For SAFMC 2016, we built a custom frame to accommodate what we needed. I call it the Tic Tac Toe.
Before working on the frame, we first decided on a navigation system. To avoid reinventing the wheel, we chose a Naze32 to provide basic flight stabilization for our quad. It is tried and proven to be reliable, especially with an open source firmware with active development.
Onboard accelerometers and gyros allow the Naze32 to provide stabilization for the quad. It is most often used for RC micro race quads (watch out for a post!). Higher level navigation will mimic an RC input into the Naze32, hence controlling the attitude of the craft.
Working on a budget, we decided to give ultrasonic sensors a try. Data from the sensors is processed by the onboard Pi with a custom algorithm to provide instructions for the Naze32.
Hence with the navigation system set, we built our frame around that. The concept of the tic tac toe frame came about when we realised it could solve a few problems for us. First, it provides a structure prop guards could easily be added to. Second, it puts the ultrasonic sensors at the edge of the frame. Third, it alleviated the need for a strong structural base plate as the arms would take load.
Made simply with 5 identical aluminium sections, the frame was simple and cheap. The center piece serves only as a platform for the electronics to be mounted to. The Naze32 sits in the middle, the Pi and camera in front, countering the weight of the battery at the back.
Initially the frame warped alot due to the thin C sections that are the arms. Those were chosen early on as they were really cheap. All the aluminium in the frame cost $6. Some attempt was made at fixing this, adding the fibreglass plates at the joints. In flight, the quad warped lesser than expected, as in flight forces seem to be quite uniform.
In short, the design process for this project started with the navigation system, then the frame built to accommodate it. Limited by a budget, many design choices was made with cost in mind. While we did not test out the navigation system this time, continuation of the ultrasonic sensor navigation is being worked on. Other methods of navigation will also be tested. An airframe is also being worked on concurrently.