“Cells in a moth antenna amplify chemical signals,” said co-author Thomas Daniel, a UW professor of biology who co-supervises Anderson’s doctoral research. “The moths do it really efficiently — one scent molecule can trigger lots of cellular responses, and that’s the trick. This process is super efficient, specific and fast.”
The team used antennae from the Manduca sexta hawkmoth for Smellicopter. Researchers placed moths in the fridge to anesthetize them before removing an antenna. Once separated from the live moth, the antenna stays biologically and chemically active for up to four hours. That time span could be extended, the researchers said, by storing antennae in the fridge.
By adding tiny wires into either end of the antenna, the researchers were able to connect it to an electrical circuit and measure the average signal from all of the cells in the antenna. The team then compared it to a typical human-made sensor by placing both at one end of a wind tunnel and wafting smells that both sensors would respond to: a floral scent and ethanol, a type of alcohol. The antenna reacted more quickly and took less time to recover between puffs.
By adding tiny wires into either end of the antenna (the arc being attached here), the researchers were able to connect it to a circuit and record its responses.Mark Stone/University of Washington
To create Smellicopter, the team added the antenna sensor to an open-source hand-held commercially available quadcopter drone platform that allows users to add special features. The researchers also added two plastic fins on the back of the drone to create drag to help it be constantly oriented upwind.
“From a robotics perspective, this is genius,” said co-author and co-advisor Sawyer Fuller, a UW assistant professor of mechanical engineering. “The classic approach in robotics is to add more sensors, and maybe build a fancy algorithm or use machine learning to estimate wind direction. It turns out, all you need is to add a fin.”
Smellicopter has two plastic fins (shown here in blue) on the back to create drag to help it be oriented so that it is constantly facing upwind.Mark Stone/University of Washington
Smellicopter doesn’t need any help from the researchers to search for odors. The team created a “cast and surge” protocol for the drone that mimics how moths search for smells. Smellicopter begins its search by moving to the left for a specific distance. If nothing passes a specific smell threshold, Smellicopter then moves to the right for the same distance. Once it detects an odor, it changes its flying pattern to surge toward it.
Smellicopter can also avoid obstacles with the help of four infrared sensors that let it measure what’s around it 10 times each second. When something comes within about eight inches (20 centimeters) of the drone, it changes direction by going to the next stage of its cast-and-surge protocol.
“So if Smellicopter was casting left and now there’s an obstacle on the left, it’ll switch to casting right,” Anderson said. “And if Smellicopter smells an odor but there’s an obstacle in front of it, it’s going to continue casting left or right until it’s able to surge forward when there’s not an obstacle in its path.”
Another advantage to Smellicopter is that it doesn’t need GPS, the team said. Instead it uses a camera to survey its surroundings, similar to how insects use their eyes. This makes Smellicopter well-suited for exploring indoor or underground spaces like mines or pipes.
During tests in the UW research lab, Smellicopter was naturally tuned to fly toward smells that moths find interesting, such as floral scents. But researchers hope that future work could have the moth antenna sense other smells, such as the exhaling of carbon dioxide from someone trapped under rubble or the chemical signature of an unexploded device.
“Finding plume sources is a perfect task for little robots like the Smellicopter and the Robofly,” Fuller said. “Larger robots are capable of carrying an array of different sensors around and using them to build a map of their world. We can’t really do that at the small scale. But to find the source of a plume, all a robot really needs to do is avoid obstacles and stay in the plume while it moves upwind. It doesn’t need a sophisticated sensor suite for that — it just needs to be able to smell well. And that’s what the Smellicopter is really good at.”
Joseph Sullivan, a UW electrical and computer engineering doctoral student, and Timothy Horiuchi, an electrical and computer engineering associate professor at the University of Maryland College Park, are also co-authors. This research was funded by the National Defense and Engineering Graduate Fellowship, the Washington Research Foundation, the Joan and Richard Komen Endowed Chair and the Air Force Office of Scientific Research with The Air Force Center of Excellence on Nature-Inspired Flight Technologies and Ideas.