Patent GB-638285-A (GB)
Title: Improvements in and relating to electro-mechanical transducers
Inventor: BRADFIED GEOFFREY
IPC: G01N29/24
Filed: 1947-02-20
Abstract: 638,285. Supersonic inspection devices. MINISTER OF SUPPLY. Feb. 20, 1947, No. 5024. [Class 118 (ii)] Supersonic testing &c. apparatus comprises a group of electromechanical transducers adapted to be coupled to spaced points on a body to be tested, means for launching wavetrains of pulse form in the transducers, such means being energized in a predetermined phase relationship, and each individual transducer being provided with means for conducting away from the launching means wavetrains which are launched in the transducer in directions other than towards its coupling to the said body, such that there is no interference with the wavetrains transferred by the coupling. Each transducer may comprise a nickel or other magnetostriction element A B, Fig. 1, of laminated form and long in comparison with the distance from the end A to a fault P in a solid material N to be tested, and mounted thereon near the end A a transmitting coil C1 and receiving coil C2. Permanent horseshoe magnets M1, M2 attached to soft iron formers D1, D2 form polarizing means. The coil C1 is connected to a pulse generator G and the coil C2 to an amplifier and cathode-ray oscillograph O. A pulse from G, only one-half sinusoid in length, by discharging a condenser through a thyratron with the coil C1 in its cathode circuit, is injected into the transducer element at Q and sets up wavetrains in the directions QA and QB. The time the wavetrain QB and the reflected wavetrain QA from the surface of the material N takes to travel to B is longer than that for the wave to travel from A to P-and back to A so that the indication of a fault P received at S is free from interference. The end B may be ribbed and surrounded by energy-absorbing material such as sand to obviate long elements AB when using low frequencies. Three such magnetrostriction elements can be combined. They are spaced equally, and a linkage arrangement enables the coil assemblies on the two outer elements to be moved relativelv to that on the centre element and to each other. The three transmitting coils are connected in parallel to a pulse generator, and the three receiving coils are connected in series to an amplifier and oscillograph. This enables the phases of the three wavetrains entering the test material to be varied. In the form shown in Fig. 3, three piezo-electric transducer elements 15, 16, 17 placed vertically and at equal distances apart on a solid test material 36, have their transmitting electrodes 18, 19, 20 connected through phasing networks 24, 25, 26 to a pulse generator 27. The direction of maximum reinforcement of the wavetrains 33, 34, 35 will depend on their relative phases. The pulses reflected from a fault or interface are received at 21, 22, 23, passed through phasing networks 28, 29, 30, a mixer and amplifier 31 and shown on a cathode-ray oscillograph 32. Since the amplitude of the integrated pulse will be a maximum when the fault lies in the direction of maximum reinforcement of the wavetrains 33, 34, 35, the bearing and range of the fault may be determined. In further modifications, two-dimensional arrays of transducers grouped over an area may be used for sweeping a large area of the test material; separate transmitting and receiving transducers may be used for estimating thicknesses of seams of coal or other material; the transducers of a set may be mounted in a carrier and each spring-pressed towards the test material, provision being made for adjusting the coil assemblies along each rod element; and in the case of a piezo-electric element, an elongated electrode applied thereto has a number of discrete electrodes thereon adapted to be. engaged by slider contacts to which the exciting signal is applied.