Patent GB-1365085-A (GB)
Title: Electromechanical transducers having piezo electric members
IPC: B06B1/06
Filed: 1971-11-09
Abstract: 1365085 Piezoelectric transducers TIBBETTS INDUSTRIES Inc 9 Nov 1971 [2 Dec 1970] 51868/71 Heading H4J A face shear responsive transducer has an elongate member of piezoelectric material and elastic means adapted to transform between a face shear stress acting at a part of the transducer and longitudinal stress in the piezoelectric member. An electroacoustic transducer 12 (Fig. 1) comprises a generally rectangular frame 14, e.g. of "INVAR" (Trade Mark) gold plated to provide good electrical contact with electrodes 30, 30 and 36, 36 at the ends of elongate piezoelectric members 24 and 26. The electrodes 30, 36 are located in apertures 40 in the frame 14 and are preferably coatings having precious metal powder bases glass bonded and fired on the piezoelectric material. Smoothly curved saddleshaped metal diaphragms 16, 18 ("diabows") have peripheral flanges 20, 22 joined by integral skirts 23 of varying height. Forces with components normal to the principal surfaces of the "diabows" are transformed at the peripheries as face shear forces 50 (Fig. 2) acting to draw two diagonally opposed corners of the frame toward one another while simultaneously forcing apart the other two diagonally opposed corners. Forces acting on a "diabow" in the opposite direction have the opposite effect. When located, the "diabows" 16, 18 are recessed into the frame 14 and secured thereto by epoxy adhesive or solder. Each piezoelectric member 24, 26 has a pair of central electrodes 28, 34. Members 24, 26 are prepolarized in the directions indicated by arrows 41 (Fig. 2). Electrical contact between facing centre electrodes 28, 34 is provided by a gold alloy curved leaf spring 42 (Fig. 3). The piezoelectric assembly has five epoxy joints 44, 46. Corner plates 48 are bonded to the frame 14 by similar adhesive and this construction increases the elastic stiffness of the joints 44. In the manner of connection described, the transducer 12 provides four essentially identical piezoelectric generators electrically in parallel. It may be used in a gramophone pickup or microphone or in the other direction of energy transduction. A transistor amplifier chip 54 is bonded to the piezoelectric member 24 over an electrode 28. External leads 58, 60, 62 pass through holes in the frame 14. Lead 56 is spot welded to frame 14. A pressure equalizing vent (not shown) may be made in frame 14 unless it is desired to keep chip 54 hermetically sealed. A microphone embodiment, (Fig. 4), e.g. for a hearing aid, has the transducer 12 supported in a tubular metal casing 64 by eight elastomer mounts 66. The casing 64 is closed at one end by a plate 68 carrying a sound inlet tube 70 extending through plate 68. A U-shaped bulkhead 74 at the other end locates a terminal plate 76, leads 56, 58 and 60 being connected to soldered terminals 78. Preferably the plate 76 has a thick film circuit on its inner face. The mounts 66 attenuate acceleration of transducer 12 relative to the casing 64 and prevent constraint on the motion of the frame 14. In a modification, a single piezoelectric member 24 is used. A single "diabow" may be used in conjunction with one or two piezoelectric members 24, 26. In another variation (Fig. 6) the piezoelectric member 100 has electrodes 104, 106 on its major surfaces, and the piezoelectric member 102 has electrodes 108, 110 on its major surfaces. Each member 100, 102 is prepolarized in its thickness direction 112. The electrodes do not extend to the frame. Electrodes 106, 108 are electrically connected by a leaf spring 42. The members 100, 102 are placed electrically in parallel by a  common connection 114 to electrodes 104, 110 and connection 116 to electrode 106. This embodiment is preferred for electrical to mechanical transduction. In the embodiment of Fig. 6 the members 100, 102 may be made from single crystal piezoelectric materials. The electrodes may be on the sides rather than on the major surfaces of the piezoelectric members, in which case the central areas of the piezoelectric members are not electroded, to avoid shortcircuiting. Then, if piezoceramic is used, the polarization is sideways. The polarization may be uniform throughout the piezoelectric member or it may have opposite directions on each side of the non-electroded central area. The resulting four pairs of electrodes may be connected all in series, all in parallel or in series-parallel combination. Similar variations are applicable to the embodiment of Fig. 6 electroded on major surfaces of the struts if at least two electrode pairs are provided on each piezoelectric member.