Scientists have developed a robot with legs inspired by the structure and gait of birds, which allows them to use fewer motors and consume less energy than designs inspired by human legs.
The next time a seagull, pigeon or goose walks by, take a close look at the movement of their legs. You will see that their knees bend in the opposite direction of ours, with the lower leg swinging forward on each step and the foot pointing down and back when it is in the air. When you lift your leg to walk, your lower leg swings to the back while your foot remains pointed ahead.
This is a major structural difference, of course, and we’ve designed our world around it. Imagine what our furniture would look like if our knees bent in the opposite direction? When you sit in a chair, your legs would point straight out in front of you. The chair would have to be more like a broad stool so you could fold your bird legs under your body to sit like a duck.
There have been many bipedal robots modelled on human legs. They require a complex system of motors and gears, and sophisticated computers to handle the motion of all the joints in a coordinated way. Toyota’s cutting edge T-HR3 is a good example.
But Birdbot, designed by researchers at the Max Planck Institute and the University of California at Irvine, uses fewer motors and much less energy by taking advantage of the mechanical action of the bird leg structure.
The secret to the bird’s efficiency lies in a series of tendons and muscles that connect all the way from the hips down through the joints to the foot, acting like long springs when the leg is bent under load. Once stretched, they then give that energy back as the leg straightens to move forward. This mechanical spring action reduces the amount of work for the muscles.
Tendons also link the movement of the foot and leg mechanically, which also helps efficiency. Bird ankles bend automatically during a stride, because of the way they are connected to the rest of the leg, which means they can do without a set of muscles to bend the ankle.
VIDEO — Birdbot from the Max Planck Institute for Intelligent Systems
Birdbot was constructed with cables and pulleys in place of tendons that go around the outside of the joints from the hip joint all the way down to the foot.
Only two motors are required, one at the hip to keep the leg swinging and one at the knee to lift the leg. The rest of the action is purely mechanical and doesn’t require sophisticated computers to control and coordinate movements at each joint, as in many previous walking robots.
Walking robots have an important advantage over machines that travel with wheels or treads: they can handle rough terrain, and navigate a world built for walking humans better. Think about walking down a staircase holding a cup of coffee. Your legs compensate for the rough surface so you don’t spill the liquid, but if you were riding down those same stairs on a bicycle you’d need excellent suspension or there would be coffee everywhere.
Engineers are constantly looking to nature for inspiration, and birds can do some amazing things, even when they don’t fly. Flightless birds like the ostrich can run up to 70 km/h over rough terrain.
They’re also the descendents of a long lineage of bipedal dinosaurs. The tyrannosaurus rex, for example, had the same leg structure as ostriches, proving that this design can carry large loads and has stood the test of time. The designers claim robots of this design could carry loads weighing many tonnes.
Bird legs have had more than 100 million years of evolution to develop efficient walking. Four-legged mammals like dogs, cats, horses, cows also have the bird-like structure on the hind legs. And they can all run much faster than humans. Clearly the creators at Lucasfilm realized this when they imagined the two-legged Imperial walkers — nicknamed “chicken walkers” — that stomped through the forest shooting at rebels in The Empire Strikes Back.
In a way, we humans are sort of the oddballs in the bipedal world, with our rearward-swinging lower legs. Science fiction usually shows robots that are designed like humans, but in reality, future walking machines could look more like birds.