| 摘要 |
1498218 Magnetic flaw detectors KENNECOTT COPPER CORP 27 May 1976 [17 June 1974] 23123/75 Heading GIN In an arrangment for detecting the presence of magnetic anomolies in ferromagnetic and non- ferromagnetic materials the output is modified to take account fo the velocity of passage of the material being tested through the detector head. A rod, wire, tube or bar 6 of material to be tested is driven by a pair of rollers 15 through a guide tube 5 extending axially through a cylindrical magnet 10 producing in tube 5 substantially uniform magnetic field. A pair of closely coupled series connected sensing coils 4, 14 are positioned around tube 5 and coupled in phase opposition by transformer 15 to amplifier 8; alternatively the coils may be wound in series opposition in which case a normal step-up transformer may be used. The e.m.f. induced in the coils 4, 14 by the passage of the material 6 therethrough will be proportional in amplitude to the magnitude of magnetic dipoles included in the anomolies and to their velocity through the field of the magnet 10. In order to make the output of amplifier 8 independent of the velocity the tachometer 12 is positioned as shown and its output is applied to one input of operational amplifier 10, the output of which is connected to a photo-conductive cell 12 which modifies the feedback impedance across amplifier 8 so that the gain of the latter varies inversely with the velocity of tube 6. Thus the output of 8, which may be fed to a CRT or strip chart recorder 9, is proportional to the size of the anomoly, external field disturbances cancelling out. Magnet 10 is made to have a sufficiently long magnetic field so that the dipoles can charge to a uniform degree independent of the magnetic constant of the ferromagnetic material and any eddy currents in the tube 6 as a result of magnetic discontinuities at the entrance of the magnet 10 have time to decay before the part passes through the coils. In an alternative circuit for providing velocity compensation Fig. 4 (not shown) the amplifier 8 has a linear gain characteristic and its output is coupled via a diode to the indicator, the diode being back biased by the output voltage from the tachogenerator. The effect of the circuit is to increase the clipping of the output of the amplifier as the velocity of the speciment 6 increases. |
