Flextensional hydrophone

• US 4,951,271 A
• Filed: 04/17/1989
• Issued: 08/21/1990
• Est. Priority Date: 04/17/1989
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Danielson, Donald A., Garrett, Steven L.
• Primary Examiner(s): STEINBERGER, BRIAN
• Application Number:
  US07/341,594
• Time in Patent Office: 491 Days
• Field of Search: 181/122, 367/140, 367/141, 367/149, 367/157, 367/159, 367/162, 367/165, 367/166, 367/167, 367/171, 367/172, 367/178, 367/180, 367/188, 73/655-657, 350/96.29, 350/96.30, 250/250, 356/345, 356/360
• US Class Current: 367/141
• CPC Class Codes: G01H 9/004 using fibre optic sensors light guides per se G02B6/00, acousto-optical devices specially adapted for gating or modulating in optical wave guides G02F1/125
• CPC Class Codes: G10K 9/121 Flextensional transducers

Abstract

An omnidirectional hydrophone having an elastic shell which is spheroidal so that the circumference of the shell about different axes changes differentially when the shell is subjected to pressure variations. The differences in circumference are advantageously measured by an optical fiber interferometer having one leg wound about the equatorial circumference of the shell and another leg wound about its meridional circumference. The shell may be an oblate spheroid having the ratio of its major axis to is minor axis greater than about (2-ν)1/4, where ν is Poisson'"'"'s ratio of the shell material, so that the shell narrows along one axis and widens along the other when the shell is subjected to a pressure change.

Claims

1. A sensor for pressure variations in a fluid, such variations having a predetermined frequency range and the sensor comprising:

an elastic and ellipsoidal shell having an interior surface and immersible in the fluid so that such variations induce in the shell deformations corresponding to the variations, and

transducer means for providing a signal corresponding to said deformations, said transducer means including

a mass disposed centrally within the shell,

a resilient cylinder extending axially between said mass and said surface and having with said mass a natural frequency of oscillation substantially lower than said predetermined frequency range so that such variations thereat cause corresponding changes in the length and in the circumference of the cylinder, and

an optical fiber having a plurality of turns wrapped in tension about the circumference of the cylinder so that the fiber has interferometrically detectable changes in length corresponding to said changes in said circumference.

2. A sensor for pressure variations in a fluid, the sensor comprising

an elastic and ellipsoidal shell immersible in the fluid so that such variations induce in the shell deformations corresponding to the variations, and

transducer means for providing a signal corresponding to said deformations, said transducer means including an optical fiber wrapped about the shell and fixedly connected thereto so that said fiber has interferometrically detectable changes in length corresponding to said deformations of the shell.

3. The sensor of claim 2 wherein said shell is substantially a spheroid having a pair of orthogonaly related axes of different lengths and wherein said fiber is one fiber of a pair thereof wrapped circumferentially about said spheroid along each of said axes.

4. The sensor of claim 3 wherein said fibers are a pair of legs of an interferometer.

5. A sensor for fluid pressure vibrations, the sensor comprising:

a generally spheroidal and elastic shell having a pair of orthogonally related axes of unequal length and having a pair of circumferential directions around the shell corresponding to said axes so that, when the shell is subjected to such vibrations, said vibrations induce in said directions differential variations in the circumference of the shell; and

transducer means extending circumferentially about the shell for detecting said variations, wherein said transducer means comprises a pair of strain detecting elements, one element of said pair being wrapped circumferentially about the shell in one of said directions, the other element wrapped circumferentially about said shell in the other of said directions.

6. The sensor of claim 5 wherein, in said transducer means, one of said strain detecting elements is a reference element.

7. The sensor of claim 5 wherein said shell is oblate and the relative lengths of said axes are such that, for increasing and for decreasing fluid pressure to which the shell is subjected due to said vibrations, the circumference of the shell about one of said axes increases and the circumference of the shell about the other of said axes decreases.

8. A sensor for fluid pressure vibrations, the sensor comprising:

a generally spheroidal and elastic shell having a pair of orthogonally related axes of unequal length and having a pair of circumferential directions around the shell corresponding to said axes so that, when the shell is subjected to such vibrations, said vibrations induce in said directions differential variations in the circumference of the shell; and

transducer means extending circumferentially about the shell for detecting said variations and having a pair of strain detecting elements, one element of said pair being wrapped circumferentially about the shell in each of said directions and said pair of elements being a pair of optical fibers of substantially equal length connected as a pair of legs of interferometer means for detecting said vibrations by detection of differences in the lengths of said fibers.

9. The sensor of claim 8 wherein said pair of optical fibers are wrapped about said shell in tension and externally thereof.

10. A sensor for fluid pressure vibrations, the sensor comprising:

an elastic shell of oblate spheroidal configuration, the shell having a pair of orthogonally related axes of unequal length and having a pair of circumferential directions around the shell corresponding to said axes so that, when the shell is subjected to such vibrations, said vibrations induce in said directions differential variations in the circumference of the shell, the relative lengths of said axes being such that, for increasing and for decreasing fluid pressure to which the shell is subjected due to said vibrations, the circumference of the shell about one of said axes increases and the circumference of the shell about the other of said axes decreases; and

transducer means extending circumferentially about the shell for detecting said variations, said transducer means including a pair of optical fibers, which are of substantially equal length and are wrapped circumferentially about the shell in each of said directions so that said pair of fibers are connectable as pair of legs of said interferometer means for detecting said vibrations by detection of said differences in the lengths of said fibers.

11. A sensor for fluid pressure vibrations, the sensor comprising:

an elastic shell of oblate spheroidal configuration, the shell having a pair of orthogonally related axes of unequal length and having a pair of circumferential directions around the shell corresponding to said axes so that, when the shell is subjected to such vibrations, said vibrations induce in said directions differential variations in the circumference of the shell, the relative lengths of said axes being such that, for increasing and for decreasing fluid pressure to which the shell is subjected due to said vibrations, the circumference of the shell about one of said axes increases and the circumference of the shell about the other of said axes decreases,

the shell being constructed of a predetermined material,

the thickness of the shell being such that, at any location thereon and in a direction circumferentially of the shell, the shell is subjected to generally the same stress radially across the shell, and

the relative lengths of said pair of axis being such that

space="preserve" listing-type="equation">A/B>

(2-ν

).sup.1/2

where "A" is the length of the longer axis, "B" is the length of the shorter axis, and "ν

" is Poisson'"'"'s ratio of said material; and

transducer means extending circumferentially about the shell for detecting said variations.

12. A sensor for a pressure variation in a fluid, the sensor comprising:

a spheroidal and elastic shell immersible in the fluid, the shell having a pair of circumferential directions therearound corresponding to a pair of orthogonally related axes of the shell, and the shell having a construction such that, when the shell is subjected to such a pressure variation, there is a differential variation in the circumference of the shell in each of said directions; and

transducer means having a pair of elements for detecting said differential variation, an element of said pair extending circumferentially about the shell in one of said directions so that one of said elements is a reference element wrapped circumferentially about said shell in the other of said directions.

13. The sensor of claim 12 wherein said construction comprises

said shell being an oblate ellipsoid of substantially uniform thickness;

said shell being formed of material having isotropic elastic properties; and

one of said axes being the shortest axis of the ellipsoid.

14. The sensor of claim 12 wherein said construction comprises the shell having greater thickness in a circumferential region corresponding to one such direction than in a circumferential region corresponding to the other such direction.

15. The sensor of claim 12 wherein said construction comprises the modulus of elasticity of the shell being greater in a circumferential region corresponding to one such direction than in a circumferential region corresponding to the other such direction.

16. A sensor for a pressure variation in a fluid, the sensor comprising:

a spheroidal and elastic shell immersible in the fluid, the shell

being of substantially uniform thickness,

being formed of material having isotropic elastic properties, and

being an oblate ellipsoid having a pair of circumferential directions therearound corresponding to a pair of orthogonally related axes of the ellipsoid, one of said axes being the shortest axis of the ellipsoid,

so that, when the shell is subjected to such a pressure variation, there is a differential variation in the circumference of the shell in each of said directions, and

the shell being a thin shell and the relative length of said pair of axes being substantially in accordance with

space="preserve" listing-type="equation">A/B>

(2-ν

).sup.1/2

where B is the length of said shortest axis, A is the length of an axis orthogonal to said shortest axis, and ν

is Poisson'"'"'s ratio for said material, so that when the shell is subjected to an increasing such variation the circumference of the shell in such a direction corresponding to said shortest axis decreases and the circumference of the shell corresponding to such axis orthogonal to said shortest axis increases and so that when the shell is subjected to a decreasing such variation the circumference of the shell in such a direction corresponding to said shortest axis increases and the circumference of the shell corresponding to such axis orthogonal to said shortest axis decreases; and

transducer means having a pair of elements for detecting said differential variation, an element of said pair extending circumferentially about the shell in each of said directions so that one of said elements is a reference for the other.

17. A hydrophone for detecting pressure vibrations in a fluid, the hydrophone comprising:

an elastic spheroidal shell, which is of oblate configuration so that the shell has a shortest axis and has a longer axis orthogonally related thereto, which is of substantially uniform thickness, and which is constructed of a material having substantially uniform elastic properties, the shell being immersible in said fluid so that the circumference of the shell about each of said axes changes in length due to said vibrations and so that, due to said oblate configuration, the circumference of the shell about the shortest axis has a different change in length during each vibration than the circumference about said longer axis;

a first optical fiber wrapped in tension about the shell circumferentially of said shortest axis so that the length of said first fiber varies during said vibrations in correspondence with such change in length of the shell about the shortest axis;

a second optical fiber wrapped in tension about the shell circumferentially of said longer axis so that the length of said first fiber varies during said vibrations in correspondence with such change in length of the shell about the longer axis; and

interferometer means connected to said first optical fiber and to said second optical fiber for detecting relative changes in lengths of said fibers due to changes in the circumference of the shell caused by said vibrations and for generating a signal corresponding to said pressure vibrations.

18. The hydrophone of claim 17 wherein said first and said second optical fibers are each disposed in a plurality of turns wrapped about the exterior of the shell.

19. The hydrophone of claim 17 wherein the ratio of the width of the shell along said longer axis to the width of the shell along said shortest axis is greater than the square root of a number which is the integer 2 minus Poisson'"'"'s ratio for said material of which the shell is constructed, whereby such changes in length of the circumference of the shell about said shortest axis are of opposite sign to such changes in length of the circumference of the shell about said longer axis.

20. The hydrophone of claim 17 further comprising:

an elastomeric envelope surrounding said shell;

an orifice defined by said shell and providing restricted fluid communication between the exterior and interior thereof; and

a quantity of liquid filling said envelope and said shell;

whereby said liquid equalizes static pressure between the exterior and exterior of said shell through said orifice.