kernelcad-sdformat

kernelCAD — SDFormat export

SDFormat is an XML model description that, unlike URDF, accepts closed kinematic loops natively and exposes a native ball-joint type. kernelCAD's export({ target: 'model', format: 'sdf-gazebo' }) writes a .sdf (spec version 1.10 — parsed by current simulator LTS releases) plus a sibling meshes/ directory with one STL per link. Ship the whole directory: the SDF references meshes/<part>.stl by relative URI, resolved against the .sdf file location, so the export loads in gz sim with no resource-path environment setup.

When to use over URDF

Pick SDFormat when:

• The assembly has a closed kinematic loop (4-bar linkages, parallel grippers, scissor mechanisms). URDF refuses these; SDFormat accepts.
• The assembly uses a ball mate. URDF must decompose into a 3-revolute chain; SDFormat emits one native <joint type="ball">.
• The downstream simulator accepts SDFormat directly.

Otherwise prefer URDF — wider tool ecosystem.

What the emitter writes (verified against gz-sim)

• Per-link <pose> from the solved mate graph. The exporter runs the mate solver at the default pose and stamps each link's world transform, so links spawn assembled instead of stacked at the model origin. If the mate graph cannot be solved, the export still succeeds but emits export.sdf-gazebo.pose-unsolved (warn) and identity poses — fix the geometry before shipping to a simulator.
• Joint <pose> relative to the CHILD link frame (the SDFormat convention), i.e. the child-side connector origin; <axis><xyz> is the child-side connector axis expressed in the joint frame.
• Mesh <scale>0.001 0.001 0.001</scale> on every visual/collision: link STLs are kernelCAD-native mm, SDFormat consumes metres, and the scale keeps geometry consistent with the (already-SI) inertials and joint poses.
• Companion mesh files: meshes/<part>.stl written next to output_path by the MCP tool and the CLI; the written paths are reported in mesh_files.
• World anchor: arm.virtualJoint(name, { type: 'fixed', parentFrame: 'world', childLink }) lowers to a native <joint type="fixed"><parent>world</parent>... so the model spawns welded instead of free-falling.
• Per-link density: prefer arm.part(name, shape, { material }) (steel/aluminum/pla/abs/pet) — one word seeds the <inertial> density from the catalog (and a matching render finish). A raw { density } in kg/m³ still works and overrides the material's; every link with neither inherits water (1000) and the export warns.