Inverted pendulum model

A normal pendulum hangs down, like a clock weight, and settles on its own. An inverted pendulum stands upright and always tips unless something keeps correcting it.

Engineers treat a two-legged robot this way: the body is the weight, the leg is the stick, the foot is the bottom end. The model is simple enough for the robot’s computer to work with very quickly when it is pushed.

The trade-off is that it leaves things out. A common version assumes the body stays at one height, so a real robot that crouches or twists behaves a bit differently from the model.