US3800275A

US3800275A - Acoustic image conversion tube

Info

Publication number: US3800275A
Application number: US00053850A
Authority: US
Inventors: J Hagemann
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1960-09-02
Filing date: 1960-09-02
Publication date: 1974-03-26
Anticipated expiration: 1991-03-26
Also published as: US3800276A

Abstract

3. An underwater acoustic device comprising in combination an annular support member, a vacuum backed circular piezoelectric plate mounted on said support member, a substantially mechanically rigid wall element mounted on said support member and forming with the front face of said plate an enclosure, and an acoustic energy propagating liquid filling the free space within said enclosure, said liquid at all times being under a pressure no greater than atmospheric but not less than the vapor pressure of said liquid at the temperature of the water in which the device is to be utilized.

Description

agemann Mar. 26, 11974 [54] ACOUSTIC KMAGE CONVERSION TUBE 2,434,666 1/1948 Mason 340/8 PC [75] Inventor: .llulius Hagemann, Panama City, Fla. [73] Assignee: The United States 01 America as l;

represented by the Secretary of the Navy, Washington, DC. 2983900 5/1961 [22] Filed: Sept. 2, 1961) Primary Examiner-Richard A. Farley [2]] AppL NO: 53 850 Attorney, Agent, or FirmLouis A. Miller; Rolla N.

Carter [52] US. Cl. 340/5 MP, 315/11 340/10 EXEMPLARY CLAIM [51] lint. C1. 11104111 11/00, H01 31/495 58 mm 111 Search 343/17; 178/6.8, 5.6, 5.8, mpmmg 178/6 SP 7.5 E 7.82 7.8' 313/148 1 bination an annular support member, a vacuum 7 backed Circular piezoelectric plate mounted on said 146 3 support member, a substantially mechanically rigid 315755 i 1 1 b wall element mounted on said support member and /1 340/5 8 d 8 6 forming with the front face of said plate an enclosure, and an acoustic energy propagating liquid fining the [56] References Cited free space within said enclosure, said liquid at all times being under a pressure no greater than atmo- UNITED sTTEs PATENTS spheric but not less than the vapor pressure of said liquid at the temperature of the water in which the de- O O O 3,414,827 1/1947 Mason .1 340/10 me Is to be utlhzed' 2,416,314 2/1947 Harrison 340/8 MM 3 Claims, 2 Drawing Figures ACOUSTIC IMAGE CONVERSION TUBE The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

The present invention relates to tubes of the cathode ray type for converting an acoustic image into a video signal and more particularly to such tubes constructed to be robust enough to withstand pressures of several atmospheres without sacrificing desired operating characteristics or performance.

A slab of piezoelectric material such as quartz or polarized barium titanate having lateral dimensions large compared to the wavelength of the acoustic energy present in an acoustic image applied to an electroded side of the slab through a coupling medium will produce localized electrical effects such that an electrical image will appear on the opposite bare side as an alternating potential pattern having the frequency of the acoustic excitation. If this piezoelectric slab, which will hereinafter be referred to as the conversion plate, is uti lized as the front face of a cathode ray tube with its bare face inward; electron beam scanning of this bare face will detect the point-by-point variations in electric charge distribution thereon corresponding to the acoustic energy distribution on its outer face and generate a time varying signal, i.e., a video signal, which by a system common to television practice can be reproduced as a visual image on the screen of a cathode ray oscilloscope to display point-by-point the intensity of the acoustic energy coming from corresponding points in the object field imaged on the front face of the conversion plate. Such a system can not compete with television in any medium transparent to light but in mediums opaque or substantially opaque to light such as turbid water such a system is useful and at the present time is the only one known which will present to remote viewers a picture of an object submerged in turbid water.

The optimum thickness for the conversion plate is a half wavelength of the acoustic energy utilized to form the acoustic image and for both quartz and barium titanate this corresponds to a thickness of approximately 0.1 inches at a frequency of 1.0 megacycle per second (mops). if a half wave conversion plate alone constitutes the end of the image tube, the maximum diameter imposed at a pressure of one atmosphere for a frequency of 1.0 mcps is about 6.0 inches for quartz and about 4.4 inches for barium titanate. Waters to a depth of 180 feet are usually assumed to be mineable and if the acoustic image tube is to be employed for mine hunting at such depths it would be subjected to pressures exceeding six atmospheres and obviously the diameter of an image tube employing only an unreinforced conversion plate is severely limited. The apparent size advantage afforded by quartz is mainly illusory because of its unavailability in sizes much greater than 3 inches except as museum pieces. Strength can be gained by dimensioning the conversion plate an odd multiple of one-half wavelength thick but in the present state of the art this introduces shortcomings which are unacceptable. it has been proposed to solve this strength problem by bonding the conversion plate to a metal front plate which forms an integral part of the vacuum tube envelope. This sandwich" type construction is difficult to bond on the molecular-scale necessary and the metal plate constitutes a necessary evil in the acoustic path.

In accordance with the present invention the image tube is constructed with a pressure relief barrier between the water medium and the conversion plate which effectively protects the conversion plate without adversely affecting acoustic performance; and, indeed, may well constitute a welcome supplement to the acoustic imaging system.

An object of the invention is to provide an acoustic image conversion tube with a window structure which will withstand the pressure differential between the substantial vacuum of the tube and the hydrostatic pressure at substantial depths under water.

Another object of the invention is to provide a pressure relief barrier between the conversion plate of an acoustic image tube and the surrounding medium.

The features of this invention which are believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its structure and operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a diagrammatic representation of an acoustic-to-visual system in which the invention is utilized; and

FIG. 2 is a fragmentary view in section ofa relief barrier mounted over the conversion plate of an acoustic image tube.

Referring now to the drawing, FIG. 1 represents an apparatus for classifying and identifying objects under water by forming from high frequency acoustic energy reflected by such object an acoustic image on a conversion plate of piezoelectric material, scanning the resulting electrical image on the opposite side of the conversion plate to develop a corresponding video signal and synthesizing a visual image from this video signal. This apparatus as shown includes an acoustic lens 10 of suitable material such as methyl methacrylate plastic positioned to image an object field on the conversion plate 11 of suitable material such as quartz or barium titanate, forming the front end of a cathode ray tube 12 comprised of a generally cylindrical envelope 13 having an electron gun l4 and a screen mesh electrode 15 positioned to collect secondary electrons resulting from the scanning of the rear surface of the conversion plate ill. The collector electrode 15 is connected through an external terminal 16 to a preamplifier 17 which preferably is included in the underwater assembly with the tube 12. The output of the preamplifier I7 is fed through a video amplifier 18 to the intensity grid 19 of a cathode ray tube 21 having on its face a fluorescent screen 22. Each of the

tubes

12 and 21 is provided with horizontal and vertical deflecting coils 23 and 24, respectively, which are connected to be energized in synchronism by suitable sweep circuits 25 in accordance with common practice in the television art. Also in accordance with common practice, the vertical return trace in the cathode ray tube 21 may be blanked through the video amplifier 18 under the control of a signal carried over a lead 26 from the vertical sweep generator in the sweep circuits 25.

Referring now to FIG. 2, wherein parts in common with FIG. I are given like reference characters, the conversion plate 1 1 is mounted on an axially compliant metal bellows 30 which is highly resistant to radial collapse. The bellows 30, which may be nickel, is fused to the open end of the glass envelope 13 of the camera tube I2. Adjacent this end of the tube 12, a circumferential flange 31 is formed, preferably integrally with the envelope 13, to serve as a support for a cup shaped member 32 having its concave surface facing the conversion plate 11 and preferably made of an impedance matching material such as molded Rho-C rubber. The central area of the cup 32 in axial alinement with the conversion plate 11 is shown as having a meniscus shape of power, i.e., its inner and outer spherical surfaces are concentric. As was indicated above, this central portion of the cup 32 may be other than 0 power to form an active part of the imaging system. It will be understood that the acoustic thickness of this central portion of the cup 32 is preferably an integral multiple of /2 wavelengths of the acoustic energy projected toward the object field being observed. The peripheral portion of the cup 32 is cylindrical and surrounded by a snug ring 33, preferably metal, for restraining the cup 32 against radial expansion when the convex surface of the cup 32 is subjected to hydrostatic pressure and inasmuch as the cup 32 is made of substantially incompressible material, it will be evident that only a very small deformation or axial displacement of the cup 32 can take place. The volume enclosed by the cup 32, the

conversion plate II, the bellows and the flange 3R is filled with a carefully degassed liquid 34, such as castor oil, having the proper impedance for transmitting the acoustic energy to the conversion plate 11. Thus any small displacement of the concave surface of the cup 32 will result merely in a slight compression of the bellows 30 without placing any additional stress on the conversion plate 111 other than the forces resulting from the compression of the bellows 30 which will be small because its compliance can be quite high and still avoid radial collapse. Also, it is preferred to have the liquid 34 at subatmospheric pressure so that even at the maximum depth during use the pressure differential on the plate RI never exceeds one atomsphere and preferably is considerably less. In the arrangement just described, the collector electrode 15 is shown as a wire mesh screen supported by the metal bellows 30 and connected to the external terminal 116 through a lead 35, the grounded electrode on the front surface of the plate 11 not being shown.

In manufacturing an image tube of the invention, when the cup member 32 and the liquid 34 are of materials which can withstand the outgassing of the tube 112, the pressure barrier assembly will be mounted on the tub 12 prior to its being evacuated and thus the bellows 30 will be in its neutral position at this time. Later, upon evacuation of the tube 12 to say 10' millimeters of mercury or less, the plate lll moves to reduce the pressure in the liquid 34 to equal the strength of the bellows 31) under compression, the pressure of the near vacuum in the tube 112 being ignored. It is thus evident that the conversion plate llll is at no time subjected to a pressure differential on its opposite faces equal to one atmosphere and in practice is made considerably less than one atmosphere by suitable selection of bellows strength but greater than the vapor pressure of the liquid 34 which is chosen to have a relatively low value.

This of course means that the area of the conversion plate 11 is no longer limited by the requirement that it withstand one atmosphere of pressure but rather has its maximum area determined by a considerably lower pressure differential, i.e., substantially the vapor pressure of the liquid employed.

When the techniques employed for fabricating and outgassing the cathode ray tube 12 would deleteriously affect both the cup member 32 and the liquid volume 34 comprising the pressure relief barrier of the invention, after the tube 12 has been outgassed these components can be assembled by the technique indicated in broken lines in FIG. 2. The glass flange 31 is originally formed with diametrically opposed passageways 36 and 37 (shown sealed off in FIG. 2) and connected respectively through

conduits

38 and 39 to a reservoir 40 of oil or other liquid under subatmospheric pressure and to a vacuum source 41. It will be evident that as the vacuum source 41 removes the air from the volume enclosed by the cup 32 and the conversion plate 11, the liquid from the reservoir 40 will flow into and eventually completely flood this volume with the liquid 34. When this volume is completely filled with the liquid 34, as may be indicated by a liquid trap 39, the

passageways

36 and 37 in the glass flange 31 are sealed off as shown in FIG. 2. By sealing the passageway 37 first, the liquid 34 is under the known pressure of the reservoir 40. This technique results in the liquid 34 being confined at less than atmospheric pressure which for the reasons pointed out above is desirable, it being remembered that the subatmospheric pressure chosen is preferably slightly greater than the vapor pressure of the liquid employed.

When the liquid 34 but not the cup member 32 would be deleteriously affected by the outgassing of the tube 12, the pressure barrier of the invention can be assembled so that the conversion plate 11 is never subjected to a pressure differential greater than a selected subatmospheric pressure by evacuating the volume to be filled with the liquid 34 simultaneously with the evacuation of the tube 12 and then after the tube 12 has been outgassed, the oil or other liquid from the reservoir 40 can be admitted to the volume 34 as above described.

While for the purpose of disclosing the invention to enable those skilled in the art to practice the same a preferred embodiment has been described in detail, it is to be understood that the invention is not limited to the precise structure shown and that many obvious modifications may be made therein without departing from the spirit of the invention, the scope of which is pointed out in the appended claims.

What is claimed is:

H. An underwater acoustic wave system of the type employing a cathode ray tube having for its front face a piezoelectric plate for converting an acoustic image formed thereon into a corresponding electrical charge pattern, characterized in that said tube has an axially compliant cylindrical extension on which said conversion plate is mounted, a substantially rigid front wall member of acoustically transparent material is secured to said tube to form with said conversion plate and said extension an enclosure, and a liquid fills the free space within said enclosure for propagating acoustic energy between said front wall member and said conversion plate.

2. Apparatus as defined in claim 1 wherein the liquid filling said enclosure is under subatmospheric pressure not less than the vapor pressure of the liquid at the temperature of the water in which the apparatus is to be utilized.

within said enclosure, said liquid at all times being under a pressure no greater than atmospheric but not less than the vapor pressure of said liquid at the temperature of the water in which the device is to be utilized.

Claims

1. An underwater acoustic wave system of the type employing a cathode ray tube having for its front face a piezoelectric plate for converting an acoustic image formed thereon into a corresponding electrical charge pattern, characterized in that said tube has an axially compliant cylindrical extension on which said conversion plate is mounted, a substantially rigid front wall member of acoustically transparent material is secured to said tube to form with said conversion plate and said extension an enclosure, and a liquid fills the free space within said enclosure for propagating acoustic energy between said front wall member and said conversion plate.