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by human minders, placed so as to catch the robot and keep it from damaging itself if its balancing abilities fail and it topples. Moreover, the walking pace is slow. Most telling of all, H6 and H7 do not swing their arms while walking, as people do, but hold their arms at their sides, bent at the elbow.This makes the robots seem slightly nerdy, as if they were too tightly buttoned up to stride freely down the street.

But even that cautious, somewhat geeky walk, though a far cry from the menacing lurch with extended arms featured in horror mov- ies, is highly significant. Its achievement has required considerable mechanical and computational development. Successful humanoid robots need mechanical frames that are highly flexible, which is de- fined in terms ofdegrees of freedom.Each degree of freedom means the capability to move a limb or other appendage in a given direction about a joint. H7 can move each leg in any of six directions, corre- sponding to six degrees of freedom, plus a seventh that comes from an adjustable toe joint in the foot. There are 23 additional degrees of freedom built into the robots body, and all its joints are moved by electric motors that drive gears.

For H7 to walk in a given direction, its legs must be set correctly and their movements coordinated in space and time.At the same time, the robots body must be constantly adjusted to maintain its balance as it walks. Moreover, the robot cannot be allowed to self-collidethat is, have one moving part strike another, such as the legs becoming entangled. With the robots humanoid shape and many degrees of freedom, there is a multitude of possible bodily configurations, each of which must be examined to ensure safe and successful walking. This is an extremely demanding computational task that must be per- formed in real time as the robot advances through the world. The necessary calculation power is provided by the equivalent of two pow- erful laptop computers built into the robot, with more power coming from other remotely linked computers. Even this computational ar- mory would not be enough without clever algorithms that minimize computation time.

Similar considerations apply to what are probably the best-known walking robots in the world, which have been undergoing develop-

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