Step one was creating the frame. I found a set of designs for the IRIS+ by 3D robotics and sent one copy of each component to the Radcliffe Science Library (RSL) here at Oxford, and 24 hours of printing later (5 days real time) the first parts were ready. It took another 13 hours before the whole frame was ready, but the whole shell only cost £47. The only problem with this approach is that there's no component list to go with the IRIS; the only thing you have to go on is general advice and a list of suggestions from the MiniFactory comments.
3D printers work by extruding a small stream of plastic about 0.1mm thick, in layers that get built up over time. The plastic used here was polylactic acid, refered to as PLA, which melts at around 200 degrees and is biodegradable. Annoyingly, this method of layering up from the base of the printer means that printing curved or otherwise non-flat surfaces is difficult. One way to solve the problem (and the method that the RSL use) is to print a flat surface anyway and then layer supports up to the point where you actually want to print. On a smaller model this isn't so bad, but removing all of this excess material from the drone case was an arduous task. In an improvement from my previous rockets project, recourse to plasters was only required once!
Normally hidden from consumers, the inner workings of the average quadcopter are quite interesting. To be able to hover in mid air, opposite pairs of rotors must spin in the same direction, with adjacent rotors rotating in contrary directions. But while this makes hovering easy, it makes turning harder - increasing power to the clockwise motors will turn the drone in one direction, and vice versa for the counter clockwise motors. Banking and moving is achieved in a similar fashion but with adjacent motors instead of opposite ones.
Things get even more involved when you move to actual electronics. Each of the four motors must be connected to an electronic speed control (ESC) unit which is responsible for using a low power control signal to alter the speed of the motors using larger currents. This type of setup would normally require two separate batteries, which is not ideal when every gram counts. Battery eliminator circuits (BEC) solve this problem by allowing one power input from the battery to power both low voltage control and high voltage motor circuits, as well as prioritising control circuits in the event of power loss. The last thing you want when the battery goes is complete loss of control! In practise, most ESC units have built in BECs.
Next we get to the brains of the operation: the flight controller. This component is responsible for turning high level commands from the remote control into voltage changes at the ESCs. The controller also has to keep the aircraft stable through gusts of wind and other types of tilt by making slight adjustments to the motor speeds (informed by its gyroscope and accelerometers). Eventually I'd like to write my own flight controller, but making high frequency real time adjustments on what would probably end up being a Raspberry Pi is tricky. For now I'm sticking to something that's more likely to work first time.
The goal with the build was to make everything as modular as possible so that it would be possible to swap out components in the future, either for upgrades or to reuse kit in other builds. Coming in part two: putting the frame and electronics together!
