Saturday, December 8, 2012

The Quadrotor Itch Strikes Again!

I was inspired by Shane zipping his small quadrotor across the office, and decided I wanted to make another, smaller quadrotor that I could fly indoors (unlike my other giant one). I wasn't too eager to cough up the relatively large sum of money that I threw down on Derpcopter (if I remember right it totaled something in the neighborhood of $800), so I designed one with cheap hobby parts that I could get for a lot less.

I decided that I wanted to 3D print the main body components so that they'd be relatively lightweight, and it would also let me do non-2D geometries (as much as I love laser cutters and waterjets, I think I eventually get sick of being stuck in 2D). I based the design on 6mm carbon fiber squaretube, since it's very lightweight but still plenty strong enough for a small quadrotor. I also wanted to try out the relatively new KK2 flight control board, so I designed my body specifically to accommodate it (3mm holes spaced on a 45mm square).

I added slots for a velcro strap on the bottom side, to hold the battery. The holes for the control board also retain the squaretube arms, which fit into the square slots on the corners. I also split the model in half across the horizontal mid plane, in order to make it much easier to 3D print.

I wanted to get a head start on picking out components to I order them from China, because shipping takes forever. Here are some links to the major components:

I decided to use the same radio transmitter that I already had for Derpcopter, there wasn't really any reason for me to buy another one. I also picked out substantially bigger ESCs than I needed, really the motors could run on ESCs half that size, but I was considering that I might want to bump up the motor size later, which I can with large ESCs.

I went ahead and ordered two sets of parts (one for me, one for a friend), then worked on the design (and my scooter!) for a few weeks while I waited for them to come. Now that I knew which motors I'd be getting, I went ahead and designed mounts for them which also serve as feet for the copter.

I also wanted a sort of dome structure on the top of the body, which would serve both to protect the control board and mount the radio receiver (mostly just make the quadrotor look more legit, though). I didn't have a very concrete idea of what I wanted the design to look like, though, so I just sketched out a few different designs until I came up with one that I liked.

I ended up going with the third design (the one on the bottom) because it looked the simplest/sturdiest, as well as the easiest of the three to print. Again notice the velcro strap slots for the radio receiver.

I went ahead and printed out the frame parts while I waited on my electronics to arrive.

Yay printers!
There wasn't much else I could do on the copter front, so I spent a couple weeks focusing on school and other projects. I was pretty excited when the giant shipping package of Hobbyking parts came in, so I laid everything out and took a picture:

I included my old radio transmitter for completeness, the orange and black one you see is the one my friend got for his quadrotor. Also, notice the extra props: I've learned a bit of a lesson there. New quadrotor = broken props, so buy extra.

I began by cutting my 75cm carbon fiber tube into 4 17cm sections. I also took rather meticulous photos of this whole build, because I'm planning on writing a quadrotor Instructable. Oh, and there happened to be a lightbox sitting at MITERS, the owners of which kindly decided to let me use it to take some pictures of quadrotor parts.

I think you're supposed to wear a mask when you do this? Not much dust came up though. I think I held my breath.
Cut arms:

Ooh, fancy.
The bandsaw didn't make perfect cuts on the CF, but it really doesn't matter for how they're being used (the faces don't need to be perfectly rectangular). Plus, I didn't really feel like sanding it down and throwing dust everywhere.

I arranged the arms and base together so that I could drill out the holes in the arms.

Sorry it's a little hard to distinguish the white base from the white background, this is why I'm an engineer and not a photographer
This was just faster than measuring out and drilling holes for the arms separately, and it works just as well.

I then did essentially the same thing with the motor mounts, except I taped around them a bit to help hold them in place on the arm.

That done, my basic frame was complete!

This one didn't quite fit in the lightbox :(
With the frame ready, it was time to work on electronics. Primarily I just had to solder lots of bullet connectors so that I could hook everything together. Oh, and I also got my battery started charging so I'd have it ready by the time I finished everything else. I took a whole lot of pictures of the process for soldering bullet connectors, because I remember they were very confusing to me the first time I had to use them (on Derpcopter). I'll be putting a full detailed guide to doing this on the Instructable, but for now I'll save you having to scroll through 20 pictures and just show you the result:

So pretty!
There are all four motors and ESCs, all soldered up and ready to go (only 32 bullet connectors between them).

Next, I discovered my first major mistake: I'd assumed that the motors used the same 3mm screw size as the control board, but actually the holes are for 2mm screws. Oh well, I didn't have the patience to do this the 'right' way and buy some screws from Amazon, so I just removed the motor mounts and drilled the holes out to 3mm.

These are annoying to fixture properly and the alloy is so soft that I just decided to beast it with vice grips.
I screwed on the control board and the power distribution board, and then started mounting the motors.

It's starting to look pretty quadrotor-y!
ESCs all wired up:

And a final picture of the finished copter, with battery, radio, and propellers attached:

I zip-tied the ESCs to the arms to clean up the wiring and prevent them from just swinging around in flight. You'll notice that the dome I designed isn't attached in this picture, this is due to the fact that I underestimated the thickness of the power distribution board when I was buying screws. The body screws I got are too short to go through everything and mount the dome, so currently I'm waiting on new screws ordered through Amazon in order to complete the body. Regardless, it's fully capable of flying without the dome, the purpose is mostly aesthetic. I did fly it for a minute or so around when I took that last picture, but it was 5am at that point and the copter was very difficult to control (due to the default PI gains of the flight control being set way too high, causing it to oscillate around whatever desired position I gave it), so I decided to call it a night.

The next day I worked out the PI (if you're not sure what this means, I recommend reading up on PID control on Wikipedia. Copters typically don't have need for the D component because it's highly affected by noise in the sensors, and it slows the system response which isn't very desirable) settings to get a much more stable copter, and came up with a name: I dubbed it the Blitzcopter, owing to how I'd completed the entire construction over the course of one manic night.

Okay, I'm not going to leave you waiting for the video any longer than that, so here it is:

Yay! The reason it's flying so loud is that I haven't yet gotten a chance to balance the propellers, which entails adding little bits of mass (i.e. tape) to the blades in order to move the center of mass of the propeller onto the rotational axis, greatly reducing the vibration of the motors.

I've also taken a couple of hard landings and snapped some of the printed motor mounts due to the impact, for now I've super-glued them back together but I'm going to be designing new sturdier mounts won't break as easily.


  1. Looks good! Where did you get your carbon fiber tubes from?

    1. Thanks! I bought 6mm tubes here on Hobbyking, they're cheap but it's based in China so shipping can cost a bit/take a while.