Note: Descriptions are shown in the official language in which they were submitted.
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ACOUSTIC UNDERWATER ANTENNA
The invention relates to an acoustic underwater antenna
of the generic type defined in the precharacterizing
clause of claim 1.
One known underwater antenna, which is referred to as a
marine-seismic streamer and forms the acoustic part of
a towed-array antenna (DE 42 08 178 Al), has a skeleton
with two traction cables and moldings which are
arranged separated from one another in the longitudinal
direction of the traction cables and are fixed such
that they cannot move axially on the two traction
cables, which run diametrically opposite on the
moldings. The skeleton is drawn into an antenna casing,
which is in the form of an elastic flexible tube, with
the molding jacket of each molding being supported via
two annular strips, which are separated axially,
composed of open-cell PU foam on the inner wall of the
antenna casing and/or of the flexible tube. An
electroacoustic transducer, which is provided by means
of a single hydrophone, is inserted in a central
aperture hole in each molding and is surrounded by a
sleeve composed of open-cell foam. Each transducer is
connected to an electrical line which is passed through
the moldings in the flexible tube interior. The
electrical lines lead to electronic assemblies for
signal processing of the electrical transducer output
signals. The amplified, digitized output signal, with
interference removed from it, is produced at the output
of each electronic assembly, also referred to as a
channel. The flexible tube is closed at the end and is
filled with a liquid. The moldings are used to provide
the dimensional stability of the flexible tube, of the
holder, which cannot move axially, for the transducers
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in the flexible tube, and for damping the liquid flow
in the flexible tube, and therefore for reducing
interference signals at the transducers.
In the case of towed-array antennas, a change has very
recently been made to packaging the electronic
assemblies in pressure-resistant housings and to
arranging them together with the transducers in the
underwater antenna, that is to say in the acoustic part
of the towed-array antenna. In this case, each
electronic assembly, which defines a channel, is
associated, in terms of space, with one transducer, in
such a way that one transducer and one electronic
housing always follow one another alternately in the
longitudinal direction of the underwater antenna. By
way of example, one such towed-array antenna is
disclosed in DE 198 11 335 Cl. Here, hydrophones and
electronic modules are arranged in one or more rows
alternately one behind the other in a gel body which
fills the antenna casing. With this configuration, an
adequate longitudinal distance is required between
adjacent transducers in which, on the one hand, the
electronic housing must be accommodated and, on the
other hand, free space must be provided for strain
relief of the electrical lines and cables, and which
therefore cannot be made indefinitely small. Since the
longitudinal distance between the transducers is
matched to the reception frequencies or transmission
frequencies of the underwater antenna in the acoustic
part in order to form directional characteristics, the
reception frequency or transmission frequency of the
underwater antenna is subject to an upper limit when
the aim is to ensure transmission or reception via a
single main lobe of the directional characteristic.
One known transducer module (DE 196 12 503 Al) has two
transducers which are separated from one another, a
separating wall which is arranged between the
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transducers and is composed of reflector and absorber
material, and an electronic circuit in which the
signals received by the two transducers are amplified
and digitized for transmission. The transducer
elements, separating wall and electronic circuit are
arranged in a cylindrical form, encapsulated with
polyurethane. The separating wall has a T-profile and
separates the transducer elements and the electronic
circuit from one another in such a way that the two
transducer elements are placed on the left and right of
the center web of the separating wall, and the
electronic circuit is placed above the lateral web of
the separating wall. The separating wall achieves
acoustic shadowing of the two transducer elements, in
such a way that each transducer element receives only
sound waves from a hemisphere in front of the
separating wall.
The invention is based on the object of designing the
physical configuration of an underwater antenna such
that the space required within the antenna casing
between the electroacoustic transducers which follow
one another in the longitudinal direction of the
antenna casing can be kept small.
According to the invention, the object is achieved by
the features in claim 1.
The underwater antenna according to the invention has
the advantage that there is an empty space after every
alternate transducer in the transducer chain as a
result of the combination of in each case two
electronic assemblies, which are associated with
adjacent transducers, to form an electronic module,
which empty space increases the flexibility of the
underwater antenna, for example when it is being wound
up, and can be used for strain relief for electrical
lines and cables, for example by crossing them over.
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The distance between the two transducers of a
transducer pair associated with the electronic module
can be utilized completely for accommodation of the
electronic module and can be minimized by an
appropriate design configuration of the electronic
module. A separation which can be minimized between the
transducers in turn results in the advantage that the
operating frequency (transmission frequency and/or
reception frequency) of the underwater antenna can be
shifted towards higher frequencies, and directional
characteristics can be provided for unambiguous
transmission in or reception from just one main
direction for operating frequencies to well above
10kHz, while avoiding grating lobes. The design of the
electronic modules as moldings which are supported on
the antenna casing and their fixing, such that they
cannot move axially, on the traction cables not only
saves the cost-increasing, sound-transparent holders
which are otherwise used for the transducers, but, by
their absence, also creates space in the longitudinal
direction of the antenna casing. At the same time, this
results in the antenna casing being stiffened.
Expedient embodiments of the underwater antenna
according to the invention together with advantageous
developments and refinements of the invention are
specified in the further claims.
According to one advantageous embodiment of the
invention, the electronic modules are advantageously
supported on the antenna casing via circumferential
strips, preferably via two strips which are arranged
circumferentially with an axial separation and are
composed of plastic. These plastic strips result in
acoustic decoupling between the antenna casing and the
electronic modules, in particular the transducers held
on them. If the plastic strips are manufactured from
open-pore PU foam, then the electronic modules - like
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the known moldings described initially - can at the
same time be used to damp the liquid flow in the
antenna casing, which is normally closed at the end and
is filled with a liquid, and thus to reduce
interference signals at the transducers.
According to one advantageous embodiment of the
invention, each electronic module has at least two
printed circuits which are fitted with electronic
components, are aligned transversely with respect to
the module axis, and are each fitted with one
transducer on a board surface which points outwards.
This manufacturing design of the electronic module
allows a very short axial physical length and therefore
the possibility of a very short distance between the
transducers in order to produce a high-frequency
underwater antenna. The printed circuit population,
which is designed using surface-mounted device
technology, with the electronic components resting
directly on the populated printed circuits, which are
aligned transversely with respect to the longitudinal
axis of the underwater antenna, not only makes it
possible to further reduce the axial length of the
electronic module but also results in the electronic
module being sufficiently pressure-resistant for use of
the underwater antenna at a greater water depth.
The electrical connections between the electronic
assemblies on the populated printed circuits are
designed to be flexible for space-saving installation
in the module housing. A so-called rigid-flexi printed
circuit is advantageously used, which comprises at
least two rigid printed circuits for population and a
mechanically firmly connected flexible conductor-track
connection in the form of a non-detachable flexible
cable.
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According to one advantageous embodiment of the
invention, the populated printed circuits have
groove-like cutouts, which are arranged distributed
over the circumference at the edge of the board, for
the traction cables and electrical lines and cables to
pass through. The populated printed circuits are held
at the edge in grooves, which are axially separated
from one another, in a module housing which is composed
of two housing halves and is open at one end. This
design configuration of the electronic modules results
in a skeleton, which is supported on the inner wall of
the antenna casing and is composed of traction cables
and electronic modules with transducer pairs, in which
case the individual electronic modules with transducer
pairs can be inserted into the skeleton or withdrawn
from the skeleton in a manner which is advantageous for
assembly and repair of the towed-array antenna, without
having to disassemble the remaining electronic modules
or the entire skeleton. Furthermore, twisting of the
underwater antenna, which is subject to tension, could
be restricted by the arrangement of the traction
cables.
According to one advantageous embodiment of the
invention, the transducers of each transducer pair in
the electronic modules lie on the module axis. The
directional characteristic which is formed from the
transducer signals has a horizontally narrow beam angle
all round. Target positions which are located using an
underwater antenna such as this are, however, ambiguous
since it is not possible to distinguish whether the
target that has been located is to starboard or port of
the longitudinal axis of the antenna casing.
According to one alternative embodiment of the
invention, all the transducers are arranged with
respect to the module axes with a radial separation on
the populated printed circuit. The transducers which
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follow one another in the longitudinal direction of the
antenna casing are offset with respect to one another
through a circumferential angle which is of any desired
magnitude and is defined stochastically. If the
transducer signals are now processed in a manner such
as that described in DE 44 45 549 Cl, then this results
in the underwater antenna having a directional
characteristic which points to only one side of the
towed-array antenna.
According to one advantageous embodiment of the
invention, each electroacoustic transducer comprises
three transducer elements with an omnidirectional
directional characteristic, which are arranged at the
same radial distance from the module axis and offset
through the same circumferential angle with respect to
one another externally in the module housing. Each
transducer element is preferably a hydrophone.
Depending on the instantaneous orientation of the
hydrophones which, for example, may be detected by
means of an orientation sensor, output signals from in
each case two of the three transducer elements are
combined, after passing through suitable time-delay
elements, to form the output signal of the
electroacoustic transducer, which signal corresponds to
the transducer having a cardioid-shaped natural
directional characteristic. These output signals from
all the electroacoustic transducers are used to form
directional characteristics whose main reception
directions each point to one side of the towed-array
antenna and thus produce unambiguous bearing results.
The signaling combination of the output signals from
the three transducer elements of an electroacoustic
transducer is described, for example, in Figure 9 of DE
31 51 028 Al.
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The invention will be described in more detail in the
following text with reference to exemplary embodiments,
which are illustrated in the drawing, in which:
Figure 1 shows a side view of a towed-array
antenna towed by a surface vessel,
Figure 2 shows a side view of a deployed vertical
antenna,
Figure 3 shows a detail of a longitudinal section
of an acoustic section of an acoustic
part in the towed-array antenna shown in
Figure 1, or in the vertical antenna
shown in Figure 2,
Figure 4 shows an enlarged illustration of the
detail IV in Figure 3, with
longitudinally-sectioned electronic
modules,
Figure 5 shows a plan view of populated printed
circuits, placed on the plane of the
paper, of an electronic module in Figure
4,
Figure 6 shows a view in the direction of the
arrow VI in Figure 5 of the populated
printed circuits fitted in the
installation position, and
Figure 7 shows an identical illustration to that
in Figure 5, according to a further
exemplary embodiment.
The underwater antenna, which in Figure 1 is integrated
in a towed-array antenna 10 and in Figure 2 is
integrated in a vertical antenna 11, in each case
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represents an elongated acoustic part 12, in the form
of a flexible tube, of the antennas 10, 11, which is
fitted with electric acoustic transducers. In both
exemplary embodiments, the acoustic part 12 is in the
form of a receiving antenna for reception of the sound
waves which are propagated in the water. The acoustic
part 12 may, of course, also be used for transmission
of sound. The acoustic part 12 is composed of a
plurality of acoustic sections 121, which are
detachably connected to one another via couplings 13.
In the case of the towed-array antenna 10, which is
illustrated schematically in Figure 1, the acoustic
part 12 is attached via a damping module 14 to a towing
cable 15 which is fixed at its end remote from the
damping module 14 to a drum 17 which can be driven and
is located on board a watercraft 16. A further damping
module 18 is attached to the other end of the acoustic
part 12, and a towing brake 19 acts on its end that is
remote from the acoustic part 12. The towed-array
antenna 10 is deployed into the water and is recovered
by means of the drum 17. The watercraft 16 may be a
surface vessel or an underwater vessel, for example a
submarine.
In the case of the vertical antenna 11, which is
illustrated schematically in Figure 2, the acoustic
sections 121, which are fitted to one another via
couplings 13, of the acoustic part 12 are once again
attached at the ends to respective damping modules 14
and 18. The damping module 14 is connected to a buoyant
body 20, and the damping module 18 is connected to a
stabilization element 21, with stabilization fins 22
and a ballast weight 23. The acoustic part 12 is
connected via an electrical cable 24 or an optical
waveguide to a control center, which is integrated in
the anchoring watercraft 16, or by radio to an
evaluation unit.
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Figure 3 shows a detail of an acoustic section 121 of
the acoustic part 12, partially in the form of a
longitudinal section. All the acoustic sections 121 of
the acoustic part 12 are designed identically. The
acoustic section 121 has an antenna casing in the form
of an elastic, strain-resistant flexible tube 25, for
example composed of polyurethane with inserted carbon
fibers or Kevlar fibers. A skeleton with two supporting
or traction cables 26, 27 and electronic modules 28
which are attached to the traction cables 26, 27 such
that they cannot move axially is drawn into the
flexible tube 25. The electronic modules 28 are in the
form of moldings for stabilizing the shape of the
flexible tube 25 and are used to accommodate the
transducers 29 (Figure 4) and electronic assemblies for
signal processing of the electrical output signals from
the transducers 29. In this case, each transducer 29
has an associated electronic assembly, which
essentially contains operational amplifiers, filters, a
sample-and-hold circuit and A/D converters or
sigma-delta A/D converters. The amplified and digitized
output signal, with interference removed from it, is
produced at the output of each electronic assembly,
also referred to as a channel. The channels are
connected to electrical lines 31 which are passed
through the towed-array antenna 10 or the vertical
antenna 11 to an electronic control center, which is
arranged in the watercraft, or to a radio transmission
path. The flexible tube 25 is closed at the couplings
13 and is filled with an electrically insulating
substance, for example gel or oil. Each transducer 29
is provided by a single hydrophone.
Figure 4 shows one of the electronic modules 28, in the
form of a longitudinal section and in detail. Each
electronic module 28 has a module housing 32 which
comprises two housing halves and has a circular cross
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section, which is open at the end. The two housing
halves are formed by two semicircular housing shells
321, 322, whose shell edges rest on one another and
produce the cylindrical module housing 32. The module
housing 32 is supported via two plastic strips 33,
which run around the module housing 32 separated
axially from one another, on the inner wall of the
flexible tube 25, and thus ensure the stability of its
shape. The plastic strips 33 are inserted into two
annular grooves 43, which are axially separated from
one another, in the outer jacket of the module housing
32 and project radially beyond the module housing 32,
as a result of which an annular gap remains between the
modular housing 32 and the flexible tube 25. The
plastic strips 33 are composed of open-pore or
open-cell PU foam and act as damping elements for
acoustic decoupling between the flexible tube 25 and
the module housing 32.
Two transducers 29 are accommodated in the module
housing 32 and are held, separated, on mutually averted
module faces of the electronic module 28. Each
transducer 29 has an associated electronic assembly
which produces the output signal of the transducer 29
or the channel for the transducer 29, such that the
electronic module 28 is formed with two channels. Each
electronic assembly comprises one or more populated
printed circuits 35. The populated printed circuits 35
are arranged axially separated from one another and
aligned laterally with respect to the module axis - and
thus with respect to the axis of the acoustic section
121 - and are fixed in the module housing 32. For this
purpose, the populated printed circuits 35 engage at
the edge in annular grooves 36, which are introduced,
axially separated from one another, into the inner wall
of the two housing shells 321, 322 and are
circumferential through 360 in the module housing 32.
The two populated printed circuits 35 which are located
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on the outside in the electronic module 28 are each
fitted with one of the transducers 29.
In the described exemplary embodiment, a total of four
populated printed circuits 35 are provided in the
module housing 32. One of the electronic assemblies is
in each case provided by two populated printed circuits
35, and these are associated with the transducers 29
arranged on the outer board faces. The populated
printed circuits 35 are mechanically non-detachably
electrically connected by means of conductor tracks
which are integrated in a flexible strip. Individual
components 38, which are arranged on the populated
printed circuits 35, on the electronic assemblies are
indicated schematically in Figure 4. These are
surface-mounted devices and rest directly on the
printed circuits. The number of populated printed
circuits 35 in the electronic module 28, with their
flexible conductor-track connections, may, of course,
vary.
Figure 5 shows a plan view of a populated rigid-flexi
printed circuit comprising four populated printed
circuits 35 with connections by means of flexible
conductor-track strips 39, before being fitted into the
module housing 32. Of the four printed circuits 35,
which are populated on both sides, those surfaces which
face away from one another in the fitted position
(Figures 4 and 6) of the two printed-circuit pairs can
in each case be seen. The two transducers 29 are
arranged on the two outer populated printed circuits
35. Each transducer 29 is in the form of a single
hydrophone which is placed centrally on the respective
populated printed circuit 35, in such a way that all
the hydrophones lie on the longitudinal axis of the
flexible tube 28. The printed circuits 35, which are
fitted with components 38 on both sides, form the
electronic assemblies. As can be seen in Figure 5, the
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populated printed circuits 35 are connected to one
another by means of the flexible conductor-track strips
39. When the populated printed-circuit chain that is
created in this way is being installed in the module
housing 32, the populated printed circuits 35 are
pushed directly against one another and the
conductor-track strips 39 are bent outwards, as can be
seen in Figure 6. In this case, Figure 6 illustrates an
arrangement which has been rotated through 900 in
comparison to the illustration in Figure 4.
A plurality of cutouts 40, 41, 42 are incorporated on
the edges in the populated printed circuits 35 in such
a way that, after the populated printed circuits 35
have been fitted into the module housing 32, said
cutouts are axially aligned with one another. In this
case, the two cutouts 40, 41 which are arranged
diametrically opposite one another are used for the
traction cables 26, 27 to pass through the electronic
module 28, and the other cutouts 42 are used to pass
electrical lines 31 through, for the connection of the
channels of the electronic modules 28.
During assembly of the acoustic section 121, the
skeleton is produced in such a way that each electronic
module 28 is hooked into the two traction cables 26,
27, and each channel of the electronic module 28 is
connected to the electrical lines 31. For this purpose,
the traction cables 26, 27 are inserted into the
cutouts 40, 41, and the electrical lines 31 are
inserted into the cutouts 42, in the populated printed
circuits 35 in the electronic module 28. The module
housing 32 is then closed by placing the two housing
shells 321, 322 one on top of the other, with the edges
of the populated printed circuits 35 engaging in an
interlocking manner in the grooves 36 in the housing
shells 321, 322. The module housing 32 then has the two
circumferential plastic strips 33 fitted to it, which
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are each inserted into the annular grooves 43 on the
upper face of the module housing 32. All the electronic
modules 28 are fixed, such that they cannot move
axially, via the module housing 32 on the traction
cables 26, 27, although this is not illustrated in any
more detail in Figure 4. The axial separation between
the electronic modules 28 is chosen such that the axial
separation between the mutually facing transducers 29
of two successive electronic modules 28 in the skeleton
is equal to the transducer separation between the two
transducers 29 which are integrated in the electronic
module 28. The empty space 44 between two adjacent
electronic modules 28 is used for strain relief for the
electrical lines 31, by routing them in the empty space
41 such that, in the next electronic module 28, they
are located in physically differently located cutouts
42 in the populated printed circuits 35 of the
electronic module 28. By way of example, the exemplary
embodiment illustrated in Figure 4 shows, for this
situation, that the lines 31 which can be seen in
successive electronic modules 28 run in cutouts 42,
which are diametrically opposite one another, in the
populated printed circuits 35.
Since the number of populated printed circuits 35 which
are used to provide the electronic modules 34, and the
axial separation between them and, otherwise, the size
of the module housing 32 can also be varied, the
separation between the two transducers 29 which are
associated with one electronic module 28 can be chosen
to correspond to a desired transmission or reception
frequency of the acoustic section 121, and can also be
sufficiently small that the reception frequency of the
acoustic section 121 can be shifted to the
higher-frequency range. The separation between two
electronic modules 28 must then in turn be chosen to
correspond to the separation between the two
transducers 29 which are integrated in one electronic
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module 28, in such a way that all the transducers 29 in
the acoustic section 121 are separated from one another
by the same axial distance.
In one alternative embodiment of the acoustic section
121, the transducers 29 are not placed on the
longitudinal axis of the flexible tube 25, as in the
case of the exemplary embodiment illustrated in Figures
3 and 4, but are arranged at a radial distance from the
longitudinal axis, on the two outer populated printed
circuits 35 of the individual electronic modules 28.
Each transducer 29 is in this case also formed by a
single hydrophone. All the transducers 29 within the
acoustic section 121 are offset with respect to one
another through a circumferential angle, with the
offset angle being different, and with the sequence of
the offset being chaotic. An acoustic section 121 such
as this allows clear distinction between a port or
starboard position of the target when finding the
bearing of a target. The signal processing for
transducer placing such as this along the flexible tube
is described, for example, in DE 44 45 549 Cl.
The modified rigid-flexi printed circuit, which can be
25 seen in the form of a plan view in the initial fitting
state in Figure 7, of the electronic module 28 differs
from the rigid-flexi printed circuit illustrated in
Figure 5 only in that each electroacoustic transducer
45 which is arranged on the two outer populated printed
circuits 35 has three omnidirectional transducer
elements 451, 452 and 453, which are placed at the
corner points of an equilateral triangle on the printed
circuit level. Each electroacoustic transducer element
451, 452, 453 is formed by means of a hydrophone. The
components of the electronic assembly are arranged on
the separate printed circuits that are connected to one
another, and are each associated with one of the two
electroacoustic transducers 45. In addition, an
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orientation sensor (not illustrated) is also integrated
in the electronic module 28. In each of the electronic
assemblies which are associated with one transducer 45,
the output signals from in each case two transducer
element pairs of the transducer elements 451, 452 and
453 are emitted as output signals from the transducer
45, which output signals are passed, as in the case of
the exemplary embodiment in Figures 3 to 6, via the
output as digitized signals to the electrical lines 31.
The respective pair of transducer elements 451, 452,
453 is governed by the output signal from the
orientation sensor. Each electronic assembly thus has
two outputs or channels, and each electronic module 28
is thus formed with four channels and is connected via
the four channels to the lines 31. The two pairs of
transducer elements each create one transducer 45 with
a cardiod directional characteristic, which has a null
point on one side of the connecting line between the
transducer elements in the transducer pair, with the
null point in the case of one pair of two of the three
transducer elements pointing to one side, and in the
case of the other pair of two of the three transducer
elements pointing to the other side, of the connecting
line. A configuration such as this of the
electroacoustic transducer 45 on the two mutually
averted end faces of the electronic module 28 makes it
possible to distinguish unambiguously between port and
starboard bearings when determining the bearings of
targets. An exemplary embodiment for the signal
processing of the output signals from the transducer
element pairs is described in DE 31 51 028 Al (Figure
9).
In one alternative embodiment of the acoustic section,
it would be possible to arrange the two transducers
separately on both faces of the electronic module,
rather than integrating them in the electronic module.
In this case as well, the advantage of design with
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short separations between transducers and the high
reception and/or transmission frequency of the
underwater antenna associated with this could then be
achieved by the combination of the channels of two
adjacent electroacoustic transducers.
