Robotics
Joint
Definition
A joint is a connection between robot links that constrains their permitted relative motion. Its kinematic type determines which rotations or translations the connected links can make relative to one another.
Also known as: Robot joint, Joints
Updated
Connections define permitted motion
A robot arm's links form a kinematic chain, with joints constraining how adjacent links move. A revolute joint permits rotation about one axis. A prismatic joint permits translation along one axis.
Modern Robotics describes these as one-degree-of-freedom joints. A universal joint permits two relative freedoms, while a spherical joint permits three rotational freedoms. One joint therefore does not always mean one independent coordinate.
A joint need not have its own motor
Joint type describes motion constraints, not the source of actuation. The reference's Stewart-platform example has legs containing universal, prismatic, and spherical joints, with the prismatic joints actuated to move the platform.
This distinction matters when reading a humanoid specification: the number of modeled joints, actuated axes, and independent degrees of freedom need not be interchangeable counts.
The complete mechanism adds constraints
A joint's permitted relative motion is only part of the model. Closing a loop between links can make several joint coordinates dependent on one another. Joint travel limits also restrict the configurations available, although a finite travel range does not by itself remove a degree of freedom within that range.
Sources
Related terms
Kinematic chain
A kinematic chain is an arrangement of links connected by joints that constrains their relative motion. Open chains have no closed link loop, while closed chains contain at least one loop.
Revolute joint
A revolute joint permits one link to rotate relative to another about a fixed joint axis. Its single relative degree of freedom is described by an angle.
Prismatic joint
A prismatic joint permits one link to translate relative to another along a fixed joint axis without relative rotation. Its single degree of freedom is described by a linear displacement.
Degrees of freedom
Degrees of freedom are the number of independent coordinates needed locally to describe a system configuration. In robotics, this count depends on the bodies, joints, and independent constraints in the model.