OPC UA is an open and royalty free set of standards designed as a universal communication protocol. While there are numerous communication solutions available, OPC UA has key advantages:
- A state of art security model (see [OPC 10000-2]).
- A fault tolerant communication protocol.
- An information modelling framework that allows application developers to represent their data in a way that makes sense to them.
OPC UA has a broad scope which delivers for economies of scale for application developers. This means that a larger number of high-quality applications at a reasonable cost are available. When combined with semantic models such as PROFINET, OPC UA makes it easier for end users to access data via generic commercial applications.
The OPC UA model is scalable from small devices to ERP systems. OPC UA Serversprocess information locally and then provide that data in a consistent format to any application requesting data - ERP, MES, PMS, Maintenance Systems, HMI, Smartphone or a standard Browser, for examples. For a more complete overview see [OPC 10000-1].
As an open standard, OPC UA is based on standard internet technologies, like TCP/IP, HTTP, Web Sockets.
As an extensible standard, OPC UA provides a set of Services(see [OPC 10000-4]) and a basic information model framework. This framework provides an easy manner for creating and exposing vendor defined information in a standard way. More importantly all OPC UA Clientsare expected to be able to discover and use vendor-defined information. This means OPC UA users can benefit from the economies of scale that come with generic visualization and historian applications. This specification is an example of an OPC UA Information Modeldesigned to meet the needs of developers and users.
OPC UA provides a robust and reliable communication infrastructure having mechanisms for handling lost messages, failover, heartbeat, etc. With its binary encoded data, it offers a high-performing data exchange solution. Security is built into OPC UA as security requirements become more and more important especially since environments are connected to the office network or the internet and attackers are starting to focus on automation systems.
OPC UA provides a framework that can be used to represent complex information as Objectsin an AddressSpacewhich can be accessed with standard services. These Objectsconsist of Nodesconnected by References. Different classes of Nodesconvey different semantics. For example, a Variable Noderepresents a value that can be read or written. The Variable Nodehas an associated DataTypethat can define the actual value, such as a string, float, structure etc. It can also describe the Variablevalue as a variant. A Method Noderepresents a function that can be called. Every Nodehas several Attributesincluding a unique identifier called a NodeIdand a non-localized name called a BrowseName. An Objectrepresenting a ‘Reservation’ is shown in Figure 6.
Objectand Variable Nodesrepresent instances and they always reference a TypeDefinition(ObjectTypeor VariableType) Nodewhich describes their semantics and structure. Figure 7illustrates the relationship between an instance and its TypeDefinition.
The type Nodesare templates that define all the children that can be present in an instance of the type. In the example in Figure 7the PersonTypeObjectTypedefines two children: First Name and Last Name. All instances of PersonType are expected to have the same children with the same BrowseNames. Within a type the BrowseNamesuniquely identify the children. This means Clientapplications can be designed to search for children based on the BrowseNamesfrom the type instead of NodeIds. This eliminates the need for manual reconfiguration of systems if a Clientuses types that multiple Serversimplement.
OPC UA also supports the concept of sub-typing. This allows a modeler to take an existing type and extend it. There are rules regarding sub-typing defined in [OPC 10000-3], but in general they allow the extension of a given type or the restriction of a DataType. For example, the modeler may decide that the existing ObjectTypein some cases needs an additional Variable. The modeler can create a subtype of the ObjectTypeand add the Variable. A Clientthat is expecting the parent type can treat the new type as if it was of the parent type. Regarding DataTypes, subtypes can only restrict. If a Variableis defined to have a numeric value, a sub type could restrict it to a float.
Referencesallow Nodesto be connected in ways that describe their relationships. All Referenceshave a ReferenceTypethat specifies the semantics of the relationship. Referencescan be hierarchical or non-hierarchical. Hierarchical references are used to create the structure of Objectsand Variables. Non-hierarchical references are used to create arbitrary associations. Applications can define their own ReferenceTypeby creating subtypes of an existing ReferenceType. Subtypes inherit the semantics of the parent but may add additional restrictions. Figure 8depicts several References,connecting different Objects.
The figures above use a notation that was developed for the OPC UA specification. The notation is summarized in Figure 9. UML representations can also be used; however, the OPC UA notation is less ambiguous because there is a direct mapping from the elements in the figures to Nodesin the AddressSpaceof an OPC UA Server.
OPC UA specification defines a very wide range of functionality in its basic information model. It is not expected that all Clientsor Serverssupport all functionality in the OPC UA specifications. OPC UA includes the concept of Profiles, which segment the functionality into testable certifiable units. This allows the definition of functional subsets (that are expected to be implemented) within a companion specification. The Profilesdo not restrict functionality, but generate requirements for a minimum set of functionalities (see [OPC 10000-7])
OPC UA allows information from many different sources to be combined into a single coherent AddressSpace. Namespaces are used to make this possible by eliminating naming and id conflicts between information from different sources. Namespaces in OPC UA have a globally unique string called a NamespaceUri and a locally unique integer called a NamespaceIndex. The NamespaceIndex is only unique within the context of a Sessionbetween an OPC UA Clientand an OPC UA Server. The Servicesdefined for OPC UA use the NamespaceIndex to specify the Namespace for qualified values.
There are two types of values in OPC UA that are qualified with Namespaces: NodeIds and QualifiedNames. NodeIds are globally unique identifiers for Nodes. This means the same Nodewith the same NodeId can appear in many Servers. This, in turn, means Clients can have built in knowledge of some Nodes. OPC UA Information Modelsgenerally define globally unique NodeIdsfor the TypeDefinitionsdefined by the Information Model.
QualifiedNames are non-localized names qualified with a Namespace. They are used for the BrowseNamesof Nodesand allow the same names to be used by different information models without conflict. TypeDefinitionsare not allowed to have children with duplicate BrowseNames; however, instances do not have that restriction.
An OPC UA companion specification for an industry specific vertical market describes an Information Modelby defining ObjectTypes, VariableTypes, DataTypesand ReferenceTypesthat represent the concepts used in the vertical market, and potentially also well-defined Objects as entry points into the AddressSpace.