simple drawing arm

this is my first try to build a drawing machine.it is a very simple 3dof arm,including 3 standard servos(one for pen up/down) and a arduino uno.software is arduino&processing.i try to use it decode some mathematical curve,so much fun.the primary image is archimedes spiral.the below is descartes to princess's letter: r=a(1-sinθ).the machine drawing three times,you can see some offset at the same point because the servo issues.