Stefano Scanzio

Evaluation of EtherCAT Distributed Clock Performance

EtherCAT is a real-time Ethernet protocol conceived explicitly for industrial applications. It is characterized by high communication efficiency, which permits control loops to be closed with short cycle times, and is provided with a suitable mechanism, known as distributed clock (DC), that enables synchronized operations to take place across the controlled system. These features can be profitably adopted, for instance, to support motion control applications. In this paper, the performance of the DC mechanism is evaluated by means of a thorough campaign of experimental measurements carried out on a real network setup. A number of factors have been taken into account that can affect accuracy and precision, and their effects studied in depth.

Performance of a Real-Time EtherCAT Master Under Linux

The adoption of open-source operating systems for the execution of real-time applications is gaining popularity, even in the networked control systems domain, due to cost and flexibility reasons. However, as opposed to their commercial counterparts, the actual performance level to be expected from them is still little known and may often depend on the kind of real-time extension being used, its configuration, and the overall software load of the system, including best-effort components. In this paper, an open-source EtherCAT master supported by a popular real-time extension for Linux, the RT Patch, is thoroughly evaluated with long-term measurements, which build confidence on the suitability of the proposed approach for real-world applications. Special attention is devoted to the unexpected, adverse effect that some best-effort components, for instance, graphics applications, may have on the overall real-time characteristics of the system. For reference, the proposed approach is also compared with RTAI, a more traditional and well-known real-time extension for Linux already in use for demanding applications.

On the accuracy of the distributed clock mechanism in EtherCAT

EtherCAT is a network protocol conceived for the use in factory automation environments, which relies on real-time Ethernet technology and is characterized by very high efficiency.

Seamless Link-Level Redundancy to Improve Reliability of Industrial Wi-Fi Networks

The adoption of wireless communications and, in particular, Wi-Fi, at the lowest level of the factory automation hierarchy has not increased as fast as expected so far, mainly because of serious issues concerning determinism. Actually, besides the random access scheme, disturbance and interference prevent reliable communication over the air and, as a matter of fact, make wireless networks unable to support distributed real-time control applications properly. Several papers recently appearing in literature suggest that diversity could be leveraged to overcome this limitation effectively. In this paper, a reference architecture is introduced, which describes how seamless link-level redundancy can be applied to Wi-Fi. The framework is general enough to serve as a basis for future protocol enhancements, and also includes two optimizations aimed at improving the quality of wireless communication by avoiding unnecessary replicated transmissions. Some relevant solutions have been analyzed by means of a thorough simulation campaign, in order to highlight their benefits when compared with conventional Wi-Fi. Results show that both packet losses and network latencies improve noticeably.

Implementation and Evaluation of the Reference Broadcast Infrastructure Synchronization Protocol

This paper describes reference broadcast infrastructure synchronization (RBIS), a clock synchronization protocol for IEEE 802.11 infrastructure wireless networks. The protocol is especially tailored for industrial and home automation networks, and in many application contexts, it offers several advantages compared with other solutions targeted at similar purposes. RBIS has been conceived to rely on conventional Wi-Fi equipment and, in particular, on unmodified access points. It is based on the master/slave approach and follows the receiver/receiver paradigm. An implementation of RBIS-carried out completely in software and based on timestamps taken at the interrupt handler level-has been developed, which achieves a synchronization error below 3 μs. Then, a simple distributed hard real-time control application has been setup, which consists in two PCs running real-time application interface for Linux (RTAI) and connected through Wi-Fi. The actuation error, measured on the generation of synchronous pulses, is strictly below 13 μs.

An enhanced MAC to increase reliability in redundant Wi-Fi networks

Wireless communications are becoming increasingly appealing for many application areas in industrial environments, including control systems at the shop-floor. IEEE 802.11 resembles Ethernet closely and, in the newest versions, it manages to provide similar throughput. Unfortunately, it fails in ensuring a comparable degree of reliability, mostly because its physical layer is quite sensitive to interference and disturbance. To overcome these limitations, very interesting solutions have appeared recently that layer a redundancy protocol above conventional Wi-Fi equipment. In this paper, a new approach is introduced and evaluated, which enhances reliability further by making redundant wireless adapters share information about the outcome of acknowledged transmissions. Doing so decreases both transmission latencies and the amount of lost packets.

A user space EtherCAT master architecture for hard real-time control systems

Recently, the open-source EtherCAT master developed by the IgH company, has been enhanced with the introduction of the EtherCAT library, which allows to develop control applications in user space. This feature makes it possible to build complex applications that need to use the floating point unit or existing user space libraries. Unfortunately, the EtherCAT library does not seem to be able to guarantee a high degree of determinism. In this paper, first the real-time performance of the user space implementation of the EtherCAT master is evaluated and compared with the one that can be obtained by implementing the same control application at the kernel level. Then, we propose an alternative hard real-time approach based on RTAI, that allows to exploit all the advantages of user space development, with a negligible impact on performance.

A distribute-merge switch for EtherCAT networks

EtherCAT is a popular industrial Ethernet solution conceived mainly for connecting control applications to remote I/Os. Because of its high communication efficiency - that achieves cycle times well below 1ms - it can be profitably adopted for motion control systems as well, which are known to have tight timing constraints. In this paper, a solution is introduced and described that is based on a suitable switch able to operate at the level of single EtherCAT telegrams. Such an approach achieves increased network performance by reducing propagation delays. In addition, a satisfactory degree of compatibility is ensured to the existing EtherCAT devices and applications.

Performance evaluation of an EtherCAT master using Linux and the RT Patch

This paper has the twofold goal of investigating the real-time performance of an EtherCAT master entirely built from open-source components (using Linux and the RT Patch at the operating system level) and assess its ability to support concurrent best-effort tasks without compromising the real-time ones, depending on kernel configuration. This is especially important for the successful adoption of the proposed approach in real-world applications. A hardware-based data acquisition system enables measurements to be taken for long periods of time, and with high resolution and precision. At the same time, this method guarantees that the measurement process does not influence the behavior of the system under test in any way.

Performance analysis of switched EtherCAT Networks

EtherCAT is a popular, real-time Ethernet network that features very high communication efficiency. However, because of the ring topology it relies on, cycle times increase when the network size grows larger (considering both the amount of process data and the network extension). In this paper a solution based on commonly available off-the-shelf switches is presented and analyzed that can improve performance for large automation systems. Such an approach (almost) does not require new custom h/w to be purposely developed.