kernelcad-urdf

kernelCAD — URDF export

URDF (Unified Robot Description Format) is a tree-shaped XML description of a multi-part assembly: links with inertial properties, joints with axes and limits, and visual + collision meshes. kernelCAD's export({ target: 'model', format: 'urdf' }) writes a .urdf plus a sibling meshes/ directory with one STL per link.

When to use

• The assembly has at least one part and one joint or mate.
• The downstream consumer expects a tree topology (one parent per link).
• You need per-link mass, centre-of-mass, and inertia for dynamics simulation.

Not for: assemblies with closed kinematic loops (4-bar linkages, parallel grippers) — see kernelcad-sdformat for those.

Quickstart

const arm = assembly('two-link');
const base = arm.part('base', box(30, 30, 8), { density: 2700 });
const link = arm.part('link', box(80, 12, 8), { density: 2700 });
base.connector('shoulderAxis', { type: 'axis', origin: { kind: 'vec3', value: [0, 0, 8] }, axis: [0, 0, 1] });
link.connector('shoulderAxis', { type: 'axis', origin: { kind: 'vec3', value: [0, 0, 0] }, axis: [0, 0, 1] });
arm.mate('shoulder', 'base.shoulderAxis', 'link.shoulderAxis', 'revolute', { limitsDeg: [-90, 90] });
return arm.model();

Export via MCP:

{ "tool": "export", "input": { "target": "model", "file": "two-link.kcad.ts", "format": "urdf", "output_path": "out/robot.urdf" } }

Result: out/robot.urdf containing one <link> per part and one <joint> per mate, plus out/meshes/<part>.stl for every link (reported in the tool's mesh_files). Ship the whole directory: the visual/collision tags reference the meshes by the configured prefix.

Geometry units

Link STL meshes are written in kernelCAD-native mm; every <mesh> tag carries scale="0.001 0.001 0.001" so consumers see metres — consistent with the SI <inertial> blocks and joint origins. Do not strip the scale attribute.

Mate-to-joint mapping

kernelCAD mate	URDF joint	Notes
fastened	fixed	Direct.
revolute with limitsDeg	revolute	Limits emitted in radians.
revolute without limits	continuous	Unbounded rotation.
prismatic with limitsMm	prismatic	Limits emitted in metres.
planar	planar	Native.
cylindrical	revolute (lossy)	Prismatic DOF dropped; export.urdf.cylindrical-lossy.
ball	3-revolute chain (lossy)	Decomposed with dummy intermediate links; export.urdf.ball-decomposed.
pin_slot	revolute (lossy)	Slot translation DOF dropped; export.urdf.pin-slot-lossy.