CXM543 User’s Manual
Doc.# 6001-0015 Rev 1.2
Page 26
8 Appendix B. Hard Iron Correction for CXM543
A hard iron offset refers to the perturbation of magnetic field experienced
by an orientation sensor caused by nearby magnetic materials which are on
the same platform as the sensor i.e. a constant offset in the magnetic field in
the frame of the sensor. This perturbation could be measured directly by
placing the sensor and platform in a magnetically shielded enclosure, but
this is often not practical. The CXM543 sensor has the capability to
measure the hard iron offset by placing the sensor (and platform) at a set of
different orientations and collecting data at each orientation. Once
measured, the hard iron offset can be subtracted from the magnetometer
readings, thus enabling a more accurate measurement of orientation. The
set of orientations used for hard iron corrections do not need to be precise,
in fact a set of random orientations will usually work. The accuracy of the
correction is better for some sets of orientations than for others, however, so
the CXM543 software leads the user through a set of orientations which are
known to work well. The ability to use orientations that are not precisely set
greatly eases use in the field. Two versions of hard iron measurement are
available, one for three-dimensional orientations and another for the case
where the platform moves in only two dimensions. The two dimensional
case would apply to land vehicles, for example, where only azimuth is
varied. On the other hand, if roll, pitch and azimuth are all varied, as is
often the case for airborne platforms, the three dimensional corrections
would be needed. Slightly different data are accumulated, depending upon
whether a 2 or 3 dimension correction will be determined, so best results are
obtained if the dimension is specified before the procedure begins. The
three dimensional data do allow for the determination of a two dimensional
correction, at slightly reduced accuracy, but the two dimensional data are
not sufficient to measure the three dimensional correction.
8.1
Setup procedure
The CXM543 is calibrated at the factory and no hard iron offset should
exist unless it is rigidly mounted to a movable platform. The movements of
the CXM543 to be described require that the entire platform be moved with
it. If the platform cannot be turned upside down then only the 2
dimensional hard iron correction will be possible.
The CXM543 must be connected to a terminal emulator program on a PC to
perform the correction. The Section 4.1 explains the procedure for setting
the CXM543 up with a terminal emulator program. The CXM543 must be
in run mode to do the hard iron correction. For this, you will need the
CXM543, a PC with a terminal emulator program, a power supply capable
of delivering 7.5 to 15 volts and cables.
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