PDM-UG: Implicit Relationships Between Assembly Components
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This approach represents the components and the set of instructions on how to build the assembly. This approach is adequate for the implicit representation of assemblies. Thus, representation_relationship_with_transformation is used to define the relative positions within an assembly. It does not allow for:
- Inclusion - the decision to create a representation, which brings all the components together, is left to the receiving system.
- The ability to define a component in one position and then replicate it in the same space.
The relationships between the assembly and the component on the product_definition level and the shape_representation level has to be linked through a context_dependent_shape_representation. This is necessary to distinguish between several occurrences of the same component within an assembly. According to Part 43 the attributes rep_1 and rep_2 of representation_relationship_ with_transformation are determined as:
- rep_1
- is defined as the representation with a context to which the transformation applies (in other words rep_1 is defined as the representation with a context to which the transformation can be applied in case of building an assembly representation) and
- rep_2
- is the representation with a context which is the result of the transformation (in other words rep_2 is defined as the representation with the context into which the component representation has to be transformed in case of building an assembly representation).
Based on this definition the attributes of representation_ relationship_with_transformation should be instantiated as follows:
- representation_relationship_with_transformation.rep_1 for the relation to the shape_ representation for the product_definition that is related by assembly_component_usage as related_product_definition (identifying the component) and
- representation_relationship_with_transformation.rep_2 for the relation to the shape_ representation for the product_definition that is related by assembly_component_usage as relating_product_definition (identifying the assembly).
Further restrictions are implied by the Part 43 informal proposition on the entity representation_relationship_with_transformation: When the transformation is an item_defined_transformation, the ordering of the representations given for the inherited attributes of representation_relationship shall be consistent with the ordering of the two representation_items given as attributes of item_defined_transformation. Therefore the following usage of the attributes transform_item_1 and transform_item_2 of item_defined_transformation is recommended:
- item_defined_transformation.transform_item_1 for the relation to the representation_ item (e.g., axis2_placement_3d) for the product_definition that is related by assembly_component_usage as related_product_definition (identifying the component) and
- item_defined_transformation.transform_item_2 for the relation to the representation_item (e.g., axis2_placement_3d) for the product_definition that is related by assembly_component_usage as relating_product_definition (identifying the assembly).
Contents |
The Instance Model: EXPRESS entities and attributes
The EXPRESS entities and attributes used to support the requirements of relating assemblies and components using item_defined_transformation are illustrated in Diagram 26.
Diagram 26: Assembly Structure with Defined Component Relationships
(usage of item_defined_transformation)
Another possibility is the definition of the assembly construction history via a cartesian_transformation_operator. The Part 42 definition of cartesian_transformation_operator also allows for scaling and mirroring. But scaling and mirroring shall not to be applied in the context of assemblies. Therefore:
- the scaling value specified by the attribute cartesian_transformation_operator.scale shall be empty (due to being optional; default scaling is equal 1.0) or shall be 1.0 and
- the determinant of the transformation matrix u (computed from the attributes axis_1, axis_2 and axis_3 in case of using cartesian_transformation_operator_3d by the function base_axis) shall equal 1.
The Instance Model: EXPRESS entities and attributes
The EXPRESS entities and attributes used to support the requirements of relating assemblies and components using cartesian_transformation_operator are illustrated in Diagram 27.
Diagram 27: Assembly Structure with Defined Component Relationships
(usage of cartesian_transformation_operator)
Context dependent shape representation
A context_dependent_shape_representation associates a shape_representation_relationship with a product_definition_shape. In the given context this allows the explicit specification of a shape of the assembly 'as assembled'. Since elements when assembled might change their shape – e.g., under pressure – this representation may differ from the geometric assembly of the individual shapes.
Attributes
- The representation_relation attribute is a reference to the shape_representation_relationship used to define the assembly.
- The represented_product_relation attribute points to the product_definition_shape that is related to the assembly_usage_occurrence.
| ENTITY context_dependent_shape_representation | Attribute Population | Remarks |
| representation_relation | type: entity = shape_representation_relationship | |
| represented_product_relation | type: entity = product_definition_shape |
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