With feet and legs like a peregrine falcon, engineers have created a robot that can perch and carry objects like a bird.

Like snowflakes, no two branches are alike. They can differ in size, shape, and texture; some might be wet or moss-covered or bursting with offshoots. And yet birds can land on just about any of them.

“It’s not easy to mimic how birds fly and perch,” says William Roderick, lead author of a new paper on the bird robot in Science Robotics.

“After millions of years of evolution, they make takeoff and landing look so easy, even among all of the complexity and variability of the tree branches you would find in a forest.”

Years of study on animal-inspired robots in the labs of Mark Cutkosky and David Lentink at Stanford University, where Roderick was a graduate student, enabled the researchers to build their own perching robot.

When attached to a quadcopter drone, their “stereotyped nature-inspired aerial grasper,” or SNAG, forms a robot that can fly around, catch and carry objects, and perch on various surfaces. Showing the potential versatility of this work, the researchers used it to compare different types of bird toe arrangements and to measure microclimates in a remote Oregon forest.

How the bird robot works

In the researchers’ previous studies of parrotlets, the second smallest parrot species, the diminutive birds flew back and forth between special perches while being recorded by five high-speed cameras. The perches—representing a variety of sizes and materials, including wood, foam, sandpaper, and Teflon—also contained sensors that captured the physical forces associated with the birds’ landings, perching, and takeoff.

“What surprised us was that they did the same aerial maneuvers, no matter what surfaces they were landing on,” says Roderick. “They let the feet handle the variability and complexity of the surface texture itself.” This formulaic behavior seen in every bird landing is why the “S” in SNAG stands for “stereotyped.”

Just like the parrotlets, SNAG approaches every landing in the same way. But, in order to account for the size of the quadcopter, SNAG is based on the legs of a peregrine falcon. In place of bones, it has a 3D-printed structure—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 and another to handle grasping. Inspired by the way tendons route around the ankle in birds, a similar mechanism in the robot’s leg absorbs landing impact energy and passively converts it into grasping force.

The result is that the robot has an especially strong and high-speed clutch that can be triggered to close in 20 milliseconds. Once wrapped around a branch, SNAG’s ankles lock and an accelerometer on the right foot reports that the robot has landed and triggers a balancing algorithm to stabilize it.

During COVID-19, Roderick moved equipment, including a 3D printer, from Lentink’s lab to rural Oregon where he set up a basement lab for controlled testing.

There, he sent SNAG along a rail system that launched the robot at different surfaces, at predefined speeds and orientations, to see how it performed in various scenarios. With SNAG held in place, Roderick also confirmed the robot’s ability to catch objects thrown by hand, including a prey dummy, a corn hole bean bag, and a tennis ball. Lastly, Roderick and SNAG ventured into the nearby forest for some trial runs in the real world.

Overall, SNAG performed so well that next steps in development would likely focus on what happens before landing, such as improving the robot’s situational awareness and flight control.

Tools to study the natural world

There are countless possible applications for this robot, including search and rescue and wildfire monitoring; it can also be attached to technologies 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—anisodactyl, which has three toes in front and one in back, like a peregrine falcon, and zygodactyl, which has two toes in front and two in back, like a parrotlet.

They found, to their surprise, that there was very little performance difference between the two. For Roderick, whose parents are both biologists, one of the most exciting possible applications for SNAG is in environmental research. To that end, the researchers also attached a temperature and humidity sensor to the robot, which Roderick used to record the microclimate in Oregon.

“Part of the underlying motivation of this work was to create tools that we can use to study the natural world,” says Roderick. “If we could have a robot that could act like a bird, that could unlock completely new ways of studying the environment.”

Lentink, co-chair of the biomimetics group and associate professor of science and engineering at the University of Groningen in the Netherlands, as well as senior author of the paper, commends Roderick’s persistence in what proved to be a years-long project. “It was really Will talking with several ecologists at Berkeley six years ago and then writing his NSF Fellowship on perching aerial robots for environmental monitoring that launched this research,” Lentink says.

“Will’s research has proven to be timely because there now is a 10 million dollar XPRIZE for this challenge to monitor biodiversity in rainforests.”

The Air Force Office of Scientific Research and the National Science Foundation funded the work.

Source: Stanford University

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How birds perch could lead to nimbler flying robots

The way birds can successfully perch on the Teflon and other materials is teaching researchers how they might create flying robots that land like a bird.

Under the watchful eyes of five high-speed cameras, a small, pale-blue bird named Gary waits for the signal to fly. Diana Chin, a graduate student at Stanford University and Gary’s trainer, points her finger to a perch about 20 inches away. The catch here is that the perch is covered in Teflon, making it seemingly impossible to stably grasp.

Gary’s successful touchdown could help researcher build better flying robots.

“Modern aerial robots usually need either a runway or a flat surface for easy takeoff and landing. For a bird, almost everywhere is a potential landing spot, even in cities,” says Chin, part of the lab of David Lentink, assistant professor of mechanical engineering. “We really wanted to understand how they accomplish that and the dynamics and forces that are involved.”

Stick the landing

Even the most advanced robots come nowhere near the grasping ability of animals when dealing with objects of varying shapes, sizes, and textures. So, the researchers gathered data about how Gary and two other birds land on different kinds of surfaces, including a variety of natural perches and artificial perches covered in foam, sandpaper, and Teflon.

“This is not unlike asking an Olympic gymnast to land on Teflon-covered high bars without chalking their hands,” says Lentink, senior author of the paper in eLife. Yet, the parrotlets made what seems almost impossible for a human look effortless.

The findings also include detailed studies of how the birds’ claws and feet produce friction. From this work, the researchers found that the secret to the parrotlet’s perching versatility is in the grip.

“When we look at a person running, a squirrel jumping, or a bird flying, it is clear that we have a long way to go before our technology can reach the complex potential of these animals, both in terms of efficiency and controlled athleticism,” says William Roderick, a graduate student in mechanical engineering in the Lentink lab and in the lab of Mark Cutkosky, chair in the School of Engineering.

“Through studying natural systems that have evolved over millions of years, we can make tremendous strides toward constructing systems with unprecedented capabilities.”

It’s all in the grip

The perches researchers used in the study aren’t your average pet store stock. The researchers split them in two, lengthwise, at the point that approximately aligned with the center of a parrotlet’s foot. As far as the bird was concerned, the perches felt like a single branch but each half sat atop its own 6-axis force/torque sensor.

This meant the researchers could capture the total forces the bird put on the perch in many directions and how those forces differed between the halves—which indicated how hard the birds were squeezing.

After the birds flapped to all nine force-sensing perches of assorted size, softness, and slipperiness, the group began analyzing the first stages of landing. Comparing different perch surfaces, they expected to see differences in how the birds approached the perch and the force with which they landed, but that’s not what they found.

“When we first processed all of our data on approach speed and the forces when the bird was landing, we didn’t see any obvious differences,” Chin says. “But then we started to look into kinematics of the feet and claws—the details of how they moved those—and discovered they adapt them to stick the landing.”

The extent to which the birds wrapped their toes and curled their claws varied depending on what they encountered upon landing. On rough or squishy surfaces—such as the medium-size foam, sandpaper, and rough wood perches—their feet could generate high squeeze forces with little help from their claws.

On perches that were hardest to grasp—the floss-silk wood, Teflon, and large birch—the birds curled their claws more, dragging them along the perch surface until they had secure footing.

This variable grip suggests that, when building robots to land on a variety of surfaces, researchers could separate the control of approaching landing from the actions required for a successful touchdown.

Their measurements also showed that the birds can reposition their claws from one graspable bump or pit to another in a mere 1 to 2 milliseconds. (For comparison, it takes a human about 100 to 400 milliseconds to blink.)

Flying robots taking off

The researchers have already begun characterizing how parrotlets take off from the different surfaces. Combined with their previous work exploring how parrotlets navigate their environment, the group hopes the findings can lead to more nimble flying robots.

“If we can apply all that we learn, we can develop bimodal robots that can transition to and from the air in a wide range of different environments and increase the versatility of aerial robots that we have today,” Chin says.

Toward that end, Roderick is working on designing the mechanisms that would mimic the birds’ gripping form and physics.

“One application of this work that I’m interested in is having perching robots that can act as a team of tiny little scientists that make recordings, autonomously, for field research in forests or jungles,” he says.

“I really enjoy drawing from the fundamentals of engineering and applying them to new fields to push the limits of what has been previously achieved and what is known.”

The National Science Foundation, the Air Force Office of Scientific Research, the Department of Mechanical Engineering at Stanford, and the Department of Defense funded the work.

Source: Stanford University

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