Undersea acoustic antenna with surface sensor

An acoustic antenna includes at least one surface sensor formed by a stack of conducting materials and dielectric layers of piezo-electric material enclosed in a sheathing of flexible material. The assembly forms a flat panel 2 mounted against the hull 5 of a navel vessel and takes the shape of the hull. The mounting of the panel on the hull is achieved by two streamlined edging sections 3, 4 while leaving an intermediate water layer 6 remaining between the panel 2 and the hull 5. The sheathing includes an envelope of flexible material filled with a visco-elastic lining material and the piezo-electric material of the dielectric layers of the sensor is preferably a polyvinylidene fluoride film.

The present invention relates to an acoustic antenna for receiving 
low-frequency undersea waves. 
Such an antenna is intended to detect and locate sources of undersea 
acoustic noise; in order to obtain good performance both in detection and 
in location, it is necessary to work over a low-frequency spectrum (by 
"low frequencies" will be understood frequencies lower than 2 kHz, 
typically lower than 1 kHz), and to make use of an antenna the gain of 
which is considerable so as to obtain a satisfactory signal/noise ratio 
(in numerous applications, a gain of 20 dB is necessary). 
These two requirements (low frequencies and high gain) necessarily dictate 
antennae of considerable dimensions. 
To that end, a first possibility consists in towing behind the naval vessel 
(ship or submarine) a streamer of hydrophones, thus forming a linear 
antenna of very great length. 
Such a type of antenna may be much longer than the submarine and thus have 
very high performance at low frequency; it exhibits numerous drawbacks, 
however, in implementation (winch system, etc. and increase in the drag of 
the submarine) and above all a complete absence of directivity in the 
vertical plane by reason of the linear configuration of the streamer. 
Another possibility consists in placing, over a large part of the length of 
the submarine, an antenna formed by an assembly of point sensors 
(hydrophones of small dimensions linked together in an appropriate way). 
It is thus possible to have available a two-dimensional array, which makes 
it possible to have directivity in the vertical plane and thus to locate 
the direction of the acoustic source in this plane. 
This hydrophone-array antenna nevertheless exhibits a certain number of 
drawbacks: 
in the first place, it is necessary to decouple the various point sensors 
constituting the antenna acoustically with respect to the vibrations and 
resonances of the hull and of the attached structures of the submarine 
(especially vibrations and resonance originating from the machinery of the 
submarine) and from the hydrodynamic flow noise of the water on the 
sensors which, in the absence of appropriate decoupling, would produce a 
perturbing acoustic pressure masking the incident signal, generally of 
very low amplitude; 
it is also necessary to ensure leaktightness and a leaktight passage 
through the hull for each sensor; 
finally, the mechanical structures used to support the hydrophones often 
badly withstand the hydrodynamic forces to which they are subjected, in 
addition to the fact that they often cause troublesome disturbance to the 
flow of streams of water along the hull of the submarine. 
In order to remedy these various drawbacks, the intention proposes an 
undersea acoustic antenna no longer produced from a set of point sensors, 
but from two surface sensors, typically of several tenths of a square 
meter of pick-up surface area each. 
The use of essentially surface sensors would make it possible, by direct 
integration effect, to mask the major part of the parasitic or flow noises 
mentioned above, which would always be more or less picked up, previously, 
with the antennae formed from an assembly of point sensors. 
It will also be seen that the antenna of the invention, despite its very 
large dimensions, only very slightly disturbs the hydrodynamic behaviour 
of the submarine, and further offers excellent resistance to hydrodynamic 
stresses and to impacts. 
To this end, according to the invention, this acoustic antenna for 
receiving low-frequency undersea waves, includes at least one surface 
sensor formed by a stack of conducting layers forming electrodes and of 
dielectric layers of piezoelectric material interposed between these 
conducting layers, this sensor being enclosed in a sheathing of flexible 
material, the assembly thus constituted forming an attached flat panel 
mounted against the wall of the hull of a naval vessel, especially of a 
submarine, this panel exhibiting a degree of freedom in bending so as to 
allow it to follow the shape of this hull. 
According to a certain number of advantageous characteristics: 
the sensor is subdivided into a plurality of elementary sensors the 
respective electrodes of which are electrically linked in parallel, the 
set of elementary sensors being placed in a common leaktight sheathing. 
the conducting layers of the elementary sensors are formed from a single 
strip machined in such a way as to divide it into separate elementary 
plates while leaving, between adjacent elementary plates, at least one 
bridge of material remaining, providing the electrical link between the 
electrodes of these various elementary sensors. 
the panel is mounted on the hull while leaving an intermediate water layer 
(6) between panel and hull, the thickness of this water layer being such 
that the distance separating the wall of the hull from the mid-plane of 
the sensor is less than a quarter of the wavelength of the maximum 
frequency of the operating band of the sensor. 
the sheathing of flexible material comprises an envelope of flexible 
material filled with a visco-elastic lining material, the visoelastic 
lining material preferably being a polyurethane material the behaviour of 
which is similar to that of water. 
the piezoelectric material of the dielectric layers of the sensor is a film 
of poly[vinylidene fluoride], the stack of conducting layers and of 
dielectric layers being preferably produced by bonding the polyvinylidene 
fluoride film onto the adjacent conducting layers. 
the material of the conducting layers is a copper-beryllium alloy.

FIG. 1 diagrammatically represents the antenna of the invention, referenced 
1. This antenna is formed by a succession of panels 2, which each 
externally exhibit the shape of a relatively thin flexible plate, which is 
applied against the wall of the hull of the naval vessel (the hull of a 
submarine, or the submerged part of the hull of a surface ship) in such a 
way as to follow the shape of the hull. 
The antenna 1 may thus consist of several tens of panels 2, for example 
sixty-four in number in one embodiment example; it therefore occupies a 
large part of each side wall of the submarine. 
The dimensions of each panel are not critical; they may, for example, be 
given a height of the order of 1 m and a width (dimension in the direction 
of the flow, of the order of 0.5 m. 
As to the thickness, it will be seen that the specific internal structure 
of the panels makes it possible, without difficulty, to give the latter a 
very slight thickness--without in any way prejudicing the performance of 
the sensor--, typically less that 10 cm. 
In FIG. 2--a, the panel 2 has been represented mounted on the wall of the 
hull 5 of the submarine: the mounting is achieved by means of two rails 3 
and 4 interacting with retaining pieces 5 or flanges. 
On the sides the panels are fixed by means of T-sections. As represented in 
FIG. 2--b, the panels are held by clamping by means of 4 flanges mounted 
on the rails at the four corners. At the upper part of the panel, in its 
centre, is the overmoulded connector followed by the connections forming a 
cable. 
The mounting is done leaving an intermediate water layer 6 providing 
mechanical decoupling between panels and hull. 
The thin hull and each panel are connected together in such a way as to 
contribute minimum hydrodynamic disturbance. 
Moreover, the electrical cables of the various panels 2 are routed under 
the thin hull above the upper rail allowing transmission of the signals 
detected by these panels 2. 
The mounting of the panels on the side wall of the hull of the submarine is 
easy because, despite their considerable dimensions, their weight is 
relatively low having regard to the fact that, as will be seen later, they 
are composed of low-density materials and are easily curved to follow the 
shape of the hull of the submarine. 
In the vertical plan, the large dimension of the panel (of the order of 1 
m, as has just been indicated), confers a significant gain in directivity 
for the highest frequencies of the band. 
Moreover, integration due to the large pick-up surface area reduces the 
sensitivity of the response to localized disturbances, leading to better 
phase control and to better track formation. 
From the point of view of the enhancement of the signal/noise ratio, the 
large size of each panel compared with the correlation length of the flow 
noise may be noted, which makes it possible to have integration effect 
which reduces the sensitivity of the antenna to the flow noise. 
In the same way, the bending waves propagated by the hull, the wavelength 
of which is smaller than the size of the panel, will be integrated, so 
that the sensitivity of the antenna to these waves will be reduced. 
Finally, the compact structure of the antenna is not intrinsically 
resonant. 
FIGS. 3 to 5 show the structure of the panel 2 in more detail. 
In essence, each of the panels 2 consists (FIG. 3) of a surface sensor 
proper 8 embedded in a lining material 9 which is itself enclosed in an 
envelope 10, 11. 
The surface sensor 8, the structure of which is represented in more detail 
in FIG. 4, is formed by an alternate stacking of conducting layers 12 and 
of piezoelectric dielectric layers 13. 
The central electrode will constitute one of the poles of the sensor, while 
the two outer electrodes, linked in parallel, will constitute the other 
pole of the sensor, as indicated at 18. This structure makes it possible 
to achieve an electrical screening effect. 
The metal layers are produced, for example, from a copper-beryllium alloy; 
the thickness of the metal electrode is of the order of 5/10 mm, for 
example. The blocking effect of the PVDF layers which results therefrom 
makes it possible to avoid it being depolarized at high temperatures 
&gt;50.degree. C. 
The piezoelectric material of the dielectric layers is advantageously a 
polymer such as a polyvinylidene fluoride (PVDF), a fluorinated polymer 
which is well known for its piezoelectric properties; the PVDF layer has a 
thickness, for example, of the order of 0.5 to 1.5 mm. 
PVDF, in addition to its piezoelectric properties, possesses the further 
advantage of excellent properties of chemical resistance and mechanical 
strength, low aging, etc, characteristic of most fluorinated 
thermoplastics. 
According to one variant, the piezoelectric material of the dielectric 
layers is a copolymer, consisting, for example, of 70% of PVDF and of 30% 
of PTrFe (PolyTrifluoroethylene). 
The PVDF film is advantageously produced according to the technology set 
out in FR-A-2 490 877, to which reference will be made for further 
details. 
Briefly, this technology consists in continuously rolling a sheet of PVDF 
so as to draw it mechanically, while simultaneously applying a high 
electric field to it, making it possible to orient the dipole moments of 
the molecules and thus to polarize the material in order to give it its 
piezoelectric properties. 
This PVDF film, cut up to the appropriate size, is bonded onto the metal 
electrodes so as to form the stack. 
The sensor thus formed is next placed in a neoprene rubber envelope 10, 
which advantageously constitutes a mould (bottom and sides of the 
envelope). The bottom of this envelope is equipped with studs 14 obtained 
during its manufacture and on which is placed the sensor, which is thus 
positioned. 
For the lining material 9, a "soft" polyurethane is used according to the 
invention. By "soft" polyurethane is understood a material the Shore 
hardness of which is typically less than 50. Its Poisson ratio is close to 
that of water 0.5. Moreover, its density acoustic propagation speed 
product is substantially equal to that of water, so as to be acoustically 
neutral with respect to the sensor. Its consistency is that of a viscous 
liquid. 
The envelope 10 consists, for example, of a pack 10 making it possible to 
constitute a mould, as has just been indicated, in which the material 9 is 
moulded. The pack is then closed off by means of a "hard" polyurethane 11, 
typically one with a Shore hardness equal to 80. 
The outer envelope 10, is, for example, a neoprene envelope of 30 mm in 
thickness. 
The only limitation is that this material is not too rigid (in order not to 
transmit the stresses applied to the region of the link to the hull of the 
submarine) and that it is more elastic than the sensor proper. 
In a variant, instead of a composite structure formed by an outer envelope 
enclosing a lining material, it would be possible to provide a homogeneous 
structure in which the sensor 8 were embedded in a homogeneous mass of 
appropriate material ("soft" polyurethane) exhibiting the necessary 
impermeability properties. 
The thickness of the lining of the sensor 9 (that is to say of the 
leaktight envelope/viscoelastic lining assembly, or of the homogeneous 
mass in which the sensor will be embedded) must be chosen so as to exhibit 
a value making it possible: 
on the inner side (hull side, sufficiently to space the sensor 8 away from 
the hull to limit the transmission of the bending waves of the hull 
towards the sensor. 
This distance must however remain small compared with a quarter of the 
wavelength of the upper frequency of the frequency band used if it is 
desired to avoid any destructive interference between the incident signal 
and the signal reflected on the hull. 
Thus, for a maximum frequency of 2 kHz, a quarter wavelength corresponds to 
18.75 cm, so that the total distance between the mid-plane of the sensor 8 
and the hull, that is to say the sum of the thickness of the lining 9 
under the sensor, of the envelope 10 and of the layer of water 6 
represented in FIG. 2, must remain markedly less than this value; in 
practice, a distance of 5 cm appears to be entirely suitable. 
on the outer side (flow side), sufficiently to space the sensor 8 away from 
the surface over which the flow is taking place, that is to say the outer 
surface of the cover 11 of the leaktight envelope, to reduce the flow 
noises picked up to an acceptable level, having regard to the level of the 
incident signal, and thus to enhance the purity of the output signal 
delivered by the sensors. 
FIG. 5 shows a particularly advantageous embodiment of the metal electrodes 
12. 
According to this embodiment, each of the electrodes 12 is formed by a 
plurality of square plates 15 linked together by thin bridges of material 
16. This structure is produced, in a conventional way, by stamping of a 
strip of metal, for example, or by cutting out with a pressurized water 
jet. 
Advantageously, the bridges 16, in addition to the fact that they provide 
the electrical continuity between the various plates 15, serve as elements 
for positioning the electrode 12 at the bottom of the envelope 10, by 
their shape in relief, illustrated in FIG. 6, which will allow the 
assembly to rest on the bottom of the envelope 10 on the studs 14 before 
pouring of the lining 10, keeping the plates 15 at an appropriate distance 
from the bottom of this envelope. 
At one of the ends of this set of plates 15, an outlet 17 is provided, 
allowing electrical connection of the electrode. 
The length L of the plates is chosen: 
to be compatible with the width of the PVDF film which it is desired to 
produce (typically, continuous strips of about ten centimetres in width), 
and also 
to preserve a certain flexibility for the whole of the sensor, allowing it 
to follow the (variable) diameter of the hull of the submarine. 
In fact, if the electrode 12 were formed by a uniform plate, its rigidity 
would make it difficult to shape the panel 2 to the profile of the hull of 
the submarine, whereas its separation into several plates 15 makes it 
possible to neutralize the rigidity of the metallic material itself. 
Finally, a sensor formed by a monobloc electrode would risk being subject 
to natural resonance over this maximum dimension, which is of the same 
order of magnitude as the wavelengths of the frequencies picked up, 
whereas by dividing the panel into cells of smaller dimensions, the 
possible natural resonances occur always at frequencies lying far above 
the upper limit of the frequency band in question. 
The electrical connection diagram is illustrated in FIG. 7, where it is 
seen that the various plates 15 are linked in parallel by the hinges 16, 
this assembly being electrically equivalent to a single electrode 12. The 
upper and lower electrodes are linked together by their connections 17, 
which form one of the poles of the sensor, while the connection 17' of the 
central electrode constitutes the opposite-polarity terminal of the 
sensor. 
From the functional point of view, this assembly corresponds to a 
column-sensor formed by a plurality of elementary cells 19; these various 
cells being mounted in parallel, so that their electrical signals are 
added. 
By way of example, the sensor 8 of each panel is formed by 21 plates of 105 
mm side, arranged into 7.times.3 and spaced apart by 128 mm. 
This embodiment example is not limiting. In fact, it is known that, in an 
antenna, it is advantageous to have a spacing between "sensors" equal to a 
half wavelength at the mean frequency of the band, so as not to be 
troubled by the image lobes. 
In the example described, each sensor consists of a panel: this is not 
obligatory. There is a separation between the "physical" panel and the 
"electrical" sensor. 
Thus, by cutting the bridges 16 between the columns and by linking to 3 
outputs, 3 column sensors of 7 plates per panel are obtained. Conversely, 
adjacent panels may be linked in parallel, in order to form sensors spaced 
apart by several panel widths. 
It is also possible to constitute an antenna formed by non-adjacent panels 
with "filling" panels between active panels, making it possible to 
preserve the hydrodynamic profile of the antenna. 
The Applicant has also produced an antenna formed by 64 panels as described 
and capable of operating at carrier speeds of several tens of knots. 
According to one embodiment variant, each elementary sensor 15 forms an 
independent sensor with an electrical outlet. In this case, each sensor 15 
is electrically connected to the outlet cable. 
Advantageously, the electrical connections are produced by means of a 
flexible printed circuit including tracks. A track arrives at a sensor by 
bonding the flexible circuit 20 to the edge between the central electrode 
and a PVDF layer, as indicated in FIG. 4. 
The positioning studs 14 are placed under certain sensors, and the flexible 
circuit is thus also embedded in the lining material 9. 
FIG. 8 represents an example of connection of 6 sensors according to this 
embodiment variant. The cut is along the central electrode, and the tracks 
correspond to the lines 21 on the strip 20. 
Other connection diagrams with several flexible printed circuit strips are 
possible without departing from the context of the invention.