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Permalink Reply by Glenn M on June 19, 2012 at 9:38pm

I'm having big problems with my compass now. It will spin, and move eratically, which makes my copter unflyable, however when I do a CLI compass test, it shows that it is fine every time. Does anyone have any ideas?

http://diydrones.com/forum/topics/compass-spins-and-behaves-eratically

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Permalink Reply by Dustin on June 19, 2012 at 9:53pm

No ideas, but I had this problem in 5.5.  In my case, it happened during flight though, not in the mission planner.  I almost lost my copter --- it landed in the woods 1/4 mile from my take off point.

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Permalink Reply by Glenn M on June 19, 2012 at 10:22pm

Yeah this is really making me want to just ditch the whole APM system. I have had nothing but trouble with it, and wasted hundreds of hours to get essentially nowhere. I still don't have a platform that even flies, let alone is reliable enough to use. If I had known there would be this many issues at every step of the way I would have just ponied up for the Wookong M 6 months ago when I started with the APM thinking it would be a better platform.

I'm supposed to be travelling for 6 months with this tricopter in 3 weeks, but it's looking more and more likely that I'm just going to end up having to run a Multiwii board instead and forget about the APM, not that I've even had much time to get the multiwii tuned nicely after spending all of my time (and money) on this flakey piece of crap.

Here's a log from a quick (hand test) flight just now showing the compass running away. CLI compass test still works fine though. How is this even possible?

Attachments:

• 2012-06-20 17-13 36.log, 35 KB

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Permalink Reply by Gustav Kuhn on June 19, 2012 at 11:04pm

Sorry to see you have big problems.

APM1 or 2 ?

Did you assemble yourself, or buy ready made?

What airframe are you using?

Post some pictures of your 'copter, maybe it is something obvious to some of the more experienced folks here can pinpoint.

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Permalink Reply by Glenn M on June 19, 2012 at 11:08pm

It's an APM1 2560 on a homemade tricopter frame. I've pretty much followed best practices for connecting everything together and building. It has been somewhat working (eg the compass was working pretty much fine) for the last few months, but it's just now that the compass has started to spin.

What is really doing my head in, is that the CLI compass test shows that the heading holds exactly, and moves correctly corresponding to movements I make by hand, but in MP, and in flight (and flight logs) it is all over the place. I don't understand where this discrepancy comes from, or how to isolate and troubleshoot the problem.

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DeveloperPermalink Reply by Andreas M. Antonopoulos on June 19, 2012 at 11:34pm

Just speculation, but it might help:

The magnetic compass is affected *very strongly* by the electrical current flowing to the motors. The magnetic field created by a running motor (and the wires running to it) is many thousands of times stronger than Earth's magnetic field. So the magnetometer will behave very differently at 0% throttle, than it does at say 75% throttle. 

So if it works under CLI (presumably on the bench, motors off), but not in flight (with motors running), then the problem sounds like it is magnetic interference from the motors influencing the compass. You either have to move the APM further away from the motor wires, ESCs etc, shield it somehow, or calibrate the motor noise "out" of the compass sensor. I believe the last option is the one you can use. 

From what I understand, if you are experiencing problems with the calibration, it is best to calibrate with a flight log (not Live Calibration) that records the magnetic field of the motors as you fly under high throttle and various attitudes. This allows the calibration to filter out the motor noise. Another alternative I have seen is to *remove the props*, then power up to 50% say and calibrate with motors running, as you do a live calibration, moving around all axes.

Hope this helps

Perhaps someone with more experience with the magnetometer can chime in, because I could be completely wrong...

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Permalink Reply by Glenn M on June 19, 2012 at 11:39pm

I understand that, but my compass is spinning and behaving eratically even when I am connected only via USB with no lipo and no motors running. Even after a fresh CLI erase and firmware load and connecting straight into MP after that.

I believe you can only do a log calibration using a .tlog, and I don't have telemetry so don't have any tlogs I can use. I have just been doing the live calibration in MP, but have also tried to enter my declination manually (are these different things?)

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Permalink Reply by Gustav Kuhn on June 20, 2012 at 12:06am

Hi Glenn, I calibrated my quad using the live method in MP, and it worked just fine.

I had two flights in stab mode, and the compass stayed true.

All that I can think is that you have a faulty Mag.

Or a bad solder joint.

As I said, a few pictures could also help.

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Permalink Reply by Glenn M on June 19, 2012 at 11:58pm

To me it seems like somehow the problem is in the layer between the raw heading values that the compass is physically putting out, and where the APM interprets those values into yaw. I say this because the raw compass heading (CLI test) never wavers and moves correctly)

I'm guessing the declination and compass offsets sit in this layer, so perhaps something is corrupt or wacky in there, but how to find out what exactly and fix it...

Edit: just to test, I've taken my props off, connected the lipo while connected to MP vis USB, run the motors up to 50% and rotated the copter around all axes, and then used the .tlog generated to generate my mag offsets, but it hasn't helped the problem.

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Permalink Reply by Gustav Kuhn on June 20, 2012 at 12:23am

Ok, I understand now what you mean by the CLI stable, but the APM saying it changes.

This is quite weird, considering that it WAS a bug in the software, everybody would have the same problem.

Or maybe this is only present in Y configuration, any other Y users have this ?

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Permalink Reply by Glenn M on June 20, 2012 at 12:32am

Yeah I am stumped at the moment really. Earlier this afternoon I had a successful flight (testing my battery duration) in the same configuration, but then this started happening.

I have taken some pictures but I doubt they will be particularly telling of anything.

I don't think the mag itself is bad, but something in the pathway of using it is.

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Permalink Reply by Dave C on June 20, 2012 at 1:23am

Hi Glenn,

Have you tried to fly it with the compass disabled? How does the yaw behave then?

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