GB779684A - Improvements in or relating to two-way ultrasonic devices with electroacoustic transducers - Google Patents

Abstract

779,684. Ultrasonic inspection devices. SIEMENS-REINIGER-WERKE AKT.-GES. May 6, 1955 [May 6, 1954], No. 13237/55. Class 118 (2). [Also in Groups XXXV and XXXIX] A device for ascertaining the degree of coupling between an electroacoustic transducer and the irradiated object comprises two transducers electrically separated but intimately associated as a unitary structure such that any variation in the degree of coupling of the one is accompanied by a similar variation in the coupling of the other, together with means for determining the radiation resistance of one transducer. A main transducer 11 (Fig. 1) of annular form is backed by an absorption element 16 and fed by screened lead 14 to supply, for example, pulses to a test piece and receive echoes from flaws therein, the magnitudes of which are deduced from the amplitudes of the echoes, which are dependent also on the degree of coupling. A central measurement transducer 10 of barium titanate or lithium sulphate is cemented to a brass or steel protective layer 12 of one wavelength thickness and connected rearwardly to a tube 13 which screens its cable 15 from lead 14 to the main transducer 11. The measurement oscillator may alternatively be in the form of an annulus around a central main transducer (Fig. 2, not shown), or it may be cruciform (Fig. 4, not shown) to ensure closer similarity in the couplings. The radiation resistance of the measurement transducer, shown at 52 in Fig. 5, is balanced against the resistance of an electrical simulation circuit 54, both being inductively fed by an oscillator 50 at a frequency of the same order as that of the main transducer operation such that the degree of coupling to the workpiece is similar, but electrical coupling is minimized. A measurement frequency of 1.12 Mc/s. is suggested for a main operation frequency of 2.5 Mc/s. with a bandwidth of 0.8 Mc/s. The potentials across resistors 58 and 59 in the measurement transducer and simulation circuits respectively are fed to two triodes 62, 63 arranged in a bridge network, as shown, with a measuring instrument 69 in its zero arm. The bridge is balanced by the variable resistor 65 with the oscillator 50 switched off. Then with oscillator 50 working and the transducer 52 perfectly coupled balance is obtained by varying resistor 56 and capacitor 57. If the transducer 52 is uncoupled the current in 69 rises to a maximum and the range 0 to 100 per cent coupling may be subdivided linearly for an approximate calibration. A more accurate calibration may be obtained by finding the change in value of resistor 56 to give zero current in 69 for various degrees of coupling and marking the calculated radiation resistance ratios at the positions occupied by the pointer prior to balancing. The output to the main transducer may be automatically controlled (see Group XXXV) to provide substantially constant power into the workpiece whatever the degree of coupling. This is effected by feeding the out-of-balance signal from the bridge network to the grid of an amplifier valve supplying the main transducer (Fig. 6, not shown).