AHRS400 Series User’s Manual
Doc# 7430-0004-01 Rev. A
Page 25
7 Appendix C. Hard and Soft Iron Calibration
7.1
Description
The AHRS400 Series of products use magnetic sensors to compute heading.
Ideally, the magnetic sensors would be measuring only earth's magnetic
field to compute the heading angle. In the real world, however, residual
magnetism in the AHRS itself and in your system will add to the magnetic
field measured by the AHRS. This extra magnetic field will create errors in
the heading measurement if they are not accounted for. These extra
magnetic fields are called hard iron magnetic fields. In addition, magnetic
material can change the direction of the magnetic field as a function of the
input magnetic field. This dependence of the local magnetic field on input
direction is called the soft iron effect. The AHRS can actually measure any
extra constant magnetic field that is associated with the AHRS or your
system and correct for it. The AHRS can also make a correction for some
soft iron effects. The process of measuring these non-ideal effects and
correcting for them is called hard iron and soft iron calibration. Calibration
will help correct for magnetic fields that are fixed with respect to the
AHRS. It cannot help for time varying fields, or fields created by parts that
move with respect to the AHRS.
The AHRS accounts for the extra magnetic field by making a series of
measurements. The AHRS uses these measurements to model the hard iron
and soft iron environment in your system. The correction algorithm is two-
dimensional. You start the magnetic calibration by sending the "s"
command. The AHRS will use all subsequent measurements to model the
magnetic environment. You should make at least one complete turn, with
your system basically level. For example, in an airplane, do a circle on the
taxiway. Multiple turns will slightly improve the estimates, but more than 3
turns is usually not helpful. At the end of this time, send the "u" command
to end the magnetic calibration process. The AHRS will calculate the hard
iron magnetic fields and soft iron corrections and store these as calibration
constants in the EEPROM.
To clear the hard iron calibration constants, send the “h” command. The
AHRS will set the hard iron offset corrections to zero. To clear the soft iron
calibration constants, send the "t" command. The AHRS will set the soft
iron correction parameters to zero. This is useful to see the performance of
the bare AHRS in your system.
For best accuracy, you should do the calibration process with the AHRS
installed in your system. If you do the calibration process with the AHRS
by itself, you will only correct for the magnetism in the AHRS itself. If you
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