The Printer robot fully assembled
The Printer robot is a fully functioning ink and paper printer that uses a pen or marker to draw on a sheet of paper. Three motors are used in the drawing. The first motor raises and lowers the pen to the paper. The second motor moves the carriage left and right along the track. The third motor moves the paper in and out of the printer. The second motor draws horizontal lines, the third motor draws vertical lines, and the two motors work in tandem to draw diagonal lines. This enables the creation of virtually any line-based drawing, including text.
For more about the book, read my Geekend post Geek Gifts 2011: The Unofficial LEGO Mindstorms NXT 2.0 Inventor's Guide.