EP0568592A1

EP0568592A1 - Flextensor acoustic transducer for deep immersion.

Info

Publication number: EP0568592A1
Application number: EP92903815A
Authority: EP
Inventors: Michel Lagier; Philippe Dufourcq
Original assignee: Thomson CSF SA
Current assignee: Thales SA
Priority date: 1991-01-25
Filing date: 1992-01-14
Publication date: 1993-11-10
Anticipated expiration: 2012-01-14
Also published as: FR2672179B1; US5431058A; CA2101053A1; WO1992013338A1; CA2101053C; DE69212806T2; FR2672179A1; DE69212806D1; EP0568592B1

Abstract

L'invention concerne les transducteurs acoustiques du type flextenseur dans lesquels un corps de section oblongue est sollicité par un moteur selon le grand axe de cette section. Elle consiste à utiliser des moyens viscoélastiques (104) pour absorber les déformations lentes de la coque (401) sous l'effet de l'immersion. Ces moyens viscoélastiques présentent une raideur importante aux fréquences d'utilisation du transducteur de manière à transmettre avec un bon rendement les vibrations du moteur à la coque. Elle permet de fabriquer un transducteur flextenseur pouvant supporter une immersion importante sans que le moteur se casse et dont le rendement est supérieur à 75 %.The invention relates to acoustic transducers of the flextensor type in which a body of oblong section is stressed by a motor along the major axis of this section. It consists in using viscoelastic means (104) to absorb the slow deformations of the hull (401) under the effect of immersion. These viscoelastic means have a high stiffness at the frequencies of use of the transducer so as to transmit with good efficiency the vibrations of the engine to the hull. It makes it possible to manufacture a flextensor transducer that can withstand significant immersion without the motor breaking and whose efficiency is greater than 75%.

Description

FLEXIBLE ACOUSTIC TRANSDUCER FOR DEEP IMMERSION

The present invention relates to acoustic transducers of the flexural type capable of being submerged to a significant depth without suffering damage and always operating correctly. It applies to the emission and / or reception of acoustic or ultrasonic acoustic waves in fluid environments such as the submarine space.

Known flextensor transducers are generally composed of a flexible, watertight shell with a cylindrical side wall of elliptical cross section, vibrated by one or more pillars or bars of piezoelectric ceramic cells. Each pillar is kept in compression between the opposite parts furthest from the side wall. In emission, an alternating electric field is applied in the longitudinal direction of each pillar and the resulting movement, which takes place along the longitudinal axis of each pillar, is transmitted, amplified, to the surrounding liquid medium, the amplitude of this movement being maximum in the plane generated by the minor axes of the ellipses formed by each cross section.

A compression preload of the piezoelectric cells of each pillar is necessary to avoid breakage of the ceramic when the pillars are stressed in extension. According to a first known embodiment, this prestressing is supplied directly by the shell when the pillars are assembled. The housings provided in the shell for the pillars have, before assembly, shorter lengths than those of the pillars. To set up the pillars, it suffices to apply two opposite external forces to the facing parts closest to the wall lateral to compress the shell at this point and cause by elastic deformation thereof a just sufficient increase in the length of the housings to allow the installation of the pillars. The prestressing force is applied when the action of the two external forces is suppressed. The pillars then remain compressed in their housings between the parts of the inner lateral wall of the shell in contact with their ends.

This embodiment requires, in order to obtain correct operation of the transducers at a determined depth, to give the amplitude of the two external forces a value greater than that which is normally exerted by the hydrostatic pressure at this depth. This has the disadvantage of limiting the use of these types of transducers to the depths for which the prestressing force of the pillar can still be ensured, to avoid breakage of the ceramic constituting the piezoelectric cells.

According to a second known embodiment, the prestressing force of each pillar can be obtained by means of a rod passing through each pillar along its longitudinal axis, the ends of the rod being held by bolting to the shell. But in this case, the hydrostatic pressure exerts, via the shell, a tensile force on each pillar which causes, when it is too strong, a rupture of the ceramic making up the piezoelectric cells.

Finally according to a third known embodiment, a description of which can be found in US Pat. No. 4,420,826, the stacking of the piezoelectric cells can be carried out along a prestressing rod which is not fixed by its ends to shell. The stack is maintained by two rails so as not to be subjected, as in the embodiment described above, to a tensile force directed along the longitudinal axis of the pillar. However, here again, when the immersion of the transducer is such that one or two sides of the pillars are no longer in contact with the shell, the transducer can no longer function properly.

The Applicant has also proposed in the French patent application n ° 88 14416 filed on 4/11/88 two other embodiments of a flextensor transducer in which a countermass is added to the ceramic pillars, which can possibly be provided by a fluidic device. These devices work correctly but these additional organs complicate their manufacture. To overcome these drawbacks, the invention proposes a flexural tensor acoustic transducer for deep immersion, comprising a hollow shell of oblong section and an electroacoustic motor intended to excite this shell along the major axis of this section, mainly characterized in that it further comprises viscoelastic means making it possible to absorb without presenting appreciable mechanical resistance the forces exerted by the hull on the engine under the effect of deformations originating from immersion, and having a significant stiffness at the operating frequencies of the engine to communicate to the hull the vibrations of this engine with good efficiency.

Other features and advantages of the invention will appear clearly in the following description given by way of nonlimiting example with reference to the appended figures which represent:

- Figure 1, a sectional view of a transducer according to a first embodiment of the invention; Figure 2, a characteristic diagram of the material making up the part 104 of Figure 1; - Figure 3, a sectional view of a second embodiment; and

- Figure 4, sectional side views from above of a third embodiment.

There is shown in Figure 1 a sectional view of a type 4 flexuring transducer according to the classification established by ROYSTER in the journal JASA N ° 38, 1965 p. 879 to 880.

This -transducteur includes a shell of elliptical section 101 in which is inserted a piezoelectric motor 102 placed along the major axis of the ellipse and which is supported by its two ends on the interior faces of the shell to make it vibrate, under the influence of an electric voltage, along an axis OX parallel to this major axis. Under this influence the whole shell begins to vibrate and the amplitude of the movement is maximum along an OY axis parallel to the minor axis of the ellipse.

When such a transducer must operate at a deep immersion, for example greater than 100m, the shell deforms by flattening along an axis 0Y, and therefore widening along the axis 0X since the interior 103 does not communicate with the outside and therefore contains only 1 air at atmospheric pressure. This enlargement tends to draw on the motor 102, formed of a stack of piezoelectric ceramics. As these do not support the traction forces, they risk breaking in dynamics.

According to the invention, a part 104 formed of a viscoelastic material whose static stiffness is low and the dynamic stiffness is high is inserted, substantially in the middle of the motor 102. In the example shown, to facilitate mechanical production, two intermediate steel plates 105 and 106 have also been inserted between this viscoelastic part and the ceramics making up the motor, but this arrangement is not essential. Similarly in the drawing, the dimensions of the viscoelastic part and of the metal plates are shown to be substantially equal to those of the ceramic plates forming the motor, but the exact dimensioning will be chosen according to the characteristics of the materials used.

So when the transducer is submerged, the shell

101 crashes and the two right and left parts of the motor located on either side of the part 104 move apart pulling on it. As the static compliance (inverse of the stiffness) of the material used is high, it gradually deforms under the influence of the deformation of the shell and it stretches without exerting appreciable traction on

^ the two parts of the engine. These are therefore not subjected to tensile forces liable to break them.

On the other hand when the motor is subjected to alternating electrical voltages intended to generate the acoustic vibration, such as the compliance of the viscoelastic material

10 used is very low for the frequencies used, which correspond substantially to the resonant frequency of the transducer, this material behaves as if it were perfectly rigid. The bar formed by the two parts of the engine, the steel plates and the part 104, thus vibrates with a

-_> single block by transmitting its vibrations to the shell of the transducer.

The material used having a difference in compliance, or stiffness, between the low frequencies which correspond to static stresses and the high

20 frequencies which correspond to dynamic stresses, we can summarize the behavior of the part formed with this material by saying that it behaves like a mechanical filter not above.

A transducer is characterized by:

25

K: stiffness of the piezoelectric motor K: stiffness of the shell

Q: quality factor 5 fr- (resonant frequency

30 on frequency band).

If V _* is the limit pressure for which the engine is detached from the hull, K _Q the static stiffness of the joint and K its complex dynamic stiffness (K = K '+ jK "), we have:

„. , G ". K"

35t _δ = ci- ^ r G being the complex shear modulus (G = G * ⁺ jG).

The stresses on the material of the joint are ^, for a hydrostatic pressure to reach equal to nP ..:

K. K_

OR - f _Q _ is the resonance frequency before the seals are put in place.

So we get:

^K ~> 3 QK „tgδ" ^* * '

This last condition makes it possible to obtain a yield greater than 75%.

Various materials make it possible to manufacture such a seal. A typical characteristic for selecting these materials is that they have a glass transition at room temperature in the frequency range considered. By way of example, a polyurethane can be used as material, of which the stiffness modulus G expressed in N / m ² and the loss factor tgδ as a function of the frequency in Hz has shown in FIG. 2.

It is noted that the transition is obtained for a frequency substantially equal to 10 ~ ² Hz, that is to say for stresses on the material moving very slowly (period 100 seconds typically corresponding to the progressive crushing of the shell of the flexor when it is immersed more and more deeply). The value G _Q of the module at this transition is then substantially equal to 4.10 N / m.

As soon as we reach a frequency of 1000 Hz, much lower than the frequencies used in the flextensor, the module reaches 1, 5.10 ⁸ N / m ² and tg δ is 5.10 " ^2. The dynamics of stiffness is then equal to 37 5 for this material, which makes it possible to obtain completely satisfactory results.

The viscoelastic material can be placed in many other places and there is shown in FIG. 3 a second embodiment in which a seal 304 is inserted between the shell 301 and the motor 302.

This motor 302 comprises a stack of ceramics subjected to a prestressing using a rod 311 which crosses the stack right through. Clamping nuts 312 are screwed to the ends of the rod to compress the ceramics via a metal support piece

313 and an insulating washer 314.

The viscoelastic seal 304 is formed by two plates inserted on either side between the shell and the part 313. In this configuration, this seal works in bending while in the previous embodiment it worked in compression, but the result is the same .

Depending on the case, the other end of the flexuring transducer of FIG. 3 may be identical to the end shown in this figure, or else the motor may be directly fixed to the hull. The realization comprising only one joint on one side is easier to manufacture but this joint is subjected to larger deformations, which are not always desirable. To fix the ideas and clearly show the orders of magnitude of the means of carrying out the invention, we will consider a class 4 flextensor transducer whose depth is equal to 10 cm long and whose attachment is in accordance with Figure 3 to both ends of this engine. The shell therefore has 4 flat seals 10 cm long (2 on each side). Typical characteristics of such a transducer are for example:

- P = 30 bars - fr = 3 kHz -K = 10 ⁹ N / m

1 m bars), we get

where is the static modulus, equal to the material described above at 4 4..1100 ⁶ 6 NN // mm ² 2 ,, SS eett ee being respectively the total surface and the thickness of the joints. We obtain for the surface of a joint (S / 4) a value

2 of 25 cm or a height (according to OX) equal to 2.5 cm. If the shell thickness is for example 15 mm, the transducer will be manufactured by thickening this shell at the connection with the motor. The dynamic stiffness is then worth

K = K _Q. ^ = 3, 1.10 ⁹ , so that K = 3 K _m .

The new resonance frequency obtained is therefore close to

2.5 kHz and we are therefore in the usable range seen above. For the performance condition, i.e.

K / tgδ> 3 Q, we have K / tgδ = 6,2.10 while 3 QK is equal to ^m 1, 26.1010. The yield is therefore clearly greater than 75%.

The invention also extends to other types of flexors, such as those of class 2 or 5.

In this case, as shown in FIG. 4, the viscoelastic filter 404 has the form of a ring placed between the motor 402, itself in the form of a ring, and the shell 401 which is in the form of two domes assembled by their circumferences.

Claims

1. Flextensor acoustic transducer for deep immersion, comprising a hollow shell (101) of oblong section and an electroacoustic motor (102) intended to excite this shell along the major axis of this section, characterized in that it further comprises means viscoelastics (104) allowing the forces exerted by the shell on the engine to be absorbed without exhibiting appreciable mechanical resistance under the effect of deformations originating from immersion, and exhibiting significant stiffness at the operating frequencies of the engine to communicate to the hull the vibrations of this engine with good efficiency.

2. Transducer according to claim 1, characterized in that the efficiency is greater than 75%.

3. Transducer according to any one of claims 1 and 2, characterized in that the material forming the viscoelastic means (104) has a glass transition at room temperature at a frequency lower than the operating frequency of the transducer.

4. Transducer according to claim 3, characterized in that the viscoelastic material (104) is a polyurethane having a glass transition at room temperature at

-2 a frequency substantially equal to 10 Hz.

5. Transducer according to any one of claims 1 to 4, characterized in that this transducer is of type 4 according to the classification of ROYSTER and that it comprises a motor (104) elongated along the major axis of the hull (101) ; this motor being cut into two parts substantially in the middle and the viscoelastic means (104) being formed of a seal connecting these two parts.

6. Transducer according to any one of claims 1 to 4, characterized in that it is of type 4 according to the classification of ROYSTER and that it comprises a motor (102) elongated along the long axis of the hull (101) and fixed to the hull by its two ends; the viscoelastic means (304) being formed of at least one seal interposed between the hull (301) and the fixing means (313) of at least one of the ends of the engine to the hull.

7. A transducer according to claim 6, characterized in that the viscoelastic means (304) are located at the two ends of the motor (302).

8. Transducer according to any one of claims 1 to 4, characterized in that the transducer is of type 2 or 5 in the classification of ROYSTER and that its section along a plane perpendicular to the plane of oblong section is circular; the motor being circular and the viscoelastic means (404) forming a ring located between the motor (402) and the hull (401).