Patent GB-812331-A (GB)
Title: Improvements in or relating to ultra-sonic methods and equipment
IPC: G01N29/34
Filed: 1954-04-12
Abstract: 812,331. Ultrasonic inspection devices. LEWIS, E. M., and FOX, J. W. July 12, 1955 [April 12, 1954], No. 10688/54. Class 118 (2). Apparatus for the examination of material by ultrasonic oscillations of a first frequency using a probe adapted to be applied to a surface of the material, comprises means for inducing oscillations of a second frequency in said material at substantially the same surface and means for indicating or responding to the effectiveness with which the oscillations are induced in the material as a check on the coupling between the probe and the material. A trigger unit 10 controls two pulse generators 11, 12 of the same pulse recurrence frequency but of different ultrasonic frequencies. These feed a probe unit 13 applied to a test specimen 14. The same transducers may or may not be used for transmission and reception, and the transducers for the different frequencies may be combined in a multiplate construction (not described). In any case the transmissions of the two frequencies are applied via substantially the same probe-specimen interface. The receiver transducers (indicated diagrammatically as on the same probe body) are connected respectively to two amplifiers 15 and 18. The oscillations of the first frequency are displayed on cathode-ray tube (C.R.T.) 16. Amplifier 18 receives the second frequency pick-up and is normally of low gain but is gated by a pulse from unit 19 to have high gain at a predetermined instant after transmission of each ultrasonic pulse from generator 11. In one method of use the amplifier 18 is gated when oscillations reflected from the probe-specimen interface are received. Such reflection varies inversely with coupling and if above a predetermined value an alarm, indicator, control or like device 20 is operated. In another method of use wherein the specimen has acceptable internal discontinuities which give rise to reflection, the amplifier 18 is gated to high gain when these reflections are received and in this case the alarm or the like is operated when the output does not exceed a predetermined limit. For convenience of setting- up a marker pulse may be shown on C.R.T. 16 by a suitable signal over line 21. The output of amplifier 18 may also be displayed on C.R.T. 16. With the same pulse recurrence frequency this may be done by suitably phasing the two transmissions and beam switching. The output of amplifier 18 may be used to step the C.R.T. trace in accordance with the coupling between probe and specimen; if a flaw signal deflects tha trace in the opposite direction it can be arranged that a signal from a significant flaw just reaches a datum line notwithstanding variations in the coupling. Alternatively the gain of amplifier 15 may be controlled by the output of amplifier 18 to compensate for variations in coupling. It is preferred that the higher frequency oscillations have a higher pulse recurrence frequency so that a plurality of pulses are transmitted between each pulse of the lower frequency. Instead of two generators 11, 12, only one may be used and the probe body split obliquely and a member resonating at a harmonic frequency inserted therein to provide oscillations at a higher frequency emerging at a different angle. Specifications 812,332 and 812,333 are referred to.