Image: PETER DILWORTH WALKING ROBOTS such as Troody (above) are harder to create than flying machines. This one took its first steps last spring.

To the high-pitched hissing sound of electric servos, Troody slowly tilts forward. Fourteen tiny motors in her limbs begin to hum as her shins lift up from her feet. She stretches her legs and rises to an upright position. Still wobbling back and forth a bit, she takes her first timid step forward, then another¿and then she is walking.

That scene played out for the first time this past spring, when Troody¿a 10-pound, two-legged robotic replica of a Troodon dinosaur from the Cretaceous period¿took her maiden stroll through Peter Dilworth's lab. As scientists continue to argue over whether it will ever be possible to clone a dinosaur ¿ la Jurassic Park, Dilworth¿a research scientist at the Massachusetts Institute of Technology's Leg Lab, a part of M.I.T.'s Artificial Intelligence Laboratory¿simply built one himself. He chose to model a Troodon for its small size and big brain: "The ratio of its brain size to its body weight makes it the brainiest of the dinosaurs," Dilworth says, "so I thought that was kind of interesting."

His objective was straightforward: "It was for the robot to stand up from a sitting position, to walk and possibly run, go back to walking and stop and then sit down and be stable, and not fall over in any phase," Dilworth explains. But as he found out, making a robot walk is a lot more challenging than it seems. "It has proven extremely difficult to make bipedal robots," says Gregory Paul, an independent dinosaur paleontologist who collaborated with Dilworth on the project. "People assume that flight is hard to do while walking is easy; actually it's the reverse."

With one look at Troody's smooth movements, however, it is clear that the researchers have succeeded. "A lot of people comment on how it [Troody] looks very biological," Dilworth says. "That's one thing that I strive for, and it's a result of the way we do the control system in the robot, a technique which uses springs in series with the motors, which softens the way the robot feels." Indeed, Troody the Troodon, like all other robots at the Leg Lab, uses so-called electric series-elastic actuators, devices invented by Lab director Gill Pratt and one-time graduate student Matthew Williamson.

Before the development of these electric series-elastic actuators, researchers relied on hydraulic actuators to create walking robots (see sidebar). In comparison, the electric actuators are lighter and require far less power, making it possible to design more self-contained machines. But they, too, have their drawbacks¿including low torque and thus a need to run very fast to deliver high power. That in turn creates a need for gears to allow the robot to move slowly. The gears, however, lower the robot's tolerance for physical shocks¿which isn't a good thing for a machine that's supposed to walk.

Pratt's team solved the problem by adding a spring between each of a robot's joints and respective gears, thus buffering the blow. The springs also became a vital part of the robots' control systems. Traditionally, most robots had control systems that only determined the position of their arms or legs in relation to their environment. "You couldn't really tell if you hit the wall if the only thing you could measure was the position of the arm," Dilworth explains. "If your position is off by a little bit, you might be stopped a little before the wall, or, even worse, you might be stopped a little inside the wall and you'd have these massive torques on your actuator, trying to position the arm inside the wall."

Image: PETER DILWORTH LIKE A REAL TROODON, Troody the robo-dino rises from a sitting position, stretches her legs and then begins to walk.