ACOUSTIC TRANSMISSION ANTENNA

The invention relates to acoustic emission antennas, in particular to acoustic emission antennas in the field of low and medium frequency systems and to a method for calibrating such an antenna. The invention applies in particular, but not in a limiting manner, to sonars with variable immersion. It can also be applied to other types of sonar such as for example fixed antenna sonar, protection or port.

Marine platforms are generally equipped with submerged sonar antennas to detect and / or locate objects underwater. A sonar antenna comprises a set of stacked transducers ensuring the transmission of acoustic signals and mounted on a support. The signals are received by a set of receivers, such as hydrophones, arranged according to a configuration chosen with respect to the configuration of the set of emission transducers.

The existing antennas for the SONAR (“Sound Navigation And Ranging”) program with variable immersion are made according to different architectures.

Flat-shaped antennas made up of an array of elementary transducers can be used. These antennas carry out the emission of SONAR signals. Their transducers are often of the Tonpilz type, which makes them bulky and heavy. Tonpilz transducers require, in fact, to equip the active element (that is to say the piezoelectric or magneto-strictive or electro-strictive material of the antenna) with cumbersome mechanical parts, such as a seismic mass at the top. 'rear, a roof and a seal. In addition, the operation in immersion of these transducers involves providing a hydrostatic pressure compensation device under penalty of greatly degrading their performance in immersion. This antenna architecture is unsuitable for the design of a towed object of low mass and implies an oversizing of the other elements of the system.

In terms of compactness and weight, other architectures are favorable, such as antennas made up of a vertical array of compact flextension-type transducers. Nevertheless, this type of antenna does not make it possible to obtain a frequency bandwidth necessary for recent wide-band SONARs, because their transducers are mono-resonant and operate in a mechanical bending mode which is very overstrained by nature. Low frequencies are therefore achieved through the use of mechanical bending. This antenna is sufficiently compact to reduce the bulk and the mass of the system, but it has the drawback of minimizing the volume of active material which can be detrimental to the acoustic power that can be delivered and therefore to the sound level. The bandwidth of these antennas remains much less than an octave, an octave being a frequency interval of the form [f; 2 F].

Antennas made up of a vertical array of transducer type "slotted cylinder" (or "slotted cylinder" in English) are also used to achieve a compact antenna and low mass. This type of transducer is also based on a mechanical bending system and therefore inherently has a frequency bandwidth equivalent to that of flextension transducers. US patents 9001623 propose an integration in a towed body and US patent 8717849 proposes a variant. This architecture makes it possible to produce a compact and lightweight antenna, but remains limited in terms of frequency band and volume of active material. To overcome this, the antenna is extended in length, but the acoustic energy is then focused in a reduced volume of fluid, which can decrease the detection performance of the sonar. The extension of the antenna in length also entails a penalty in terms of navigation of the towed body, especially at high speed. In addition, its integration on the towed body is complex and increases the mass of the towed body and subsequently increases the complexity of operational use.

It is also possible to use antennas made up of a vertical array of compact and wideband FFR (“Free-Flooded Ring”) type transducers to increase the width of the transmission frequency band. This type of antenna can be present on SONAR towed from surface vessels. Patent FR 2776161 gives an example. The operation of these transducers is based on the coupling of two resonance modes which makes it possible to obtain bandwidths of the order of an octave. In addition, the ratio of active material is very high compared to the total mass, and therefore it is possible to achieve high power emissions, which is favorable to the noise level. However, these antennas cannot cover several octaves.

It is also possible to use antennas made up of a vertical array of transducers divided into groups of at least s two transducers to optimize the transmission bandwidth and the sound level (FR 3026569). However, as before, it is not possible to cover several octaves.

In order to increase the useful bandwidth, it is possible to combine several FFR transducers of different sizes (WO 2015/019116), but this leads to an increase in the mass and therefore in the power requirement, which complicates the system. Compared to the antenna of patent FR 2776161, the mass and the power requirement are 2.5 to 3 times greater. In addition, this solution is limited to the acoustic level because there are acoustic interactions between the different transducers and there is an acoustic masking effect of the small transducers by the larger transducers.

The invention aims to overcome the aforementioned drawbacks and limitations of the prior art. More precisely, it aims to propose an acoustic antenna having a wide frequency band without altering the sound level, while remaining in a dimension similar to the prior art in terms of mass, size and power.

An object of the invention is therefore an acoustic antenna intended to equip a sonar, the antenna being centered around a first longitudinal axis and comprising at least a first set of at least one transducer and a second set of at least two transducers stacked along said longitudinal axis, each transducer having at least one radial mode having a resonant frequency, called radial frequency, as well as a cavity mode having a resonant frequency, called cavity frequency, characterized in that the transducers of the first set are configured to emit sound waves in a first continuous frequency band extending at least between the cavity frequencies and the radial frequencies of the transducers of the first set and the transducers of the second set are configured to emit waves sound in a second continuous frequency band extending at least between the cavity frequencies and the radial frequencies of the transducers teurs of the second set, in that the cavity frequency of a transducer of the second set is substantially equal to the radial frequency of a transducer of the first set plus or minus (frl-fcl) / 10, fri

being the radial frequency of the transducer of the first set and fcl being the cavity frequency of the transducer of the first set.

According to embodiments of the invention:

the first set of transducers comprises at least two transducers and the transducers of the second set are placed between the transducers of the first set;

the transducers of the second set are divided into sub-groups, each sub-group comprising at least two transducers of the second set, the spacing between each sub-group being greater than or equal to the spacing between two transducers of the same sub-group. group, and each subgroup having at least one cavity mode having a resonant frequency, called the group cavity frequency;

the second set comprises seven transducers divided into three sub-groups, the first sub-group comprising two transducers, the second group comprising three transducers, the third sub-group comprising two transducers, and the second sub-group being placed between the first and the third subgroup.

the group cavity frequency of at least one subgroup is equal to the radial frequency of the transducers of the first set plus or minus (frl-fcl) / 10 and the group cavity frequency of at least one other subgroup group is equal to the cavity frequency of the transducers of the first set plus or minus (frl-fcl) / 10, fri being the radial frequency of the transducer of the first set and fcl being the cavity frequency of the transducer of the first set;

the antenna comprises passive elements stacked along the first longitudinal axis, surrounding the transducers of the second set and having at least one radial mode having a resonant frequency, called the radial frequency, equal to a radial frequency of the transducers of the second set more or less 0.1 x fr2, advantageously equal to a radial frequency of the transducers of the second set plus or minus 0.05 x fr2, with fr2 the radial frequency of the transducers of the second

together and also having at least one cavity mode having a resonant frequency, called the cavity frequency, included in the first frequency band;

the passive elements are made of a material such that the E / p ratio of this material is higher than that of the material making up the transducers of the second set, E being the Young's modulus and p the density of the materials;

the passive elements are cylinders having a diameter greater than that of the transducers of the second set;

the transducers are FFR ("Free-Flooded Ring") transducers produced in n piezoelectric ceramic or magnetostrictive ceramic or electrostrictive ceramics;

the transducers of the first set and of the second set are of circular, trapezoidal or polygonal section;

the antenna comprises at least a third set of at least two transducers stacked along K longitudinal axes parallel to the first longitudinal axis, K being greater than 1, the transducers of the third set having at least one radial mode having a resonant frequency, called radial frequency, as well as a cavity mode having a resonant frequency, called the cavity frequency equal to the radial frequency of the transducers of the second set plus or minus (fr2-fc2) / 10, fr2 being the radial frequency of the transducers of the second together and fc2 the cavity frequency of the transducers of the second set, the transducers of the third set being configured to emit sound waves in a third continuous frequency band extending at least between their cavity frequency and their radial frequency, the third band frequency having at least one frequency higher than the frequencies of the first and second frequency bands, and the meeting of the first st, second and third frequency bands forming a continuous frequency band;

the K longitudinal axes coincide with the first longitudinal axis; the antenna comprises at least a first phase shifter arranged so as to introduce a first phase shift between an excitation signal of the transducers of the first set and an excitation signal of at least one sub-group of transducers of the second set;

the antenna also comprises at least a second phase shifter arranged so as to introduce a second phase shift between excitation signals of different subgroups of transducers of the second set; and the antenna comprises N + 1 groups of transducers of the same type and N phase shifters arranged so as to introduce a phase shift between an excitation signal from the transducers of the first group and an excitation signal from another group, N being a integer greater than 1.

Another object of the invention is a method for calibrating an acoustic antenna according to the invention, characterized in that it comprises the following steps:

at. Excite a first group of transducers of the same type and short-circuit the other transducers;

b. Measure in the far field the phase of the pressure waves generated by the transducers of the first group;

vs. Excite a second group of transducers of the same type and short-circuit the other transducers;

d. Measure in the far field the phase of the pressure waves generated by the transducers of the second group;

e. Calculate the phase difference between the phase obtained in step b and the phase obtained in step d;

f. Adjust a phase shifter so that it introduces a phase shift equal to the difference calculated in step e to the excitation signal sent to the transducers of the second array.

Other characteristics, details and advantages of the invention will emerge on reading the description given with reference to the appended figures given by way of example and which respectively represent:

FIG. 1, an acoustic antenna according to a first embodiment;

FIG. 2, an acoustic antenna according to a second embodiment;

FIGS. 3a, 3b and 3c, an acoustic antenna according to, respectively, a third, a fourth and a fifth embodiment;

FIG. 4, an acoustic antenna according to a sixth embodiment;

FIG. 5, a calibration method according to an embodiment of the invention; and

FIG. 6a, results of simulations with an acoustic antenna according to an embodiment of the invention presented in FIG. 6b.

Throughout the description, the term “cylinder” is used in the general sense and designates a ruled surface the generatrices of which are parallel, that is to say a surface in space made up of parallel lines. In the embodiments illustrated by the figures, the transducers and passive elements have an annular shape, that is to say of a cylinder of revolution.

FIG. 1 shows an acoustic antenna ANT according to a first embodiment. The antenna ANT is centered around a first longitudinal axis A1 and comprises a first set of at least two hollow cylindrical transducers T1 and a second set of at least two hollow cylindrical transducers T2. In this first embodiment, the first set includes two T1 transducers and the second set seven T2 transducers. The cylindrical transducers T1 and T2 are formed around the same longitudinal axis A1. The transducers T2 are placed between the transducers T1 without there being any physical overlap between the transducers T1 and T2. This makes it possible to avoid harmful acoustic interactions, such as the masking of the T2 transducers by the transducts. urs T1. Each transducer (T1, T2) has at least one radial mode having a resonant frequency, called the radial frequency, and at least one cavity mode having a resonant frequency, called the cavity frequency. The T1 transducers of the first set are configured to emit sound waves in a first frequency band extending at least between the cavity frequencies and the radial frequencies of the T1 transducers, and the T2 transducers of the second set are configured to emit waves sound in a second frequency band extending at least between the cavity frequencies and the radial frequencies of the transducers T2. The T1 and T2 transducers have different physical dimensions, in particular the T2 transducers have smaller physical dimensions than those of the T1 transducers, so that the cavity frequency of a T2 transducer of the second set, fc2, is substantially equal to the radial frequency of a transducer Tl of the first set, fri, with a tolerance not greater than (frl-fcl) / 10, i.e. fc2 = fri ± (frl-fcl) / 10 with fcl the cavity frequency of a transducer T1. This makes it possible to obtain a continuous transmission frequency band comprising the frequencies of the first and second frequency bands.

The T2 transducers of the second set can be divided into subgroups comprising at least two transducers. In this first embodiment, the transducers T2 are divided into three subgroups (SGI, SG2, SG3). The first sub-group SGI comprises two transducers T2, the second sub-group SG2 comprises three transducers T2 and the third sub-group SG3 comprises two transducers T2. The SG2 subgroup is placed between the SGI and SG3 subgroups. The spacing between each subgroup, that is to say between the subgroups SGI and SG2 and the subgroups SG2 and SG3 for this first embodiment, is greater than or equal to the spacing between the transducers T2 of the same subgroup. This makes it possible to perform several functions with the T2 transducers.

Each subgroup (SGI, SG2, SG3) has at least one cavity mode having a resonant frequency, called the group cavity frequency. Indeed, when two identical annular transducers are arranged one above the other at a small distance with respect to the acoustic wavelength of their cavity modes, these modes interact and their frequency decreases (the frequency of the radial mode n 'is not affected). Thus, since the transducers T2 have equivalent physical dimensions, it is the spacings between the transducers T2 of the same subgroup which make it possible to modify the group cavity frequency of a subgroup.

Claims

1. Acoustic antenna (ANT) intended to equip a sonar, the antenna being centered around a first longitudinal axis (Al) and comprising at least a first set of at least one transducer (Tl) and a second set at least two transducers (T2) stacked along said longitudinal axis, each transducer having at least one radial mode having a resonant frequency, called radial frequency, as well as a cavity mode having a resonant frequency, called cavity frequency , characterized in that the transducers of the first set are configured to emit sound waves in a first continuous frequency band extending at least between the cavity frequencies and the radial frequencies of the transducers of the first set and the transducers of the second set are configured to emit sound waves in a second continuous frequency band extending at least between the cavity frequencies and the radial frequencies of the transducers of the seco nd together, in that the cavity frequency of a transducer of the second set is substantially equal to the radial frequency of a transducer of the first set plus or minus (frl- fcl) / 10, fri being the radial frequency of the transducer of the first set and fcl being the cavity frequency of the transducer of the first set and characterized in that the transducers of the second set are placed between the transducers of the first set and in that no transducer of the first set is placed between the transducers of the second set.

2. Acoustic antenna according to claim 1, wherein the first set of transducers comprises at least two transducers and the transducers of the second set are placed between the transducers of the first set.

3. Acoustic antenna according to one of claims 1 to 2, wherein the transducers of the second set are divided into sub-groups, each sub-group comprising at least two transducers of the second set, the spacing between each sub-group being greater than or equal to the spacing between two transducers of the same subgroup, and each subgroup having at least one cavity mode having a resonant frequency, called the group cavity frequency.

4. Acoustic antenna according to claim 3, wherein the second set comprises seven transducers divided into three sub-groups, the first sub-group (SGI) comprising two transducers, the second group (SG2) comprising three transducers, the third sub-group. group (SG3) comprising two transducers, and the second subgroup being placed between the first and the third subgroup.

5. Acoustic antenna according to one of claims 3 and 4 wherein the group cavity frequency of at least one subgroup is equal to the radial frequency of the transducers of the first set plus or minus (frl-fcl) / 10 and the group cavity frequency of at least one other subgroup is equal to the cavity frequency of the transducers of the first set plus or minus (frl-fcl) / 10, fri being the radial frequency of the transducer of the first set and fcl being the cavity frequency of the transducer of the first set.

6. Acoustic antenna according to one of the preceding claims comprising passive elements (PI) stacked along the first longitudinal axis, surrounding the transducers of the second set and having at least one radial mode having a resonant frequency, called radial frequency, equal to a radial frequency of the transducers of the second set plus or minus 0.1 x fr2, advantageously equal to a radial frequency of the transducers of the second set plus or minus 0.05 x fr2, with fr2 the radial frequency of the transducers of the second set and also having at least one cavity mode having a resonant frequency, called the cavity frequency, included in the first frequency band.

7. Acoustic antenna according to claim 6 wherein the passive elements are made of a material such that the E / p ratio of this material is higher than that of the material making up the transducers of the second set, E being the Young's modulus and p the density of the materials.

8. Acoustic antenna according to claim 7 wherein the passive elements are cylinders having a diameter greater than that of the transducers of the second set.

9. Acoustic antenna according to one of the preceding claims wherein the transducers are FFR ("Free-Flooded Ring") transducers made of piezoelectric ceramic or magnetostrictive ceramic or electrostrictive ceramics.

10. Acoustic antenna according to one of the preceding claims wherein the transducers of the first set and of the second set have a circular, trapezoidal or polygonal section.

11. Acoustic antenna according to one of the preceding claims comprising at least a third set of at least two transducers (T3) stacked along K longitudinal axes (A2, A3) parallel to the first longitudinal axis (A1), K being greater than 1, the transducers of the third set having at least one radial mode having a resonant frequency, called the radial frequency, as well as a cavity mode having a resonant frequency, called the cavity frequency equal to the radial frequency of the transducers of the second set plus or minus (fr2- fc2) / 10, fr2 being the radial frequency of the transducers of the second set and fc2 the cavity frequency of the transducers of the second set, the transducers of the third set being configured to emit sound waves in a third continuous frequency band extending at least between their cavity frequency and their radial frequency, the third frequency band having at least one frequency higher than the frequencies of the first and second frequency bands, and the union of the first, second and third frequency bands for mant a continuous frequency band.

12. Acoustic antenna according to claim 11 wherein the K longitudinal axes coincide with the first longitudinal axis.

13. Acoustic antenna according to one of the preceding claims comprising at least a first phase shifter (Dl) arranged so as to introduce a first phase shift (Dfΐ) between an excitation signal of the transducers of the first set and an excitation signal of at least one subgroup of transducers of the second set.

14. Acoustic antenna according to claim 13 also comprising at least one second phase shifter (D2) arranged so as to introduce a second phase shift (Df2) between excitation signals of different subgroups of transducers of the second set.

15. Acoustic antenna according to one of the preceding claims comprising N + l groups of transducers of the same type and N phase shifters arranged so as to introduce a phase shift between an excitation signal of the transducers of the first group and an excitation signal of another group, N being an integer greater than 1.

16. A method of calibrating an acoustic antenna according to one of claims 13 to 15, characterized in that it comprises the following steps:

at. Excite a first group of transducers of the same type and short-circuit the other transducers;

b. Measure in the far field the phase of the pressure waves generated by the transducers of the first group;

vs. Excite a second group of transducers of the same type and short-circuit the other transducers;

d. Measure in the far field the phase of the pressure waves generated by the transducers of the second group;

e. Calculate the phase difference between the phase obtained in step b and the phase obtained in step d;

f. Adjust a phase shifter so that it introduces a phase shift equal to the difference calculated in step e to the excitation signal sent to the transducers of the second array.