Model data extraction. Mass property data. Mass property data. Mass property data. Integral Processes. Units and volume

Transcription

1 Model data extraction Most solid modelers permit information to be extracted directly from the model database. Graphic profile data 2D drawings annotation Solid model database details the distribution of solid volume in space. Mass property information may be obtained through direct model query If volumetric data known, only requires input of material density to calculate mass properties Typically, extracted mass property data may be displayed alphanumerically, graphically or both. e.g. a list of properties or placing a symbol at the CG of the model Some systems may allow assignment of MP data to a parametric variable to permit optimization. e.g. allow specified model dimensions to vary within a range to optimize the location of the CG Units and volume Solid modeling systems use generic units internally Units are user specified either at creation of model or at point of output Integral Processes As name implies, calculation by adding things up Is used for area and volume calculation 1

2 Surface area calculation Calculate areas of individual facets (for triangular facets, absolute value of half the cross product of two sides) Sum the individual areas to get a total Techniques typically easier for boundary representations than CSG Answers not exact but often within 0.5% Volume Calculation One easy method based upon Monte Carlo Technique. Consider 2D example Given a 2D area Generate bounding box of known dimensions Randomly cast rays (vectors) and note which intersect area Monte Carlo Technique Monte Carlo Technique The area is then found by multiplying the ratio of hits/total rays cast by the area of the bounding box Can be repeated in the other coordinate directions and analyzed to approximate volume. Variation On Technique Generate Minimum Bounding Box in 2D view of solid Use Quadtree technique to sub-divide box Cast rays (vector) through center of each sub-division Variation On Technique Distance between intersections of hits can be found. For multiple intersections, distance between odd hit to even is solid; even to odd is void Multiply distances by area of subdivisions to get volume 2

3 Moments of Inertia Not computed analytically due to complexity of geometry Computed numerically through algorithms which divide a solid into smaller, less complex volumes (octree volumes or ray casting) calculates moments of smaller volumes sums elemental results Other calculations Radii of gyration, products of inertia, principal axes Calculated from moment of inertia Principal moments Calculated using principal axes and products of inertia Accuracy Usefulness of MP data dependent on accuracy of model definition. MP Calculation may be based upon exact mathematical definition or faceted representations. Accuracy may vary by volume calculation technique employed. for example, ray casting versus numerical analysis Accuracy checks Check against known simple volume Subtraction technique query volume of 2 geometries subtract (Boolean) and query 2nd value should equal actual subtraction of 1st two 2D drawings Documentation of solid model differs with company and application. Many companies still use traditional 2D working drawings for product manufacture. Also require documentation for: presentation, manuals, advertising Graphic profile data 2D drawing frequently created through profiling process 2D views created based upon line-ofsight (similar to display processes) Software determines visibility and allow display control of hidden features show hidden lines as dashed, invisible, etc. 3

4 Graphic profile data (con t) Because line-of-sight (LOS) linked to display, any view orientation is possible. Typical to create standard orthographic views top, front. right side, left side, etc. Start with a base view and project base view sets LOS for projected views Graphic profile data (con t) Possible to create multiple base views with different display orientations e.g. top view as base for projections and an isometric view Most systems also support: auxiliary projections section views with applied cross-hatching view based upon cut feature defined at userdefined location of cutting plane Quality Extraction only as good as source model. Errors in solid can (and will) show up in extractions 2D views may require: clean-up (view positioning, etc) annotation (dimensions, notes) view display (tangent intersections from fillet and round blend features) Not all data required for manufacturing is geometric Other information required such as: tolerances surface finishes thread specifications in constraint -based systems, construction dimensions may be imported from solid model other required dimensions may be created these are cosmetic ; they have no link to the model (see associativity ) the default annotation settings for dimensions and notes may not conform to current ANSI/ASME standards software cannot make decisions on annotation type, form and placement therefore, the user must understand the principles and standards involved 4

5 Associativity Systems may have associative connectivity to between extracted drawings and model non-associative uni-directional (model to graphic) bi-directional (model to graphic and back) Non-associative No direct link between model and drawing. 2D drawing created by examining solid and using 2D CAD system Not easy to update drawings if model design is changed. Unidirectional Link from model to drawing 2D data generated from profiling solid Change in solid requires updating 2D output added through traditional 2D CAD techniques. Bi-directional Link between 3D model data and 2D output May be simply a modified view Construction dimensions for model shared with 2D extraction Modification of 3D model modifies extraction and vice-versa. Bi-directional Powerful design tool but dangerous. May not be expedient to modify model from 2D extractions. Design group access must be controlled. 5