GB1067764A - Method and apparatus for determining hardness of ferromagnetic material - Google Patents

Abstract

1,067,764. Measuring hardness of ferromagnetic material. ASSEMBLY PRODUCTS Inc. May 4, 1964, No. 18506/64. Heading G1N. An apparatus for testing the hardness of a moving strip of magnetic material comprises a magnetizing head 12 (Fig. 2) and a pick up head 13, whereby the retentivity of .the material is determined as a measure of its hardness, the greater the retentivity the greater the hardness. The strip 10 is magnetized in spots by the magnetizing head 12 which is energized in pulses by a constant frequency A. C. source, e. g. at 40 cycles, in the form of a pulse generator. At a second location the pick up head 13 receives signals induced therein by the retained magnetic field; the signals are amplified, filtered to remove those not at the pulsed frequency, rectified and fed to a modulator. An erase head 15, powered by D. C. or relatively high frequency current, is provided and the degree of erasing is modulated by the integrated picked up signal so that as the signal becomes larger the erase field becomes stronger tending to reduce the picked up signal to its original level, and is in effect a negative feedback arrangement. The amount of feedback is measured on a meter 14 and is taken as a measure of hardness. The erase field compensates for inadvertent change in distance between the head 13 and the strip. For instance as the distance increases the picked up signal weakens, the erase field weakens tending to restore the picked up signal. In a further embodiment Figs. 3, 4 (not shown), so that change in distance between the head 12 and the strip may be compensated for, the erase coil is wound on the same core as the magnetizing head coil. The measured value may be used for process control.