Piezoelectric transducer

Abstract: To reduce costs and improve piezoceramic transducers, use of a spool (4) with nail-shaped contacts (5,6) forced thereinto, pressed against the vibrating diaphragm (12) by an elastic O-ring to eliminate solderings and/or spring contacts.

Description: The invention relates to an electroacoustic transducer of the piezoceramic type, which is suitable for use as a piezoceramic capsule or as a tone caller in telephone instruments. Piezoceramic transducers currently used in telephone instruments comprise essentially a transducer diaphragm, maintained perypherally in position in a cavity between a bottom of the case with one wall perforated in order to allow the transmission of sound and a cover to which terminals for external electrical connection are applied. The electric connection between the diaphragm and these terminals is established either by means of thin wires or extentions of the terminals themselves both soldered on the poles of the diaphragm. Soldering on the diaphragm causes drawbacks. In fact, the solder joint on the ceramic layer often detach; moreover, efficiency problems may arise since the solderings are not perfectly equal in all the pieces of a production series and the efficiency is not constant; last, but not least, costs are high. In order to avoid soldering on the diaphragm several attempts have been made. From the European patent application EP-A-0 336 860, there is known a telephone transducer with a piezoelectric unit in which the electrical connections to the diaphragm are established through two contact terminals which are arranged in such a way as to apply pressure, one on the annular portion of the diaphragm and one on the central portion of the diaphragm. With structures like that the soldering of the diaphragm becomes unnecessary; however, they are not free from drawbacks. First of all the connection members manufactured by punching and bending from a matrix of rolled material, do not allow a constant contact pressure on the diaphragm because of manufacturing tolerances, and this create problems of efficiency for the diaphragm. Moreover, the bends could change their angulaticn during the assembling of the unit; the spring require adjusting and consequently the connecticn members which cannot be forced into the termini fitting holes of the bottom require a successive sealing of such holes in order to avoid the leakage of the unit. Further, the spring contact on the ceramics applies pressure on the inner area of the diaphragm and this worsens the situation. From Patent Abstracts of Japan, vol. 10, no. 46 (E-383)(2103), 22 February 1986, there is known a piezoelectric acoustic transducer in which a piezoelectric vibrator is constituted of a unimorphic vibrator where a piezoelectric ceramic element is bonded to a metallic plate and arranged so that the side of the element will point the opening side of a case. A support ring is formed by an insulating part and a conductive part and a spring terminal is fitted to a notch. The terminal consists of a contact part and a fitting part formed with a spring material and temporarily fixed after a folding piece of the fitting part is fitted in a not-notched thin part of the insulating part. A damping film is applied to a copper foil patterns of a cover. External terminals are inserted into terminal fitting holes of the cover and connected to the copper foils respectively. Also with this solution soldering on the diaphragm becomes unnecessary; nevertheless it is still not free from drawbacks since it uses spring terminals. Moreover, the contact between the ceramics and the corresponding terminals is established at the inner zone of the diaphragm which is not allowed to vibrate freely. It is therefore an object of the present invention to provide a piezoelectric transducer which is not subject to the same deficencies of the aforementioned prior art transducers. Another object of the present invention is to provide a piezoelectric transducer of simplified and economical construction. These and other objects which will become more apparent hereinafter are achieved by a piezoelectric transducer having the characteristics set forth in the characterizing portion of claim 1. The invention will now be described with the aid of two embodiments shown in the accompanying drawings, attached merely by way of an indicative and not limitative example, in which: Fig.1 is a perspective exploded view of a piezoceramic transducer designed to work as a piezoceramic capsule in a telephone instrument receiver according to the present invention; Fig.2 shows the transducer of Fig.1 where the case is formed directly on the bottom of the handset housing, and Fig.3 shows a piezoceramic transducer designed to work as a tone caller in a telephone subset. Referring now to the mentioned figures in detail, like numerals indicate like elements throughout the several views. Fig.1 shows a first embodiment of the piezoceramic transducer, according to the present invention, in the configuration to be used as a piezoceramic capsule in a telephone instrument receiver. In it a case 1 houses an elastic element 2, a piezoceramic transducer diaphragm 3 and a bottom base 4 which carries the contacts 5, 6, 7 accessible from outside. The case 1, made of insulating material, is realized by molding. In an embodiment particularly advantageous it can be molded directly on the bottom of the handset housing thus further lowering costs of the transducer. Preferably the case has a circular section and the cover wall is provided with openings 8 for the transmission of acoustic waves generated by the vibrations of diaphragm 3. Inside on the cover wall, is a circular rib delimiting an annular housing 9 for the elastic element 2 and which is provided optionally with a reference tooth 13 that engages in a corresponding notch (not shown in the figure) provided on the inner end of the base 4. At the upper end of the inner wall of the case is an annular stop tooth 14 designed to snap on the upper edge of the bottom base 4. The piezoceramic diaphragm 3 is of composite type and consists of a metallic discoid plate 11 having a central portion of one of its sides covered, in a known and conventional manner, with a piezoceramic element 12 plated on its free surface with a layer of conductive material. The piezoceramic element is provided with a terminal area 10 which extends through the circular annulus and forms a pole of the vibrator while the other pole is formed by the remaining annulus portion 11a not covered by the ceramic layer. The elastic element 2 consists preferably of an O-ring of elastomeric material which assures a uniform elastic coupling contact between terminals 5, 6 and the diaphragm poles 10, 11a respectively. The capsule bottom base 4 consists of a circular spoon of insulating material provided with two through holes extending axially and located at diametrically opposite positions and at a distance which is comprised between the diameter of the ceramic element 12 and the diameter of the metallic plate 11. Two feedthrough terminals (5,6) are forced into said through holes and constitute the contacts accessible from outside. Advantageously such terminals have a nail-shaped configuration and are made of plated (e.g. silverplated) metal. The nail heads abut from one side on the end face of the spool and from the other on the diaphragm 3 when the transducer is assembled. A through-hole 16 axially extending is designed to realize damping microleaks of diaphragm 3. Fig. 3 illustrates an embodiment in which the transducer is used as a ringer in a telephone subset. In it a case 1′ houses the elastic element 2, the piezoceramic transducer diaphragm 3 and a bottom base 4′ which carries the contacts 5, 6 accessible from outside. The case 1′ is of plastic material and consists of two cylindrical, coaxial and adjacent portions of different diameters. The lower diameter portion is closed at one end by an end-wall having an opening 8′ for transmission of acoustic waves generated by the vibrations of the diaphragm 3. Inside the free end of the greater diameter portion is an annular tooth 14′ designed to snap on the bottom base 4′. Both the portions define, at the inner interface zone, an annular housing 9′ for the elastic element 2, which besides assuring the uniform elastic coupling contact between terminals 5, 6 and diaphragm poles 10, 11, provides for sealing in order to avoid leakage of the transducer. On the annular step outside the case are two bosses 19 provided with holes 20 for fixing the transducer to the frame of the telephone subset. The assembly of the transducer is carried out in a very simple manner. After the positioning of the elastic element 2 inside the annular housing 9, 9′, the diaphragm 3 is laid down thereon with the ceramic element 12 pointing the opening side of the case 1,1′ (the correct orientation of pole 10 is determined automatically by the machine which performs the positioning of the diaphragm). Thereafter the bottom base assembly 4, 4′ is positioned with the terminal heads pointing the diaphragm and pushed in, thus compressing the elastic element 2 until the annular tooth 1, 14′ snaps on the base 4, 4′. In such a way the heads of terminals 5 and 6 make an ohmic contact respectively with the poles 10 and 11 of diaphragm 3, whilst the elastic reaction of the element 2 assures a uniform elastic coupling for maintaining such ohmic contact. The operation of the piezoceramic transducer is well known to those skilled in the art and therefore will not be described here. Thus the invention full achieves the above mentioned objects. The piezoceramic transducer according to the present invention besides eliminating all the drawbacks of the prior art transducers is very easy to manufacture and assemble with very reduced costs as compared with those of conventional piezoceramic transducers. This is due to the fact that spring shaped terminals have been eliminated. While the invention has been described with reference to specific embodiments thereof, it should be noted that it is susceptible to several modifications and variations. For instance, the bottom base 4 of the transducer when used as a piezoceramic receive capsule may be provided with a circumferential groove along the lateral surface for housing a coil 15 electrically connected to the terminal 6 contacting the metallic plate 11 and to a third terminal 7 provided on the end of the spool 4 and also accessible from outside, said coil 15 being designed to generate an auxiliary magnetic field which may influence the deaf prothesis. Notches 17, 18 are suitably provided on the base 4 for the passage of the coil leads to terminals 6, 7.

Claims: A piezoelectric transducer of the type comprising a piezoceramic transducing diaphragm (3) comprising a support plate (11), a piezoceramic element (12) and two contact areas (10,11a) forming two poles of said diaphragm (3) and being situated on the same side of the support plate, said diaphragm being maintained peripherally between a perforated case (1,1′) and a bottom base (4,4′) by an elastic element (2), said bottom base (4,4′) carrying a pair of contacts (5,6) accessible from outside, all arranged so that the contact areas (10,11a) of the diaphragm will point said contacts (5,6), said contacts (5,6) consisting of feed-through terminals so designed to have one end in abutment on said diaphragm when the transducer is assembled, the electric connections between the diaphragm and the contacts being established by pressure contact, characterized in that the contact pressure is produced by the action of said elastic element (2). A transducer according to claim 1, wherein said feed-through terminals (5,6) consist of nail-shaped metallic pins forced into respective through-holes axially extending in said bottom base, so that the nail-head is in abutment on the inner end surface of the bottom base (4,4′) from one side and on a peripheral position of the diaphragm from the other side when the transducer is assembled. A transducer according to any of the preceding claims, wherein said perforated case (1,1′) at its opening end is provided with an annular tooth (14,14′) designed to snap on the upper end of the bottom base (4,4′) when the latter is placed into said case (1,1′) and pushed in. A transducer according to any of the preceding claims, wherein said elastic element (2) is an O-ring made of elastomeric material and located inside an annular housing (9,9′) formed inside the case (1,1′). A transducer according to claim 1 and 2, characterized in that said bottom base (4) comprises a circular spool provided with a further contact (7) accessible from outside and with a circumferential groove designed to house a metallic wire winding whose leads are connected to said further contact (7) and to that one (6) of said feed-through terminals which is in contact with the metallic plate (11). A transducer according to claims 1 and 6, wherein said spool (4) and said case (1) are provided with reference elements (17,13) designed to engage each other for the correct positioning of the spool during the assembling. A transducer according to claims 1 and 6, characterized in that said case (1) is formed directly on the bottom of the telephone handset housing.

Inventors: D AVOLIO ANTONIO, PESENTI LUIGI, D'AVOLIO, ANTONIO, PESENTI, LUIGI
Applicants: ALCATEL DIAL FACE SPA, ALCATEL DIAL FACE S.P.A.
Application year: 1991
Main classification: H04M1
IPC codes: H04M1, H04R17, H04R1