Piezoelectric Transducer

Abstract

This disclosure relates to the novel construction of a single- or a multi-element underwater sound transducer comprising one or more cylindrical, capped, piezoelectric ceramic elements (barium titanate, lead zirconate-titanate, or other material of similar properties) in a linear configuration. The structure results in acoustically decoupling each element from adjacent elements and from all of the supporting structure while retaining acoustic stability of the transducer over the broadest possible frequency range and with wide variation in temperature and hydrostatic pressure.

Description

BACKGROUND OF THE INVENTION

This invention is directed to an acoustical transducer and more particularly to a piezoelectric type transducer which is stable, of broad frequency range, and a nonresonant line type transducer.

Heretofore, piezoelectric ceramic cylinders have been mounted in various ways to form underwater sound transducers. One method is to slip the cylinders over a rod for structural support and to separate them from the rod and from each other by compliant washers or spacers made of natural rubber, neoprene, or a mixture of cork and neoprene. When these normally compliant materials are exposed to hydrostatic pressure or to extremes of temperature, however, their acousto-mechanical properties change, and the electroacoustic characteristics of the piezoelectric assembly change. The sensitivity of the line transducer then changes as hydrostatic pressure or temperature changes.

SUMMARY OF THE INVENTION

The line transducer and the sensor element described herein consist of one or more end-capped piezoelectric ceramic cylinders slipped over a thin-walled steel tube. The ends of each cylinder are sealed by O rings and separated from adjacent cylinders by thin Nylon spacers. The entire assembly is made waterproof by an air-free butyl rubber hose, or boot, molded from a special compound having low water-vapor permeability and good hydroacoustic characteristics. Castor oil fills the steel tube and the space between the ceramic cylinders and the butyl boot to provide acoustic coupling between the sensor elements and the boot. The air space between the steel tube, the piezoelectric cylinders and the end caps provides an acoustic mismatch between the air space and the end caps which prevents the acoustic sound pressure from reaching the inside surface of the cylinder.

STATEMENT OF THE OBJECTS

It is therefore an object of the present invention to produce a stable, broad frequency range, nonresonant line transducer.

Another object is to isolate the transducer elements such that hydrostatic and acoustic pressures do not have a deleterious effect on the elements.

Still another object is to provide a structure which has high sensitivity and acoustic isolation between the interior and the exterior of the radiating ceramic elements.

Yet another object is to provide a transducer which may be made as a single unit or with a plurality of units joined together linearly.

Other objects and advantages of the invention will become obvious from a more careful review of the following specification considered in view of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a cross-sectional view of the transducer.

FIG. 2 illustrates two piezoelectric elements separated from each other illustrating the parts that separate the two elements when assembled in place.

FIG. 3 illustrates the outer end of one cylinder in combination with the outer end cap and the electrical connectors in the end cap and spacers in between.

FIG. 4 illustrates a typical free-field voltage sensitivity of a transducer comprising 20 piezoelectric cylindrical elements.

DESCRIPTION OF THE DRAWING

Now referring to the drawing FIG. 1, there is shown by illustration a multi-element piezoelectric type transducer made in accordance with the teaching of this invention. As shown, the device includes a central thin walled cylindrical tube 11 such as steel about which are placed two cylindrical linearly aligned piezoelectric ceramic cylinders 12 coaxial with the steel tube. Each ceramic cylinder is provided with polycarbonate end caps 13 bonded thereto by epoxy adhesive. The end caps are stepped with a portion separating the ceramic cylinder from the steel tube and a portion which extends outwardly along the end of the ceramic cylinder. The portion that extends along the end of the cylinder is spaced from the steel tube and receives therein an O-ring 14 which provides a seal between the end caps and the steel tube. Each of the end caps of each adjacent cylinder are separated by a Nylon ring or spacer 15. The ends of the steel tube are threaded and receive thereon outer end caps 16 and 17. End cap 16 has a thick bottom that has an aperture therein through which the device may be filled with oil and a screw plug 21 screwed into the aperture to prevent oil leakage through the aperture. The end cap 17 is cylindrical with a threaded portion 22 that threads onto the steel tube and a portion 23 that extends in the opposite direction from the central solid portion 24. Nylon spacers 25 and 26 are placed between the outermost end caps 13 on the piezoelectric cylinders 12 and the outer end caps 16 and 17 in order to hold the piezoelectric cylinders firmly in place. An acoustic boot 27 of butyl rubber or any other suitable type such as neoprene or polyvinyl is spaced from and encloses the piezoelectric cylinders and is secured at its ends to the outer end caps 16 and 17 by metal bands 28 or any other suitable method.

Each of the outer end caps is provided with suitable passages 29 to provide a passage from the inner area of the steel tube to the area between the boot and the ceramic cylinders. The space between the boot and ceramic cylinders and the space within the steel tube is filled with a substance having substantially the same acoustical properties as that of water such as castor oil, a silicone fluid or an elastomer compound such as polyurethane. The fluid is filled through the filler aperture in the outer end cap 16 and through passages 29. The spacings 30 between the ceramic cylinders and the outer surface of the steel tube are filled with air.

An electrical connector 31 is secured to the outer extending end of end cap 17 by suitable screws 32 and an O-ring is provided between the electrical connector and the outer extending end to prevent leakage therebetween. The electrical leads 33 and 34 supplied through cable 35 pass through suitable apertures in the end cap 17 and connect with high pressure glass-to-metal seal connectors 36. One electrical conductor is connected between the connector 36 and the inside surface of each ceramic cylinder and another conductor is connected between the connector 36 and the outside surface of each of said ceramic cylinders. The electrical cable 35 is bonded or otherwise secured to the electrical connector 31 to prevent leakage into the area in which the conductors are secured to the glass to metal seal connectors.

FIG. 2 is an enlarged view of the central thin-walled cylindrical tube with two ceramic cylinders 12 spaced from each other thereby illustrating the end caps 13 of the ceramic cylinders, the O-rings 14, and the Nylon spacer 15 that separates the cylinders when they are secured into place.

FIG. 3 illustrates a view of the end of the central thin-walled cylindrical tube secured to the outer end cap 17 which illustrates the electrical connectors 36 secured thereto by the glass to metal seals. Further, there is shown, one end of the ceramic cylinder 12, the end cap 13 adjacent thereto with the Nylon spacers 26 that force the ceramic cylinders firmly into place. An O-ring prevents leakage of oil into the air space between the ceramic cylinder and the tubular member and electrical conductor 40 connects with the inside of the ceramic cylinder.

In operation, the transducer is made to contain as many piezoelectric cylindrical elements aligned in axial alignment as desired. An electrical voltage signal is applied to the piezoelectric elements which causes the elements to expand and contract radially in accordance with the applied signals, as is well known in the art. The expansion and contraction of the cylinders causes pressure on the castor oil which transmits the pressure to the surrounding water. Thereby producing a compressional wave in the water corresponding to the signal applied to the piezoelectric elements.

The transducer will also operate as a receiver by compressional waves on the transducer causing the piezoelectric elements to contract and expand thereby producing an electrical output which is transmitted to a receiver. The operation is not unique and is not different from that of the prior art.

The piezoelectric ceramic cylinder assembly described herein is considered to be unique. The annular polycarbonate end caps provide strong, compliant support for each cylinder and, together with the O-ring, acoustically isolate the ceramic cylinder from the supporting thin-walled steel tube. Because the ends of the ceramic cylinder are exposed to the acoustic sound pressure at all hydrostatic pressures and temperatures and are not in contact with materials whose acoustic characteristics change with hydrostatic pressure or with temperature, the electroacoustic characteristics of the piezoelectric ceramic cylinders are not changed by changing boundary conditions, as can happen in other assembly methods. The thin Nylon spacer between adjacent cylinder assemblies acoustically decouples the cylinders and prevents a low-frequency resonance in the operational frequency band.

The castor oil inside the thin-walled steel tube makes the device acoustically transparent in water by providing a medium with acoustic characteristics similar to those of water; by introducing the oil at the bottom of the line transducer when it is being filled, air is not trapped in the space between the outside acoustic boot and the ceramic cylinders. Additionally, the air space between the ceramic cylinders and the outer surface of the steel tube produces an acoustic mismatch between the air space and inner end caps to prevent the sound pressure from reaching the inside surface of the ceramic cylinder.

By use of a structural assembly as set forth above, one can provide a line transducer whose electroacoustic characteristics are stable over a wide frequency range at temperatures from about 2.degree. C to about 30.degree. C at hydrostatic pressures up to 6.9 MPa (1,000 psig). It has been determined that a transducer contructed of 20 piezoelectric ceramic cylinders, 3.0 cm long .times. 3.0 cm diameter such as shown, assembled in an axial arrangement will produce a typical free-field voltage sensitivity as shown in FIG. 4.

Other materials such as aluminum, magnesium, aluminum oxide, and beryllium oxide can be used for the annular end caps, but maximum decoupling will be obtained when polycarbonate end caps and Nylon spacers are used. The thin-walled tube can be made of aluminum, but greater strength is provided by steel. Silicone fluid or an elastomer compound such as polyurethane, both of which have acoustic properties similar to those of water, can be used instead of castor oil. Instead of butyl rubber, the acoustic boot can be made of neoprene or polyvinyl chloride; however, butyl has the lowest water permeability.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims

What is claimed is:

1. An acoustical transducer for operation in a liquid medium; which comprises,

a metallic elongated tubular member,

a plurality of piezoelectric cylinders mounted around said tubular member coaxial therewith in end-to-end alignment with each other and forming a radial spacing between said cylinders and said tubular member,

an end cap assembled onto each end of each of said cylinders,

a spacer of noncompressible electrically insulative material separating the adjacent end caps of each of said cylinders,

an O-ring on each side of each of said spacers between said spacer and said end caps on adjacent ends of said cylinders,

means securing said cylinders firmly in position on said tubular member,

an acoustic boot secured about said cylinders, and

an acoustic transmitting medium within the area between said boot and said cylinders and within said tubular member.

2. An acoustical transducer as claimed in claim 1; wherein,

the area confined by the inner surface of each of said piezoelectric cylinders and the outer surface of said tubular member is filled with air.

3. An acoustical transducer as claimed in claim 2; wherein,

the spacer between said end caps on said cylinders are made of Nylon.

4. An acoustic transducer as claimed in claim 3; wherein,

said end caps on said piezoelectric cylinder are made of polycarbonate.

5. An acoustic transducer as claimed in claim 4; wherein,

said tubular member is a thin-walled steel tube.