Patent GB-758647-A (GB)
Title: Electro-mechanical transducers and systems incorporating the same
Inventor: BRADFIELD GEOFFREY
IPC: H03H9/30
Filed: 1953-05-14
Abstract: 758,647. Acoustic delay lines. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 6, 1954 [May 14, 1953], No. 13580/53. Class 40 (8). Use is made in an acoustic delay line of mechanical waves propagated in a torsional mode. The line may consist of a tube A, Fig. 1, of a piezo-electric material such as barium titanate or a mixture of barium and other titanates. The signal to be delayed is introduced by means of electrodes at B and is picked up by means of electrodes at D, E, &c., successive reflections of the signal from the ends of the line being prevented by means of tapered layers C, F of grease, or " Plasticine " (a Registered Trade Mark). The grease or other damping material may be held in short tubes surrounding the ends of the delay line. Each set of electrodes, Fig. 2, at B, D, E, &c. is arranged on the outer surface of the tube. Electrodes c and e lie on one circle normal to the axis of the tube, f and d on another such circle and a and b on a third circle symmetrically located with respect to c, d, e and f. Relative distances between the electrodes are given in the Specification. The stretch a to b round either half of the tube is polarized electrically either by the cold-polarization method or by applying fields of 12 to 20 kV/cm. (for barium titanate) at a temperature of 145‹ C. for about one hour under silicone oil. If small percentages of lead titanate are present fields of 24 to 28 kV/cm. are used. If electrodes e, d and f, c are connected in common a signal applied between these two pairs of electrodes will excite a torsional wave. In an alternative method the stretch between c and d is polarized in one axial direction and the stretch between f and e in the opposite direction. Signals applied between a and b then give rise to two torsional waves simultaneously propagated in opposite axial directions. In a further method the inner surface of the tube is silvered at r, Fig. 6, and opposite pairs of electrodes a, n and f, i applied to the exterior. Polarizing voltages are applied from a to r, from r to n, from i to r and finally from r to f. A signal applied between the inner electrode r and all the outer electrodes in common gives rise to a torsional wave. In another form of the invention the delay line comprises a length 42, Fig. 8, of finegrained magnesium alloy enlarged at its ends to provide end faces of relatively large diameter. Torsional waves are launched and picked up by means of terminal piezo-electric arrays consisting of four or more spaced segments 43 of Y-cut quartz arranged on the end faces of the line with their x directions all in the same circumferential sense. The quartz segments may be replaced by segments of barium titanate polarized tangentially. In a further modification the launching and pick-up means comprise tubes 10, Fig. 9, of a magnetostrictive material such as nickel-zinc ferrite and the delay line consists of a tube 11 of an alloy steel having a low coefficient of expansion. The tubes are provided with tapered joints and are secured together by means of an epoxy resin cement or by polyvinyl acetate. Circumferential polarization of the ferrite tube 10 is effected by means of a temporary axial conductor along which is passed currents of the order of 100 amps. in bursts of a second or so. Signals are applied to the tube by means of an axially adjustable coil 14. Coils may be arranged at different positions along the line, lengths of magnetostrictive material being used at the coil loca. tions. Alternatively the whole of the line may consist of a magnetostrictive, e.g. ferrite, tube. The Specification gives suitable dimensions for delay lines of these types and figures indicating their performance. A modification is described (Fig. 11, not shown), in which the delay line consists of a wire whose ends are axially drilled and expanded to form a smooth transition to the larger diameter of the ferrite tubes. The coils may be mounted in shells made of a ferrite having a high permeability but a low magnetostrictive constant. In a modified form of magnetostrictive transducer an axial polarizing field is provided by means of a winding 36, Fig. 14, a circumferential signal field being applied by means of a toroidal winding 29 on a ferrite tube 28. The toroidal winding is applied to a detached portion of the ferrite tube which is subsequently cemented to the end of the main tube. The tube 28 is supported within an iron shell 31 by a mica washer 33 in an iron cheek 34 and by another cheek 35 of insulating material. The interior of the casing is filled with grease and may contain a matching transformer 37. A delay line is secured to the end 29 of the ferrite tube. In the various forms of the invention damping may be improved by tapering the end portions of the tube to which the damping material is applied. The line may be keyed to the damping material by means of longitudinal grooves which increase in depth towards the ends of the line. Specification 661,049 is referred to.