Category: flying doobobs

accidentally or purposefully aeronautic

The Once and Future Ballcopter

chorl4 Comments

First off, some minor updates on the not-Land-Bear-Shark (a.k.a the quadrotor, deathcopter, and myriad other pet names I and my peers have assigned to it). I’ve tuned the Z-axis rate controller some more such that now the yaw rate is much more controlled and the servos less twitchy. Powering those fans on injects massive amounts of mechanical noise into the sensors, so much that I’ve begun to consider coding more robust discrete-time filters in the software to target the vibrational frequencies that the fans so readily emit. I was ready to move onto testing the pitch and roll control software, though, when the problem suddenly became a little less virtual:

Oh boy. When you hear the scratchy-fan sound and look up to see a fan rotor hanging halfway out of the duct, it’s time to stop hovering. I guess this is why cheap ducted fans are cheap.

The entire motor mounting face seemed to have broken off; luckily the motor was held in by its power leads, otherwise there would probably be a hole in the ceiling somewhere. I’m not sure what plastic the casing of the fan is made of, but it does seem quite brittle. With the motor mounting face being the focal point of all the stresses the fan rotor imparts on the system, any manufacturing defect (like voids or local shrinkage) or increased loads (like from an off-balance rotor) would make the area prone to failure.

This fan is the most unbalanced of the entire set that had their shafts replaced, so I’m not surprised it was the first to go. However, it’s still a bummer – luckily only $40 lost and not $400 (then again, perhaps a better made fan would have never failed or been off balance). Otherwise, it’s also a hassle to have to replace the whole thing – the rotor is also well-trashed by being ground against the housing at a few thousand RPM. Fortunately, I have a spare (the very first one tested, in fact).

Now, I naturally gravitate away from projects as they enter the software-only stage. During the coding of the yaw controller, I was also thinking of distractions to embark on so I have an excuse to not write code. With this thing temporarily out of mechanical service now, I’m going to take a little break to recharge my code-monkey-pacitor with a project that only requires more software.

I knew I couldn’t leave Ballcopter alone for long. Despite its parts now having taken two and a half weeks to get back to the East Coast (still having not managed it as of right now… seriously, USPS?), I do have enough parts back here at base to independently build version two. Not that I would want to do that, since I did order an entire new radio for the thing…

So what’s different about BallCopterTwo? Hopefully it will be 1000% less terrible. What ultimately failed on the first design was my complete failure at creating control surface linkages. Airplane parts and building methods are still weird to me, which is part of the reason why Chuckranoplan hasn’t moved much from where it was in May. For the new design, I had intended to move to 8 independent, direct-driven flaps, mixed in software, such that there were no linkages involved, but figured that would negatively impact the vehicle weight. However, it took a new video of the original Japanese ball drone design for me to seriously pursue the 8-servo route. Here’s a screencap from the video.

Those sneaky bastards. Well, you can only get spiral deflection of the flaps like that if each one is actually independently driven. Two can play at that game:

Other changes to the version 1 design are a sleeker center column thing which should reduce weight even more and control flaps that aren’t way oversized. Because processing 4 channels of radio control into 8 servo channels needs software, I’ll probably whip up a quick board that carries an Arduino Nano and an IMU module – basically a condensed version of what’s on the quadrotor, stripped to the bare necessities. As long as I have a microcontroller onboard, it will be gyro-stabilized using the gyros on the IMU – I have plenty of practice now from making the quadrotor yaw controller.


Remember kids, don’t cut foamcore on the laser cutter.

 

Ballcopter!

chorl

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 Hobbypartz.com, 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!!