Engineers have created a robot with the feet and legs of a peregrine falcon that can perch and carry objects like a bird. No two branches are alike, just like snowflakes. They can vary in size, shape, and texture; some may be wet, moss-covered, or overflowing with offshoots. Despite this, birds can land on almost any of them. This ability piqued the interest of Stanford University engineers Mark Cutkosky and David Lentink, who are now at the University of Groningen in the Netherlands and have both developed technologies based on animal abilities.
“It’s not easy to mimic how birds fly and perch,” said William Roderick, Ph.D. ’20, who worked in both labs as a graduate student. “After millions of years of evolution, they make takeoff and landing look so easy, even in the midst of all the complexity and variability of tree branches found in a forest.”
Years of research in the Cutkosky Lab on animal-inspired robots and in the Lentink Lab on bird-inspired aerial robots enabled the researchers to create their own perching robot, which is detailed in a paper published in Science Robotics. Their “stereotyped nature-inspired aerial grasper,” or SNAG, when attached to a quadcopter drone, forms a robot that can fly around, catch and carry objects, and perch on various surfaces. The researchers used this work to compare different types of bird toe arrangements and to measure microclimates in a remote Oregon forest, demonstrating its versatility.
One of the underlying motivations for this work was to develop tools that we can use to study the natural world. If we could create a robot that could mimic the behavior of a bird, we could open up entirely new avenues for studying the environment.William Roderick
A bird bot in the forest
Previously, the researchers studied parrotlets (the second smallest parrot species), which flew back and forth between special perches while being recorded by five high-speed cameras. The perches, which came in a variety of sizes and materials such as wood, foam, sandpaper, and Teflon, also had sensors that recorded the physical forces associated with the birds’ landings, perching, and takeoff.
“What surprised us was that they did the same aerial maneuvers regardless of the surface they were landing on,” said Roderick, the paper’s lead author. “They give the feet the ability to deal with the variability and complexity of the surface texture itself.” The “S” in SNAG stands for “stereotyped,” because this formulaic behavior is seen in every bird landing.
SNAG, like the parrotlets, approaches each landing in the same manner. SNAG, on the other hand, is based on the legs of a peregrine falcon to account for the size of the quadcopter. It has a 3D-printed structure in place of bones, which took 20 iterations to perfect, and motors and fishing line stand-in for muscles and tendons.
Each leg has its own motor for moving back and forth, as well as another for grasping. A similar mechanism in the robot’s leg absorbs landing impact energy and passively converts it into grasping force, inspired by the way tendons route around the ankle in birds. As a result, the robot has a particularly strong and fast clutch that can be triggered to close in 20 milliseconds. When SNAG’s ankles lock around a branch, an accelerometer on the right foot reports that the robot has landed and activates a balancing algorithm to stabilize it.
During COVID-19, Roderick relocated equipment from Lentink’s lab at Stanford to rural Oregon, where he established a basement lab for controlled testing. He sent SNAG along with a rail system, which launched the robot at different surfaces, at pre-defined speeds and orientations, to see how it performed in different scenarios. Roderick demonstrated SNAG’s ability to catch objects thrown by hand, including a prey dummy, a corn hole bean bag, and a tennis ball while holding SNAG in place. Finally, Roderick and SNAG ventured into the nearby forest for some real-world practice runs.
Overall, SNAG performed so well that future development efforts will most likely concentrate on what happens before landings, such as improving the robot’s situational awareness and flight control.
Back to nature
This robot has numerous applications, including search and rescue and wildfire monitoring; it can also be attached to technologies other than drones. The proximity of SNAG to birds allows for unique insights into avian biology. For example, the researchers tested the robot with two different toe arrangements: anisodactyl (three toes in front and one in back, similar to a peregrine falcon) and zygodactyl (two toes in front and two in back, similar to Parrotlet). Surprisingly, they discovered that there was very little performance difference between the two.
One of the most exciting potential applications for SNAG, according to Roderick, whose parents are both biologists, is in environmental research. To that end, the researchers fitted the robot with a temperature and humidity sensor, which Roderick used to record the microclimate in Oregon.
“One of the underlying motivations for this work was to develop tools that we can use to study the natural world,” Roderick explained. “If we could create a robot that could mimic the behavior of a bird, we could open up entirely new avenues for studying the environment.”
Roderick’s perseverance in what turned out to be a years-long project was praised by Lentink, the paper’s senior author. “This research was really launched by Will talking with several ecologists at Berkeley six years ago and then writing his NSF Fellowship on perching aerial robots for environmental monitoring,” Lentink said. “Will’s research is timely because there is now a $10 million XPRIZE for this challenge to monitor biodiversity in rainforests.”