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How to calibrate eCompass for hard, soft iron effects

Posted: 15 Sep 2011 ?? ?Print Version ?Bookmark and Share

Keywords:calibration? smartphone? electronic compass? soft iron?

Here's a document tackling the theory for the in-situ calibration of a smartphone electronic compass (eCompass) for hard and soft iron effects.

Here some important points:
? It is possible for a smartphone eCompass to be calibrated by the owner in the street with no a priori knowledge of location or the direction of magnetic north.
? Magnetometer measurements subject to hard and soft iron distortions caused from ferromagnetic materials on the PCB lie on the surface of an ellipsoid which can be accurately modeled by ten parameters.
? The calibration process consists of fitting the ten model parameters to the magnetometer measurements. Three model the hard iron offset, six model the soft iron matrix and one models the geomagnetic field strength.
? When the ten model parameters are known, it is a simple procedure in software to transform the magnetometer measurements from the surface of the ellipsoid to the surface of a sphere centered at the origin. This transformation removes the hard and soft iron interference allowing an accurate eCompass heading to be computed.
? Reference C source code for the calibration can be licensed free of charge from Freescale for use in any product using a Freescale magnetometer. Please contact your Freescale sales representative for the license agreement.

View the PDF document for more information.

Originally published by Freescale Semiconductor Inc. at as "Calibrating an eCompass in the Presence of Hard and Soft Iron Interference".

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