lula-description-generator

name	lula-description-generator
description	Generates Lula robot description YAML from a URDF file for Isaac Sim motion planning (CuMotion/RMPflow/Lula), including automatic collision sphere generation via mesh repair + voxelisation + medial-axis sphere packing

Lula Robot Description Generator

Generates Lula robot description YAML files from URDF files for use with Isaac Sim's CuMotion / RMPflow / Lula motion planning backends.

What this skill does

A Lula description file configures:

• C-space — the joints the motion planner directly controls
• Default joint positions — home configuration
• Acceleration & jerk limits — per-joint motion constraints
• C-space-to-URDF rules — how joints outside the c-space are handled
• Collision spheres — per-link convex approximations for obstacle avoidance

The collision spheres are the hardest part to create manually. Isaac Sim's built-in sphere generator frequently fails on non-watertight meshes. This skill automates sphere generation using:

1. Mesh repair (pymeshfix → trimesh fallback)
2. Voxelisation of the mesh interior
3. Euclidean distance transform (voxel value = distance to nearest surface)
4. Greedy medial-axis sphere packing

MCP tool

lula_generate_robot_description(
    urdf_path,               # absolute path to URDF
    output_path=None,        # defaults to <urdf_dir>/<name>_lula_description.yaml
    mesh_search_paths=None,  # extra dirs for mesh resolution (incl. package:// URIs)
    controlled_joint_names=None,  # explicit cspace list; auto-detected if omitted
    max_spheres_per_link=16, # 8–32 is typical; more = tighter fit but slower planning
    voxel_fraction=0.04,     # 4% of max bounding-box dim; lower = finer but slower
    skip_sphere_links=None,  # links to exclude from sphere generation (virtual frames)
)

Returns: success, output_path, cspace, num_links_with_spheres, total_spheres, warnings, summary.

Standard workflow

Phase 1 — Inspect the URDF

Use urdf_inspect to understand the robot structure before generating:

• How many links and joints?
• Which joints are controllable (revolute/prismatic/continuous)?
• Are there gripper/finger joints separate from the arm?
• Are mesh paths resolvable (package:// URIs need search paths)?

Phase 2 — Decide the c-space

For manipulator arms, the c-space is typically the arm joints only (NOT gripper fingers). Gripper joints should appear in cspace_to_urdf_rules as fixed entries.

Example: UR5e — c-space is 6 revolute joints:

controlled_joint_names=["shoulder_pan_joint", "shoulder_lift_joint",
                        "elbow_joint", "wrist_1_joint",
                        "wrist_2_joint", "wrist_3_joint"]

Example: Piper — 6-DOF arm (joint1–joint6); gripper joints (joint7, joint8) go in cspace_to_urdf_rules:

controlled_joint_names=["joint1", "joint2", "joint3", "joint4", "joint5", "joint6"]

When controlled_joint_names is omitted, ALL non-fixed joints are included.

Phase 3 — Resolve mesh paths

If the URDF uses package:// URIs, provide mesh_search_paths pointing to the directory that contains the package folder. For example:

URDF: package://piper_description/meshes/base_link.STL
workspace: /home/user/ws/piper_isaac_sim/
→ mesh_search_paths=["/home/user/ws/piper_isaac_sim"]

For relative mesh paths (e.g. UR5e), the URDF directory is searched automatically — no mesh_search_paths needed.

Phase 4 — Generate

result = lula_generate_robot_description(
    urdf_path="/abs/path/to/robot.urdf",
    controlled_joint_names=[...],
    mesh_search_paths=[...],   # only if needed
    max_spheres_per_link=16,
)

Check result["warnings"] for any links where sphere generation failed or meshes could not be found. These links will be absent from collision_spheres.

Phase 5 — Review & tune

Open the generated YAML and verify:

1. C-space: correct joints listed?
2. default_q: sensible home position? Zeros are usually fine for upright configs.
3. acceleration/jerk limits: generated as heuristic (5× and 2500× velocity limit). Tune for your robot's real specs if needed.
4. collision_spheres: visually inspect in Isaac Sim's Robot Description Editor (Tools → Robotics → Lula Robot Description Editor) — load the YAML and use the 3D viewport to verify coverage.

Common tuning knobs:

• Increase max_spheres_per_link (e.g. 24–32) for better coverage on complex links.
• Decrease voxel_fraction (e.g. 0.02) for finer mesh sampling on thin geometry.
• Add problematic links to skip_sphere_links and hand-author their spheres if the automatic result is poor.

Known limitations & pitfalls

• Mesh repair repairs topology but may slightly alter geometry. Sphere centres reflect the repaired mesh, which is usually fine for collision purposes.
• Concave links (e.g. U-shaped brackets) require more spheres for good coverage. The medial-axis packing naturally handles concave shapes but may need max_spheres_per_link ≥ 24.
• package:// URIs without a matching search path will silently skip sphere generation for those links. Always check warnings.
• Very small links (e.g. sensor frames, fingertips < 1 cm) may produce only 1–2 spheres. This is usually fine.
• YAML heuristic limits: acceleration_limits and jerk_limits are heuristic estimates. For production use, replace with values from the robot datasheet or identify them via system identification.

Output format

api_version: 1.0

cspace:
  - joint1
  - joint2

default_q: [0.0, 0.0]

acceleration_limits: [15.0, 10.0]
jerk_limits: [7500.0, 5000.0]

cspace_to_urdf_rules:
  - {name: finger_joint, rule: fixed, value: 0.0}

collision_spheres:
  - base_link:
    - center: [0.0, 0.0, 0.05]
      radius: 0.047
  - link1:
    - center: [0.0, 0.0, 0.1]
      radius: 0.04

Loading into Isaac Sim

1. Import your URDF via File → Import → URDF
2. Open Tools → Robotics → Lula Robot Description Editor
3. Load the generated YAML
4. Inspect collision spheres in the 3D viewport
5. Re-export once satisfied (YAML or XRDF format)

Backlog / future improvements

• Support for MJCF input format
• Auto-detection of mimic joints for cspace_to_urdf_rules
• Sphere quality scoring and iterative refinement
• Direct XRDF output format