GB872096A - Improvements relating to the correction of magnetic compasses - Google Patents

Abstract

872,096. Magnetic compass correction. MINISTER OF AVIATION. Jan. 13, 1960 [Jan. 16, 1959], No. 1768/59. Class 40 (1). The magnetic compass system of Specification 775,440, wherein two magnetic detectors are located at different points on an aircraft or other vehicle and the output of one used to correct the other for errors due to the proximity of the structure of the vehicle, is modified by feeding the outputs of the two detectors to the same synchro stator; the signal from the synchro rotor is passed via a phase-sensitive circuit to a servomotor which turns the rotor until the input to the motor is zero; the output of one detector is made adjustable in phase or amplitude relative to the other to achieve the desired correction so that the final angular position of the synchro rotor will indicate the corrected bearing of the vehicle. In the Figure the two detectors 1 and 2 are of the flux-gate type, and are mounted in pendulous holders on a projecting boom so that the former is further away from the body of the vehicle than the latter. The energizing coils 3 and 4 are supplied with current at 400 c/s. with a phase difference depending on the setting of a phase-shifter 17. The signals induced in the detectors by the magnetic field of the vehicle will be different in amplitude and it can be shown that for a given setting of the phase-shifter the output from the synchro rotor 12 will be independent of the contribution from this field. The output is compared with the doubled supply frequency in a phase-sensitive rectifier 19 and any difference is passed to a servomotor 24 which turns the rotor until the difference signal is zero. The unit 17 may introduce a fixed phase-shift, e.g. 45 degrees, and a variable phase-shifter is then included in the lead to the frequency doubler. The outputs of the detectors can be made to differ in amplitude instead of phase by winding the corrector unit 2 with fewer turns or by including resistances in the leads from this unit to the synchro stator. Final adjustment can be made with a three-gang variable resistor in series with the unit. The detector units may have two arms instead of three, with appropriate modification of the synchro. According to the Provisional Specification the adjustment can be carried out by a resistor across the synchro rotor.