Design of electro-mechanical transducer elements

Aug. 18, 1959 μ. μ. palo 2,900,536

DESIGN OF ELECTRO-MECHANICAL TRANSDUCER ELEMENTS

Filed Nov. 18, 1954

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ATTORNEY

United States Patent Office

2,900,536

Patented Aug. 18, 1959

2,900,536

DESIGN OF ELECTRO-MECHANICAL TRANSDUCER ELEMENTS

Maurice M. Palo, Conneaut, Ohio, assignor to The Astatic Corporation, Conneaut, Ohio, a corporation of Ohio

Application November 18, 1954, Serial No. 469,617

Claims. (Cl. 310—9.6)

The present invention relates generally to the art of converting mechanical vibrations and movements into electrical signals, and has particular reference to improvements in the design of piezo-electric transducer elements of the type commonly employed in phonograph apparatus, microphones, and the like.

As an overall object, the present invention seeks to provide piezo-electric transducer elements of novel and practical design, having improved physical and electrical characteristics rendering the elements more suitable for specific intended applications.

More specifically, it is an object of the present invention to provide an improved transducer element which substantially approximates certain desired ideal characteristics of a physical and/or electrical nature, while at the same time being capable of economical commercial manufacture. In this respect, the invention contemplates the provision of an improved transducer element which may be formed of extruded ceramic piezo-electric material, such as barium titanate, for example, and which is possessed of certain ideal characteristics heretofore impossible or difficult of obtainment.

Another specific object of the invention resides in the provision of a novel and improved piezo-electric transducer element adapted upon bending or flexing thereof to generate electrical charges or signals wherein the charges or signals produced therein are uniform, or substantially so, over the entire extent of the active portion of the transducer. It will be understood, .of course, that electrical charges or signals generated by piezo-electrical elements are proportional to the severity of the flexing or bending thereof. It follows that where the bending or flexing of the element is of changing severity over the active extent of the element the charges produced in different areas or sections of the element will vary accordingly. Thus, it is one of the objects of this invention to provide transducer elements of such design that the stressing of the active piezo-electric portions thereof is uniform throughout, as is the electrical charge or signal produced thereby.

The above and other objects and advantages of the invention will become apparent upon full consideration of the following detailed specification and accompanying drawing wherein are disclosed certain preferred embodiments of the invention.

In the drawing:

Figures 1-4 are enlarged cross section views of certain piezo-electric elements of heretofore known design;

Figure 5 is an enlarged cross section view of a piezoelectric element of theoretical design, having certain ideal characteristics;

Figure 6 is an enlarged cross section view of a piezoelectric element embodying the teachings of my invention and closely approximating the theoretically ideal design of Figure 5;

Figure 7 is a perspective view of a modified element of the type shown in Figure 6;

Figure 8 is a perspective view of a further modification of the element shown in Figure 6;

Figures 9 and 10 are enlarged section views of transducer elements incorporating further teachings of my invention; and

Figures 11 and 12 are perspective views of further modifications of the transducer element of Figure 9.

Referring now to the drawing, and initially to Figures 1-4 thereof, the numeral 10 designates an extruded ceramic piezo-electric member of substantially circular tubular cross section. The member so represented is a common type of element, known in the art, and is provided with a continuous inner electrode surface 11 and separate outer electrode surface portions 12 and 13. The complete element is in the form of a tube, which, upon side-to-side bending movements is placed under compression along one side and under tension along the opposite side, the arrangement being such that electrical charges of opposite polarity are built up along the opposite sides of the element, the charges being drawn off through the electrodes 12 and 13 for amplification in any wall known manner. It will be observed, of course, that in any side-to-side bending movements about a vertical axis the extreme side portions of the element will be most greatly flexed, while the upper and lower portions, being close to the axis of flexure, will be but slightly affected.

In the element of Figure 2 the tubular ceramic body member 110 is of a more or less oval cross section, having flattened side portions, so that during side-to-side hp-nHing movements about a vertical axis there is a more uniform stressing of the side wall portions over any cross sectional portion of the element. Moreover, the structural characteristics of the element of Figure 2 are more desirable for certain purposes since, as will be readily observed, there will be substantial compliance to bending in transverse directions, with little compliance to bending in vertical directions. As in the case of the element of Figure 1, a continuous inner electrode 111, and separate outer electrodes 112 and 113 are provided, the inner electrode being employed in initially polarizing the element while the outer electrodes are employed to pick up the charges or signals developed during flexing of the element.

In Figure 3, there is an essentially oval extruded ceramic body member 210 having outer electrodes 212 and 213 and a plurality of core filaments 211' in tubular openings 211 provided therefor. In this instance, the core filaments 211' take the place of the inner electrode. Otherwise, in its essential characteristics, the element of Figure 3 is equivalent to that of Figure 2.

In Figure 4 there is shown a transducer element having preferred operating characteristics, but which is somewhat undesirable from the standpoint of cost of mannfacture. In this element a pair of flat ceramic plates 310 and 310' are provided with outer electrode coatings 312 and 313 and inner electrode coatings 311. The plates 310 and 310' are soldered or otherwise bonded to a center plate or vane 315 of brass or similar material. The vane 315 constitutes a neutral plane about which the ceramic plates are bent to generate the desired electrical signals

In Figure 5 there is shown a theoretically ideal transducer assembly wherein two flat ceramic piezo-electric plates 410 and 410', having the proper electrode coatings 411, 412 and 413, are maintained in relatively widely spaced relation by means of a theoretical infinitely compliant material, indicated by broken lines 414. In this theoretically ideal element lateral or side-to-side bending forces will efficiently place the spaced ceramic plates under tension or compression, depending upon the direction of the bending force; and, as will be readily understood,

2,900,536 ³ the tension and compression forces so produced will be substantially uniform throughout the whole of the cross section of the plates 410 and 410', as compared to the element of Figure 4, for example, where bending forces will cause negligible tension or compression forces adjacent the vane 315, constituting the neutral bending plane.

In accordance with the teachings of the present invention, as represented in the embodiment of Figure 6,1 may realize the various production and other advantages inherent in the designs of Figures 1-3, while realizing the various operating advantages of the designs of Figures 4 and 5. Thus, in the cross sectional view of Figure 6 there is shown a transducer element which has a body portion of H-shaped section, which is preferably formed by extrusion processes or suitable ceramic piezoelectric material. The vertically extending flanges 510 and 510' of the H-shaped section constitute the active operating portions of the transducer element, and are equivalent to the spaced ceramic plates 410 and 410' of the theoretically ideal transducer element of Figure 5. Suitable electrode coatings 512 and 513 are provided upon the outer surfaces of the flanges 510 and 510' in the conventional manner.

Connecting the flanges 510 and 510', and maintaining the same in spaced relation, is a center web section 515 which is preferably of a minimum thickness consonant with the required physical strength of the element for the application intended. An inner electrode is of course required for initially polarizing the element, and for this purpose separate conductive coatings 514 are provided about the upper and lower inside surfaces of the H-shaped section, in the manner shown in Figure 6.

It will be observed that the transducer element of Figure 6 closely approximates the theoretically ideal section of Figure 5, being modified only by the provision of a thin connecting web 515. The arrangement is such that an efficient push-pull stress relation is produced during side-to-side bending movements of the transducer element so that most effective use is made of the active portions of the element.

In Figure 7 there is shown a modification of the element of Figure 6 wherein further provisions have been made to insure even distribution of bending stresses throughout the whole of the element. In many applications of piezo-electric elements the same are fixedly mounted at one end while being drivingly connected at the other end to a movable member such as the record engaging stylus of a phonograph apparatus, for example. The element is thus supported in the manner of a cantilever beam, and when the free end is flexed bending stresses are set up within the beam or element which increase from the free to the fixed end of the element. And, of course, since the magnitude of the electrical signal generated varies in accordance with the severity of the stress imposed upon the ceramic material the signal will vary over the unsupported length of the element. In such cases the full capacity of the element is not realized, and in some instances the fidelity of reproduction may be less than could or should be realized from the element.

In the element of Figure 7 the upper end of the element, designated as “B,” would be the fixed end in a complete transducer assembly, while the lower end, designated as “A,” would be the free end, being connected by suitable means to a phonograph stylus or other detecting device, not shown. As will be observed, the web member 515 is provided with a series of holes or apertures 516^5103 along its length, the size of the apertures decreasing from the free end A toward the fixed end B of the element. In this manner the physical strength of the element is progressively reduced toward the free end A; and when side-to-side vibratory forces are applied to the end A the element will be flexed substantially uniformly along its entire length. It is thus possible to realize a maximum effective operating efficiency from the transducer element.

In the embodiment of Figure 8 there is shown another embodiment of the element of Figure 6 wherein means 5 are afforded for utilizing the areas of the transducer element which are most greatly stressed during operation thereof. Thus, in the embodiment of Figure 8 the element is of substantially uniform strength throughout its length, so that when assembled as a cantilever beam, 10 with the fixed end at B and the free end at A, stresses caused by side-to-side forces applied at the free end A will cause the greatest stresses at and adjacent to the fixed end B. Accordingly, I propose to provide only the portion of the element adjacent the fixed end B with conductive elec15 trode coatings 512 and 513. As will be readily understood, this will render the element more sensitive or responsive to movements of the free end A, but will correspondingly reduce the electrical capacity of the overall unit. Therefore, the requirements of various particular 20 applications will determine the most proper balance of the two factors.

Referring now to Figure 9, there is shown a preferred type of transducer element having special contours so that when bending forces are applied thereto a uniform stress 25 is created in the active ceramic portions thereof in the manner desired. The element of Figure 9 is fixedly mounted at its left hand end B, in any suitable manner, and is connected at or near its free end A with a record engaging stylus or other device for detecting vibratory 30 mechanical movements. The center portion of the element 715 may be of brass or other conductive material and is provided along each outer surface with a layer of ceramic piezoelectric material, as indicated at 710 and 710'. At the outer surfaces of the ceramic layers 71® 35 and 710' are provided conductive electrode coatings 712 and 713, the conductive vane or center member 715 constituting an inner electrode for polarization purposes in this case.

In accordance with the teachings of the invention the 40 vane 715 and/or ceramic layers 710 and 710' are so contoured with respect to the load applied thereto that upon application of the load the transducer element will bend uniformly throughout its length. In this manner the ceramic layers are uniformly stressed throughout and 45 most efficient use is made of the element. It will be understood, of course, that actual longitudinal cross section of the element taken along the plane illustrated will vary according to the type and manner of application of application of the side-to-side force, as well as the longi50 tudinal cross section taken along other planes. The basic principles of forming a beam having uniform strength in bending are well known in the structural arts, for example, and in general these same principles are applied in the present case in obtaining a beam-like trans55 ducer element having a primary characteristic of uniform strength in bending throughout its entire active length.

In Figures 11 and 12 are shown modifications of the element of Figure 9 wherein the element is anchored at 60 opposite ends, B and B', with the vibratory activating force being applied intermediate such fixed ends. The general structure of the elements of Figures 11 and 12 is similar to the element of Figure 9, having reference again to the nature and application of forces thereto in 65 order to arrive at a cross section which uniformly resists bending throughout its unsupported length.

In Figure 10 there is shown a further modification of the element of Figure 9 wherein the center portion or vane 815 is formed with a portion of reduced cross sec70 tion adjacent its fixed end B. The arrangement is such that upon application of lateral forces at the outer or free end A of the element the bending stresses are highly concentrated at the portion of reduced cross section. Electrode coatings 812 and 813 are applied only at the 75 area of reduced cross section, and accordingly the ele

2,900,536 ment is highly responsive or sensitive to any movements of the outer end A. In addition, the area or reduced cross section is so contoured, in accordance with the teachings set forth above, that the stresses produced therein are substantially constant throughout the entire length of the area.

It should thus be apparent that I have fulfilled the objects initially set forth. The invention consists basically in the provision of transducer elements having improved physical and electrical characteristics, and to this end I have provided a novel ceramic piezo-electric element of H-shaped cross section having operating characteristics closely approximating a theoretically ideal transducer element, and having the further important advantage of being capable of manufacture by economical extrusion processes. The element referred to is so arranged that a substantially uniform push-pull stressing of the active elements results at any cross sectional layer thereof during side-to-side bending movements of the element.

It should be particularly observed that since the piezoelectric elements of the present invention are of the ceramic type, preferably of barium titanate, the lattice structure of the material is polycrystalline and as such can be and is polarized in desired orientations throughout various desired sections of the physical body. Thus, in the body of Figure 6, for example, it is possible to orient the polerization in the two slab sections 510 and 510' in such a manner that the generative effect across the thickness of these slabs upon bending of the element as herein discussed is in series and thus additive when the electrical output of the element is taken from the two outer electrodes 512 and 513. Such polarization of piezo-electric elements comprising spaced slabs of polycrystalline material is in accordance with standard practice and is well known to those skilled in the art as is evidenced by the prior art patent to Howatt, No. 2,640,165.

In addition, the invention teaches a novel arrangement for proportioning a transducer element throughout its length so that the bending stresses therein are uniform under the load applied. In this manner the electrical charges produced are uniform throughout the length of the element, and a maximum operating capacity may be realized therefrom. In combination with this latter feature, or in some cases independently thereof, I may provide a conductive electrode coating over only a portion of a complete transducer element, that portion being at and adjacent the area at which the greatest stress occurs during bending of the element. By this arrangement the transducer is rendered accurately responsive to applied bending forces.

It will be understood, however, that the embodiments herein shown and specifically described are intended to be illustrative only, and reference should therefore be made to the following appended claims in determining the full scope of the invention.