Patent GB-1372352-A (GB)
Title: Transducers
IPC: H04R19/00
Filed: 1971-10-28
Abstract: 1372352 Electrostatic transducers UNITED KINGDOM ATOMIC ENERGY AUTHORITY 28 Oct 1971 [28 July 1970] 36580/70 Headings H4J and H4D A polymeric film 12 (Fig. 5) is situated between and contacts two electrically conductive surfaces, e.g. provided by conductive layers, 14, 14A. One face of the film 12 is opposed to a face of a member 10 (e.g. a workpiece under test) over at least substantially the whole extent of the one face of the film 12. That area of the face of the film 12 which is opposed to a face of the member 10 is mechanically coupled to and conformed to the contours of the opposed face of the member 10. An electrical disturbance between the conductive layers 14, 14A, creates acoustic disturbance in the portion of the member 10 adjacent the film 12 and vice versa, The layers 14, 14A may be provided by aluminium or gold coatings. Adhesive secures one coating to the member 10. The film 12 is of polyethylene terephthalate, nylon or epoxy resin. Either the film 12 is an electret or a polarizing voltage is applied across the layers 14, 14A. If the member 10 is conductive the layer 14A may be dispensed with and the film 12 held against member 10 by electrostatic attraction. An electrically conductive screen 18 is provided in which is mounted an insulating block which engages layer 14. In an alternative arrangement (Fig. 3) electrically insulating arms 30, 32 of a spring clamp have electrically conductive protuberances 34, 36 which enable electrical connections to be made to a metal coating 14 on a polymeric film 12 and to a conductive member 10. A polymer paint 37 may be applied around the edge of film 12 with the aim  of increasing the permissible polarizing voltage. Where the member 40 (Fig. 4) is non-conductive it may have a conductive layer 42 of silverloaded paint. A metal coating 14 on a polymeric film 12 makes electrical contact with a contact block 44. A further contact block 46, insulated from block 44, is bored to receive a conductive plunger 48 which is urged into contact with layer 42 by a spring 50. A grub screw 52 allows limited movement of plunger 48. In another transducer (Fig. 8) a polymeric film 12 has a metallic coating in the form of a conductive spiral 72 produced by etching. North and South polepieces 74, 75 produce a magnetic field around the film 12. In operation, a polar. izing voltage E is applied between coil 72 and member 70. Film 12 is held by electrostatic attraction against member 70. When a voltage pulse is applied, as by a blocking oscillator 76 between the ends of coil 70, an acoustic pulse is produced in member 70. The magnetic field provided by the poles 74, 76 increases the motion by means of a motor effect. An alternative transducer which makes use of a coil (84), Fig. 9 (not shown), to provide a magnetic field acting substantially perpendicular to a film 12 is also described. The transducer arrangement may be used in conjunction with a charge amplifier (26), Fig. 2 (not shown), to detect an acoustic pulse in a member such as member 10 (Fig. 5) or with a blocking oscillator (56), Fig. 6 (not shown), to project an acoustic pulse in the member 10. Two transducers, one operating in the acousticto-electrical mode and the other in the opposite mode, can be used to form equipment for measuring the velocity of acoustic pulses in a member. A pulse-echo single probe crack detector (Fig. 7) comprises a film 12 having a polarizing voltage E applied between a connector 20 and earth. This provides electrostatic attraction between member 10 and film 12. A blocking oscillator 60 applies a high voltage pulse to develop an acoustic pulse in member 10. The acoustic pulse is reflected by any crack in member 10. The reflected pulse causes production of a charge change in the transducer and this produces a signal which is passed through a buffer stage 62 to a charge amplifier 64, from where an output signal is derived.