This overview explains the need and scope of OPC UA Companion Specifications.
OPC UA Companion Specifications are developed for various reasons:
- To publish specific information models (e.g., for specific industries, specific devices, specific use cases)
- To specify how to use OPC UA in specific environments. Examples are
a. The mapping of OPC Services to IEC 61131-3 Function Blocks to be used as OPC UA Clients. See OPC 30001.
b. The definition of a cloud-based library of OPC UA Information Models and or Address Spaces. See OPC 30400.
c. Profiles that express requirements for specific use cases (for instance features like auditing or redundancy, specific transports or security policies)
The majority of Companion Specifications define specific information models are defined in the following paragraphs in more detail.
The Base: OPC UA infrastructure
OPC UA has been designed for scalability and supports a wide range of application domains, ranging from field level (e.g. devices for measurement or identification, PLCs), to enterprise management support. To achieve these design goals, the OPC UA standard provides a multi-layered architecture as shown in the following figure:
OPC UA is built on the following Infrastructure:
- Discovery which allows Clients to find OPC UA Servers, their supported protocols, security policies and other capabilities.
- Transport which defines protocol mappings that allow establishing a connection and exchanging well-formed messages between OPC UA Applications.
- Information Access which comprises the means to expose object-based Information Models in an Address Space and the Services to access this information.
- Security and Robustness, which are integrated into Transport and Information Access.
Information models are layered on top of this infrastructure. OPC UA specifies a number of base information models (DataAccess – DA, Alarms&Conditions – AC, and more) that define commonly used objects including both real-time and historical data variables and alarms.
Companion Specifications for Information Models
New Information Models can be created based on the OPC UA Data Model and eventually derived from OPC UA Base Information Models. Companion specifications of such Information Models are often called “Industry standard models” because they typically address a dedicated industry problem. The synergy of the OPC UA infrastructure to exchange such industry information models enables interoperability at the semantic level.
A number of such industry standard models have already been created.
Ways to produce Companion Specifications
OPC Foundation differentiates three ways of producing companion specifications:
These are specifications created by OPC-internal working groups. They can be found here https://opcfoundation.org/developer-tools/specifications-unified-architecture following the core OPC UA parts.
These are specifications that are created in a joint working group between the OPC Foundation and another organization. These joint specifications represent the majority. The released joint companion specifications can be found here: https://opcfoundation.org/developer-tools/specifications-opc-ua-information-models
The JOINT working group program is defined here: https://opcfoundation.org/joint-working-groups/
Companion specifications can also be created independent of the OPC Foundation.
To support creating companion specifications, the OPC Foundation created several guidelines and templates. They are available for download here: https://opcfoundation.org/Guidelines-And-Templates/.
Furthermore, this list of experts (https://opcfoundation.org/experts/) offer services to vendors or organizations.