The hardest part of any hanging-pen drawing machine is getting a straight line to actually come out straight, and that is exactly where this build got interesting. The maker first converted x and y coordinates into the two belt lengths and interpolated between them in a straight line, which produced heavy distortion. The fix was to chop every line into 1mm segments and interpolate the belt lengths between those tiny steps, which cut the warping down dramatically. That kind of stubborn, incremental debugging is what makes this project worth following.
The machine itself is a polargraph: it hangs from two cords fixed near the upper corners of a wall, and a pair of motors winds and releases them to carry the pen anywhere on the surface. Inside there is an ESP32, two stepper motors with drivers, and a buck converter, all packed into a 3D-printed shell in translucent grey with a central pen mount. Assembly apparently needed nothing more than a screwdriver and tweezers to seat the parts in the frame, and the whole thing comes in under $100.
The results look genuinely good. It takes SVG files as input and has produced portraits, an owl, a comic strip and a cartoon character, including one portrait in purple ink on a pale wall. The maker recommends washable markers so the art can be wiped off, and imagines stencilling or even large-scale embroidery on fabric stretched across a wall.
One problem remains open. The pen holder is not a rigid body, so modeling its position from the two belt lengths is unsolved, and sagging cords follow catenary curves that likely make the boundary rectangle taper toward the bottom, with edge distortion worst up top where the cords pull tightest. The current plan is a fit-based correction: draw a grid of known size, overlay the measured result, and compute a de-warp matrix, possibly dropping the tilt compensation code and treating the warp as a trapezoid. It will be interesting to see whether existing CNC firmware already has an answer.
Project by Nik Ivanov. Watch it on their YouTube channel: