Patent GB-1290389-A (GB)
IPC: G01N27/90
Filed: 1969-10-31
Abstract: 1290389 Eddy-current flaw detectors COMMISSARIAT A L'ENERGIE ATOMIQUE 31 Oct 1969 [2 Oct 1969] 53479/69 Heading G1N In an eddy current test apparatus an induction coil 1 is energized from an A.C. voltage generator comprising a fixed frequency oscillator 8, an oscillator 9 having a frequency which can be adjusted by means for example of a variable capacitance diode, and a mixing circuit 7 to which the voltages derived from the oscillators are applied. As shown, a selector switch 16 serves to apply to the diode a bias voltage. In position A a saw-tooth generator causes the voltage V to carry out the cyclic sweeping of a range of frequencies and by inserting tubes 4 having known defects it is possible to determine for each type of defect (i) the frequency Fo of the maximum value of sensitivity (ii) the frequency at which sensitivity is low or zero, and (iii) the value of voltage delivered to peak-to peak detector 15 as a function of frequency. In position B a saw-tooth generator 18 causes the frequency of V to vary in a series of uniformly spaced steps, the duration of one step corresponding to the time necessary for the motor 6 to cause tube 4 to carry out a single pass (few cms.) through the coil 1. In this way a different operating frequency is applied for each of n passes of a tube having a known defect and the n values of the voltage detected by 15, as displayed on C.R.O. 13, represents the characteristic curve of this defect. This procedure is repeated with tubes having different types of defects and a series of curves define in the case of each type of defect frequencies F 0 , F 1 , F 2 , and F 3 . The operation is synchronized by a sawtooth oscillator 19 which times each frequency step and switches an analog store connected to receive the signals from detector 15. The read out of the store to C.R.O. 13 is controlled by a clock 31. In position C, a D.C. voltage generator 20 maintains V at the fixed frequency F 0  corresponding to the type of defect being detected, and a control circuit 24 moves tube in either direction, the voltage delivered by amplifier 14 being applied to a threshold circuit 25 having a level adjusted to this type of defect and when said voltage is sufficient to trigger 25, the motor 6 is stopped. In this way the presence of the defect is accurately indicated. In position D, a D. C. voltage generator 21 and a selector switch 22 apply four different voltages to the diode which are fixed at frequencies F 0 , F 1 , F 2 , F 3 , and a program device 23 controls, for each tube to be inspected a sequence of four pass of the tube through the coil 1 and controls the four positions of the switch 22 so that each of the four passes is carried out in respect of each of the four frequencies. If a memory device 26 having a switch 27 controlled by the program device 23 indicates that the circuit 25 has been triggered at least once during the four passes, the tube in that case is automatically removed by selection apparatus 28. A field-effect transistor 2 is included in the Hartley oscillator, and a feedback capacitor 32 of the oscillator may be connected in series with a switch 33 which serves to convert the oscillator to a selective amplifier.