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A unified solution for enterprise-wide data exchange

25 November 2018

Industrial organisations are now able to bring systems, applications and devices together in a unified architecture to exchange information throughout the enterprise thanks to the integration of FDT and OPC UA standards.

FDT Group and OPC Foundation have been working together to provide greater access to critical information throughout the enterprise. 

FDT is an established integration standard of FDT/FRAME-enabled control and asset management systems and FDT/DTM-enabled field devices, while the OPC Unified Architecture (UA) provides an infrastructure to make enterprise information available to thousands of other applications and platforms.

Data challenges
In November 2016, FDT Group and OPC Foundation announced the release of an FDT for OPC UA companion specification/annex for information modelling. More recently the FDT Group worked with OPC Foundation to enable native integration supported both by OPC UA and FDT 2.x technologies. Instead of writing this integration capability into the FDT specification, the two organisations collaborated on a companion specification describing how to implement an OPC UA Server in an FDT/FRAME as part of the emerging FDT IIoT Server (FITS) architecture. Most of the companion specification is devoted to outlining the data mapping between the two sides.

The FITS solution takes advantage of the FDT for OPC UA companion specification in enabling sensor-to-cloud, enterprise-wide connectivity for industrial control systems. It combines native OPC UA integration, web services and rich control network interoperability to optimise connectivity and information exchange for the next generation of automation. The solution also features robust layered security addressing all components of the server architecture.

Progress on integration
Standard integration of information provided by FDT/DTMs into the OPC UA information model is essential for device diagnostics, configuration and remote asset management, as well for integration with higher-level business applications. This document defines an OPC UA Information model to represent the FDT architectural models. This allows an FDT/FRAME or FDT IIoT Server to expose project structure and device specific information through standard OPC UA mechanisms. 

As part of the integrated FDT/OPC UA solution, the built-in OPC UA Server can read and write device information. Any OPC UA Client can access the FDT/OPC UA Server and obtain data as long as it has the right credentials. 

From the FDT standpoint the aforementioned approach exposes its project tree to the OPC UA client so that it an see what devices are accessible. As users click on each device they can view and access its specific attributes and information. 

OPC UA provides a uniform information exchange methodology between applications, whereas FDT provides network/device configuration and access to devices. The combined FDT/OPC UA approach enables unification of system engineering, configuration and diagnosis in Industry 4.0.
The capabilities for OPC UA integration were introduced with the FDT 2.0 specification. Additional enhancements were made with the subsequent 2.1 version and will be strengthened in the 2.5 standard (also known as FITS), which is set to deploy very soon.

Optimising communication
Within a traditional client-server (i.e., request-response) communication model, a client computer or software requests data or services, and a server computer or software responds to the request by providing the data or service.

Another model for systems and devices to communicate on a network is publish-subscribe messaging which sees any message published to the network on a topic immediately received by all of the subscribers to the topic. Clients that publish the data send it only when the data changes. Clients that subscribe to the data automatically receive it from the server, but again, only when it changes.

The publish-subscribe extension enables public subscriptions for larger numbers of devices where the client-server model struggles, as a large number of connections would have to be established, each client would need to provide memory for storing the connection information, and high processor load would be generated in the server for encoding the individual messages per established connection.

OPC Foundation recently announced the release of a publish-subscribe (PubSub) specification to make the OPC UA standard compatible with emerging IIoT applications. Its mission is to provide a mechanism for publishing server data to many clients. With OPC UA PubSub, applications do not directly exchange requests and responses. Instead, publishers send messages to a message-oriented middleware, without knowledge of what, if any, subscribers there may be. Similarly, subscribers express interest in specific types of data, and process messages that contain this data, without knowledge of what publishers there are.

Among other things, PubSub allows peer-to-peer communication between industrial controllers, and between controllers and HMIs. The peers are not directly connected and do not need to know about the existence of each other. It also enables things like asynchronous workflows and OPC UA Servers to stream data to applications hosted in the cloud.

Improving information exchange
Thanks to ongoing FDT/OPC collaboration there is now a choice of methodologies for implementing a network communication model. Both client-server and publish-subscribe models are included in the FDT for OPC UA companion specification.

With the client-server approach, the client goes through OPC UA to access current data values but must keep asking to verify the information. This is done either through a program in the OPC UA Client or by having an individual do a manual ‘refresh’.

Alternatively, the emerging FITS architecture can employ a publish-subscribe methodology allowing sensor, network and topology information to permeate the enterprise, including mobile devices, DCS, PLCs, MES, ERP systems, the cloud, IIoT and Industry 4.0.

The publish-subscribe methodology eliminates the burden of request-response communication. Clients essentially say, ‘I’m interested in a particular piece of data, so please tell me whenever it changes.’ Multiple clients can subscribe and receive notifications at once. The server will automatically notify all the subscribed clients when the specified information has changed according to the pre-defined parameters. This approach has been proven to save system bandwidth. 

A typical use case for the publish-subscribe communication model is employing OPC UA and an FDT/DTM to monitor device health. By requesting notification only on changes in device condition, system and network resources are freed from continually polling the device to ascertain its health.
A key advantage of the FDT architecture is that OPC UA is an easy plug-in. There is no need for changes to the communication, gateway or device DTMs. Plant or factory personnel can see all the networks on the server, as well as all FRAME applications and devices with DTMs through OPC UA. In addition, developers writing communication or device DTMs do not have any added requirements to support OPC UA.

The FDT/OPC UA information model was designed to provide expanded integration capabilities along with ease of implementation. When industry stakeholders implement OPC UA, however, they may have challenges using the technology within their automation architecture due to the data trafficking role of the PLC or DCS. This could require the assistance of a process or control engineer to expose the required data from the control system to OPC UA.

FDT/OPC collaboration is intended to eliminate the typical constraints in industrial communication. When most engineers think of OPC UA they envision it running at the Ethernet level – but device hardware is needed to reach a compatible network. With the addition of FDT technology, users can take advantage of their existing infrastructure, bypassing any PLC or DCS that’s in the architecture and communicating directly with end devices through OPC UA. As long as the device has a DTM in the FRAME or Server, the user will be able to access the device and all of its data through OPC UA.

If the user already has an FDT/OPC UA-enabled application in the architecture there is no need for additional configuration other than assigning security credentials. At that point, all data inside FDT is accessible through OPC UA. Engineers do not need to modify ladder logic or write rules for DCS systems. With an OPC UA Server inside an FDT application, all of the information is available. 

Furthermore, IT departments utilising MES or ERP systems do not need to o consult with a PLC or DCS programmer to access specific types of control data. They are able to browse the server structure and find the necessary information. 

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