GB2025616A - Magnetic compass - Google Patents

Abstract

A magnetic compass 10 with eddy-current damping, e.g. a hand- bearing compass, has an annular electrically- conducting damping barrier shell 20 of apertured spherical form at whose centre 17 the assembly of the magnet system 18 and compass card 15 is pivoted at the centre of gravity of the magnet system 18. The wall of the damping shell 20 extends symmetrically or approximately symmetrically above and below a horizontal plane through its centre 17. The centre of gravity of the whole pivoted assembly of compass card 15 and magnet system 18 is below the pivot point 17. This damping arrangement provides greatly improved damping and lower resonant frequency of the magnet/card assembly in pitch and roll, giving greater ease of reading. The compass card 15 is annular and surrounds the barrier shell 20 in a plane passing through the pivot point 17, and the card 15 is supported by a spider 28, 29 which extends below the shell 20 and up through a bottom aperture 22 in the shell to the magnet system 18 to which it is attached. <IMAGE>

Description

SPECIFICATION Magnetic compasses This invention relates to magnetic compasses, of the kind having an assembly of a magnet system and a compass card universally pivoted for angular movement in pitch, roll and yaw, and having eddy-current damping of that assembly. The invention is particularly although not exclusively applicable to handbearing prismatic compasses.

For a hand-bearing compass to be readable under normal conditions of pitch and roll on a small boat at sea, the compass card must remain steady, i.e. be well damped.

For most applications a viscous translucent liquid has provided very effective damping for this purpose. However, on a flat compass card the damping effect of the liquid is far greater in the vertical direction than in the horizontal direction; thus yaw movements are less well damped than pitch or roll movements. The use of liquid has other disadvantages. The solutions have a high coefficient of expansion and a tendency to deteriorate with age.

Containing the liquid without leakage makes construction of such a compass expensive, and eventual leakage is common.

Alternative methods of damping have been used without resort to the use of liquid. One such method is 'eddy-current' damping. In this case the damping effect is a result of eddy currents which are set up in a metallic barrier having a high electrical conductivity. The barrier is shaped and positioned in such a way as to cut the lines of magnetic force of the compass magnet system when this is in motion. The eddy currents which are set up in the barrier as a result of this 'flux cutting' when the magnet system is in motion produce a force on the magnet system opposing its motion. This opposing force gives rise to a damping effect in much the same way as viscous forces damp the card of a liquid-damped system.

Up until now the damping performance of such an eddy-current-damped compass has been satisfactory in the horizontal plane (i.e., in yaw), but poor in the vertical direction (i.e., in pitch or roll). This has arisen because of the shape and position of the damping barrier, arising from the custom of positioning the compass magnets below the compass pivot point for reasons of balance. For example, in the known so-called Ritchie compass a shallow bucket-shaped damping barrier is provided which barely extends above the level of the magnet system in its equilibrium rest position, the pivot point of the magnet system being at a level substantially above the upper edge of the barrier as well as being above the magnet systems itself.With the compass mag nets in this position it is a straight forward matter to design a barrier which provides adequate eddy current damping in the horizontal plane. However, due to the non-symmetrical arrangement of the magnets in relation to the pivot point, the provision of a damping barrier which will be effective in pitch and roll as well as in yaw is rendered difficult.

One object of the present invention is to provide a magnetic compass with eddy-current damping having good damping performance as regards oscillations in pitch and roll as well as in yaw.

According to the present invention, a magnetic qompasswith eddy-current damping having a pivotally-mounted assembly of a magnet system and compass card is provided with a damping barrier in the form of a spherical electrically-conducting shell fixedly mounted in the housing around the magnet system, within which shell the said assembly is universally pivoted about a pivot point at the centre of the shell, the wall of the shell extending between latitudes above and below the level of the pivot point.

Preferably the magnet system itself is pivoted with its centre of gravity substantially at the pivot point at the centre of the shell with its poles close to the wall of the shell; and preferably also the centre of gravity of the assembly of the magnet system and card is below the pivot point to provide gravity-stabiiisation of the card. This arrangement enables a considerable reduction in the natural frequency of oscillation of the magnet and card assembly in pitch and roll to be achieved, as compared with the Ritchie compass, and this greatly improves the readability of the instrument especially if it is of the prismatic type.

Preferably the wall of the shell extends between latitudes which are at least 15 degrees, and possibly 20 degrees or more, above and below the equatorial plane of the shell, around the whole shell.

Thus for angular displacement of the magnet system in pitch and roll within the range of the upper and lower edges of the shell (corresponding to the expected maximum angles of pitch and roll), the distances from the poles of the magnet system to the shell wall will remain unaltered, both above and below the equatorial plane, as the magnet system pitches and/or rolls, thereby providing greater effectiveness and uniformity of the eddy current damping than is possible with the aforesaid Ritchie compass.

In a preferred construction, the barrier shell may comprise an annular section of a spherical shell defined between upper and lower planes of section which are parallel to the equatorial plane of the shell.

For the purpose of compactness these planes are preferably at equal, or approximately equal, distances from the centre, producing a symmetrical or approximately symmetrical shell arrangement, the shell having upper and lower circular apertures of equal or approximately equal diameter. Through one of these apertures, for example the lower, the supportforthe pivot may extend.

The magent system itself may take various forms, but one having at least two pairs of poles in a non-linear disposition should be provided in order to provide damping in roll as well as in pitch. Preferably a cruciform magnet system is used, with limbs of equal lengths and with four poles respectively situated at the outer ends of the four limbs, like poles being in adjacent limbs. This arrangement is symmetrical in its plane and will provide similar dynamic performances in both roll and pitch modes. This is also simple to manufacture. However it would be possible to use an H-shaped magnet assembly.

The compass card, in one construction of the invention, is of annular form and surrounds the barrier shell, being supported by means of a depending support for example of spider form which extends below the barrier shell and has a central portion secured at its upper end to the magnet system, the central portion of the support extending upwardly through an aperture in the lower side of the barrier shell. The support is shaped to provide adequate clearance from the lower edge of the apertured shell as the card moves in roll and pitch.

The compass point indication and/or scale graduations are marked on the annular card. The card may lie in a plane containing the pivot point at the centre of the shell, the mass of the depending support which lies below that plane ensuring that the centre of gravity of the whole pivoted card/magnet assem blywill be below the pivot point.

The invention may be carried into practice in various ways, but one specific embodiment will now be described by way of example only with reference to the accompanying drawings, in which: Figure lisa sectional elevation through a handbearing magnetic compass with eddy-current damping; and Figure2 is a plan in section on the line ll-ll of Figure 1, showing the cruciform magnet system of the compass.

In the illustrated embodiment, the compass comprises a housing 10 having a dished bottom 11 on which a vertical pivot needle 12 is mounted. The housing 10 has a glass top 13 provided with the usual prismatic sight 14 for reading the scale markings on the annular compass card 15, with the compass sighted on an object.

Pivotally mounted on a pivot point constituted by the sharp upper end 17 of the upstanding pivot needle 12 is an assembly 19 of the compass card 15, its supporting spider 16 and a cruciform magnet system 18, the pivot point 17 being at the centre of a sectioned spherical copper barrier shell 20 for eddycurrent damping. The shell 20 forms an annulus surrounding the magnet system and extending both above and below the horizontal equatorial plane through the centre of the shell, the upper and lower edges of the shell being defined by parallel upper and lower planes of section which are at equal or approximately equal latitudes above and below the equatorial plane through the centre, determined in accordance with the expected maximum angles of pitch and roll, e.g. 15"-20" of latitude or more.Thus the sectioned shell has upper and lower circular apertures 21 and 22 of equal or approximately equal diameter defined by the parallel section planes, the rim of the upper aperture 21 being secured as by adhesive to the underside of the window glass 13 and the lower aperture 22 being open with the upstanding pivot needle 12 extending up through it, the point 17 of the needle 12 being situated at the centre of the shell 20. A buffer 23 is also secured to the underside of the window glass 13 to retain the card/magnet assembly 19 on the pivot pin 12.

The magnet system 18 is of planar cruciform shape, shown in plan in Figure 2, having four limbs of equal length at right angles to one another, two adjacent limbs 24,24 being magnetised to provide north poles adjacent their outer ends and the opposite two adjacent limbs 25, 25 being magnetised to provide south poles adjacent their ends. The overall axis of magnetisation of the cruciform magnet system 18 is indicated by the arrow A in Figure 2.

The cruciform magnet structure 18 has a central recess 26 in the underside of its central portion or hub, the recess 26 being formed with a bearing seat at or approximately at the centre of gravity of the cruciform magnet structure 18 which seat rests ori the pivot point 17 of the needle 12 at the centre of the part-spherical shell 20. The outer ends of the four limbs 24,24, 25, 25 of the cruciform magnet structure 18 lies close to but spaced from the internal surface of the shell 20 in which the limbs extend diametrically.

The compass card 15 surrounds the barrier shell 20 and is mounted on the magnet-system 18 by means of a depending spider 16 whose limbs are of downwardly-angled form as shown, forming a central armature column 29 whose upper end is attached to the underside of the magnet structure 18 symmetrically around its central bearing seat. The pivot needle 12 extends upwardly through the column 29 to support the whole card/magnet assembly 19 on its point 17 in a universally-pivotable manner. The depending spider 29 extends around the lower rim of the shell 20 with sufficient clearance to accommodate the movement of the card/magnet assembly in pitch and roll as well as in yaw.The card 15 lies in a generally-horizontal plane passing through the pivot point 17 at the centre of the barrier shell 20, and the depending spider 16 acts as a balance weight which brings the centre of gravity of the whole card/magnet assembly below the pivot point 17, thereby giving the pivoted assembly gravitational stability.

The copper barrier shell 20 is symmetrical or approximately symmetrical about the horizontal equatorial plane through its centre and is highlyconducting electrically, and intersects the magnetic fluxes associated with the poles of the cruciform magnet assembly, so that rotational movement of the magnet assembly about the universal pivot 17 in yaw, pitch and/or roll will generate eddy-currents in the shell 20 by which the rotational movements of the magnet structure will be damped in accordance with the well known eddy-current damping princr- ples. The arrangement described, with the magnet assembly pivoted symmetrically with respect to the shell 20 at or near its own centre of gravity, stabilised by the depending spider 16, allows the restoring torques in the roll and pitch modes to be controlled equally, without appreciably affecting the moment of inertia of the whole pivoted assembly in these modes. The pivoting of the compass magnet structure at or near its centre of gravity thus not only allows a more effective pitch and roll damping system to be used, than in known constructions, but also enables the pitch and roll resonance frequency to be adjusted or predetermined conveniently.

In a prototype compass constructed substantially in accordance with the above description and the drawings, a substantial improvement in damping performance was achieved over other known eddy current-damped instruments. The settling time in the yaw mode was about 6 seconds, much the same as for a typical known design of Ritchie eddy-currentdamped instrument but much better than the typical 11-15 seconds settling period of known liquiddamped instruments, while the settling time in pitch and roll of the prototype was reduced to about 1 second as compared with about 3 seconds for the known eddy-current-damped instrument. Moreover tne resonance frequency in the pitch and roll modes of the prototype was about half that of the known eddy-current-damped design.These two improve ghent factors, namely settling time and resonance frequency, combine to give in effect a sixfold improvement in the pitch/roll performance and hence in the case of reading of the compass under conditions of pitch and roll disturbance, with the result that the prototype compass embodying the invention can be read comfortably with a prismatic sight. This is not possible with the known designs of Ritchie eddy-current-damped instrument, in which a substantial disturbance in pitch or roll causes the compass card to oscillate up and down around its pivot so many times that the view through the prism sight is generally out of focus and unreadable.

Thus the prototype embodying the invention provides an eddy-current-damped instrument which combines the good yaw performance of that type with the good pitch and roll performance of the liquid-damped type of instrument but without the disadvantages arising from the use of the liquid medium in the latter.

It will be understood that whilst the invention has been specifically described and illustrated in an embodiment constituting a hand-bearing compass, it is also applicable to other kinds of compasses such as steering compasses, which do not have a prism sight. In particular, it may be applied to compass used in conjunction with radio direction finding apparatus.

Claims (13)

1. A magnetic compass with eddy-current damping, which includes an assembly of a magnet system and a compass card, said assembly being pivotally mounted in a housing, and a damping barrier fixedly mounted in the housing around the magnet assembly, the damping barrier comprising a spherical ,electrically-conducting shell which contains the magnet system, the said assembly being universally pivoted about a pivot point at the centre of the shell, and the wall of the shell extending between latitudes above and below the level of the pivot point.

2. A compass as claimed in Claim 1, in which the centre of gravity of the magnet system is located substantially at said pivot point, and the magnet system has-magnetic poles which are close to the wall of the shell.

3. A compass as claimed in Claim 2, in which the centre of gravity of the said assembly of magnet system and card is below the said pivot point.

4. A compass as claimed in any one of Claims 1 to 3, in which the wall of the shell extends between latitudes which are at least 15O above and below the equatorial plane through the centre of the shell.

5. A compass as claimed in Claim 4, in which the said latitudes are at least 20O above and below the said equatorial plane.

6. A compass as claimed in any one of Claims 1 to 5, in which the barrier shell comprises an annular section of a spherical shell defined between upper and lower planes of section which are parallel to the equatorial plane of the shell.

7. A compass as claimed in Claim 6, in which the said planes of section are at approximately equal distances from the centre of the shell, the shell being a substantially symmetrical body with upper and lower circular apertures of approximately equal diameter.

8. A compass as claimed in Claim 7 having a pivot needle on whoe pointed end the said assembly of magnet system and card is pivotally supported, the needle extending through the lower circular aperture and being fixedly mounted in the housing.

9. A compass as claimed in any one of Claims 1 to 8, whose compass card is of annular form and surrounds the barrier shell, and in which the said assembly includes a support which depends from the magnet system within the shell, extends through an aperture in the lower part of the shell, extends upwardly outside the shell, and is attached to the card to support it, the support being shaped to provide clearance from the shell as the said assembly pivots in roll and pitch.

10. A compass as claimed in any one of Claims 1 to 9, in which the magnet system has at least two pairs of magnetic poles arranged in a non-linear disposition.

11. A compass as claimed in Claim 10 in which all the magnetic poles are coplanar.

12. A compass as claimed in Claim 11 in which the magnet is of cruciform shape with four limbs of equal length each having a magnetic pole near its outer end, poles of like polarity being in adjacent limbs.

13. A magnetic compass with eddy-current damping, substantially as specifically described herein with reference to the accompanying drawings.