This is a very simple robot that uses a simple switch as a sensor and stands on only two wheels with inverted pendulum mechanism.
When the robot is going to fall the motor starts and moves the robot to the direction it is going to fall, so the motor torque about the center of gravity that is higher than the motor makes the robot balanced.
You can find the tutorial of making this robot at http://mcop.netfirms.com/balrobo.html
You can also find the tutorial of making this robot at http://www.instructables.com/id/balancing_robot
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