Patent GB-772083-A (GB)
Title: Improvements in and relating to the transmission of ultrasonic vibrations
Inventor: BRADFIELD GEOFFREY
IPC: G01N29/26
Filed: 1952-09-20
Abstract: 772,083. Ultrasonic inspection devices. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Dec. 21, 1953 [Sept. 20, 1952], No. 23655/52. Class 118 (2). [Also in Group XXXIX] In a pulse-echo flaw detector in which a beam of ultrasonic waves is produced by an array of piezo-electric transducer elements composed of barium titanate or a mixture of barium and lead titanates, the direction of propagation is switched by varying the relative phase of energization of the elements by discrete amounts. The direction of propagation may also be varied by mechanical movement of the array. The array may comprise (1) parallel strips of piezoelectric material spaced apart 0.35#-0.62# and cemented to corresponding metallized strips on a backing member, shallow grooves being cut in the backing member between the metallized strips to reduce mechanical mutual impedance, Fig. 3 (not shown), or (2) a grooved sheet of piezo-electric material as described in Specification 703,508, [Group XL (b)]. A metal diaphragm or coupling member is interposed between the array and the workpiece. Dimensions of the array are given for operation at frequencies of 1.25 and 2.75 Mc/s. The metallized strips are connected directly or via individual capacitors to tappings on a coil so that the transducer elements B, Fig. 8, constitute the shunt elements of an electrical delay line, Figs. 4-6 (not shown), the output from the transmitter electrical pulse generator 62 being applied selectively to one end of the line or the other by means of a relay 61 or manuallyoperated switch, Fig. 7 (not shown), to vary the relative phase of energization of the elements. The delay line is preferably of the constant-m derived type with m=1.3. Side lobes of the directive beam may be reduced by progressively reducing the energization of the elements away from the centre by inserting attenuator sections in the shunt arms of the line. As shown in Fig. 8, each end of the line is terminated by its characteristic impedance Z0, but alternatively a single impedance can be connected through a switch to the unfed end of the line, Fig. 7 (not shown). In Fig. 8, the array is used for transmission and reception, the delay line being energized through a transformer 63 and balancing resistance 65 equal to Z0/2 and the receiver being connected between the centre of the secondary and the transducer end of resistance 65. Alternatively, as in Fig. 10, each end of the delay line is tapped into a resistance shunted by a choke, the transmitter output is applied across the resistance, the receiver input is connected between tappings on the choke and resistance and relay contacts are provided to connect the transmitter and receiver to either end of the line. In Fig. 11 (not shown), separate receiving elements are provided at each end of the array, the resultant directivity pattern of these elements being broad enough to include the angular sweep of the transmitted beam. Fig. 12 shows an array of 18 elements, B1 . . . B18, which may be connected alternatively by means of ganged switches D1 . . . D18 and D20, D22 . . . D36 to tappings 1 . . . 18 or to tappings 2, 4, 6 . . . 36 giving two beam positions, two more beam positions being obtained by energizing the delay line at the left- or right-hand by means of switches A1, F2 and A2, F1 and a fifth position being obtained by closing the ganged switches E1 . . . E18 to energize the elements in phase; compensating capacitors C1 . . . C17 and C20, C22 . . . C36 are connected to tappings 1 . . . 17 and 20, 22 . . . 36 respectively by means of the switches when no transducer elements are connected to these tappings. By interconnecting the contacts of switches D1 . . . D17 and D20, D22 . . . D36 one set of capacitors C1 . . . C17 or C20, C22 . . . C36 could be dispensed with. Indicator. The received echo pulses are applied to deflect the trace of a cathode-ray oscilloscope. When the transmitted beam has two positions, the corresponding echoes may be displayed with opposite polarities, Figs. 14 and 15 (not shown), and when it has more than two positions the trace may be displaced synchronously with the beam switching; the cathode-ray is preferably blanked during the change-over of the switches, Fig. 9 (not shown). Mechanical adjustment of array, Fig. 1. The invention is applicable to an arrangement of the type described in Specification 766,984 in which the transducer array 45 and its backing member 46 are mounted on a " shoe " 44 which is movable by means of a bell-crank lever 53 over the cylindrical or spherical surface of a coupling block 41. The delay line 56 and its relay 55 are mounted on the upper surface of the backing member 46 which may be shaped to minimize the reflection of waves therefrom and the whole assembly is enclosed in a metal screening box 53 secured to the block 41 by pegs 59. The shoe 44 and coupling block 41 may be metal or plastic and are coated with absorbent material, e.g. plastic loaded with asbestos fibre, powdered mica or tungsten, the backing member 46 being formed of similar material. Alternatively, the array may be applied selectively to the surfaces of a prismatic coupling block or two test heads may be employed, the beam positions of one head being intermediate three of the other head, Fig. 13 (not shown). Specifications 713,339 and 758,631, [Group XL (b)], also are referred to.