US1869556A - Method and means for vibrating crystals - Google Patents

Description

Aug. 2, 1932. E. GIEBE ET AL 1,869,555}

METHOD AND MEANS FOR VIBRATING CRYSTALS Filed Jan. 20, 1928 2 Sheets-Shet l 0 Z Iyiat 1 5 16 1 Z INVENTOR ERKIH GHEBE BY ADQLF SCl-lElBE 1932- E. GIEBE ET AL 1,869,556

METHOD AND MEANS FOR VIBRATING CRYSTALS Filed Jan. 20, 1928 2 Sheets-Sheet 2 0 I I T [3 f INVENTOR ERKH GlEBE BY AboLF scams TORNEY Patented Aug. 2, 1932 UNITED STATES PATENT eFFICE ERTGI-I GIEBE AND ADOLF SC HEIBE, OF BERLIN, GERMANY, ASSIGNORS TO RADIO COR- PORATION OF AMERICA, A CORPORATION OF DELAWARE METHOD AND. MEANS FOR VIBRATING CRYSTALS Application filed January 20, 1928, Serial No. 248,145, and in Germany January 28, 1927.

All of the methods adapted to produce piezo-electrically, elastic vibrations in crystals, are based upon the fact that analternating electric field cutting across the crystal,

by virtue of the reciprocal piezo-electric: ef feet, sets up periodical dilatations and com pressions. In other words, expansional or longitudinal oscillations of the crystal are caused whose amplitude is large as soon as the frequency of the exciting field is in agreement or tune with the frequency of one of the longitudinal natural oscillations which may be either the fundamental or a harmonic of the crystal. Contradistinct therefrom, according to the present invention, fiexural (transversal vibrations) and torsional vibrations are used. The particular means whereby vibrations of this sort are induced constitute the subjectmatter of the present invention which shall be explained hereinafter with the aid of the accompanying drawings in which Figs. la'and 16 show a known method of mounting a crystal so as to set it into longitudinal vibration;

Figs. 2a and 26 show a method according to my invention for causing a crystal to vibrate flexurally;

Figs. 3 and l exemplify other arrangements ao for causing the crystal to vibrate flexurally, the crystals in these arrangements extending materially beyond the electrodes, which extensions by virtue of. the loading effects on the crystals will cause them to vibrate at lower frequencies;

Figs. 5a and 5b are end and plan views respectively of an arrangement for causing torsional oscillations in a crystal;

Figs. 6a and 6?) show arrangements to induce transverse vibrations in a crystalline ring; and

Fig. 7 is another arrangement for producing transverse vibrations in a crystal.

The essential feature of the present invention shall first be explained by a number of arrangements cited by way of example. The piezo-electric material shall be assumed to consist of rock crystal, for example, quartz, of the simplest possible geometrical form, 60 i. e., a rod of rectangular cross-section, al-

where at the same angle (see Fig. 6a) are the neutral axes, and the direction of one thereo as the Y-direction.

The three directions X, Y, Z constitute a rectangular coordinate system.

To more clearly'understand what follows, a sketch is first shown in Figs. 1a and lbof a known arrangement for the purpose ofin-V ducing longitudinal vibrations. The same represents essentially a condenser comprising the field-generating coats or armatures or electrodes E1 and E2, and the quartz rod Q, whose orientation can be seen from the coordinate system X, Y, Z, shown conjointly therewith, constituting the dielectric.

The efiect of the electrical field, that is, reciprocal piezo-electric effect, consists here of a dilatation of the rod in the Y-axis and a simultaneous compression in-the X-axis, or vice versa. The electrical field. in this arrangement in homogeneous, both in each con stituent cross-section as well as throughout the length of the rod. As a result, longitudinal vibrations-are set up. However, even if the electrodes E1 and E2 are shorter than the length of the rod, so that, for instance, they cover only a small fraction of 1 the length of the rod (see, e. g., the arrangements of E. Giebe and A. Scheibe published in Elektrotechnische Zeitschrift VOL/1:7, p. 380, 1926) so that the field is no longer homogeneous, the electrical field inside each cross-section is everywhere appreciably equal as regards intensity and direction, and it changes from onesection to another only in intensity, while the direction remains every-- where appreciably the same. The small volthe rod is therefore uniformly deformed. at

'ume element between the short electrodes of V each point, with the result that only longitudinal vibrations can be produced.

(1). An arrangement adapted to produce piezo-electrically, transversal vibrations according to the disclosures of the present invention is shown by way of example in Figs. 2a and 2b. The orientation of the quartz rod with relation to the crystal axes is the same as in the known arrangements illustrated in Figs. 1a and lb. However, instead of the two electrodes as in Figs. 1a and 15, four electrodes E1 to are here used which are connected in pairs in a way as indicated by polarity signs and and which are brought to an alternating potential source.

By means of such an arrangement and mode of connection of the electrodes, there arises a non-uniform electrical field such that the latter in one half of the cross-section has the opposite direction compared with the other half. The electrical field between E1 and E2 produces a compression in the X-axis the upper half of the rod, and a dilatation in the Y-a-xis (or vice versa). The field between E3 and IE4 at the same instant produces a dilatation in the X-axis in the lower half, and a compression in the Y-axis (or vice versa). By virtue of such deformations, flexures are caused in the rod, and these result in resonance oscillations whenever the frequency of the exciting alternating potential equals one of the natural transversalperiods of the rod. Bending of the rod takes place in the direction of the Z-axis.

' These transversal vibrations, though to a lesser degree, may be set up also when done of the pairs of electrodes, say, the upper one E1132 is removed.

In this case, to be sure, the electrical field in the upper half has not the opposite direction, but a very small and nearly vanishing intensity compared with the field between E3 and FA. As a result, only the lower half of the rod is subject to deformation, and thus also a flexure is caused. V I

(3'). In what way, accordlng to the present invention, a rod'orientated in relation to the crystal axes as shown in Figs. 2a and 2b, can be made to undergo transversal vibrations in the direction of the electrical axis is shown by way of example by the arrangement in Fig. 3, which comprises again four electrodes. But these, contradistinct to Fig. 2a, are not disposed in the Z-axis above each other, but in the Y-axis side by side. The electrodes are connected in pairs in a way as indicated by the and signs, and are brought to an alternating potential source. The essential feature also in this case is that by virtue of the disposition and connection of the electrodes, an electrical field isproduced in a volume element of the rod between the electrodes subject to variati'ns both as to size and direction and of such nonperiodical uniformity that fiexures and thereby transversal vibrations in the X-axis are caused.

(4). The orientation of the rod by no means has to be chosen in a way as assumed in Figs. 1 to 3, indeed, according to the present invention also rods of different orientation can be induced to undergo transversal vibrations. This is shown by way of example in Fig. 4 where the longitudinal axis of the rod falls in the direction or the X-axis and not in the direction of the Y-axis as before shown. Also here four electrodes connected as shown may be employed, and these electrodes are disposed side by side in the X- axis. Inthis case, for instance, dilatations may be set up in the upper half of the rod as shown in the drawing in the portion of the rod between the electrodes E1 and E2. while simultaneously in the lower half a compression in the direction of the X-axis is produced, and this again results in flexural vibrations in the Y-axis. It is to be noted that in Figs. 3 and l the crystal extends beyond the electrodes for some distance. In this way loading of the crystal is accomplished and accordingly lower frequencies of flexural" vibrations are obtained.

(5). In an arrangement as shown 1n Fig. 4, for the same reasons as mentioned in (9.)

transversal vibrations can be set up by means of two electrodes, E1, E2 or E3, E4, instead of of by four-electrodes, although less readily so.

Turning the rod with an arrange- 4 an angle of 9O degrees about the X-axis, whilethe electrodes remain in their plane so that the Z-axis comes to coincide with the plane of the drawing. whereas the Y-axis is at right angles thereto, transversal vibrations falling in the direction of the optical axis will be produced with the same connec tion of the exciting electrodes.

(7). Torsional vibrations may be generated, for instance, in an arrangement as shown in Figs. 5a and 5?) when the rod is orientated as shown in the drawing. The four electrodes which are disposed above one another with reference to the Z-axis are a ain connected in iairs accordin to like signs. The non-uniformity of the-field is of such a nature that twisting about the longitudinal axis of the crystal is produced.

This may be explained more fully as follows. piezoelectric reversal effect, that what is de cisive is the action of an electric field in the direction of the electric axis, that is to say, in the designation here used as a basis, in the X-axis, while an externally applied electrical fieldin the direction of the optical axis (Z) or in the direction of the neutral axis (Y) occasions no appreciable pressure alteration in the crystal. Hence, a potential difierence set up between the electrodes E1- 1 It is known in connection with the and E2 in the scheme shown in Fig. a is unable to occasion either a dilatation or a compression in the direction of the Y-axis. Since the electric field between the superposed electrodes E1 and E3 or between the electrodes E2 and E4, which are likewise superposed acts in the direction of the optical axis (Z), no compression or dilatations need be expected by these fields either in the various parts of the crystal. Indeed, appreciable mechanical forces can become effective only in direction X.

What electrical fields exist in direction X can be seen from the arrangement Fig. 5?). Appreciable components in direction X are furnished by the stray fields from the left and the right lateral surface of the electrode E1 to the left and the righthand lateral face, respectively, of the electrode E3. 'The components of the force lines acting in the direction of axis X going from the left and the (ill typical of tortion.

right lateral surface of the electrode E2 to the left and the right lateral surface of the electrode E l have to be taken into account upon the posterior face of the crystal bar in the shape of compression a dilatation, as the case may be, in the direction of axis X.

Now, if the assumption be made that at certain instant the components of the electric ,field, actingat the right-hand lateral surface of E1 decreasing in the direction of the positlve X-axis results 111a compression 1n direction X, then a dilatation is obtained on the right lateral surface of electrode E3, further at the left lateral surface of-electrode E1 a dilatation and at the left lateral surface of the electrode E3 a compression.

Since the corresponding charges upon the posterior electrodes are exactly interchanged or transposed, it'will be noted that for each compression upon the posterior face there is a dilatation upon the anterior face, and for each dilatation upon the posterior face a compression occurs upon the anterior face. And this is the typical tensional distributi on for torsion.

Nhat'happens is that a small portion of the surface of the bar coordinated to the right-hand lateral surface of electrode E1 undergoes compression and is incidentally lifted out forwardly from the plane of oscillation. The same situation holds true of a small particle of the crystal surface coordinated to the left-hand lateral surface of the electrode E3,'while small portions of surface upon the left-hand side of the electrode El and the right-hand side of electrode E3 are bent back. Since upon the posterior face the tensions are interchanged or reversed, there is obtained a saddle surface Thus the crystal bar illustrated in Figs. 5a and 5b is twisted about its longitudinal axis.

The arrangements hereinbefore described are merely given by Way of example. There according to like signs.

electrodes must then be chosen so as to suit the particular piezo-elect'ric properties of the kind of crystal that is used. Instead of rods, also other geometrical forms, for instance, plates or rings, may be employed.

I (8). For instance, in Fig. 6a, an arrangement adapted to excite transversal vibrations in'a quartz ring is shown. The optical axis is at right angles to the plane laid through the ring, in other words, it falls in the plane of the drawing. The electrical axes are indicated with their signs. In such a ring,

the piezo-electric vector .changes along the circumference as to size and direction. It

is of opposite sign uponboth sides of a neu- .tral axis. 'Hence, in this case, non-uniform deformations required for'the production of transversal vibrations in-the plane of the ring can be produced by means of two electrodes E1 and E2 disposed at a neutral axis V as shownin Fig. 6a.

The ring may also be excited in an electrical arms. In this case,"four electrodes are suitable to be used, as shown zin Fig-. 6b,

the electrodes having to be connected in pairs The flexural oscillations in such a ring are to be defined in analogy with the flexural oscillations in a rod or bar. For this .pur-

pose one has to conceive the annular bar to be cut open at a certain cross-section and to be stretched out. For instance, if the ring Fig. 6a is cut open at onepoint and is then stretched orstraightened out, in the middle of the bar or at one of the ends thereof and then, according to the position of the electrodes, axial orientation or fiexure of the bar as in Figs. 2 is obtained. Hence, it is feasible to bend the ring out of its plane forwardly or rearwardly, although it is likewise possible to periodically compress it in its own plane or to expand it (drawit apart). In short, all of the considerations made with referenceto bars or rods apply equally to a ring.

. By adopting similar electrode arrangements as for rods, also rings may be caused to undergo torsional vibrations or transversal vibrations at right angles to the ring plane.

(9). An entirely different shape of the crystal together with an electrode arrangement suited for this case for the purpose of producing transversal vibrations is illustrated in Fig. 7. i

(10). In the arrangements hereinbefore shown, either two or four-electrodes have -been provided. However, to insure stronger vibrations, it may often be an advantageous plan to use a greater number of electrodes whose fields are brought to act at different points of the rods, and which, by choosing proper polarities, may be made to assist each other in their effects.

(11). The presence of resonance between the frequency of transversal or torsional vibrations of the rod and the frequency of the alternating field that is applied as well as the ordinate number of the overtone or harmonic which is active, can be seen from the arising of luminous actions at the crystal rod when vibrating inside a vacuous space and from the luminous figure (see our United States Patent Number 1,685,810 or our German Patent 467529). The presence of a Vacuum at the same time tends to diminish the damping of the rod.

As regards the technical and practical usefulness of the present invention the following may be pointed out: a

1. Crystals which are undergoing transversal or torsional vibrations may be used for all such purposes for which heretofore longitudinal vibrations have been used.

2. Transversal vibrations and torsional vibrations which may be produced piezoelectrically according to the disclosures of the present invention, oifer this-advantage over longitudinal vibrations that a far larger range of frequencies of elastic natural vibrations is obtained. It is a well known fact that owing to the natural limitation of crystal sizes, quartz rods of more than 10 to 15 cm. length are practically notproducible. The lowest'longitudinal natural frequencies of such rods lie betwen 20000 and 30000 Hertz units. In the case of transversal natural frequencies, the lowest figures for rods of equal length range between 1000 and 3000 Hertz, in other words, they fall insi-dethe audible range of waves.

3. In the case of torsional and transversal vibrations, the limits of resonance frequencies can be influenced and varied if desired by securing weights on the crystal pieces, so that very low frequencies can be obtained. This is not possible with longitudinal vibrations. In Jigs. 3 and 4, it is shown how the'crystal itself may be utilized for loading so that lower frequencies may be obtained.

4. Quartz rods of low natural frequency can be used for the same purpose for which tuning forks are employed. Hence, when excited piezo-electrically, they can be used as high-precision acoustic standards.

5. In so far as the radio frequency range in radio-telegraphy is concerned, this advantage can be insured that the tranversal natural frequencies, owing to their far higher ordinate numbers fall much closer together inside this range than the longitudinal natural frequencies. Hence, when using transa crystal, having a relatively thick portion intermediate its ends and relatively long extensions projecting from said thick portion, and, electrodes positioned at the thick portion of the crystal.

2. A flexural piezo-electric vibrating system adapted to be set into continuous flexural vibration by the application of alternating potentials thereto comprising a piezo-electric crystal element having a relatively thick center and relatively long extensions attached thereto; and, electrodes for applying alternating potentials to the thick center of the crystal for cauing stresses therein, setting the crystal into flexural vibration, the frequency of which is decreased due to the mechanical loading effect of the crystal extensions.

3. In a flexural vibrating system wherein alternating potentials are piezo-electrically transformed into flexural vibrations, a crystal having a relatively thick center and relatively long extensions attached to the center; and, pairs of electrodes positioned at the center of the crystal for applying potentials thereto to cause stresses therein which in turn tend to flex the crystal, said stresses being set up when said electrodes are excited with alternating potentials such that each pair of adjacent electrodes are of opposite instantaneous polarity.

4. A resonator adapted to have elastic vibrations induced therein for undulatory electrical current purposes comprising a piezoelectric crystal long relative to its Width.

and thickness, and having a portion intermediate its ends projecting therefrom transversely to the longitudinal dimension of the crystal.

5. A resonator adapted to have elastic vibrations induced therein for undulatory electrical current purposes comprising a piezoelectric crystal long relative to its width and thickness and having rectangular portions projecting transversely therefrom relative to its longitudinal dimension, said rectangular portions being located substantially at the geometrical center of said crystal.

6. Resonant apparatus adapted to have elastic vibrations induced therein for undulatory electrical current purposes comprising a piezoelectric crystal long relative to its breadth and thickness having a portion intermediate its ends projecting therefrom transversely to its longitudinal dimension, in

combination with exciting electrodes placed adjacent said transversely extending portion.

7, Resonant apparatus adapted to have elastic vibrations induced therein for undulatory electrical current purposes comprising a piezo-electric crystal of angular shape, said crystal being long relative to its Width and thickness and having rectangular pro- 5 jecting portions projecting from the center thereof and substantially at right angles to its longitudinal dimension in combination with exciting electrodes placed adjacent said projecting rectangular portions. 7

ERICH GIEBE.

ADOLF

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