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All my projects & progress are available at marginallyclever.com. This is my mural drawing art robot. The principle is simple: if you know the distance between the motors and the distance down to the pen you can find the string length with pythagorean theorem. We can do that anywhere on the wall, which means we can find how much to change the strings to move from A to B. Once I had it moving in straight lines I added GCODE support. I didn't want to type 10,000 gcode commands ...
This is a companion discussion topic for the original entry at https://community.robotshop.com/index.php/robots/show/makelangelo-polargraph-art-robot