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Because wheeled units are comparatively simple to design, they are used as test-beds by those who develop robots. Students and fac- ulty in Carnegie Mellon Universitys Robotics Institute, for instance, have gotten used to seeing wheeled cylindrical robots rolling through their building, testing the robotsability to map the environment and navigate through it.

Also for the sake of simplicity, present-day commercially available robotsother than those that simulate four-legged petsroll on wheels. At least one, the vacuum-cleaning robot called Roomba, placed on the market in 2002, is a consumer item. It is made by the iRobot Corporation whose co-founder, chairman, and chief technol- ogy officer is Rodney Brooks, the MIT roboticist responsible for Genghis and Cog. Roomba is a 6-pound, disc-shaped unit, slightly more than a foot across, that rides on two wheels and an agitator brush. Unlike conventional vacuum cleaners that draw their electrical power from a cord plugged into a wall socket, Roomba operates from rechargeable batteries and hence has complete freedom of motion.

Put Roomba down in the center of a room, and it begins to cover the floor in a pattern of broad spiral sweeps (which give Roomba its name) until the robot encounters a wall.Then its behavior changes to track along the wall and clean the wall base with side brushes, until it makes another foray across the room. If Roomba encounters an ob- stacle such as a chair leg, it stops and goes off in a new direction, and it is also intelligent enough to halt and turn around at the top of a flight of stairs. Eventually the robot cleans the entire area, and then it stops, ready to be placed in another room or recharged.While the robot has sensors and a level of intelligence called heuristic learning logicthat was first developed for units used to clear minefields, the fact that it does not need to maintain balance as it rolls simplifies its design and lowers its price.

The iRobot company has also developed tracked robots in the form of small units called PackBots.These were designed to function in difficult environments, and so were well suited to enter the wreck- age of the World Trade Center after the terrorist attack of September 11, 2001. Equipped with video cameras, they sent back images as they

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