Robotics
Prismatic joint
Definition
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.
Also known as: Linear joint, Prismatic joints
Updated
Sliding along one axis
A telescoping link or linear slide illustrates the motion of an ideal prismatic joint. The connected links keep the same relative orientation while their separation changes along the joint axis.
Modern Robotics classifies this as a one-degree-of-freedom joint, alongside the rotational revolute joint. The permitted axis can move in world coordinates when earlier links in a chain move.
Displacement replaces joint angle
A prismatic joint's coordinate has length units rather than angular units. In a Denavit-Hartenberg model, the link offset d varies while the corresponding joint angle remains a geometric parameter. The Robotics Toolbox documentation provides separate prismatic definitions for standard and modified DH conventions.
A model also needs a zero position, positive direction, and travel limits to interpret that coordinate.
A mechanism can mix joint types
Revolute and prismatic joints can occur in the same kinematic chain. The Stewart-platform example in Modern Robotics combines prismatic legs with other joints to move a platform. The joint label describes its motion constraint. Actuator, transmission, friction, and inertia properties are additional model choices, as the Toolbox definitions make explicit.
Sources
Related terms
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.
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.
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.