IF she could buy a pair of legs for $5,000, Una-May O'Reilly would probably do it. No doubt a nice set of working limbs would help Cardea, the humanoid robot she is building at the Massachusetts Institute of Technology, look the part.
But robotic legs are time-consuming and very expensive to create. It took Honda 16 years to produce its current prototype of Asimo, a 4-foot-tall $1 million robot that can negotiate stairs.
"Mobility is really complicated in a humanoid," said Dr. O'Reilly, a research scientist and a principal investigator on the Cardea project, as she lifted her own leg in front of her and tried to balance on one foot. "It becomes an endeavor unto itself."
So when she and Rodney A. Brooks, director of the M.I.T. computer science and artificial intelligence laboratory, were offered a Segway Human Transporter to serve as Cardea's mode of mobility, they took it. Cardea, named for the Roman goddess of thresholds and hinges, can now roll up to a door, recognize it and open it with its robotic arm.
It turns out that the Segway Human Transporter, the two-wheeled, motorized, super-balanced scooter designed to whisk people around an urban landscape, may have a second career as a humanoid transporter.
With a footprint about two feet wide, a Segway can navigate through doorways and turn on the spot in hallways. It can go up hills, over bumps and carry hundreds of pounds.
Most important to the M.I.T. researchers, the Segway's gyroscopic balance system allows Cardea to stand as tall as Dr. O'Reilly without the need for an awkwardly wide base to insure stability. M.I.T.'s goal is to create a robot that can act as a kind of personal assistant, and Cardea's height and maneuverability should help it interact with humans at their level.
The Defense Advanced Research Projects Agency, or Darpa, a Pentagon agency, purchased 15 modified Segways this summer for robotics labs at M.I.T., Carnegie Mellon, Georgia Tech, Stanford and nine other universities and research centers as part of its broader Mobile Autonomous Robot Software project, aimed at creating robots that can assist military operations without constant human supervision.
"We've had really, really good results with this platform," said Sebastian Thrun, an associate professor of computer science at Stanford in Palo Alto, Calif. Most of the wheeled robots that Mr. Thrun works with have to stay low to the ground, which limits their ability to survey their surroundings. With a taller robot, he can place sensors and cameras in a higher position.
"It allows us to build relatively complex robots with relatively small footprints that can still see very nicely," he said, adding that the Segway's price tag, about $5,000, makes it an economical choice.
Two days after receiving the platform this summer, the Stanford team had its Segway-robot hybrid navigating between boxes in the hallways. Within a month, the "Segbot" was roaming the campus, using a laser range finder to survey the area and create a three-dimensional map of the grounds.
At Carnegie Mellon in Pittsburgh, where researchers have taught smaller robots to play soccer, researchers hope to team their Segway robot with Segway-riding humans for a soccer demonstration this summer with the goal of studying their interaction.
The Segway Robotic Mobility Platform, as it is called, has its quirks. James F. Keller, a project engineer coordinating the Segway research at the University of Pennsylvania, said the platform performed well when tested on varying terrain and could easily travel a country road or go anywhere a wheelchair-bound person might go. But, he added, "it is somewhat prone to falling on its face if it has a problem."
In September Segway recalled its machines because they could lose their balance when their batteries run low, a problem the company is fixing by upgrading the software. But the Segway robots used in the study took most of their tumbles when turning at too high a speed (the Segway can reach about 10 mile per hour), catching air on a bump or accelerating on sandy and slippery roads. At least one team reported a crash that destroyed the hard drive in the onboard laptop.
John Morrell, chief development officer at Segway, said the scooter was a challenge for researchers accustomed to robots with a lower center of gravity.
"A dynamically stabilized robot can fall down," Mr. Morrell said. "To some extent, that's a new problem to think about."
Indeed, most of the teams, concerned for the delicate equipment they entrust to the platform, were quick to draw up plans for kickstands, bumpers and even air bags that deploy when the robot is in danger of falling or crashing.
Darpa approached Segway with the platform idea in February; it took a few months for the company to develop a modification kit for its commercial product. Unlike the Segway for humans, which the rider directs by leaning forward or backward, the robotic platform has its movements controlled by a computer program. The handle bars are tucked out of the way, and there is a shelf that holds sensors, cameras, robotic arms and anything else the researchers devise.
As a company, Segway has not hung out its shingle as a robo-parts supplier, but Mr. Morrell said he has gotten calls from robotics labs interested in obtaining their own Segway platforms. Any new sales are bound to be welcome because Segway has sold only 6,000 or so of its human transporters.
Meanwhile, M.I.T. has a long way to go before Cardea can act as a true personal assistant. The robot spends much of its time in pieces as 11 researchers work on its various systems. (Facing the first deadline to show Darpa a functioning robot, the M.I.T. team rushed a bit and now has to clean up some of its mess.)
Cardea's one arm ends in a stump, and its head is a faceless camera. But there are plans to add more sensors, an improved vision system, two more arms and an expressive head. Eventually, researchers hope Cardea will be able to take instructions and cues from the people it encounters, and learn about its environment by handling objects. Earlier M.I.T. robots did somewhat similar things, with one difference - they couldn't scoot around and explore on their own.
"Our humanoid robots had never moved," Dr. O'Reilly said.