GB870487A - Flaw detecting device - Google Patents

Abstract

870,487. Electric tests. REPUBLIC STEEL CORPORATION. Jan. 24, 1958, No. 2413/58. Class 37. In a flaw detector of the type in which a coilforming part of an oscillator tank circuit is moved over the material under test an amplifier fed from the oscillator output provides a feedback signal to the oscillator through a circuit of time constant sufficiently long to ignore small variations to maintain the oscillator amplitude constant and at the same time to provide an indication of flaw depth. As shown, the search unit 10 is moved over the object under test and includes a coil 16 forming part of the tuning circuit of an oscillator and a neon lamp which glows when flaws of more than a certain depth are detected. The unit 11 is carried by the operator and comprises the circuitry shown in and described with reference to Figs. 4 and 5 and a meter in the top of the unit which indicates flaw depth. Circuits (Figs. 4 and 5).-Search coil 16 forms with capacitor 49 the tuned circuit of an oscillator comprising triodes V1, V2. The setting of potentiometer 58 feeding the grid of feed-back triode V2 determines the initial amplitude of the oscillations. The oscillator output is fed to cathode follower V3 and high-frequency (oscillator frequency) amplifier V4. The output of the high-frequency amplifier V4 is fed through transformer 82 to full-wave rectifier V5. The high-frequency components are removed by filter circuit 87 and the low-frequency signal remaining is representative of the variation in amplitude level of the oscillations. Part of the signal is tapped from potentiometer 91 and fed through a circuit 63, 60, 62 whose time constant is such that amplitude variations of relatively low duration are ignored, to the grid of tube V2 to maintain the oscillation amplitude constant. At the same time the demodulated signal is fed through clamp V7 which ensures that only positive impulses are applied to low-frequency amplifier V8 and from there to a further amplifier V10, a clamp V9 being provided so that only negative impulses are applied to potentiometer 114. This potentiometer controls the amplitude of the pulses applied to amplifier V10 which supplies neon 27 in the search head so that a flaw of greater than a particular depth causes the neon to light. The negative impulses from clamp circuit 30 are also fed through potentiometer 130, the setting of which controls the amplitude on the signals reaching meter 14 which indicates flaw depth. The signals are amplified at V11, fed through transformer 137 to a rectifier comprising diodeconnected triode V12, through long time constant circuit 148 to produce a relatively steady bias and the grid of cathode follower V13. The output of the cathode follower actuates the flaw depth meter 14, the arrangement being that the cathode current moves the meter to the full scale reading but is reduced by the measuring voltage. Specification 727,217 is referred to.