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In 2008 I have built a simple balancing robot, mostly a copy from a balancing robot made using a Basic Stamp 2 SX, found on the Parallax Forum, that I have adapted to work on Arduino. The robot uses servos modified for continuous rotation and a Ping))) sensor for balancing. A potentiometer was added to adjust the balancing point, because it depends greatly on the level of the surface. Also I installed a serial LCD to display the distance and the PID values. I wrote a tutorial on Society Of Robots ...
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/robots/show/balancing-roboduino