FR2492142A1 - ACOUSTIC ANTENNA WITH GEODESIC LENS - Google Patents

Abstract

ACOUSTIC ANTENNA CONTAINING A GEODESIC LENS AS A FOCUSING DEVICE FOR A SONAR, EMITTING AND RECEIVING WITH LOW ANGULAR WIDTH DIAGRAMS ON SITE IN AN ANGULAR FIELD CAPABLE OF REACHING 360. GEODESIC FORMED BY TWO WALLS 14 AND 16 SEPARATED BY SPACERS 20. ON EXIT OF THE LENS, THE R-RAYS EXIT THROUGH TORIC LIPS H. THERE ARE DIFFERENT SHAPES OF SURFACES FOR STIGMATIC LENSES. TRUNCONIC OR DISC SURFACES ARE ALSO USED. THE PROPAGATED ACOUSTIC WAVES ARE OF THE STONELEY TYPE. APPLICATION TO UNDERWATER SURVEILLANCE SONARS.

Description

- 1 -

  The invention relates to acoustic antenna devices

  sonar ticks, immersed in water, being capable of emitting and / or receiving with angular low angular range directivity diagrams in an angular field of up to 360? The devices comprise, according to the invention, acoustic lenses called geodesic lenses which produce a focussing by propagation of the acoustic waves on a surface.

left surface.

  It is known in the underwater acoustics technique to monitor an angular sector of up to 3600 by a cylindrical antenna having columns of transducers. The signals emitted and received by the transducers are out of phase or delayed and then added to form

  lanes in sector directions, from the guarded angular sector.

  Each channel corresponds to a particular phase shift or delay for the transducers concerned and this results in the disadvantage of a large volume of electronic circuits, if the angular sector

is tall.

  It is also known to form channels through acoustic lenses, the transducers being in the focal plane of these lenses. Cylinders are generally used

  filled with a liquid whose acoustic index of refraction with respect to

  port to water must be greater than 2. The lenses called lens

  "balls" have the disadvantage of requiring a heat regulation

  liquid and only allow angular

  of the order of 10 in an angular sector of 600.

  It is also known to use so-called lenses

  thin lenses similar to those of optics, these lenses

  solid matter of index less than 1 have the disadvantage, given the geometric aberrations, of having only one sector

useful angle of the order of 100.

  To overcome these disadvantages of the prior art, the device according to the invention uses a geodesic lens for focusing the beams of the antenna. This kind of

  lens is known in the radar technique.

  Briefly, according to the main characteristic, it is

  an acoustic antenna device immersed in water, which is

  characterized by the fact that it includes:

  a device for focusing acoustic waves that is

  Paged along a surface of revolution, called geodesic lens, electromagnetic transducers to excite and detect these. acoustic waves as well as means for coupling surface waves to volume waves propagating in the water. Other benefits and features will emerge from the

  description given by way of example and illustrated by the

gures that represent;

  FIG. 1, the schematic diagram of a stigma lens

tick Luneberg type,

  FIG. 2, the schematic diagram of a stigmatic lens

  geodesic datum of the "Rinehart" type, - figure 3, the profile of the meridian of a geodesic stigmatic lens of the "helmet" type (Tin Hat), - figure 4, the schematic diagram of a waveguide

of the type "Stoneley" to a wall.

  FIG. 5, the curve giving the speed of propagation

  of a Stoneley wave depending on the product thickness-

  FIG. 6, the block diagram of a double-wall waveguide,

  - Figure 7, the schematic diagram of a geo-

  2-wall waveguide type "helmet"; FIG. 8, the schematic diagram of the elevation of the radiation angle in elevation of a helmet-type lens, FIGS. 9 and 10, the schematic diagrams of the lenses

frustoconical and discs.

- 3 -

2492 1 42

  FIG. 11, the schematic diagram of a cylindrical lens

drique with couplers,

  - Figure 12, the principle dems of a geo-

  multi-surface desiccated, called conflexion lens.

  Acoustic lenses are known and use the

  same principles as optical or electromagnetic lenses.

  An antenna with an acoustic lens has the advantage

  to form a channel with a single transducer placed at the focal point or source of the sound rays. To obtain a high resolution and a large angular field the lens must be close to the characteristics of a perfect lens,

  that is, stigmatic, for two spheres.

  A lens is perfectly stigmatic for two spheres, one called "object sphere" and the other "image sphere", if PO is a point of the object sphere and P the conjugate point of the image sphere, the time taken to go from Po to Pi is the same

for all the departments.

  A known example where the object sphere is the plane of infinity

  is the Luneberg lens, shown in Figure 1.

  This lens has a refractive index with spherical symmetry

  n (r) such that: 2 n (r) = 2 - -2 (1) R2 where R is the radius of the outer sphere S and r is the radius of a sphere s, with ri R. Then a radius Do incident

  by the center 0 of the sphere S. All the rays such as Di.

  D2 and D3 parallel to C focus in one point.

  It is at this point that we place the transducer T which corresponds to

  in the direction of the spokes Do, D1i D2 and D3.

  This law of index, given by the relation (l), can be reproduced only by a series of layers with constant index which introduces defects. In addition, in acoustics, the production of a series of layers with index greater than 1 can be practically implemented only by the use of rubber-like elastomers which make it possible to obtain low speeds but which are very absorbent materials, at. beyond a few kHz. So there are no so-called perfect lenses

reproducing this law of index.

  The two main categories of lenses according to art

  prior art are "ball" lenses and "thin" lenses.

  They do not allow to obtain at the same time a good resolution

and a large angular field.

  "Ball" lenses are spheres or cylinders

  and they are isotropic, that is to say that they have the same properties for all directions for the sphere and for all directions perpendicular to the axis of symmetry, in the case of the cylinder: they therefore allow a large angular field. In these lenses the marginal rays do not focus at the same focus as the rays located near the axis and to reduce these aberrations the index must be high, greater than 2. These indices can be obtained only with liquids. the family of fluorocarbons that are very sensitive to

  variations in temperature leading to focal aberrations

  if one does not take care to stabilize the temperature.

  As a result, such lenses do not make it possible to obtain a diameter / wavelength ratio greater than 100

  thus limiting the resolution, at an angle of the order of the degree.

  "Thin" lenses are similar to those used in geometrical optics and can be made in

solid material.

  A convergent acoustic lens is bi-concave. These lenses make it possible to achieve very good performances in resolution but not being isotropic the angular field

  remains limited to about + 10 degrees because aberrations occur

  rations identical to those of optical lenses; astigmatis-

me, coma, etc.

  According to the invention, the antenna comprises a geometrical lens

  désique. FIG. 2 shows such a lens equivalent to that of Luneberg, which has been proposed in microwaves in an article by R. F. Rinehart in "Journal for Applied Physics",

flight. 19, page 860, September 1948.

  This lens consists of a surface of revolution (,

  around an axis Oz having meridian curve (M).

  Waves propagating along the surface (s) and issuing from a point P on this surface in the plane Oxy, follow the

  Geodetic lines of (<). The lens called "Rine lens-

  hart "has a meridian (M), such that the phases of the waves coming from P arriving on the circle (C) in the plane Oxy are the same, as on the circle (S) of FIG. 1. All the rays arriving

  on the circle (C) parallel to Oz, it is necessary for the len

  Rinehart is equivalent to the Luneberg lens that these rays are rotated at the exit of (C) from an angle of

  around the Ox axis.

  The equation of the meridian (M) of the Rine-

  hart is given by: 2 r + arc sin (L (2) where R is the radius of the circle (C), e the length PQ on this meridian for the current point Q and r the distance from the point Q to the axis Oz The waves coming from the point P and which propagate on the surface () end up at the circle (C) and leave after rotation according to the parallel rays such as D10, D11, D12

and D13.

  According to a preferred version of the invention, geodesic lenses having a surface in the form of a "helmet" ("Tin Hat" in the Anglo-American literature) are used.

  will be shown later. The theory of these bins for applications

  cations in the radar technique was made in R.G.F. Warren and S.F.A. Pinnel entitled "The 1Iathematics of the Tin Hat Scanning Antenna" RCA Victor Co, Ltd (Montreal, Canada)

  Technical Report 7, 1951, Contract DRBS - 2 - 1 - 44 - 4 - 3.

26- 2492142

  It is also possible to make geo-

  desicned with simpler forms, like truncated cones, cylinders or disks. But for these forms, he

  There is no more rigorous stigmatism and there are aberrations.

  According to the invention, it is also possible to use lenses formed by two cones having the same axis, which intersect in a circle. The lenses are described in the article by Toraldo di Francia published in "Journal of Optical Society of Americavol 45, NO8 page 621. The passage from one cone to another produces a deviation of a radius similar to a refraction by a diopter in optics, which we call "conflexion" By associating several conflexions, we can obtain a geodesic lens of low aberration called multiplet

of confession.

  According to the invention, the propagation of acoustic waves guided along a surface is obtained by a waveguide

  acoustic, consisting of one or two walls.

  FIG. 4 shows the principle of a waveguide with a solid wall 2 of thickness e. The interfacial wave that propagates in the wall and the surrounding environment, which is usually water, is called the "Stoneley Wave." These waves are described in Stoneley's article in "Proceedings of the Royal".

  Society, 106, page 416 (1924).

  While only one mode of propagation is established in the

  two modes of propagation are established in the solid.

  The amplitude of the wave in the solid and in the liquid is respectively exp + sy and exp - dLy, where Oy is an axis perpendicular to the wall 2 in the direction of the liquid, cds etcoL are the coefficients of weakening in both environments. To simplify, we only take one value (for

two modes in the solid.

  The penetration depths PL and Ps in the two media are given by: PL = 1, 2 / a L and P = 1.2 / ds L s The Stoneley wave is produced by a transducer T which can be placed at the end of the wall 2. The electrical signals

  applied to the transducer T are provided by the generator 1.

  The transducer T is positioned so that its radiating face is perpendicular to the Ox axis of the wall 2 of the guide. The

  height of this face being Hp the transducer is placed at a dis-

  Dp rate of the wall, with Dp <H2pt o which is the distance of FRESNEL

  o to is the wavelength in the liquid. Moreover, the transduc-

  tor T is excited by the generator 1 so that its beam face

  nante vibrates in compression, all the points of this face being in

  phase. This results in equiphase plane waves capable of

  a wave guided by the wall, which is of the Stoneley type.

  The velocity V of the Stoneley wave as a function of the product e.f is given in FIG. 5, f being the frequency. This velocity V for large values of e.f tends to an asymptotic value

  Vs. It is known that if we take a thickness e equal to a value of

  their e, equal to the penetration depth PC = 1.2 / .C 6 c is the attenuation coefficient for the shear mode, ec can be calculated by taking V = Vs by the relation: e = 0-2 o0 = c / c0 (3) ccj YO and Co are the wavelength and the velocity in the liquid o and Av = co Vs, cs and c are the sp speed waves of shear and compression in the solid, silt = cs / cp and sip where o are the densities of the solid and water, the ratio Av / cO is equal to: = 2 o Lt (eo 2 _ l0 (4) go P 2 (y) _ (o O with F (o0) = (1 -2W2) 2-4 y2 (W6 _ 1) k (n2 2 - 8- For the steel-water interface o = 2.153, d = 0.552 and 0'0, 128 and the value of 4v / c is close to 2.5 1073

  according to (4). The thickness e is then equal to 0.22.O according to (3).

  According to the invention, the thickness of the solid wall is chosen at least equal to e, that is to say to the depth of penetration Pc; for example for a frequency equal to 750 kHz

  with co = 1500 m / s it is chosen greater than 0.5 mm.

  Since the speeds c and c are independent of the frequency s o f, the speed of Stoneley is independent of the frequency

  according to the expression (4), for e> ec.

  The variation of this speed as a function of the temperature of the water is small, the gradient for the ratio Av / c as a function of the temperature being of the order of 8 x 10-6 per C If the geodesic lens has strong slope variations, use is preferably made of two-walled guides, shown in Figure 6. The two solid walls 10 and 11 are separated by a distance h, the space 12 between the two walls 10 and 11 is generally filled with liquid. If plane waves are generated perpendicular to the interfaces of the liquid-solid walls, interfacial waves are established

  of the Stoneley type. As for the single-wall guide, the thick-

  Both sides of the walls must be chosen larger than the critical thickness ec of the Stoneley wave in the solid wall. In addition, the distance h between the two walls must be chosen for a frequency determined so as to propagate a wave that combines the energies conveyed by the two interfacial waves. The propagation mode that is excited in such a guide is nondispersive and its velocity, which is that of Stoneley, does not depend on the distance h. Poulr steel-water interface at the frequency of 750 kHz for example, the two walls are at least 0.5 mm thick

  while the distance h can vary between 2 and 8 mm approximately.

  The propagation mode is suitably excited from a transducer T, placed identically as for the one-wall guide. The transducer is placed at the end of the guide, its radiating face being perpendicular to the axis Ox and it is

  positioned symmetrically with respect to the two walls.

  Advantageously, to increase the energy emitted, the ends of the walls are beveled 13 so as to use a

  transducer having a larger transmitting surface.

  The transducer must be positioned perpendicular to the axis of the guide to avoid exciting parasitic propagation modes, and its radiation face has a height Hp substantially equal to

  the distance between the ends of the bevelled walls.

  Figure 7 shows a geodesic lens according to the

  which has two metal surfaces 14 and 16 having

  walls thicker than the critical thickness e.

  The distance between surfaces 14 and 16 is maintained

  aunt by spacers 20 of small dimensions relative to the acoustic wavelength. The volume between the two surfaces

  14 and 16 is usually filled with water.

  The shape of the surfaces can be that of the lens

  from Rinehart shown in Figure 2. In a preferred version

  In the context of the invention, a "helmet" type lens (Tin Hat) shown in FIG.

  circle (C) are made in such a way that the spokes of

  tie (ô) are parallel to the axis oy or slightly inclined relative to the plane Oxy, below this plane. FIG. 3 shows the meridian of a helmet lens connected in a circle (C) by a quarter circle 35. In this case the radius (R E) is parallel to

the axis OY.

  The advantage of this lens compared to that of Rine-

  hart is due to the fact that the direction of radiation of this lens coincides with the proper direction of the exit slot

of the guide.

  On this same circle (C) are the transducers, such as the opposite of the lips, which sends the acoustic energy

  in the guide formed by the two surfaces 14 and 16.

- 10 -

  In this case, the output circle (C) called "pupil circle" is at the same time the input circle or "source circle The angular aperture in the bearing (in the Oxy plane) of the 3 dB attenuation diagram expressed in radians, of the geodesic lens-at the exit of the circle <C) is of the order of XO / D o D

is the diameter of the circle (C.

  The angular aperture in site (in the plane perpendicular to

  dicular to the Oxy plane) is about V-8>, - 7D. We can increase

  the opening in site by extending the waveguide following the sur-

  face c'nique or plane 26 shown in Figure 8. The output of the guide is then by a circle (C '). L and L2 being tangents to the circle (C) parallel to the axis Oy. The lines L1 and L2 intersect the circle (C ') at E and E IH is the arrow of the arc E E2. The opening of the deposit of the lens being

  then equal to 2 VKX / 1H is increased.

  The lenses for which the pupil circle (C) and the source circle (S) coincide, as shown in Figure 7, have a field limited to at most about 90 because of the masking by the

transducers.

  The production of lenses of this type is, for example, carried out by forming two metal plates, made of steel, for example, in a mold whose profile of the meridian is machined according to a law such as that represented for example in FIG. 3; the two plates are then held at the desired distance by spacers 20 of small dimensions in front of XO to avoid

diffraction.

  The frustoconical and cylindrical lenses have the advantage of having an angular field in the Oxy plane of 3600. Indeed, in

  In this case, the rays emitted or received by the source circle (S) do not

do not pay this circle.

  Figure 9 shows a sectional view of the lens of this type according to the invention. The waveguide formed by a wall is of frustoconical shape. The circumference of the upper base forms

- he -

  the source circle (S) of radius (R1) while the circumference of

  the lower base forms the pupil circle (C) of the radius (R2.

  The propagation guided on this surface from a point of the source circle makes it possible to obtain a focus in a direction (R) whose angle of inclination θ <with respect to the plane Oxy is given by the relation; i = arc cos ((4) Z being the height of the truncated cone, the lens thus obtained is obviously more easily achievable than the "Tin-Hat" type of lens, but it does not make it possible to obtain performance equivalent to a perfect lens,

  in particular, it has spherical aberrations.

  The base of the cone resting on the pupil circle (C) is connected to

  corded to a curved portion 22, which directs the axis of the guide

in the direction (R6) -.

  On the source circle (S) are the N transducers

  T1, T2, ... TN (We assume N even to simplify).

  In the figure, the two transducers T1 are shown

  and TN / 2 + 1 which are in the cutting plane.

  Two variants of conical lenses can be made

  embedded image: the cylindrical lens, that is to say Ri = R and the 1 2 disc lens represented in FIG. 10, equivalent to a trunk;

cone height H = R2-R1.

  This figure shows a two-wall guide coupled to the transducers T1, T2, ... TN; T1 and TN / 2 + 1 being

in the cutting plane.

  According to a variant of the invention is used (Figure 11) several source circles at different heights on a cylindrical lens to form lanes in site. In the figure only two source circles have been represented for simplicity

  (S) and (S 'L Transducers T1, T2 .. coupled to the

  1 "1 2 .. N coulenaty linder by a coupler c and transducers T1, T2,

.TN..DTD: by-a coupler c1.

- 12 -

  Circles (S1) and (Si) correspond to different site angles. Finally, a coupler c2 at the base of the cylinder has its end oriented towards the mean direction of the elevation angles. According to another embodiment, the set of transducers T1, T2, T * with the coupler C1 are translated parallel to the axis of revolution z z ', which has the effect of changing the angle of elevation of the spokes (% 6 According to another variant of the invention, a geodesic lens is produced by associating several conflexions, which are called multiplets of conflexion, the surface

  obtained with the waveguides described above.

  cédemment. An exemplary embodiment is shown in FIG. 12. This lens comprises two reflections 21 and 22 and the geometry of the waveguide comprises a disc portion of radius R and a cylindrical portion of height H. The transducers are located

  on a circle source ray R In a preferred version

  we take H = 3R / 2 and R4 = 2R / 3, for these dimensions the

  focus is obtained at infinity.

  For geodesic lenses operating at frequencies below 100 kH, a wall coating of polyurethane-type viscoelastic material is preferably used, in which the speed of the acoustic waves is close to that in the water. This protects the steel walls from corrosion. If the waveguide has two walls, the gap will be filled with polyurethane ensuring a greater holding

  crushing and greater constancy of the interval.

  The composition of such a material is described in a patent

  French de la plaintiff, N0 of deposit 69 40589.

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Claims (24)

  1. An acoustic antenna immersed in water, characterized in that it comprises a focusing device of the guided acoustic waves which propagate along a surface of   revolution, called a geodesic lens, of electronic transducers   acoustics to excite and detect these acoustic waves as well as means for coupling guided waves to volume spreading in the water.   An acoustic antenna according to claim 1, characterized   in that the surface of revolution forming the geodesic lens is a metal wall and the radiating faces of the electro-acoustic transducers are placed at the ends of the wall at a distance less than the so-called FRESNEL distance, these compression-vibrating faces and generating plane waves and producing interfacial guided waves on the   wall surface called "Stoneley" waves.   An acoustic antenna according to claim 2, characterized   the fact that the thickness of the wall is greater than the depth of penetration of the Stoneley waves into the wall, in the shear mode.   Antenna according to claim 1, characterized   the fact that the geodesic lens has two walls separated by a distance h and filled with a medium and that the radiating faces of the transducers are placed at   ends of both walls at a distance less than   FRESNEL and symmetrically with respect to these walls, the vibrating faces in compression generating plane waves that produce Stoneley waves on both internal surfaces both sides.   An acoustic antenna according to claim 4, characterized   because the thickness of the two walls is greater than the depth of penetration of the Stoneley waves in both walls, for the shear propagation mode and the distance between the two walls is about twice   the depth of penetration into the filling medium. - 14 -   6. Acoustic antenna according to claim 4 characterized in that the ends of the walls are beveled and that the radiating faces of the transducers have a height equal to about the distance between the ends of the bevelled walls.   Antenna according to claim 1, characterized   the fact that the coupling of the guided waves to volune waves is obtained. by connections to the surface of revolution oriented at its ends in the directions of radiation of volume waves.   An acoustic antenna according to claims 1, 2, or 3,   characterized by the fact that the shape of the lens is of the type Rinehart lens.   9. Acoustic antenna according to claims 1, 2 or 3,   characterized by the fact that the shape of the lens is of the type "helmet" (Tin Hat).   Antenna antenna according to claims 1, 2 or 3,   characterized in that the lens has a cylindrical shape.   Antenna according to claims 1, 2 or 3,   characterized in that the lens has a frustoconical shape.   An acoustic antenna according to claims 1, 2 or 3,   characterized by the fact that the lens has the shape of a disc.   Antenna according to claim 10, characterized   owing to the fact that a coupler c1 formed by a surface near the walls makes it possible to excite the Stoneley waves from the transducers and that a coupler c2 also formed by a surface near the walls constitutes the coupling means of the guided waves to the waves of volume.   Antenna according to claims 1, 2 or 3,   characterized by the fact that the surface forming the lens is composed of several cones having the same axis of revolution forming   thus geodesic lenses called multiplets of conflexion.   15. Acoustic antenna according to claim 14, characterized   rised by the fact that the geodesic lens of - 15 -   consists of a cylinder of radius R and height H connected to a disk and that the transducers are at the disc level on a circle of radius R4   16. An acoustic antenna according to claim 15, characterized   characterized by H = 3R / 2 and R4 = 2R / 3.   17. Acoustic antenna according to claim 9, characterized   in that at least two circles at different heights have transducers coupled to the cylinder   thus making it possible to form lanes in site.   18. Acoustic antenna according to claim 9, characterized   laughed at the fact that the transducers with the coupler c1   can be translated parallel to the generator of the cylin- dre.   19. Acoustic antenna according to claim 4, characterized   terized by the fact that the medium that fills the space between two walls is a liquid.   20. Acoustic antenna according to claim 19, characterized   characterized by the fact that the filling liquid is water.   21. Acoustic antenna according to Claim 4, characterized   by the fact that the medium that fills the space between the two -   walls is a viscoelastic material of the polyurethane type in which the acoustic propagation velocity is equal to that some water.   22. Acoustic antenna according to claim 2, characterized   rised by the fact that the walls have a coating of material   visco-elastic type polyurethane in which the speed of pro-   acoustic pagation is equal to that in the water.   23. Antenna according to claim 7, characterized   due to the fact that the meridian of the couplings forms a quarter circle.   Antenna according to claim 8, characterized   risqué by the fact that the helmet lens is connected to a guide frustoconical shape.   25. Acoustic antenna according to claim 8, characterized   that the headset lens is connected to a