| Charter |
Develop specifications for programmable logic controllers (PLCs) concentrating on IEC 61131-3. This includes control program, tasks, controller variables, structured data, function blocks.
Interoperability of the new Generation – The How and What
Overview
PLCopen and OPC Foundation combine their technologies to a platform and manufacturer Independent information and communication architecture. This mix of OPC Unified Architecture (UA) and IEC 61131-3 creates interesting options for the realization of automation structures.
Goal #1: Semantic Interoperability by mapping IEC61131-3 into OPC UA Namespace
The first goal was to map the IEC 61131-3 software model to an OPC UA information model. This goal can be described in the following way: when today an IEC 61131-3 control program is loaded on different control platforms from different control suppliers one can communicate with these controllers by using OPC UA and access process variables. The representation in the namespace of the OPC UA servers is however different for each platform: A visualization program must each time be adapted for each controller although the control code is identical. However, the expectation from the customers is that an identical control project also is accessed via OPC UA in the same way. Besides the accessibility to the instances of the controller variables, further metadata needs to be available. Descriptions how complex data structures are constructed, the type of function blocks used, and which variable are in or out parameters. Other metadata can be the number of tasks and their cycle times. The entire IEC
61131-3 software model and the content of the controller programs are mapped into the OPC UA namespace. This namespace can be provided by an OPC UA server which is integrated into an embedded controller.
But why use OPC UA? With the Unified Architecture, OPC Foundation answers questions
like:
- How to find a communication partner?
- How are data, metadata, and functionalities of the communication partner made available?
- How can communication be reliable, performant and independent of the operating system or the programming language?
- How can security aspects such as authentication and encrypting be ensured?
OPC UA offers the basis for universal, secure and durable network communication, i.e. provides the monitoring of configurable timeouts and connecting interrupts, encrypted communication and different communication protocols, are part of OPC UA. Many other organizations use OPC UA as a modern and efficient transport layer.
PLCopen quickly recognized the possibilities, and adds to OPC UA by describing the contents, thus „what “, which should be exchanged: not only variables but also functions and metadata of the control logic.
The results from the working group are an important basis: A HMI for example can now provide a picture template for a function block – this graphic element can now communicate directly with all controllers on which the function block is present. This clearly simplifies the engineering process.
Goal 2#: OPC UA CLIENT FOR IEC 61131-3 (PLCopen)
The interaction between IT and the world of Automation is certainly not revolutionary, but is based on the long-established model of the automation pyramid: The upper level initiates a data communication (as a client) with the level below, which responds (as server) cyclically or event-driven: A visualization, for example, can request status data from the PLC or transfer new production recipes to the PLC. With Industrie 4.0 this strict separation of the levels and the top-down approach of the information flow will start to soften and mix: In an intelligent Network each device or service can autonomously initiate a communication with other Services.
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