Patent GB-1095625-A (GB)
Title: A method of and apparatus for establishing the presence of a material having known electro-magnetic properties
IPC: G01V3/08
Filed: 1965-12-13
Abstract: 1,095,625. Detecting electromagnetic material. RIO TINTO MANAGEMENT SERVICES SOUTH AFRICA (PROPRIETARY) Ltd. Dec. 13, 1965 [Dec. 14, 1964], No.52858/65. Heading G1N. Stray metallic objects of known electromagnetic characteristics in a stream of a second magnetic material of known electromagnetic characteristics are detected by creating two primary A. C. magnetic fields in the material, of different frequencies. Secondary fields are thereby produced by each material whose magnitudes and directions depend in different ways on the frequencies of the applied fields. The resulting fields are then detected, the difference in magnitude of the fields at the two frequencies giving an indication of whether a stray object is present or not. In the embodiment, which is used for the detection of excavator teeth in magnetite rock, coplanar coils 12, 13, Figs.2, 3 (former not shown), surrounding a conveyer (15) carrying the magnetite rock, supply the two A. C. magnetic fields. A third coplanar coil 14 detects the resultant magnetic fields. The frequencies of the applied fields are chosen to be 80 c/s and 5À1 Kc/s. for which it is found that, for magnetite rock, the secondary fields produced are approximately equal to each other and in the direction of the primary field, while for an excavator tooth the fields are again equal but in opposite directions; at the low frequency the secondary field aids the primary field while at the higher frequency it opposes this field (see Fig. 1, not shown, in which curve 11 represents a piece of magnetite rock and curve 10 an excavator tooth). The high frequency current in the coil 13 is supplied by an oscillator 16, Fig. 3, oscillating at 5À1 Kc/s per second. The oscillator output is amplified at 17, and applied through a filter and matching network 18 to the coil 13. The output from the oscillator is also passed through a frequency dividing circuit 19, which provides an output signal of 80 cycles per second, an amplifier 20, a filter and matching network 21 to the coil 12. The detecting coil 14 is connected to an amplifier 22, whose output is applied to two filter circuits 23, 24 which separate the low and high frequency signals obtained from the coil 14. The outputs of the filters are then applied to two phase sensitive rectifiers 25, 26, designed to give maximum response to the signals at 5À1 Kc/s per second and 80 cycles per second respectively in the coil 14. A comparator 27, to which the output signals of the rectifier are applied, then derives the difference between the two output signals which is finally applied to an alarm circuit 28, set to operate when the difference voltage exceeds a certain value. The circuit is so arranged that when the conveyer belt (15) is empty, the voltages induced in the received coil are cancelled by means of two back-off signals obtained from networks 29, 30, so that there is no difference output from the comparator 27. When only magnetite rock is present, the induced fields at the two frequencies are approximately equal so that again no current will flow in the alarm circuit 28. But, when an excavator tooth passes between the coils, at 80 c/s, the secondary fields in the two materials at 80 c/s both aid the field, while at 5À1 Kc/s the two fields are in opposite directions thus partially cancelling each other out. Since unequal signals are therefore being fed into the two filter circuits the comparator 27 will provide a difference signal which will actuate the alarm circuit to indicate the presence of an excavator tooth. This alarm circuit (not shown in detail) may be arranged to stop the conveyer belt, mark the belt, sound an alarm or even eject the tooth.