IMPROVED MAGNETIC COMPASS CARD

Abstract

The balancing righting moments and dampening of the compass card and directive magnets in a liquid damped magnetic compass are separated to reduce the tilt and oscillation of the compass card when subjected to lateral acceleration, such as pitching and rolling in a seaway. The compass card is connected to the pivoted directive magnetic assembly by a card gimbal arrangement. The compass card is connected to the outer side of the gimbal ring rotating on a first axis. The magnetic assembly is connected to the inner side of the gimbal ring for rotation along a second axis oriented 90 degrees from the first axis.

Claims

1. A magnetic compass comprising: a liquid filled spherical compass bowl; a major gimbal within the bowl supporting a central pivot; a compass card; a magnet assembly; and a two-axis card gimbal, coupling the compass card to the magnet assembly; wherein the magnet assembly rests on and freely rotates about the central pivot.

2. The magnetic compass as claimed in claim 1 in which the two-axis card gimbal comprises a gimbal ring with inner swivel pins coupled to a housing of the magnet assembly and outer swivel pins coupling the gimbal ring to the compass card.

3. The magnetic compass as claimed in claim 1 in which the magnet assembly comprises two parallel cylindrical rare-earth magnets straddling the pivot axis and having a centre of mass below a pivot point of the central pivot.

4. The magnetic compass as claimed in claim 2 in which the magnet assembly comprises two parallel cylindrical rare-earth magnets straddling the pivot axis and having a centre of mass below a pivot point of the central pivot.

5. The magnetic compass as claimed in claim 2 in which the outer side of the gimbal ring is coupled to the compass card rotating on a first axis while the magnetic assembly is connected to the inner side of the gimbal ring rotating along a second axis oriented 90 degrees from the first axis.

Description

PREFERRED EMBODIMENT

[0011] The invention may be better understood with reference to the illustrations of a preferred embodiment of the invention in which:—

[0012] FIG. 1 shows a cross-section of a compass

[0013] FIG. 2 shows a side view of the compass card with gimballed directive magnets viewed along the north-south axis.

[0014] FIG. 3 shows a plan view of the compass card with gimballed directive magnets

[0015] Referring to FIG. 1, a spherical compass bowl 1 comprises two hemispheres 2 and 3 held together by clamp rings 4. The hemispheres can be fabricated from injection-moulded polycarbonate or glass. A transparent dampening fluid such as Isopar L paraffin fills the void 5 within the bowl acting as a dampening agent for the components within the bowl. The clamp rings also provide a securing means to a vessel's ceiling, console, pedestal or bulkhead. Within the two hemispheres a set of major gimbal rings 6 support a pivot pin 7 and a pointer or lubber line 8. Resting on the pivot pin is a compass card assembly 9 which can be more clearly seen with reference to FIG. 2.

[0016] In FIG. 2, directive magnets 10 are attached to a plastic or aluminium housing 12 that holds the magnets. The directive magnets are of the rare-earth variety and are cylindrical with north-seeking ends. Typically they are 3-5 mm in diameter with a length of 20-40 mm. An optional flotation volume 13 assists in providing a righting moment and in lessening the load on the pivot pin. The housing has a central conical depression 14, also known as a jewel cap, to rest on a pivot pin 15. Alternatively the housing can include a pivot pin that rests on a jewel cap. The pivot pin and jewel cap are common components for compasses and do not form part of the invention. The housing 12 is kept level with the righting moment being achieved with the centre of mass being below the pivot point and or the centre of buoyancy being above the pivot point. The latter can be achieved by including on the magnet housing a buoyant or flotation chamber above the jewel cap.

[0017] The magnet housing supports a gimbal ring 16 that has internal swivel pins 19 aligned centrally along a north-south axis, while external pins 18 aligned along an east-west axis engage with a compass card 9. The compass card 9 is kept level with the righting moment of the card being achieved with centre of mass being below the external pins 18 and or the centre of buoyancy being above the external pins.

[0018] Referring to FIG. 3, these same components are shown along with the same corresponding numerical labels.

[0019] The above description is for illustration only and those skilled in the art of compass construction will find other methods to achieve the object of the invention while embodying the essence of the invention.

[0020] It will be appreciated from the above that the invention better separates the tilt of the magnet with that of the compass card, and thereby minimises unwanted compass card movements and oscillations as result of the lateral accelerations from rolling or pitching of a vessel.