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Another Magnetic Measurement

One of the fun things about the MEMS and sensors space is that there are often many ways of skinning the many proverbial cats. Magnetometers are one example.

When Bosch Sensortec introduced its BMC050 6-axis sensor last year, they made particular note of their FlipCore technology for detecting the strength of the earth’s (or any local) magnetic field. Detailed information, however, wasn’t readily available.

At the recent MEMS Executive Congress, I was able to talk with Bosch Sensortec’s Marcellino Gemelli to get the next level of understanding. And, in principle, it’s surprisingly straightforward. It simply relies on the familiar equation V=L di/dt.

They essentially build a transformer. One coil has a magnetic material in it, and they run a current through the coil; the secondary coil acts as the detector. The trick is that they periodically reverse the current through the primary coil. At the point where they reverse the current, you get a voltage spike in the secondary coil.

But the timing of that spike relates to the entire magnetic field, not just the one created by the primary. That includes the earth’s magnetic field (as well as any local “anomalies”). If the device is facing East/West, then the contribution from the earth’s field is orthogonal and has no impact; it has maximal impact if the device is facing North/South. The amount of that component delays (or advances) the voltage spike with respect to the time when the current in the primary coil was changed.

By measuring this phase shift in the output pulse train, they determine the heading of the device.

Of course, there are details with respect to calibration, and it has to be done in three axes, so the reality is somewhat more complicated than the theory, but they appear to have tamed it; they’re in production. With a 1000 microtesla field (which, I guess, would be 1 millitesla), they claim roughly 0.5 degree angular resolution.

The device itself combines the magnetometer with an accelerometer, which is essential for providing the tilt compensation necessary for an eCompass. You get acceleration data out of it as an additional bonus.

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