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Compass calibration - bad fit, using previous
#1
In the Pypilot calibration app I've performed the following steps:

- IMU heel pitch calibration: OK
- Accelerometer: Turned the IMU on 6 axes until green dots came on every side of the sphere and then locked the calibration
- Compass: Sailed out and turned the ship a few sharp rounds in a rolly/wavy condition, but then I got the following notification:

Bad fit, using previous setting. After that I locked the calibration for the moment, but I could see on the sphere that is animated in the calibration tool that the green dot or lines that then are projected, would not fit the entire circle outline. At one corner of the sphere it did not fully "fill" the sphere. So it looked more like an oval on one side.

Don't know what the mistake is, could this be because deviation and that I have a steel boat?
Any way, I made 3 circles while calibration was not locked and I even cleared the previous setting. Before I can "rely" on the autopilot I guess I need to first finish proper calibration, but I don't know how to do it correctly, or maybe it is already correct?

Looking forward to any opinion or knowledge on the matter.
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#2
did the calibration update at all? Can you post a screenshot of the calibration plot with the sphere? It may be you have too high of magnetic distortions and should relocate the sensors.
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#3
I'm not sure, it did complete a calibration I think but then said "bad fit" and I think reverted to some old values. I couldn't make a screenshot because the plot didn't remember the previous calibration values. I can however share the current one, but I don't know if this would have any value:

[Image: 2020-07-27-18-52-40-Aye-Aye-Systems-2-op...Viewer.png]

[Image: 2020-07-27-18-54-06-Aye-Aye-Systems-2-op...Viewer.png]

[Image: 2020-07-27-18-55-10-Aye-Aye-Systems-2-op...Viewer.png]
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#4
ok, this is either not a good fit, or only a 2d fit (will get errors when heeling) It's impossible to tell without more data points (rotating the boat)

I suggest you try again mounted in the boat and make circles. You will know the calibration is correct from the plot. The next version of pypilot has a much faster calibration and improved display.
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#5
Omg, I am so ashamed. I just realized there was an external GPS antenna mounted (MAGNETICALLY) on the case. The magnet seems very powerful, when I hold it even 50 cm from the casing, it will be visible on the 3d plot. 

I have now (at home) tried some calibration just by moving the prototype casing around it's own axis and heeling and pitching a bit, now I have these results:

[Image: 2020-07-28-12-48-19-Start.png]

Don't know what this notification means "fit found field outside of normal earth". Is this good?

I will start calibration over again on the boat for more real results.

I hope this was the only cause, any power lines have been fitted with the rule of 2cm per amp.
I am however looking very much forward to the next pypilot version, curious about the improved display!

I'll post the results of todays calibration and tests soon.
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#6
Well, tryout of today results:

- removed the GPS magnet
- unchecked compass calibration lock
- sailed out the harbor and did 2 rounds starboard and 2 rounds portside
- still bad fit Sad

But, the autopilot seems to hold the compass course OK (in some regions) but not all.
Sometimes the compass gives a good reading, and the heading in OpenCPN points the ship in the right direction, but in some directions it does not.
I think I'll try another case and / or another IMU.
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#7
The gps magnet is definitely a problem, and the screenshot you posted has garbage results.

You should be able to verify calibration works by slowly and carefully rotating the sensors in 360 until a fix is found. You should see a circle of dots on the screen. You can even do this turning the sensors and pi in your hand without turning the boat to see how it works, but once it does, you will want to calibrate by turning the boat when the sensors are fixed to it.
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#8
(2020-07-30, 01:48 PM)seandepagnier Wrote: The gps magnet is definitely a problem, and the screenshot you posted has garbage results.

You should be able to verify calibration works by slowly and carefully rotating the sensors in 360 until a fix is found.  You should see a circle of dots on the screen.  You can even do this turning the sensors and pi in your hand without turning the boat to see how it works, but once it does, you will want to calibrate  by turning the boat when the sensors are fixed to it.

I've removed the GPS and went on the water again to recalibrate. It did visualize the yellow dots in a circle, gave a fix but it was the same as previous it said. Maybe I should start all calibration (also accelerometer) all over again, I didn't do the accelerometer again after removing the magnet. I'll just clear everything and try it again. Smile
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#9
well if it makes a neat circle and fixes to it, then how does the displayed compass heading compare to reality?
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#10
I tried another IMU today from another brand and on another Pi. Jumper wires and enough distance to the Pi, used no enclosure and tried recalibrating everything, but still bad fit. No interference and the power supply was well away from the Pi. I tried the calibration at home still so I don't know if it will have a better fit when performing real rounds on a boat. When comparing the compass readings to reality it did move in the right direction, but deviated between 20-50 degrees most times.

I also downloaded a compass app on my android phone to compare the test results. Also tested this by looking at the readings as I moved my phone closer to the metal enclosure, and indeed it interfered clearly. Readings scrambled a bit as it seemed to be going through a field, and when closed to the enclosure it returned to normal "isch". So, I'll get rid of the enclosure and revert to a PVC enclosure.

The "bad fit, using same as previous" notification is still something I want to understand more. I know more and better readings can result in 2d and 3d fix with more movement. So my question is, can this be accomplished by calibrating at home just by turning the IMU around it's own axis, or do you really need more range (on a boat).
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