Trust in Software

I had some fun with the Quad this evening.  I’ve been flying it in my father-in-law garden (the only fairly large private space that I have access to) testing the new 2 axis gimbal and the fake SJ4000 that I was given by Rob.  The first flight went really well, but I wasn’t doing anything more advanced than hovering in loiter mode and giving it a bit of yaw now and then.  Everything was fine (except for the stupid hard landing)

I went out about an hour later to have another go.  Immediately I’d noticed that the wind had picked up, but if I kept fairly low, this shouldn’t be a problem, and certainly not outside the envelope.  I armed in stabilise mode and took off in Loiter.  Everything looked really sweet so I let the altitude climb a little to get a nice aerial of the garden. I probably went up to about 15m (I’ve not checked the logs yet) and did two slow spins in yaw.  It was obvious the the wind was pushing the quad around quite a lot, but the GPS seems solid and Loiter was holding position nicely. I decided to come down.

Nothing happened.

My first thought was that the wind was quite blustery and must be keeping the quad aloft, so I dropped the throttle even more, in fact, I had it right down, but the quad still hung there, perhaps coming down very slowly, but certainly not listening to me. Adrenaline was pumping and figuring the best thing to do now was to try and remove any autonomous behaviour from the FC, I went back to stabilise mode.

Despite the heart thumping adrenaline, I had the presence of mind to put the throttle back up to mid stick, so that the quad wouldn’t just fall out of the sky.  Nothing. The quad still hung there.  It was still coming down very slowly and was probably about 10m when dramatically, and 5 or so seconds after commanding it, stabilise kicked in.

What happened next is never going to be a coherent story, I was panicked and outside my range of experiences with quad flying. Despite having about 50% throttle, the quad came down rapidly, not quite free fall or autorotation, but far faster than I’d hoped for.  It hit the edge branches of a small tree at about 5m which knocked it off level and it hit the ground hard but still upright, breaking both front leg extensions and burying the gopro clone lens down in the lawn.  The damage looked not much more than cosmetic so I started breathing again and pushed the throttle down and to the left to disarm, as the props were still spinning.

But they didn’t stop. I looked down at the TX in my hands and was horrified to see it dead.  No power at all.  I cycled the power button, but nothing, still dead.  I looked at the damaged quad on the lawn. I couldn’t approach it like this to pull the battery without disarming it,  and with a dead TX, it was a very dangerous and live machine.

As I watched it spinning at fairly low revs and wondering how long it would take for the batteries to die, or it to disarm it’s self, everything suddenly got much, much worse.  Without any warning, the quad throttled up to full power, and shot upwards, hitting the same tree as it had done on the way down.  It lurched wildly in the air, and I watched it, with the dead TX still in my hand shoot up and away from me at full power into the sky.  Every story, every post, every video I’d seen about fly-aways came into my head at that moment, and I knew not only had I lost it, but there was a significant chance it might damage some property on the way down.

As I think back to it now (this all happened about 2 hours ago) I can almost see the branching control structures in the code and the variables updating from the Accelerometers and gyros as the Flight Controller followed logic and paths that I had no idea about.

It dawned on me slowly as I watched the rapid gain in altitude back up into the strong winds, and the damaged legs swinging around in the down-draft from he props, that all may not be lost. The Quad was under it’s own control, in Fail-safe, attempting to execute a “return to launch”.  The first element of which is a rapid ascent to 15m in order to get clear of any danger. Still powerless to control it, with no life magically returned to the TX, I crossed fingers as the software brought the quad back to above it’s first received GPS coordinates, and a slow stable descent back to Terra Firma. Ir disarmed within a few seconds of coming to rest on the two remaining legs and the camera gimbal.  At this point, I knew it was over.

Flying multirotors is about gaining experience and tonight was a valuable lesson.  Not only about getting some new, better nicads in my TX but in how to deal with emergencies and the partnership between the human operator and the onboard software.


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