PDM-UG: Geometric Shape Property
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This page belongs to the PDM Usage Guide.
Part geometry is identified in the PDM Schema as a representation of a property of a part definition. As with general part properties, a representation of a property is identified and linked to the product definition by the entity property_definition. For the part shape property, property_definition is specialised to the subtype product_definition_shape.
The external part shape is represented by the entity shape_representation, a subtype of representation used in general part properties. Shape_representations model external models that are referenced in files associated to the PDM master file.
STEP Application Protocols define various subtypes of shape_representations with differing constraints on the allowable representation_items to explicitly represent the detailed geometric model. In the context of the PDM Schema, it is generally assumed that these subtypes of shape_representation are not used.
In the scope of the PDM Schema it is generally assumed that shape_representations are placeholders for externally defined geometry. This geometry may be defined in STEP format as well as in native CAD format. In general it is not recommended to instantiate dedicated geometric elements as items of the shape_representations for PDM Schema files. However, certain detailed elements of the shape are required to be able to place and relate the external geometric models together. Therefore, placement information is placed in the set of items of each shape_representation. Placement information is modeled using the entity axis_placement, a subtype of geometric_representation_item.
Part geometry is identified in the PDM Schema as a representation of a property of a part definition. As with general part properties, a representation of a property is identified and linked to the product definition by the entity property_definition. For the part shape property, property_definition is specialised to the subtype product_definition_shape.
The external part shape is represented by the entity shape_representation, a subtype of representation used in general part properties. Shape_representations model external models that are referenced in files associated to the PDM master file.
STEP Application Protocols define various subtypes of shape_representations with differing constraints on the allowable representation_items to explicitly represent the detailed geometric model. In the context of the PDM Schema, it is generally assumed that these subtypes of shape_representation are not used.
In the scope of the PDM Schema it is generally assumed that shape_representations are placeholders for externally defined geometry. This geometry may be defined in STEP format as well as in native CAD format. In general it is not recommended to instantiate dedicated geometric elements as items of the shape_representations for PDM Schema files. However, certain detailed elements of the shape are required to be able to place and relate the external geometric models together. Therefore, placement information is placed in the set of items of each shape_representation. Placement information is modeled using the entity axis_placement, a subtype of geometric_representation_item. Part geometry is identified in the PDM Schema as a representation of a property of a part definition. As with general part properties, a representation of a property is identified and linked to the product definition by the entity property_definition. For the part shape property, property_definition is specialised to the subtype product_definition_shape.
The external part shape is represented by the entity shape_representation, a subtype of representation used in general part properties. Shape_representations model external models that are referenced in files associated to the PDM master file.
STEP Application Protocols define various subtypes of shape_representations with differing constraints on the allowable representation_items to explicitly represent the detailed geometric model. In the context of the PDM Schema, it is generally assumed that these subtypes of shape_representation are not used.
In the scope of the PDM Schema it is generally assumed that shape_representations are placeholders for externally defined geometry. This geometry may be defined in STEP format as well as in native CAD format. In general it is not recommended to instantiate dedicated geometric elements as items of the shape_representations for PDM Schema files. However, certain detailed elements of the shape are required to be able to place and relate the external geometric models together. Therefore, placement information is placed in the set of items of each shape_representation. Placement information is modeled using the entity axis_placement, a subtype of geometric_representation_item.
Contents |
The Instance Model: EXPRESS entities and attributes
The EXPRESS entities and attributes used to support the requirements of independent property identification are illustrated in Diagram 8.
Product definition shape
Product_definition_shape represents the shape of a product. The shape may be a conceptual shape for which a specific geometric representation is not required.
Attributes
| ENTITY product_definition_shape | Attribute Population | Remarks |
| name | type: label = string | no standard mapping exists should be instantiated as empty string |
| description | type: text = string | OPTIONAL |
| definition | type: entity = product_definition | reference to an instance of product_definition to which the shape information is attached |
===Preprocessor Recommendations=== None specified.
===Postprocessor Recommendations=== None specified.
===Related Entities=== None specified.
Shape definition representation
A shape_definition_representation links the definition of a shape, i.e., the product_definition_shape, with its representation, i.e., a shape_representation. For a given product_definition_shape alternative representations might be given by multiple occurrences of shape_definition_representation that each link a representation to one product_definition_shape.
Attributes
- The definition attribute is used to point to the product_definition_shape for which the representation of shape is provided.
- The used_representation references an instance of shape_representation that provides a representation of the part shape.
| ENTITY shape_definition_representation | Attribute Population | Remarks |
| definition | type: entity = product_definition_shape | |
| used_representation | type: entity = shape_representation |
===Preprocessor Recommendations=== Some APs allow the assignment of a role ('detailed representation' or 'idealized representation') to a shape_definition_representation by using name_attribute.named_item. However, it is recommended to use a document content property (see Document content property) to record this type of information instead of using name_attribute.attribute_value.
===Postprocessor Recommendations=== None specified.
===Related Entities=== None specified.
Shape representation
Shape_representation is a subtype of representation that is specifically used to represent shape information. The shape information represented is either the complete shape of a part or corresponds to a specifically identified portion of a part shape. For the PDM schema it is generally recommended to explicitly link instances of shape_representations to the corresponding part master data. This linkage can either be established via product_definition_shape or the usage of shape_aspect together with product_definition_shape.
Attributes
- The name attribute unambiguously identifies the geometric model within the CAD environment.
- The items attribute collects the items of the shape_representation.
- The context_of_items attribute references a geometric_representation_context that establishes the coordinate space for the shape_representation.
| ENTITY shape_representation | Attribute Population | Remarks |
| name | type: label = string | |
| items | type: set of representation items | a set of representation_items that are related in the context_of_items |
| context_of_items | type : entity = geometric_representation_context | reference to an instance of geometric_representation_context or an complex instance of (geometric_representation_context, global_uncertainty_assigned_context) |
===Preprocessor Recommendations=== Some APs allow the assignment of a version to a shape_representation by using version_assignment.assigned_id with fixed value: version_assignment.role.name = 'version id'. It is recommended not to assign versions to shape_representations.
===Postprocessor Recommendations=== None specified.
===Related Entities=== A shape_representation is in the PDM Schema always referenced by at least one instance of shape_definition_representation. The shape_definition_representation relates the shape_representations either to a property_definition (defining the shape property of an identified portion of a part shape) or an instance of product_definition_shape representing the complete shape of a part.
Geometric representation context
The geometric_representation_context defines the coordinate space in which the geometric model defined by a shape_representation resides.
Attributes
- The context_identifier attribute identifies the coordinate system.
- The context_type attribute specifies the type of context.
- The coordinate_space_dimension attribute defines the dimensionality of the coordinate space.
| ENTITY geometric_representation_context | Attribute Population | Remarks |
| context_identifier | type: label = string | |
| context_type | type: text = string | should be instantiated with 'external' |
| coordinate_space_dimension | type: dimension_count = INTEGER | it is recommend to instantiate with 3 |
===Preprocessor Recommendations=== The explicit representation of geometry is not in the scope of the PDM Schema. The PDM Schema supports exclusively shape_representations that are externally defined. Thus the context_type attribute should be instantiated with 'external'.
===Postprocessor Recommendations=== None specified.
===Related Entities=== geometric_representation_context specifies the coordinate system for the use by shape_representations.
Geometric representation item
Geometric_representation_items are items for product data representation with the additional meaning of having geometric context. Geometric_representation_items are subtyped to a variety of entities in the context of geometric representations. The subtypes relevant for the PDM Schema are axis2_placements that can be used to define transformations between shape_representations.
Attributes
- The name attribute is used to name the geometric_representation_item.
| ENTITY geometric_representation_item | Attribute Population | Remarks |
| name | type: label = string |
===Preprocessor Recommendations=== only subtype of geometric_representation_item should be instantiated.
===Postprocessor Recommendations=== None specified.
===Related Entities=== most relevant in the scope of the PDM Schema is axis2_placement_3d to define transformation information.
Axis2 placement 3d
Axis2_placement_3d is a geometric_representation_item that specifies the location and orientation in three-dimensional space of two mutually perpendicular axes.
Attributes
- The name attribute provides a name for the placement.
- The location attribute defines the spatial position of the reference point and origin of the associated placement coordinate system.
- The axis attribute defines the exact direction of the local Z axis.
- The ref_direction attribute can be used to determine the direction of the local X axis.
| ENTITY axis2_placement_3d | Attribute Population | Remarks |
| name | type: label = string | |
| location | type: entity = cartesian_point | |
| axis | type: entity = direction | OPTIONAL |
| ref_direction | type: entity = direction | OPTIONAL |
===Preprocessor Recommendations=== None specified.
===Postprocessor Recommendations=== If necessary an adjustment to ref_direction has to be made to maintain orthogonality to the axis direction. If axis or ref_direction are omitted, these directions are taken from the geometric coordinate system.
===Related Entities=== None specified.
The Instance Model: STEP exchange file format (ISO10303 Part 21 syntax)
#290 = PRODUCT_DEFINITION('p2v', 'view on part2', #280, #130);
/* Entities #600 to #660 define the geometric model for 'part2' */
#600 = PRODUCT_DEFINITION_SHAPE('shape_p2',$,#290);
#610 = SHAPE_DEFINITION_REPRESENTATION(#600,#620);
#620 = SHAPE_REPRESENTATION('sol2',(#670),#630);
#630 = GEOMETRIC_REPRESENTATION_CONTEXT('c2','external',3);
/* Entities #640 to #670 are the elements of shape representation */
#640 = CARTESIAN_POINT('', (3.0E+001, 0.0E+000, 1.0E+001));
#650 = DIRECTION('', (0.0E+000, -1.0E+000, 0.0E+000));
#660 = DIRECTION('', (1.0E+000, 0.0E+000, 0.0E+000));
#670 = AXIS2_PLACEMENT_3D('', #640, #650, #660);
