Ethernet: The Ticket for High-Speed Motion Applications

Get on Board With One of Ethernet's Many Variants That Works Best for You

By Jim Montague

1 of 3 < 1 | 2 | 3 View on one page

October 2012 IssueAll aboard! You might want to catch this train. Ethernet has rolled over the world, and now it's arriving at the last few pockets of resistance, or at least appearing on the horizon.

It's no longer a question of whether Ethernet can handle increasingly higher-speed motion; it's just a question of when and in what applications. But even though you can travel blazingly fast via some Ethernet-based networks, your data packets also have to stay on the rails and stop at the right stations to get where they're going. In machines, trains and on the plant floor, speed is nothing without accuracy. Ease of use helps a lot, too.

"We've been using EtherNet/IP for motion in our palletizers for six or eight years," says Kevin Davis, electrical design manager at Production Automation (PAI) in Montgomery, Ala. "Today, even in our high-speed areas running eight-axis with servos at 80 cases per minute with a half-second cycle time, it's still EtherNet/IP. We just make sure to isolate our high-speed motion from other Ethernet traffic by using an Ethernet module with a ring topology."

Joey Stubbs, North American representative of the EtherCAT Technology Group (ETG), adds, "The days of dedicated networks for motion, I/O and other tasks are effectively over, since all of these tasks can be serviced by one network simultaneously from one controller. Traditional motion protocols don't offer anywhere near the performance, diagnostics, ease of use, configuration and cabling of well-implemented, Ethernet-based protocols. This isn't just due to performance. It's also because machine-level communications now take advantage of consumer-based technologies such as Cat. 5/6 cabling, connectors, transceivers, standard NICs as masters, standard diagnostic tools, etc. This is instead of having to use special, costly hardware required by legacy protocols. This piggybacking of physical-layer components drives down the cost of systems and increases product selection."

PAI's palletizers have to quickly but gently stack and wrap cases of super-thin plastic bottles, and so they use robot arms and cranes on EtherNet/IP to replace traditional physical diverters.

So, how can machine builders choose the right type of Ethernet and related networking components to satisfy end users who need increased speed and throughput, better handling, increased flexibility and more open networking? Follow a good role model. There seem to be just as many Ethernet-based solutions as users have problems. You just have to get on the right train — and avoid the brawl in the club car.

Slice It or Stamp It?
Basic, vanilla Ethernet wiring and its pure TCP/IP came from IT and office realms, where data was blasted to all parts of the network and speed wasn't as crucial or possibly dangerous as on the plant floor. Ethernet's more recent industrial protocols adopted increasingly intelligent switches and addressing to achieve determinism, and then adjusted their communication methods and rules for increased speed by prioritizing and synchronizing how they transfer data. In general, tightly dedicated Ethernet networking permits greater speed, but means less flexibility. Meanwhile, less-restricted Ethernet enables more flexible communications, but typically runs slower.

Historically, there were two main schools of thought about manipulating Ethernet to serve in higher-speed motion: time slicing and time stamping. To prevent message collisions, boost speed and achieve determinism, time slicing ensures that critical devices and functions are assigned specified time slots to transmit information. A managing node on the network handles time allocation, so the others can transmit without interference when it's their turn. Time slicing is used mostly by EtherCAT, Powerlink, SERCOS III and Profinet Isochronous Real Time (IRT).

The second method, time stamping, is based on the IEEE 1588 standard's Precision Time Protocol (PTP), which defines a method for sub-microsecond synchronization of the clocks in sensors, actuators and other terminal devices on a standard Ethernet-based network or other distributed application. It's used primarily by EtherNet/IP with CIP Sync and CIP Motion. IEEE 1588's basic function is to have the most precise clock on a network synchronize all the others, and then time stamp each data packet moving on the network.

Not surprisingly, the different methods of altering Ethernet for higher-speed motion have led to some arguments. The main debate seems to be between the time slicers and the time stampers. The slicers say that time-stamped Ethernet is basically an Internet Protocol (IP) that's not dedicated enough to truly handle high-speed motion, and that it must run slower because its master device is always on and all data must run through it. However, the stampers counter that time-slicing might be quicker, but it risks losing data because its nodes can't run and communicate when the master device is talking.

1 of 3 < 1 | 2 | 3 View on one page
Show Comments
Hide Comments

Join the discussion

We welcome your thoughtful comments.
All comments will display your user name.

Want to participate in the discussion?

Register for free

Log in for complete access.


No one has commented on this page yet.

RSS feed for comments on this page | RSS feed for all comments