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Document MT0605P.2018.E
© Xsens Technologies B.V.
MTi User Manual
4.2.2 Using the Earth magnetic field to stabilize yaw
By default, yaw is stabilized using the local (earth) magnetic field (only in the AHRS product types). 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. In that case, there are solutions to use the magnetometers after
all.
Refer
to
https://base.xsens.com/hc/en-us/articles/115004479409-Magnetic-distortions-and-
4.2.3 Estimating gyro bias in magnetic disturbed environments
The gyroscope bias is continuously estimated. For the rate of turn around the x-axis and the y-axis (roll
and pitch axes), the gyroscope bias is estimated using gravity (accelerometers). In a homogenous
magnetic field and with filter profiles using the magnetometer, the gyroscope bias around the z-axis will
successfully be estimated.
In some situations, the heading cannot be referenced to the (magnetic) north. This is the case when the
magnetic field is not used or when the magnetic field is not homogenous. There are several ways to
mitigate the drift in heading (rotation around the z-axis):
1. The gyroscope bias can be estimated using Active Heading Stabilization (AHS). As the heading
is not referenced, there is no guarantee for this performance under all circumstances. Refer to
https://base.xsens.com/hc/en-us/articles/210133105-Use-of-Active-Heading-Stabilization-
AHS-in-MTi
2. When the MTi has sufficient movement in roll and pitch (>30 deg for more than 10 seconds),
the gyroscope bias will be estimated for the z-gyroscope. When rotating the MTi back to roll and
pitch around 0, the heading will be more stable than before the roll/pitch movements.
3.
When the MTi cannot or is not rotated around roll and pitch, it is possible to let the gyroscopes
bias to be estimated when the MTi does not rotate (a so-called no-rotation update). Refer to
https://base.xsens.com/hc/en-us/articles/203307252-Bias-repeatability-of-gyroscopes
4.2.4 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.
4.2.5 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].
