Variable frequency transducer

Aug. 6, 1957

Filed Aug. 24, 1954

F. J. FRY ET AL

VARIABLE FREQUENCY TRANSDUCER

2,802,196

Sheets-Sheet 1

<img file="US2802196A_D0001.tif" />

Aug. 6, 1957

F. J. FRY ETTAL

VARIABLE FREQUENCY TRANSDUCER

2,802,196

Filed Aug. 24, 1954

Sheets-Sheet 2

<img file="US2802196A_D0002.tif" />

Aug. 6, 1957

F. J. FRY ET AL

VARIABLE FREQUENCY TRANSDUCER

2,802,196

Filed Aug. 24, 1954

Sheets-Sheet 3

FIG. 4

<img file="US2802196A_D0003.tif" />

<img file="US2802196A_D0004.tif" />

<img file="US2802196A_D0005.tif" />

2,802,196

Patented Aug. 6, 1957

United States Patent Office portions of one pattern of an arrangement of the piezoelectric elements dividing some of the unit spaces into one or more pieces per unit, but providing for a symmetrical arrangement of the elements in an entire rectangular plane unit;

Fig. 3 is an enlarged fragmentary view showing the construction and assembly of piezoelectric crystals in the supporting frame and the electrical contact space for engaging the mercury;

Fig. 4 is a perspective view of the transducer piezoelectric assembly with common electrical connections for the different units thereof;

Fig. 5 is a face view of a fragmentary portion of the piezoelectric unit showing connections of the divided piezoelectric units; and

Fig. 6 is a fragmentary sectional view showing the electrical connection of the parts shown in Fig. 5.

In this relatively large size sound transducer, which may be used for underwater sound, the matrix arrangement presents a square radiating face which is attached to a shell holding a mercury column having one dimension which is varied by means of a mechanically driven piston. The whole arrangement is enclosed in a steel tank with a sound rubber window so that it can be used as an underwater sound radiating transducer.

Although the piezoelectric element used in this construction is synthetically grown ammonium dihydrogen phosphate (ADP) crystal, it is equally possible to apply the same technique to a system using any electrochemical vibrating element. This includes the magnetostrictive vibrating elements as well as the newer ceramic elements such as barium titanate.

Referring now more particularly to the drawings, a relatively large underwater sound transducer is herein described which has a square crystal array mounted in a face plate 10 located at the front of a mercury housing which is also square, and the sides of which are four pieces 12 of solid metal, such as steel, carefully finished and surface ground on all of the exposed surfaces. These steel sides are painted with a suitable adhesive (Hysol 6020, Houghton Laboratories) in the area where they will come in contact when they are bolted and attached together. These sides are bolted together and the adhesive left to set so that liquid tightness in the corners is assured. With this construction, it is possible to make a housing with dimensional variations of less than %&gt;oo of an inch. At the back of the housing is a metal plate 14 secured to the plates 12 and having gasket grooves 16 to set the gaskets therein which engage the edges of the plates 12 to make a fluid-tight joint. The face plate 10 is also provided with gasket grooves 18 about the edges of the inner side which is secured to the mercury housing to also make a fluid-tight joint.

Within the housing is a square piston 20 preferably formed with marginal flanges 22 on each of its four sides and the corners of the piston supported in the housing by guide rails 24 in each of the four corners of the housing. At the center of the piston and tightly secured therethrough is a hollow piston shaft 26 which extends through a central opening 28 in the back plate 14 as a guide. The front piston face is surface ground so that an accurately planed and movable face is presented in the back of the entire crystal array which is mounted in the plate 10 bolted to the front face of the square housing.

To provide the necessary acoustical decoupling of the mercury column in the housing from the supporting chamber, a pressure-release material 30 is applied to the walls of the housing and to the face of the piston which preferably comprises a layer of balsa wood, approximately ’/t inch thick, which is held accurately planar by glu1

2,802,196

VARIABLE FREQUENCY TRANSDUCER ₅

Francis J. Fry, Champaign, and William 3. Fry, Urbana, III., assignors, by mesne assignments, to the United States of America as represented by the Secretary of the Navy

Application August 24,1954, Serial No. 451,992

Claims. (Cl. 340—10)

This invention relates generally to a transducer in 15 which a mercury column of variable length is intimately coupled to a piezoelectric, electrically driven element so that a unit-vibrating element is formed, the frequency of which may be made resonant and is capable of being varied as desired within its prescribed limits. The in- 20 vention is more particularly described as a sound-radiating area composed of a matrix in which a relatively large number of vibrating elements is employed.

More particularly, this invention relates to the novel construction and operation of a transducer of relatively 25 large size having a plurality of piezoelectric units, electrically connected together so that a continuously variable resonant frequency characteristic is possible in the operation thereof. In the present invention, the variable frequency characteristic has been realized without greatly 30 increasing the size over that of a fixed frequency transducer of the same radiating area.

An important object of the invention is to provide a transducer construction for large size transducers in which a continuously variable resonant frequency char- 35 acteristic is possible.

A further object of the invention is to produce a transducer in which the variable feature resulting in a dimensional increase occurs mainly in a direction at right angles to the radiating area of the transducer which has this ⁴θ radiating area in one plane.

More particularly, the invention comprises a matrix arrangement of a plurality of piezoelectric elements in a relatively large square radiating face which is attached to a shell holding a mercury column whose dimension is 45 varied by means of a mechanically driven piston.

A further object of the invention is to provide a transducer arrangement in which the piezoelectric elements comprise a pattern of single elements or a division thereof into two, three, or more pieces which occupy the same 50 space as a single element but together are arranged in a symmetrical planar pattern, usually with the elements with the greatest number of pieces for each unit at the center of a rectangular pattern. This arrangement is to produce a desirable beam pattern of sound. 55

Still a further object of the invention is to provide a variable resonant frequency transducer system having a housing for supporting a mercury column continuously variable in one direction with a drive mechanism for varying the mercury column length and an overall hous- 60 ing enabling the system to radiate into the liquid medium.

Still a further object of the invention is to provide an improved mounting for the piezoelectric units and means for connecting them intimately and positively to a variable mercury column. 65

Other objects of the invention will appear in the specification and will be apparent from the accompanying drawings, in which:

Fig. 1 is a sectional view of a transducer construction in accordance with this invention; 70

Fig. 2 is a face view of a transducer element as shown in Fig. 1, also illustrating somewhat diagrammatically,

2,802,198 ing the wood to the steel with a surface-ground plate resting upon the wood during the setting process.

The mercury housing is mounted in a solid casing, preferably made of metal plates and comprising a bottom 32 with supporting side rails 33 upon which the mercury housing is rigidly supported at the sides thereof. At the front of the casing directly in front of the face plate 10 is a front plate 34 having a window aperture 35 therein about the same size as the face plate. Extending over the outside of the window aperture and overlapping the outer edges thereof is a sound-transmitting window 36 of rubber or other suitable material which is tightly secured about all of the edges of the window aperture by a marginal clamp 37 suitably secured in place by fastening bolts 38.

At the rear of the casing is a back plate 39 with a discharge opening 40 having a valved pipe extending therefrom at the bottom thereof and at the sides are side plates 41 which extend upwardly to a common level with the front plate 34 where they are covered by a top plate 42. A tubular extension 43 is suitably secured to the top of the casing surrounding the openings therethrough by a flange 44 and fastening bolts 45. The top of this tubular extension 43 may be sealed by a threaded cap or in any well-known manner, and fluid seals may extend through the top plate 42 into the extension to be accessible therethrough when the cap is removed. Extending through the top plate 42 is a fill pipe 46 having a control valve 47.

To reciprocate the piston 20 in the mercury chamber, a hollow sleeve 48 is attached to the rear plate 14 extending rearwardly therefrom and surrounding the central opening 28. One end of a threaded shaft 49 extends within the hollow piston shaft 26 engaging internal threads 50 thereof, and the outer end of the threaded shaft 49 is connected to a gear 51 by a key 52 disposed between supporting plates 53 and 54 which are secured to the outer end of the sleeve 48 allowing free movement of the gear 51 and the shaft 49 in the rotary direction but confining the longitudinal movement of the shaft. A gear train 55 is connected to the gear 51 and is suitably supported by the mercury housing with a terminal drive shaft 56 extending through the top plate 42 and sealed in its passage therethrough by a packing gland 57 so that the outer end of the shaft projects into the extension 43 where it may be engaged by a tool A, similar to a socket ring or any other suitable means for turning the shaft and the gear train. The rotation of the shaft 56 in one direction or the other correspondingly moves (he piston 20 at the back of the front plate 10 thereby varying the mercury column at the back of the front plate. By maintaining a fluid-tight connection through the top of the casing, the pressure in the casing may be varied as desired. This may be controlled through the inlet pipe 46, and the fluid usually deposited in the casing is degassed which completely fills the casing surrounding the mercury housing.

In order to maintain a supply of mercury in the chamber in the mercury housing, an inlet opening 58 is provided in the top plate 12 with a passage 59 extending to the front and back of the chamber, and connected to this opening is a pipe 60 which extends through the casing, preferably through the rear wall 39 thereof, with a suitable controlling valve 61 at the outside of the casing. A similar connection may be provided in the bottom plate 12 comprising an outlet opening 62 having a passage 63 extending to the front and rear of the mercury chamber. To this opening a pipe 64 is connected, which also extends through the rear wall 39 and is provided with a controlling valve 65. By this or any other suitable means, mercury may be supplied to the mercury housing and withdrawn therefrom if desired. In pipes 60 and 64, couplings are provided so that the mercury housing can be removed conveniently from the outer chamber.

The front plate 10, which is attached to the mercury housing, comprises a steel matrix having a plurality of partitions 66 forming rectangular or square cells in which piezoelectric crystals 67 are mounted. These crystals are adhesively connected at the rear side of the plate to a sheet 68 of silver palladium alloy (comprising 60 percent Ag and 40 percent Pd), which is supported by the matrix plate and its partitions. This alloy sheet 68 is of the order of .010 of an inch thick and is spot welded to the plate 10 and its partitions in a plurality of places to produce an extremely well-knit structure. The alloy sheet 68 provides the necessary intimate electric coupling between the crystals and the mercury column. The coupling is obtained by amalgamating the alloy with the mercury on the side opposite the glued crystals.

’ After completing the above assembly of the plate 10 with partitions 66 and alloy sheet 68, the back surface of the alloy sheet is thoroughly cleaned with a scouring powder in preparation for electroplating of certain areas. The electroplated area covers the alloy sheet 68 and ¹ overlaps the marginal edge thereof onto the back of the front plate 10 except in the 100 regions immediately behind where the crystals are glued. A thin layer of nickel 69 of the order of .0005 of an inch is deposited leaving the regions immediately behind the crystals for contact ' and amalgamation with the mercury. The regions with the nickel coating will not amalgamate, neither does the mercury show a creeping tendency under the nickel. An adhesive fillet 70 is provided between the edges of the alloy and of the front plate 10 as clearly shown in Fig. 3. This fillet assures liquidtightness of the alloy sheet.

A paper mask 71 is applied by glue or another adhesive over the regions in which the nickel coating has been electroplated. This paper acts as an acoustical decoupling material, and about .0015 of an inch in thickness is taken up by the nickel glue and paper layers. The adhesive Hysol 6020 previously mentioned is satisfactorily used for these gluing operations with a room temperature set.

The piezoelectric crystals of various numbers of plies are glued into the matrix plate 10 between the partitions with a suitable adhesive (Hysol 6020) as shown more clearly in Figs. 2 and 3. During the adhesive-setting period, the crystals 67 are held in position spaced from the plate 10 and the partitions 66 thereof by balsa wood strips 72. After the crystals are set, these strips are removed and a decoupling material 73 (Hycar ebonite) is installed. While positioning the crystals, the adhesive layer which connects them to the alloy sheet 68 is closely observed and worked so that no visible bubbles remain between the crystals and the alloy sheet. The crystals are provided with gold electrodes and tabs 74 *o which silver conductor wires 75 are soldered and connected. In the slots between the crystals and the steel matrix and its partitions, the balsa wood strips may be replaced by the acoustical decoupling material 73, and in the region between the crystals above the steel plate, the same material is installed. This material serves to acoustically decouple the crystals from steel matrix and from each other.

When the electrical connections between the crystals and the conductor wires 75 are completed, the exposed alloy surface of the sheet 68 is amalgamated with mercury. The amalgamation with mercury is accelerated by the use of a very small quantity of nitric acid which is wiped on the alloy plate and then wiped off by a slight scraping action of a knife blade over the various surfaces. A good amalgamation is one on which lint-free material can be used to wipe off the surface mercury, and then upon application of a small drop of clean mercury, there is immediate covering of the entire amalgamated region with a bright and shiny mercury film. In a complete set-up of the crystal assembly, the materials which come in contact with the mercury are steel, cast iron, balsa wood, Hysol adhesive, nickel, the paper mask, the silver _7g palladium alloy, and the gasket material, such as neoprene on the front and back plates and in the piston drive

3,803,106 . . 5 mechanism, the tubings 60 and 64 which may be plastic material, and the outside valves which enable the mercury to be placed into the mercury housing after the completed transducer assembly. Materials such as zinc and aluminum must be avoided because of their contaminating influence on the mercury.

The crystals in the cells of the matrix may be in the form of single unitary crystals 67, or the crystals for each cell may be divided into two portions 67a with suitable separating insulation 76, or they may be divided into four insulated parts 67b, each connected by suitable electrodes, usually arranged in a pattern as suggested in the upper left quadrant but extending over the entire matrix in Fig. 2 in which four of the solid units 67 are at each comer, then two line crystals 67α extend in the two outer rows between each comer, and the four section crystals 67b are in the thirty-six central cells. In this entire crystal system, the individual crystals or the divided parts thereof filling each cell are connected by the gold electrodes 74 either to a conductor wire 75 or 20 to an opposite conductor wire 77. These wires are supported in a raised position above the crystals by supporting posts 78 as shown in Fig. 4 and connected to common conductors 79 and 80. These conductors 79 and 80 extend through a fluid-tight connection 81 in the top plate 25 42 of the conductor casing and into the space in the tubular extension 43. This arrangement is for crystals in which the voltage is applied at right angles to the utilized direction of motion. There are many ways of r»« *1.^ ____ _ _____ , 5 , . . * portions thereof for obtaining different beam patterns.

In the piston 20, a plurality of openings 82 may be provided, and the marginal flanges 22 are spaced from the sides of the mercury housing to provide for a quick _ ^_ -“ — —“.I *·-. wav H1V1VU1JI VllCtXUUCl from one side of the piston to the other in varying the frequency of the transducer.

For this type of transducer, a relatively slow speed drive is provided having a total stroke of approximately 4 inches. The mercury housing is completely filled with 40 mercury, and the casing enclosing the housing is completely filled with degassed castor oil. Sound is transmitted from the crystal assembly to the sound window 36. Pressure may be placed upon the castor oil through the fill openings and also upon the mercury, if desired. In ⁴⁵order to fill the transducer casing with castor oil under the most favorable conditions, the casing is made air tight so that it may be completely evacuated of air whereby the oil is pumped in or admitted under vacuum. To accomplish this result, a solid metal window which may be in the form of a plate 83 with a peripheral flange 84 may be applied to the outside of the marginal clamp 37, and an air-exhausting pipe 85 may be inserted from the outside and connected through a valve 86 with a suitable vacuum line. At the same time, vacuum connections are ⁵⁵made with the top of the transducer casing and with the mercury housing through its pipe 60 since it is also emptied during the oil filling. With this technique, the hydrostatic pressures on both sides of the sound rubber window 36 are approximately equal. The degassed castor oil is ⁰⁰evacuated to the same pressure as the transducer, and the oil flows by gravity or by pumping it into the transducer housing. After filling, the entire system is brought simultaneously to atmospheric pressure, and the valves leading introduced into the mercury chamber through the valve 65 and the pipe 64. The valve 61, which is the airexhaust valve for the mercury housing when it is being filled with mercury, may be closed when the mercury chamber is filled. After the hydrostatic pressure is equalized, the window plate 83 is removed and the sound window 36 is free for sound vibrations.

To support the transducer beneath the surface of the water for test purposes, the tubular extension 43 may be of a length desired for the depth in the water. This pipe ⁷¹ '6 provides passage for the tool A used as a turning rod for changing the piston location in the mercury chamber and for access to the terminals for the electrical supply conductors 79 and 80. If the transducer casing is located sufficiently below the surface of the water to make a material change in the pressure applied to the sound window 36, a balancing pressure may also be applied to the transducer casing through the fill pipe 46.

This transducer design is thus able to meet the requirements of sonar (underwater sound) apparatus for use on naval and other vessels. For this purpose, a remote drive mechanism for varying the mercury column length is used, together with means for controlling the pressure in the transducer casing in the mercury housing as above . described. The holes 82 through the piston and the spaces at the edges of the piston are provided for rapidly displacing mercury around the piston for adjusting the movement thereof.

With this construction and the adopted arrangement of crystals, a voltage is applied at right angles to the utilized direction of motion, the column of mercury may be varied and has an intimate acoustical coupling with a plurality of piezoelectric, electrically driven elements capable of vibrating the variable resonant frequencies. The vibrations thus produced will pass through the flexible sound window and may be thus projected for remote pickup. This action also may be reversed and varied to receive sound through the transducer window which activates th* piezoelectric units to produce an electric signal, the resomercury column. ” '

While a preferred embodiment has been described in some detail, it should be regarded as an example or embodiment of the invention and not as a restriction _Or construction and arrangement of the parts without departing from the spirit and scope of the invention.