Bitbeam is a 3d printable, LEGO Technic-compatible construction system. More info at http://bitbeam.org Use this shaft encoder wheel to measure the rotation of a wheel. (You'll also need an LED and a phototransistor.) Shaft encoders are especially useful when using a DC motor for motion. DC motors are cheap and wonderful for making little robots move, but DC motors do not come with a built-in feedback mechanism for precision control. Shaft encoders are a great way to get that feedback. For an explanation on how to use shaft encoders in your projects, I recommend Fred Martin's book "Robotic Explorations". In the download, I've included both a printable STL and the original OpenSCAD source code. If you use this with infrared LEDs and phototransistors, be sure to print this in black or other dark color. Through trial and error, I noticed the encoder sensing did not work when the wheel was printed in natural (light-colored) ABS.