Ethernet-based Motion Hot and Real-time

Jan. 13, 2007
One of the hottest topics in industrial networking these days is Ethernet-based motion control. Protocols may run solely on Ethernet—Layer 2—of the International Standardization Organization’s (www.iso.ch) Open System Interconnection (OSI) 7-layer Reference Model. Some use the model’s transmission control protocol (TCP)/Layer 4 and Internet protocol (IP)/Layer 3. Some bypass those two to get real-time responses. Regardless, they’re all unique.
For example, SERCOS III (for Serial Real-time Control System) bypasses TCP/IP. But it provides hard real-time for motion, explains Scott Hibbard, vice president of technology for vendor Bosch Rexroth Corp.’s (www.boschrexrothus.com) Electric Drives & Control Division, Hoffman Estates, Ill. Hard real-time implies time-critical tasks that must be completed within the specified time.“What is unique in SERCOS III is that it has been so specified that if a system can tolerate jitter (flicker or fluctuation in a signal) higher than 1 microsecond, the specification can be supported with off-the-shelf hardware,” Hibbard adds. “This means [it can be used] for ‘lower performance’ applications, such as point-to-point motion or I/O (input/output) control.” For configuration and diagnostics, ProfiNet uses TCP/IP, notes Carl Henning, deputy director of the Scottsdale, Ariz.-based Profibus Trade Organization (www.us.profinet.com). But when real-time data is involved, TCP/IP gets bypassed. Remarking that a ProfiNet motion-control application is open to TCP/IP traffic, he adds that studies have shown that 280 microseconds would be required to pass real-time data through TCP/IP, but ProfiNet only needs 40 microseconds to bypass those layers.Predictable behaviorThough it also uses TCP/IP if timing is non-critical, motor maker Baldor Electric Co.’s (www.baldor.com) Powerlink finds best use in a managed network to achieve deterministic or predictable behavior. But Powerlink devices can also operate as traditional TCP/IP devices, complying fully with the Institute of Electrical and Electronics Engineers IEEE 802.3 standard—which includes 10 megabits per second (Mbps) Ethernet, 100-Mbps fast Ethernet, gigabit Ethernet and 10-gigabit Ethernet—explains David Smith, manager of Ethernet products for Baldor UK Ltd., Bristol, U.K. And because Powerlink is a software-based solution using standard hardware devices, “it will make use of gigabit Ethernet devices,” he adds, forecasting this functionality will be shown at EMO Hannover 2007, the big September trade fair in Hannover, Germany.  Communicating in real time to 100 axes in 100 microseconds is Beckhoff Automation GmbH’s (www.beckoff.com) EtherCat fieldbus, which has no IP addresses or physical address switches. The bus also can send data packets every 10 microseconds, explains Rob Rawlyk, applications and engineering manager for Beckhoff Automation LLC, Burnsville, Minn. With EtherCat, each device in the bus system reads an outgoing data packet. This cyclic redundancy check allows end-users to specifically locate a problem area if the data packet has been corrupted, he explains. From the Open DeviceNet Vendors Association (ODVA, www.odva.com) is CIP Motion, an extension of Common Industrial Protocol (CIP), which provides services to support motion servo drives as well as volts per hertz, says Steve
Zuponcic, chair of ODVA’s CIP Distribution Motion Joint Special Interest Group and commercial engineering manager for vendor Rockwell Automation Inc. (www.rockwellautomation.com
), in Mayfield Heights, Ohio. Noting that ODVA “didn’t do anything to bastardize the standard OSI (model’s seven) layers,” he explains that the organization didn’t try to slice the traditional motion-control model as has been done. Instead, it changed the model. “We said, ‘All we need to understand is what time it is,’ ” Zuponcic recalls. “We can have drives and controllers on the wire that understand, to a high degree, what time it is within +/-100 nanoseconds.”    C. Kenna Amos, is an Automation World Contributing Editor.

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