The inverted pendulum is a pretty classic dynamics problem and reaction wheels are cool. That’s why we like [Mike Rouleau]’s self-balancing stick.
The video, viewable after the break, was fairly sparse on details, but he furnished some in the comments. The little black box on the top is a GY-521 Gyroscope module. It sends its data to an Arduino attached to the black cord which trails off the screen. The Arduino does its mathemagic and then uses a motor controller to drive the reaction wheels at the correct speeds.
[Mike] mentions that he didn’t do anything too fancy with the dynamic model of the demonstration. He even hand-tuned the PID values rather than resorting to fancier training methods. The Arduino simply runs a bit of code, and, optionally, streams some data back to the computer for visualization.
The stick can stay up until the power goes out, and is a pretty cool demonstration. As some mentioned in the comments; this would make a fetching desk ornament.