Like snowflakes, no two branches are alike. They vary in size, shape, and texture; Some may be moist or mossy-covered or bursting branches. Birds, however, can land in almost any of them. This ability intrigued engineers Mark Cutkosky and David Lentink at Stanford University – now at the University of Groningen in the Netherlands – who have both developed techniques inspired by animals' abilities.
"It's not easy to mimic how birds fly and perch," said Dr. William Roderick, 20, a graduate student at both LABS."They have evolved over millions of years to make taking off and landing look so easy, even among all the complex and varied branches you would find in a forest."
Years of work on animal-like robots in Cutkosky's lab and bird-like aerial robots in Lentink's lab have enabled researchers to build their habitat robots, as detailed in a paper published on Dec. 1 in Science Robotics. When they are mounted on a quadcopter, their "nature-inspired Aerial grabber" (SNAG) forms a robot that can fly around, capture and carry objects, and perch on a variety of surfaces. Researchers used it to compare the alignment of different types of birds and measure microclimates in remote forests in Oregon, showing the potential diversity of the work.
A bird robot in the forest.
In previous studies of parrots – the second smallest species – the tiny birds were filmed by five high-speed cameras as they flew back and forth between special perches. The perches — representing a variety of sizes and materials, including wood, foam, sandpaper, and Teflon — also contain sensors that capture the physical forces associated with the birds landing, perching, and taking off.
"What surprised us was that no matter what surface they landed on, their air movements were the same," said Roderick, lead author of the paper."They let the feet deal with the diversity and complexity of the surface texture itself."This formulaic behavior, seen in every bird landing, is why the "S" in SNAG stands for "stereotype."
Just like the parrot, SNAG landed the same way every time. But, to take into account the size of the quadcopter, SNAG is based on the legs of a peregrine falcon. In place of the bones are 3D-printed structures — perfected after 20 iterations — and motors and fishing line in place of muscles and tendons.
Each leg has its motor to move back and forth, and the other controls the grip. Inspired by the way tendons move around birds' ankles, a similar mechanism in the robot's legs absorbs landing impact energy and passively converts it into grabbing force. As a result, the robot has an especially powerful and high-speed clutch that can trigger a shutdown in 20 milliseconds. Once wrapped around the branch, SNAG's ankle locks and an accelerometer on its right foot reports that the robot has landed and triggers a balancing algorithm to stabilize it.
During the COVID-19 pandemic, Roderick moved equipment, including 3D printers, from Rentink's lab at Stanford to rural Oregon, where he set up a basement lab for controlled testing. There, he fired SNAG along with an orbital system to different surfaces, at predetermined speeds and directions, to see how it behaved in different scenarios. With SNAG stationery, Roderick also demonstrated the robot's ability to catch objects with its hands, including a prey dummy, a cornhole beanbag, and a tennis ball. Finally, Roderick and SNAG venture into a nearby forest for some real-world test runs.
Overall, SNAG performed very well, so the next step in development is likely to focus on what happens before landings, such as improving the robot's situational awareness and flight control.
Return to nature
The robot has countless possible applications, including search and rescue and wildfire monitoring; It could also be used for technology other than drones.SNAG's proximity to birds also allows for unique insights into avian biology. For example, the researchers ran the robot with two different toe arrangements: a dactyl, which had three toes in front and one in the back, like a peregrine falcon; The other is a pterodactyl, which has two toes in front and two in back, like a parrot. To their surprise, they found little difference in the two men's performance.
For Roderick, the son of two biologists, one of the most exciting possible applications of SNAG is environmental research. To that end, the researchers also attached a temperature and humidity sensor to the robot, which Roderick used to record Oregon's microclimate.
"Part of the underlying motivation for this work is to create tools that we can use to study the natural world," Roderick said."If we could have a robot that could act like a bird, that could open up a whole new way of studying the environment."
Lentink, the senior author of the paper, praised Roderick's perseverance in what proved to be a multi-year project."It was Really Will talking to a couple of ecologists at Berkeley six years ago and then writing his NSF fellowship to study airborne environmental monitoring robots that started this research," Rendink said."Will's research is proving timely, as there is now a $10 million XPRIZE for this challenge of monitoring biodiversity in tropical rainforests."
Co-author Mark Cutkosky is the Fletcher Jones Professor in the College of Engineering and a member of the Stanford Bio-X and Wu-Chua Institute for Neuroscience. David Lentink is co-chair of the Bionics group and an associate professor of science and engineering at the University of Groningen in the Netherlands.
The research was funded by the Air Force Office of Scientific Research and the National Science Foundation.