A self-balancing scooter
A Segway stands on just two wheels side by side, so on its own it would tip forward or back. It stays up by constantly driving its wheels back under the rider.
An upside-down pendulum
Rider and machine form an inverted pendulum pivoting on the wheel axle. The heavy centre of mass sits high up, so any small tilt makes it fall further, like a broom balanced on your hand.
Sensing the tilt
Gyroscopes and tilt sensors in the platform measure the pitch angle and how fast it is changing, about 100 times every second.
The computer decides
Two control boards compare the tilt with upright and work out how hard to drive each motor. They are duplicated so that one can take over if the other fails.
Lean forward, roll forward
When you lean forward, the motors turn the wheels forward to get back under your centre of mass. Keep leaning and it keeps rolling: that is how you accelerate. Lean back and it slows down or reverses.
Turning with LeanSteer
To turn, you tilt the handlebar sideways. A sensor at its base tells the controllers, which run one wheel faster than the other, so the Segway turns, even on the spot.