The Magnetic Field Mapper can be used by executing the following steps:
Mounting of the MT
Recording of calibration measurement using the Magnetic Field Mapper software or other logging software
Processing of measurement data by the Magnetic Field Mapper software
Writing the results to Motion Tracker using the Magnetic Field Mapper software or by transmitting a specialized message to the Motion Tracker
In the case that, after mounting the MT, it cannot be directly connected to a PC that runs the Magnetic Field Mapper software, it is possible to perform magnetic field mapping off-line. This is described in Non-default calibration procedures. The standard procedure is described in the following sections.
The MT is attached to the object for which the magnetic field mapping procedure is carried out. Make sure that wherever the MT is placed, it cannot move with respect to the object. There are no specific requirements in terms of mounting orientation of the MT with respect to the object.
Remarks:
During a 3D calibration measurement, the object to which the sensor is attached has to be rotated through as many different orientations as possible. It may help to think about 'scanning' the surface of a sphere with the MT x-axis. It is important to cover as many orientations as possible, at least that many that will cover the envelope of motion of your application.
In the case of a 2D calibration measurement, the object has to be rotated through at least a full 360° circle. It is recommended to do this at constant and low (<15 km/h) speed.
Note: The MFM algorithm will always try to find a solution, even if there is only a partial capture. This means that when the MFM has been performed in the orientations of the intended application only, the heading will be accurate in those orientations, but possibly not outside that captured envelope.
The Magnetic Field Mapper will tell you if the measurement was OK.
Calibration remarks:
It is required to rotate the MT in as many different orientations as possible. To reach all points, as can be seen in Figure 10, as a rule of thumb, a calibration trial of around three minutes should suffice, provided that the object is rotated over a sufficiently large angle and held sufficiently still. If one of these requirements is not met, a longer calibration trial may prove to be useful.
For a 2D calibration measurement, it is recommended that the object moves through a full 360° circle.
If the MT can be directly connected to a PC, the Magnetic Field Mapper software can be used to record the calibration measurement data. If this is not the case, refer to Non-default calibration procedures to setup the off-line procedure.
To start the recording of the calibration measurement, start the Magnetic Field Mapper from the Xsens folder in Program Files or the Start Menu.
Choose the 'Write results to Motion Tracker non-volatile memory' option if the Motion Tracker is directly connected to the PC running the Magnetic Field Mapper software. For MTw, first establish a wireless connection using the MT Manager (100 Hz) and then close MT Manager to allow the MTw to communicate via Magnetic Field Mapper.
Home screen of the Magnetic Field Mapper
Press 'Scan' to scan for the connected MT. If the MT is not found, the following dialog is shown.
Error message indicating no devices are found
Scan the required port and click Next. If an MT is found, you should see the following dialog.
Screen where you can select the MT to use
Click 'Next' and select the location where the software stores the log file of the calibration measurement.
Screen where you select the logging directory
The software is now ready to start the calibration measurements. Read #How to perform a calibration measurement for more information about how to perform the measurement. Click 'Start' to begin the measurement. A window will open, visualizing the magnetic field data points, as they are being collected. Make sure to cover the surface of the globe as much as possible by rotating the MT in all possible directions.
Screen showing the data capturing process
When the sequence of rotations is completed, press 'Stop' to end the measurement and to start the analysis of the data. A window will pop up, visualizing the calibration results.
Screen indicating the overview results of the calibration
The Overview presents general information regarding the calibration.
The 'Result' value indicates the quality of the calibration, and can be one of four values: Good, Acceptable, Bad or Fail.
The amount of 'Used points' indicates how many magnetic field data samples were used by the Magnetic Field Mapper software.
'Mode' indicates whether a calibration has been performed in 3D or 2D.
'Status' indicates whether the calibration parameters have been written to device memory.
The Before MFM and After MFM sections state the most important specifications related to the calibration:
- Standard deviation of the norm; this value should be as low as possible;
- Average of the norm: this value should be as close to 1 as possible;
- Maximum error w.r.t. a norm of 1: this value should be as low as possible.
By clicking Advanced Results, the quality of the calibration can be reviewed in more detail, see #Explanation of the Advanced Results.
Upon review of the results, they can be stored in MT memory by clicking 'Write to selected devices'. This is necessary for the MT to be able to use the updated calibration parameters in the future.
The Advanced Results window of the Magnetic Field Mapper generates several reports which are discussed below.
In the Advanced Results window, this figure shows the total movement of the industrial motion tracker over all directions of an imaginary sphere. In the left image, the original magnetic field measurements are shown. The right image shows the magnetic field measurements after the MFM procedure. The rounder this sphere is, the better the MFM results are.
In the Advanced Results window, this figure shows whether the MFM was able to apply the new magnetic field model to all corrected magnetic field measurements of the file or measurement. When large spikes are visible, the data set used for the MFM may result in a magnetic field model of lower quality.
In the Advanced Results window, this figure shows the residuals of the corrected magnetic field vector of the file or measurement with respect to the new magnetic field model. When measurements are visible outside the Gaussian distribution, the data set used for the MFM had errors, which may result in a magnetic field model of lesser quality. Ideally, the residual histogram follows the black line, indicating that the distribution of the residuals follows a Gaussian distribution.
In the Advanced Results window, this figure shows the norm of the magnetic field before and after Magnetic Field Mapping. It also shows which data points were used for the Magnetic Field Mapping, indicated by '+'. The more flat the blue line is (and the closer to 1.0), the better the calibration.
If the calibration procedure does not give the desired results, it may be caused by one of the following error sources:
|
Cause |
Explanation |
|---|---|
|
Non homogeneous magnetic field in measurement volume |
The effect of a non-homogeneous measurement field shows in large residuals even though you follow procedure.
To remedy this problem, try to perform the calibration measurement in a different place or remove nearby metal objects. |
|
Saturation |
The disturbance of the magnetic field can be so extreme that the magnetometers are saturated. In this case, a warning will be given.
Reposition the MT on the object, away from the ferromagnetic material and not close to sharp edges, to remedy this problem. |
|
Large accelerations |
If the object is accelerated too much during calibration, this will cause an error. If large accelerations cannot be avoided, contact the Xsens support team at http://www.xsens.com/support. |
|
Limited rotation |
The calibration procedure is designed to process measurements in which the MT is rotated through a large amount of possible orientations, even though measurements with a limited range of motion will most likely give good results as well. |
|
Extreme disturbance of magnetic field |
It can be that the disturbance of the magnetic field is so extreme that the program cannot find any function to correct the disturbance. This could occur more easily when one of the other error causes play a significant role.
The result of such an error will become apparent in very high residuals or an error message. |
The Magnetic Field Mapper algorithm will always try to produce a result. Only when the internal results are unrealistic, the Magnetic Field Mapper will not output the results.
This chapter provides several example reports in order to be able to recognize disturbances.
Correct Magnetic Field Mapping
This report shows an even distribution of points on the sphere, has a normal distribution of residuals (fit within the Gaussian model), has small residuals in all directions and a norm after MFM of close to 1.
Too much magnetic disturbance
The points in the right sphere hover just above the surface of the model; the points do not fit to the model (and many points even fall outside the graph, see the red bars), residuals are large and the norm is not straight and/or close to 1.
Large accelerations
The points in the sphere do not form solid lines, almost all points are outside the graph in the Gaussian model plot, the residuals are very large in all directions and the norm after MFM is not 1.
Not enough points
The spheres show hardly any points and are poorly distributed, resulting in large residuals. The norm after the MFM procedure is close to 1, but not constant.
Depending on the application in which the MT is used and the amount of dynamics, calibration reports may differ in appearance. This section provides several (successful) example reports for various use cases.
2D calibration: Wheeled robotic ground vehicle
Example report for a 2D Magnetic Field Mapping. The application is a wheeled robotic ground vehicle consisting of a large metal structure with batteries and motors nearby. During the calibration, the vehicle drove two full circles. Clearly, the calibration has been successful: the centre of the sphere has been transformed to 0 and the magnetic norm is nearly constant at 1.
Semi-3D calibration: Drone
Example report for a semi-3D Magnetic Field Mapping. The application is a drone. During the calibration, the drone tilted towards all possible directions. After calibration, the magnetic norm is nearly constant at 1 except for some noise possibly caused by the drone's on-board electronics and motors.