Crash test

Well I feel better now. I'm always a little nervous to fly a new ship until the first crash. You never know how she'll deal with adversity. 

I was flying FPV in loiter at ~15m when it suddenly pulled hard right and back making a nice arc into the ground at about 20kph according to the telemetry log. The current jumped from the ~30A normal to over 60A in the few seconds of the death plunge. When I got there  the motors were spinning and wouldn't react to transmitter commands including disarm. All the props were broken of course.

Looked through the logs a bit, but I can't tell what happened. If anybody cares to take a glance and give me some insight, I would be very grateful. I don't mind when things go terrible if I can at least understand why it happened. 

The damage seems to be limited to a couple of cracked arms and a few other frame bits. The GoPro was fine thanks to the waterproof housing. Not even a broken antenna on the 3dr radio or video Tx! I'm taking it all apart tonight and checking the electronics, they got covered in snow, but no physical damage that I can see so far. I'll upload some pics once Iphoto stops crashing every time I try to get them off my camera. There must be weird solar weather or something going on today, I've never had a problem getting pics off of my camera before, really strange... 

I'll have to upload my tlogs from the windows machine, just a minute.

: )

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  • I have everything disassembled at this point. I'm going to just rebuild into MkII with some fundamental layout changes.

    I will definitely do more testing in the future. I have been a fool for not using this excellent logging and tuning capability. Having flown RC for so long I think that I tended to rely on in flight tuning methods almost exlusively. With this telemetry stream and a tuning stand I could have really eliminated a lot of potential issues, it won't eliminate crashes, but diagnosing fault could be a lot easier if there weren't so many possible causes on the list. 

  • @ Randy correct me if I am wrong but I would think it is moving the nav way point in the direction of command related to the magnetic heading. The commands during FPV to fly south caused the way point to move west from the compass error. This type of error is often seen as a toilet bowl effect during loiter if the declination is off. 

    Joshua the only way to see if the instillation is affecting the magnetometer would be to do some tests. Run one motor at a time to full throttle in each of the cardinal points an see if you get a swing in the compass heading. This is something I will do on my new build since everything is so tight in the avionics bay.

  • @Randy, What does sticks input do when vehicle is flying in Loiter mode? During stabilize mode it sends pitch up/down roll left/right command. During loiter mode this inputs are mapped with position, right? How those inputs are mapped with latitude and longitude commands?

  • I just checked mp hud, bearing stays rock solid even with the APM tilted to extreme roll. Of course this is out of the airframe now just held in my hand with USB connected.

  • 3692597487?profile=original

    I'm using the uBlox LEA-6 and the auto-learn tune.

    In this build all the ESCs and their power distribution were ~35mm above the APM with the mag on the daughter board, I have ferrite on everything, but I worry that messes with the poor mag even more.

    I'm rebuilding in a new configuration with everything spread out much further.

    I get that nav reacted to correct it's position in a too aggressive way, that's probably my fault for flying in loiter without tuning the nav PIDs enough. This machine is fairly powerful (4s/770kv/12x4.5 prop). 

    Why did it go against my stick inputs though? I don't understand why it thought the loiter point was somewhere else?

    Here's the ESC layer which sits above the flight control layer-

    3692597812?profile=original3692597837?profile=originalBottom side

  • Developer

    Oops..little typo above.

    5. AC dev team will make loiter less aggressive when far from the loiter point.  Really the response needs to be non-linear....when you're close in it needs to try to blance aggressively but we need to back off that aggressiveness a bit when you're far from the target.

  • Developer


        Yes, the MP always shows a + configuration even if your copter is in the x configuration.

        I've been thinking about this a bit more and I think we have a combination of things we need to do to solve this:

    1. if you're using the mediatek you should reduce your loiter rate P to 2.5 (unless you've upgraded to the mediatek 1.9 firmware in which case a higher number might be ok).  Because of the 1sec lag on the mediatek and the lower accuracy you can end up with an unstable loiter if loiter rate P is much above that.
    2. leave your compass in auto-learn instead of trying to calibrate it yourself using the mission planner.  I think the inflight method is just as good as the mission planner method (maybe better).  Also please try checking the MP HUD what happens to the compass heading when you lean your copter over 45 degrees.  Being off by 10~20 deg is not big deal but if it's off by 90 then the compass offsets are way off.
    3. Check the motors in a stable hover and try and figure out which motors are tilted.
    4. try and reduce influence of the motors on your compass
      • move APM away from the wires, etc. use ferrit cores etc
      • AC dev team will include compensation for motor throttle vs compass bearing in 2.9.1
    5. AC dev team will make loiter less aggressive when far from the loiter point.  Really the response needs to be non-linear....when you're close in it needs to try to blance aggressively but we need to back off that aggressiveness a bit.

    All this together has surely got to solve the issue completely.

  • Nice. The ferrites only reduce ac type noise. The high amp DC lines in close proximity to the magnetometer could induce an error related to the current flow. But I think the root cause is related to the compass error.

  • Emile, disregard my above comment about the video not being aligned with flight log, starting at :43 it's a continues clip with no edits until the touchdown , so yes, even with the .5 sec lag it's obvious that actual heading was almost 90deg counterclock to what the nav_bearing graph shows in last part of the flight.

    Also, it's not shown in the above photos, but I had a small 10A v-reg giving 5v into the minimosd on the video side, it was fed by the 3s battery powering the vid- Tx, and was mounted between the  Tx and the minim board. One more source of noise in the picture.

  • 3692597124?profile=original3692597219?profile=original

    I put ferrite everywhere, some are just token attempts as I only have a couple of wraps on them. My intention was to remake the short cables with a reasonable number of wraps on the next version, just wanted to get everything laid out. It's tight especially when the upper ESC plate is mounted. I really need an external mag(early APM 2 adopter penalty). 

    I looked at the "magfield" log and see a change during the crash event, but I assume that is just because the craft is moving. If the current jump to 60A caused interference that makes sense, but why did the auto pilot ask for this dramatic increase in a direction opposite my inputs? I assume that in loiter my stick inputs are supposed to be constantly changing the desired loiter position.

    If my GPS lag is only .5sec then the nav bearing graph is definitely off from what was actual. Am I correct that the icon in the MP flight data map is + even if config is set to x?

    The video has been truncated in a few spots so it will not match up with the flight log in real time.

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