US 3153156 A
Description (OCR text may contain errors)
Oct. 13, 1964 F. w. WATLINGTON 3,153,156
PRESSURE-PROOF CERAMIC TRANSDUCER Filed May 1'7, 1962 if-h I 26/ 3 INVENTOR. Eemvz W. 1144704 470 United States Patent 3,153,156 PRESSURE-FROG CERAMMZ TRANSDUCER Frank W. Watlington, Pembroke, Bermuda, assignor to the United tates of America as represented by the Secretary of the Navy Filed May 17, 19:?2, Ser. No. 195,dt) 7 (Ilaims. ill. 31ti--8.6)
This invention relates to pressure-proof ceramic trans ducers and especially to improvements in ceramic transducers of the type having two ceramic discs which are separated by a centrally excised spacer.
In the construction of ceramic piezoelectric transducers, it is advantageous from the point of view of sensitivity to employ two bimorph plates, or discs, of ceramic piezoelectric material separated by a gasket, so that an air space exists inside the ring between the two discs. The transducer is then encapsulated for use in fluid media such as the ocean. As the transducer is lowered farther and farther into the ocean, increasing hydrostatic pressure flexes the discs inwardly until the elastic limit of the discs is exceeded and one of the discs fractures under the strain. Thus, accidentally exceeding the depth limit of the transducer results in permanent injury and worthlessness. The present invention prevents fracture of the discs of this type of transducer if the depth limit of the transducer is exceeded.
The objects and advantages of the present invention are accomplished by decreasing the thickness of the air gap so that inward fiexure of the discs causes them to contact each other before the elastic limit of the discs is exceeded.
An object of this invention is to prevent fracturing of the discs of a transducer, of the type which has ceramic discs separated by an air space, when said transducer is subjected to excessive pressure.
Another object is to simplify the construction of a pressure-proof ceramic transducer of the type which has multiple ceramic discs separated from each other by air spaces.
Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
FIG. 1A is a cross-section of the preferred embodiment of the invention;
FIG. 1B is a top view of the embodiment shown in FIG. 1A;
FIG. 2 is a cross-section of another embodiment of the invention;
FIG. 3 is a diagram illustrating the important dimensional relationships in the transducer; and
FIG. 4 is a cross-section of a second embodiment of the invention.
In FIG. 1A, a pair of piezoelectric ceramic discs, or birnorphs, 1t) and 12 are separated by a spacer 14 comprising a fiat metallic ring so that an air space 24 exists between the inner surfaces of the two ceramic discs. The outer surfaces 16 and 1S and the inner surfaces 26 and 28 of the ceramic discs carry metallic coatings which act as electrodes.
Each disc is fabricated from a pair of wafers of a ceramic material such as barium titanate. For example, the upper disc 12 consists of an upper and lower wafer, 11 and 13 respectively. During the manufacturing process, polarizing potentials are applied to the surfaces of the wafers so that induced charges will thereafter appear on the surfaces under the influence of compressive and tensive forces.
In the embodiments shown in FIGS. 1A and 2, the polarizing voltages applied to the contacting surfaces of the wafers are of the same sign, e.g., positive, and those applied to the non-contacting surfaces (the outer surfaces 16 and 13 and the inner surfaces 26 and 7.8v of the bimorphs) are of the other polarity, e.g. negative. Under a compressive force applied to the outer surfaces 16 and 18, the outer surfaces 16 and 18,. and therefore lead 26), will exhibit a positive induced polarity and the inner surfaces as and 28, and therefore lead 22, will exhibit a negative induced polarity. This unit is thus a parallel connection of the two bimorphs and provides maximum current output.
The two outer surfaces 16 and 13 are electrically joined together and a wire lead 20 is brought out for connection to external circuit. In the preferred embodiment, the ring spacer 14 is made of electrically conductive material such as brass or silver and electrically connected to the coatings on the inner surfaces 26 and 28 of the ceramic discs iii and 12 by soldering or using a conductive epoxy for bonding. A wire lead 22 is soldered to the ring spacer 14 and brought out for connection to the external circuit. The use of the metallic spacer l4 eliminates the need for soldering leads to the metallic coatings on the inner surfaces 26 and 28 of the discs.
However, as illustrated in FIG. 2, the spacer 14' can be fabricated from an insulating material such as rubber or fibre. in this case, wire leads are soldered or otherwise electrically connected to each of the inner-surface metallic coatings 2d and 28 and brought out through radial grooves in the ring spacer 14' to be joined with leads from the outer surfaces 16 and 18 of the discs 10 and 12 to form a single lead 22 for connection to the external circuit. Two other wires are bonded to the coating between the. contacting surfaces of each dis-c and joined together to form a second lead 20 for connection to the external circuit. The polarities of the surfaces of corresponding wafers in FIG. 2 is the same as that of the surfaces in FIG. 1A.
The proper thickness of the ring spacer 14 is determined after testing the ceramic discs to ascertain the inward distance from its normal unflexed plane through which either disc can be flexed before it breaks. If this distance is designated as (a), then the spacer should have a thickness which is just slightly less than (2a), so that the discs will contact each other before each has flexed a distance (a) from its normal position.
In FIG. 3, which is exaggerated for clarity, the dotted line 26' shows the position of the lower surface of the upper disc it! when it has been flexed inwardly. The distance between its normal unflexed position and its position when it reaches its elastic limit is shown as (a).
It is obvious of course, that the closer the spacer thickness approaches the dimension (2a), the greater will be the depth to which the transducer can be lowered and still operate, keeping in mind, however, that the spacer thickness cannot be made equal to (2a) Without danger of having one of the discs fracture.
For immersion in a liquid medium such as sea water, the unit is encased in a sheath and encapsulated in potting material so that the unit is watertight.
Another type of transducer which utilizes a series connection of the two bimorphs is shown in FIG. 4. This type of connection provides maximum voltage output from the transducer. Here the contacting surfaces of the upper bimorph are originally polarized positively and surfaces 16 and 25 are polarized negatively. The contacting surfaces of the lower bimorph are polarized negatively and surfaces l8 and 28 are polarized positively. Under the influence of compressive forces at surfaces 16 and 18,
surfaces 16 and 28 will have an induced charge of posi- 1 live polarity and surfaces 26 and 18 an induced charge of negative polarity. 'I hus lead 2-1 will carry a positive J) polarity and lead 23 a negative polarity. ring 14 is of the electrically conductive type.
In. operation, the unit is lowered into the medium and the hydrostatic pressure upon it increases with the depth. As the hydrostatic pressure increases, the ceramic discs bow inwardly in proportion to the pressure. Before the elastic limit of the ceramic material is reached, however, the inner surfaces 26 and 25 of the discs come into contact with each other and further stressing of the discs is prevented. Damage to the transducer is thereby obviated although the unit will operate with reduced output when such contact is made.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
1. A pressure-proof, piezoelectric transducer comprising, in combination:
at least two discs of piezoelectric material, the fiat sur faces of said discs bearing electrically conductive coatings; and
at least one annular spacing ring for insertion between said piezoelectric discs so that said discs and ring form a sandwich structure,
the thickness of said spacing ring being smaller than twice the distance through which either of said discs can be flexed without exceeding its elastic limit.
2. A transducer as set forth in claim 1, in which said spacing ring is fabricated from an electrically conductive material.
3. A pressure-proof, piezoelectric transducer comprising, in combination:
at least two discs of piezoelectric material, the fiat outer surfaces of which bear electrically conductive coatings; and
at least one disc spacer having an excised central por- The spacer tion, said spacer being inserted between said piezoelectric discs to form a sandwich structure,
the thickness of said spacer being smaller than twice the distance through which either of said discs can be flexed without exceeding its elastic limit. 4. A transducer as set forth in claim 3, wherein said spacer is fabricated from an electrically conductive material.
5. A pressureroof, piezoelectric transducer comprising, in combination:
at least two discs of piezoelectric material the flat surfaces of which bear electrically conductive coatings;
at least one disc spacer having an excised central portion, said spacer being inserted between said piezoelectric discs to form a sandwich structure,
the thickness of said spacer being smaller than twice the distance through which either of said discs can be flexed without exceeding its elastic limit; and
a pair of conductive leads, one being electrically connected to said spacer and the other being electrically connected to the conductive coatings on the outer surfaces of both said piezoelectric discs.
6. In a piezoelectric transducer of the sandwich type in which an annular spacing ring separates two piezoelectric discs, the flat surfaces of said discs bearing electrically conductive coatings,
the improvement wherein the thickness of said spacing ring is less than twice the distance through which either of said discs can be flexed without exceeding its elastic limit.
7. A transducer as set forth in claim 6, in which said spacing ring is made of an electrically conductive material.
Crownover May 27, 1958 Parssinnen et al. Sept. 11, 1962