Patent GB-853942-A (GB)
Title: Improvements relating to the correction of magnetic compasses
Inventor: HARVEY GILBERT NATHANIEL
IPC: G01C17/38
Filed: 1957-01-28
Abstract: 853,942. Electric indicating systems. AVIATION, MINISTER OF, [formerly MINISTER OF SUPPLY]. Jan. 28, 1957, No. 2933/57. Class 40 (1). General principle.-The direction of a component of the earth's magnetic field is determined in a vehicle by means of two magnetic field detector units mounted at spaced points on the vehicle, the outputs of the two detectors being so combined that the output of one is " weighted " relatively to the other and used to operate a device which indicates the true position of the earth's field independently of the disturbing fields within the vehicle. The Specification states that, in a vehicle consisting largely of magnetic material, pairs of points may be found at which the fields tending to deviate the earth's field bear a constant ratio to each other differing significantly from unity and are parallel to each other and remain so under all reasonable conditions of magnetization of the bodies. The detectors are mounted at such points and a proportion of the output of one is combined with the total output of the other. The Specification shows that if this proportion is correctly chosen the combined output may be used to indicate the true deviation of the earth's field uneffected by the various fields due to the vehicle. Detail operation.-As shown, Fig. 1, magnetic devices 10, 11, described in detail below, are mounted at locations of the type referred to above. Each detector is A.C. energized and produces a three-line double frequency output which is fed to the appropriate stator of synchros 20, 21, the rotors of which are coupled together by a shaft 22 which also joins them to a servomotor 26 and indicator 23. The relative angular position of the resultant field and detector controls the output from the appropriate rotor which is zero only when it is at right angles to this field. The output of rotor 20 is reversed and fed through a potentiometer 24 when a proportion of it is combined with the output of rotor 21. The combined output is fed to an amplifier 25 where it is compared for phase sense with a reference and used to drive servo 26 until the amplifier input is zero. Meter 23 then indicates the direction of the earth's field. In a modification (Fig. 2), compass needles 112, 113 position a vane in a conductive liquid to vary the current distribution between electrodes 116, 117 and thus to produce outputs in transformers 133, 134. The two outputs are then unequally combined by potentiometer 135 as described with reference to Fig. 1 and drive a servomotor 26. This motor positions the rotor of synchronous 120, 121 to a common shaft 22 to which is coupled indicator 23, while the outputs from the stators of these synchros feed repeater synchros 120a, 121a to reposition the compose bowls until the amplifier input is zero.- Detector construction (Fig. 1).-Each detector consists of a pair of parallel three-arm plates 13, 14, 15 carrying at their ends flux collectors 13a, 14a, 15a. The two plates are joined together by a central hub of magnetic material. An exciter winding is mounted on the hub and pick-off windings on each spoke. The assembly is pendulously mounted and is energized by 400 cycles A.C. the peak values of which saturate the core to produce an 800 cycle output upon the pick-up coil. Specification 409,027 is referred to. Reference has been directed by the Comptroller to Specification 775,440.