26 April 2011
EtherCAT and Safety over EtherCAT get popularity boost
While many people are familiar with the application of Ethernet technology in the office and in the wider communications market, fewer may be aware of the protocol's application in the industrial sector. But Ethernet has been playing a central role for some time, taking industrial communications forward when existing fieldbus technologies reached their limits.
One particular implementation of Ethernet in the industrial world is EtherCAT, or Ethernet for control automation technology. This protocol was developed by German automation company Beckhoff and has since been taken forward by the EtherCAT Technology Group, which was founded in 2003.
An indication of EtherCAT's growing popularity can be gauged by the increase in membership of the technology group. Currently, there are more than 1350 companies in the group and membership grew by 36% between May 2009 and May 2010.
EtherCAT is optimised for process data, which can either be transported directly in Ethernet frames or packed into UDP/IP datagrams. Each Ethernet frame may contain several EtherCAT messages and the data sequence is independent of the physical order of the EtherCAT terminals in the network. Broadcast, multicast and communication between slaves are possible.
According to EtherCAT, its Device Protocol operates differently to other Ethernet solutions. Instead of a packet being received, interpreted and coped as process data at each connection, Ethernet frames are processed 'on the fly'. An EtherCAT slave controller at each node reads the data addressed to it while the frame is being forwarded to the next device in the network. And data for transmission is treated similarly, inserted in frames as they pass through the node.
Beckhoff claims an EtherCAT network can update 1000 distributed I/Os in 30µs and up to 1486byte of process data can be exchanged with an Ethernet frame. This, it says, is equivalent to almost 12,000 digital I/Os and transferring this data only takes 300µs.
Communications systems can be set up using a range of topologies; EtherCAT claims there are no practical limitations, subject to a limit of 65535 nodes per network segment. And it is possible to extend the link between nodes using optical fibre.
Addressing safety concerns, the EtherCAT organisation has developed the Safety over EtherCAT protocol (see fig 1). EtherCAT is a single channel system, which allows safe and non safe data to be transmitted over the same links. It says the transport medium is regarded as a 'black channel' and not included in safety considerations. However, safety frames can be included within the Ethernet message, allowing safety related data to be handled using 'containers'. These containers are said to be analysed in devices at the application level.
The frame is designed such a 6byte container is sufficient for transferring all error detection information, including 1byte of safe process data. But the protocol does not impose limits on the length of safe process data. This means that safety components with a large amount of safe process data can be supported.
The Safety over EtherCAT protocol has been tested by TuV as being suitable for use in applications requiring Safety Integrity Level (SIL) 3 certification.
The group notes that previous fieldbus communications systems did not have the necessary performance or capacity, which required high speed control loops to be closed locally, often by proprietary systems. This brought a number of problems, including closed technologies and complex communication schemes.
It says EtherCAT allows the performance of modern control systems to be applied to high speed control loops, overcoming previous problems. The bus interface is also simplified.
An indication of the growing support for EtherCAT came recently from Texas Instruments, which announced it had taken an EtherCAT license for its embedded processors. From the latter part of 2011, TI's Sitara range of ARM based processors will feature the technology.
"EtherCAT sets new standards for real time performance and flexibility, which is critical for TI embedded ARM devices geared toward the industrial market," said Matthias Poppel, director for embedded processing, EMEA, with TI. "As a leading industrial communication protocol, the growing popularity of EtherCAT in industrial drive and I/O applications is based on its robustness and simplicity, allowing master and slave controllers to communicate with each other without a host computer in high noise industrial environments. The TI approach to EtherCAT, with the programmable real time unit on ARM based processors, allows for flexible implementation that can more easily adapt as industrial standards evolve over time."
Martin Rostan, executive director of the EtherCAT Technology Group, added: "We are thrilled about this milestone development, which will open entirely new markets for EtherCAT. I am convinced that this will help EtherCAT to establish a strong position in a wide range of embedded applications and further accelerate the adoption of EtherCAT in the automation market."
A similar endorsement has come from industrial board developer Kontron, which has recently signed a cooperation agreement with Softing Industrial Automation under which the latter will provide Kontron with a range of IP cores and stacks, including EtherCAT, for implementation in fpga based boards.
According to Kontron, fpga based real time Ethernet protocols are often used to reduce the number of hardware variants, in turn reducing development effort. But other application areas include motion control, where customer specific algorithms can be implemented in the fpga alongside real time Ethernet protocols. This combination allows fast central response times to be achieved when several axes are being controlled synchronously.
"Our aim is to provide our customers with application ready designs which have the interfaces implemented to suit their specific needs and which are already thoroughly tested and can be directly certified with the target application," said Dirk Finstel, Kontron's chief technology officer. "Customers don't want to have to deal with specific I/O interfaces and communications protocol layers; their core competence is in the application. As a supplier of boards and systems, it is our job and our core competence to provide all the relevant interfaces and tested communication protocols. Our cooperation with Softing lets us offer this on the basis of its fpga stacks and IP cores for real time Ethernet."
Softing's EtherCAT IP cores and stacks – along with those for Profinet, EtherNet/IP and MODBUS TCP – will be available for implementation in the fpga based I/O hubs of Kontron's latest processor boards, but they can also be licensed from Kontron.
Softing's ceo Dr Wolfgang Trier noted: "A strong feature of the Softing solution is the modular architecture of our protocol stacks. For example, for fpga based platforms, customers can choose from any combination of standard and softcore cpus for their system and application concept. This architecture supports Kontron's fpga based embedded computer offerings. With Softing's protocol stacks on board, customers will significantly reduce the development and verification effort on their way to an industrial Ethernet capable product."
According to Kontron, sample implementations of Softing stacks and IP cores for EtherCat and the other industrial communications protocols will be available on the Kontron PCIe/104 MICROSPACE MSMST and the Kontron COM Express FPGA Starterkit.
Author
Graham Pitcher
Supporting Information
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Kontron UK Ltd
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