This paper proposes a systematic design approach for a precision-guaranteed motion control system. We develop a delay-guaranteed motor drive with our new software implementation and real-time Ethernet, which can be used as a building block to build up a multi-axis motion control system. Our drive software implementation provides a probabilistic guarantee on drive-local processing delays to motor actuation, while real-time Ethernet provides a deterministic guarantee on message communication delays from a motion control host to each drive. In the paper, we address the precision of a motion control system in two terms: host cycle time and simultaneous actuation deviation. The host cycle time is a period with which the host can periodically release motor control messages while the average drive utilization does not exceed 1, and the simultaneous actuation deviation is the difference between the earliest and the latest actuation at different drives in response to the same message. In our approach, the main objective is to minimize the periods of tasks in each drive, using our stochastic analysis, which gives us a minimum possible host cycle time. Together with an existing delay analysis of real-time Ethernet, we analyze the end-to-end delay from message release to motor actuation and in turn the simultaneous actuation deviation. Through experiments, we show that for various requirements on the deadline miss probabilities of the tasks, we can successfully reduce the host cycle time and evaluate the resulting distribution of the simultaneous actuation deviation depending on the number of drives.

OPC unified architecture (UA), a communication standard for the manufacturing industry, enables exchanging control and management data among distributed entities in industrial automation systems. OPC UA wrapper is a migration strategy that provides UA clients with seamless access to legacy servers having OPC classic interfaces. This paper presents the design of a standalone OPC UA wrapper and discusses its performance through extensive experiments using a prototype implementation. The wrapper consists of two main components, i.e., UA server and classic client, which communicate with each other via shared memory and semaphore. One important feature of the design is that it employs a distributed component object model runtime library implemented in Java for platform independence. This makes it possible to build a cost-competitive wrapper system by using commercial off-the-shelf non-Windows solutions with low-cost microprocessors. Another key feature is the event-driven update interface between the UA and classic components, which we propose as an alternative to the sampling-based mechanism for the reduced delay. Through experiments using workloads from an industrial monitoring system, we present a systematic approach of identifying the system parameters having a direct impact on the wrapper performance and eventually tuning them such that the read and subscription services of OPC UA exhibit the best performance.

Toward a Holistic Delay Analysis of EtherCAT Synchronized Control Processes

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.

Load scheduling over cyclic electrical devices can reduce the peak power demand. In this paper, we propose a machine-learning-integrated load control (MILC) scheme for improved performance and reliability. By dynamic capacity adjustment and interactive load heuristic, MILC tries to reduce the power deviation while keeping the temperature violation ratio and switching counts within an acceptable range. A prototype of the proposed scheme has been implemented and, through experiments using load traces from a real home, we evaluate the performance of MILC. The results show that MILC reduces the peak demand from 4993 W to 4236 W and successfully decreases the power deviation by 12.1% on average.