I was testing out my new quadcopter today and it was working well, not perfectly, but I was using only Stabilize and AltHold modes. Stabilize worked fine, and AltHold worked for a while...
It did a pretty good job of maintaining altitude for the first 15 minutes, then after swapping batteries it decided to shoot up about 150 feet out of range of my tx (it's a cheap tx with low range), I panicked and tried swapping to Stabilize mode and threw down the throttle, but it didn't respond and kept going up. Then all of a sudden, it seemingly disarms and plummets to the ground. Fortunately a tree broke it's fall or it woulda been toast. Only a broken arm to boot.
I came home and tested the failsafes to make sure they work, and they do. I used the ArduPilot failsafe guide for checking them and it swaps to Land mode once I turn off the tx. So I've no clue what happened once it went out of range.
That being said, this was a catastrophic failure. I will add the flight log here, I'm not sure what to analyze to get to the bottom of the problem. It notes a battery failure, and also a RC failure, and I've taken a look at the barometer data and nothing looks out of the ordinary.
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Here's a flight test from today. I am trying with this foam that I had (before considering purchase of the expensive foam Bill mentioned). It looks like there are still a lot of vibrations, and some of them go very low but I think that might be due to landing on the ground several times during the flight.
Overall it felt pretty good. I think I just need to do the AutoTune and AutoTrim features on it, especially AutoTune as it seems to have problems self correcting when wind hits it (it will tilt way too much to self correct, to the point of almost flipping over). I tried doing the AutoTune but it didn't seem to start when I flipped my ch7 in AltHold, odd.
Does it look better/worse?
flight_test_9-14-2015.png
2015-09-14 13-38-35 61.bin.log
Looking at the log in chronological order:
1. RC radio link went down
2. APM went into RC Failsafe - LAND mode (as per FS_THR_ENABLE = 3)
3. APM set throttle to 100% and climbed to 30m
4. Due to the high current draw, the battery voltage dropped below 10.5V
5. APM went into battery failsafe - LAND mode (as per FS_BATT_ENABLE =1). Continues landing.
6. APM regained RC radio link
8. As per RC controls, APM goes to STABILIZE mode, 0 throttle and falls rapidly until it hits the ground.
I note that you had disabled all the pre-arm checks - was there a particular reason for doing this?
I also noticed that there's no GPS data in your logs - did you have a GPS module connected?
I'm honestly not sure why the APM climbed when it went to LAND mode. Looking at the baro alt:
Hope this helps!
The real baro readings are correct, the copter thinks it's not moving upwards (Alt doesn't move), but it is, as you can see in the log. The vibrations are messing up the Z-axis navigation. Look at the vibrations, they get bad at the point the copter starts to rise. Land triggers, but the throttle is max just to keep stablizie, so it does't descend.
Vibration really messes up the whole thing.
Would having the FCB on top of the frame rather than nested inside have any effect on vibrations? I'm wondering if the wind from the props might be doing something to it (aside from not having proper foam of course).
Hey Stephen!
Thanks for the detailed report, that is incredibly helpful. I do have some questions that hopefully you might be able to answer.
When the RC radio link first went down, it looks like the altitude was still quite low (not sure of the difference between BarAlt and Alt, but either way they're both low). Does this mean that there may have been a power issue with my rx? I have been powering my rx via a PPM encoder straight to the RC port on the servo rail, could this be a problem? Since my tx didn't go out of batteries, and this should still be well within operating range, it strikes me as very odd that the failsafe would be triggered.
Yes, I disabled pre-arm checks because I don't have a GPS, I should probably limit it to only ignore the GPS rather than the entire pre-arm check (I think that's possible, right?). Could not having a GPS somehow be related to what happened?
As for the mystery of why it went up rather than down.... all I can hope is that it's some sort of noise or vibration issue effecting the barometer. With Bill's advice, I'll be sure to put some good foam between the Pixhawk and the frame. I may also consider putting it inside the frame rather than having it rest on the top, since pressure buildup from props could effect the baro's readings.
BarAlt = altitude according to barometer
Alt = Filtered altitude (after combining with other altitude sensors and putting through a software filter)
There may well have been a power issue with the Rx. Given that it came back on again, I'd lean towards reception issues. Definitely run some tests with the Rx and see if you narrow down the issue.
Any suggestions what sort of tests I should run?
Thanks :)
It looks like you have some bad vibrations and they go crazy just before you switch to land. Bad vibes will make the copter climb and switch to stabilize is the only option. You did that and the copter came down, good thing, then the log stops. It like you had complete power failure at that point.
Thanks for the explanation Bill. I was using bubble wrap as a dampener under the Pixhawk, but maybe that's not enough. I'm gonna try using some insulating foam used in doorways instead once my new arms come in.
I read here (https://www.youtube.com/watch?v=4TmSn9WBOag) changing the MPU filter frequency to 10Hz may help the vibration issue as well, is that worth a shot?
You need some proper foam. I use the foam for mounting gyros in helicopters. it's like this https://store.3drobotics.com/products/pixhawk-foam. You local hobby shop should have some. I think that is better than changing the filter (I would do that as a last resort). When you get flying again, just do a short flight and then verify your vibrations. I'm posting a new version of APM Planner and MP Beta should have it also which is a vibration display for Copter3.3 release (if using an earlier release you need to look at the logs)
I also notice that your throttle is at 0% when you switch the stabilize, that triggers to motors to cut as a safety feature. it's best to keep the throttle at about 40-50% when you switch modes as that is enough throttle to stabilize but should be not enough to go up, but enough to slow a descent.
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