Welcome to the Bus Wars, Part Tres

You may not be aware, but a new bus war is being waged in your plant — the battle of the motion networks. Senior Tech Editor Dan Hebert, PE, provides analysis and commentary on the newest battle over fieldbus.

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Dan Hebert, PEBy Dan Hebert, PE, Senior Technical Editor

YOU'VE PROBABLY been an unwilling participant in the fieldbus wars. Even if you’ve picked your preferred digital network, you still must cope with competing standards, and hope your pick will emerge as one of the winners.

Part two in the bus wars is now in full swing, and the battle is among different wireless networks. There is a tremendous state of flux in this area as new players and new technologies jostle for leadership. It’s hard if not impossible to pick winners and losers at this point, causing many to wait for the dust to settle.

You may not be aware, but a new bus war is being waged in your plant. Welcome to Bus Wars, Part Three—the battle of the motion networks. Process plants don’t employ the level of motion found in discrete parts manufacturing facilities, but it’s a good bet that your plant has quite a few motion applications.

If your plants package in quantities smaller than railcars or super sacks, you probably buy packaging machines, each with its own motion network. Material handling systems of all types are another area where motion networks are typically used. Various types of process machines also often employ motion networks.

    

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What does this mean to you? It means you must have at least a general overview of various motion networks. If you don’t, you may find that your plants end up with many different motion networks, incompatible not only with each other, but also with your existing process control system and fieldbus networks.

Most motion networks are based on Ethernet. That’s the good news. The bad news is that one Ethernet-based motion network is almost never compatible with the next. The other bad news is that many of the Ethernet-based motion networks modify Ethernet hardware to improve performance. These modifications can yield non-standard Ethernet hardware.

All Ethernet networks use standard cabling, usually Cat 5 or a closely related variant. All Ethernet networks use the standard seven-layer Ethernet protocol. But, different networks modify at least the upper three layers of the Ethernet protocol to fit specific applications. This yields different flavors of Ethernet.

One leading machine control vendor explains the situation this way: “Different real-time motion networks bear the name Ethernet, but it’s fatal to assume that there is plug-and-play connectivity among these networks,” says Markus Sandhoefner, sales manager of B&R Industrial Automation. “The development of real-time Ethernet motion networks reflects the fieldbus wars of the past.”

As with process networks, most motion networks were created by one company or by a small group of companies. At some point, the network was usually turned over to an independent organization charged with supporting and promoting that network. You know the drill. Emerson creates Foundation fieldbus, and then turns the network over to the Fieldbus Foundation. Siemens does the same with Profibus.

In theory, the independent organization isn’t tied to founding members. In practice, each vendor backs one organization strongly, and supports the other organizations weakly, or not at all.

A similar situation exists with motion networks. Rockwell Automation created DeviceNet, and then turned it over to an independent organization named ODVA. ODVA now supports multiple networks including DeviceNet, Ethernet/IP, and CIP Motion.

Profibus supports motion through two of its networks. “We have Profidrive for Profibus and Profinet,” comments Carl Henning, Profibus’ deputy director. “In both cases, the motion control runs on the same network as the normal fieldbus network,” he notes.

For your process plants, expect that purchased machines will use some type of motion network, probably one of those mentioned above. Make sure that your existing Ethernet network can communicate with the motion network without major software or hardware modifications. You also need to ensure that plant personnel can support each machine’s motion network. This is more problematic. In an ideal world, all of your machines would use the same motion network. In the real world, the best you can probably do is limit the number of different motion networks used on your machines.

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