81 lines
4.1 KiB
Plaintext
81 lines
4.1 KiB
Plaintext
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Article 36322 (112 more) in sci.physics:
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From: pad@galaxy.nsc.com (Paul Denny x8349)
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Subject: fluxgate compasses
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Keywords: how it works
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Date: 4 Sep 92 16:51:43 GMT
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Organization: National Semiconductor, Santa Clara
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Lines: 70
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I recently asked here how fluxgate compasses work and received the
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following explanation by email from Franklin Antonio. I felt that
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it was an elegant explanation that would be of interest to sci.physics
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and as such I have reproduced his explanation. All credit for this
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posting are due to him and any errors remain mine - flame me not him.
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enjoy! (I did)
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From: Franklin Antonio <ANTONIO@qualcomm.com>
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In a recent sci.physics posting, you ask how fluxgate compasses work.
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The trick that requires explanation is how you can get a coil to produce
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a signal which is proportional to the strength of one component of a
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STATIC magnetic field. (ie the Earth's magnetic field) Obviously, if you
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can pull that off, you can do it twice, with two coils oriented 90 degrees
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apart, then take the 4-quadrant arctangent of the two signal levels, and get
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the direction of the static field. (or if you prefer doing it in 3d, you
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can do the same with three coils.)
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To make a signal which is a measurement of a static magnetic field, the
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fluxgate compass gates the flux of the earth's magnetic field.
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In the absence of local magnetic materials, the earth's magnetic field
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looks locally uniform. That is to say the lines of force are curved
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but gently so, so that locally you can think of them as uniform and
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straight, like the lines on a piece of writing paper. Now, consider
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a piece of magnetic material of high permeability. Perhaps a piece of
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iron or ferrite. Put this into the previously uniform field, and now
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the lines of force bend, because the lines would rather go thru the
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high permeability material than thru free space. If we had a way to
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gate the permeability of that hunk of iron, that is to vary it in a
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regular patern high/low/high/low/... then we would make the lines of
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force of the earth's magnetic field move in and out of the iron. Now
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we would have a varying (rather than static) magnetic field which
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is proportional to the strength of the static (Earth's) magnetic field
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which we were trying to measure. Of course, now that it's varying, we
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can measure it easily.
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How to make the permeability of that magnetic material gate on and off?
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Since permeability is in general a nonlinear function of total field,
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you can do this with bias. Consider a toroidal magnetic core (iron or
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ferrite) with two coils wound on it. One coil is for bias, the other
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for sensing. Put a large gated signal into the bias winding. Here
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large means large enough so that the nonlinear properties of the magnetic
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material come into play. When the bias signal is off, the core has a
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high permeability. When the bias signal is on, the core has a lower
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permeability. Now observe the sense winding. On it, we will see two
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signals. One is the bias signal obviously, and we need to filter that
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out. The bias frequency is chosen to be high enough that we can easily
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lowpass filter the sense output to eliminate the bias. The second
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component in the sense winding will be caused by some of the earth's
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magnetic field being alternately pulled into the core (by the permeability
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of the core material), and displaced (when the permeability drops).
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This signal is then rectified, and measured.
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This is usually done using two (or three) cores, oriented at right angles,
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so that the resulting signals are the components of the field in the
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corresponding directions.
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In a practical implementation, the bias might be something like a 100 kHz
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sine wave, which is gated on/off at a rate of a few per second. Bias level
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would be many ampere-turns, so to put the core well into saturation when
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bias is on.
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Accuracy is limited by your ability to wind a very uniform (symmetrical)
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sense coil, and other such non-ideal characteristics of the implementation.
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There is a NASA patent. I had a copy once, but no longer have it handy.
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Check your favorite patent database, and search on "flux-gate".
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End of article 36322 (of 36434)--what next? [npq]
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