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* claim: #6025 * fix(engine): account for sheet perforations in physical properties * fix(inspect): retain kernel topology counts for sheet bodies
111 lines
7.0 KiB
Markdown
111 lines
7.0 KiB
Markdown
# Assembly physical properties
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The Browser's component **Properties** action measures the selected assembly placement,
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including its nested components. Select the root design to measure every instance in the
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assembly. **Inspect → Measure → Properties** opens the same window for the one selected
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placement, or else for the active component. The window shows material-aware mass, volume,
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surface area, center of mass, first moments, the moment of inertia and the world bounding box.
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Pending occurrence moves are included without capturing them. While the window is open, the
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view marks the center of mass with its principal axes, numbered and colored as the window
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lists them. **Copy matrix**, under the tensor, puts it on the clipboard: three lines of
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tab-separated values at full precision, in the units shown, ready to paste into a spreadsheet
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or code. The window for the whole design is titled just **Properties**; one opened on a
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component, placement or bodies carries its name.
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The same measurements are available through commands:
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```json
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{"command":"component.properties","params":{"component":"Bracket"}}
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{"command":"component.properties","params":{"component":"root"}}
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{"command":"component.properties","params":{"component":"Subassembly","descendants":false}}
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{"command":"occurrence.properties","params":{"occurrence":"Bracket:2"}}
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{"command":"component.properties","params":{"component":"root","about":[0,0,0]}}
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```
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`component.properties` defaults to the active component and measures its first placement.
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`occurrence.properties` measures the chosen instance. A nested occurrence can appear beneath
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several instances of its parent; supply `path` (the complete list of occurrence ids from the
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root) to choose one of those placements. An ambiguous occurrence without a path returns an
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error. Both commands accept `descendants: false` to measure only directly owned bodies.
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Results contain a `bodies` list and a `total` object. Each body's entry identifies the original
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body name, owning component and occurrence path. Physical material density is resolved from
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that original body, so copies keep their material and mass. Values use millimetres, square
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millimetres, cubic millimetres and grams; coordinates use the world frame.
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Mass and center of mass account for each body's density. Volume and area are summed over
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placed bodies, including overlapping bodies and repeated instances. Measurements use the
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kernel's tessellation-based body measurement. An empty component reports zero totals and
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null center of mass and bounding box. A failed body measurement returns an error instead of
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presenting an incomplete assembly total. These commands leave geometry, pending placements
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and undo history unchanged.
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Sheet-metal perforation fields contribute their physical holes to volume, mass, surface area
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and center of mass even though they do not cut the B-rep. Hole centers follow the panels when
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folded or unfolded and follow each occurrence's placement. Measure and component/occurrence
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Properties use the same correction. Bounding boxes and B-rep topology still describe the
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uncut body. A failed perforation calculation returns an error rather than an uncorrected total.
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The inertia tensor also removes each analytic hole cylinder, including its offset from the
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sheet's center of mass.
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Measuring a perforated sheet after scaling or shearing its metadata is not supported and
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returns an error. Rigid and mirrored placements preserve the analytic hole dimensions.
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## Moment of inertia
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Each closed body's entry has `inertia_g_mm2`: its inertia tensor about its own center of mass,
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in world axes, in g·mm². The `total` object has:
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- `first_moment_g_mm`: the first moments of mass, Σ m·c (the total mass times the center of
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mass).
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- `inertia_g_mm2`: the inertia tensor about the total center of mass, in world axes. Each body's
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own tensor is moved there with the parallel-axis theorem.
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- `principal_moments_g_mm2` (smallest first) and `principal_axes` (unit vectors, one per
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moment). The first two axes point along the world axis each is nearest to; the third is the
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first × the second, so the three make a right-handed frame. Moments within 0.1% of the largest
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count as equal, as a round body's two equal moments do. Their axes are then the world axes
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nearest the plane they share.
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- `inertia_about`, only when the command has `about: [x, y, z]` (mm, world): `{point,
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inertia_g_mm2}`, the tensor about that point in world axes.
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The tensor is the matrix that turns an angular velocity into angular momentum. Its diagonal holds
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the moments of inertia (Ixx = ∫ (y² + z²) dm …), and the off-diagonal terms are the negated
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products of inertia (Ixy = −∫ x y dm …), the convention URDF and most dynamics libraries take. For
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kg·m², divide by 10⁹.
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The tensor is integrated over each body's welded export mesh, the one URDF export takes its
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inertia from. It is then scaled to the body's measured mass, so it agrees with the mass and center
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of mass shown. Planar bodies are exact. A round body is within about 0.1% at the standard export
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quality. Each body is tessellated once, however many times the assembly places it; each placement
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turns its tensor. A closed body whose mesh does not close has `inertia_g_mm2: null`, and so does
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the total. Open bodies have no volume, so they have no tensor either, and the total leaves them
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out.
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## Per occurrence and per rigid group
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`assembly.mass_properties` (no parameters) gives what a dynamics solver needs of each moving
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part, measured as above, in kg, mm and kg·mm²:
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```json
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{"command":"assembly.mass_properties","params":{}}
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```
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- `occurrences`: the design's own bodies (`occurrence: 0`) and then every occurrence, each with
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its `frame` (origin and unit axes in world coordinates), its own bodies (material, density,
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volume and mass), and `mass`: its bodies' total `mass_kg`, `center_of_mass_mm` (world) and
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`center_of_mass_local_mm` (in its frame), and the tensor about that centre in world axes
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(`inertia_world_kg_mm2`) and in its frame's axes (`inertia_local_kg_mm2`). Only the bodies the
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occurrence's component owns count, not those of occurrences nested in it.
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- `groups`: occurrences that move as one body, each with the occurrences it holds, the same
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`mass` totals (combined with the parallel-axis theorem) in the frame of its lowest
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occurrence (`frame_occurrence`; 0 is the world), and `fixed` when it holds the design itself
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or a grounded occurrence. An unsuppressed rigid joint (Rigid Group makes them) puts its two
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occurrences in one group, and a nested occurrence that no joint places goes with the
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occurrence it sits in, since the joint solver leaves it where its parent puts it.
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- `warnings`: open bodies (no mass), closed bodies whose mesh does not close (`inertia_*: null`),
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scaled placements (no tensor in their own axes) and subassemblies placed more than once (the
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occurrences inside them are listed at the first placement, where joints place them).
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A body without a material assignment has the default material's density, as Properties shows.
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Pending occurrence moves count. Rust callers get the same numbers, typed, from
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`Session::mass_properties()`.
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