16 August 2011
Fieldbuses are an established standard technology in automation; distributed I/O has replaced parallel wiring to centralised I/O cards in most applications. Users benefit from improved diagnosis, simplified installation and less wiring effort. It was fieldbus technology that enabled the wide-scale application of PC-based control systems. While the performance of controller CPUs – particularly for IPCs – is increasing rapidly, conventional fieldbus systems have become ‘bottlenecks’ that limit the performance of control systems. An additional factor is the layered control architecture, consisting of several subordinate (usually cyclic) systems: the actual control task, the fieldbus system and perhaps local expansion buses within the I/O system or simply the local firmware cycle in the peripheral device. Reaction times are typically between three and five times higher than the controller cycle time – an unsatisfactory solution. Above the fieldbus system level – i.e. for networking controllers, Ethernet has been employed for some time. Ethernet has also entered the drive or I/O level areas traditionally dominated by classical fieldbus systems. The main requirements for this type of application are high real-time capability, suitability for small data quantities, and cost-effectiveness. EtherCAT meets these requirements and at the same time makes Internet technologies available at the I/O level.Ethernet at I/O levelFeatures of Ethernet which make it less suitable for automation technology applications should be considered very carefully, and circumvented wherever possible, when considering the use of this technology in automation applications. The main differences between the different approaches for adapting Ethernet to automation technology are how they cope with these Ethernet features:* High overhead for communication with devices that frequently have to exchange small quantities of data.* High interface costs compared with classic fieldbus nodes (transformer, PHY, MAC and required processor performance).* Lack of real-time capability which, on closer inspection, is caused by runtimes of the software stacks, rather than by the fact that Ethernet is used as the transmission medium.* Unfavourable topology. Ethernet now commonly uses a star topology which is not ideal when it comes to system wiring and can lead to excessive cabling effort or highly cascaded communication dependencies.EtherCAT functional principleBy applying the functional principle of ‘Processing on the Fly’, EtherCAT technology overcomes the inherent limitations of other Industrial Ethernet solutions: The Ethernet packet is no longer received then interpreted with process data then being copied at every device. The EtherCAT slave devices read the data addressed to them while the frame passes through the node. Similarly, input data is inserted while the telegram passes through (see Fig. 1) with the frames hardly being delayed at all. The frame sent by the master is passed through to the next device until it reaches the end of the segment. The last device detects an open port and so sends the frame back to the master.An EtherCAT frame comprises the data of many devices both in send and receive direction within one Ethernet frame. The usable data rate increases to over 90%. The full-duplex features of 100BaseTX are fully utilised, so that effective data rates of >100 Mbit/s (>90% of 2 x 100 Mbit/s) can be achieved. But what are the resulting EtherCAT features? And even more important… What are the end user benefits of these features?More efficient machinesEtherCAT is not substantially faster than traditional fieldbus systems, but is the fastest of the industrial Ethernet solutions. Typical EtherCAT cycle times are 50…250 µs, while traditional fieldbus systems take 5..15 ms for an update. EtherCAT makes the processing power of an industrial PC available for the application, and faster controls provide more accurate results. Canadian injection moulding machine manufacturer Husky, for example has reported that by using EtherCAT it was able to reduce the part weight variation and thus the wall thickness of the plastic cup saving material worth over US $180,000 per machine per year. However, it is not just closed loop controls that benefit from faster communications: in many applications there are so called transitions, where it is necessary to wait for the part to arrive, the cylinder to reach its end position, or the pressure to reach a certain threshold. With a faster network, waiting times are reduced and the throughput of the machine or plant is increased. Better motion controlEtherCAT is also very precise. Its distributed clock allows measurement values to be sampled and outputs set in a synchronized manner network-wide. Jitter substantially smaller than one microsecond. In most cases deviation is even below 100 nanoseconds. This accuracy is ideal for synchronized motion control applications and for integration of measurement tasks within the same network.EtherCAT is also inexpensive: On one hand there are the software implemented masters on standard Ethernet ports, on the other the low-cost slave controller chips. The cabling side also saves money, as EtherCAT does not require switches or other active infrastructure components and uses standard cabling and connectors. Engineering effort is also reduced as network tuning is not required and the diagnosis features of the technology provide exact error localization and require less trouble shooting time. Technology acceptance There is another factor for low component prices –acceptance of the technology, wide choice of products and competition among the suppliers. EtherCAT is supported by the EtherCAT Technology Group, which has over 1700 member companies from 52 countries. EtherCAT also has the fastest adoption rate among the Industrial Ethernet technologies. The product guide on www.ethercat.org currently features over 300 entries with over 600 products.EtherCAT networks have no practical limitations regarding topology – line, star, tree, redundant ring and all those combined with up to 65535 nodes per segment. In case the 100m distance between two nodes are not sufficient, fibre optic cables are used. Even wireless technologies can be integrated aAd hot connect enables connection and disconnection of nodes during runtime. The speed of EtherCAT allows classical fieldbus systems to be integrated easily and 20 systems are already supported. With these components it is possible to integrate existing devices into an EtherCAT network and realise interfaces to neighbouring or higher level systems. Migration from previous systems is facilitated and at the same time the complex interface variety of the central controllers is omitted: other systems are simply integrated via EtherCAT, and not via PCI, cPCI, PCIe, PC104, etc.Functional safety option Conventionally, safety functions are realised separately from the automation network, via hardware or using dedicated safety bus systems. Safety over EtherCAT, however, enables safety-related communication and control communication on the same network. The safety protocol is based on the application layer of EtherCAT, without influencing the lower layers. It is certified according to IEC 61508 and meets the requirements of Safety Integrity Level (SIL) 3.With EtherCAT networks, there is no need for manual address settings via dip-switch or rotary switch at every device because at boot-up the addresses are assigned automatically. Even if devices are added later, the original addresses can be retained. EtherCAT masters can also feature an automatic topology recognition, comparing the actual network configuration with the one expected by the application program. After device replacement, all parameters can be downloaded automatically. Almost every fieldbus and Ethernet technology claims to be open. With EtherCAT, however, openness does not just mean international standardization, but also free implementation support, clear guidelines regarding interoperability, master and slave implementations for a variety of operating systems and controllers, openness of the configuration tools for third party devices and specifications also for the application interfaces. Furthermore, Texas Instruments recently announced the implementation of EtherCAT slave controller functionality in several ARM based product lines – which makes EtherCAT the first dedicated industrial Ethernet to be integrated into standard microprocessors, turning it into a mainstream technology.EtherCAT technology is not only fully Ethernet-compatible, but also characterised by particular openness ‘by design.’ The protocol can transport other Ethernet-based services and protocols on the same physical network. Such Internet technologies are tunneled via the EtherCAT protocol, so that the real-time characteristics are not impaired. All internet technologies can therefore also be used in the EtherCAT environmentEtherCAT is used worldwide in a wide range of applications, including robotics, packaging machines, presses, printing machines, semiconductor and FPD manufacturing equipment, logistic systems, power distribution, wind energy, automotive test beds, assembly lines, food processing systems, woodworking machines, metal forming and plastics machines.
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