US1823329A - Piezo electric device - Google Patents

Description

' Sept. l5, 1931. w. A. MARRlsoN 1,823,329

PIEZO ELECTRIC DEVICE Filed Aug. 26, 1924 /nvenlor Waffe/7A Marr/san Patented Sept. 15, 1931 UNITED STATES PATENT OFFICE WARREN AQMARRISON, OF ORANGE, NEW JERSEY, ASSIGNOR TO *WESTERN ELEC- TRIC COMPANY, INCORPORATED, F NEW YORK, N. Y., A CORPORATION OF NEW You AIEIEZO ELECTRIC DEVICE yApplication `led August 26, 1924. Serial No. 734,189.

- kThis invention relates to piezo electric devices and more particularly to means for increasing the frequency range of such devices. Heretofore in the design of piezo electric devices used as the frequency control elements in space discharge tube systems, it has been customary to prepare the piezo active element of these devices in the form of a .parallelepiped The piezo crystal is usually arranged between metallic plates upon which alternating electromotive forces are impressed. When so arranged, the piezo element is set into vibration by the action of the electric field between the plates. If the crystal is cut with its longer dimension in the direction of a line perpendicular to one of the natural faces of the mother crystal and the metallic plates are positioned on opposite sides of the crystal on the line of its shorter axis, that is its thickness, the crystal being cut so that this dimension is parallel with an electrical axis, then the crystal will be set into longitudinal vibration in the direction of its longer axis to produce what is known as the transverse effect. Of course, the crystal will also tend to vibrate in the direction of the electric field, that is, as in accordance with what is known as the longitudinal effect although this type of vibration is not of significance in connection with this invention. The natural frequency of the crystal in this mode ,of vibration depends upon the elastic constant of the substance as Well as its density yand length. This frequency is given approximately by an equation Where is a constant approximately equal t0,l

must be greatly increased to obtain low frequencies. For example, a crystal designed to vibrate at a frequency of 10,000 cycles per second would be approximately 27 centimeters in length. Obviously, it is extremely difficult to obtain quartz or other crystals of these dimensions and even Were they readily obtainable, the apparatus employing them would necessarily be cumbersome and diflicult to'handle.

It is an object of this invention to cause vibration of lpiezo electric elements at low frequencies.

Another object is to produce transverse that is, exural vibration of a piezo electric element.

An additional object is to produce piezo electric elements having a ratio of restoring force to massv which is smaller than has heretofore been obtained.

A feature of the invention relates to piezo electric crystals of smal dimensions having a shape such that it has a natural frequency lower than has heretofore been obtainable from crystals of equivalent dimensions.

Anotherfeature relates to a piezo electric device having its various elements so arranged that transverse vibration of the vibrating element is produced.

It is found that by cutting piezo electric crystals from quartz in suitable shapes, the ratio of restoring force to mass may be decreased'for forces acting in the direction of the longer axis of the crystal. Such crystals are found to have a natural frequency of vibration, in the direction of the longer axis, that is'lower than that of crystals cut in the form offparallelepipeds having equivalent dimensions. In addition, it is found that a piezo electric crystal may be set into transverse vibration y properly positioning the metallic plates arranged adjacent to opposite sides of the crystal.

In the drawings, Fig. l illustrates the method of cutting a dumbbell shaped piezo electric crystal in accordance with this invention.

Fig. 2 is a transparent view illustrating the method of cutting the dumb-bell shaped crystal of Fig. 1.

I .belllshaped'crystal l1 is illustrated in lthe Fig. 3 is an enlarged view of a piezoelectric device employing a dumb-bell shaped crystal.

Fig. 4 is an end view of the device shown in Fig. 3.

Fig. .iillustrates azpiezo electric device arranged Sforxtransverse vibration of the vcrystal. Y

Fig. 6 illustrates the mode of vibration of the crystal of Figx. Y y i 'Fi'gj-' is an end viewlof the'piezo electric device of Fig. 5. Y o

Fig. 8 illustrates a modicationo-fthe devicey shown in Fig. 5. v

In Fig. 1, a dumbbell shaped crystal l'l'is f position vfrom which it is cut. f The dotted l flines 1 and 2 represent the longer .andy-shorter axes respectively, of this crystal.

InFigB, the dumb-bell shaped crystal 1l is shown positioned between the metallic plates 13.

An end View of the piezoelectric deviceas shown .in Fig. ..3 is illustrated ,in Fig. L in which the position of the plates V:13V is fniore clearly shown. ll/hen aV source of altern nat'ingfelectromotive force is applied to the terminals 14, .the crystal 11 is set v.in longitudinal vibration in the direction of -the dotted aXis 1 of Fig. 3.*.5By virtue of theV enlarged ends @provided on thecrystal 11,

Vthe mass of .the crystal is increased without substantially,increasing the restoring force. t is thus seenftliat .the rationf restoring force to mass is correspondingly,reduced and as a result the natural frequency ofthe vi- Y `bration of the crystal 111l is .alsoV reduced;

The Vfollowing table kgives .thennatural fre- Vquency of vibration r'or several samplesof "crystals of dumb-bell shape having a `length of `cm., a width atthe end sections in the direction of axisQ of Fig. 2 of V12cm. and a tliiclmess of .5' cm. The column headedfA is the width of .the reduced vportion .in the direction or' axis 2v in centimeters. u

Natural frequency 414, v.73 Waeoo f,

.-54 1e-37,580. .3s ss,aso l ,.29 Y .-2s,aeo .18 24,960

lnFigs'. 5 and 7, a piezo electric device arranged 'to 'produce low l frequency Vvibra'- This mode of vibration is-produced by virtue crystal maybe applied -zto ltliegplatf'es 13 and tions by another method is illustrated. In this device, the crystal 11 is cut 'in the orm of a parallelepiped. The plates 13 are positioned along one edge at opposite sides of the crystal 11. VWith this arrangement of :f

the plates the V crystal l1 is `set into. vibra tion, A.upon :the application -ofa'lterriatmg electromotive forces to the plates 13, inthe manner illustrated by the curves of Fig. 6.

of the Atact that thelportion of -fthe crystal directly opposite the plates 13 undergoes -contractionand expansion whereas the portion ot' the crystalV away from the'plates lfha's substantially no strains produced therein. r.The result is a'transverseliexingof the crystal 11. It is well known thatin-one. inode a Vfree-free V,bar vibratesabout "two nodes such as nodes 15 lillustrated finFigrG, at Vapproirimately 0:22a of .the `length ofthe bar from its ends. The natural (frequency of the `b'arfor crystal Yinfthis mode of {vibra-v tion can be accurately determined Arom the elastic constant, the density, and the Idimensions of the bar. See Bartons textbook on Sound, 1191s edition, kpagesE280 to 288 inclusive published by MacMillan t'Company, Ltd., St. Martin Street, London, wherein onpage 281 the frequency N is shown equal to l Y in which N is .a quantity, the valuelof which is to 'be determined by further calculations ,.160

not essential `to'thevpresent disclosure, K is the radius of gyration, Z is the length of the bar, gisr `Youngsinodulus of elasticity and p is the density of the material.

The crystal Velement VA11 ispreferably cut .,105 r from Vthe. mother crystal as already en plained with respect '.tothe crystal or Fig. 3 having its longer axis-perpendicular to `one-of the natural ifaces of the mother crystalas shown in Figs. 1 and'2.

no 1ra-Fig. -8 a modication ofthe piezo electric device Ashown in Fig.` 5 is illustrated wherein additional `plates 13VV areprovided for `vutilizing both sections of `the crystal.

By means ofthe `device illustrated in this n.5

igu=re,'a greater lresponse or" the crystal may be obtained for a given impressed alternating electromotive i'orce' wave. lf desired, an electromotive force of VVfrequency correspondingto the natural frequency of the a corresponding.electromotive force vhav-ing its `Vfrequencydetermined solely by the natural vfrequency of the crystal finay be obtained fromplates'l. l

Piezo electric devices nconstructed in Y accordance withth'is invention maybe applied asl `frequency .control elements in space discharge tube systems 'for frequencies within the audible range. Y

Although the invention has been described in connection with certain specific embodiments, the principles disclosed are applicable to other practical arrangements and the invention is therefore to be limited only as indicated by the scope of the appended claims.

What is claimed is:

l. A piezo electric device comprising a vibratory element of piezo electric material and conducting plates, subjected to electromotive forces, laced adjacent corresponding portions o opposite sides of said element parallel to its longer axis and unsymmetrically with respect to said axis whereby said element tends to vibrate transversely. f

2. A piezo electric device comprising a vibratory element of quartz material, and conducting plates, subjected to electromotive forces, placed adjacent corresponding portions o opposite sides of said element parallel to its longer axis and unsymmetrically with respect to said axis whereby said element tends to vibrate transversely.

3. A piezo electric device comprising a quartz crystal body in the form of a parallelepiped and conducting plates, subjected to electromotive forces, placed adjacent corresponding portions of opposite sides of said crystal parallel to its longer axis and unsymmetrically with respect to said axis whereby said crystal tends to vibrate transversely.

4. A piezo electric device comprising a parallelepiped of quartz material having one dimension greater than another, and conducting plates, subjected to electromotive forces, placed adjacent corresponding portions of opposite sides of said parallelepiped parallel to its longer dimension and unsymmetrically with respect to the axes of the respective sides whereby said parallelepiped tends to vibrate transversel at a lower frequency than can be obtaine by longitudinal vibration in the direction of said longer dimension.

5. A piezo electric device comprising a plurality of pairs of conducting plates subjected to electromotive forces from a single source, and a parallelepiped of quartz material therebetween, each of said pairs of plates being placed adjacent corresponding portions of opposite sides of said quartz parallelepiped parallel to its longer axls and unsymmetrically with respect to said axis, whereby said parallelepiped tends to vibrate transversely of its longer axis.

6. The method of operating a piezo electric oscillator` comprising a single homogeneous quartz piezo electric element, which consists of simultaneously producing mechanical stresses of opposite sense in different parts of said element.

7. The method of operating a piezo electric quartz crystal which comprises simultaneously impressing differences of potential of relatively opposite sign across different parts of a crystal whereby there is induced a compressive stress in one part of said cr stal and a tensile stress in another part o said crystal.

8. The method of operating a piezo electric crystal having a conformation of a rectangular parallelepiped, the long dimension of which is perpendicular to a face of the natural crystal which comprises simultaneously impressing differences of otential of relatively opposite sign across di erent parts of the crystal and so as to electrostatically stress the crystal in a direction parallel with a side of the natural crystal whereby there is induced a compressive stress in one part of said crystal and a tensile stress in another part of said crystal.

In witness whereof, I hereunto subscribe my name this 22nd day of.August A. D.,

WARREN A. MARRISON.

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