F-SAM: a flexible drone that folds up like a scroll
It turns out that building a flying robot does not require much equipment. Flying robots are usually very complex in design and have many components, such as two full wings or four separate motors.
Such devices may be built for people with large budgets, but for anyone trying to stick to a reasonable budget, all that is really needed to build a flying robot is a single airfoil and an attached fixed-pitch propeller. And if you make this airfoil flexible, you can even fold it entirely into a kind of flying robotic Swiss roll.
This type of unmanned aerial vehicle is called a monocopter, and its design is based on the seeds of the samara plant, which are single seed pods that have a "wing." The ability to rotate steadily slows the seed's descent to the ground, allowing it to spread many kilometers. This is an inherently stable design that will spin on its own and do so neatly and predictably, which is a good feature for a drone: if something in the hardware breaks, it simply spins gently down to land by default.
The Singapore University of Technology and Design created a monocopter called F-SAM, which stands for Foldable Single-Actuator Monocopter. In terms of hardware there may not be much to it, but that is part of the charm of the device's design. A single actuator provides full directional control: the throttle increases the rotation speed, causing the device to gain altitude.
Controlling a monocopter may seem complicated, since it requires repeated motor pulses at a point during the device's rotation. F-SAM works in a video motion capture environment to explore its potential for precise autonomy, but it is not limited to this environment and does not require external control to operate.
Although the F-SAM control panel was custom designed and the wing requires some machining, the other parts are inexpensive and sold separately. The total weight of F-SAM is only 69 g, of which almost 40% is the battery, which provides a flight time of about 16 minutes.
IEEE Spectrum spoke with the creators of the monocopter and asked them several questions. The scientists explored various materials and material combinations for the flexible wing structure. The editor asked why they settled on a mix of balsa wood and plastic.
“The structure of a monocopter wing needs to be stiff so it can be controlled in flight. Although a monocopter can fly using more flexible materials that we tested, such as flexible plastic or flexible polymer, they allow the wing to twist freely in flight, which reduces the effectiveness of cyclic motor control. Plastic-laminated balsa provides sufficient stiffness for effective control and also allows folding”
According to the researchers, F-SAM can fly outdoors. It is passively stable, so its flight does not require feedback control. The motion capture environment provides its absolute position for station and waypoint flights indoors. When flying outdoors, an electronic compass provides the relative heading for primary cyclic control. The team of scientists is working on a prototype with built-in GPS for autonomous outdoor flights.
“A camera can be added (we have done this before), but because of its rotating nature, images may come out blurred. 360-degree cameras are becoming lighter and smaller, and we may try to mount one on F-SAM or the other monocopters we have. Other possible sensors include a LiDAR sensor or a ToF sensor. With LiDAR, the platform has an advantage because it already rotates at a known speed. A conventional LiDAR system requires a special actuator to create rotational motion. As a rotating platform, F-SAM already has natural rotational dynamics, which makes LiDAR integration easy and more efficient”
Today, F-SAM can be folded into a compact form and stored inside a container. However, a person still has to unfold it and either launch it by hand or place it on the floor to take off. In the future, the scientists envision F-SAM being placed in a container with a mechanism (for example, compressed gas) to launch the folded unit into the air so it can immediately begin unfolding thanks to the elastic materials used. The motor can initiate the rotation that allows the wing to straighten out due to centrifugal forces.
The scientists believe F-SAM may be a good toy but cannot be a good alternative to quadcopters if the goal is ordinary aerial photography or videography. However, it may be a good competitor for disposable GPS-guided reconnaissance missions. Since only one actuator is used for flight, it can be made relatively cheaply.
In addition, it is very quiet in flight and easily camouflaged after landing. Lightweight sensors for various tasks, such as climate monitoring, can be integrated into the platform. Several F-SAM devices can be deployed from the air, since they can automatically spin during descent and also fly during certain periods to collect extended meteorological data in the air.
“We have several interesting projects, most of which are devoted to the theme "Do more with less." This means our projects aim to perform multiple tasks and flight modes while using as few actuators as possible. Similar to F-SAM, which uses only one actuator for controlled flight, we are working on another project, a fully autorotating version called Samara Autorotating Wing (SAW)”
According to him, this platform can operate in two flight modes with just one actuator. It is ideally suited for deploying disposable sensors in remote locations. For example, the platform could be used to deploy sensors for forest monitoring or a wildfire warning system.
The sensors can land on tree canopies, and after landing the wing provides the necessary area to capture solar energy for continuous operation for several years. Another interesting scenario is using the autorotation platform to guide radiosondes back to a collection point after they finish their upward journey.
The researcher said that currently many radiosondes are launched with hydrogen balloons from weather stations around the world (more than 20,000 each year from Australia alone), and once the balloon reaches high altitude and bursts, the sensors fall back to the ground. Guiding them back to a collection point could save millions of dollars every year and also help protect the environment through reduced pollution.
