I just finished a long debugging session with Andrew Tridgell and he has discovered the cause of my yaw (and attitude) issues. In case you don't know Andrew Tridgell, he is the genius developer who made the the latest release fly so well with his DCM improvements.
I think the majority of people out there are experiencing this issue can fix the problem very easily.
"So, what is it?!?!"
It turns out that the automatic compass calibration code on the APM does not work so well. I suspect it is working even more poorly under 2.5 because of the improvements to the DCM, but that correlation has not been confirmed. Like most of the users out there, I have never given more than a moments thought to compass offsets because they have been set automatically for as long as I have been a user, but it turns out they have an enormous impact on the performance of one's copter.
First, what are compass offsets? The offsets are three values that account for the difference in the magnetic field of the earth and the magnetic field that your copter experiences due to interference created by the ESCs, motors, etc. So, the magnetic interference can't possible effect heading that much, right? Wrong. If you don't believe me take a look at this graph:
You can see that towards the beginning of the flight when I am not doing much, the red and green lines are not that far off. As the flight continues and I start using more throttle, they are way off. At about 5:20:20 my heading is off by about 250 degrees. That is massive!!!! So why does it change throughout the flight? The more throttle that is applied the more current is flowing through one's electronics which increases the magnetic interference. Normally this field offset is accounted for, but recently I suspect that some users have bad offsets like I did. My offsets were essentially 1,1,1 when they should have been -180,3,52. Check out the next graph which shows the effect throttle has on the magnetic field:
This is a log from another user, Marco Robustini. The green line shows the throttle and the red line shows the magnetic field. You can see that as the throttle increases, the magnetic field more than doubles.
So how do you fix it? There is a new feature in the latest Mission Planner that allows you to use a telemetry log to determine the offsets OR you can calibrate your compass in real time by holding and spinning your copter 360 degrees. It is highly recommended that you use the telemetry log method. The reason is that when you are holding your copter and spinning it, you are not accounting for the amount of mag interference from the motors (because they are hopefully not running :-D ). Follow these steps:
With Telemetry Log (recommended)
1. Connect to your copter via telemetry.
2. Take off and fly around for a few minutes like you would normally fly. At this point we are collecting data about the magnetic fields during flight.
3. Land and disarm.
4. Download the tlog from that flight. (Instructions here: http://code.google.com/p/arducopter/wiki/AC2_Logs)
5. Connect to your board
6. Goto the Configuration tab
7. Click on the Setup tab
8. Click on the Hardware tab in the popup
9. Click the "Calibrate" button
10. Click "No" when asked if you want to use live data.
11. Browse to the fresh tlog
12. Your offsets are now saved
Without Telemetry Log
1. Follow the above steps 1 through 9
2. Click "Yes" when asked to use Live Data
3. Rotate the copter 360 degrees
The calibration utility is leagues ahead of the automatic calibration on the board that most of us are relying on. Many thanks to Andrew Tridgell, Michael Oborne, and all the devs who had a hand in the compass calibration utility in the planner. There is some serious brain power behind these features we take for granted.
Andrew Tridgell is working on improving the automatic calibration routines that reside on the board based on the updated (and original) algorithms by Bill Premerlani, but for now the Mission Planner calibration is everyone's best bet for improved flight performance.
If you have any questions just leave a comment.
Thanks,
Adam
Comments
@John Stuart, No problem. I hope it helps you. If it doesn't though, please post logs back here for me to look at. Thanks!
@Jan Schermer, That is interesting. The Mission Planner calibration automatically disables the compass learning as far as I understand it. There is a parameter called COMPASS_LEARN. Make sure that is set to 0 on the Configuration tab of the Mission Planner.
I tried now the new code, i think there's some bug. My yaw is pendular and not stable. I put my tlog in the dev list.
COMPASS_LEARN in the parameters list, 1 is enable, 0 is disable.
Isn't there a parameter called "Compass_Learn" ?
Where do you disable in flight calibration?
Marco, I'm talking about the old (current) behaviour - in that case the offset calibration is performed whenever the copter is armed, and this is the default behaviour. If I want to keep the learned offsets I'd have to recompile with compass learning disabled. Isn't that right? So even if we do calibration from tlogs on the ground, it will continue learning offsets when we fly - I guess it's pointless then.
Andrew Tridgell has release on the GIT the new automatic calibration routines that reside on the board based on the updated (and original) algorithms by Bill Premerlani.
It's a preview, is to be tested.
Jan, for the "learning" flight you do with the automatic calibration enabled, then land, load the tlog and the planner sets the correct offset, then you can take off but first disabling the automatic calibration in flight or the procedure would be useless (i think).
Hmmm, this explains some problems I have, didn't think of looking at the compass cal...... nice work, thxs
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