US2403692A - Piezoelectric device - Google Patents

Description

July 9, 1946. G. c. TIBBETTS PIEZOELECTRIC DEVICE 2 Sheets-Sheet Filed Dec. 29, 1944 a zkh zk rww y 1946- e. c. TIBBETTS 2,403,692

PIEZOELECTRIC DEVICE FiledDec. 29, 1944 2 Sheets-Shet 2 Imam? 602 6 6ZZZZZ9 z ywf; a?

Patented July 9, 1946 UNITED STATES PATENT OFFICE 10 Claims.

This invention relates to piezo-electric devices for converting electrical energy into mechanical vibrations or converting mechanical energy into electrical impulses. and more particularly to devices of the type disclosed in the patent of Raymond W. Tibbets, No. 2,386,279, granted October 9, 1945, comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic forces, a flexible member or toggle extending along one side of the device from edge to edge in one of the directions, another flexible member or toggle extending along the other side of the device from edge to edge in the other direction, and means interconnecting the slab and members at the aforesaid edges, the members being arched in said directions respectively so that each member tends to straighten when the slab expands in the direction in which it is arched.

Objects of the present invention are to provide a piezoelectric device which will faithfully detect and reproduce acoustic vibrations, which is capable of a very high order of straight-line response to audio-frequencies, which reduces harmonic distortions to a minimum, which has relatively high sensitivity, which is relatively insensitive to shock pick-up, which is smple in construction, which has a high order of mechanical strength, which is capable of being used either as a transmitter or a receiver, which requires a minimum amount of crystalline piezoelectric material in its construction, which can be manufactured at low cost, which can be readily sealed to exclude moisture, in which the sound Waves impinge substantially in phase on all its actuating surfaces and in which the lead extensions are held securely in position.

According to the present invention the toggles of the aforesaid application are in the form of diaphragms extending throughout substantially the entire area of the piezo-electric slab, instead of ribbons covering only parts of the sides of the slab. In a more specific aspect each diaphragm is arched away from the slab in one of the aforesaid directions and toward the slab in the other of the directions. While each diaphragm may be arched away from the slab in the direction in which the other diaphragm is arched toward the slab, with means interconnecting the central portions of the two diaphragms through an opening in the slab, preferably the diaphragms are arched away from the slab in the same one of said directions and toward the slab in the other of the directions so that the central portions of the diaphragms move toward the slab at the same time. In a still more specific aspect the device comprises means for mounting two units with corresponding dimensions of the two units disposed in planes approximately at right angles to each other so that the voltages produced in the two units by shock, tend to cancel each other, together with means electrically interconnecting the two units so that the voltages produced in the two units by sound waves are cumulative.

For the purpose of illustration typical embodiments of the invention are shown in the accompanying drawings in which Fig. 1 is a perspective view of a crystal from which one or more piezo-electric slabs may be cut;

Fig. 2 is a perspective View of a slab cut from the crystal;

Fig. 3 is a similar view with electrodes and leads mounted on the slab;

Fig. 4 is a perspective view of a complete unit;

Fig. 5 is a diagonal section of the unit, taken on the line 5-5 of Fig. 4;

Fig. 6 is a similar section taken on line 6-6 of Fig 4;

Fig. 7 is a section taken on line 'l-J of Fig. 4;

Fig. 8 is aperspective view of one of the diaphragms viewed from the inside;

Figs. 9 and 10 are sections like Figs. 5 and 6, showing a modification;

Fig. 11 is a section showing the unit of Figs. 4 to '7 mounted in a double diaphragm device;

Fig. 12 is a similar View showing a unit mounted in a single diaphragm device;

Fig. 13 is a similar view showing the unit mounted in a case of the type used in a hearing aid receiver;

Fig. 14 is a perspective view like 4 except in that the unit is circular instead of square;

Fig. 15 is a perspective view like Fig. 3 showing the slab of the circular unit;

Fig. 16 is a front view of a shock-proof device; and

Fig. 17 is a section on line l7l'| of Fig, 16.

While any piezo-electric material may be used in the present invention, Rochelle salt crystals are preferred, such a crystal being shown in Fig. 1 in which a-a is the electrical axis, 12-12 the transverse axis and 0-0 the longitudinal axis. In cutting a square slab from such a crystal for use according to the present invention, the crystal is so cut so that the diagonal lines mm and 11-72 bisect the angles between the axes b-b and cc. Thus when the slab expands along the line m-m, it contracts along the line n-n and vice-versa. While the crystalline slab may have any desired shape, it is preferably square as shown in Fig. 3 or circular as shown in Fig. 15. As disclosed in the aforesaid application the slab may be formed in one layer or in a plurality of layers, and each layer may comprise a single piece or a plurality of pieces disposed in edgewise alignment.

As shown in Fig. 3 the piezo-electric slab I has electrodes 2 mounted on its opposite faces and leads 3 connected to the electrodes respectively. The electrodes are preferably formed of gold-leaf as described and claimed in Patent 2,287,950 and leads 3. which are preferably formed of metal foil, may be anchored to the electrodes in any suitable way as for example the manner disclosed in copending application Sr. No. 559,522.

Mounted on each side of the slab l is a toggle diaphragm 4 which may be formed of Bakelite, Bakelite-metal powder mixtures, molded natural amber, special metallic alloys or other material having a good modulus of elasticity and a coefficient of expansion approximating that of Rochelle salt crystals. As shown in Fig. 5 each diaphragm is arched away from the slab along the diagonal n--n and towards the slab along the diagonal 77t-7)l, Thus the thickness of the diaphragm varies around its periphery from a minimum at the corners on the diagonal mm to a maximum at the corners on the diagonal 11-41. On the inside each diaphragm is provided with a peripheral flange or shoulder 6 which varies in depth so that its exposed face lies in the same plane throughout the entire periphery of the dia-- phragm. Thus each diaphragm seats fiatwise against the slab. as shown in Figs. 5, 6 and 7, without distortion. The abutting faces of the slab and diaphragm shoulders, are secured together with Duco or other suitable cement. While the central portion of each diaphragm inside the shoulder 6 may vary for structural and functional reasons it is preferably uniform in thickness throughout its entire area. While the optimum thickness depends on the character of the material employed and the function of the unit. when using Bakelite the thickness is preferably of the order of four thousandths inch to fifteen thousandths inch.

The modification shown in Figs. 9 and 10 is like that shown in the preceding figures except in that the central portions of the diaphragms are interconnected by means of a pin 1 extending through an opening in the slab, as in the aforesaid Patent No. 2,386,279. and in that one diaphragm is turned 90 degrees with respect to the other, so that along one diagonal both diaphragms arch downwardly and on the other diagonal both diaphragms arch upwardly. As disclosed for example in the aforesaid application 451,690 the rod 1 may be connected to a phonograph stylus, a microphone diaphragm or a sound-reproducing diaphragm. The device may be mounted on rubber feet I0.

Figs. 11, 12 and 13 illustrate various ways of utilizing the unit shown in Figs. 3 to 8. The device of Fig. 11 comprises an annular frame 8 upon each side of which is mounted an arched diaphragm 9. Interposed between the central points of the diaphragms 4 and the diaphragins 9 are posts II which may merely seat in recesses in the diaphragms or may be secured at the ends by means of cement. When used as a microphone the sound waves impinging upon the diaphragms 9 move the diaphragms 4 back and forth toward and from each other. As the diaphragms 4 move toward the slab from their normal positions shown in Figs. 5 and 6 the slab is stretched along the diagonal iz-n and compressed along the diagonal m-m; and when the diaphragms 4 move away from the slab from their normal positions shown in 5 and 6, the slab is compressed along the diagonal 12-11 and stretched along the diagonal mm. Thus each diaphragm exerts a force on the slab along each diagonal as the diaphragm moves back and forth. When using the device as a microphone the sound waves impinging upon the diaphragms 9 produce corresponding electrical waves in the circuit connected to leads 3 by virtue of the alternate compression and stretching of the slab along its diagonal; and when using the device to convert electrical impulses into sound waves the action is reversed, the electrical impulses causing the slab alternately to expand and contract along each diagonal, thereby actuating the diaphragms 9 through the medium of the posts H.

The device shown in Fig. 12 is similar to that shown in Fig, 11 except in that it has only one diaphragm l5, and instead of the other diaphragm it has a case 14 secured to the periphery of the diaphragm I3. The pieZo-electric unit is mounted between a lug 16 on the casing and a post IT engaging the diaphragm. When the diaphragm 13 moves inwardly it compresses the piezo-electric unit between the abutment l6 and post ll, thereby moving both diaphragms 4 inwardly toward the intermediate slab I, and when the diaphragm [3 moves outwardly beyond its normal position it causes both diaphragms 4 to move outwardly away from the slab.

The device shown in Fig. 13 is like that shown in Fig. 12 except in that the casing 18 has a cover l9 and the diaphragm 2| is flat instead of cone shaped. The cover l9 has an outlet 22 for the sound waves produced by diaphragm 2!, this device being particularly suited for use as a hearing aid.

In devices such as shown in Figs. 11, 12 and 13 the large diaphragms 9, I3 and 2| may be normally biased toward the unit sufiiciently to keep the intermediate post H or H under compression throughout the entire range of vibration, in which case the post and abutment l6 may merely seat in recesses in the diaphragms without being cemeneted. However, the large dia' phragms are preferably not normally biased and the parts are cemented together so that the large diaphragms can pull on the small diaphragms when sound waves cause the large diaphragms to flex outwardly from their normal positions. and the small diaphragms can pull inwardly on the large diaphragms when actuated by electrical impulses through the slab l.

The device illustrated in Figs. 14 and 15 is like that shown in Figs. 3 and 4 except in that it is round instead of square. As in the first embodiment the device of Figs. 14 and 15 comprises a central piezo-electric slab 23 carrying electrodes 24 and leads 25, and diaphragms 27 having peripheral shoulders cemented to the slab throughout the periphery of the device. As shown in Fig. 14. each diaphragm arches away from the slab along the diameter 28 and towards the slab along the diameter 29.

The device shown in Figs. 16 and 17 comprises a base 3|, two posts 32 mounted on the base, two rings 33 mounted on the posts. one above the other, and two units 34, such as shown in Figs. 3 to 8, mounted in the rings 33 by means of gauze or other porous flexible material 36 cemented to the rings 33 and the peripheries of the units. The units 34 are identical with each other but one is turned degrees about its vertical axis with respect to the other so that, along one vertical plane extending along one diagonal of the units, the diaphragnis of one unit are arched toward the associated slab and the diaphragms of the other unit are arched away from their slab. Thus when the units are connected in circuit, either in series or parallel, so that the voltages or currents produced in the two units by sound waves are cumulative, the voltages produced in the two units by mechanical shock tend to cancel each other.

Inasmuch as piezo-elcctric crystalline plates are Water soluble, it is desirable to protect the crystalline portion of the units from moisture. Therefore, after cementing the various elements, such as the diaphragms, the slab and the electrodes and leads, I propose to coat the exposed surfaces of the crystalline plate or plates and the electroded portions first with a suitable liquid plastic such as, for example, cellulose nitrate in a suitable solvent. Then I apply a final coat of a solution of polystyrene in a suitable solvent. Polystyrene has very little water absorbing properties and a very thin coat provides exceptional moisture-proofing for a device of this type. However it is difficult to cement surfaces coated with polystyrene. I therefore apply the cellulose nitrate or similar material to the crystal portion first, then cement to this coated surface and then apply the moisture-proofing coat or coats of polystyrene, This moisture-proofing procedure is described and claimed in the aforesaid patent.

It should be understood that the present disclosure is for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. A piezo-electric device comprising a slab which expands in one edgewise direction when subjected to electrostatic forces, a diaphragm extending along one side of the device from edge to edge in said direction, and means interconnecting the slab and diaphragm at said edges, the dia phragm being arched in said direction so that it tends to straighten when the slab expands as aforesaid.

2. A piezo-electric device comprising a slab which expands in one edgewise direction when subjected to electrostatic forces and a diaphragm extending along one side of the device from edge to edge in said direction, the diaphragm being cemented to the slab at said edges and being arched in said direction so that it tends to straighten when the slab expands as aforesaid.

3. A pieZo-electric device comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic forces, a diaphragm extending along each side of the device from edge to edge in one of said directions, and means interconnecting the slab and diaphragm at said edges, each diaphragm being arched in one of said directions so that it tends to straighten when the slab expands in that direction.

4. A piezo-electric device comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic forces, a diaphragm extending along one side of the device from edge to edge in each of said directions, and means interconnecting the slab and diaphragm at said edges, the diaphragm being arched away from the slab in one of said directions and toward the slab in the other of said directions.

5. A piezo-electric device comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic forces, a diaphragm extending along each side of the device from edge to edge in each of said directions, and means interconnecting the slab and diaphragms at said edges, each diaphragm being arched away from the slab in one of said directions and toward the slab in the other of said directions.

6. A piezo-electric device comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic forces, a diaphragm extending along each side of the device from edge to edge in each of said directions, and mean interconnecting the slab and diaphragms at said edges, the diaphragms being arched away from the slab in the same one of said directions and toward the slab in the other of said directions.

7. A piezo-electric device comprising a slab which expands in one edgewise direction when subjected to electrostatic forces and a diaphragm extending along one side of the device from edge to edge in said direction, one of said parts having a peripheral shoulder mounted on the edge of the other part and the diaphragm being arched in said direction so that it tends to straighten when the slab expands as aforesaid.

8. A piezo-electric device comprising a slab which expands in one edgewise direction when subjected to electrostatic forces and a diaphragm extending along one side of the device from edge to edge in said direction, the diaphragm having a peripheral shoulder mounted on the edge of the slab and the diaphragm being arched in said direction so that it tends to straighten when the slab expands as aforesaid.

9. A piezo-electric device comprising a slab which expands in one edgewise direction and contracts in another edgewise direction when subjected to electrostatic force, a diaphragm extending along each side of the device from edge to edge in each of said directions, means interconnecting the slab and diaphragms at said edges, each diaphragm being arched away from the slab in one of said directions and toward the slab in the other of said directions, and each diaphragm being arched away from the slab in the direction in which the other diaphragm is arched toward the slab, and means interconnecting the central portions of the two diaphragms through an opening in the slab.

10. A piezo-electric device including two units each comprising a slab which expands in one edgewise direction and contracts in another edgewise didection when subjected to electrostatic forces, a diaphragm extending along each side of each slab from edge to edge in each of said directions, and means interconnecting the slab and diaphragm of each unit at said edges, each diaphragm of each unit being arched away from the slab in one of said directions and toward the slab in the other of said directions, means for, mounting said units with correspunding directions of the two units disposed in planes approximately at right angles to each other so that the voltage produced in the two units by shock tend to cancel each other, and means electrically interconnecting the two units so that the voltages produced in the two units by sound waves are cumulative.

GEORGE C. TIBBEI'IS.

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