US1977169A - Piezo-electric system - Google Patents

Description

Oct. 16, 1934. w CADY 1.977,16 9 I PIEZO ELECTRIC SYSTEM Filed Dec. 1931 3 Sheets-Sheet l E :1. c:-r... 1.

ATTCJRN EY Z INVENTOR x Walter I [13:15

7 Oct. 16, 1934. w. G. CADY PIEZO ELECTRIC SYSTEM 3 Sheets-Shet 2 Filed Dec. 1931 IDLE- 7 80 EEI- 1EI L] I i ATTORNEY Patented Oct. 16, 1934 rmzonmormo SYSTEM Walter G. Cady, Middletown, Conn. Application December 17, 1931, Serial No. 581,630

6 Claims.

My invention pertains in general to piezoelectricity and specifically relates to means for producing vibrations in, piezo-electric bodies by shearing stresses.

One of the objects of my invention consists in providing an electrical system having a piezoelectric bodyf-s'et in vibration by shearing stresses.-

posite directions to produce shearing stresses for causing piezo-electric vibrations in predetermined modes, such as ilexural or torsional vibrations.

I accomplish these and other desirable objects in a novel piezo-electric system having means for causing shearing stresses in a piezo-electric element to produce vibrations therein.

In the drawings which accompany and form a part of this specification and in which like refer ence numerals designate corresponding parts throughout:

Fig. 1 is a perspective view of several crystal plates showing their orientation with respect to the crystallographic axes of the natural crystal;

Fig. 2 is a schematic plan view of a Rochelle salt crystal'plate, together with an electrical circuit employed therewith, in one embodiment of my invention;

Fig. 3 is a side elevation of the representation of Fig. 2;

Fig. 4 is a representation corresponding to Fig. 2 but showing a different arrangement of the electrical circuit; H Fig. 5 is a theoretical representation of the deformation of a crystalplate in accordance with my invention;

Fig. 6 is.a'schematic plan view of a Rochelle salt crystal'plate and circuit employed therewith, for producing overtone-frequericies;

Fig. 7 isa side elevation of the representation of Fig. 6; 3

Fig. 8 is a schematic plan view of a quartz crystal and circuit employed therewith for pro ducing flex'ural vibrations in accordance withmy invention;

Fig. 9 is a schematic elevation of the representation of Fig. 8 showing the deformation of-a crystal at a particular instant;

Figs. 10-12 are schematic representations corresponding. to Fig. 9 but indicating different orientations of crystals that may be used in the system shown in Fig. 8;

Fig. 13 is a schematic representation of a quartz crystal and circuit employed therewith in a further embodiment of my invention;

Fig. 14 is an end elevation of the crystal and electrodes of Fig. 13;

Fig. 15 is a plan view of a quartz crystal and the electrodes employed therewith in one embodiment of my invention for producing torsional vibrations;

Fig. 16 is a side elevation of the crystal of Fig. 18 together with the electrical circuit employed therewith; and

Fig. 1'? is an end elevation of the crystal of Fig. 18 showing the deformation of a transverse section thereof in accordance with the principles of my invention.

My invention is directed to providing means for producing shearing stresses in a piezoelectric body for setting up vibrations therein in pre-- determined modes. The use of such vibrations as produced according to my system may be of extremely low frequencies as compared with the frequency of piezo-electric vibrations produced by systems heretofore known. In the present state of the art, in order to produce, by modes of vibrations now known, piezo-electric vibrations of frequencies even approaching those which I have obtained in the systems of my invention, it would be necessary to provide very large piezo-electric elements as compared with the size of those which I employ. It will be easily understood that use of large piezo-electric elements, especially of quartz and Rochelle salt crystals, is undesirable and prohibitive owing to their high cost and easy breakage.

In the present specification I will disclose how shearing stresses may be produced in both Rochelle salt crystals and quartz crystals for producing predetermined modes of vibrations therein. It will be understood, of course, that although I illustrate my invention in connection with Rochelle salt crystals and quartz crystals, other piezo-electric bodies having like characteristics may be similarly employed. Further, it is to be understood, throughout the following description, that thesource of energy for supplying voltages to-excite the crystals is adjusted so that the' frequency of the applied alternating voltage is equal to the natural frequency of vibration of a particular crystal with reference to the desired mode of vibration. This natural frequency is, of course, dependent upon the material of the crystal, that is, whether it is comshown. I

are centered at the nodes 7a and'7b of the plate.

the. crystal.

chelle salt crystals and quartz crystals, and the. production of torsional vibrations in Rochelle salt crystals and quartz crystals.

Flexural vibrations in Rochelle salt crystals Referring to the drawings, Fig. 1 is a perspective representation of several crystal plates of different orientations showing the relation to the X, Y, and Z axes of a natural crystal. The three axes here referred to astheX, Y, and Z axes are the same, respectively; as the a, b, and c, axes in the terminologycommonly employed by crystallographers. Plates A and B are X-cut plates, sincethey are the form of plates'or. flat slabs cut perpendicular to the X axis. Similarly, the plates C and. D are Y-cut, and the plates E andFare Z.-cut. Such nomenclature-is in accordance with accepted usage.

Heretofore in the art it has been customary in'the' case of X-cut, Y-cut, and'.Zcut plates, to apply'the electricfield in a direction parallel to the X, Y, and Z axes, respectively. In accordance with my system for producing fiexural vibrations, the electric held for any given crystal platev is appliedin a direction parallel to the breadth of the plate. r

Fig. 2 illustrates how a particular one of the plates shown in Fig. 1 may be utilized in accordance with the principles of my invention for producing fiexural vibrations. Withrespect to orientation, the plate" 1 in Fig. 2 will be considered as corresponding toplate E of Fig. 1. Plate 1 is therefore a Rochelle salt Z-cut plate having the length and breadth in the .XY plane and the thickness in the Z plane. Fig.5 is a side elevation of the plate 1 of Fig. 2 taken in the XZ plane. Y 1

Referring to Fig, 3, two triangular strips '7 and 8 are provided having their thin edges supporting the plate 1 across the breadth of the plate 1 at the regions wherethe nodes of vibration occur. These nodal regions are indicated in Fig. 2 by the dotted lines 7a and 7b. Obviously we may use for '7 and-8 twonarrow strips of any suitable solid material extending across the breadth of the crystal plate. Referring to Fig. 2, thereare provided electrodes 2, 4 and 3, 5 disposed upon opposite sides of plate 1. as The pairs of electrodes 2-4 and 35 1, respectively, for producing electric fieldsin directions parallel to the breadth of, the plate 1 mounted as closely as possible to the crystal with-1 out actually touching it. If preferred, verythin conducting electrodes may be applied directly to Itwill be understood that these electrodes, together with the supporting strips '1 and 8; are mechanically mounted in any suitable housing structure, which, for the sake of'sim plicity, is not shown in the drawings.

In Fig. 2, the electrodes 2 and 5 are connected directionsg" d-e Fo n rse iq lle t a a in i. 1..; s- .w e-eon de iel at: e r

together, and electrodes 3 and 4 are connected together in circuit with a source of energy 6 for supplying alternating voltages across electrode pairs 24 and 35. It will be evident, from the connection, that adjacent electrodes 'on either side of the crystal plate will be of opposite instantaneous polarityrand that, therefore, the fields produced between the pairs of electrodes 2-4. and .3 5,;respect ivel w' opposite Fig.4 illustrates an alternative method of conmeeting" the electrodes 2-4 and 35. It will be seen that the-electrodes 2 and 3 are connected withthe source. of energy 6 and that the electrodes 4 and 5 are interconnected. This form of 'connectionwillflalso produce fields of opposite directions between the electrode pairs when alterhating voltages are supplied from the source of energy 6, vThe plus and minus signs in Figs. 2 and 4 are indicative of the relation of the polarity f he e t des I a pe i ul rrin an r Fig} 5 an exaggeratedre 'r esentation 0 deformation-of pla te l due..- to ;shearing as produced in the circuit arrangements .of liigs.

. el e d en i d as the left half,- under imue nce oft duced between the s s trpde s -r 13. ree respectively. Under influence of an elec,tri in ed r ct o e-fBf-iha Q1 h- 2l3e- W be deformed by shearing stresses"intoa parallelo grarnzxconfiguraticn shown, while. the half,v under influence of. an electric, field {i p i m-t n. l ..b :d 9 l b S1 res n a ara elo a t nnage opposite direction; When the fields all. indirectiomthe 5B and ffLZhalves-oflthe "1 l will. b e o medbr.s a n ire f' es d sQ Q as howabrth & it allelograms in Figl,;5 Underinflue o hating electric. fields; ff. opposite directions, lop posed shearing stresses ,will pesetlup in theplate -1.whereby-theplatewill'beset. brations. The representation of Fr sponds to that of Fig. ,3,.the. .plate 1 being" as an elevation inxthfi p ane and the electric fields being applied indirectionslpe to thepaper; p H .1

Although the initial; Stresses produc w plate l -areof the naturev onpu'rezsh argt e ate presulting flexural vibrations are det rrn ned .,,as to. frequency by; he elasticity.of compression of the piezo-electric material. It will e 9fi d1 n-Fi I etthe lat l. 1 ha l n th approximately, t i I he-b1" which I found, in practice, to .beja, anv s 7 ratio.. I Iowever,. the exact alue of bieadt the plate is a matter of i"? 'pnsegu frequency or theyibratior'i is essence only'f'o'ii the lengthand. thickness, There is nothing principle to prevent ha virr "the" breadth'ev en greaterthan' thelength, if desired. rerexam re; in the caseof plate -1' which" corresponds to"'t'he plate of Fig.- 1, we-"might regard thefdiiri'e sions parallel tothe Y and X axes as the asset-n and'breadth, respectively?- We would then place the electrodes at the ends of ==the plate; instead of i along thesides,'-so that the electric field's would be in directions parallel to the X axis. The plane 3H5 of flexure would then -be-Jin the planecontaining .the Y and :Zax'es Si'milarly,by placing-the' -elec trodes so that the-electric field-is parallel 'tQEJthe Z axis 'fle'xural vibrations; as produced-by shearing stresses, would take placein the.XY.. plane-r 5 Thus, from one Rochelle salt plate, we may obtain three different fundamental frequencies of fiexural vibration. In each case, the particular frequency will depend, according to formula: for

.flexural vibration, on the two dimensions of the platethat are at right angles to theelectric field,

and on the value of Youngs modulus in the direction of the effective length of the plate.

Where only one frequency is desired it may be found advisable, for economy of material and also to secure a greater field strength with a given supply voltage, to make the breadth of the plate, that is, the dimension parallel to Y in Fig. 2, even less than the thickness. This applies to all of the types of flexural vibrator described below.

In the foregoing, consideration has been given to. the use of plate E of Fig. 1 for producing flexural vibrations by shearing stresses. The other plates shown in Fig. 1 may also be utilized for the production of fiexural' vibrations in a similar manner. For example, for plate F, the electrodes would be placed so as to cause the electric ing, in dotted lines, in an exaggerated manner, the

deformation of thecrystal plate, at a particular instant, due to shearing stresses.

I Referring to Fig. 6, the plate is provided with pairs of electrodes 1112, 13l4, 15-16, and 17-18. In the embodiment shown, the electrodes 11, l4, l5, and 18 are connected together and the electrodes 12, 13, 16, and 17, are connected together in circuit with a source of energy 6. The source of energy 6 is any suitable means for supplying voltages to the electrodes just mentioned for producing alternating electric fields therebetween. Owing to the connections of the electrodes, adjacent pairs of electrodes produce fields having different instantaneous directions.

The nodes of vibration of the plate 10 are indicated by the dotted lines 22, 23, 24, and 25.

, As in Fig. 3, the plate is supported by strips 7 and 8 positioned transversely of the crystal at two of the. nodes as 23 and 24. The electrodes positioned on either side of the plate 10 are of a size such as to extend substantially from anti-node to anti-node.

To excite the nth overtone of the fundamental frequency, it has been found that, for best results, n+2 pairs of electrodes should be used, the connections with the source of energy being such that electric fields simultaneously produced by the adjacent pairs of electrodes on either sicle are of opposite direction.

Owing to the electric fields between each pair of the electrodes, shearing stresses will be set up in each portion of the crystal'plate 10 between each pair of electrodes whereby the crystal as a whole will be deformedas exaggeratively depicted for 'a particular instant by the dotted line 10b 7. The deformation will take place inthe opposite direction when the fields are reversed, whereby the plate 16, under the influence of the alternating electric fields will be set into fiexural vibrations at a frequency which'is an overtone of the fundamental flexural frequency,

Flewural vibration in quartz crystals In accordance with my invention use is made of opposed electric fields applied to a quartz plate for producing shearing stresses for causing flexural vibrations in accordance with either of two equations, relating electrical fields with mechanical shears, which, for quartz, may be written thus:

In the two foregoing equations, E1 and E2 represent electric fields parallel to the X and Y axes,

respectively, and Yz and 2x are the shearing stresses about the X and Y axes, respectively, while an is a piezo-electric constant of quartz. In applying these equations I find it most convenient to express the stresses in dynes per square centimeter and the other quantities in electrostatic units.

In accordance with one embodiment of my system, I provide a quartz plate 40 having its length, breadth and thickness parallel tothe Z, X, and Y axes, respectively, as shown in Fig. 8. Two pairs of electrodes 4l42 and 43-44 are positioned as shown. The electrodes 4244 are connected together, and the electrodes 4143 are connected to the source of energy 6. The source of energy 6 supplies voltages for producing alternating electric fields between the electrode pair 4l42 and the electrode pair 4344, the field between the electrodes 41 and 42 being in a direction opposite to the direction of the field produced between the electrodes 43 and 44. The plus and minus signs in Fig. 8 are indicativeof the relative polarities of the electrodes for a particular instant. The electric fields traversing the plate 40 in directions parallel to the X axis produce shearing stresses about the X axis, that is, in the YZ plane. Electric fields produced between the adjacent pairs of electrodes are in opposite directions so therefore the shearing stresses are also in corresponding directions.

Fig. 9 is a side view in the YZ plane of plate 40. Figs. 10, 11, and 12, are similar views of quartz plates having different orientations with respect to the crystallographic axes and which may be utilized in accordance with my invention.

The dotted line 40b in Fig. 9 is exaggeratively indicative of the deformation of the plate 40, at a particular instant, as caused by shearing stresses in the right and left hand halves of the plate produced by the electric fields of different directions between the electrode pairs 41-42 and 4344.

In conformity with Equations (1) and (2) the method of flexural excitation of quartz plates is applicable to plates cut with any of the orientations shown in Figs. 9-12, inclusive, when electric fields are applied in directions perpendicular to the plane of the paper. In each instance the shear takes place about the third axis perpendicular to the paper, that is, the plane of the shear is in the plane of the paper. In accordance with the principles of fiexural vibration, the natural frequency of the plate will be decreased as the longest dimension of the plate is increased, and the shorter dimension of the plate is decreased. Equation 1 applies to Figs. 9 and 10 and Equation 2 applies to the Figs. 11 and 12.

In experiments, a plate such as in Fig. 9 was employed, in which the dimensions parallel to the X, Y, and Z axes were 20 millimeters, 3 millimeters, and millimeters, respectively. Such a piezo-electric plate may be used as an oscillator as disclosed) in *my' Patent Number 1,472,583,,issued October 30, 1923.

Another method for producing fiexural vibrations by shearing stresses ,in quartz plates is based on the equation wherein E2 is the impressed electric field parallel to the Y axis, e11 is the appropriate piezo-electric constant, and X represents a shearing stress about the Z axis. In thisinstance the equation is the same that governs the vibrations commonly employed in the well-known Y-cut plates. However, according to my invention, I dispose the electrodes in a manner such as to cause flexural vibrations to be generated about the Z axis, that is, in the XY plane. This arrangementis illustrated inFig. 13. l

In Fig. 13, aquartz plate is provided'having the length and thickness parallel to the Y and X axes. Electrodes 5 1and 52 are positioned at opposite ends around rod 5 3 while an encircling band electrode53 is centrally disposed around the plate 55. Electrodes 51' and 52 are connected together in circuit with the source of alternating voltage 6, which is also connected to the electrode 53. In accordance with my invention, forthe fundamental frequency of vibration, theplate should be so mounted that supports land 8 come approximately 0.22 of the length from 'each end at the points 55 and 56' in Fig. 20. .Tlie node's of-vibration'are located in these regions. 'When the source of energy 6 supplies voltages of the proper frequency to the electrodes, electric fields are set up traversing the rod 50 in directions extending from the electrode 53 to the electrodes '51 and 52, or vice versa, depending upon the polarity of the electrodes. ,Opposit'e shears are thereby produced in the 'rod'50'for producing fiexural vibrations. The source of energy 6 supplies alternating voltages preferably of a fre quency for producing resonant flexural vibrations of the fundamental frequency of "the crystal rod 50. In order tostrengthen the electric field and to concentrate it most effectively it may be found advantageous to have electrodes 51, and 52 in the form of bands (later described in connection -with Rochelle salt) 72 and '73 s'hownin Fig. 17

havingX, Y, and Z, dimensions of 0.1 cm, 9.?

cm, and 0.5 cm, respectively, which, as a master oscillator produced'flexural vibrations at a frequency of 3,000 cycles per second. This ire--v quency is believed to be, by far, the lowest frequency yet'produced in a quartz plate utilizedas an oscillator.

Torsional vibrations in Rochelle salt crystals 1 Torsional vibrations in Rochelle saltplates take These three equations are, of course, thesame that express the stresses employed in the flexural vibrations with quartz described. above. In Figs.

1-l2the plates-are so cut and the electrodes so disposed that thestresses set up in accordance with these equations produce, under the influence of' alternating electric fields of the proper fre-. 'quency, iflexural vibrations .as previously de? scribed. .I shall now show. how, by a different arrangement of electrodes, the same shearing stresses maybe caused to set up torsional vibrations. It is, of course, assumed that the applied voltages shall in each case be ofa frequency substantially equal to the natural frequency of torsional vibration of the crystal plate. In the foregoing: three equations E1, E2, and E3 represent impressed electric fields parallel to the X, Y, and-Z axesrespectively, e14, 025, and cat, are the threepiezo-electricconstants, and Yz, Zx, and X are shearing stresses about the-X, Y, and Z'axes respectively; The plate is preferably in the form of a rod with its length parallel to one of the crystal axes... The cross-section may be circular, square, or" of other convenient shape. In the present description the cross-sectional form is, for convenience, assumed to 'be rectangular.

, Torsional'vibmtions in quartz crystals Consideration will now be given to producing torsional vibrations in quartz crystals by means of shearing stresses in'accordance with my invention; To obtain torsional vibrations about the X or Yaxes the quartz plate is orientated and'the electrodes disposed so as to conform to Equations (1) and (2), respectively. For producing torsional vibrations about the Z axis, conditions must prevail in accordance with Equation (3). As explained above in connection with torsional vibrations in Rochelle salt, so also in the case of quartz itis true'that the same shearing stresses that produce flex'ural vibrations can, with proper orientation of" the crystal plate, disposition of electrodes, and choice'of frequency, be made to produ'cetorsional vibrations'as well. The arrangement based'on Equation (3) will be considered first;

-A quartz crystalplate 80, Figs. 15 and 16, is provided having length, breadth, and thickness parallel to the Z, X, and Y axes, respectively. The two pairs of electrodes 8182, and 83--84, are provided as shown on opposite sides of the plate 80 and-are connected to the source of energy 6 for producing electric fields which, in Fig. 15 will be perpendicular to'the plane of thepaper but always 'in opposite directions. The field parallel to the Y axis produces a shearing stress about the Z axis. Fig. 17 represents the resulting deformation of a rectangularsection of plate 80 into the" dotted parallelogram. Such a deformation will takeplace for both right handand left hand halves of the crystal but in opposite directions due 1 of energy 6 supplies alternating voltages of a frequency proper. for. producing resonant torsional vibrations in the plate 80. The plate may then be used as 'a resonator or oscillator. This mode of vibration is especially useful and desirablefor the production of extremely low frequencies of vibration; For' producing low frequencies, the Z L dimension should" be relatively other dimensionssma'll. Y

In order to producel-torsional vibrations in large and the quartz about the X'or Y axes, in conformity with Equations (1) or (2), I 'dispose'the electrodes as "lol in Fig. 13. The quartz plate is so oriented that the electric field lies in the direction of that axis about which torsion is to take place, which may be either the X or Y axis.

It will be apparent from the foregoing that my invention provides means for producing torsional and flexural vibrations of low frequency in piezoelectric bodies through the agency of shearing stresses.

In practicing my invention, it is not essential that the quartz or Rochelle salt plates or bars be precisely oriented with respect to the crystallographic axes. In general there will be a component of electric field capable of producing the desired result even though the geometric axes of the plate depart somewhat from the crystallo graphic axes. Further, it is not essential that the cross-section be everywhere the same. For example, in order to secure still lower frequencies it is possible to load the plate or bar at each end, either by cutting it from the parent crystal with the ends relatively wide or thick, or by afiixing a mass of metal or other suitable material at each end.

One of the chief advantages to be derived from my invention resides in that vibrations of extremely low frequencies can be produced with great stability and precision. Any of the crystal plates herein disclosed can be utilized as resonators or oscillators in conjunction with appropriate electrical circuits. Forexample, the crystal plates can be incorporated in the electrical circuit disclosed in my Patent Number 1,472,583, as before stated. Although I have disclosed my invention in certain forms and modifications, I do not desire to be limited thereto except insofar as may be a pointed out in the appended claims.

What I claim as new and original and desire to secure by Letters Patent of the United States is:

1. A piezo-electric system comprising a Rochelle salt plate having its length parallel to one of the crystallographic axes, a plurality of electrodes disposed upon opposite'sides of said plate and in planes parallel to said crystallographic axis and one of the other two crystallographic axes, a

source of alternating voltage, connections between said electrodes and said source of voltage whereby adjacent electrodes on either side of said plate are of different relative polarities for producing electric fields traversing said Rochelle salt plate in opposite directions to produce adjacently opposed shearing stresses in said plate to cause the same to vibrate fiexurally, said electrodes being of a configuration such as to extend approximately from anti-node to anti-node of the vibration.

2. In a piezo-electric system, a Rochelle salt plate, and means for causing said plate to vibrate at the nth harmonic of the fundamental frequency of flexural vibration comprising, n+2 pairs of electrodes, each of said pairs comprising electrodes disposed on opposite sides of said plate, and means for energizing said electrodes to produce alternating electric fields traversing said piezo-electric element, adjacent electrodes on either side of said piezo-electric element being of opposite instantaneous polarity.

3. A piezo-electric system comprising, an elongated piezc-electric element the major dimension of which is in a direction substantially perpendicular to the optical and electrical axes thereof, means for producing an electric field traversing said element alternately from the central portion thereof towards both ends and from the ends thereof towards the central portion in directions substantially parallel to the direction of said 95 major dimension for setting up opposite shearing stresses in said element to cause the same to vibrate.

4. A piezo-electric system comprising, a rectangular plate of Rochelle salt having its dimensions parallel to the three crystal axes respectively, pairs of electrodes operatively related to said plate to cause said plate to flexurally vibrate in a particular plane and at a particular predetermined frequency, and means including said electrodes to produce opposing alternating electric fields traversing said plate in directions perpendicular to said plane of flexural vibration.

5. A piezo-electric system comprising, a rectangular plate of Rochelle salt having its dimen- 7 sions parallel to the three crystal axes respectively, two pairs of electrodes operatively related to said plate to cause said plate to vibrate fiexurally in a particular plane and at a particular predetermined frequency and means including said electrodes to produce opposing alternating electric fields traversing said plate in directions perpendicular to said plane of flexual vibrations.

6. A pieao-electric system comprising, a quartz plate and means including electrodes positioned lftil on either side of said plate for simultaneously producing alternating electric fields traversing one of the smaller dimensions of. said plate for producing opposing shearing stresses in said plate to cause the same to fiexurally vibrate in a plane at right angles to a line drawn between the centers of said electrodes.

WALTER G. CADY.

its

Images