Industry Groups Pave Way for Field-Bus Standard
Industrial Internet Consortium Members, IEEE, OPC Foundation and Shapers Group collaborate on open industry standard.
Industrial networks (also known as field buses) come from simpler times and were originally used to provide deterministic connectivity between PLCs, I/O points, and sensors over relatively longer distances. At that time traditional mechatronic systems used centralized architectures. In these centralized architectures a central CPU connected over a backplane (such as a VME or ISA bus), which also connected to various I/O boards. The boards would then connect to sensors using TTL logic. As machines grew larger and faster and sensors needed to be read at higher frequencies, such architectures started to fail due to noise susceptibility.
The concept of remote I/O was developed to solve these technical issues, and various field buses evolved using technologies such as RS485 and CAN. These technologies proved effective and reliable until the next wave of machine design, where the I/O and throughput requirements increased again, forcing the industry to look at alternative means to connect.
The Evolution to Ethernet
The need for faster machines, more stringent I/O requirements, and larger amounts of data forced the industry to move to a faster medium. The industry adopted Ethernet as a well-understood medium. While Ethernet is fast, it is based on the concept of CSMA/CD, which allows any network component to communicate at any time. When two or more components communicate at the same, instant packet collision occurs (causing data to be corrupted). The sender senses that a collision has occurred and retransmits the packet after some random time. This process is repeated until a packet is transmitted without a collision. The more components exist on the network, the higher the probability of a collision, which leads to larger amounts of network jitter. In order to minimize jitter, various automation vendors came up with their own protocols to manage network collisions. For competitive reasons, some vendors developed their own proprietary protocols.
Some automation vendors resorted to using managed network switches, while others made modifications to the Layer II of the Ethernet standard, thus breaking way from the standard. Using managed switches introduces more complexity, as these switches have to be configured for the specific machine architecture. These switches were also expensive and introduced significant network delays that hindered the performance of motion applications. In all cases on the factory floor, the Operational Technology application (OT) had to run on a separate network from the rest of the IT infrastructure, creating the IT/OT divide, and significant overhead in network and machine management and maintenance.
An Opportunity to Simplify
With the creation of the new Time-Sensitive Networking (TSN) standard, factories now have the opportunity to significantly simplify their networks and their IT infrastructure. A new set of IEEE networking standards will now allow network components to communicate with real-time performance across a wide area network (WAN) without compromising data integrity or security. Three new IEEE standards are responsible for this achievement.
IEEE 802.1ASrev: This standard allows network nodes and switches to have a common sense of time across a wide area network. One such application for this standard provides the capability for a network device to publish and deliver its data to specific destinations on a network in a cyclic manner with minimal jitter.
IEEE 802.1Qbv: This standard provides capabilities and techniques to forward or queue network packets based on their destination and required arrival deadlines.
IEEE 802.Qcc: This standard allows for definition and configuration of a system of nodes on a network that must communicate in real-time. Paths between nodes are calculated to guarantee that each network path can meet its arrival deadline requirements.
The creation and implementation of these three standards allow factory network hierarchies to collapse into one flat open network. Here it will be possible for real-time components (such as drives, sensors, and I/O banks) to co-exist on the same network with non-real-time components such as printers, office desktops, etc.
Figure 1 shows an example of a small factory network. Here we only see a representation of the OT network where the machines are connected together via standard Ethernet. The network mainly consists of a network of controllers that communicate using standard Ethernet protocols. On the other side they communicate using some flavor of real-time Ethernet, not compatible for a standard office network.
Figure 2 illustrates how a network with TSN may look. Here you no longer need a separate network for automation. All the components will sit on the same factory TSN network. Another side effect is that individual machines no longer need to have a PLC. The PLC function may be used into a local fog. The PLC functions may simply become a software function on a server or a local fog.
The IIC TSN Testbed and the Shapers Group
The IIC Testbed was originally proposed within the IIC framework as a physical platform where IIC members interested in adopting TSN may jointly collaborate, shares tools and best practices and test out the TSN concepts between their mutual devices. The original physical testbed was built and hosted at National Instruments (NI) in Austin, TX. NI would regularly host testbed members to get together a few times a year to connect their components together. The lab space provided by NI proved to be a very effective and collaborative environment where engineers from various companies jointly developed their respective TSN capabilities. Later another testbed was hosted at Bosch Rexroth in Germany to provide a lab area more accessible to European members. Other testbeds will also be added in China.
The Shapers Group is a special interest group formed within the TSN testbed members who recognized that while TSN provides a very effective delivery mechanism, they still needed a standard for managing and abstracting the data being communicated. The Shapers Group started to focus on the data definition and abstraction to provide seamless interoperability between machines and automation components on the network, based on open standards. OPC UA was chosen as the data modelling and communication capability. There were several aspects that led to the choice of using OPC UA. These included:
Independence and Openness: The OPC UA standard is maintained by the OPC Foundation which is an industry consortium that creates and maintains standards for open connectivity of industrial automation devices and systems.
Standardization: OPC UA has a high degree of standardization with very clear definitions on how to use the standard for data modelling and communications.
Powerful Object Model: OPC UA has a very powerful object model that allows data to be abstracted using an object-oriented paradigm. OPC UA also allows objects to define “method calls” that can be activated remotely by other connected entities.
Worldwide Acceptance and Pervasiveness: OPC UA is now available on the majority of modern controllers (PLCs, Industrial PCs, etc.) provided by vendors around the world.
Abundance of Tools and Stacks: Device Manufacturers and OEMs can easily start adopting OPC UA, thanks to a large number of technology vendors who provide commercial software and programming libraries. There is also a significant amount of open-source components for building OPC UA servers and clients.
Scalability: OPC UA can be built to scale from small embedded controllers with small memory, to larger CPU with lots of memory. While OPC UA has a huge number of features and functions the OEMs or device manufacturers can choose to build servers with only the required functionality for their application. A minimal functionality server may require as little as 50kBytes of RAM.
Integrated Security: Data encryption and security is defined and is an integral part of the OPC UA infrastructure allowing networked components to communicate securely. This is necessary to meet modern IIoT (Industrial Internet of Things) standards.
Figure 3 shows the original set of companies that collaborated in the Shapers Group and saw the opportunity to define OPC UA as a new machine interoperability standard in machine automation, using TSN as the real-time transport layer.
In order to efficiently meet the required interoperability performance between machines and components the Shapers also recognized that the OPC UA client/server model was an inappropriate for high-performance real-time applications. Some members worked closely with the OPC Foundation to add the Pub/Sub (publish/subscribe) functionality into OPC UA. This capability allows for one-to-one or one-to-many relationships between networked nodes, thus allowing components to publish data periodically. This is based on the concept of a bus cycle, where data can be shared without being explicitly polled.
The monumental effort put forth by the various working groups at IEEE, The OPC Foundation, the Shapers Group, and the IIC testbed members has now paved the way for a new field-bus standard that is based on open technology and open industry standards. Various vendors have started to develop products based on the new OPC UA /TSN standard. This includes network components such as I/O, industrial controllers, TSN switches, etc. Over time we will soon see drives with TSN network interfaces allowing OEMs and machine builders to deploy machines with very high-end motion performance requirements. Commissioning these machines will become simpler as well due to the plug-and-produce capabilities of this new field bus.
The Shapers Group is also continuing to expand. On April 24th, 2018 Rockwell Automation joined the Shapers Group recognizing that this new technology allows easy and secure sharing of information across different vendor technologies while the TSN suite of standards helps improve latency and robustness in converged industrial networks. The adoption of this communications technology is continuing to grow, and it is now highly likely over time to become the dominant standard in industrial automation.
Sari Germanos is part of the business development and technology marketing teams at B&R Industrial Automation Team. He is responsible for open source technologies and open standards for machine interoperability. He also has significant experience in applying simulation technologies to improve the efficiency of developing large-scale distributed systems. Germanos is chairing the work group developing the OPC UA Companion Specification for ISA TR-88. He also represents B&R at the Industrial Internet Consortium where he co-chairs the Networking Task Group. Sari received his MS in Computer Science from Boston College.