GB1155817A - Magnetic nondestructive testing system - Google Patents

Abstract

1,155,817. Magnetometers; detecting flaws. F.M.O. FORSTER. June 7, 1968 [June 9, 1967], No.27192/68. Heading G1N. A MAGNETOMETER comprises a torroidal core 32 with input and output windings 36, 38 and preferably biased by a permanent magnet 40 so that an external magnetic field varies the degree of coupling between the coils. In one arrangement Fig. 2 the input coil is fed from an oscillator and the variation in the amplitude of the output is measured. In an alternative arrangement Fig. 4 (not shown) both coils are connected in the oscillator circuit so that the variation in inductance and coupling varies the oscillator frequency. To detect FIELD GRADIENT pairs of magnetometers such as that in Fig. 2 may be disposed side by side as illustrated in Fig.6, or Fig. 7 or Fig. 8 (not shown) with their output windings connected in series opposition. DETECTING FLAWS. Fig. 1. An elongated workpiece 28 is mounted for rotation while the magnetometer 10 is traversed along its length, thereby making a spiral scan of the surface. The workpiece is first magnetized by passing current through it from a D.C. source. In an alternative arrangement (Fig. 9) the magnetometer may be arranged to rotate around a workpiece passing through its support member. For use in detecting flaws the input and output windings may be tuned to resonance by capacitors 66, Fig.2, while the magnetometer probe is spaced from the workpiece. As the probe approaches the workpiece its resonant frequency changes and the sensitivity decreases as the field from the workpiece increases thereby maintaining the overall sensitivity of the probe substantially constant. Signal output from a rotary probe may be transmitted by coupled bifilor windings instead of by slip rings Fig. 10 (not shown).