US2166713A - Piezoelectric crystal mounting - Google Patents

Description

y 13, 1939- s. A. BOKOVOY 2,166,713

PIEZOELECTRIC CRYSTAL MOUNTING Filed March 30, 1935 clampzln pres-Sure osczlZZa tin, dimension maximum movement t r of crystal zero movemeV of cryatal l M INVENTOR:

Samuel ALBo/iovoy WMM BY HTTOHNEY Witness Patented July 18, 1939 UNITED STATES PATENT OFFICE Samuel A. Bokovoy, Audubon, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 30, 1935, Serial No. 13,887

Claims.

This invention relates to piezoelectric crystal apparatus, particularly to electrode plates for quartz bar or width-oscillating crystals, and has special reference to the provision of an improved 5 electrode assembly for so-called air-gap, pressure, node-mounting of such crystals.

Numerous mounting arrangements for piezoelectric crystals are known in the art and various advantages are claimed for each type mounting. Thus the gravity or simple air-gap holder, wherein the crystal is simply laid without clamping restriction on the inner bottom surface, or on a riser, within a closed container, is said to possess the advantage of ensuring substantial constant frequency of oscillation. This desirable charac teristic, however, can only obtain when the box is highly insulated from shocks or tremors tending to dislocate the crystal. Such mountings are obviously entirely unsuited for use in moving vehicles and in other installations where the crystal and its electrodes might be subject to relative movement.

Another type mounting provides a pair of spaced, fiat surfaced, electrodes with the oscillator clamped in face to face relation therebetween. Such mounting ensures rigidity but at a considerable sacrifice of frequency-accuracy and amplitude of oscillation. These disadvantages are only slightly less in degree when the crystal is mounted at a plurality of distributed points, especially so if. any of the supporting points are adjacent the area of maximum movement in the oscillatory dimension.

Where, as is usually the case, freedom from mechanical displacement and freedom of oscillatory movement are prime desideratum it has been proposed to support the crystal adjacent its nodal axis and to apply the clamping pressure, which is necessary to secure the crystal against displacement, through the quartz center. One such known arrangement requires the formation of a detent in the surface of the crystal, another employs an adjustable screw mechanism, still another requires that the crystal be electroplated; all such node mounting arrangements are expensive and some are quite complicated.

The recent development of crystals having a substantially zero temperature coefficient of frequency has made practical the use of piezoelectric elements in fields which, up to now, have been closed because of the high cost of the necessary thermostatically controlled casings and associated frequency-fixing apparatus. With old type crystals the cost of the electrodes per se comprised only a small percent of the total cost of the apparatus so that the manufacturer was justified in employing plates and mountings of the above described expensive construction. Similar plates used with modern crystals would, however, constitute a major factor in the total cost of the apparatus and render uneconomical the application of crystals in those fields Where cost is a prime consideration.

Accordingly, principal objects of the invention are to provide an inexpensive, simple and trouble-free nodal zone mounting of maximum rigidity and minimum contact for piezoelectric crystals and further to ensure maximum output efficiency for such crystals.

Other objects will be apparent and the invention itself will be best understood by reference to the following description taken in connection with the accompanying drawing wherein:

Figure 1 is a perspective view of one. of a pair of electrode plates within the invention and shows oppositely located risers adapted to support a crystal at spaced points within its nodal zone;

Fig. 2 is a modification within the invention showing an electrode plate having a support riser along an axis corresponding to the nodal axis of a crystal;

Fig. 3 is an electrode plate for supporting a crystal adjacent its mechanical center;

Fig. 4 is a plan view of, a modification of the plate of Fig. 3;

Fig. 5 is a vertical section taken on the line 5---5 of Fig. 4 and indicates one method of forming a riser;

Fig. 6 is a diagram of a crystal indicating certain areas of stress, of interest in connection with the invention;

Fig. 7 is a side elevation of a dustproof casing containing a piezoelectric crystal mounted be tween electrode plates of the present invention, the casing itself forming no part of the invention; and

Fig. 8 is a sectional View taken on the line 8-8 of Fi 1.

While the invention will be explained in connection with a quartz crystal cut to oscillate along one of its greater dimensions, it is to be understood that the electrode plates of the present invention may be employed in the mounting of so-called thickness oscillators and when so used will permit substantially complete freedom of, oscillation.

In a piece of quartz, designed to oscillate along one of its greater dimensions, the movement is greatest adjacent the terminal ends of the oscillating dimension and standing waves may be said to be present approximately midway of the crystal along an axis parallel to these ends, as represented in Fig. 6. The electrode plates of Figs. 1 to 5 inclusive are designed to contact the crystal along this nodal axis, or line of zero movement. While theoretically this axis is a mere line, for all practical purposes there is a small area or nodal zone against which clamping pressure may be applied without appreciable damping effect upon the crystal.

Referring now to Fig. l, which shows a preferred embodiment of the invention. The electrode of this figure comprises a rectangular wafer or plate l l, formed of brass or other metal. It is pref erably substantially coextensive in length and breadth with the crystal associated therewith; it may be of wafer thickness but should be sufficiently rigid to obviate prolonged parasitic oscillations if jolted. The major face a, a of plate ii is flat, and there are a pair of sharply defined, inwardly directed, oppositely located, wedge shaped risers l2-l2' thereon. The axis of symmetry s--s of these risers l2 l2' and of the plate H itself, corresponds to the previously described nodal axis of the crystal to be supported thereon.

A convenient way of manufacturing the plates of Fig. 1 in quantity, is to mark off a number of roperly oriented circles on a sheet of the electrode material, cut out the rectangular blanks and machine the surface of the blanks, following the circular markings, to the desired depth. The top surface of risers lZ-I2' should ordinarily extend .000'75 to .002" above the major surface of the plate but for special applications may projectfrom .00025" to say .02.

The electrode plate 20 of Fig. 2 has a plurality of rectangular risers 22, 22a, 22b spanning the entire width or length, as the case may be, of the plate so that the crystal may be positively clamped at spaced points along its entire nodal axis.

The crystal contacting riser 32 of Fig. 3 is disposed at the center of the plane of the surface of the plate 30 which center corresponds to a point midway of the nodal axis ss of the crystal. The riser shown is circular but may be of any other convenient shape.

In Fg. 4 two risers 42 and 42 are shown, oppositely located on the axis of symmetry s-s of electrode plate 40.

Fig. 5 shows how, instead of cutting away the surface of the main body of the plate as described in connection with Fig. 1, the plates may be simply formed from sheet metal by pressing or punching with a tool of the desired shape on the reverse side b of the metal, so that a section on the major surface a will protrude. The surface of the protruding metal may then be ground, if necessary, to the exact height and flatness required.

Fig. 7 shows a crystal i clamped by means of a spring clamp 2 between two electrode plates I l--l l, the plates of this embodiment being similar to that shown in detail in Fig. 1. The opposite major faces of the crystal I each contact only that portion of the adjacent plate constituted by risers l2 and I2 so that there is an.

air-gap separating the crystal from each plate except for the small area within the nodal zone of the crystal embraced by the risers. Since, as above set forth, risers l2-I2 are positioned within the nodal zone of the crystal, the pressure exerted by spring 12 and the weight of the upper electrode plate II in no way damp its natural period of oscillation.

The entire assembly is preferably enclosed within a dustproof box or casing 5.

As a number of possible embodiments may be made of the above invention, and as changes may be made in the embodiments set forth, without departing from the spirit or scope of the invention, it is to be understood that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the appended claims and by the prior art.

What is claimed is:

1; An electrode for piezo-electric crystals comprising a substantially flat metal plate having a pair of sharply defined, flat surfaced, wedge shaped, crystal supporting risers thereon, the apex of each wedge being inwardly directed from opposite edges of said plate and defining an axis 5 crystal supporting risers thereon, said sections.

being positioned on axis of symmetry of said plate.

3. An electrode in accordance with claim 2 and wherein said risers are circular in shape.

4. An electrode in accordance with claim 2 and wherein said risers are rectangular in shape.

5. The combination with a piezo-electric crystal element of a pair of metal electrode plates, each plate having an axis of symmetry coinciding with a nodal axis of said element and 'hav-- ing a well defined raised portion raised .00025 inch to .02 inch from its main surface on said axis of symmetry, said raised portion contacting a crystal face of said element along only a portion of its said nodal axis, and means for clamp;

ing said element between said plates.

SAMUEL -A. BOKOVOY.

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