Almost got killed in the office today. Log file attached..

I have a large octo With powerful motors. It's been in the air many times.

I did some work on it today, changing the camera gimbal. I also installed a New 5,8ghz video tx and a trigger Circuit for operating the shutter on the camera. The shutter and video tx were both Connected to the camera. Shutter was Connected to ch6 on the receiver.

I decided to Power it up in the office to adjust the camera shutter channel and test that everything worked. The telemetry system(Frsky Taranis) warned about low RSSI. Since the only new Connection I had made to receiver/apm was the shutter cable, I removed the servocable from the receiver ch6. The telemetry system stopped warning about low RSSI. So, just to check again, I reconnected the servo cable to ch6. I know I should not do this kind of stuff on a live machine With propellers, and luckily I was taking precautions by keeping out of Reach of propellers. Then all hell broke loose...

8 Heavy lift motors With large APC propellers, capable of lifting a total of 20kg went into full throttle. I just let it go and took shelter, while the machine was doing its thing to the ceiling and Office Furnitures, before shutting Down. Most of the propellers and one motor destroyed. Glad to be alive With all my body parts intact. I've been working on multirotors since "the beginning", and had my share of propeller cuts, but thats a long time ago..  It's obvious I needed a reminder again... :)

So... what happened? I'm not concluding With anything, this may not be related to the APM at all. I suspect that the camera shutter in some way made the receiver bring channels high/low. Logfile Attached. Anyone care to do some analyzing?

2014-10-28 17-59-05 6.log

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  • I would be willing to bet that the cable cause a minor brownout and a "reboot" (or something rather similar) in flight. This has happened to me a few times with my quad, only this involved the telemetry radio. The telem cable had a tiny cut in it that cause it to very randomly disconnect under vibration, and it just so happened that under the perfect conditions on a particular takeoff, it disconnected and caused the APM to "reboot". All of the outputs to the ESC's were either kept constant, or the stabilization was still working but the RC input wasn't working, and the quad slowly and steadily drifted into a tree.

    I was able to reproduce these results in my lab, and this is what I found. When the disconnect hit, while the APM was armed and the motors were spinning at quarter throttle, the TX and RX LEDs on the APM would stay constantly on, and the red status light stayed on and the blue one flashed. The motors stayed locked on, and unchanged over the space of a few minutes. The RC receiver still registered as connected, and never lost power, but the MAV packets stopped permanently, even after being given a few minutes in this state, and I never regained control. Log files also stopped being recorded when the disconnect hit. I was never able to find the source of the problem, and after a week of working on it, I replaced the cable and have yet to see this behavior again, over 300 flights later. Morale of the story: connecting or disconnecting cables with the APM powered can cause random and unpredictable behavior. 

    My advice to everyone: Don't connect the main battery inside if you have props on. If you need to power your APM and anything attached to it at the bench, instead of using the main battery and the ESC's to power it, cut a USB cable and solder headers onto the power and ground. Plug it into your APM, and you instantly have a 5v power supply. 

    • BEWARE! Doing this with a Pixhawk will blow your 3.3V regulator!

  • Hi Kristian,

    This is a shot to dark but I try,

    Do you have one drive battery or two batteries in serial connection? There is a change to mix battery earths by taking for instance 3S from another battery (3S+3S total) for gimbal. That makes smoke for a short time and that current peak could cause anything.

  • http://youtu.be/T7F2bsd2bIk?t=7m22s

  • Thank's Kristian to share your bad experience, I never had this problem but I'm not an exception, you alert me that this accidents are more common than I think, I'm going to be more carefull now.

  • Moderator

    Having received my own 'drone tattoos' (more than one) I never do bench work with the props on.   Just too much risk.  I've built a couple of large octo's with 18 inch props and I swear they're powerful enough to sever a finger if not a part of your hand.  

    This is just a guess, but I think your receiver lost power, forcing the APM into whatever failsafe you had setup.   I'm guessing it was trying to RTH inside your office.    

    As we begin to work on larger and more sophisticated drones,  we need to start treating them with the same respect as we would firearms.  The difference is that it is easy to see when a drone is 'loaded' - it's loaded when the props are on.

    • Another quick and cheap positive mechanical safety that was shown hereabouts a while back (so now lost in the misty swamps of this feeble forum software, which was designed to handle complexities such as chatting within a two-room schoolhouse) involves a length of cord with loops tied in at appropriate intervals. Slip the loops over the props and they're muzzled and totally safe. Way quicker than removing/replacing props and is also nice to leave on at the field until the last moment (so for example when the mains are connected and you're leaning over the aircraft while pushing the arming button you're still totally safe).

      • Moderator

        That would work too, except when you want to run the motors on the bench.  

        • LOL, right, you could thus test your smoke alarms! No, it's just a safety...

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