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!

[seandepagnier]

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?

It means you do not have sufficient data to perform a calibration update. There must appear yellow points on the sphere from each of the 6 orientations for this to work. Many sensors come pre-calibrated so this is only a slight correction but some have significant biases and it is essential to calibrate them out.

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

Not start again, but continue to rotate. Normally if the boat makes a circle it will get sufficient coverage so I am not sure what you are doing.

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

Mostly you can ignore it, but it means the calibration is sub optimal but did update.

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

This means that the calibration points all land on a plane meaning technically only 2 dimensions of the compass bias can be computed, and the remaining one is unknown so compass errors may exist with higher levels of pitch and roll. If you can perform a 3d calibration, a flat calibration (boat in a circle) will update just the dimensions that it can.

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

Did you ever see the boat?

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

Thank you for your clarification!

It is not supposed to be useful but maybe it is. It would be useful if you know there is compass interference when you are not using the autopilot.

[gponcon]

Hello Sean, thank you very much for these answers. For once the sea is flat this morning and I just recalibrated...

Accelerometer: I still have a "calibration distance too small" with one or two cyan dots per side and yellow and red dots on the current side. I stayed 1 minute per side... maybe it is still not enough?

To calibrate the compass, I make a complete turn and then another complete turn upside down (to untangle the electric wires...). At anchor, the compass works perfectly.

Locking: I noticed that the screwdriver used to immobilize the RPI is magnetic... so I lock the calibration when I use it.

Update :

I forgot: there was a sailboat on the visual, which has disappeared for a week or two.

We set sail for 2 or 3 days with our new calibration. To be continued.

[seandepagnier]

Sorry, the "calibration distance too small" actually just means that the new calibration is the same as the old calibration and there is no significant update to apply. So it should be good.

Perhaps this message should be changed.

[gponcon]

Thank you again for all this information and for the clarification concerning "calibration distance too small".

Since I have conscientiously calibrated my IMU everything is much better, I have just sailed several days in many different weather conditions and pypilot is behaving very well. Hence the importance of this step to take advantage of the power of pypilot!

The only situations where the pilot still has a little difficulty are in tight upwind conditions (maybe I haven't found the right settings yet, in this case locking the helm is more efficient) and in areas of strong eddies (which seems normal, it requires a very fast reaction time with violent strokes of the steering wheel to maintain the course).

With the right settings, the boat's trajectory is straight.

[seandepagnier]

If locking the helm is more efficient you could get the same effect with zero gains. So probably you just need lower gains to still give some feedback (more efficient sailing)