Patent GB-1468852-A (GB)
Title: Method and device for eddy-current detection of a change in magnetic behaviour of a material
IPC: G01R33/12
Filed: 1974-06-12
Abstract: 1468852 Electro-physical testing INSTITUT DE RECHERCHES DE LA SIDERURGIE FRANCAISE 12 June 1974 [29 June 1973] 25965/74 Heading G1N In an eddy current testing arrangement for detecting the transformation of a conductive material from a first non-magnetic state, e.g. paramagnetic, to a second magnetic state, e.g. ferromagnetic, the effects of variation in the distance between the probe and the test coil are mitigated. A coil 4, maintained at a constant temperature, is positioned adjacent a moving steel sheet to be tested, Fig. 6 (not shown) and supplied with a constant A.C. from a constant A.C. voltage source 1, waveform a Fig. 7, and current regulator 3. The output f across the coil, representative of its impedance is fed to differential amplifier 2 to the other input of which is fed a signal from a balancing circuit 5 which includes a differentiating circuit 6, providing a #/2 phase shift, variable gain amplifiers 7a, 7b and an adder 8 from the output of which is obtained signal e which can be adjusted in phase and modulus. Initially, in the absence of the test object, the phase and modulus of the signal e is adjusted to be equal to that of signal f, representative of the self impedance of the coil, so that the output of amplifier 2 is zero. This balancing step has the effect of transferring the axis of the impedance vector diagram of Fig. 1 from the point O to a point K oo  corresponding to the self-impedance of the coil 4. Under these conditions in the event of a change in magnetic behaviour of the material subsequently tested the point M is displaced along the curve C3 between curves C1 showing the variation of impedance of the coil for a non-magnetic material for differing air gaps and curve C2 showing the variation for a magnetic material. The phase displacement of the signal represented by the vector K oo M with respect to a reference is then representative of the magnetic behaviour change irrespective of the air gap, between the probe and sheet. Phase detection. After the balancing step the probe 4 is placed adjacent the steel sheet whereby eddy currents induced in the steel result.in a change in the effective impedance of the coil 4 which gives rise to an unbalance signal at the output of the amplifier 2, signal g. The signal is filtered at 16 and fed to the positive input of flip-flop 10a to generate the signal i. At the same time a reference signal c derived from channel B is fed to the negative input of flip-flop 10b to generate the signal h. Signals l and h are fed to AND gate 11 to generate signal j the width of which is directly proportional to the phase difference between signals c and g. After integrating at 12 the signal k is fed to threshold detector 12 which controls the lighting of lamp 15 when the threshold level l is exceeded. Instead of using circuit 5 a coil identical to coil 4 may be used. The invention may be used in thermo-mechanical or thermal treatment operations to detect the commencement of crystalline transformation of a steel from γ structure to α structure or to determine Curie point temperature.