The gyroscope bias is continuously estimated. For the rate of turn around the x-axis and the y-axis (roll and pitch axes), the gyroscope bias is estimated using gravity (accelerometers). In a homogeneous magnetic field and with filter profiles using the magnetometer, the gyroscope bias around the z-axis will successfully be estimated.
In some situations, the heading cannot be referenced to the (magnetic) north. This is the case when the magnetic field is not used or when the magnetic field is not homogeneous . There are several ways to mitigate the drift in heading (rotation around the z-axis):
- From MT Software Suite 4.3 and firmware revision 1.4, the sensor fusion algorithm includes Active Heading Stabilization (AHS) (disabled by default). This means that the heading drift in the VRU_General filter profile will be reduced to approximately 1 deg/hour (MTi 100-series) or approximately 3 deg/hour (MTi 10-series). As the heading is not referenced, there is no guarantee for this performance under all circumstances. In this case, the options below remain. From MT Software Suite 4.4, AHS is available for all filter profiles.
- When the MTi has sufficient movement in roll and pitch (>30 deg for more than 10 seconds), the gyroscope bias will be estimated for the z-gyroscope. When rotating the MTi back to roll and pitch around 0, the heading will be more stable than before the roll/pitch movements.
- When the MTi cannot or is not rotated around roll and pitch, it is possible to let the gyroscopes bias to be estimated when the MTi does not rotate (a so-called no-rotation update). For this, make sure that the MTi doesn’t move and send the command (SetNoRotation (0x22)) to the MTi in measurement mode. It is recommended to apply a no-rotation update for 6 seconds. The MTi must absolutely not be rotating during the given period of time that the procedure is active. We also recommend waiting for the MTi to warm up for at least 10 minutes as bias changes over temperature.
Understanding Sensor Bias (offset)
Active Heading Stabilization (AHS)