Patent GB-926851-A (GB)
Title: Improvements in and relating to electro-mechanical transducers
Inventor: BRADFIELD GEOFFREY
IPC: G01N29/24
Filed: 1958-05-15
Abstract: 926,851. Flaw detecting probes; local circuits. NATIONAL RESEARCH DEVELOPMENT CORPORATION. May 26, 1959 [May 15, 1958; Feb. 19, 1959], Nos. 15706/58 and 5771/59. Class 40 (4). [Also in Groups XL (b) and XL (c)] In order to reduce the variation of sensitivity with range in an ultrasonic flaw detector, use is made of a piezo-electric transducer excited with an intensity which is graded to fall off in steps from its centre or central axis outwards. The transducer comprises a single element (of e.g. quartz, lithium sulphate or a ceramic such as barium titanate) and may be in the form of a disc or a rectangular bar. To obtain graded excitation, one of the electrodes is subdivided, e.g. into a central electrode and one or more annular electrodes in the case of a disc, or into a number of longitudinal strips or transverse bars in the case of a rectangular element. The electrode widths are a substantial multiple of the wavelengths at the working frequency. Where there are three annular electrodes, the diameter of the central electrode may be 1/3 of the overall diameter d, the external diameter of the first annular electrode may be d/2 and that of the second 3d/2. The excitation applied to the electrodes may diminish from the centre, e.g. it may be graded according to the sequence 1, 2/3, 1/3, 1/9 in the case of the four-electrode arrangement referred to above. Alternatively, the same excitation may be applied to all electrodes but the annular one may be broken up, e.g. by means of circumferentially spaced gaps. The excitation of the outer electrodes may be in phase with or may lead the excitation of the central electrode. Alternatively, where there are only two electrodes, the annular electrode may be excited in alternate transmitter pulses with voltages which lead and lag the voltage on the central electrode by a quarter period. The electrodes may be separated by V-shaped grooves. The element may be provided with a backing of epoxy resin loaded with sufficient tungsten powder to make it conductive and the subdivided electrode may be applied to the outer face of the backing. The backing may be in two or more concentric portions separated by cylindrical insulating members of, e.g., polymethyl methacrylate or polytetrafluoroethylene whose inner edges lie within grooves in the surface of the element. The backing may be applied by moulding under heat and pressure. The exciting means may comprise a thyratron arranged to discharge a condenser into a tuned circuit consisting of the crystal capacitance and a coil mounted close to the crystal. The outer electrodes may be excited from adjustable tappings on the coil. The transmission surface of the element is uniformly electroded and may carry a number of matching layers, e.g. two layers the innermost of which has a characteristic impedance # c <SP>¥</SP>C c <SP>¥</SP># # <SP>“</SP>C # <SP>“</SP> and the outermost a characteristic impedance # c  <SP>“</SP>C c  <SP>“</SP># #  <SP>¥</SP>C #  <SP>¥</SP> where # c ,# #  are the densities of the element and the liquid transmission medium and C c , C #  are the corresponding wave velocities. Graded excitation of the crystal may be obtained by a gradual or stepwise tapering of the thickness of the matching layers towards the periphery of the element. Alternatively, the outer portions of the matching layer may be in the form of annuli divided by gaps into segments. Specifications 766,982 and 841,512 are referred to.