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
Rigel: Do you have telemetry? Tlogs only contain data if you are using telemetry. Regardless, you should update to 2.6.
Nice Magneto image BTW :-D
I was noticing significant changes in my compass readings as a function of motor drive. I have a current sensor on my quad so it got me thinking that maybe there would be a nice relationship between current and the compass offsets. First thing was to run the compass calibration with live data (with motors off). I then placed the quad on the floor, held it down, and ran up the throttle. Rotated the quad 90 degrees and did the same thing, then another 90, then one more. After extracting motor current, heading and the three mag_x/y/z and dumping them into excel I plotted the result - it is shown here as "Uncorrected Data".
I then worked out how much mag_x, mag_y, and mag_z changed as a function of the current - it worked out to +2.28 units/amp for X, -1.36 for Y, and -0.4 for Z. I then modified compass.cpp to make the correction based on the current.
I then re-ran the test and got the following results:
The extent of the improvement varies, but I was running at about a 30 degree heading error at a heading of around 240 degrees, and after the correction was below 10 degrees. Seems to fly fine in stabilize mode, but too windy to head out to the field for a real flight. Maybe tomorrow,
Presumably the errors are also a function of how the quad is being manouevered (different motor currents in each motor), but this is a still a reasonable first order correction. I think that it would be fairly easy to collect the three parameters needed as a setup function - going through tlog to Excel was a bit painful.
Thanks Adam,
Yes I'm waiting for on-board compass calibration indeed. Anyway till today I never had yaw issues since I loaded 2.5.3 version. And now I see the problems of all persons that have written aboutt it.It was really awkward.
Thats why I tried to do with it something.
For now I'll try live data for calibration, even if these don't regards noise from ESCs. But better than nothing.
@palo sestak: No. But you can try to calibrate the compass without the tlog. That can be achieved by clicking "Yes" when asked, "Use live data or log. Yes for live data." Then take your copter and rotate it around all of its axis. That should be better than nothing that this point. As I am sure you have read in this thread, a new on-board compass calibration is coming soon.
Is there a way how to convert log files to tlog? Since I have no xbee modules I can't get tlogs from real flight, only store log files from flight. But when I want to calibrate compas there is only tlog option to load ;o( one can't load log file.
WHOA! 100 props?! That is epic! :-D You sound like the perfect beta tester... If you need help pulling from git and getting set up just shoot me a PM and I will try to help you out.
@Paul Emous: I think a release is slated in the next couple weeks, but its not officially planned quite yet. If you want to fly the latest code though, you can always pull it from git! :-D Just fly at your own risk; the bleeding edge of development can be risky, but the current git has been tested by the developers.
@Paul Emous: That isn't surprising. Each flight will have different magnetic field attributes and thus generate different offsets. That is why you should choose a tlog that is representative of your normal flight habits. When the onboard automatic offsets code gets released, you won't have to worry about all this. This is an interim solution.
I should have been more clear. You need to have wireless telemetry installed/enabled in order for the tlog method to work. The "t" in "tlog" stands for telemetry. If you do not have telemetry, that log will be empty and the calibration will not work properly. @Paul Emous: Is this the case with your setup?
Hi Paul,
I'm also having some similar problem on my Y6, yesterday I tried it with quite a bit of wind, and I had to keep pushing the Y6 forward, as the rear arm tended to fall.
Yaw eeps moving all the time -+ 10º/15º. I'll try to apply afernan's magnetic solution.
As for the rear arm falling back, I've had a quick look at the code in motors_Y6, and it seems to me that both rear arm motors are given less power. For left and right motors we get + (g.rc_2.pwm_out * 2 / 3), but for the rear motors we get - (g.rc_2.pwm_out * 4 / 3). Can someone show some light on this??
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