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Comment by hogu | Hacker News Reader

well you can do a single inverted pendulum pretty easily with a PID controller, it's the double inverted pendulum that is hard. But anything on a quadrocopter is really hard.

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A free-flying small quadcopter is incredibly difficult as on-board sensors generally consist of noisy, drift-prone MEMS gyros and accelerometers, small and interference-sensitive magnetometers, bad civilian GPS, and barometers that are prone to environmental fluctuation. In addition, a free-flying quadcopter requires either complete autonomy or an RF link with some range or reliability, which generally requires a good amount of power.

But these aren't free-flying quadcopters; the researchers used a very high-end motion capture rig and a PC in close proximity, making everything a lot less difficult - with motion capture, positional accuracy is quite easy and sensor noise is easy to correct for, and with enough money someone else has already solved the latency issues in motion-capture for you.

I certainly agree this video is impressive but I also believe lab-only motion capture-based demos skew the hobbyist and public vision of what small quadcopters can do.

Did you see the show from Cannes last year?

http://www.youtube.com/watch?v=cseTX_rW3uM&t=50

I don't know how much of that was motion-capture, it seems like a quite difficult environment to have done that...

Read the video description: "Thanks to Vicon for the tracking system" - the same system used in this juggling video, as far as I know.

I think all of the KMel Robotics projects (at least all of the ones I've seen on video, like the Bond theme) have used the same basic design, which involves a Vicon system.

I wonder how they get it to work in a dark room with some _very_ bright moving lights? IR with really good visible light filters maybe?

I think so. Like most motion-capture systems, Vicon systems use high-frequency strobe lights to illuminate markers being tracked. IR strobing is definitely an option for these systems. I, too, am quite impressed that they worked in that situation, as the range on IR strobes is usually much worse than NIR and especially red ones. I suppose with the way the lighting is set up the cameras could have been in relatively close proximity behind the stage.

All the issues you mentioned ring a bell. Is there any progress in the hobbyist area on developing inline smoothing filters and adaptive kalman filters to clean the signals and compensate for drifts using all the instrumentation readings available?

I think Paparazzi probably have the most interesting adaptive control algorithms and sensor fusion code, although their community is perhaps smaller than other open-source flight control solutions: http://paparazzi.enac.fr/wiki/Main_Page . I guess they're not strictly hobbyist since as a university project, various contributors also publish several papers a year.

Pretty much everyone in the open-source arena (Ardupilot, OpenPilot, Paparazzi, PARIS/WiiCopter) experiments with various filtering approaches and sensor fusion strategies.

One interesting challenge is that more sensors increases cost and hobbyists are often cash-strapped (especially after crashing a lot!), so sensor fusion isn't as heavily pursued as adaptive control and basic filtering strategies for 6-axis input.

Ah, thanks for that.

I had a lot to do with the question of "where the hell are we & how the hell do we get to where we should be" in real time and to fairly tight tolerances (at scale) - sadly/luckily at the time it was left to the pilot to follow the blinking light rather than deal with the mechanics of moving control surfaces directly.

The main thrust of surveys like [1] though was enhanced final product accuracy through sensor fusion strategies, using calibration flights to tune filters to remove induced magnetic field effects from aircraft headings, using lidar, microwave, accelerometers, differential ground GPS, temperature, pressure, and humidity to get good estimates for actual height clearances, estimates for the mass of air column, etc, etc, blah, blah, old guy stuff.

That's a single survey of many, performed over decades, and it was interesting in the evolution of processing to watch it move from post processing to in line processing. Which of course prompted the question.

The paparazzi (seriously france? this is for drone RC craft to snap pics of celebrity boobs?) project pages look interesting, so I'll dive into those.

[1] Fiji, 1997 "A combination of helicopter and fixed wing aircraft was used to fly the survey with a total of 160, 000 line kilometers of data acquistion (~80,000km per aircraft type)"

http://webcache.googleusercontent.com/search?q=cache:http://...

Occasionally, quadcopters surprise people and seemingly inexplicably do hard things without trying:

http://diydrones.com/profiles/blogs/why-doesnt-the-ball-fall...

You can only control a inverted pendulum using a PID if it's near one of its equilibrium points. You need a linearized model of the dynamics and this is usually done using the small angle aproximation for the sine. I don't think they are using a PID controller for the pendulum in that video.

Oh, you're right. I was thinking of the double inverted pendulum.

actually you can control a single inverted pendulum with an on/off controller that is fast enough...

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