8/10/14
Now there is some slightly better footage of the quad in action. One of the Hobbyking motors needed replacing after it began internally shorting last time. With the new motor installed, CarbonCopter is back in action!
6/18/14
Taking a detour from the ongoing 3d printer build report, I'd like to introduce: CarbonCopter (or at least the crude, motor-less CAD model seen above). CarbonCopter is a simple, no frills, semi-custom quadricopter I put together in the first few weeks after spring term ended this summer. Intended to be a platform for future motor controller/positioning system/balance algorithm projects, it was also just a fun and easy craft that allowed me to enter the world of DIY flying things.
After checking out all the applicable RC supplies available in the US HobbyKing warehouse, I ordered the motors, props, carbon fiber boom arms, battery, and MultiWii controller board. I wanted this copter to be cheap and easy to build. Higher performance parts and optimized component pairing can happen later if I decide to invest a lot more money in an expensive RC hobby! While it would be cooler to try to make a controller board from scratch, I doubt I could top the user interface and hardware you get with the MultiWii for only $27.15. I choose the Aerodrive SK3 2822-1275 outrunners with generic 10x4.5 props because they were in stock and in the right size range. Some 15 amp "Multistar" escs and two 3s Lipos rounded off the order. The two Lipos were actually an accident; I had only intended to order one 3s 4000mAh pack, but now I guess I've also got a 3s 5000mAh pack to play with too.
That covered most of the moving parts and electronics. When it came to designing the chassis (if one can even call it that), I stuck to parts I could reasonably mill on the ShapeOko CNC mill. I went with the "sandwich" design school of thought, squishing the boom mounts between two stiff plates. I like this because it does a decent job of minimizing bending torque on the plates, and instead converts a lot of the force into tension and compression. With relatively long 320mm booms, I wanted to keep things as rigid as possible, to minimize oscillation effects on the gyro and accelerometer. I can personally attest to how frustrating it can be when vibrations screw with your control system. The spacers between the carbon fiber plates are two piece HDPE shaft collars, with the material sourced from my favorite cutting board supplier. Seriously, if you have a CNC router and haven't tried HDPE as a milling material, you are missing out.
Sourcing the carbon fiber plates was surprisingly harder than I expected. There were no good US warehouse HobbyKing offerings at the time, and most of the other online sources I found that could ship quickly seemed overly expensive. So I finally decided that the benefits of purchasing some epoxy and cloth from my local Tap Plastics (which could allow me to make more interesting things in the future) outweighed the hassle. I layed up the carbon fiber plates five layers thick, using standard twill weave cloth. I used wax paper and a petroleum jelly and acetone mix to help keep the epoxy from sticking to my work station. This worked actually worked decently, and gave me satisfactory (if not perfectly smooth and glossy) results.
Next up, I exported the Solidworks drawings to g-code, and began milling.
I was happy with how well the parts came out, and especially with how little fraying I saw. The cutting process was very quick, but I unfortunately neglected to document it. After the plates were done, I also got to work milling a foam mold for a carbon fiber electronics cover. This is visible in the first image on this post, but it was really more of an neat thing than actually useful.
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| In the milling process |
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| Done and sanded! |
Oh well, at least now I know how not to do it. This didn't end up being used. While this was happening, the ShapeOko was milling out even more pieces. Specifically, 16 chassis spacers and 4 motor mount clamps were produced from the HDPE. The motor mount clamps are what connect the 4 outrunners to their respective booms. They were my first attempt at doing real 3d profiling work (2.5d milling), and they worked great. Here is a closeup of their design:
They are not particularly elegant or weight conscious, but they were easy to mass produce on the mill. Not knowing the exact bolt pattern of the outrunners (as they had not arrived yet), I just made a large hexagon of space that could be drilled through later.
When it came to fastening the pieces together, I think my time spent designing a Formula Hybrid racecar is beginning to show, because it's a little over the top for the application. 4-40 hardware everywhere, with abundant nylock fasteners.
The frame is very rigid, even when flexed from the far ends of the booms. Once this and the motor mount clamps were in place, electronics install went very quickly. The MultiWii was simple to wire to the escs and my cheap HobbyKing receiver. As for the battery, it is just slung beneath the quad with velcro. I thought about adding legs for graceful, less jarring landings. Who was I kidding though; legs only work if you plan on not crash landing occasionally, and I expect to crash a lot. Besides, the entire quad is very flat and wide, so the risk of a propeller contacting the ground if I were to make a proper landing seems fairly low. I booted up the MultiWii software and confirmed that I was getting good sensor readings from the quad. Initially I had trouble triggering the motors to arm and start up, but some tweaking on the transmitter adjustment slides fixed that.
I managed to get a video of the first test flight. It isn't pretty, and I didn't do justice to how well it can actually fly due to my inexperienced piloting. I've since taken it for some really fun runs and put it to more of a test. Below are my introductory "touch and go" landings.
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