A three-legged robot with an unconventional and graceful walk has been developed by US researchers. Like humans, it exploits gravity to save energy with each step, but it also flips its entire body upside-down with each stride.
The fearsome alien tripods in HG Well's book The War of The Worlds are described as moving like "a milking stool tilted and bowled violently along the ground".
STriDER (Self-excited Tripedal Dynamic Experimental Robot) has a far more graceful, and acrobatic, gait (see video top right) that sees its body flip 180 degrees with each step. A second video (bottom right) explains more about the project and shows the initial prototype.
To take a step forwards, the robot shifts its weight onto two of its legs, allowing itself to fall forwards away from the third leg. Its body then flips upside-down and the third leg swings up between the other two just in time to catch the ground and return STriDER to a stable tripod stance. To change direction, the robot simply switches its choice of swinging leg.
Although STriDER's body shape is like nothing in nature, its gait is meant to closely mimic the way biological organisms walk. The aim is to minimise the complexity of controlling each limb, and to reduce overall energy use. The first prototype stands 1.8 metres tall, while the latest incarnation is slightly shorter, at 0.9 metres.
"STriDER's gait is closer to that of a human walking than most bipedal humanoid robots you see today," explains Dennis Hong at Virginia Tech, US, who leads the project. "This is how we humans walk, we do not actively control our knees, we just let them swing."
The robot is not designed to be a load carrier, Hong explains. "Its purpose is for deploying sensors to difficult-to-access areas," he told New Scientist. "Since STriDER is tall by nature, it is great for deploying cameras."
The most successful autonomous robots often resemble remote-control cars, he points out, and cannot see above obstacles.
"I've never heard of a robot with a three-legged design before, but I can see the advantage," says Dave Barnes, who works on locomotion for planetary rovers at Aberystwyth University, UK. "It's like a biped with a walking stick."
Although bipedal humans and robots can move quickly, they have to expend energy just to keep upright when they are not moving, he points out: "A tripod stance is very stable, you can just lock the joints."