Both a mapping and a match [made in Heaven]
Mapping PROFINET data to the OPC UA information model has been one of the main tasks of PROFIBUS & PROFINET International (PI) for the past few months. Analyzing the real-world requirements that can be addressed by focusing on such a mapping has yielded a number of use-cases. These use-cases show that the effort spent collaborating on a Companion Specification with the OPC Foundation will be beneficial across the Industry 4.0 landscape.
The Use Cases
Interoperability has been a main tenet of OPC from its humble beginnings as a means to connect PLCs and HMIs from different vendors. While classically performed via client/server, the publication of Part 14 of the OPC UA specification now allows for a publish/subscribe model. With it, PLC to PLC communication becomes much more attractive. Horizontal integration or Controller-to-Controller (C2C) is actually nothing new for PI. In fact, 15+ years ago the very first version of PROFINET, Component Based Automation (CBA), satisfied this exact need. But as Ethernet reached deeper into the field, PROFINET for I/O was created and proved much more popular. Now that the time has come again to address C2C, instead of reinventing the wheel, the choice is clear to adopt a well-established purpose-built standard for this task: OPC UA.
Functional safety at the C2C level has yet to be addressed in a truly open vendor-neutral format, using widely adopted technologies. To close this missing link in the chain, PI has provided the PROFIsafe technology to the OPC Foundation for use with OPC UA. It makes perfect sense. PROFIsafe, as an Application Profile ‘lives above’ the 7-layer ISO/OSI Model. By virtue of the black channel principle, it doesn’t care about the underlying transport or physical layers, merely ensuring messages get from one point to another. PROFIsafe has required either PROFIBUS or PROFINET for a foundation. Now, mapped to OPC UA, there is a single safety concept for both the controller level and the field level.
Vertical Integration: Controller Level
Mapping PROFINET to OPC UA in a PLC enables vertical integration. First, high-speed data gets buffered and collated. Doing so turns it to information. Then, that information gets sent vertically to higher level systems. If the PLC is short on memory or computing power, a dedicated edge gateway can be installed to perform the task.
Vertical Integration: Edge Gateway
The methods and results are the same when mapping PROFINET to OPC UA in an edge gateway instead of in a controller. For end-users particularly in the process industries, the NAMUR Open Architecture describes a way to do this outside the traditional automation pyramid. The goal here is to leave the existing process controls untouched and create a second data channel apart from existing connections. The mapping and decoupling occur in the edge gateway.
Vertical Integration: Device Level
Similarly, mapping to OPC UA directly in a PROFINET device or IO-Link master can achieve the same goal of vertical integration. And since both PROFINET and OPC UA are based on standard Ethernet, they can share a common infrastructure. Both protocols are passed on the same wire. This comes with a warning however: compared to PROFINET frames, OPC UA frames are relatively large and can eat up bandwidth. That’s where Time Sensitive Networking (TSN) comes in -but not for this discussion.
During the course of work mapping PROFINET to OPC UA, it was determined that existing OPC UA information models do not sufficiently address all use-cases. Often times there are multiple ‘views’ for one entity. For example, information about the components, assets, or location of the entity would be considered the Physical View. The Communication View of the entity would contain information regarding its communication relationships and the PROFINET device. Since each View looks at the automation entity from a different perspective, these can be considered Facets. A complete picture can be created through the aggregation of each Facet.