Saturday, July 9th, 2011 @ 18:53 | Ballcopter, Project Build Reports

Hello everyone, I am pleased to introduce Ballcopter.

It doesn’t work.

But it sure is cute, though. Look at how cute it is!

Ballcopter was inspired by a recent Japanese experimental ball shaped drone which made the rounds on the Internets about a month and some ago (though it appears to have been in development for a year or more). Here’s some more test videos:

Needless to say, I was instantly enamored with the spherical design. It’s so much simpler than a multirotor machine (like A Certain Emergency Quadrotor) and appears to be even more agile. And being Japanese, infinitely cuter. Having no details about how it was built besides just staring at videos, I sort of mentally reverse engineered it and designed one, but did not go about actually building it.

…until two weeks ago.

visiting the other coast

I was extended an invitation to visit Makani Power, an airborne wind turbine (AWT) research and development company based in Alameda, California, by MIT alums affiliated with the company. Lots of MIT graduates (and recently, MITERS alumni) end up in the San Francisco Bay area for some reason, and I found the environment at their shop/facility very much reminiscent of what I’m used to here. While I’m not someone who is enamored with wind power, I did take the opportunity to talk to the engineers and learn a bit more about aerodynamics and wing design, and also tried my hand at laying up carbon fiber.

Those two things can only result in more desire to build flying objects from me, so I’m not sure if the outcome can be considered positive or negative.

But I also built a ballcopter. I spent a total of 6 days at Makani, and while not discovering that carbon fiber fabric actually does deform and change shape (I had wondered how on earth smooth CF surfaces were made before), or looking at how composite airfoils were manufactured, I whipped up a design for Ballcopter, ordered parts (and had them overnighted to the facility), and used the shop to cut out the frame and finish it up. The parts cost was  about $150 including the whole overnighting thing, and the total time of build from start to finish was only about 60 hours.

And it kind of shows….

This is a picture of the first few hours of design. Again, there is absolutely no science to this at all – it’s a crude visual pirating of the vehicle shown in the test videos – I just sat down and started CADing. The frame was designed to be made using foamcore (stiff polystyrene foam overlaid with heavy paper), also known as posterboard, foamboard, etc..

I’ve added the control flaps here. The way I guessed that the JSDF drone worked was using the upper set of flaps to control direction (tilt and movement in XY, or I guess in this case XZ) and the bottom set as a rudder (spinning about the polar axis). Opposite diagonals of the upper flap set move with eachother, and all the rudder flaps displace the same relative angle. I was intending to just use a stiff tape as the hinge, which seems to work out for most small airborne implements. The servos are 9 gram miniservos, and the propeller mockup is standing in for a 10 x 4.7 inch prop.

The design progresses a little more with the addition of the… Tropic of Capricorn? The wide band adds structural rigidity to the outer shell and also acts as a crude duct to funnel air over the rudder flaps.  I have no clue what they actually do, but J-ballcopter had it too!

Because the timeframe was so short, I pretty much just went downstairs and heaved foamcore on the laser cutter and went for it.

Wait, foamcore on a laser cutter? Now, MIT’s rules have always said that cutting foamcore on a laser is an instant death sentence followed by permabanning from everything ever. You won’t even be allowed to use the shop in Hell, even though it’s already on fire and people are already dying around you. Foam tends to melt and burn, then drip and burn more, until a serious fire develops. I was not entirely convinced that the rules were founded on technical barriers, just peoples’ stupidity, so I elected to experiment with foamcore cutting settings on a foreign laser cutter.

And you know what? Like I suspected, it worked just fine. The key is to use many passes of a low power cut with very high head speeds so the laser never parks long enough on one spot to burn the foam. I ended up finding that 4 passes of 20% power (on a 150W system) and “30%” speed made for a very clean cut in 5.5mm foamcore, with the last pass just barely breaking through the paper backing on the far side, resulting in a good ‘puzzlebot’ sheet part. These settings are for a Universal Laser ILS1275, so I don’t know how that translates to Epilog or any other manufacturers.

Disclaimer: if you’re an MIT student, DON’T EVER CUT FOAMCORE ON THE LASER CUTTER!

Instant ballcopter. Just add hot glue! Notice that one side panel has been left off so I can actually service the thing.

And the parts arrive. I got all of these from, which seems to be a U.S. variant of Hobbyking (you know, the one that isn’t Hobbyking’s U.S. office). I used to see them spammed all over the place, but hey, they’re legit. The lineup includes a shady 6 channel 2.4ghz radio (functionally identical to the 6 channel HK radio I bought for the robots last year), eight 9 gram miniservos (because they’re like 2 bucks), a 28mm outrunner with 25A controller, a 1Ah 3S lipo battery, and a few little linkage parts.

The one thing which I could not get because it was out of stock, and which I couldn’t find at the local hobby shops because they were useless, was a propeller hub for the motor. So I actualled machined something (!) and made one. It’s based around a short 1/4-20 bolt that was drilled out and had a set screw added. Stock locknuts and washers take care of the fastening of the propeller. Sure, it’s heavier than an aluminum prop collet, but could I find an aluminum prop collet?

A laser-cut buffer plate made of plywood secures the motor to the frame. The servos were also given little laser-cut mounting plates because let’s face it – foamcore is just not structural.

And here it is!

Notice the small carbon fiber rods spanning the the upper flaps in an attempt to get them to synchronize.

The word “attempt” is key across all aspects of Ballcopter here. It really didn’t work, and after a bit of head scratching (and “duh” moments), I concluded that…

  • Those flaps are just too rigged and horrible. I ended up using electrical tape for the hinge material because it was in front of me. Protip: Electrical tape stretches. Instead of rotating the flap about the hinge when the servos moved, it was more liable to just displacing the flap without much rotation.
  • The servo linkages were not well thought out at all. In fact, they weren’t thought out – just built on the fly. The linkage is a 45 degree bell crank of sorts. While all aircraft control surfaces seem to rely on the small angle approximation, this only works if there are not like 3 different small angles involved and one of them isn’t based around displacement from 45 degrees. This arrangement is clearly seen in the above photo. Basically the servos were more yanking on the flaps sideways after only a few degrees of rotation. The same goes for the rudder flaps, where 2 flaps were actuated by a servo between them.
  • As soon as the prop started and the vehicle took off, those sad little linkages and hinges just collapsed and the result is not very much flap displacement at all. The vehicle was therefore barely controllable, and tended to just run away in one direction (or run away while twirling if I applied any rudder control)

The JSDF drone is gyro-stabilized, too, and mine was totally and utterly open loop.

So over all a great execution of Cheap Chinese Imitation that would probably have worked if it wasn’t crudely reverse engineered and then even more crudely reassembled. I do want to try my hand at this again some time, though – direct drive flaps (no loose and bendy links), real hinging, reducing weight a little (there is really no need to carry a 1Ah battery on something this small), and adding some intelligent stabilization using IMUs.

It’s so cute that I have to try making it work. Before I left, I stripped down Ballcopter back into parts, and hopefully those parts are en route to me in Boston. The frame was left mostly untouched and would make a great lamp or something.

Onwards ボールコプター!!



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