FMT1000 Datasheet, PDF (25/29 Pages) Fairchild Semiconductor – Motion Tracking Module with Output of Orientation, Inertial Motion Data and Magnetic Field

7 Magnetic Interference
Magnetic interference can be a major source of error for
the heading accuracy of any Attitude and Heading
Reference System (AHRS). As an AHRS uses the
magnetic field to reference the dead-reckoned
orientation on the horizontal plane with respect to the
(magnetic) North, a severe and prolonged distortion in
that magnetic field will cause the magnetic reference to
be inaccurate. The FMT1000-series module has several
ways to cope with these distortions to minimize the
effect on the estimated orientation.
7.1 Magnetic Field Mapping
When the distortion is deterministic, i.e. when the
distortion moves with the FMT, the FMT can be
calibrated for this distortion this type of errors are
usually referred to as soft and hard iron distortions.
The Magnetic Field Mapping procedure compensates
for both hard-iron and soft-iron distortions.
In short, the magnetic field mapping (calibration) is
performed by moving the FMT together with the
object/platform that is causing the distortion. On an
external computer (Windows or Linux), the results are
processed and the updated magnetic field calibration
values are written to the non-volatile memory of the
FMT1000-series module. The magnetic field mapping
procedure is extensively documented in the Magnetic
Field Mapper User Manual, available in the Fairchild MT
Software Suite.
7.2 Active Heading Stabilization
(AHS)
It is often not possible or wanted to connect the
FMT1000-series module to a high-level processor/host
system, this makes the use of the Magnetic Field
Mapping procedure less desirable and cumbersome.
Also, when the distortion is non-deterministic the
Magnetic Field Mapping procedure does not yield the
desired result. For all these situations, the on-board
XKF3 sensor fusion algorithm has integrated an
algorithm called Active Heading Stabilization (AHS).
The AHS algorithm delivers excellent heading tracking
accuracy, improving heading tracking in almost all
cases. There are rare occasions where environmental
conditions (e.g. specific movements in combination with
specific magnetic distortions) that could lead to a lesser
performance than expected. In most cases, heading
tracking drift in the FMT1000-series can be as low as 1
deg per hour, while being fully immune to magnetic
distortions.
AHS is only available in the VRU_general filter profile.
This filter profile is the only filter profile in the FMT1020
VRU and one of the 5 available filter profiles in the
FMT1030 AHRS.
© 2015 Fairchild Semiconductor Corporation
FMT1000-series • Rev. 1.0
25
www.fairchildsemi.com