Patent GB-638120-A (GB)
Title: Improvements in electromagnetic inspection
IPC: G01N27/90
Filed: 1948-02-16
Abstract: 638,120. Magnetic tests and measurements. MAGNETIC ANALYSIS CORPORATION. Feb. 16, 1948, Nos. 4501 and 4502. Convention dates, Oct. 18, 1941, and Nov. 6, 1941. [Class 37] Apparatus for the electromagnetic inspection of metallic material to indicate defects or changes in structure, composition, physical dimensions, &c. comprises an oscillator with an energizing coil in its output circuit, an auxiliary system with a coil coupled to the output circuit of the oscillator or fed by a separate oscillator, a secondary circuit coupled to the oscillator and auxiliary system, and an indicating device also coupled to both, with adjustable means for balancing the voltage in the secondary circuit derived from the oscillator with the voltage derived from the auxiliary circuit. As shown in Fig. 1, a feedback oscillator 1, controlled by potentiometers 24, 31, feeds into a tuned circuit consisting of coil 3 and condenser 5. An amplifier valve 6, also fed from oscillator 1 via potentiometer 32, feeds into a similar tuned circuit consisting of coil 8 and condenser 10, coils 3, 8 being connected in series aiding and inductively coupled through the test material 20. Two secondary windings 11, 12 coupled respectively to coils 3, 8 are also tuned to the oscillator frequency by condenser 13, and are connected in series opposition, so that under normal conditions the voltages in 11 and 12 are equal and opposite and there is no indication on a meter 15 fed by an amplifier 14 from coils 11, 12, fine adjustment of this balance being provided either by a variocoupler 4-9, 34 or by providing a decade potentiometer at 32. If one coil 3 is affected by a change in material 20, the voltages in 11 and 12 become different, and there is an indication on meter 15. In a modification, Fig. 2 (not shown), the coils fed by amplifier 6 are not inductively associated with the test material 20, thereby producing a standard voltage opposing the induced voltage in secondary winding 11, and thus supplying a continuous indication of a change in the test material. The above two systems may be combined (Fig. 5, not shown). A further embodiment, Fig. 8, replaces the amplifier 6 of Fig. 2 (not shown), by a separate oscillator 101, similar to oscillator 1, and also substitutes a cathode-ray oscillograph 44 for the meter 115 and amplifier 114, the horizontal plates H being fed with the voltage from the auxiliary oscillator 101, and the vertical plates V being fed with the voltage across condenser 113, i.e. the combined voltages of the secondaries 36 and 111 associated with the auxiliary and main oscillators respectively. Thus, perfect balance with standard material 20 gives a straight line on the oscillograph 44, and variations in the test material 20 are shown in magnitude and character by the modification to this figure. The two oscillators 1, 101 are tied by a common variable resistance 45 in the leads to the anodes of oscillator valves 21, 121. The system of Fig. 8 may be combined with that of Fig. 1. In all the above embodiments the test material, if ferrous, may be supplied with a superimposed D.C. field from coil 16. The main and auxiliary oscillators 1, 101, may be of the heterodyne or negative resistance type. The coils may be arranged in two groups 3, 11, 111 and 8, 12, 112, Fig. 5 (not shown), the coils in each group being wound concentrically or end to end, and the groups being arranged displaced either axially along the test material or axially of the test material. The test material is fed spirally through the fields of the coils. Specification 4500/48 (as open to inspection under Sect. 91), is referred to.