6 OPC UA ObjectTypes ToC Previous Next

6.4 Referable and Identifiable ToC Previous Next

The identification concept serves to clearly identify all AAS elements. In principle, a distinction can be made between the following partial identification concepts, which differ in their validity area (Figure 10).

• Globally unique identification, e.g. IRDI or URI - this is called “Identifiable”

• Locally unique identification (instance-related ID valid only in the inner context of an AAS) - this is called “Referable”

• semantic referencing (reference to a conceptual library (dictionary / repository))

In Figure 10 these are shown. The light blue backed rectangles are each AASs. The attributes of the AAS named in the upper section (AAS - Asset Administration Shell, Asset and Submodel) have a globally valid identifier. In addition to the identifier itself, the identifier is also given its type, which can be URI, IRDI or Custom (manufacturer-specific). The identifiers of these AAS elements are thus unique. This is necessary because it is the entry point into the AAS. If this is compared with an OPC UA Server URI, then the same applies since the validity in the namespace of the OPC UA server must also be unique.

readme_files/image014.png Figure 10 – Identification concepts for AAS

The middle section of Figure 10 shows AAS elements that have locally unique identifiers. This allows for e.g. the same properties can be present several times and also submodels can be used again and again. The validity of this identification concept is only in its context (namespace), i.e. along the composition from the globally unique identifiers until the individual instances. An example of idShort is “MaxTemp” and for the composition in the identification: “urn: PROFIBUS: PROFIBUS-PA: V3-02: Parameter: 1: 1: MaxTemp # 0002”.

With these two concepts, the identification of an AAS, i.e. the addressing is clearly possible. However, as with the example of the PROFIBUS PA profile, the name MaxTemp is unique only in this profile. In the planning documents of the plants there are also specifications for the maximum temperature, as well as in the manual of the transmitter. All three parameters correspond to the same understanding, but they are not automatically interconvertible by machine. This is where the third identification concept of AAS comes in. The lower part of Figure 10 shows the semantic references that refer to dictionaries, which are feature catalogs that store the understanding of the properties and important attributes (data type, unit of measurement, …). From selected AAS elements e.g. submodel and property can be checked if the same thing is meant despite different naming. It is possible to specify only the semantic reference and it is expected that the user can interpret this. However, it is also possible to embed the corresponding dictionary in the AAS.

For these three identification principles, there are corresponding model elements in the AAS metamodel. It is also possible that an element e.g. a submodel has both a local identifier and a semantic reference. The reserved word “keys” encapsulates each of the attributes of the identifier. The attribute “local” allows elements to be located outside the AAS where it is identifiable. Local = “TRUE” means the identifier value “Value” refers to the same AAS, Local = “FALSE” means the identifier value “Value” refers to an external location. The “Local” variable is defined in AASKeyDataType (Table 70).

Figure 11 shows an overview about the IAASReferableType ObjectType.

readme_files/image015.png

Figure 11 – OPC UA mapping of the identification concept

For the identification concept, a mapping to the OPC UA information model is shown in (Figure 11). It can be seen that “Identifiable” is a specialization of “Referable”. The identifiable identification contains not only the identifier but also the type and optionally a version and revision. The Category for the Referable Identifier are inherited. What is striking is that in OPC UA modelling at this level, the formal idShort designation is not visible. idShort is always mapped to the browsing name of an OPC UA node by the generally valid rule (see). In this way, the locally valid identification customary in OPC UA can be used.

Overall, this AAS identification concept is an extension of the OPC UA BrowseName Identification (valid in the namespace of the OPC UA Server).

6.4.1 IAASReferableType ToC Previous Next index

Table 15 defines the AASReferable ObjectType.

Table 15 – IAASReferableType Definition

Attribute Value        
BrowseName IAASReferableType        
IsAbstract False        
References Node Class BrowseName DataType TypeDefinition Others
Subtype of the BaseInterfaceType defined in OPC 10000-5, i.e. inheriting the InstanceDeclarations of that Node.          
0:HasProperty Variable Category 0:String 0:PropertyType M
0:HasDictionaryEntry Object Admin-shell.io/aas/2/0/Referable   AASIriConceptDescriptionType  

The OPC UA node attribute “description” is used for the IAASReferableType instances.

The components of the IAASReferableType have additional references which are defined in Table 16.

Table 16 – IAASReferableType Additional References

Source Path Reference Type Is Forward Target Path
Category 0:HasDictionaryEntry True Admin-shell.io/aas/2/0/Referable/category

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