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Defining Your System as a USD Scene
June 4, 2025
Before any simulation can run, you need a digital representation of your system. In NVIDIA Omniverse, that representation is a USD stage—a hierarchical file format that stores geometry, physics properties, and custom metadata. This post explains how to convert your CAD data into USD, set up physics attributes, and attach user-defined tags so that Omniverse understands your system’s state at every step.
CAD → USD: The Essential First Step
Unlike URDF importers, most CAD→USD tools focus on “geometry + basic physics properties” and do not automatically compute a full inertia tensor unless explicitly instructed or unless they have a built-in physics step. In practice:
- Tools like Champion (our CAD→USD pipeline) or Pixyz primarily convert geometry (meshes, subassemblies) into USD prims. They preserve the assembly tree (e.g.,
/RobotArm/Elbow/Wrist) and often auto-assign a default mass or density, but they do not by default calculate the full 3×3 inertia tensor for you.
- You usually assign a
physxRigidBodyAPIwith either:- A fixed
mass(e.g., 2.5 kg) plus let Omniverse compute inertia from that mass and geometry at runtime, or - A
densityvalue (e.g., 7800 kg/m³ for steel) and let PhysX compute both mass and inertia based on the mesh’s volume.
- A fixed
- In either case, Omniverse’s PhysX integration will calculate the principal moments of inertia (Iₓₓ, Iᵧᵧ, I𝓏𝓏) from the convex approximation of your mesh. However, this only happens once you load the USD stage into an Omniverse physics context—not during the CAD→USD conversion step.
- If you need to supply a custom inertia tensor (e.g., because your CAD part has nonuniform density, or you want “mass at pivot” to match a physical prototype), you can explicitly set the
inertiaTensorattribute on the rigid-body prim:
def Xform "MyLink"
{
float mass = 2.5
vec3f inertiaTensor = (0.350, 0.450, 0.300) # principal Iₓₓ, Iᵧᵧ, I𝓏𝓏 in kg·m²
...
}
Why Converters Don’t Auto-Compute
- CAD formats (STEP, IGES, Parasolid) often lack a consistent density field—they export only geometry and perhaps a “material name” (e.g., “aluminum”), but not an actual numeric density.
- Converters typically don’t guess density. They leave it to you (the engineer) to either:
- Assign a uniform density after import, or
- Use a downstream physics tool (e.g., Omniverse PhysX) to compute inertia from assigned mass/density.
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