Patent GB-520826-A (GB)
Title: Improvements in or relating to compasses
IPC: G01C17/30
Filed: 1938-07-13
Abstract: 520,826. Magnetic tests. SIEMENS APPARATE UND MASCHINEN GES. July 13, 1938, No. 20839. Convention date, July 13, 1937. [Class 40 (i)] A compass comprises one or more rods of permalloy or other high permeability material, each rod carrying one or more windings energized by 'a periodically varying current and being rotated at constant speed about a vertical axis a measure of the angular position being obtained electrically from the impedance variations of the one or more windings due to the modulation of the electrically produced flux in each rod by the magnetic field during rotation. A single rod of permalloy arranged to rotate horizontally, and stabilized by means of gyroscopic, pendulum or other devices may carry a single coil energized with A.C., or, in order to operate at a more favourable point on the magnetization curve the rod may, in addition, be polarized by an arrangement of permanent magnets or by a supplementary coil energized with D.C. Alternatively, the coil on the rod may be energized with periodically variable D.C. Fig. 10 shows an arrangement of two rods 1, each carrying a coil 2 energized from a D.C. or A.C. source 10 through devices 7, 8, to convert the current through the coils into periodically variable D.C., the coils 2 being connected in a bridge circuit with non- inductive resistances or impedances 9 which, e.g., may comprise rods and coils similar to 1, 2. 'The currents in coils 2 due to the source 10 are balanced out in the bridge, the output across a, b, being a sinusoidal e.m.f. due only to the rotation of the coils 2 in the magnetic field. The two coils 2 are as shown in Fig. 11, mounted on parallel arms of a magnetic element 16 (the rods 1 of Fig. 10) and the components 7, 8, 9, are mounted in a part 18 connected to the element 16 by a frame 22, the whole being rotated about a vertical axis 20 by a synchronous motor 19, or airscrew, &c. The current supply 10 is led in by slip rings (not shown) and the points a, b, Fig. 10, are connected to the segments of a two part commutator 11 engaged by diametrically opposed brushes carried in a frame 25 rigid with a compass disc 29. The resultant e.m.f. across the brushes varies in known manner according to the angular setting thereof relative to the magnetic field, zero e.m.f. position being arranged to correspond to the position in which the northdirection'of the compass card 29 coincides with the true magnetic north and the longitudinal axis of, e.g., the aeroplane carrying the compass. The brushes may be connected, if desired, through an amplifier to an ammeter and the brushes rotated by hand until zero indication is reached, or, as shown, the brush frame 25 may be set automatically by a motor 27, which, in known manner, responds to the voltage across the brushes and rotates the brush frame through worm gearing 28, 26 in either direction until the voltage across the brushes is zero, when the angular setting of the brushes indicates by means of the compass card read against an index 30 the direction of the aircraft. Stabilization. The vertical shaft 20 is mounted by Cardan joints in fixed bearings 36, 37, and is stabilized in the vertical position by causing both shafts 32, 35 to follow up the movements of the corresponding shafts of a gyroscopic device 38 carried in a Cardan frame 39 rotatably mounted in bearings 40, 41. The shaft 42 is caused to follow the movements of the gyroscope axis corresponding to 32 by means of a stirrup, the shaft 42 carrying a contact arm 43 co-operating with countercontacts 45, 46 mounted on a sector plate 44 rotatable about axis 42. Movement of the arm 43 into contact with either contact 45, 46, energizes a motor 47 to rotate the sector plate through worm gearing 51 in one direction or another to break the motor circuit at 43-44 or 45, the motor, at the same time, driving the shaft 32 through worm gearing 48, 49, shaft 50 and stirrup to follow-up the shaft 42. A similar follow-up device may be arranged for the shaft 35. Modifications. The relative angular position may be determined by the e.m.f. induced by transformer action in at least one winding by the e.m.f. induced in at least one of the windings 2 due to rotation thereof in the magnetic field. A part of or the entire secondary output can be rectified and used to produce a magnetic reaction effect by energization of a supplementary polarizing winding on the rod or rods 1. Magnetic reaction may also be used in the embodiment described above in detail in which case the D.C. polarizing coil may be dispensed with. The alternating current windings on two rods may be arranged as two adjacent arms of a bridge, the other two arms being formed by condensers, the measuring device being connected so that a winding and condenser forming an oscillatory circuit constitutes each half of the bridge. Instead of a single rotating system, two or more such systems may be arranged at an angle. For example, three systems may be arranged at 120 degrees to each other and rotated by a three-phase motor. The three systems supply a three-phase current which produces a rotating magnetic field. The indicator system may be oppositely rotated at the same speed as the triple system, thereby giving in the indicator system a vector of the magnetic field coinciding with the vector of the magnetic field in which the triple system rotates. This field vector may influence a magnetic needle or a winding to give an indication of its position. The error caused by the deviation of the geographical from the magnetic meridian may be compensated for by gearing as described in Specification 490,162, [Group XX].