Document MT0605P.E
© Xsens Technologies B.V.
MTi User Manual
16
3
MTi System Overview
3.1 Calibration
A correct calibration of the sensor components inside the MTi is essential for an accurate output.
Because of the importance of the calibration, each Xsens’ MTi is calibrated and tested by subjecting
each product to a wide range of motions and temperatures.
The MTi 10-series and the MTi 100-series feature different gyroscopes and a different sensor fusion
algorithm. Therefore, the high-performance MTi 100-series require a more elaborate calibration
method.
The individual calibration parameters are used to convert the sensor component readout (digitized
voltages) to physical quantities as accurately as possible, compensating for a wide range of
deterministic errors. Additionally, the calibration values are used in both Xsens sensor fusion
algorithms, as discussed below.
3.2 Xsens Kalman Filter (XKF3i) for MTi 10-series
The orientation of the MTi 10-series is computed by Xsens Kalman Filter. XKF3i uses signals of the
rate gyroscopes, accelerometers and magnetometers to compute a statistical optimal 3D orientation
estimate of high accuracy with no drift for both static and dynamic movements. XKF3 is a proven
sensor fusion algorithm, which can be found in various products from Xsens and partner products.
The design of the XKF3i algorithm can be summarized as a sensor fusion algorithm where the
measurement of gravity (by the 3D accelerometers) and Earth magnetic north (by the 3D
magnetometers) compensate for otherwise slowly, but unlimited, increasing (drift) errors from the
integration of rate of turn data (angular velocity from the rate gyros). This type of drift compensation is
often called attitude and heading referencing and such a system is referred to as an Attitude and
Heading Reference System (AHRS).
3.2.1
Using the acceleration of gravity to stabilize inclination (roll/pitch)
XKF3i stabilizes the inclination (i.e. roll and pitch combined) using the accelerometer signals. An
accelerometer measures gravitational acceleration plus acceleration due to the movement of the
object with respect to its surroundings.
XKF3i uses the assumption that on average the acceleration due to the movement is zero. Using this
assumption, the direction of the gravity can be observed and used to stabilize the attitude. The
orientation of the MTi in the gravity field is accounted for so that centripetal accelerations or
asymmetrical movements cannot cause a degraded orientation estimate performance. This
assumption is surprisingly powerful, almost all moving objects undergo accelerations if they are
moving, but in most cases the average acceleration with respect to the environment during some
period of time is zero. The key here is the amount of time over which the acceleration must be
averaged for the assumption to hold. During this time, the rate gyroscopes must be able to track the
orientation to a high degree of accuracy. In practice, this limits the amount of time over which the
assumption holds true. For the class of miniature MEMS rate gyroscopes used in the MTi-10 series
this period of time is about 10-20 seconds maximum.
However, for some applications this assumption does not hold. For example an accelerating
automobile may generate significant accelerations for time periods lasting longer than the maximum
duration the MT’s rate gyroscopes can reliably keep track of the orientation. This will degrade the
accuracy of the orientation estimates with XKF3i somewhat, because the application does not match
