Stanford Scientists Observe Birds to Refine How Robots Touchdown

Birds are capable of perching on a wide range of surfaces, thick or thin, slick or rough. But will they be able to find stable footing if a branch is covered in Teflon? Keen to build better robots, Stanford researchers found out.

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

Gary’s successful landing on the Teflon—and on other perches of different materials—is offering scientists insight into how they might build machines that touchdown like a bird.

“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,” said Chin, who is 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.”

Even the most advanced robots do not come anywhere close to the grasping ability of animals when handling objects of varying sizes, shapes, and textures. So, the scientists collected data about how Gary and two other birds touchdown on various kinds of surfaces, including a range of natural and artificial perches covered in sandpaper, foam, and Teflon.

“This is not unlike asking an Olympic gymnast to land on Teflon-covered high bars without chalking their hands,” said Lentink, who is senior author of the paper. Yet, the parrotlets made what appears nearly impossible for a human look easy.

The team’s study, published August 6^th in eLife, also contained thorough studies of the friction produced by the birds’ feet and claws. From this work, the scientists learned that the secret to the parrotlet’s perching adaptability 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,” said William Roderick, a graduate student in mechanical engineering in the Lentink lab and lab of Mark Cutkosky, the Fletcher Jones 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.”

(Non)sticking the landing

The perches in this study were not the average pet store stock. The scientists divided them in two, lengthwise, at the point that approximately lined up 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 on top of its own 6-axis force/torque sensor. This meant the scientists could measure the entire forces the bird put on the perch in many directions and how those forces varied between the halves—which showed how hard the birds were squeezing.

After the birds flew to all nine force-sensing perches of various softness, size, and slipperiness, the team started examining the first stages of landing. Comparing various perch surfaces, they expected to see variances in how the birds moved toward the perch and the force with which they touched down, but that is not what they discovered.

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. 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.

Diana Chin, Gary’s Trainer and Graduate Student, Stanford University

The degree to which the birds wrapped their toes and curled their claws differed based on what they came across upon landing. On squishy or rough surfaces—such as the sandpaper, medium-size foam, and rough wood perches—their feet could produce high squeeze forces with minimal help from their claws. On perches that were toughest to grasp—Teflon, floss-silk wood, and large birch—the claws of the birds curled more, and they dragged them along the perch surface until they had safe footing.

This variable grip indicates that, when designing robots to land on different surfaces, scientists could separate the control of approaching touchdown from the actions necessary for a successful landing.

Their measurements also revealed that the birds can reposition their claws from one graspable pit or bump to another in just 1 to 2 milliseconds. (For comparison, a human requires about 100 to 400 milliseconds to bat an eyelid.)

Birds and bots

The Cutkosky and Lentink labs have already started characterizing how parrotlets liftoff from different surfaces. Coupled with their earlier work investigating how parrotlets steer through their environment, the team hopes the findings can pave the way to more agile 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.

Diana Chin, Gary’s Trainer and Graduate Student, Stanford University

Toward that end, Roderick is involved in engineering the mechanisms that would imitate 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. 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.

William Roderick, Graduate Student in Mechanical Engineering, Lentink lab, Stanford University

(Credit: Stanford University)