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Industrial Ethernet Book Issue 102 / 14
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OPC UA TSN: universal language for real-time data exchange

The goal of OPC UA is open and easy communication. But now, OPC UA TSN is providing seamlessly shared information between all kinds of machines, devices and sensors in real-time. It looks to be a foundational element of machine interoperability and information transparency for the 4th wave of industrial revolution.

THE BABEL FISH IS REMARKABLE, although sadly it is a fictional creature. Readers of The Hitchhiker′s Guide to the Galaxy will know that placing the fish in your ear allows you to understand anyone, speaking any language, instantaneously. In the context of Industry 4.0 which sets out to achieve interoperability and information transparency between all machines, devices and sensors, the Babel Fish would be an extremely valuable tool if applied to the factory floor.

Now, with the combination of two open technology standards, OPC UA and IEEE TSN, industrial automation has an equivalent, entirely non-fictional solution for the real-time transmission of information between machines and devices from any vendor. Let′s take a closer look at the two standards and the ways in which their integration will benefit industrial automation users.

OPC UA: data into information

OPC UA, standardized and maintained by the OPC Foundation, provides a common framework that sets the rules on how to turn data into information and how to manage that information in a distributed environment such as a "machine to cloud" scenario.

As the value of data has become increasingly well understood by industrial users via initiatives like Industry 4.0 there has been a rapid rise in the adoption of OPC UA, with all of the major industrial automation vendors now implementing it in their products. By representing data in a uniform way, OPC UA enables interoperability between devices that could not previously share data, and gives users new insight into a wealth of information.

OPC UA was initially designed to be used within a client-server architecture, with a user (the client) such as a human operator or a cloud-based information management system requesting information from a device (the server) in a standardized way, e.g. with specific methods and controlled by defined access rights. The server then provides appropriate access to the data for the client in a standard format. With the new PubSub extension planned for release this year, OPC UA now has the capacity to be used for multicasting information too.

In this case, a device (the publisher) provides standard format data to a set of other devices (the subscribers). The publisher can provide this information at set time intervals so subscribers know how often or even when to expect new machine or production data. Subscription to published data can be altered dynamically, enabling devices to receive new data from additional sensors as systems expand or change.

These features are particularly useful for industrial users as they can flexibly use PubSub to automate the operation of systems made up of multiple devices or machines that were not previously interoperable. Many automated industrial systems operate at extremely fast speeds, these systems require not only the regular exchange of data but for the data to be provided within just a few milliseconds (known as real-time). This is where IEEE TSN fits into the picture.

TSN deterministic communications

IEEE TSN provides an open, standard method for communicating data in real-time over Ethernet by extending existing Quality of Service mechanisms in Ethernet with additional capabilities for latency control, determinism, and high availability. This is the platform upon which industrial users can take advantage of the OPC UA PubSub extension to implement automated, interoperable systems for real-time applications. The combination of OPC UA PubSub and TSN offers a complete solution for realizing Industry 4.0 concepts in industrial systems.

TSN itself comprises a selection of IEEE standardized mechanisms that act to guarantee that data streams arrive at their intended destination on time, no matter what else is going on in the communication network. The most important of these TSN mechanisms are time-synchronization (IEEE 802.1AS) and scheduled message release (IEEE 802.1Qbv). Sharing a common time-base between communication network elements such as Ethernet switches makes it possible to create a dedicated "time sensitive virtual data path" for each data stream as it passes through each switch along the way to its destination.

OPC UA PubSub and TSN offers a complete solution for realizing Industry 4.0 concepts for factory automation.

As OPC UA PubSub now allows for the publishing of data at set intervals, this creates the opportunity to schedule those data streams using the TSN traffic scheduling mechanism. In turn, this means that OPC UA subscribers can now expect to receive published data at a guaranteed time with a very high level of precision, with no risk of delays or message loss due to network congestion or high-priority conflicts on the shared network. While OPC UA PubSub relationships need not use or require such precise and reliable communication in general, it is expected that many machine-to- machine use cases will benefit from this kind of guaranteed networking service, supporting vendor independent, real-time interaction in "teams" of machines on the factory floor.

Use Case

Take a packaging line for example. There are stations along the line executing tasks such as shrink wrapping. Today, OPC UA is used to extract uniform information from the various machines in the line, but OPC UA data cannot be shared in real-time between machines along the line. Real-time communication is restricted to Industrial Ethernet fieldbuses that can vary by industrial automation vendor, which makes synchronization and interoperability between machines very difficult.

OPC UA TSN offers open communication with precision timing for synchronization and interoperability across packaging line machines from multiple vendors. The PubSub extension to OPC UA enables machines at each station to publish data in a multicast manner and for machines at other stations to subscribe to the data that is relevant for their operation. TSN ensures that this data is delivered from publisher to subscribers with sub-millisecond latency and with extremely tight jitter. Using PubSub and TSN to share OPC UA information in real-time means that machines can work in harmony no matter the name printed on the controller. OPC UA TSN can thus help to improve solutions for packaging line challenges such as product tracking, line speed synchronization, robot movement synchronization, quality management and track & trace.

Testing for use cases like this and others is currently ongoing within the Industrial Internet Consortium′s Time Sensitive Networking testbed. Here, vendors are able to test their latest OPC UA PubSub and TSN supporting products with those from other industrial automation companies. This facilitates interoperability and adherence to standards, ensuring that customers are reaping the full benefits of the two technologies.

OPC UA PubSub TSN integration

To achieve the marriage between OPC UA PubSub data exchange and TSN scheduled communication, a mechanism is required that assigns TSN schedules to published OPC UA data. This mechanism is currently known as the PubSub TSN Configuration Broker. The broker can be considered as both a network management function and an API for the OPC UA layer; it is tasked to interface with OPC UA publisher and subscriber devices and the TSN scheduling software. This interface ensures that three critical elements are in place:

  • The TSN scheduler knows the publishing interval of OPC UA data (and in some cases an offset within the publishing interval).
  • The OPC UA publishing device knows when the network is ready (configured according to the schedule) so that it may start publishing.
  • The OPC UA subscribers are primed to receive data to which they are subscribed within the requisite timeframe.

Once the TSN scheduler has confirmed that it can assign a path through the network to each OPC UA PubSub data stream requested via the broker, the broker then provides configuration relevant feedback to the OPC UA publishers and subscribers.

To combine OPC UA PubSub data exchange and TSN scheduled communication, a mechanism is required that assigns TSN schedules to published OPC UA data.

Initial configuration of the OPC UA publishers and subscribers is still done via traditional engineering tools. The OPC UA PubSub Broker is an automated function that resides as a software module hosted on an Ethernet switch or another networked device like an industrial PC, this means that there is no additional burden on the user when configuring OPC UA PubSub over TSN. The system also allows for dynamic reconfiguration of OPC UA data streams over the TSN network.

When new publishers are added to the network, the brokerage process is repeated to instruct the TSN scheduler to look for available slots in the schedule and then provide feedback to the publishers and subscribers. The PubSub TSN Configuration Broker mechanism is currently being specified within the OPC Foundation. When complete, it will provide the link that enables industrial users to easily connect and configure OPC UA TSN compliant devices from any vendor.

A guiding principle of OPC UA is the facilitation of open and easy communication. OPC UA TSN now provides a way to seamlessly share information from all kinds of machines, devices and sensors in real-time.

Simon Brooks, Industrial Product Marketing Manager, TTTech.

Source: Industrial Ethernet Book Issue 102 / 14
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