revolute joint

A revolute joint is a joint that lets one link spin around a single fixed line, like a door swinging on its hinge. It does just one thing — rotate about one axis — and nothing else: the link cannot slide back and forth or tilt sideways, only turn. Because it allows exactly one independent motion, we say it has one degree of freedom. The single number that describes its state is simply the angle it has turned to, the way a clock hand is fully described by where it points.

Revolute joints are by far the most common joints in robots, for the same reason hinges are everywhere in daily life: rotation is easy to build and easy to power. An electric motor naturally spins, so you can couple one straight onto a revolute joint and the link turns. A typical robot arm is mostly a stack of revolute joints — shoulder, elbow, wrist — each adding one more way for the arm to bend, so that together they sweep the hand through a wide reach of positions and orientations.

Engineers often draw or label a revolute joint with the letter R. An arm described as RRR, for example, is one made of three revolute joints in a row. Knowing each joint is purely rotational keeps the geometry simple: the robot's overall pose is just the list of all those turning angles.