Your normal, everyday quadrotor drone is able to maintain stable flight through two methods: opposing rotor rotation direction, and speed control. The motors opposite each other spin in opposite directions, which keeps the drone from going into a hard spin. The speed control keeps the drone level by adjusting the RPM of each rotor, and changing that speed on a particular side allows for directional control. In his newest YouTube video, Tom Stanton takes a completely different approach and attempts to control a drone with reaction wheels.
This is a very physics-heavy video, so dust off your learning caps and strap in for some math. The most important concepts here are momentum — specifically angular momentum — and the moment of inertia of a spinning disc. Angular momentum is the same principle behind an inverted pendulum, and is what keeps a Segway upright. You can control the angular position of mass on a pivot by the torque created by spinning a weighted reaction wheel in the opposite direction. Do that with precision, and you can keep the mass balanced.
For this project, Stanton has taken that idea and applied it to a drone that only has two rotors. One rotor is mounted on top of the body, and the other is below. They turn in opposite directions the yaw, so that the drone doesn’t just spin like a top. As you’d expect, that is an incredibly unstable design, as there is nothing to keep the drone from tipping and crashing — that’s where the reaction wheels come in.
The reaction wheels are mounted on two sides of the 3D-printed drone in order to control the roll and pitch. If it starts tipping left or right, one wheel spins to correct the roll. If it starts tipping forward or back, the other wheel spins to correct the pitch. As Stanton demonstrates, it works — sort of. He was only able to keep the drone airborne for about ten seconds before the reaction wheels could no longer keep up. But, it’s a great proof of concept, and could potentially provide stable flight with a refined design and a lot of PID tuning.