Parrot-inspired robot flies, grabs, and perches

The ability of birds to grasp onto branches of almost any size, shape, and texture should not be taken for granted. It was this ability that fascinated teams of Stanford University engineers who had previously developed technologies inspired by animal abilities.

“It’s not easy to mimic how birds fly and perch,” said Dr William Roderick, who was a PhD student at Stanford. “After millions of years of evolution, they make take-off and landing look so easy, even among all of the complexity and variability of the tree branches you would find in a forest.”

However, years of study on biomimetic robots in Stanford labs has enabled the construction of a perching robot. When attached to a quadcopter drone, this “stereotyped nature-inspired aerial grasper” (SNAG) forms a flying, catching and carrying, perching robot. The researchers used their bird-like robot to compare bird toe arrangements and to measure microclimates in a remote Oregon forest.

In previous studies of parrotlets – one of the most Lilliputian members of the parrot family – the researchers recorded the birds flying back and forth between special sensor-loaded perches with high-speed cameras. This allowed them to explore the forces associated with the birds’ landing, perching, and take-off on perches representing a range of sizes and materials. They found that the parrotlets performed the same aerial manoeuvres regardless of surface, letting their feet handle the variability of the surface texture.

Just like those parrotlets, SNAG approaches every landing in the same manner. However, to account for the size of the quadcopter, SNAG is based not on the legs of a parrotlet but a peregrine falcon. Instead of hollow bones, it has a 3D printed structure with motors and fishing line to stand in for muscles and tendons. Each leg has a motor for moving back and forth, and another motor for grasping. Inspired by the way tendons route around the ankle in birds, a similar mechanism in SNAG’s leg absorbs landing impact energy and passively converts it into grasping force.

The result is an exceptionally strong, high-speed clutch. Once wrapped around a branch, SNAG’s “ankles” lock and an accelerometer attached to the right foot reports that the robot has landed, triggering a balancing algorithm to stabilise it.

During the heights of the Covid-19 pandemic, Roderick shifted equipment – including a 3D printer – from one of the Stanford labs to rural Oregon, to set up a basement lab for controlled testing.

SNAG was sent along a rail system that launched the robot at different surfaces at set speeds and orientations, to test how it performed under various scenarios. With SNAG held in place, Roderick also confirmed its ability to catch hand-thrown objects such as dummy prey, a bean bag, and a tennis bag. Finally, he brought the bird-like robot into a nearby forest for trial flights.

Overall, SNAG performed so well that next steps in development would likely focus on refining pre-landing processes, such as improving the robot’s situational awareness and flight control. There are many possible applications for perching robots, such as in search and rescue operations and wildfire monitoring.

Roderick and his colleagues additionally explored bird biology using SNAG, testing it with two toe arrangements: anisodactyl (three toes in front, one at the back, like a falcon) and zygodactyl (two toes in front, two at the back, like a parrot). They found surprisingly that little performance difference between the two. Finally, they used SNAG for environmental research, attaching a temperature and humidity sensor and using it to record the microclimate in the Oregon forest.

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