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Document MT1600P.2019.A
© Xsens Technologies B.V.
MTi Family Reference Manual
6
Installation tips and tricks
6.1 Transient accelerations
The 3D linear accelerometers in the MTi are primarily used to estimate the direction of gravity to obtain
a reference for attitude (pitch/roll). During long periods (more than tens of seconds) of transient “free”
accelerations (i.e. 2
nd
derivative of position) the observation of gravity cannot be made. The sensor
fusion algorithms can mitigate these effects to a certain extent, but nonetheless it is impossible to
estimate true vertical without additional information.
The impact of transient accelerations can be minimized when you take into account a few things when
positioning the device when installing it in the object you want to track/navigate/stabilize or control.
If you want to use the MTi to measure the dynamics of a moving vehicle it is best to position the
measurement device at a position close to the centre of rotation (CR) of the vehicle/craft. Any rotations
around the centre of rotation translate into centripetal accelerations at any point outside the centre of
rotation. For a GNSS/INS device with a valid GNSS-fix, the detrimental effect of transient accelerations
on orientation estimates is overcome by integrating GNSS measurements in the sensor fusion engine.
6.2 Vibrations
The MTi samples IMU signals at high frequency per channel, processing them using a strapdown
integration algorithm with coning/sculling compensation. Proper coning/sculling compensation already
mitigates errors that poorly designed signal processing pipelines introduce when the device is under
vibration. For best results however, it is recommended that the MTi be mechanically isolated from
vibrations as much as possible: since vibrations are measured directly by the accelerometers, the
following two conditions can make the readings from the accelerometers invalid;
1. The magnitude of the vibration is larger than the measurement range of the accelerometer. This
will cause the accelerometer to saturate, which may be observed as a “drift” in the zero
-level of
the accelerometer. This will show up as an erroneous roll/pitch.
2. The frequency of the vibration is higher than the bandwidth of the accelerometer. In theory, such
vibrations are rejected, but in practice they can still give rise to aliasing, especially if close to the
bandwidth limit. This can be observed as a low frequency oscillation. Further, high frequency
vibrations often tend to have large acceleration amplitudes (see item 1).
There is an effect on the gyroscopes as well and especially when the vibrations include high-frequent
coning motion, the gyroscope readings may become invalid.
6.3 Magnetic materials and magnets
When an MTi is placed close to or on an object that is either magnetic or contains ferromagnetic
materials, the measured magnetic field is distorted (warped) and causes an error in the computed
heading. The earth magnetic field is altered by the presence of ferromagnetic materials, permanent
magnets or power lines with strong currents (several amperes) in the vicinity of the device. The distance
to the object and the amount of ferromagnetic material determines the magnitude of disturbance
introduced. Errors in estimated yaw due to such distortions can be quite large, since the earth magnetic
field is very weak in comparison to the magnitude of the sources of distortion.
By default, the AHRS and the GNSS/INS versions (when using the GeneralMag filter profile) stabilize
heading using the local Earth's magnetic field. In other words, the measured magnetic field is used as
a compass. In addition, the gyroscope biases are continuously estimated by the MTi's on-board filter.
For the rate of turn around the x-axis and the y-axis (roll and pitch axes), the gyroscope bias is estimated
