Digitalization of machines requires standardized language for machine communication – and OPC UA is the perfect choice.
In 2019, the VDMA Glass Technology Forum established a joint working group to define the requirements of the flat glass industry in an OPC UA companion specification (CS) for flat glass processing machinery. Now, four years later, we are proud of the standardized communication for flat glass processing machinery that has been established. But our ambition did not stop there: We have continued to develop the standard, focusing on harmonizing recipe structures for a broad variety of glass processing machines and machine feedback, allowing compliance with KPI (key performance indicators) standards such as ISO 22400. Our input to the superposed OPC UA CS for machinery is integrated into the current version of the specification – proof that the challenges of flat glass processing are not much different from those of production machinery in other domains.
Introduction
One fundamental requirement for the digitalization of machine compounds is a common language that allows information to be passed on clearly, exactly as humans do. As part of Industry 4.0, the digitalization of production, OPC UA has become established over the last 20 years as a major language for machine-to-machine communication (just as English is used as a common language for specialist literature and conferences). Like in any language, there is a domain-specific dialect in which specific words have defined meanings. In the digital world, this dialect is covered by domain-specific companion specifications (see Figure 1). Using standards like this eliminates the need for expensive and high-maintenance proprietary interfaces between machines – saving time and money when integrating machines into the production environment and minimizing the risk of failures/data loss, especially during production ramp-up.
As a result of the very positive response to the initiation, various major, well-established European suppliers of flat glass processing solutions played an active role in establishing the standard. The group defined common targets for standardization, such as:
1. uniform two-way communication of job metadata between processing machines and superposed manufacturing execution systems (MES)/production planning systems (PPS),
2. detailed processing information incorporated into a “recipe specification,” and
3. feedback in “Machine & Production Monitoring” (see Figure 2).
The intensive cooperation within the group resulted in better understanding of sub-domain requirements, such as glass cutting, milling, glass lamination as well as assembly of insulating glass units (IG/IGU). The first OPC UA Companion Specification 40301 for Flat Glass Processing was released in January 2022, specifying the Job Sending and Status module.
Why do we need an OPC UA specification for job transmission?
If you look at a modern flat glass processing facility – or indeed any other processing facility – you will find machines from different brands communicating with MES/PPS to obtain jobs. All this communication is, in most cases, based on proprietary standards defined by whoever is in charge. Replacing machines or adding further equipment to expand production entails lengthy protocol definitions, implementation costs and production risks during start-up. Furthermore, current job sending is in most cases one-way communication, leaving the MES system without a trace of the current job status. Additional procedures like manual “ready messaging” (using product scanning or a display) are therefore used to keep track of production.
Creating a uniform standard for job communication that allowed two-way communication was therefore defined as the objective. The core element is the job state machine (Figure 3) – an algorithm that allows jobs to have different states. Prior to production, the jobs are in an initializing buffer from “Idle,” meaning that they have just been transferred to the machine. “Queued” means that they have been placed in the production sequence, while “Released” shows that all materials are available for production to start. Once released, the job switches to “Running,” meaning that it is being executed. Ideally, it will then be “Ended” successfully. The states remain similar for all processing types (from cutting to IG assembly).
With the OPC UA CS standard, the communication is now properly defined. A major benefit is that the superposed MES is informed about the change and current job status in real time, making production tracking more accurate and enabling last-minute interventions in the job sequence.
Is the job model a complete description of my job or do we need more?
The job model holds the essential information such as the name of the job, time constraints, the requested machine (type) for the job, and a bulk of information containing the specific manufacturing details. In the flat glass industry, this bulk of information is usually transferred as a file in specific format (comparable to the g-code you send to your 3D printer). Given our strong ambition to fully standardize job transfer, the definition of a uniform standard for working instructions for all machines started with the “Recipe” definition. We soon discovered that many of the requirements we face are hard to encode in a way that meets the OPC foundation standards. We invented a “loadable Enums” structure to provide the required flexibility. To achieve our definitions on a formal level, we ended up with the specification VDMA 24124. Our basic idea is to get this definition into the field first, before collecting those Enums actually used in practice in an OPC foundation-type standard for later use.
The work related to VDMA 24124 started with the challenge of achieving a global standard for our industry while ensuring that the resulting definitions can still be read by humans in the field. Based on the requirements, the decision was quickly taken to use JSON-type tag value-based descriptions. JSON formats can be easily expanded and read by humans. Unfortunately, JSON itself is such an open standard that it is quite difficult to agree on a specific format. We therefore decided to use Google Protocol Buffer definitions as a guideline (see www. https://protobuf.dev). Various tools and libraries exist within this environment to ease development.
Starting with the cutting process, we identified the related objects and used them to define our structure. One of the fundamental objects within the definition is a rectangle that may contain other rectangles (recursive definition). Therefore, an example of a cutting pattern might look like the one presented in Figure 4.
The level of definition achieved in our first release comprises all requirements for the cutting process, including specialties like laser marking, edge and area deletion, and surface defects (scratches etc.). Assembly processes (IGU) are also covered, even including very special, rare applications. A reasonable level of processing definitions is already defined, too.
Our next targets for refinement are total coverage of assembly processes and a reasonably high level of processing requirements (CNC machinery for cut-outs, nudges, drill holes, etc.).
Will I receive standardized information about the machine and production?
The main objectives of Industry 4.0 using OPC UA CS in various domains are a systematic approach for obtaining machine status information and detailed feedback from production. To ensure standardization on a global level, discussions about feedback from flat glass processing machines were integrated at a higher level into the companion specification for all machinery in the OPC 40001 series (see Figure 1). Since the machine status and modes are already defined in this specification, we investigated how to use those parameters as a base KPI calculation. Integrating the standard of KPI for MES Systems, the ISO 22400 specifications, provides a clear definition of how the machine information shall be used to calculate such values based on machine feedback. In short, KPIs such as overall equipment effectiveness (OEE) and net overall equipment effectiveness (NEE) can only be calculated in a correct normative manner if information from machine and production planning are combined (see Figure 5). For example, for OEE calculation, there is a difference between a machine being “out of order” during regular production (where it counts against) and “off times” like official breaks or other non-production times (where it does not count against). Examples of how to correlate times are published in the CS OPC UA for Machinery VDMA 40001-1.
In order to gain a deeper insight into the production executed and the transfer of information required for materials backtracking and quality control (basis of the future product passport), a feedback model for machine data was developed and harmonized with the ISA 95 job control (OPC 10031-4) standard. The main information blocks of this model contain the following for each object produced:
· General production information (start and end of job, general status)
· Bill of used material
· Quality-related production parameters
Results of this model are released in the CS OPC UA for Machinery 40001-3:2023 Job Management, including an example based on an insulating glass element.
What are the next steps?
Specification of standards is just the first step. Standards only come to life once they are applied. Therefore, we are now working with our group to simplify the OPC UA CS implementation and further consolidate the CS for flat glass with the CS for machinery.
To sum up, we are pleased that we were able to form this group of named flat glass processing suppliers for machinery and software and to establish an initial specification in such a short time. Challenges will certainly arise, but we are certain that we can overcome them. We look forward to the first live applications of our standard, benefitting machinery suppliers and, in particular, their respective customers.
