Patent GB-910556-A (GB)
Title: Improvements in or relating to correction of the deviations of magnetic compasses
Inventor: BIRD WILLIAM GEORGE
IPC: G01C17/38
Filed: 1957-10-29
Abstract: 910,556. Electrical indicating systems. BIRD INSTRUMENTS Ltd. Jan. 29, 1959 [Oct. 29, 1957], No. 33636/57. Class 40 (1). [Also in Group XXXVI] In a method of correcting deviation of a magnetic compass, the compass is shielded from substantially all stray magnetic fields and from the earth's field, and is subjected to a field which is derived from a field-sensitive signal generator exposed to the earth's magnetic field at a point magnetically remote from stray fields due, e.g. to the iron of the vehicle in which the compass is installed, and which is identical in direction and sense with at least the horizontal component of the earth's field at the compass. In a preferred embodiment, Figs. 6, 7, 8, for use in a ship, the signal generator is mounted high on a mast and comprises three detector elements in the form of saturable inductors 18, 19, 20, detector element 20 being responsive to the vertical component of the earth's field and detector elements 18, 19 sensing mutually perpendicular horizontal components. The output from each detector element is amplified in one of three negative-feedback amplifiers 23 having ganged gain-controls, and rectified in one of three phase-sensitive rectifiers 24, the outputs of which are fed to three pairs of coils 15, 16, 17 disposed mutually orthogonally around the compass bowl 7 (Fig. 2). A cylindrical shield 14 of high permeability magnetic material, the walls of which may be solid, or may be formed of thin strips wound on a former or of thin-walled concentric cylinders, surrounds the compass box 10 on a binnacle 11 to which are attached the conventional deviation-correcting devices. Coils 15, 16, wound on rectangular formers, are pressed into the compass box 10 while the coils 17 are wound on cylindrical formers and lie in parallel planes above and below the compass needle 6; alternatively, the coils 15, 16 are toroidally wound on the magnetic shield 14. The magnetic shield 14 is preferably detachable so that, in the event of failure of any of the detector elements (which failure may operate a warning relay) the compass may be used in the conventional manner. Modifications.-Detector elements. These may utilize the skin effect in high permeability alloys, nuclear resonance or may be rotating coils or, as shown in Fig. 10, Hall effect devices having a Hall plate 32 (which may be composite strip) of semi-conductor material such as germanium, silicon or indium antimonide, sandwiched between two conducting electrodes 33, 34, e.g. of copper or aluminium, the longitudinal axis of the device lying at right angles to the component of the earth's field to be measured. In an armoured vehicle, the detector elements may be fixed to a retractable mast, and in an aircraft they may be in the tail or at a wing tip. Transmission circuit. A common amplifier and a common phase-sensitive rectifier may be used, the signal from each detector element in turn being transmitted cyclically by an arrangement of time-controlled gating circuits. If simple amplifiers are used, D.C. windings may be energized, thereby to give negative magnetic feedback to the detector elements. The components of the earth's field sensed by the detector elements may be differently disposed relative to the vehicle and to one another, e.g. three horizontal components at 120 degrees to each other may be sensed, and in stabilized vehicles the vertical component may be omitted. The shield may be modified to improve the screening of the compass from the ambient vertical field. If necessary, the detector elements may be provided with conventional deviation-correcting devices or compensating direct currents may be supplied to auxiliary coils on the detector elements. A plurality of shielded compasses may have field reproducing coils connected in parallel to common detector elements, and any such compass may have repeaters.