Determinism: Don't Change A Thing

Ethernet has displaced many industrial networks over the past few years, as many users made the transition from a range of fieldbuses.

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But now that they’ve extended Ethernet beyond central backbones, users don’t want to make any changes to gain the benefits of determinism.

Gaining true determinism requires some changes to either hardware or software, but equipment developers want to minimize those alterations. One lure of Ethernet is that a single cable can link together many types of equipment that used to require a range of cabling and connection schemes while also carrying a number of protocols.

Engineers who need determinism, which isn’t inherent in the standard, don’t want to give up that commonality. When Mag Industrial Automation Systems searched for a network that provided precise timing, the first items on its checklist were compatibility and flexibility. Determinism had to mesh into existing applications seamlessly without locking integrators into a single networking topology.

 “We want to stay away from anything that’s proprietary; we need a common design that’s flexible enough to change,” says Pawan Seth, senior project engineer for control at Mag Industrial Automation Systems, of Sterling Heights, Mich. Mag picked EtherCat for the environments that require determinism.

In applications as diverse as machining and laying out composite aircraft wings, Mag’s users need to be sure that many different pieces of equipment are communicating efficiently. When they’re not, the systems have to shut down immediately so they’re not building scrap. “We have several machines tied together, so if the programmable logic controller (PLC) is not getting the right information, you need to know immediately,” Seth says.

Often, customers need to add determinism to established networks. Few want to rip and replace entire segments of their infrastructures to obtain improved timing. The fewer hardware changes they can make, the better.

“If these same customers can retain their current software drivers and communications interfaces and still get the added value of determinism, they will do this more readily, and that technology will be more easily adapted,” says Steve Zuponcic, application engineering manager for supplier Rockwell Automation Inc. in Mayfield Heights, Ohio. “Suppliers that can provide determinism transparently will have the best differentiation. Their customers won’t have to think about it as they build their architectures.”

While hardware changes are taboo to many customers, some networking suppliers find that users are willing to alter software to gain determinism. That approach is taken by CC-Link, which was originally developed by Mitsubishi.
 
“Without requiring any costly hardware changes, only a minor application layer software modification, manufacturers with a transmission control protocol/Internet protocol (TCP/IP) product can have their device operate on a CC-Link IE Field network by connecting to an Ethernet Adapter module,” says Chuck Lukasik, director of the CC-Link Partner Association-Americas, based in Vernon Hills, Ill. “CC-Link IE Field operates at 1 Gbps (gigabit per second), which is 10 times faster than competitive industrial Ethernet networks.”

As with the shift to Ethernet, safety in numbers is a key factor when plant managers select networking technologies. Strong support from many manufacturers coupled with the security of standardization were important factors when Kollmorgen had to add determinism for its time-critical installations.

“We support CAN over EtherCat, using CanOpen. We picked EtherCat partially because it is an international standard that’s used by a lot of companies,” says Carroll Wontrop, senior system engineer based in Radford, Va. “The distributed clocks let us handle multi-axis systems for motion, synchronizing the relationship between each axis.”

Another reason engineers and managers like standards is that it’s fairly certain that the technology and products will continue to advance. That’s certainly true for the overarching Ethernet specification. The 100 megabits per second (Mbps) version is common in industrial applications, and many users are moving to 1 Gigabit (Gbit) Ethernet.

This transfer rate moves the core technology closer to speeds that would have been considered real time a decade or so ago. Some system designers will be able to meet their timing demands when they implement Gbit Ethernet. But the need for true determinism will continue, and users will continue to expect more from their networks.

“When you go beyond full duplex communications at 100 Mbps to 1 Gbps, faster speeds will allow for even more precise levels of determinism,” says Rockwell’s Zuponcic.“The faster components go, the faster people will want to do things.”

While faster versions of Ethernet will have some impact on the technologies that offer determinism, it’s expected to be minimal. Some observers feel that even the next version, 10 Gbit Ethernet, won’t make a big dent when it eventually moves into industrial applications. That won’t be any time soon, since it’s still a small portion of the commercial market. But technologists are already factoring it into their plans.

“People will use faster versions of Ethernet when they become common, but it won’t be all that big a deal for many applications other than the backbone,” says Carl Henning, deputy director of PI North America, formerly the Profibus Trade Organization, in Scottsdale, Ariz. “If you have a total transmission time of 10 microseconds and you gain an order of magnitude improvement by going from 1 Gbit to 10 Gbit Ethernet, how many people will notice a nine microsecond difference?”

Related Feature - Determined To Improve Precision
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