It turns out that you don’t need a lot of hardware to make a flying robot. Flying robots are usually way, way, way over-engineered, with ridiculously over the top components like two whole wings or an obviously ludicrous four separate motors. Maybe that kind of stuff works for people with more funding than they know what to do with, but for anyone trying to keep to a reasonable budget, all it actually takes to make a flying robot is one single airfoil plus an attached fixed-pitch propeller. And if you make that airfoil flexible, you can even fold the entire thing up into a sort of flying robotic swiss roll.
This type of drone is called a monocopter, and the design is very generally based on samara seeds, which are those single-wing seed pods that spin down from maple trees. The ability to spin slows the seeds’ descent to the ground, allowing them to spread farther from the tree. It’s an inherently stable design, meaning that it’ll spin all by itself and do so in a stable and predictable way, which is a nice feature for a drone to have—if everything completely dies, it’ll just spin itself gently down to a landing by default.
The monocopter we’re looking at here, called F-SAM, comes from the Singapore University of Technology & Design, and we’ve written about some of their flying robots in the past, including this transformable hovering rotorcraft. F-SAM stands for Foldable Single Actuator Monocopter, and as you might expect, it’s a monocopter that can fold up and uses just one single actuator for control.
While F-SAM’s control board was custom designed and the wing requires some fabrication, the rest of the parts are cheap and off the shelf. The total weight of F-SAM is just 69g, of which nearly 40% is battery, yielding a flight time of about 16 minutes. If you look closely, you’ll also see a teeny little carbon fiber leg of sorts that keeps the prop up above the ground, enabling the ground takeoff behavior without contacting the ground.
You can find the entire F-SAM paper open access here, but we also asked the authors a couple of extra questions.
IEEE Spectrum: It looks like you explored different materials and combinations of materials for the flexible wing structure. Why did you end up with this mix of balsa wood and plastic?
Shane Kyi Hla Win: The wing structure of a monocopter requires rigidity in order to be controllable in flight. Although it is possible for the monocopter to fly with more flexible materials we tested, such as flexible plastic or polymide flex, they allow the wing to twist freely mid-flight making cyclic control effort from the motor less effective. The balsa laminated with plastic provides enough rigidity for an effective control, while allowing folding in a pre-determined triangular fold.
Can F-SAM fly outdoors? What is required to fly it outside of a motion capture environment?
Yes it can fly outdoors. It is passively stable so it does not require a closed-loop control for its flight. The motion capture environment provides its absolute position for station-holding and waypoint flights when indoors. For outdoor flight, an electronic compass provides the relative heading for the basic cyclic control. We are working on a prototype with an integrated GPS for outdoor autonomous flights.
Would you be able to add a camera or other sensors to F-SAM?
A camera can be added (we have done this before), but due to its spinning nature, images captured can come out blurry. 360 cameras are becoming lighter and smaller and we may try putting one on F-SAM or other monocopters we have. Other possible sensors to include are LiDAR sensor or ToF sensor. With LiDAR, the platform has an advantage as it is already spinning at a known RPM. A conventional LiDAR system requires a dedicated actuator to create a spinning motion. As a rotating platform, F-SAM already possesses the natural spinning dynamics, hence making LiDAR integration lightweight and more efficient.
Currently, F-SAM can be folded into a compact form and stored inside a container. However, it still requires a human to unfold it and either hand-launch it or put it on the floor to fly off. In the future, we envision that F-SAM is put inside a container which has the mechanism (such as pressured gas) to catapult the folded unit into the air, which can begin unfolding immediately due to elastic materials used. The motor can initiate the spin which allows the wing to straighten out due to centrifugal forces.
Do you think F-SAM would make a good consumer drone?
F-SAM could be a good toy but it may not be a good alternative to quadcopters if the objective is conventional aerial photography or videography. However, it can be a good contender for single-use GPS-guided reconnaissance missions. As it uses only one actuator for its flight, it can be made relatively cheaply. It is also very silent during its flight and easily camouflaged once landed. Various lightweight sensors can be integrated onto the platform for different types of missions, such as climate monitoring. F-SAM units can be deployed from the air, as they can also autorotate on their way down, while also flying at certain periods for extended meteorological data collection in the air.