Patent GB-642649-A (GB)
Title: Apparatus for measuring the values of magnetic fields
IPC: G01R33/02
Filed: 1946-12-02
Abstract: 642,649. Automatic control systems for magnetometers. GULF RESEARCH & DEVELOPMENT CO. Dec. 2, 1946, No. 35606. Convention date, Nov. 1, 1943. [Class 38 (iv)] [Also in Group XXXVI] Alignment control.-In a device for measuring the intensity of magnetic field, a detecting magnetometer of the flux-valve type is automatically aligned in the direction of maximum field intensity by electromechanical servomotor means controlled by one or more orienting magnetometers in response to deviations of the position of the apparatus with regard to the magnetic field direction. The detecting magnetometer may also act as the orienting magnetometer. In Fig. 1 detecting magnetometer 10 and a similar orienting magnetometer 11, both of the A.C. energized flux valve type, are mounted on discs 12, 13 at the ends of a frame 14 fixed to a turbine gyroscope housing 15, the disc 12 being adjustable in a vertical plane so that the detecting magnetometer axis is aligned with the dip of the earth's magnetic field. The gyroscope housing is mounted on gimbals in a U-shaped bracket 21 carried by a turntable 22. The axis of rotation of the frame 14 is maintained vertical by the gyroscope and when the axis of the detecting magnetometer 10 lies along the earth's field in a north-south direction, that of the orienting magnetometer 11 lies in an east-west direction, so that the component of the earth's field threading it is zero and its output signal is also zero. On movement of the mounting of the apparatus deviating the detecting magnetometer 10 from alignment with the earth's field in a horizontal plane, the orienting magnetometer 11 is also deviated from the east-west line and positive or negative signal pulses are developed according to the sense of the deviation. The output is fed to amplifier 25 so as to energize the reversible electric motor 24 which may have two-phase windings fed respectively from the amplifier and from the flux valve excitation source to rotate the turntable 22 in the appropriate direction to restore the orienting magnetometer to the east-west line and the detecting magnetometer to the north-south line of the earth's field. The output of the detecting magnetometer is amplified and indicates the magnetic field intensity on a meter 35. In a modification (Fig. 3, not shown) three mutually perpendicular flux-gate magnetometers energized by oscillators are fixed in gimbal rings which are respectively rotatable about two mutually perpendicular axes by two reversible electric motors. The first two magnetometers are orientated in a neutral magnetic plane, their outputs operating central circuits for the motors, which direct the gimbal rings into a position at which the magnetometer outputs are zero, while the third or detecting magnetometer output is amplified and applied to an indicating instrument measuring the intensity of the earth's field. Fig. 4 shows a modification in which a single magnetometer 65 energized by oscillator 66 is pivoted within a ring 74, perpendicular to the earth's field, at a small angle to an axis normally parallel to the earth's field, about which it nutates, driven by mutually perpendicular flexible rods 73 twisted by rochelle salt piezo-electric crystals 75, 76 energized 90 degrees out of phase through phase-shifting networks from an oscillator 77 where frequency is small compared with that of oscillator 66. When the nutatory axis is parallel to the earth's field, the magnetometer output is constant, but on deviation of the axis the output is modulated by a signal whose amplitude indicates the magnitude of the deviation and whose phase indicates its direction. The output is amplified at 68, energizes an indicating circuit 69, the meter 70 of which measures the magnitude of the earth's field and is rectified at 80, the output of which is split at 81 into two antiphase voltages which are combined vectorially with two other voltages of the same frequency fed from oscillator 77 through phase-adjusting circuits 82, 83. The combined voltages are rectified, and corresponding D.C. voltages appearing across condensers 89, 91 combine in opposition to energize the field of D.C. motor 93 which rotates ring 74, one of the mutually perpendicular supports of the magnetometer, the other support being correspondingly rotated by a similarly controlled motor 96. The motor control systems are phased so that the modulated signal produced by deviation of the axis of nutation of the magnetometer from the direction of the earth's field energizes the motor fields in such directions as to rotate the magnetometer supports to restore coincidence between the magnetometer nutatory axis and the direction of earth's field. Applications to the detection of submarines, landmines and other magnetic bodies from aircraft and vehicles are described. Specification 642,650 is referred to.