Robots are capable of far more precise and accurate motion than humans are. That is great when you want a robot to assemble a PCB or perform heart surgery. But it’s a problem when you want robots to do something creative — they’re just too perfect. The lack of human-introduced flaws makes robot artwork feel soulless. To overcome that, Georgia Tech graduate students built a graffiti robot called GTGraffiti that paints with the fluidity of a human.

GTGraffiti is a cable-driven robot can cover huge canvasses and even entire walls. Four winch mechanisms connect to the four corners of the robot via cable wires. By increasing or decreasing the length of each cable, the robot can position itself at any XY coordinate on the work surface. The robot end effector that hangs from the cables carries a can of spray paint, which it can actuate on demand. ODrive 56V motors turn the winches and a Teensy development board controls those through ODrive drivers. The spray can actuator utilizes a hobby servo motor controlled by an Arduino Nano board, which receives power from a 10,000mAh USB batter pack.

With that hardware alone, the robot would still act like a robot. In order to give it human-like motion, they turned to an actual human graffiti artist. The used a mocap (motion capture) system to record the artist’s hand movements as he painted a series of basic shapes. In software, the researchers could combine those shapes into more complex art. When the robot paints, it follows the recorded motions for each shape. That results in human-like “brush strokes” that look natural to our eyes and ultimately graffiti murals that feel like real art.

Image credit: Georgia Institute of Technology