Patent GB-611027-A (GB)
Title: Improvements in electromagnetic apparatus of the flux-gate type
IPC: G01R33/02
Filed: 1944-10-31
Abstract: 611,027. Inductances. SMITH & SONS (ENGLAND), Ltd., S., and MEREDITH, F. W. Oct. 31, 1944, No. 21298. [Class 38 (ii)] [Also in Groups XXXVI and XL (c)] An electromagnetic apparatus of the fluxgate type has the magnetic core in the form of a bridge analogous to a Wheatstone bridge but substituting magnetic flux paths for electrically conducting paths and magnetic reluctances for electrical impedances. The flux-gates and the local windings which control the local flux therein are such as to unbalance the flux bridge in one direction or the other to allow more or less of the main flux to flow through the pick-up or electrical output coil in one direction or the other. A flux bridge is shown diagrammatically in Fig. 1, in which the channel 16 may be a path of the earth's magnetic flux or may be an actual core part energized by an input coil 17. Windings 19, 20, 21, 22 surround the core parts forming the arms of the bridge, an output coil 18 surrounding the core part constituting the diagonal 14, 15. If the flux-gates 19, 21 are closed while 20, 22 are open, the M.M.F. between 12, 13 causes a flux along the path 12, 15, 14, 13, whilst if the gates 19, 21 are open and 20, 22 are closed the flux passes along the path 12, 14, 15, 13, so that if these two fluxes alternate at a predetermined frequency an alternating voltage is generated in the coil 18, the amplitude being double that produced by a single pulse of flux in one direction. A fluxgate compass is shown in Fig. 2 and has three parallel limbs 23, 24,.25 joined by limbs forming the four arms of the bridge. Flux collector plates 26, 27 project from the outer limbs 23, 25, the inner limb 24 carrying the output or pick-up coil 18. A flux-gate is provided on each arm of the bridge by dividing the arm into two parallel core members each carrying a coil 19, 20, 21, 22 for driving a local flux round the local circuit formed by the two parallel members so that the reluctance of these members is varied in accordance with variations of the local flux, which may be made up of a direct and an alternating component. The alternating component during one half cycle raises the induction above the bend to saturation and during the next half cycle lowers the induction, so that the flux-gates are alternately closed and . opened. The earth's flux therefore flows in alternate directions through the pick-up coil 18 and induces an alternating E.M.F. therein, the amplitude of which is a measure of the inclination of the members 26, 27 to the earth's field and can be used to operate indicating or control mechanism. Alternatively, the local control voltages may be rectified A.C. voltages. The voltages in the coil pairs 19, 21 and 20, 22 may be in quadrature, in which case the frequency of reversal of the flux through the coil 18 is double the frequency when the voltages are 180 degrees out of phase. A closed core may constitute the diagonal 12, 13 instead of the collector plates 26, 27 in order to detect small changes of current in the coil 17, Fig. 1. A construction suitable for a variable inductance is shown in Fig. 3, the core being built up from the laminations shown in Fig. 4. The parts forming the flux-gates comprise portions 26, 27, 28, 29, integral with the limb 23, which form butt joints with similar portions 261, 27<SP>1</SP>, 281, 29<SP>1</SP> integral with the limb 25. These laminations alternate with L-shaped laminations 16, 23 and 16, 25. The core part 24 is formed of components 24a, 24b, which overlap the butt joints. The cores 26, 261, &c. of the flux-gates consist of only half the number of laminations used in the limbs 23, 24, 25, 16. Specifications 451,850, [Group XXXVI], and 611,016 are referred to.