Patent GB-659869-A (GB)
Title: Improvements in or relating to electro-mechanical transducers employing dielectric elements
IPC: H04R17/00
Filed: 1949-06-03
Abstract: 659,869. Piezo-electric crystals. BRUSH DEVELOPMENT CO. June 3, 1949, No. 14948. Convention date, June 12, 1948. [Class 40 (viii)] Piezo-electric transducer elements are formed of polycrystalline dielectric material, each element being in the form of a plate which is curved, or corrugated in one direction only so as to stiffen the plate in another direction. Then, bonding the plate in the said other direction causes some regions to expand and other regions to contract, thus giving opposed voltages on electrodes placed on opposite surfaces of the respective regions. The elements may be made of barium titanate ceramic with which some strontium titanate may be mixed. In Fig. 5, an element 11 is straight in the longitudinal direction and is curved in the transverse direction to increase its stiffness. The element 11 is firmly fixed to a base 24 so that force applied along the length direction of a shaft 25 tends to bend the element about its longitudinal axis. When the bending is to the left the outer portions 14, 16 of the element are compressed and the central portion 15 is extended and vice versa. Thus, if pairs of separated electrodes are placed on opposite faces of the three zones, the voltages developed at the outer zones will be opposite to the voltage developed at the central zone. The pairs of electrodes may be connected in aiding sense, in series or in parallel. Alternatively, the element may also be slightly curved in the longitudinal direction, Fig. 6 (not shown). In Fig. 7 the transverse curvature decreases towards the free end which is flat. This makes the stiffness increase as the bending movement increases towards the base, thus giving a uniform stress distribution. A similar effect may be produced by keeping the transverse curvature constant, but by reducing the width of the element towards the free end, Fig. 8 (not shown). A waved or corrugated transverse section has the effect of a number of alternately oppositely curved elements connected side by side, alternate electrodes being connected together, Fig. 12 (not shown). The curvature may not be uniform. Fig. 10 shows in transverse section an element with two side limbs at right angles to a central plane portion. On bending, the side limbs contract when the central portion expands and vice versa, thus giving a voltage of one polarity on electrodes 64, 66, and an opposite voltage on electrode 65, relative to a common electrode 68. For interconversion of a rotational movement and an electric voltage, use is made of an element as in Fig. 13 in which the side limbs extend in opposite directions from the central portion. A rod 105 is fixed to a cap 106 on the top of the element. On turning the rod in a plane parallel to the base 104, the two side parts of the element are bent in opposite ways. Thus for a clockwise rotation of rod 105, the side limbs are compressed and the central portion is mainly extended, to produce corresponding voltages on electrodes 107, 108, 109 mounted on the several portions of the element. Specifications 648,403 and 648,404 are referred to.