User Manual

© Xsens Technologies B.V.
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NOTE: To be able to accurately measure orientations as well as position in applications
which can encounter long term accelerations, Xsens offers a solution that incorporates a GPS
receiver (the MTi-G).
11.4.2 Using the Earth Magnetic Field to Stabilise Heading (Yaw)
By default, the heading is stabilised using the local (earth) magnetic field. In other words, the
measured magnetic field is used as a compass. If the local Earth magnetic field is temporarily
disturbed, XKF-3w will track this disturbance instead of incorrectly assuming there is no
disturbance. However, in case of structural magnetic disturbance (>10 to 20 s) 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.
If the MTw is rigidly strapped to an object containing ferromagnetic materials, structural
magnetic disturbances will be present. Using an Xsens a technique 'magnetic field mapping'
(i.e. a 3D calibration for soft and hard iron effects), these magnetic disturbances can be
completely accounted for, allowing the MTw to be used as if it would not be secured to an
object containing ferromagnetic materials.
11.4.3 Initialisation
The XKF-3w algorithm not only computes orientation, but also keeps track of variables such
as sensor biases or properties of the local magnetic field. For this reason, the orientation
output may need some time to stabilize once the MTw 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. To reduce stabilizing time, the last
computed gyroscope bias can be stored in the sensor unit non-volatile memory. If the MT is
used after only a short period of power-off the gyro biases will generally not have changed a
lot and the stabilizing time will typically be less than 10 seconds. Furthermore, XKF-3w will
converge faster and reach optimal robustness faster if it is started in an area without
magnetic disturbances.
11.5 Strap Down Integration
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Strap down integration (SDI) is a method used to compute an orientation/position change
given an angular velocity and linear acceleration of a rigid body. Angular velocity and
acceleration data in the MTw is sampled at a very high frequency to maintain accuracy under
very dynamic conditions such as vibrations and impacts. The sampling frequency is too high
to be transmitted wirelessly, and would typically present a computational load that is too
high on the receiving host device (e.g. PC). Therefore the data that has been sampled at a
high frequency is digitally filtered and down-sampled to 600 Hz and calibrated using the
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Further information about strap down integration for the MTw is available in the MTw
Whitepaper.