Patent GB-579439-A (GB)
Title: Improved electromagnetic direction indicator systems
IPC: G01R33/02
Filed: 1944-03-31
Abstract: 579,439. Magnetic tests and measurements. GENERAL MOTORS CORPORATION. March 31, 1944, No. 5957. Convention date, May 19, 1943. [Class 37] An electromagnetic direction indicator system comprises a bar of highly permeable magnetic material having on two legs thereof, respectively a coil for carrying direct current and a pick-up coil forming part of a bridge circuit which is energised by alternating current and the output of which is amplified to operate a direction indicator or controller. A laminated bar 2, Fig. 1, of nickel-iron or similar high permeability alloy has an elongated opening 4 in the centre so as to form two spaced legs. A coil 6 is wound on one leg and is connected in series with a battery 8 and resistance 10 so as to provide a D.C. magnetising current to bring the flux density in the bar to the knee of the magnetisation curve. A coil 12 is wound on the other leg and is connected in one arm of an impedance bridge 20. The output of the bridge is connected to a band pass amplifier 22, the bridge being fed from an A.C. generator 24 with the frequency to which this amplifier is adjusted. The output of the amplifier 22 is connected to the primary winding 26 of a coupling transformer, the secondary winding 28 of which is connected to the grids 32, 38 of amplifying triodes 34, 40. The centre tapping of the secondary 28 is connected to a battery 42, the opposite terminal of which is connected to the filaments 46, 48 of the valves 34, 40. The anodes 50, 66 of the valves 34, 40 are connected through ammeters 54, 70 to relay operating coils 58, 74, the common junction of which is connected to a phase shifting network 64. The relay coils 58, 74 operate armatures 76, 78 which perform switching operations to steering or indicating means. The bridge 20 is balanced with the bar 2 in a given orientation in the earth's field so that there is no current flowing in the amplifier 22 or primary 26. If the position of the bar 2 changes with respect to the earth's field the flux through it changes and the inductance of the coil 12 alters and unbalances the bridge. Current then flows in the primary 26 and voltages in phase opposition are applied to the grids 32, 38. At the same time a voltage is applied to both anodes 50, 66 in phase from the phase shifting network 64 in order to give a direction sense to the impedance bridge. The E.M.F. applied to the grids by the battery 42 is set to prevent any normal flow. The valve of which the grid is made more positive conducts and completes a circuit through its relay coil to close its signal or control switch 76 or 78, so that one of these switches is closed when the inductance of the coil 12 increases and the other when the inductance decreases. The switches 76, 78 may control steering mechanism. In a modification, Fig. 2, two similar inductor bars 88, 90 are mounted parallel to one another on a rotatable plate 84 which has an index 86 to indicate its angular position with respect to a base plate 82. Each bar carries two coils 92, 94, 96, 98. The coils 94, 96 are connected in series with a battery so as to magnetise the bars 88, 90 in opposite directions. The inductances of the coils 92, 98 vary oppositely, and are connected in adjacent arms of the bridge.