Stanford Engineers Build Bird-Inspired Drone With "Bird Feet" That Can Perch on Branches and Carry Objects
No two tree branches in the world are alike. They can differ in size, shape and texture, and some may be wet or covered with moss or shoots. Yet birds can land on almost any of them.
This ability sparked strong interest in the labs of Stanford University engineers Mark Cutkosky and David Lentink, who now work at the University of Groningen in the Netherlands. Both had already developed technologies inspired by animal abilities.
“Imitating how birds fly and land on surfaces is not easy. After millions of years of evolution, they make takeoffs and landings look effortless, despite the complexity and variety of tree branches found in the forest”
Years of research on animal-inspired robots in Cutkosky's lab and bird-inspired aerial robots in Lentink's lab allowed the researchers to create their own bird-like drone, described in detail in a paper in Science Robotics. When the "legs" are attached to a quadcopter, their "stereotyped aerial grasp," or SNAG, forms a robot that can fly, catch and carry objects, and land on various surfaces.
To demonstrate the potential versatility of this work, the researchers used it to compare different toe arrangements on bird feet and to measure microclimates in remote forests of Oregon.
In previous studies of parrots, tiny birds flew back and forth between special perches while five high-speed cameras recorded them. The perches, representing different sizes and materials, including wood, foam, sandpaper and Teflon, also had sensors that captured the physical forces associated with landing, perching and takeoff.
“What surprised us was that they performed the same aerial maneuvers regardless of the surface they landed on. They allow the legs to cope with the variability and complexity of the surface texture itself”
This stereotyped behavior, observed when every bird lands, is why the S in SNAG stands for "stereotyped." As with the parrots, SNAG approaches every perch the same way. But to account for the quadcopter's size, SNAG is based on the legs of a peregrine falcon (a bird of prey from the falcon family). Instead of bones, it has a 3D-printed structure, which took 20 attempts to perfect, plus motors and fishing line that replace muscles and tendons.
Each leg has its own motor for moving forward and backward, and another for grasping. Inspired by how tendons run around a bird's ankle, a similar mechanism in the robot's leg absorbs the energy of impact on landing and passively converts it into gripping force. As a result, the robot has a particularly strong, high-speed clutch that engages within 20 milliseconds. After wrapping around a branch, the SNAG ankle lock and an accelerometer on the right leg signal that the robot has landed and trigger a balancing algorithm to stabilize it.
During the coronavirus pandemic, Roderick moved equipment, including a 3D printer, from Lentink's lab at Stanford to rural Oregon, where he set up a basement lab for controlled testing. There he sent SNAG along a rail system that carried the robot across different surfaces at set speeds and orientations to see how it performed in various scenarios.
After setting up SNAG in place, Roderick also confirmed the robot's ability to catch objects thrown by hand, including a dummy, a bag of corn beans and a tennis ball. Finally, Roderick and SNAG ventured into a nearby forest for several real-world test runs.
Overall, SNAG worked so well that the next development steps will likely focus on what happens before landing, such as improving situational awareness and flight control of the robot.
This robotic device has countless possible applications, including search and rescue operations and wildfire monitoring. It can also be used with technologies other than drones. SNAG's closeness to birds offers a unique view into avian biology.
For example, the researchers operated the robot with two different toe arrangements: anisodactyl, with three toes in front and one behind, as in the peregrine falcon, and zygodactyl, with two toes in front and two behind, as in parrots. To their surprise, they found that the performance difference between them was very small.
For Roderick, whose parents are both biologists, one of the most interesting applications of SNAG is environmental research. To this end, the researchers also attached a temperature and humidity sensor to the robot, which Roderick used to record the microclimate in Oregon.
“Partly, the main motivation behind this work was to create tools we could use to study the natural world. If we had a robot that could act like a bird, it would open entirely new ways to study the environment”
Lentink, the senior author of the paper, praised Roderick's persistence in this project, which was designed to span years.
“In fact, Will, who spoke with several ecologists in Berkeley six years ago and then wrote his NSF fellowship proposal on placing aerial robots for environmental monitoring, sparked this research. Will's work proved timely, because XPRIZE has now allocated $10 million to tackle this challenge of monitoring tropical forest biodiversity”
