Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
17
the assumptions made in the algorithm. Note however, that as soon as the movement again matches
the assumptions made, XKF3i will recover and stabilize. The recovery to optimal accuracy can take
some time.
NOTE:
To be able to accurately measure orientations as well as position in applications which can
encounter long term accelerations we offer a solution that incorporates a GPS receiver (the MTi-G-700
GPS/INS).
3.2.2
Using the Earth magnetic field to stabilize yaw
By default, yaw is stabilized using the local (earth) magnetic field (only in the MTi-30 AHRS). In other
words, the measured magnetic field is used as a compass. If the local Earth magnetic field is
temporarily disturbed, XKF3i will track this disturbance instead of incorrectly assuming there is no
disturbance. However, in case of structural magnetic disturbance (>10 to 30 s, depending on the filter
setting) the computed heading will slowly converge to a solution using the 'new' local magnetic north.
Note that the magnetic field has no direct effect on the inclination estimate.
In the special case the MTi is rigidly strapped to an object containing ferromagnetic materials,
structural magnetic disturbances will be present. Using a so-called 'magnetic field mapping' (MFM, i.e.
a 3D calibration for soft and hard iron effects), these magnetic disturbances can be completely
calibrated for, allowing the MTi to be used as if it would not be secured to an object containing
ferromagnetic materials.
3.2.3
Initialization
The XKF3i algorithm not only computes orientation, but also keeps track of variables such as sensor
biases or properties of the local magnetic field (magnetic field: MTi-30 AHRS only). For this reason,
the orientation output may need some time to stabilize once the MTi is put into measurement mode.
Time to obtain optimal stable output depends on a number of factors. An important factor determining
stabilizing time is determined by the time to correct for small errors on the bias of the rate gyroscopes.
The bias of the rate gyroscope may slowly change due to different effect such as temperature change
or exposure to impact.
3.2.4
XKF3i filter profiles
As described above, XKF3i uses assumptions about the acceleration and the magnetic field to obtain
orientation. Because the characteristics of the acceleration or magnetic field differ for different
applications, XKF3i makes use of filter profiles to be able to use the correct assumptions given the
application. This way, XKF3i can be optimized for different types of movement. For optimal
performance in a given application, the correct filter profile must be set by the user. For information on
how to specify a filter profile in XKF3i, please refer to the MT Manager User manual [MTM] or the MT
low-level communication protocol documentation [LLCP].
The different filter profiles are divided in several types of motion and are discussed below.
Number
Name
IMU
Magnetometer
Product
39
General
•
•
30-AHRS
40
High_mag_dep
•
•
30-AHRS
41
Dynamic
•
•
30-AHRS
42
Low_mag_dep
•
•
30-AHRS
43
VRU_general
•
30-AHRS, 20-VRU
