Dig it -- the Telerobotics department of NASA has started running a weekly series called "Cool Robot of the Week." Every seven days, they find a new robot and point to the project. This week it's David Anderson's two-wheeled, balancing "nBot":

The basic idea is pretty simple: drive the wheels in the direction that the upper part of the robot is falling. If the wheels can be driven in such a way as to stay under the robot's center of gravity, the robot remains balanced. In practice this requires two feedback sensors: a tilt or angle sensor to measure the tilt of the robot with respect to gravity, and wheel encoders to measure the position of the base of the robot. Four terms are sufficient to define the motion and position of this "inverted pendulum" and thereby balance the robot. These are 1) the tilt angle and 2) its first derivative, the angle velocity, and 3) the platform position and 4) its first derivative, the platform velocity. These four measurements are summed and fed back to the platform as a motor voltage, which is proportional to torque, to balance and drive the robot.

Do not fail to check out the videos on this page of the nBot driving around! It makes your heart glad to see a robot roll around on only two wheels.

(Thanks to Boing Boing for finding this one!)