A Proposal to Combine OPC UA and DDS

Amid an array of industry calls for greater interoperability, Real Time Innovations and Beeond are advocating for a new standard to combine OPC UA and DDS to unify the automation networking industry.

Interoperability of automation devices has long been the holy grail sought by end users in every industry. Technology suppliers have, over the years, delivered various levels of interoperability, but none have delivered it in a way that has fully met end user expectations.

But now, as industry’s digital transformation demands high levels of interoperability, the fire has been lit under both the end user and supplier sides to address the issue head on. Ongoing work around this can be seen in the activities of the Open Process Automation Forum, the Open Industry 4.0 Alliance, and the OPC UA Down to the Field Level with Time-Sensitive Networking (TSN) initiative.

Adding to all this activity is a call from Real-Time Innovations (RTI), a supplier of DDS connectivity software for Industrial Internet of Things (IIoT) applications, and Beeond, a provider of training and platforms for IIoT based on OPC UA, to combine the best aspects of DDS, OPC UA and TSN to “connect the factory floor to the top floor, sensors to cloud, and real-time devices to work cells.”

In a white paper titled A Converged Approach to Standards for Industrial Automation) that details the companies’ approach to this idea, the authors state: “The industry needs a clear, technically optimal, proven standards approach. This document proposes a new technical architecture to build this future. The design combines the best of the OPC UA, DDS and TSN standards...to define and standardize the architecture to unify the industry.”

As a bit of background to this, the white paper reminds readers that the OPC Foundation's OPC UA is the most prevalent technology for moving data and information from field devices to the cloud, and is well-known for its client-server model for applications to interact with a complex system. DDS is a connectivity framework standard, developed by the Object Management Group, to provide an integration framework for software applications. DDS, using a publish/subscribe databus, serves as both a control bus and an edge-to-cloud connectivity framework.

You’ve likely heard—with the announcement of the OPC UA down to the field level initiative—that OPC UA is also adopting the publish/subscribe method at the core of DDS and industrial communication methods like MQTT. While that would appear to address much of the issue raised by the call for an integrated standard around OPC UA and DDS, the authors of the white paper note that “the current OPC UA pub/sub extension approach to extend the OPC UA object-oriented client/server with some ‘bolted on’ pub/sub concepts cannot deliver the performance or scale necessary for complex environments. The lack of a data-centric databus will result in limited facilities that do not scale.”

This proposed combination of technologies also highlights some shortcomings of DDS. The authors note that simply extending the DDS databus ‘decoupled’ data centric model of publish/subscribe topics and services to ‘bolt on’ a more tightly-coupled object-oriented concept would not provide the rich feature set currently available in OPC UA.

The white paper states that OPC UA’s object-oriented client/server and DDS’s data-centric publish/subscribe methods both have their sweet spots: “The object-oriented model is a good match for device configuration because each vendor can define and expose its own information model. The object-oriented model is also useful as a way to provide a consolidated interface to a more complex subsystem (e.g., an external machine or plant interface)."

This model fits systems with topologies that remain fairly static during operation. Meanwhile, the data-centric model is a good match for loosely coupled systems that need to operate more autonomously. It can handle changes of topology and system reconfiguration without impacting the running applications. It is optimal for publish/subscribe and real-time control through its direct (serverless) communication for critical data flows.

Brett Murphy, senior director, market development, IIoT at RTI, made it clear that RTI and Beeond are not advocating replacing OPC UA publish/subscribe) everywhere. "However, the OPC UA pub/sub standard is designed mostly to connect workcell devices on local networks. It is not designed to support extensive software development" he adds. "As systems become more intelligent, software will become more important in the final integration. DDS is designed for such use cases. The proposed design combines the familiar OPC UA client server capability, the OPC UA information model, the proven DDS databus, and TSN's real-time networking.”

With that noted, what kind of standard are RTI and Beeond advocating for?

Because OPC UA and DDS already co-exist and interoperate—as evidenced by OMG’s OPC UA/DDS Gateway Specification—the intent of RTI and Beeond is to expand beyond this gateway specification with a more comprehensive integration proposal.

Their white paper proposes implementing the OPC UA information model in DDS. According to the paper, this provides an alternative to the current OPC UA publish/subscribe function, replacing it with a DDS-based publish/subscribe design. The result will bring proven software and control integration together with industrial automation expertise. It will support multiple physical network types, allowing systems to transparently use Ethernet, TSN, or other transport technologies.

More granular details of how this can be enacted in automated systems via software development kits are provided in the paper.

As of now, the standard concept described in the white paper is being floated as a way to “invite others to join us in fleshing out the next level of detail to address specific use cases,” said Costantino Pipero, chief technology officer and founder, Beeond.

When asked about what types of industrial applications the authors see as being ideal for this standard, he offered three examples:

  • Packaging/Serialization: A packaging line needs the ability to detect the presence of a package and scan identifiers via cameras or RFID receivers. A programmable logic controller (PLC) will typically implement sorting logic in a tight control loop with very high speed, low-latency information exchange among all the actors involved (20 packages per second is not uncommon). Time synchronization is therefore required to coordinate the various controllers.
  • Robotic Tool Changes: Low-latency and time synchronization are desirable to coordinate the actions between the robot and the tool. Tool configuration usually happens in less than a second on a typical 1-5 mS cycle time requirement for control loops.
  • Autonomous Material Movement Equipment: Coordinate processes between carrier and tool to control flow of material, products, and packaging (requiring identification, tracking, and positioning). Multi-carrier systems increase the need for time synchronization.

Pipero envisions the same system developers and device vendors using either OPC UA or DDS today being users of the combined solution they are proposing.

“For the larger scale, software-intensive industrial systems coming along now and, in the future, system integrators will play a larger role in developing and integrating the software and systems we expect,” he added.

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