What does it do?
Knowing the length of each string and the distance between the servos gives us everything we need to use trigonometry to find the XY coordinates of the pen. We can also do the reverse, which means we can find how much to change the strings to move from A to B. From there I added gcode support. Lastly I wrote some Java apps to create a nice GUI. If your arduino comes with the firmware already loaded then you don't even need to install the Arduino app. :)
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[original story: Let's Make Robots]