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.

Ethernet-based motor drives are hard real-time control systems used to operate servomotors through the industrial Ethernet. Recently, Ethernet-based drives have drawn attention as a solution for industrial transport systems where numerous linear motor drives move magnetic shuttles individually or collectively to accurately position production parts. This paper presents the design and implementation of an Ethernet-based motor drive that enables delay analysis for synchronized motor actuation and sensing to build a scalable and precise industrial transport system. Our software design constructs the drive function using periodic tasks run by a rate-monotonic real-time scheduler and performs worst-case response analysis to determine the end-to-end delay required for the control host to actuate or sense the motor in the drive. Based on the calculated drive delays, clock-based I/O using Ethernet-provided global time realizes synchronized motor operation across multiple drives. In the Ethernet-networked control system, different phases of the host cycle with respect to the drive cycle can result in different actuation and sensing delays. To reduce the delay, we propose a phase-shifted loop method and present a heuristic to find the best phase that minimizes the normalized drive delays. Experimental results obtained using a prototype EtherCAT drive show that the phase-shifted loop significantly reduces the difference between the commanded and feedback currents while properly managing tracking errors. Performance evaluations are performed to investigate the impact of different Ethernet technologies on delays. Elaborated delay models are developed for EtherCAT and Ethernet Powerlink, and a comparative study of delay performance is conducted for various parameters, such as the number of drives, the message size, the network topology, and the bandwidth.