16
Document MT0605P.2018.E
© Xsens Technologies B.V.
MTi User Manual
4 MTi System Overview
4.1 Calibration
A correct calibration of the sensor components inside the MTi is essential for an accurate output. The
quality and importance of the calibration are of highest priority and so each Xsens’ MTi is calibrated and
tested by subjecting each product to a wide range of motions and temperatures.
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 Xsens sensor fusion algorithms, as discussed
below.
4.2 Xsens Kalman Filter for VRU and AHRS product types
The orientation of the VRU and AHRS is computed by Xsens Kalman Filter. XKF3™ is a proven sensor
fusion algorithm, which can be found in various products from Xsens and partner products. The industrial
applications version is XKF3i: it 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.
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).
4.2.1 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. 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.
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 the assumptions
made in the algorithm. Note however, that as soon as the movement again matches the assumptions
made, XKF3i 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 a solution that incorporates a GNSS receiver, the MTi-G-
710 GNSS/INS.
