US2285143A - Mounting piezoelectric elements - Google Patents

Description

Jime 2, 1942, OKQVOY 2,285,143

MOUNTING PIEZOELECTRIC ELEMENTS Filed Jan. 2, 1940 lhwentoi sanzzzellflpl a ya -of Fig. 3 with the in position.

Patented- June 2, 1942 UNITED sTATEs PATENT OFFICE MOUNTING PIEZOELECTBIC Samuel A. Bokovo'y, Audubon, N. 3., assignor to Radio Corporation of America, a corporation of Delaware Application January z, 1940, Serial No. 312,060

9Claims.

This invention relates to the piezo-electric-art' and particularly to the mounting of piezo-electric crystal elements for use as resonators andoscillators. Recent advances in the art of, cutting quartz make it possible to manufacture, commercially,

crystal oscillators. and resonators having a frequency-characteristic which is constant within i 10 cycles at any desired "broadcast" or "com" mercial frequency. In fact, it now is entirely practical to turn out quartz crystal elements which are accurate to within, say, one cycle per million and this too irrespective of temperature changes over at least a limitedrange.

The principalimpediment to the general adoption of the most rigid standards of accuracy is the practical difficulty incident to the mounting of such nicely cut crystals.

According y. the principal object of the present invention is to provide an improved'method and means for mounting a piezo-electric crystal element.

Another and related object of this invention is to provide an improved air-gap mounting which is applicable to both thickness-mode and contour-mode piezo-electric crystals and one wherein the crystal is constrained to remain in or, if accidentally displaced, return to its normal" position with respect to its retaining pins so that the frlction-therebetween is of a, constant (and hence calculable) Figure 3 is a plan view looking in the-direction of the straight arrow of Fig. 4' of a crystal element which has been accordance with the principle of the present invention, and

Figure 4 is a side elevational view of the crystal top electrode of the "holder It is now appreciated by those skilled in the art that the ideal crystal-holder would be one cut and mounted in wherein there can be no shifting of the crystal 55 with .respect to its electrode or other support and one which at the same time permits perfect freedom of oscillation. Referring to Figs. 1 and 2: In an effort to meet these seemingly contradictory requirements," it has previously been proposed to simply place the crystal C without clamping pressure upon the bottom electrode plate E of a simple air-gap holder H which is provided with retaining pins i, 2; 3, 4, positioned at spaced points adjacent two diagonally opposite corners of the crystal and to tilt the holder with respect .to the horizontal so that there is no pressure upon thecrystal but merely the small restraining force incident to the crystal being urged (by gravity) into contact I with the lower retaining pins l and 2.

It will be observed upon a close inspection of the solid outline of Fig. 2 that in this prior art mounting the edges of the crystal 0 are in contact not only with the two bottom retaining pins l and 2 but also in contact with an upper pin 3. This fact would be of almost negligible moment provided the crystal would remain contiguous this pin 3, but it will not, evenduring normal operating conditions. Thus, owing either to self-oscillation or to tremors ofexternal origin, the crystal will shift its position so that at one moment it, ma be vibrating contiguous the pin 3 and at another moment at a point contiguous the pin 4, or at some position intermediate the pins 3 and '4. It is this unpredictable movement of the crystal while oscillating that gives rise to changes in frequency. Such frequency changes maybe either in the nature of a hop or a drift of as much as ten cycles. Another ob- Jection to this particular'type, of mounting and which is encountered most frequently when the crystal is a heavy one and its angle of tilt is large, is that"the corners of the element may become wedged between the bottom retaining pins land 2 so that an appreciable damping force is exerted against the crystal. I

The above described and other lessapparent objections -to tilted air-gap mountings are substantially obviated in accordance with the present 'invention by so arranging the crystal and its retaining members or pins, that the crystal is biased as by gravityin a direction calculated to always urge the crystal .into contact with the same retaining pins, and this too in spite of the presence of such counter biasing force as may-be generated as an incident to the normal operation of the piezo-electric element. I

To this end, referring now to'Figs. 3 and 4, a tilted air-gap holder H is provided as in the prior art, the angle of tilt of at least the bottom electrode E being less than 90 and more than 3". In the-embodiment of the invention selected for illustration, the bottom electrode is provided with six retaining members which may conveniently be in the form of inverted L-shape pin-like elements l I to Hi, inclusive, similar to those de- ;scribed in U. S. Patent 2,139,998 to Herbert A.

Clarke and having inwardly directed rounded and preferably polished ends. At least two of the retaining members, e. g., pins I2 and l'3,,re-

' vention, the pins I2 and I3 which are allotted to .the edge e are so spaced with respect to the crystal that they are off-set in different directions from its center of gravity 9. This is indicated in Fig. 3-wherein a line p erected normal to the horizontal and in line with,the pin I2 is to the left of the center of gravity 9 of the crystal and a similar line 12' adjacent the pin it is to the right of the said center of gravity. As also indicated in the drawing these pins l2 and i3 should preferably be so arranged that a line drawn between them and parallel to the edge ,e forms an angle of more than 3 and less than 90 with respect to the horizontal.

When the crystal C is arranged on the electrode surface'E in the manner above described it is apparent that the greater proportion of its' when as shown in Fig. 3, the piezo-electric crystal comprises an oblong element, since in this casean even greater proportion of the weight of the .crystal is exerted in the direction of the pins l2 and I3.

- It is more-or less standard practice to make the supporting electrode surface and the supported crystalline surface as nearly optically flat as possible, with a view to obviating the possibility -of the crystal rocking on its support when in operation. .To reduce the friction between these two surfaces, it has also been proposed to polish both the crystal surface and the surface of the electrode. This practice is recommended in carrying the present invention into effect.

However, it is preferable to provide the crystal.

surface with a relatively higher polish than that of-its supporting electrode, since this modiflca-- tion of 'the standard practice has been found to reduce the possibility of adhesion between these Various other modifications of the invention will suggest themselves to those skilled in the art. Accordingly, the foregoing is to be interpreted as illustrative and not ina limiting sense except as required by the prior art and by the" spirit of the appended claims.

3. The-invention as set forth in claim 1 and wherein the intensity of said biasing force is sufllcient to maintain said piezo-electric element in contact with said certain selected of said retaining members in spite of the presence of disturbing forces incident to the normal operation of saidpiezo-electric element.

4. A device of the character described comprising, an electrode surface, a piezo-electric element upon said surface, a plurality'of spaced retaining members for said element,- at least two of said retaining members being mounted adjacent to a common side of said element, said two retaining members being offset in different directions from the center of gravity of. said element and said electrode surface being tilted in a direction calculated to bias said crystal into contact with said two retaining members.

5. The invention as set forth in claim 4 and wherein said piezo-electric element comprises an oblong crystalline body. a

6. The invention as set forth in claim 4 and wherein the angle of tilt of said electrode'surface is more than 8 and less than with respect to the horizontal.

7. The invention as set forth in claim 4, and wherein a line connecting said two retaining members forms an angle with the horizontal of less than 90 and more than 8. I

8. The invention as set forth in claim 4 and wherein said mean-electric element comprises an which said element is urged by gravity, at least flat surfaces and also to reduce wearing of the metal incident to the friction between it and the relatively harder quartz surface. To. this end the bottom and side surfaces of the crystal are preferably provided with what is known in the art asa "mirror finish}? and the electrode surface with a polish which may be defined as a step or two removed from a mirror finish.

two of said members being mounted in spaced relation adjacent a common long edge of said element on opposite sides of its center of gravity and at least one of said retaining members being mounted adjacent the bottom short edge of said element;

9. In a device of the character described a quartz piece-electric element having a polished electrode face, a supporting member having. a polished surface contiguous which the polished face of said quartz element is adapted td be i moved, said electrode face of said quartz element having a higher polislithan the polished surface of said supporting member.

saucer. A. goxovor.

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