Patent GB-1535609-A (GB)
Title: Method of magnetising and adjusting the magnetisation of a permanent magnet
IPC: H01H51/22
Filed: 1976-08-02
Abstract: 1535609 Electromagnets; magnetization ELMEG - ELEKTRO - MECHANIK GmbH 2 Aug 1976 [1 Aug 1975] 32142/76 Heading H1P A magnetic system, e.g. that of a polarized relay (Fig. 1) comprises U-shaped yokes 1, 2, between whose bases is placed a permanent magnet 3 having a parallel 2-part shunt of U- shaped cross-section embracing the magnet with a gap of air or non-magnetic material 6, and between whose limbs an armature 7 is movable by excitation of surrounding coil 8 so that its lateral end faces bear against diagonally opposite limbs of the yokes 1, 2. The permanent magnet is magnetized from a D.C. source 9 (Fig. 4) connected to energize electromagnet 12 over switch 10 and a changeover switch 11 to develop a field between its poles to magnetize the shunted permanent magnet of, e.g., a relay placed therebetween. A clock-operated timing device 14 connects the electromagnet to the source until the permanent magnet is magnetized to saturation, and then reverses switch 11 to connect the electromagnet 12 to gate switch 16. A measuring device 17 responsive to the induction of the permanent magnet determined by the reluctances in series with the field thereof feeds output through a gate 18 to a comparator 19 receiving a reference signal from source 20, which defines a value calculated from the envelope of a hypothetical demagnetization curve. The difference signal operates function control device 21 for a variable resistor 22 passing a current proportioned to the difference, which in turn controls direct current from source 23 through the electromagnet in opposite polarity to source 9 so as to reduce the magnetic induction in the permanent magnet. If the difference signal from comparator 19 falls below a critical level, a Schmitt trigger discriminator 24 and amplifier 25 actuate switch 26 to disable source 23. Clock pulse generator 27 alternately opens and closes gates 16, 18 so that comparator 19 and source 23 are alternately connected to control resistor 22; thus the magnetic induction is measured with the electromagnet 12 de-energized to determine the demagnetization produced in the permanent magnet by source 23. After magnetization by source 9 and reversal of switch 11, the permanent magnet induction measured at 17 in the appropriate cycle of clock pulse generator 27 is compared at 19 with the reference value, and the device 21 (e.g. an operational amplifier) generates a signal controlling the flux of electromagnet 12 due to source 23. A large difference increases the electromagnet flux and the demagnetization of the permanent magnet is correspondingly increased but remains less than the initial flux due to source 9, so that the magnetic induction of the permanent magnet after demagnetization does not fall below that defined by reference source 20, and proceeds by steps which diminish successively with the difference between the measured and reference values of induction, until these values coincide approximately; when the difference indicated by discriminator 24 falls below a critical value. The procedure is applicable to unshunted permanent magnets, or plural permanent magnets disposed between the yokes; which may themselves be annular.  The permanent magnets may be magnetized externally of the magnetic system and fitted therein subsequently. Plural permanent magnets may themselves be magnetized together within the magnetic system. A lengthy theoretical discussion of the magnetic properties of the system is included.