US2399820A

US2399820A - Piezoelectric apparatus

Info

Publication number: US2399820A
Application number: US457055A
Authority: US
Inventors: Adolph R Morgan
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1942-09-02
Filing date: 1942-09-02
Publication date: 1946-05-07
Anticipated expiration: 1963-05-07

Description

May 7, 1946. A. R. MORGAN 2,399,820

PIEZOELECTRIC APPARATUS Filed Sept. 2, 1942 mam/on PHTTE/F/YS cum/5p CEYSTHL 3nnentor (.Ittomeg I are, however,

estates PIEZOEnnorm assist a. Mo

to Radio Co ericaa actionof Delaware A Application September This invention relates to. piezoelectric swimtus and has special reference th the provision of an improved form oi crystal andeholder for use in underwater signaling systems.-

1 In the usual underwater signaling system, a .quartz resonator convertselectric oscillations-of a given frequency (usually about 40,000 cycles per second) .intoelastic waves of the same frequency. These elastic waves are reflected back after impact on a distant obstacle, are amplified by the resonance phenomenon and then translated into electric signal ener The higher limit of frequency employed in such signaling systemsis fixed by the distance to be traversed or, stated another"way,by the absorption that can be allowed. The general ru1e,is:

, penetration of the waves in water is inversely proportional to thesquare of the frequency,

. Thus, it is knownthat for 100,000 cycles per second the total energy is decreased to one-third of s its initial value at 5 kilometers, while 'thecorrespending distance of .30 kilometers.

approximately, about 2,000; times larger, and at 100,000 cycles per second the duced to one-third at 2 meters.)

The above considerations point toward an up- 40.000 cycles per second is However, it must be borne in mind that when (In air, the absorption would be energy would be re-,

2, ms. 's No. 452,055

g system,will exhibit a radiation pattern considerably wider than that exhibited by pres- .ent day crystals of the samefrequency and di- J ameter.

- .per limit of 70.000 cycles per second for the frean ordlnaryquartz plate is employed the radiated energy is localized in a narrow to the plate, and of a formula:

cone perpendicular half-angle Agiven by the sin a=1s where d is the diameter of the piezoelectric plate and A is the wave length. Hence, one selecting convex surface 8a of the a conventional crystal of a relatively high frequency for use in a signaling system of limited range has heretofore been obliged to accept the very narrow radiation pattern peculiar to a fiat plate of that frequency. This, of course, is not always desirable since practical considerations may dictate the use of a relatively wide radiation pattern.

Accordingly, the principal object of the present As will hereinafter more fully appear, the crys-'- tal employed in achieving the foregoing object is preferably in the form of a. circular portion of a spherical shell. Therefore, a related object of the invention is to provide a suitable mounting arrangement for such an odd-shape crystal and one which will permit the crystal to be vibrated,

subaqueously, in its intended mode and at its maximum practical emciency,

Other objects and advantages will be apparent and the invention itself will be best understood by reference to I the following specification and to the accom w drawing wherein: g

Figure l is a fragmentary sectional view of an underwater signalingunit'incorporating a pianoelectric crystal assembly within the invention;

- Flgurez is a view in perspective of a quartz piezoelectric crystal cut in accordance with the principle of the invention, and similar to the crystal Shown in Fig. 1.

' Figure 3 is a plan view of the crystal of Fig. 2.

which, when used in an underwater sis.-

Figure 4 is a sectional view of the crystaltaken on the line of Fig. 3.

Figure 5 is a graph showing a narrow radiation pattern characteristic of prior art crystals, and

Figure 6 is a graph showing the wide radiation pattern characteristic of crystals cut in accordance with the principle of thepresent invention.

The broad objects of the invention may be achieved in an otherwise conventional underwater signaling unit, indicated generally. at i (Fig. l), by substituting a suitably mounted concavo-v convexpiezoelectrie element a for the usual flat plate crystal or crystals of the prior art. As shownin Fig. l, the crystal 8 forms part of the end wall of the watertight casing i and it is the crystal which is presented to the sea water or other ambient medium surrounding the casing. It is this arrangement watertight as by a clamping n. asduring normal operation, the crystal This electrode I3 is preferably in the form of a vertex is its center of curvature, is substantially sussest themselves of piesoelectricity. By way oftexample; the ize ness dimension. i and the curvature of the crystal may be greater 7 silver or other metallic coating applied to the inner or concave face of the crystal and electrically connected to the voltage supply '(not 10 wide radiation pattern is required, it is entirely practical to employ a bank or mosaic of crystals.

shown, but which may include an amplifier mounted within the casing I) by means of a wire I! which may be biased as by means of a loop or bend lt intoengagement with the said metalized concave face of the crystal.

As shown more clearly in Figs. 2, 3 and r4, the

.pieaoelectric element employed in carrying the invention into-effect preferably comprises a circularportion' of a spherical shell. This "dished" vibratile element 3 preferably comprises an 2:- cut quartz element cut to respond to a fundamental frequency which is a function of its thickness dimension. That is to say, a crystal exhibiting a i ongitudinaP type of vibration (as does an x-cut crystal) is to be preferredto one exhibiting a .shear type of vibration and where the desired frequency canbe achieved with either a thi -mode" or a contour-mode crystal element, thickness-mode" element should ordinarilybe used.

. As previously indicated. the frequency to which the plead-electric element is cut will ordinarily be dictated by the distance to be traversed by the elastic waves it sets up in the sea wateror other ambient medium in which the device is to invention wherein the frequency employed was of the order of 500,000 per second, (i. e. 500 kilocycles) the crystal specifications were as follows:

- The crystal was an x-cut-quarts element of the formshown inFlgs. 2, 3 and 4 whereinit will be 1 noted (see especially Fig. .0 that the vertex halfangle of the cone whose .base. is the spherical outer convex surface of the crystal. and whose 45 degrees. The greater diameter of the said was 1.25, the radius of its outer or conwater-tight casing constitutedsin part y an x-'.

along differentradii. The said crystal when vibrated, subaqueously. set up a radiation pattern similar to that shown in Fig. 6 wherein the elastic\.

waves are shown as spanning an arc-of 60 degrees. This radiation pattern is approximately four times as wide as the pattern (shown in Fig. 5) achieved with a conventional 'ilat-surf/a ced crystal tested inthe same apparatus.

Various. modiilca ons of the invention will those skilled in. the art or less that given in the foregoingexampie, as determinedby the exact pattern desired. Further it is not essential to the practice of the in-- vention that the crystal be of uniform thickness,

5 sinceinsomecasesthedesiredpatternmaybe Accordingly, the foregoing description of one practical embodiment of the invention should be interpreted asillustrative and not in a limiting lo sense except'as'required by the prior art and by.

a spherical shell and w erein the vertex halfis the spherical surl faceof said crystal. and whose vertex is its center angle of the cone whose of curvature, is substantially tiideineea y 2. Theinvention as setforth in claim 1 and wherein the thichness of. said piezoelectric eleq ment is substantially; uniform as measured along different radii of said spherical alien.

- 3. The invention as set forth in claim 1 and wherein said piezoelectric crystal is constituted v so of quartz and is adapted to respond to a predetermined fundamental thickness-mode frequency.

A 4. The invention as set forth in claim 1 and wherein said piezoelectric crystal comprises an x-cut quarts element adafited to respond to a be operated. in one practical embodiment of the predetermined fundamental thickness-mode freq a '5. Asignaling device comprising a fluid-tight" casing constituted in part by a piezoelectric crys- 40 tel out in the form of a concavo-convex element,

" the convex face of said piezoelectric crystal being presented to'the ambient surrounding saidcasing. and means for vibrating said piezoelectric element. I q

6. The inventionas set forth in claim 5 and wherein the inner concave face of. said piezoelec-v tric element is provided with a metallic electrode coating. the outer convex face of said element be.

in: uncoated.

7. An underwater signaling unit comprising a cut piezoelectric quartz crystal element cut in the permanent form of a circular'portion of a spherioal shell. the convex face of said crystal element v being presented to the aqueous medium normally I surrounding said casing, and means comprising an electrode presented to the'inner concave surface of said element for vibrating said quartz at a frequency which is a function of its thicki l snonrn n; MOR AN: