PBO - Fluxgate question

[Magic_Sailor]

Question - "How does a fluxgate compasss work?

Answer - "The electronic compass makes use of four fluxgate valves, basically vectoral magnetomometers".

So, that's cleared that one up then!

Anyone care to enlighten me further?

Magic

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[Cornishman]

Tim Bartlett has written an excellent description of how a fluxgate compass works. A bit too long to reproduce here but you will find it at the bottom of page 17 in the latest edition of the RYA Navigation handbook

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[IanPoole2]

Magic

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[ParaHandy]

Re: Magic circle .. ?

people and boats with these devices seen to be perpetually going in circles trying to get them to work (calibrate in instruction manual-speak) ...

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[Nich39Nige]

Re: Magic circle .. ?

Only took 38 circles to calibrate on my boat. Got some very strange looks from Stinkies in Poole Harbour.

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[peterb]

A fluxgate is a device that measures the magnetic field strength in a specific direction.

By mounting one fluxgate to measure the field strength along the boat, and a second to measure the strength across the boat, it is possible to work out the direction of the field relative to the boat. For instance, if the fore-and-aft strength were high but the athwartships strength were zero, then the boat would be facing magnetic north (or south); if the athwartships strength were high but the fore-and-aft strength were zero then the boat would be facing east (or west); if the two strengths were equal then the boat would be facing north-east (or NW or SE or SW). In practice the sense (+ or -) as well as the strength of the field component is measured, so that the ambiguities are resolved.

Because the two fluxgates measure the strengths of the components in two perpendicular directions, it is possible (by using Pythagoras's theorem) to calculate the total horizontal strength of the earth's field. Theoretically this should be the same whatever direction the boat is pointing in, but if the boat has some magnetism of its own then the total field will vary as the boat is turned. By turning the boat in circles and measuring the changes in observed field strength, it is possible to measure and compensate for the boat's field. Hence the requirement on most fluxgate compasses for a setting up procedure in which the boat makes slow circles.

On one boat I sail, the fluxgate head is mounted on a bulkhead forming part of a hanging locker, with the usual warnings about ferrous material. On one trip the autopilot (controlled by the fluxgate) seemed to be making unusually large and frequent movements, and its error relative to the main steering compass seemed large and variable depending on tack. It turned out that someone had moved our normal plastic coat hangers and replaced them with wire hangers. Yes, fluxgates can compensate for deviation; but they can't compensate for deviation which changes as the hangers rock about in the locker.

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