The direction of the measured earth magnetic field is used as a (3D) compass to determine the direction of the north (heading or yaw), used as an absolute reference in the calculation of 3D orientation.
Disturbance caused by objects in the environment near the sensor, like magnet or vehicles that move independently, is non-deterministic and cannot be fully compensated for. However, the amount of error caused by the disturbance can be reduced by optimally using the available sensor information and valid assumptions about the application. This is the task of the Xsens Kalman Filter (XKF) running in the motion processor.
NOTE: Never expose the sensor to strong magnetic fields. The sensor contains the absolute possible minimum of ferromagnetic materials. Nonetheless, some minor components can be magnetized permanently by exposure to strong magnetic fields. This will not damage the unit but will render the calibration of the magnetometers useless, typically observed as a (large) deviation in heading.
If one of the following situations is observed in homogeneous magnetic field:
- persistent drift in yaw
- over 10% deviation from magnetic norm =1, see Figure 5
- fluctuation when rotating the sensor in different directions, see Figure 6
you can use Magnetic Field Mapper (MFM) to map the magnetometer to the local magnetic field to find the north in the local frame. Homogeneous environment here means there should be no ferromagnetic objects within at least three meters from the place in which the measurement is carried out.
Figure 5 Norm deviation
Figure 6 Norm fluctuate when rotating
Before carrying out the MFM it is extremely important that the measurement is carried out in a homogeneous magnetic field. Keep in mind that the structure of the building you are in (floor and ceiling) is likely to contain magnetic materials.
The MFM can be executed in a few minutes and yields a new set of electronic datasheet values (extended Motion Tracker Specification (eMTS) data) that can be written to DOT’s non-volatile memory. Follow these steps to implement the MFM process in demo App:
- Connect target sensors
- Choose Magnetic Field Mapper in Advanced tab
- Select connected sensor to start MFM
- Rotate the sensor with constant and low (<15 km/h) speed in x, y, z directions to collect calibration measurement
- Stop the MFM when all directions are covered and check the result. If the result shows Good or Acceptable, write the measurement values to device. Otherwise (Fail or Bad) a re-do is required to achieve a successful MFM result
An mtb file containing the calibration data will be created during the MFM. You can find it in the same directory of the logging data: Android/data/com.xsens.dot.android/files