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Document MT1600P.2019.A
© Xsens Technologies B.V.
MTi Family Reference Manual
accuracy of the orientation estimates because the application does not match the assumptions made in
the algorithm. Note however, that as soon as the movement again matches the assumptions made, the
algorithm 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 solutions that use aiding data from a GNSS receiver: the
MTi-7 GNSS/INS, MTi-670 GNSS/INS or MTi-G-710 GNSS/INS.
4.4.3 Heading/yaw estimation
By default, yaw is referenced by using the local (earth) magnetic field (e.g. in the AHRS product
versions). In other words, the measured magnetic field is used as a compass. If the local Earth magnetic
field is temporarily disturbed, the algorithm will track this disturbance instead of incorrectly assuming
there is not disturbance. However, in case of structural magnetic disturbance (>10 to 30 seconds,
depending on the filter profile settings) the computed heading will slowly converge to a solution using
the 'new' local magnetic north. Note that the magnetic field has not direct effect on the inclination
estimate.
The filter profile ‘Fixed Mag Ref’ will assume a magnetic reference upon startup and keep that reference
regardless of new magnetic environments (available only on MTi 600-series, see
MTi 600-series
Datasheet
).
In the special case the MTi is rigidly strapped to an object containing ferromagnetic materials, structural
magnetic disturbances will be present. In that case, Xsens offers an easy-to-use solutions to recalibrate
the magnetometers based on those structural magnetic disturbances (refer to chapter 6.3 of this
document.
Next to the solutions described on the article
Estimating Yaw in magnetically disturbed environments
mitigate effects from magnetic disturbances, the sensor fusion algorithm in a GNSS/INS device makes
use of data from the GNSS receiver. This means that the GNSS/INS device has an increased resistance
towards magnetic disturbances. It is for example possible to estimate the heading based on comparison
between accelerometer data and the GNSS acceleration. For GNSS/INS devices, the magnetometer
data is only actively used in the GeneralMag filter profile, the other filter profiles are completely
independent of the magnetic field.
4.4.4 Velocity and Position estimation
Transient accelerations
The GNSS/INS algorithm adds robustness to its orientation and position estimates by combining
measurements and estimates from the inertial sensors and GNSS receiver in order to compensate for
transient accelerations. It results in improved estimates of roll, pitch, yaw, position and velocity.
Loss of GNSS
When the GNSS/INS device has limited/mediocre GNSS reception or even no GNSS reception at all,
the sensor fusion algorithm seamlessly adjusts the filter settings in such a way that the highest possible
accuracy is maintained. The GNSS/INS MTi will continue to output position, velocity and orientation
estimates, although the accuracy is likely to degrade over time as the filters will have to rely on dead-
reckoning. The GNSS status will be monitored continuously such that the filter can take GNSS data into
account again when available and sufficiently trustworthy. In case the loss of GNSS lasts longer than a
specific period (depending on product type, e.g. 45 seconds), the device will enter a state in which it
stops outputting position and velocity estimates, and no longer uses velocity estimates in its sensor
fusion algorithms until GNSS reception is re-established.
