IMU / Compass calibration questions

[gponcon]

Hi everyone. A little topic so that we can know everything about IMU / compass calibration, including novices like me .

To calibrate my IMU, I used the official instructions:

- https://openplotter.readthedocs.io/en/2....ation.html

As well as information on this forum:

- https://forum.openmarine.net/showthread.php?tid=3124
- https://forum.openmarine.net/showthread.php?tid=4240
- etc.

But it is possible that I still can't calibrate my IMU correctly... maybe that's the problem I've had all along. After several attempts, here are my personal notes:

Accelerometer bias

Open the accelerometer page and orient the IMU on the 6 faces of the cube whose sides are perpendicular to the axes. Stay a few seconds on each axis until a "need more range" message appears. At the end, the coordinates of the sphere appear. There may also be an "insufficient coverage" message indicating that you should start again (perhaps after calibrating compass?).

Compass

Slowly turn the IMU 360° flat to calibrate. In principle the compass is "self-calibrating". It loses north if the accelerometer is out of adjustment. The compass display window often shows a message that data is missing. Calibrating the compass by simulating a heeling also works, I understand it is useful...

Alignment

When the boat is flat, press the "Boat is level" button. In principle the boat that appears should be correctly positioned in relation to the compass.

And the questions I still have (because I can't find answers or they are contradictory...):

- At the end of the accelerometer calibration, I get the message "calibration distance too small". What does it mean?

- After the compass calibration, I systematically get the message "insufficient coverage: xx need 14". Is this important? Do I have to start again?

- I also sometimes get the message "allowing bad fit" during the compass calibration. What does it mean?

- What does "sigmapoints flat, 2D fit only" mean?

- On the level calibration screen, the boat has disappeared. What does this mean? A screen :



- Once the calibration is complete, is it useful to lock the calibration (of the accelerometer, the compass, or both)?

Thank you for your clarification!