, on its opposite faces, the metallic layers being arranged to act as electrodes for the acoustic elements.

  Preferably, the acoustic element comprises suspension means for supporting the sheet of polymeric material around a support member, the suspension means comprising a plurality of tubes of elastic material, preferably three, arranged so that their longitudinal axes are substantially parallel to that of the tubular shape.

  Advantageously, the elastic material of the suspension means 30 comprises rubber based on polysulphide, rubber based on fluo) ro-silicone or rubber rich in nitrile. In addition, the metal layers constituting the electrodes may comprise, in their regions which cover the suspension means, electrically insulated zones, so as to define active zones and inactive zones on the electrodes, in order to ensure that the suspension means do not come into contact with the active areas of the elements.

  Particularly advantageously, the polymeric material is. consisting of polyvinyl fluoride, either with voids or without voids.

  The bore in the tubular form can further be used to accommodate a tension member and coupling means for coupling a plurality of the acoustic elements to form a towed system.

  The present invention will now be described, by way of example, with reference to the accompanying drawings in which: FIG. 1 schematically illustrates a sheet 15 of piezoelectric polymer material and shows the drawing axis along which the sheet has been stretched, and the resulting sensitivity plans; Figure 2 schematically illustrates a drawn tube of piezoelectric polymeric material, showing the configuration of the planes of sensitivity which result from the tube drawing process; FIG. 3 schematically illustrates an embodiment of an acoustic element according to the present invention; 4 shows a partial end view of the acoustic element shown in FIG. 3; Figure 5 shows a schematic cross-sectional view of an acoustic element according to another embodiment of the present invention; 30 and FIG. 6 schematically illustrates the system for suspending the active part of the element shown in FIG. 5.

  Referring to Figure 1, a sheet 2 of piezoelectric polymeric material, such as polyvinylidene fluoride (PVdF) is produced by stretching along a stretching axis 4. The stretching process gives Birth of a thick film sheet, which typically has a thickness of about 0.6 mm and which, taking into account the properties of PVdF, sensitivity planes which are generally designated by sensitivity planes "31 "," 32 "and" 33 ". The designations "31", "32" and "33" for the sensitivity planes and their orientation relative to the drawing axis are assumed to be known to those skilled in the art and will therefore not be described in the this request.

  May be provided on the sheet 2 of coatings 6 and 8 of a metallic material such as copper or nickel, which are capable not only of acting as electrodes, but also of stiffening the polymeric material in the plane of sensitivity stretch "31" and in the stretch sensitivity plane "32" and to help keep the material relatively rigid in the shapes imparted. If the sheet 2 is subjected to a deformation force, such as hydrostatic pressure 20 produced by acoustic signals propagating through water, then an external charge Q is generated which can be detected by detection means appropriate, with Q = q "33" + q "32" + q "31" of q is the charge generated in each of the three sensitivity planes, q "33" being a negative quantity and q "31" and q "32" being positive quantities. The sheets 2 have therefore, with a substantially planar shape, found application as hydrophonic elements attached to the exterior of the hulls of ships.

  For towed systems, each hydrophonic element must be as long as possible to effectively integrate current noise, while retaining an ability to conform according to the radius of curvature required when the system is stored on a drum on board. of na-ire. The use of PVdF-5 tubes as active elements in towed systems has been considered. The PVdF can be stretched into a tubular shape 10, as shown in FIG. 2. However, it can be observed in FIG. 2 that, due to the stretching process, the relatively sensitive sensitive plane "31" is oriented in the axial direction of the tube 10. In addition, a tube drawn in PVdF does not reach the regularity and the sensitivity of the plane drawn materials and, moreover, the wall stiffness induced by the coating 10 with electrodes mt a] I iques on the inner and outer surfaces of the tube further decrease the sensitivity.

  The towed systems, in (use), are exposed to acceleration forces along the tow line. thus it is understood that drawn tubes 10 are not suitable for use as elementary hydrophones in such systems, since the output load Q generated by the variations in hydro) static pressure resulting from relatively weak acoustic signals would be eclipsed by the loads arising from acceleration forces along the tow line, that is, aligned with the plane of sensitivity "31".

  It has been realized, according to the present invention, that the elementary hydrophones could be manufactured by configuring the sheet] 2 shown in Figure 1. in a tubular shape 12 as shown in Figure 3, which is preferably a cylinder shape right circular, in which the plane of sensitivity 30 "31", which is aligned with the axis of stretching of the material, is arranged according to the circumference of the cylindrical shape 12. Typically, the cylindrical shape 12 has a external diameter of approximately 22 mm and a length of 140 mm. It can be observed that the elementary hydrophones 35 can be made relatively long, and therefore relatively sensitive, in comparison with the systems - 6 of current design with ceramic discs, and: that in addition a system of substantially reduced diameter can be obtained because the the tensioning member of the system, the interconnections of elements and the associated electronic processing assemblies 5 can be housed inside the central space of the tubular element 12. By adopting the principles of the present invention, elementary hydrophones of tubular shape with a diameter of about 22 mm were made, using a sheet 10 of PVdF 0.6 mm thick.

  PVdF is a relatively rigid material which will not keep a tubular shape without holding forces.

  The tubular shape 12 shown in FIG. 3 can be manufactured by providing a number of notching lines 15 along the length of the PVdF material and by bending the sheet around the notching lines 14 beyond the plastic limit PVdF to form a plastic hinge, as shown in Figure 4.

  By this technique, it is possible to form a polygonal tube 20 comprising a certain number of longitudinal plates. The tubular shape can be obtained by bending the sheet around a mandrel of suitable size. In the embodiment shown in FIGS. 3 and 4, the sheet 2 is preferably provided with metal electrodes 25, for example nickel electrodes, before the formation of the notching lines 14. The interior electrode thus formed is therefore continuous, the dishes 16 each having an external electrode el] ect-rically insulated. The external electrodes can be connected by any suitable connection means, for example conductive retaining rings or simply metallic connection wires. Alternatively, the notching lines 14 can be stopped before the longitudinal ends of the sheet to provide electrical coupling in the end portions of the outer electrode. - 7

  The elementary acoustic hydrophones can also be manufactured by rolling the sheet 2 in the sensitivity plane "31". In this embodiment, copper electrodes are provided, for example by plating, on the opposite sides of the sheet 2, and the sheet 2 is rolled without notching the copper plating. Only a relatively thin veneer is necessary to keep the rolled sheet in its cylindrical shape, which allows the acoustic elements 10 to be produced in a simple and relatively economical manner, and with a relatively low density by nature, which makes it possible to give easily to the assembled system the floating character. Realistic tests with rolled sheets in the sensitivity plane "31" demonstrated with satisfaction that the inherent sensitivity of the plane-shaped PVdF material is retained, contrary to what might have been expected previously.

  The acoustic elements according to the present invention are extremely rigid, by virtue of their laminate and tubular nature, and if they are made symmetrical when assembled, form the hydrophones; elementary of a towed system, no output signal is obtained when loaded at midpoint. The elements 25 operate essentially in the single mode "33". The plated electrodes provide rigidification in the sensitivity planes "31" and "32", which effectively reduces the contribution of these planes to the output signal and, in correspondence, the hydrostatic sensitivity tends towards the sensitivity plane "33 ". Undesirable contributions to the signal, which are generated by accelerating forces along the towing axis and by the curvature of the elements, appear respectively in the sensitivity planes "32" and "31", but are minimized because the structural rigidity of the element and the orientation of the sensitivity plane "31". Such selectivity and such sensitivity of the structure cannot be obtained with the drawn tube material as mentioned previously with reference to FIG. 2.

  To form a towed system, the PVdF elements of tubular shape 12 are supported around central supports 18 by suspension units 20 arranged so as to minimize the orthogonal transmission of the axial vibration, that is to say from the towing axis to the "active" "33" sensitivity plane. The suspension units 20 comprise three tubes mounted with a mutual spacing of 120 between the supports 18 and the elements 12 as shown in FIGS. 5 and 6. The heart 22 of the system is used to house a tension member 24 and the wiring through 26 from other elements of the system, the assembly being coated with an outer sheath 28 to prevent the penetration of water.

  The suspension units 20 can be made from rubber, such as polysulfide-based rubber, fllutorosilicone-based rubber or nitrile-rich rubber, the main requirements being moldability, compliance and resistance to kerosene, which is the fluid used for filling the sheath 28 with the system. It is fundamental that the attenuation of the transverse axes to the axis is important, because any acceleration force in this direction generates a reaction to the pressure in the active plane of sensitivity of the tube 1.2, which generates it - even undesirable output signals whose amplitudes depend on the asymmetries of the system.

  The suspension units 20 can be considered very soft transversely and hard axially.

  These damping characteristics were obtained by using the deformation action of the soft rubber tube in the transverse direction and the inherent rigidity - 9 depending on the length of the tube in addition to the high rigidity of the tubular form 12 at which the suspension units are] .iés. The rigidity of the tubular form 12, which is increased by the plated electrodes 5 6, 8, the softness transversely of the suspension units 20, as well as]. orientation of the sensitivity plane "31" obtained by the use of a PVdF sheet of planar shape, combine to ensure that the axial acceleration forces are confined almost exclusively in the sensitivity plane "32" , i.e. the least significant sensitivity plane of the PVdF material.

  Furthermore, the electrodes 6, 8 respectively have circumferential grooves 30, 32 so as to define active zones 34, 36 for the electrodes, which are electrically isolated from inactive zones 38, 40 which are in contact with the suspension units 20. The suspension units 20 are therefore not in contact with the active or sensitive zones 34, 20 36, which further improves the rejection by the acoustic element of the undesirable transverse forces.

  It can therefore be observed that by using an acoustic element made from a PVdF sheet material brought into a cylindrical shape with oriented sensitivity planes in accordance with the present invention, a compact, robust and robust floating towed system can be produced. relatively economical, and furthermore having a good voltage sensitivity, typically -199.5 dB for 1 V / Pa l dB, low noise, good surface integration ability, high source capacity , typically 1000 pF, a sensitivity to axial vibrations of -80 dB Hz / g, and a sensitivity to transverse vibrations of -90 dB Hz / g, as well as a large active range which, for a system with a diameter of 35 mm, is located in the vicinity of 47000 mm2 / meter - 10 linear, which represents about 43% of the surface of the system.

  Although specific embodiments of the invention have been described, it will be understood that modifications can be made without departing from its scope. For example, the suspension units 20 can be made differently from the three mutually spaced rubber tubes. In addition, to ensure that the sensitive areas 34, 36 of the electrodes 6, 8 do not cover 10 the suspension halves 20, the electrodes 6, 8 can be stopped at the position of the grooves 30, 32 or in their vicinity. In addition, other ductile conductive metals or alloys can be used to manufacture the electrodes 6, 8.

  The tubular shape 12 can also be made from the sheet 2 wound in a spiral so as to increase the source capacity of the system, and the term "tubular shape" is intended to cover the acoustic elements made in this way. - 11

Claims (10)

REVENDTICATIONS   1. An acoustic element, characterized in that it comprises a sheet of piezoelectric polymer material (2) stretched on. along a drawing axis (4) and configured in a substantially tubular shape (12), the drawing axis 5 being oriented substantially in the circumferential direction of the tubular shape.   2. An acoustic element is] claim 1, characterized in that the tubular shape (12) is cylindrical.   3. An acoustic element according to claim 1, characterized in that the tubular shape (12) is polygonal.   4. An acoulstic element according to claim 1, characterized in that the tubular shape (12) is defined by a spiral configuration of the sheet of material (2).   5. An acoustical element according to one of claims 1 to 4, characterized in that the sheet material includes d (the metal layers (6, 8) on its 20 opposite faces, said metal layers being arranged so to act as electrodes for the acoustic element.   6. An acoustic element according to claim 5, characterized in that suspension means (20) are provided for supporting said sheet of polymeric material around a support member (18), said suspension means comprising a plurality of tubes of elastic material arranged in such a way that their longitudinal axes are substantially parallel to that of the tubular form (12).   7. An acoustic element according to claim 6, characterized in that the suspension means (20) are made from any one of the materials chosen from the group comprising caulcholuc based on - 12 polysulfutre, rubber based fluoro-silicone and rubber rich in nitrile.   8. An acoustic element according to resold-icat-io) n 6 on 7, characterized in that the metal layers (6, 5 8) constituting said electrodes include, in the regi ons thereof located to the right of suspension means, electrically isolated zones, so as to define inactive and: active zones (38, 40) of the electrodes to ensure that said suspension means 10 do not come into contact with the active zones of l. acoustic element.   9. An acoustic element according to one of claims I to 8, characterized in that said polymeric material is polyvinylidene f] uoride.   10. An acoustic element according to one of claims I to 9, characterized by the presence in the tubular form of a bore which houses a tension member (24) and coupling movens (26) for coupling a pl] urality acoustic elements to form a towed system.   1il .. A towed system, characterized in that it comprises a plural i t of acoustic elements according to one of claims 1 to 10.