Patent GB-645868-A (GB)
Title: Improvements in or relating to apparatus for inspecting material
IPC: G01N29/07
Filed: 1947-01-08
Abstract: 645,868. Supersonic inspection devices. SPERRY PRODUCTS, Inc. Jan. 8, 1947, No. 644. Convention date, Jan. 17, 1946. [Class 118 (ii)] [Also in Group XL (c)] ; Relates to pulse echo systems for testing materials for the presence of defects comprising means for transmitting recurrent pulses of supersonic energy through one surface thereof and for receiving echo pulses reflected by a defect, and a receiver responsive to said transmitting and receiving means adapted to actuate an indicating circuit including a meter indicating the distance of the defect below said surface. According to the present invention in order to prevent indications due to the transmitted pulse and the echo pulse reflected from the opposite surfaces of the material, a control circuit is provided for rendering the indicating circuit responsive to the receiver only during an operative period commencing at the termination of a transmitted pulse and terminating immediately prior to the reception of the echo pulse from the opposite surface of the material. As shown in the Figure recurrent exponentiallydamped electrical wave trains from generator 10 are applied to a piezo-electric crystal 11 in contact with surface S of the material 12 and the crystal is also coupled to a receiver 15, the pulse output from which is applied to the grid of a thyratron 60 connected in series with a pentode 70 which is biassed to cut-off. Positive square-wave gating pulses from generator 80 corresponding in timing and duration with the required operative period are applied to the grid 9<SP>11</SP> of pentode 70 to render it conductive so that an echo pulse produced by a defect D or D<SP>1</SP> triggers thyratron 60 and discharges condenser 61 producing a negative saw-tooth voltage which is terminated when pentode 70 is cut off at the end of the operative period so that the duration of the saw-tooth and hence its peak amplitude is inversely proportional to the distance of the defect from the surface S. When the saw-tooth terminates, condenser 62 is rapidly charged through diode 63 by an amount proportional to the peak amplitude and this charged leaks to earth through the meter 16 which is calibrated to indicate the required distance. The amplitude of the echo pulse which indicates the magnitude of the defect is measured by applying the pulse output from receiver 15 through a condenser 96 to a meter 98 connected in a similar manner to meter 16, the cathode of the associated diode 97 being negatively biassed so that the diode is blocked except during the operative period when the bias is counteracted by the positive gating pulse applied from generator 80 to the diode anode. The generator 10 comprises a saw-tooth generator 31, the output from which is applied through an inverter valve 35 to trigger a thyratron 37 and excite a tuned circuit 38, 39 and the generator 80 comprises a cathode-coupled flip-flop 82, 81 triggered from valve 35 of generator 10 the timing of the output positive square-wave from inverter valve 87 being adjusted by a variable condenser 85 and the duration being adjusted by condenser 90 and resistance 91. Specifications 641,081 and 641,095 are referred to. According to the Specification as open to inspection under Sect. 91 the invention is applicable to aircraft pulse echo altimeters for indicating whether the aircraft is within a predetermined distance from the ground. This subject-matter does not appear in the Specification as accepted.