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