Servo - December 2014

18 SERVO 12.2014

With an eye toward making better running robots, researchers have made surprising new findings about some of nature's most energy-efficient bipeds: running birds.

Although birds are designed primarily for flight, scientists have learned that species that predominately live on land and scurry around on the ground are also some of the most sophisticated runners of any two-legged land animals. These characteristics may have been evolving since the time of the dinosaurs and, some would say transcend the ability of other bipedal runners, including humans.

In a study published in the Journal of Experimental Biology, researchers from Oregon State University, the Royal Veterinary College, and other institutions outline how running birds have achieved an impressive ability to run while minimizing energy cost, avoiding falls or injury, and maintaining speed and direction.

"Birds appear to be the best of bipedal terrestrial runners, with a speed and agility that may trace back 230 million years to their dinosaur ancestors," said Jonathan Hurst, an associate professor and robotics expert at the OSU College of Engineering.

Running birds come in an enormous range of sizes, from tiny quails to an ostrich that has 500 times as much body mass. Most — but not all — can fly, but spend a lot of their lives on the ground, and they don't always look that graceful when they run. However, researchers found that they maximize the results while keeping their priorities straight: Save energy and don't break a leg.

In the wild, an injury could lead to predation and death, and in like fashion, when food resources are limited economy of motion is essential.

"These animals don't care that they appear a little unstable or have a waver in their gait," Hurst said."Their real goal is to limit peak forces, avoid falling, be safe, and be as efficient as possible. If their upper body seems to lurch around a little as a result, that's okay. What they are accomplishing is really quite elegant."

Even more surprisingly, a wide variety of ground-running bird species with very different body sizes use essentially the same strategy to accomplish these sometimes conflicting tasks. In order to hop over obstacles on uneven ground, they use a motion that's about 70 percent a "vaulting" movement as they approach the obstacle, and 30 percent a more crouched posture while on top of the obstacle.

"Evolution has shaped running birds into all different sizes and skeletal structures," said Christian Hubicki, a doctoral student at Oregon State who co-authored the study."But we found their behavior in how they run is essentially the same."

In collaboration with Monica Daley at the Royal Veterinary College in London, the researchers studied five species of birds and developed a computer model in OSU's Dynamic Robotics Laboratory that closely matches that behavior.

"We should ultimately be able to encode this understanding into legged robots, so the robots can run with more speed and agility in rugged terrain," Hubicki said. "These insights may also help us understand the walking and running behaviors of all the common ancestors involved, including theropod dinosaurs such as the velociraptor."

The researchers began the study with a hypothesis that body stability would be a priority since it might help avoid falls and leg injuries. That's not what they found, however.

Instead, running birds have a different definition of stability. They do avoid falls, but also allow their upper bodies to bounce around some, just so long as they don't tumble.