OPC-UA Agent for Legacy Programmable Logic Controllers

Open platform communications (OPC) unified architecture (UA) is a communication standard increasingly used in industrial automation systems to enable the exchanging of control and management data between distributed entities. This paper proposes the design of an OPC-UA agent to enable UA information service and client functionalities in legacy programmable logic controllers (PLCs). The agent runs on a separate machine connected to the PLC using a dedicated link and maintains shared memory for certain variables in the PLC. Based on the periodically synchronized variables, the agent services the OPC-UA information model and executes client function blocks on behalf of the PLC. One important design feature is the remote procedure call of IEC 61131-3-based function blocks using synchronized variables. This allows the standard OPC-UA client functions to be used in existing PLCs which only support numeric types and do not support strings or complex structures. To validate the proposed design, we implement an agent prototype and demonstrate the successful monitoring and control of an industrial robot controller via OPC-UA. Through experiments, we evaluate the performance of UA functions in terms of the latency of read services for an increasing number of items. The evaluation results are believed to provide useful insights into agent-based approaches for integrating legacy PLCs into the OPC-UA framework.