The Field Level Communication Initiative (FLC) initially started with 23 founding members. In the meantime, Murrelektronik, Festo and Lenze have joined as additional supporters in order to develop the requirements for a universal, OPC UA-based communication standard in the field of factory and process automation in various working groups. The requirements analysis not only takes into account the different life cycles of automation components and production systems, but also the needs of the various users along the value and supply chain: technology providers, automation manufacturers, machine builders and system integrators, end users and data analysts. The FLC Steering Committee also presented the jointly agreed roadmap at last year's SPS: The first specification release will focus on the controller-to-controller (C2C) use case.

The FLC Interaction Model with the various 'abstract' OPC UA use cases.

© OPC Foundation

The subsequent specification release will then contain corresponding extensions for the controller-to-device (C2D) use case. The fact that all FLC devices (controllers and devices) are based on OPC UA means that a standardized, consistent level of communication is available across all automation levels, which also covers the use cases shown in the image - controller-to-compute, device-to-compute and compute-to-compute. This opens up completely new possibilities, particularly with regard to the various Industry 4.0 application scenarios and IT/OT convergence.

Technical working groups

Over 150 experts from more than 50 companies are currently involved in the technical working groups of the FLC initiative, which are open to all members of the OPC Foundation. Due to the Covid-19-related cancellation of several multi-day working meetings, which could not be fully compensated for by virtual web meetings, the specification work has slowed down a little in recent weeks. Nevertheless, the release candidate of the first specification version is within reach. According to current planning, it should be ready in the 3rd quarter.

Base Device Facets

The FLC system architecture based on the OPC UA framework.

© OPC Foundation

The underlying FLC system architecture is based on the OPC UA framework (IEC 62541), which enables secure, reliable and manufacturer- and platform-independent information exchange, see image on the right. FLC devices support both the connection-oriented client/server communication model and the publish/subscribe extensions, which are essential for communication at field level due to the corresponding requirements for flexibility, efficiency and deterministics. FLC is also based on the security mechanisms specified in OPC UA, which support authentication, signing and encryption of the data to be transported and can be used for both client-server and pub/sub communication relationships.
The central element for the extensions specified by FLC is the OPC UA metamodel. This is used to specify corresponding information models for FLC devices and to make the modelled information accessible via standardized OPC UA services.

The facets

Facets describe the functionality (functions, interfaces and behavior) of automation devices. The FLC roadmap envisages that in a first step, the Minimum Viable System (MVS), the basic functionalities of an FLC device are specified, which enable the first FLC products to be fully usable and at the same time backwards-compatible with regard to future specification updates. The base device facet plays a decisive role here. It describes basic device functions that are independent of the specific characteristics, such as device identification or basic diagnostics. Building on the base device facet, device-specific or function-specific facets are then specified, such as for safety and motion.

Safety Facet

Functional safety requirements are covered by OPC UA Safety. The first OPC UA Safety specification has already been adopted for this purpose, which is based on client-server mechanisms and was developed from a joint working group with the Profibus User Organization (PNO). There will soon be an extension that describes the mapping to PubSub and the parameterization of safety participants. One of the special features of the safety concept for OPC UA is that safe participants can also be integrated into the communication during operation, which is not possible in this form with conventional safety protocols.

Motion Facet

Just a few weeks ago, a new working group was set up to specify the Motion Facet (OPC UA Motion). The Motion Facet comprises the specification of motion control functions for various types of motion devices, such as controllers, standard drives, frequency inverters and servo drives. The FLC Steering Committee has agreed to build on the CIP Motion and Sercos specifications and to adapt them to modern concepts of data modeling and real-time requirements, as well as to consider corresponding Industry 4.0 use cases.

Offline engineering

Offline engineering is also on the FLC initiative's to-do list. The aim is to integrate field devices into projects via corresponding device description files without them having to be physically present first. With the help of product and configuration descriptors, the device descriptions are made accessible to corresponding configuration tools. However, the specification work does not start 'from scratch', but is based on the corresponding preliminary work and experience of the supporting automation manufacturers and fieldbus organizations.

Communication Facet

Peter Lutz is Director Field Level Communication at the OPC Foundation.

© OPC Foundation

An important topic for FLC is the question: How are connections established between different automation devices? And how can corresponding data be exchanged via these connections using Pub/Sub?
Modeling the quality of service (QoS) offers a way of flexibly mapping services to subordinate communication protocols - layer 3 mapping via UDP and direct layer 2 mapping to Ethernet TSN - and transmission physics (SPE/APL). This approach also allows easy extension to other subordinate transmission standards, including wireless data exchange via radio, such as 5G and Wi-Fi 6.

The combination of OPC UA with direct mapping to an underlying Ethernet TSN is particularly important: this is the only way to enable deterministic data transmission via OPC UA at field level. A working group under the leadership of the FLC Steering Committee is currently working out which TSN sub-standards should be made mandatory for FLC end devices and infrastructure components in order to meet the defined requirements for performance, flexibility and user-friendliness. The OPC Foundation has made a clear commitment to the TSN-IA profile, which is being developed in the IEC/IEEE 60802 working group and aims to ensure that different protocols and traffic types can be transmitted via a common network infrastructure.
This coexistence is not only essential for the convergence of IT and OT; it is also an important aspect in the migration of existing 'brownfield' solutions based on conventional fieldbus protocols.

Status quo and next steps

Work on the first version of the specification has made good progress in recent months - despite Covid-19 and the associated restrictions. The basic concepts have largely been approved and incorporated into the first draft specifications. An initial technical document explaining the underlying system architecture of FLC and the components it contains is due to be published in June. The first release candidate of the FLC specification with a focus on C2C is then planned for July/August, on the basis of which prototypes can then be implemented and the draft specification validated.
At the same time, a working group will be set up to generate corresponding test specifications, which will then be converted into corresponding test cases for the OPC UA certification tool (CTT) in a second step.