Sensors, measuring devices, RFID chips, PLCs and other embedded systems provide important data for production in industrial applications. In conventional control architectures, data requests are initiated either cyclically or event-triggered and are always in response to requests “from above”, i.e. from the client level. The lower level always acts as a server and responds accordingly. In other words, an RFID reader or a PLC controller are “dumb” in communication terms. In the “Smart Factory” physical, real systems and virtual, digital data merge to form intelligent, self-organising production units. They acquire the data required for this purpose autonomously. This means that all devices and services must be able to communicate with each other independently, irrespective of the manufacturer, operating system, hierarchy and topology.
PLC controller initiates horizontal and vertical communication as OPC UA client
Beckhoff has integrated the OPC UA client function blocks, which are standardised by PLCopen in cooperation with the OPC Foundation, in the PLC. This initiative was first proposed by Beckhoff in 2006. Beckhoff has chaired the group since 2009. The specification for the function blocks was released in 2014. As an OPC UA client the controller can play the active, leading part, in addition or as an alternative to the conventional role allocation. The PLC is thus able to exchange complex data structures horizontally with other controllers, or it can vertically call methods in an MES/ERP system via an OPC UA server, e.g. to retrieve new production orders or write data to the cloud. This enables the production line to become active autonomously. In combination with integrated OPC UA security this is a key step towards Industry 4.0.
Effective, data-consistent services from the SOA-PLC
At present, data exchange between the MES level and the PLC usually takes place via a time-consuming handshake procedure: The MES system signals the transfer of a recipe to the controller, for example, and the PLC acknowledges readiness. Once the recipe data have been transferred, the transfer is acknowledged. SOA-PLC now makes it possible to transfer data to the controller with a single communication: Data values are no longer exchanged in multiple transactions, but handled as a single service with input parameters (the recipe) and output parameters (acknowledgement by the PLC). “Via OPC UA we make the Remote Procedure Call (RPC) available right down to the programmed PLC function”, explains Stefan Hoppe, TwinCAT Product Manager at Beckhoff. “This will shorten the communication round-trip times between the PLC and MES systems significantly and can lead to higher production throughput. It will definitely reduce the engineering costs for establishing a data link between the shop floor and the top floor quite dramatically.” The implementation is very simple for PLC programmers: A PLC method (with any input/output parameters) is simply available as a service call in the OPC UA server, which is integrated in the PLC. Each OPC UA client can call this service with the IT security functions and authorisation integrated in OPC UA while preserving data consistency, irrespective of the operating system. The calls to the PLC can also be initiated from other devices or services at the IT level or the cloud.
Some customers have already implemented the first pilot projects: For example, around 560 Beckhoff Embedded PCs control pumps and water towers in the water treatment plant for the Vogtland region in Germany, which covers an area of 1400 km². They act as distributed intelligences that make decisions independently and exchange information with each other or query statuses and process values for their own processes, in order to ensure uninterrupted process flow.