Bus War III: Process meets motion

If you work in a process plant, you likely have a few motion applications. In this article, Senior Tech Editor Dan Hebert, PE, provides a general overview of the various motion networks competing today.

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

 

Without even knowing it, you probably are an unwilling participant in the real or imaginary fieldbus wars. You picked your preferred digital network, but still must cope with competing standards and hope your pick will emerge as one of the winners.

Bus War II currently flares among different wireless networks. The state of flux in this area makes it nearly impossible to pick or predict winners. Users wait impatiently for the “dust” to settle.

Part three of the bus wars is being waged in your plant—the battle of the motion networks. Process plants don’t employ the amount of motion found in discrete parts manufacturing, but it is a good bet that if you work in a process plant, you have quite a few motion applications.

So this means you need at least a general overview of various motion networks. If not, you might end up with many different motion networks, incompatible with each other and your existing control system and fieldbuses.

Most newer motion networks are Ethernet-based—good news. One Ethernet-based motion network almost never is compatible with another—bad news. Many of the Ethernet-based motion networks modify hardware to improve performance, but create non-standard Ethernet hardware.

Ethernet networks use standard cabling, usually CAT 5 or a related variant. Ethernet networks use the standard seven-layer Ethernet protocol. But, different networks modify at least the upper three layers to fit specific applications. Some of the Ethernet flavors are Ethernet/IP, Modbus TCP/IP, and Profinet. A single Ethernet network could accommodate an Ethernet/IP PLC, a Modbus Ethernet TCP/IP PLC, Ethernet/IP I/O, and Modbus Ethernet TCP/IP I/O. But, the only seamless communication would be between the PLC and the I/O using the same flavor of Ethernet.

 

“It is fatal to assume that there is plug-and-play connectivity among these networks,” says Markus Sandhoefner, sales manager, 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 a single company or small group of companies. You know the drill. Emerson creates Foundation fieldbus, then turns the network over to Fieldbus Foundation. Siemens does the same with Profibus.

In theory, the independent organization is not tied to founding members. In practice, each vendor supports one organization strongly and the other organizations weakly, if not at all.

 

It’s the same with motion networks. Rockwell Automation created DeviceNet and turned it over to ODVA. ODVA now supports multiple networks including DeviceNet, Ethernet/IP, and CIP Motion.

“We have Profidrive for Profibus and Profinet,” comments Carl Henning, deputy director at Profibus. “In both cases the motion control runs on the same network as the normal fieldbus network.”

 

Other visible motion networks include Ethernet Powerlink, EtherCat, SERCOS, Mechatrolink, and SynqNet, among others.

For process plants, expect purchased machines to use some type of motion network, probably one of the above. Make sure that the existing Ethernet network can communicate with the motion network without major software or hardware modifications. This usually can be done through Ethernet-based TCP/IP communications.

It’s also important—and more problematic—that plant personnel can support each motion network. In an ideal world, all of the machines would use the same one. In the real world, try to limit the number of different motion networks used on those machines.

 

As one possible detour around an existing bus pileup, Hilscher claims a possible solution for both the machine control and the end user’s system. “The netX chip supports Profibus, DeviceNet, CAN, CANopen, Ethernet, EtherCat, Powerlink, AS-i, InterBus, Profinet, SERCOS III, EtherNet/IP and serial based protocols,” says Hilscher North America COO Phil Marshall. “It also has built-in hub or switch and the IEEE 1588 timer so it is a complete Ethernet Motion chip. These protocols are all downloaded on boot up of the chip, so in theory an OEM can support all the Ethernet protocols and the end user could select what he wanted.”

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