Most balancing robots reflect an inverted pendulum design.  By moving its center of gravity higher the wheels are able to create more effective torque.   This effect can be demonstrated by balancing an upside down broom in your hand then turning the broom over and trying to balance the broom handle side up.   A good example of the inverted pendulum design is the French robot named Joe shown on the right.  Joe is a radio-controlled robot that is particularly adept over non-level terrain.

   Two-wheel balancing robots have a particular advantage over robots with three or more wheels. If the center of gravity gets over either the front or back wheels, the robot risks toppling over end.

    The robot to the left is another good example of an inverted pendulum design.  It is called "Autobender."  The 'auto' stands for autonomous.  It is equipped with a sonar rangefinder and a board mounted CCD chip camera mounted on a pan and tilt motion table.  Again, the weighty equipment is mounted nice and high to better enable the wheels to stay under the center of gravity. 

    Most balancing robots are equipped with solid-state gyroscopes and accelerometers.  These sensor inputs are mounted inline with the robots center of gravity.  The gyroscopes tend to be very accurate in measuring small changes in angular velocity in the short term but tend to drift over time.  The accelerometers are very accurate in determining level over the long run so is repeatedly used to zero the gyro.  This sensor data is fed into the Microchip PIC18F452 controller unit in a feedback loop.

    Not all balancing bots need to have two wheels.  This contraption is called the Loony-Cycle.  It was built as a design project by an English technical boarding school.  It uses a gyroscopic effect to stay upright similar to a coin rolling on edge.  It has two small fans that pushes the wheel from side to side using the camber of the tire to turn automatically.  While the Loony-Cycle does not use a microchip for balance it is a excellent example of creative thinking.