Patent GB-674074-A (GB)
Title: Method of and apparatus for eliminating harmonics and other proximity effects
IPC: G01C17/38
Filed: 1949-05-24
Abstract: 674,074. Magnetic compasses. BELLON, F. B., and LENOIR, J. L. M. H. May 24, 1949, No. 13800/49. Class 97 (ii). [Also in Groups XXXV, XXXVI, XL (b) and XL (c)] A method of suppressing unwanted effects such as harmonics in apparatus comprising at least one active unit, e.g. the sensitive element of a magnetic compass and in which the intensity of actions produced or sustained by this unit is a non-sinusoidal periodic function of a parameter of position or of time comprising the forming of the active unit as a plurality of pairs of elements, all geometrically identical and acting simultaneously, the values of the parameter which define the respective positions of these pairs of elements as well as the intensity of the action produced or sustained by each pair of elements, being so selected as to annul the undesired harmonics of the periodic action. With a finite number of n pairs of elements disposed symmetrically in an angular relationship to a reference axis it is shown that the disposition of the pairs according to angles p being an integer such that 1#p#n and the pairs of elements located at angles +αp and - αp having an intensity factor proportional to cos q αp, will result in all harmonics other than of the order q being annulled. Alternatively the same result may be attained if the active unit is replaced by an infinity of pairs of elements symmetrically arranged about a common axis so that the element located between the angles α and α+dα has an intensity factor proportional to cos q α. In applying the method of the invention to magnetic compasses the poles of the magnet, or the magnet itself may be regarded as the active element. In the first instance a sensitive element is constructed which comprises a series of pairs of parallel magnets 60 - 60a, 61 - 61a, Fig. 32, with their poles m<SP>1</SP>, m<SP>2</SP> arranged on the circumference of a circle so that α<SP>1</SP>= 18‹, α<SP>2</SP> = 54‹ and m<SP>2</SP>=0.620 inch, which arrangement will annul the 3rd, 5th and 7th harmonics. In the second instance an arrangement of magnets 62-62a, 63-63a, 64-64a, 65-65a, Fig. 33, is obtained wherein for the values of α and m given above the same harmonics will be annulled. This arrangement is dynamically balanced and further has a magnetic field of the form shown in Fig. 7 with equipotential surfaces U=0 in the vicinity of which the field is weak and reverses its direction as one moves along the axis of symmetry. By placing semicircular and heeling error corrector magnets in the vicinity of such zero field points the amplitude of their correcting effect may be varied through zero, and if soft-iron quadrantal correctors are placed at the points of zero field the induction effect thereon from the sensitive element is annulled. The sensitive element of the compass may also be constructed as a homogeneous body of revolution in accordance with the second general solution given above and comprising an infinite number of pairs of elements either magnetic poles or magnets. In the first instance an element 67, Fig. 35a, is derived having uniform magnetization normal to the axis of revolution, and in the second instance a sensitive element 15, Fig. 36a, having a radial magnetization Jo cos α where Jo is the intensity in the direction α=0 is obtained. In both these instances all harmonics other than the fundamental are annulled and the intensity of the action produced on the sensitive element is a perfectly sinusoidal function of its angular position about its axis of symmetry. By constructing such sensitive elements of elementary magnetic particles embedded in a non-magnetic substance of appropriate specific gravity and shape a floatless element may be obtained for use in fluid compasses. By combination of such bodies of revolution the intensity of the field in the vicinity of the points of zero field may be reduced. Any of the above constructions may be applied to the various compass corrector magnets or soft iron elements. Thus a single semicircular magnet comprising an oblate ellipsoid magnetized in accordance with the method of the invention 17, Fig. 13, may be used, the amplitude of its correction effect being varied by translation along the axis XX<SP>1</SP> in the vicinity of the point of zero field and the phase of the effect being varied by rotation about its own axis. Alternatively the corrector may be formed of two elements 77- 78, Fig. 40, whose relative rotation with respect to each other is controlled by a knob 79 and the phase of the correction effect being changed by rotating the assembly as a whole. The introduction of the Flinders bars introduces a quadrantal error or second harmonic which may be eliminated according to the invention by utilizing a plurality of pairs of Finders bars for which ## cos 2α=0. Thus for the Flinders bars 18, Fig. 15, where α<SP>1</SP>=22.5‹ and α<SP>2</SP>= 67.5‹ and #<SP>1</SP>=#<SP>2</SP>, the 2nd, 4th and 6th harmonics of the effect are annulled. In the form shown in Fig. 17a the bar is shaped as a semicircular annulus so as to annul all even harmonics and if this form be constructed of telescopic segments 25-28, Fig. 17, the amplitude of the correcting factor may be varied. In order to reduce hysteresis the bars may be laminated or made of wires or other elements of small dimensions. Examples are also given of the construction and arrangement of quadrantal corrector units according to the method of the invention. By combination of mutually orthogenal quadrantal correctors an astastic system is obtained which re-inforces the effect of the Earth's field as the sensitive element and at the same time annuls the quadrantal effects induced in the correctors by the sensitive element. Where quadrantal correctors in the form of bodies of revolution are being employed they may also be combined with appropriate non-magnetic metallic elements to obtain eddy current damping effects.