There are a few situations in which the magnetometer in an IMU needs a re-calibration. This re-calibration of a magnetometer can be done with our software called; 'Magnetic Field Mapper' (MFM). A few situations in which this is recommended:
- When you mount or un-mount a tracker on ferromagnetic material
- If strange orientations are observed in the avatar in MVN, even though a high quality calibration was performed (see this article how to do that and refer to the paragraph 'Quality environment during calibration'). An MFM is recommended when this strange orientation sticks to the same tracker(s), also when calibrating at other locations.
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
A locally disturbed (warped) magnetic field causes an error in orientation that can be quite substantial.
The earth magnetic field is altered by ferromagnetic materials (e.g. steel), permanent magnets or very strong currents (several amperes).
For more in dept information on the procedure and results of the Magnetic Field Mapper please refer to the Magnetic Field Mapper documentation.
Since the MVN Awinda system contains a high number of trackers, you can either perform the MFM with only 1 or the few trackers which need this procedure, or you can use all trackers at the same time. When you MFM multiple trackers at once, it is easier to mount them on a non-ferromagnetic object (see picture). In the picture the MVN straps were used to prevent them from falling off during the procedure. For convenience switch them on before you strap over them.
- Work on a laptop (you need to be portable because you will be going outside later).
- Download and install MT Software Suite: https://www.xsens.com/mt-software-suite-mtw-awinda/
- Switch on the trackers on the tennis ball (e.g.) and connect the Awinda station to the laptop.
- Open MT Manager. You can find this program within the MT Software Suite installation folder.
- Connecting the MTw's in MT Manager: On the left (Device List) you already see the Awinda station. Click on 'Wireless configuration' .Within the next pop up window, click 'enable wireless master'. Now, the Awinda station starts searching for the trackers. You can move the ball with the trackers a bit to trigger them to connect to the Awinda station. Within 20 sec you should see a list of all trackers with a green square in front. When the amount of items in the list correspond to the amount of trackers on the tennis ball, click 'start measurement'.
To check if all trackers are online you can click the '3D orientation' icon . This is the icon in the top bar that shows a 3D box with a pink and purple surface. Move the ball, and you will see all the boxes (trackers) move in the window as well. If this is indeed the case, frankly close MT Manager only by pressing the red X in the top right corner. The trackers will continue to be in measurement mode.
Performing the MFM:
- Make sure you are in a homogeneous magnetic field: it;s recommended to go outside with the current setup. Make sure not to disconnect the Awinda station while moving and keep the ball within wireless range. Choose a location outside, at least 20m away from any building or car. Also make sure to not cause a magnetic field by wearing clothes/items made of ferromagnetic materials. Leave your cell phone, watch and keys behind.
- Run the Magnetic Field Mapper (within the MT Software Suite folder again). Click through the pages and check if all trackers are found. You can increase the length of the window by dragging it down for a better overview. If all trackers are there, have the ball in your hand while keeping it away as far as possible from the laptop. Now, click the bottom right to start capturing.
- The movements during capture: Keep the ball centered and move it in 3D along all axes twice, slowly and fluently. Take for each rotation maybe 5-10 seconds. End the movements with a few slow rotations in random directions. The capture should not last longer than 30-60 seconds. To end the capture, keep the ball centered in the same area away from the computer and click 'Process'. (second person might come in handy) This can take a while, because there is a lot of data to process.
- Reaching the next page, you can check the results. If all trackers were mounted on 1 object, you only need to check the results of a few trackers. Click 'Show results'. In the results on the top left you will see 2 grey spheres. The rounder the sphere on the right side, the better the results of the MFM (see picture below).
Next, the 'normal distribution curve' or 'Gaussian distribution curve' on the top right of the window 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 Gaussian distribution, the data set used for the MFM had errors, which may result in a magnetic field model of lesser quality (see picture below).
- If this is ok for a few trackers, click 'write results to MT's'. You will see 'successful' appear with each tracker. You can close the MFM software now.
- After performing a successful MFM, disconnect the Awinda and open MVN Animate/Analyze. Go to Options > Preferences > Interface > User Interface and click 'Discard estimated tracker data'. This will reset the estimated sensor biases on your computer, and your system will restart building new sensor biases again with the new magnetometer calibration in place. Make sure to restart MVN after removing the tracker data.
These are results of an incorrect MFM (big residuals in red on both sides of the Gausian curves. Magnetic environment during MFM procedure might not be homogeneous):
These are results of a MFM with too few points (not enough 3D rotations and capture time too short):