US 3421137 A
Abstract available in
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Description (OCR text may contain errors)
Jan. 7, 1969 H. L, RATHBUN, JR 3,421,137
ECHO REPEATER AND/OR TARGET SIMULATOR Filed Feb. 2s. 1966 Sheet str2 fi roe/ve Ys Ham/sy L. /P/m/u/v, Je. v
H. RATHBUN, JR
ECHO REPEATER AND/OR TARGET SIMULATOR Filed Feb. 234 196e Jan. 7, 19.69
Sheet INVENTOR. /M/e uw L, Ran/ um, Je.
United States Patent O Claims ABSTRACT OF THE DISCLOSURE An underwater acoustic repeater array wherein three equispaced transducers are supported concentrically between the apexes of a pair of truncated equilateral metallie pyramids. The outer surfaces of the pyramids are coated with an acoustic reflecting material and the active faces of the transducers are directed toward the coated faces. A housing having an amplifier therein connected :between two such arrays and including electrical means connecting in parallel one set of the transducers to the amplifier input and the output thereof to the other three transducers. The acoustic signal received #by one set of transducers will be amplified and re-radiated by the other transducer set at a higher level with a minimum of coupling therebetween.
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.
This invention relates to acoustic arrays and more particularly to an improved acoustic echo repeater for underwater communication and retransmission omnidirectionally in a horizontal plane.
In general a typical underwater acoustic repeater array consists of a hydrophone array for receiving and a projector array for transmission. The term array for purposes of this specification is defined as one or more transducers arranged in a particular geometry. Inherent in all repeaters is the fact that a certain proportion or level of signal is fed back from the projector to the hydrophone. A basic prerequisite of any such repeater is that for a given acoustic gain the level of the signal fed back from the projector to the hydrophone through the acoustic medium and the mounting and support structure must be less than that of the original signal received by the hydrophone.
Solutions to the problem of feedback in acoustic repeaters have involved the yuse of rather large transducers in terms of wavelength and large baies between the hydrophone and projector. Further, the separation therebetween is made large so as to produce a bulky, weighty and unwieldy structure. The separation distance is directly limited by the Fresnel diffraction around the array baille. Typically, for presently available equipment operating at approximately 12 kc. (}\(wavelength)=5 inches) with a 25 db acoustic gain, the transducers are separated a distance of l2 feet (28.23A) and two circular aluminum air iilled rubber batiies (diameter (7.2 \)=3 feet) are disposed between the projector and hydrophone.
Where the above dimensions are impractical or Where the wavelength is long, the only other solution has been to hold the receiving hydrophone inoperative during transmission by the projector. Another solution involves the use of transponders where a fixed, permanent signal level is projected independent of the received signal characteristics. The drawback here is quite obvious since the projected signal is independent of the received signal, whereas the repeater should provide only a fixed gain or level in reference to the received signal.
In view of the foregoing, it is an object of this invention to provide an inexpensive, simple and reliable improved acoustic echo repeater having a xed gain and of a minimum size and weight.
Another object is to provide azimuth oriented omnidirection acoustic repeater or target simulator having exteremely low coupling 'between the hydrophone and projector elements.
A further object is to provide an acoustic repeater array wherein the separation between the active elements is minimized and the structures thereof are of simple geometric shapes and proportions.
Other objects and advantages will appear from the following description of an example of the invention, and the novel features will be particularly pointed out in the appended claims.
In the accompanying drawings:
FIG. 1 shows a transducer suitable for use in the embodiment of the invention;
FIG. 2 shows a perspective view of the bilaminar electrostrictive elements with their support structure and electrical connection;
FIG. 3 illustrates in -plan an embodiment made in accordance with the principle of the invention;
FIG. 4 is a view taken approximately along 4-4 of FIG. 3;
FIG. 5 is a side view taken approximately along line 5-5 of FIG. 4, and
FIG. 6 is a graph illustrating the relative feedback between elements for various separations therebetween.
In the transducer of FIG. 1, bilaminar electrostrictive elements 10 manifest dishing distortion in response to a driving alternating potential. Methods and materials for fabricating the laminae 11 and 12 for electrostrictive elements 10 are well known in the art. For example, U.S. Patent #2,486,560 describes electrostrictive transducers, particularly of barium titanate and methods of making the same, which description may be utilized for fabricating the transducer elements. Also, a paper published by Sperry Gyroscope Co. of Great Neck, N.Y., and presented at the 14th Annual National Electronics Conference, Chicago, Ill., Oct. 13, 1958, entitled The Electro-Acoustic Transducer and Its Application to Sonar Systems, by George Rand and John Devine, includes `further information on electrostrictive transducers and methods of making them. The laminae 11 and 12 are conventionally fabricated of a material that is or that can be rendered electrostrictive and their opposed faces are coated with separate electrode films and are polarized transverse to the electrode tilms as is well known in the art, and the polarization may -be carried out as described in the above-mentioned references. Paired laminae 11 and 12 are bonded face-to-face with an adhesive. The particular adhesive is not critical but the following physical properties in the adhesive to some degree determine operational characteristics 0f the transducer. The more firmly that the adhesive bonds to the facing electrode surfaces of the electrostrictive element 10 and the tougher the adhesive, the greater the power handling capacity of the resultant transducer without rupturing at the adhesive bond. The greater the flexibility of the adhesive bond and the thinner the adhesive bond, the greater the efiiciency of the transducer because less power is lost in driving the adhesive bond material. One example of a commercial adhesive that has satisfactory properties for the purpose described is Eastman 910 cement. There is considerable literature on adhesives from which information on other satisfactory adhesives may be obtained. For example, a book entitled Adhesives by Felix Braude, published by Chemical Publishing Company, and a periodical entitled Adhesives and Resins, published in Great Britain at 329 Grays Inn Road, London, W.C. 1, provides information on adhesives and their properties from which information on other adhesives satisfactory for the purpose may be selected.
The arrangement shown in FIG. 1 wherein one terminal is connected to both facing electrodes is simple to assemble. One bonding procedure that has proved satisfactory is to select a matched pair of electrodesurfaced and polarized laminae, apply adhesive to one face of each of the matched laminae, and with a thin ilat strip of copper foil disposed between the adhesive coated faces of the laminae, press the laminae firmly together. By applying pressure not only is a good adhesive .bond obtained, but the copper foil 15 is forced into electrical contact with the facing film electrodes of the two laminae to a suflicient extent satisfactory for the purpose. lConductors are conventionally soldered to the outside film electrodes and are connected in common to provide one electrical terminal of the transducer, and the foil 15 or a conductor connected to the foil 15 provides the other electrical terminal of the transducer.
The two transducer elements are attached together in line. A spacer 16 that gives satisfactory results is a ring formed with slots on its inner and outer surfaces at equiangularly spaced intervals and consecutive angularly spaced slots occurring alternately on the inner and outer surfaces. The ring 16 may be formed from a stiff resilient material, e.g., brass tube stock, eg., SAE 74. To forni the ring, a length of the tubing stock is mounted in a band saw with an indexing means and its outer surface is formed with slots. To form the inside slots, the cutting saw band is severed, threaded through the tubing, and its ends welded together and with the aid of indexing means the inner surface of the tubing is formed with slots. The wall thickness of the tubing is about s inch and slot depth is on the order of 176,2 inch. Successive slots may be on the order of ten degrees apart, the slot spacing is related to the circumferential length of the ring. For a 11/2 inch ring diameter, ten degree slot spacing is suitable. After the tube stock is slotted, the tubing is sawed into thin rings. The ring thickness may be on the order of 1/16 inch. Two transducer elements 10 are so arranged on opposite sides of the ring for flexure in opposite directions when an alternating potential is applied and the ring is bonded to the marginal areas of the inner faces of both transducer elements 10; an air space is sealed in between the transducer elements 10. The spacing ring is radially compliant to dynamic forces but is radially stiff to static forces, and it has high dynamic and static stiffness in the axial direction. The radially compliant support afforded by the slotted ring endows the double bilaminar disk with excellent electromechanical transducing properties. The strain and the forces developed in one disk correspond to that in the other disk and with a radially compliant ring therebetween, efficiency is high. If the metal ring were not radially compliant, i.e., if the ring were not slotted, it would prevent radial motion of the disk edges when excited by an applied alternating potential and would thereby inhibit or even prevent bending or dishing action. Because the ring has high dynamic stiffness in the axial direction, each disk has, at its bonded margin, a node of axial motion. If the ring material were very compliant axially, for example, if it were of rubber to provide good radial compliance, then the nodal circle of axial motion for each disk would move inwardly from the edge of the disk, and the portion of the disk outside of this circle would vibrate out of phase, resulting in poor radiation loading.
With the above arrangement there is obtained an electroacoustic transducer with high electromechanical coupling coeflicient and with high strength.
When an alternating potential is applied to the double bilaminar transducer to drive the elements 10 in the fundamental exural mode, each element 10 manifests a dish-like distortion. The electrical connections to the electrodes and the directions of polarization are such that distortion in the two bilaminar disks are degrees 0ut of phase. By driving two bilaminar elements back to back as edge supported disks, the transducer radiates from both outer faces. Substantially no energy is consumed by the included air space. This unit when placed in an acoustic medium such as water will couple most of the acoustic energy into the water with little loss in the included air cavity pressure release. This arrangement has the advantages of transducer elements anchored with heavy mounting fixtures without the disadvantages.
The mechanical resonance of the described transducer is determined by the physical dimensions of the disks. The first resonant frequency in air of the mode of an edge supported diaphragm is dened as follows:
Berra-a f=resonant frequency, c.p.s. t--thickness R=radius cp=velocity of sound in the material E=modulus of elasticity p=density of the material r=Poissons ratio A transducer in accordance with this invention designed for resonance at about 9 kc. is about 11/2 inches outside diameter and slightly more than i716 inch thick overall (inside diameter 15/16 inches). Because of small size and light weight, these transducers can be assembled in arrays or mounted in appropriate reflective baies as hereinafter described to produce desired beam width and power handling capabilities. The transducing material is used to best advantage; substantially all of it is effective. Because the two opposed faces of the transducer radiate acoustic energy when alternating potential is applied, the radiation loading that is obtained is greater than for a disk radiating from only one surface resulting in higher electroacoustic efficiency. A transducer provides a pattern which is approximately omnidirectional and which approaches that of a spherical sound source.
The bilaminar piezoelectric ceramic transducers 20, 18, 19 described hereinbefore are arranged coplanar and at the vertices of an equilateral triangle as shown in FIG. 2. The transducers are supported by a centrally situated support member 21 via the outer brass rings or tubing 22 although other support means and/or structure could be equally well employed. The active transducer elements are cast in an acoustically transparent resin (generally indicated at 23) and electrically joined in parallel via cables 24. The support member 21 which may be a heavy walled brass tube also supports a pair of oppositely extending truncated triangular equilateral pyramids 25 as shown in FIG. 3. These pyramids serve as reflectors or baliles 26 and are generally of some suitable metal, such as aluminum coated with a reflective rubber material 27. The pyramid walls 25 form 90 corner reflectors with the transducers disposed in the bisecting angle thereof. The layer of conventional compressional wave reflective material 27 is bonded to the inner surfaces of the walls 26 which are the outer pyramid surfaces. Two properties of the material selected should be greatly different from the corresponding properties of the medium in which the array is used, namely, density and the velocity of the compressional wave energy therethrough. Isoper, a product of B. F. Goodrich Industrial Product Company, Akron, Ohio, which is a relatively stiff rubber-like material, is one example of a suitable commercial material for layer 26 where the array is intended for use in water.
The two pyramid structures are joined at their apexes by the support member and the transducers centrally disposed thereof with their active faces directed toward the coated pyramid surfaces so as to form one-half of a repeater or echo array. An identical unit is joined in axial alignment therewith by a shaft 28 and an electronic section 29'. The transducer elements are connected to the electronic section via cable 29 as shown in FIG. 2. The dimensions shown in FIGS. 3, 4, and 5 are all in terms of wavelengths and in this regard for use at approximately 9.0 kc. one-half inch thick Isoper cemented onto 3%6" thick sheet aluminum was found to provide satisfactory results. A center support shaft 30 which can, as illustrated, extend outwardly from the array and support the array in the water.
Measurements were made to ascertain the effect of separation(s) between the pyramids or baffles on the feedback loss. The results were plotted and are graphically illustrated in FIG. 6. This shows that a slight increase in attenuation is achieved by separating the baflies but that overall a minimum of 30 db obtained even yfor small baie separations. In general the array is employed as an echo repeater and/or target simulator so that one Set of transducers are used as hydrophones while the other set acts as the projectors. The system is omnidirectional in the plane of the transducers and since the units are identical they may be interchanged. With a self-contained electronic system 29 the signals received by one set of transducers (hydrophones) are `amplified and applied to the projector set for retransmission at higher power without appreciable feedback.
summarizing the overall system, a clear prime requisite of a repeater array is that for a given acoustic gain the level of the signal fed back from the projector to the hydrophone, through the acoustic medium and other structure must be less than that of the original signal received by the hydrophone. The array essentially comprises a pair of identical units, the total, made up of four truncated equilateral pyramids having base lengths approximately 2\/ 3 wavelengths, two transducer units and a watertight housing containing sutiicient electronics, etc. The truncated pyramids are lined or coated on the outside with an acoustic reflective material. Bilaminar piezoelectric ceramic discs, edge-supported back-to-back, operating in the fundamental flexural mode are employed since they are both small and eflicient both as projectors and hydrophones. The transducer units consist of three coplaner discs located at the vertices of an equilateral triangle whose side length should be slightly less than one-half a wavelength. The center mount of the transducer unit is at the center of the triangle and is keyed so as to perform two functions, positioning the unit elements (discs) in the baflie and accurately positioning the two bafe elements which when assembled so that one pyramid is inverted with respect to the other, form three 90 corner retiectors with apertures of approximately two wavelengths (27x). The discs are therefore positioned in the bisecting plane of the 90 corner reflector. It should -be noted that since the 4distance between a disc and its opposite baffle edge is not constant (varying from V5 to \/5), the length of the ray paths also vary and thus the effect of any Fresnel diffraction is reduced.
The units are mounted one above the other or in axial alignment with the electronic housing therebetween. If the array is used with the axis in the vertical then each unit or the array as a whole, is omn1d1rect1onal1n azlmuth.
With this array structure the system provides:
A nearly omnidirectional pattern in the horizontal plane,
A pattern 30-50 wide between the half-power points in the vertical plane,
A feedback level between the projector and hydrophone units at least 30 db down,
An operating source level of at least db/ /microbar at one yard,
Minimum component and array size dependent on the wavelength.
It will be understood that various changes in the details, materials, and arrangements of parts (and steps) which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.
1. An acoustic repeater array having first and second units, wherein each of said units comprises:
a pair of truncated equilateral metallic pyramids having the outer surfaces thereof coated with an acoustic reliecting material,
three acoustic transducers,
ring support means holding said transducers at the vertices of an equilateral triangle concentric therewith,
said support means disposed between said pyramids and supporting said pyramids with their apexes adjacent one another and in axial alignment whereby said transducers are positioned symmetrically therebetween and having their active faces directed toward said coated outer surfaces,
amplifier means confined in said housing and having input and output terminals,
electrical means connecting in parallel said three transducers of said first unit to said input terminal of said amplifier means and connecting the output terminal thereof to the three transducers of said second unit,
whereby the acoustic signals received by said first unit will be amplified and re-radiated at a higher level by said second units.
2. The acoustic array according to claim 1 further including:
structural means for supporting said housing between said units and maintaining said units in axial alignment.
3. The acoustic array according to claim 2 wherein said transducers are bilaminar piezoelectric ceramics.
`4. The acoustic array according to claim 3- wherein the height of said pyramids is approximately equal to the wevelength of said signal and the distance between the bases of said pyramids of a unit is 2h.
References Cited UNITED STATES PATENTS 3,031,644 4/1962 Hisserich et al. 340-3 3,054,084 9/1962` Parssinen et al. 340-8 3,117,318 1/1964 Jones 343-18 3,243,768 3/1966 Roshon et al 340-10 RICHARD A. FARLEY, Primary Examiner.
U.S. Cl. X.R. 340-5, 8