OPC unified architecture: From the-plant floor to the corporate enterprise
By Ian Verhappen
By Ian Verhappen
The OPC Foundation is in the midst of an ambitious project called Unified Architecture (UA). When completed, UA will expand the realm in which OPC, open connectivity via open standards, is used from beyond the traditional regime of field control, SCADA and control systems to include the corporate enterprise. The grey area in Figure 1, from the OPC Foundation presentation “UA Vision,” shows how the new OPC will encompass everything from the interface to the data concentration devices in the field up to the offices in the corporate LAN.
The team preparing this specification has broken it down into more manageable parts, each of which, much like the OSI model, can be developed with some degree of isolation, but still remain fully connected to provide the system as a whole. The key deliverables of these individual parts are as follows:
- Architecture. The system architecture has been designed so that it can integrate all the forms of data presently supported by the various OPC standards, including direct access, alarms and events, commands, complex data, and object types.
- Designed for federation. The data types are defined as abstract information and, as shown in Figure 1, these data types will be valid from the plant floor, through various information models and intermediate systems, up to the enterprise systems.
- Information modelling. Standard information models will be developed and deployed to address industry domains and specific requirements.
- Complex data. OPC UA will support standard, domain and vendor-specific data formats to provide the maximum degree of flexibility required for specific industries, and as a mechanism for suppliers to differentiate their products and offerings. The same concept is used by other foundations, including the Fieldbus Foundation with its different types of function blocks.
- Security. Transport level security can be used to encrypt and sign messages. Encryption and signatures protect against disclosure of information, and protect the integrity of messages. Encryption capabilities are provided by the underlying communications technology used to exchange messages between OPC UA applications. This result is achieved through the use of SecureChannel Services, which are typically implemented via the communication stack rather than OPC UA directly. This enables the use of existing security designs such as virtual private networks to be the basis for the secure connection between client and server.
- Enterprise integration. OPC UA uses a standard messaging system. Messages may be encoded as XML text or in binary format for efficiency purposes. These messages may then be transferred using multiple underlying transports (e.g. transmission control protocol or web services over hyper text transfer protocol). The OPC UA model represents objects in terms of their variables, events and methods, as well as their relationships with other objects. For example, OPC UA servers are able to represent a temperature transmitter as an object that is composed of a temperature value, a set of alarm parameters and a corresponding set of alarm limits. This model allows data, alarms, events and their history to be integrated into a single OPC UA server.
- Robustness and reliability. Reliability is the key to acceptance of data transfer tools for control and automation, so robustness and reliability are built into the UA architecture. This includes more rigorous test procedures for protocol compliance and interoperability. The result will be minimal, and there will hopefully be no failures. Sequence numbers, keep-alives, resyncing and support for redundancy are some of the tools and ways in which the expected degree of high reliability will be achieved.
- Companion standards. The OPC UA object model allows servers to provide type definitions for objects and their components. Type definitions may be further subclassed. They also may be standardized or system-specific. Object types may be defined by the OPC Foundation, other standards organizations, vendors or end-users.
The OPC Foundation created 11 separate standards to implement the above functionality, and separated these 11 standards into two groups. The first group, “Core Specifications,” contains seven standards, while the second, “Access Type Specifications,” contains four standards that deal with various ways of accessing information.
For more information on this, check out the OPC “UA Vision” presentation on the OPC Foundation’s website, www.opcfoundation.org.
Ian Verhappen is an ISA Fellow, ISA certified automation professional, adjunct professor at Tri-State University and director of Industrial Networks at MTL Instruments, a global firm specializing in fieldbus and industrial networking technologies. E-mail him at Ian.Verhappen@ICE-Pros.com, or visit his website, www.ICE-Pros.com.
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