Physical → digital

Begin with the question the model must answer

A model for checking screw access needs accurate nearby geometry. A model for predicting balance needs credible mass distribution and joint behavior. A model for a website can emphasize appearance. One representation rarely serves all three purposes equally well. Decide what you need to predict before adding detail.

If you start from a physical object, choose reference faces and axes, measure dimensions and record uncertainty. Photographs help document shape but do not automatically establish scale or hidden geometry. Scanning can provide a surface, yet joints, material properties, electrical interfaces and internal mass still need interpretation. If a trustworthy CAD source exists, inspect and reconcile it instead of remaking every surface.

Geometry has several jobs

A native CAD model preserves editable design structure. STEP exchanges solid geometry. A mesh approximates surfaces as triangles. A robot description ties bodies together and assigns physical meaning. Converting one file type to another does not invent missing joint axes or validate the manufacturing process.

For simulation, visual geometry is what you see. Collision geometry is what the contact system uses. A carefully chosen box or capsule can be more useful for contact than thousands of tiny cosmetic triangles. Simplification must preserve the surfaces that matter: a foot should meet the floor where the actual sole does, and a nearby body should not gain invisible clearance because its collision shape was shrunk too aggressively.

Mass and inertia explain how motion responds

Mass influences translation. Center of mass locates its weighted balance point. Rotational inertia describes resistance to angular acceleration about specified axes. Two objects with equal total mass can rotate very differently if one concentrates material near the axis and the other spreads it outward. A decorative head change can therefore affect walking even when its joint commands stay the same.

For a uniform solid box of full dimensions x,y,z, inertia about its center is Ixx = m(y²+z²)/12, with analogous expressions for the other axes. Use meters and kilograms. A hollow enclosure is not a uniform solid box; treat that calculation as an approximation or model its material distribution more accurately. CAD density-based mass estimates also depend on the correct material and representation of purchased or hollow parts.

A twin earns trust through comparison

A digital twin is useful when its relationship to the physical system is maintained with measurements and versioned changes. Start with dimensions and mass, then compare joint directions, range, static posture, free or constrained motion and controlled responses. Update one suspected mismatch at a time and preserve the earlier baseline.

Do not tune a model only until one attractive clip looks right. Reserve additional tests to check whether the update generalizes. A model fitted to a single standing posture might still predict turns or impacts badly. The course’s BDX-R simulation is a reference model with known limitations; its exact relationship to a future physical build must be established during commissioning.

Keep a small model record with four headings: purpose, measurements, assumptions and validation tests. Purpose says what you want to predict; measurements describe the physical evidence; assumptions identify what you have approximated; validation tests compare the predictions with observations. Together they explain what the model can currently be trusted to answer.