Revolute Joint

A joint that only lets two links rotate relative to each other about a fixed axis, giving 1 degree of freedom.

A revolute joint — often labeled R, and also called a hinge joint — connects two links and only lets them rotate relative to each other about a shared axis, with no sliding, so it contributes exactly 1 degree of freedom. It's the most common joint in arms and humanoid robots: a motor drives the output side through a reducer, and an encoder measures the rotation angle, which is what's meant by the joint angle. Its counterpart is the prismatic joint (P), which only allows translation along an axis. In URDF, a joint of type “revolute” must specify its axis direction and limits — upper and lower angle bounds in radians, maximum torque, and maximum velocity; a joint with no angle limit that can spin forever, like a wheel, is typed “continuous” instead. What people call a “6-axis arm” is usually six revolute joints chained in series.

ExampleA door hinge is a revolute joint. All 7 joints on a Franka Research 3 are revolute; the values q1 through q7 the controller reads out are each joint's rotation angle, in radians.

Also called

Hinge Joint, R Joint

Related

Link · Prismatic Joint · Spherical Joint · Joint Limits · Degrees of Freedom (DoF) · Rotary Encoder

Sources

Wikipedia: Revolute joint
Modern Robotics (Lynch & Park, 2017), 2.2.1 Robot Joints
ros/urdf_tutorial: 07-physics.urdf