Date: 08/23/2002 at 11:34:09 From: Josh Subject: Digital compass Dr. Math - I'm not sure if you can answer questions like this, but I was just wondering how a digital compass works. I've looked everywhere and I cannot find any information on it. Any help would be appreciated. -Josh
Date: 08/23/2002 at 16:04:54 From: Doctor Rick Subject: Re: Digital compass Hi, Josh. I'm not up on all the latest technology, but I looked around the Web a bit, and one commercial site mentioned Hall-effect technology. If that's what they use, I can tell you the basics of how it works. The Hall effect works like this. You make an electric current flow through a metal bar or film of some sort - something that has width as well as length. A magnetic field (such as the earth's) deflects the electrons sideways in their travel along the metal film, toward one side of the film. This makes the electrons sort of pile up on the side they are pushed toward, and when there are more electrons in one place than another, you can measure a voltage between the two places. The voltage, called the Hall voltage, is proportional to the current in the film, and it is also proportional to the magnetic field strength. What I've said is a bit too simple. If the magnetic field is perpendicular to the film then what I said is true. But if it is not, it is the *component* of the field perpendicular to the film that matters. If you've seen anything about vectors, you'll probably know what I mean; if not, you can ask and I'll try to explain more. In any case, you can set up two (or better, three) Hall-effect sensors, perpendicular to one another. Then the ratios of the Hall voltages in the sensors can be used to reconstruct the direction of the magnetic field. If you know about vectors, you can work out exactly how the direction would be determined from these ratios. One tricky thing is that we don't really want to know the angle between the direction we're headed and the magnetic field. That's because the magnetic field isn't just pointed north, it's also pointed down (has a downward component) in the northern hemisphere (up in the southern hemisphere). In vector terms, we want to know the angle between the horizontal component of our direction and the horizontal component of the magnetic field. I suppose this is why I saw ads for hand-held digital compasses that had bubble levels to help you hold the compass horizontal. - Doctor Rick, The Math Forum http://mathforum.org/dr.math/
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