Synchronized: EtherCAT achieves event synchronization with precision down to less than 1 microsecond using exact adjustment of distributed clocks. Drive and I/O commands are synchronized and time-stamped while collecting information from the machine.
Ethernet technology is making a move to become the dominant network for high-performance servodrive and machine control networking. Not all of the cards are on the table yet, but supplier groups totaling more than 100 companies worldwide are pouring huge development dollars into Ethernet-based solutions.
The target applications for suppliers involve high-end machine and motion control using a single, high-speed network for motion, I/O, and IP communications to streamline configuration, operation, and maintenance. But the problem for engineers is: With the wide array of networking solutions, company alliances and standards groups, which solution is best for them?
According to Sal Spada, an analyst for ARC Advisory Group, OEMs should be thinking about network architectures for machine control and about how third-generation digital drives are bringing standard networking capabilities to the mid- and low-end drive market.
"There is no one answer in this market," Spada says. "But Ethernet itself is providing the basis for more powerful solutions. If an OEM's supplier is not headed in the direction, they are definitely behind the eight-ball," he adds.
Crowded playing field
Three solutions already in the marketplace and shipping product—Ethernet-Powerlink, EtherCAT, and SynqNet—are attempting to broaden support by recruiting other companies to implement the technology. And two proposed technologies backed by established players, Profibus with Profinet IRT and SERCOS-III, are working on next-generation Ethernet solutions for their respective platforms.
With 20,000 nodes in the field, Ethernet-Powerlink can be used for drive-to-drive and drive-to-controller communications, and also for connections to HMIs, I/O, sensors, and other devices on the same network. Originally developed by B&R Automation (www.br-automation.com), Ethernet-Powerlink technology has been opened up, and a version 2 specification has been developed by a coalition of 40 companies, including automation suppliers, machinery builders, and end users.
At the 2004 Hannover Fair, a demonstration showed five servodrives (Lenze, AMK, Hirschmann, Baldor, and B&R) operating on a single Ethernet-Powerlink network. Drive product announcements, and other intelligent devices such as encoders, sensors and actuators, are anticipated.
B&R Automation says Ethernet-Powerlink is unique because it doesn't require any specialized hardware and provides determinism by implementing a time-slicing system that prevents network collisions in software. The system is a decentralized automation approach that uses peer-to-peer communications broadcast information within one cycle from either the drives or the central CPU. In some motion applications, the ability to take encoder feedback into a drive and broadcast position information, for example, removes network load and provides faster position update since it only takes one cycle to complete the operation.
Ethernet-Powerlink also uses CAN device profiles (for drives, I/O, sensors, and other components) developed by the CAN in Automation group (www.can-cia.de) that define variables to transfer and where they will be located within the communications telegram.
EtherCAT Technology Group
EtherCAT was developed by Beckhoff (www.beckhoffautomation.com) and has nearly 2,000 nodes in the field. According to Gerd Hoppe, CEO of Beckhoff North America, the protocol has been published and submitted to IEC and ISO for standardization in the upcoming real-time Ethernet standards. An EtherCAT Standardization Group (www.ethercat.org) has been formed, and Hoppe expects members to announce new products including controls, drives and encoders at the SPS Show in November.
While the technology can be implemented for distributed control, its primary strength is centralized control, and it has a strong heritage using X86 processors and the Windows operating system.
"EtherCAT has the ability to execute telegram content on-the-fly and the advantage of not needing microprocessors in the drives or I/O modules to execute telegram content," says Hoppe. Processing is done with a hard-coded protocol chip, so that the application side is completely off-loaded from processing the IP protocol stack, an approach that Hoppe says keeps EtherCAT chip costs in the $5-$6 range, lower than its competitors.
He says that compatibility with the IEEE 802.3 specification allows the technology to be used with regular TCP traffic, so conflicts between regular and real-time traffic is managed. The system uses automatic address assignment to eliminate manual switches, plus MAC ID and IP address settings.
With the speed for 100BaseT Ethernet, EtherCAT executes motion control and other functionality directly on the CPU and replaces Profibus, ControlNet, and DeviceNet scanner cards that would otherwise normally plug into the PCI bus with software. As a result, Beckhoff is looking to introduce a small PC (3×5 inch) with no rotating media, no fans, and Celeron/ Pentium M processors. The unit will have two Ethernet jacks, one for IT communications and the other to communicate with the machine, a PCI connector for an LCD display, a 24V power supply, and a CF memory slot.
Motion Engineering Inc. (www.motioneng.com) designed SynqNet to support high-performance, centralized control systems. In addition, a number of OEMs and drive vendors have embedded the technology into their products. According to Spada of ARC, he believes that the recent acquisition of MEI by Danaher Motion could help them propel the network.
Implementing SynqNet in a ring topology offers unique, "self-healing" fault tolerance that guarantees the system will operate if there is a cable break, a loose connection, or a complete fault of any node. Even if two out of five nodes fail, SynqNet can control the remaining nodes and execute alternative motion parameters. A closed ring insures a redundant data path for transmitted data through the entire ring, which SynqNet uses as a secondary data channel.
Profinet IRT (for "isochronous real-time") is targeted for high-performance, motion control applications, such as Web handling applications, that require precise, high-speed synchronization. According to Michael Bryant, executive director of the Profibus Trade Organization, Profinet IRT delivers high performance using switch technology in order to achieve 150-microsec cycle times.
The key distinction of IRT is the deterministic performance and its compatibility with standard, off-the-shelf TCP/IP communications, according to Bryant. He says that the new ASIC for Profinet IRT has been in development for three years, and is currently in beta testing. He adds that he expects first chips for high-performance applications—a four-port switch called the ERTEC 400 that uses an ARM 9 processor—to be available in the marketplace by the end of this year. In addition, first Profinet IRT products are expected to be available as early as the middle of 2005.
The proposed SERCOS-III specification will provide an Ethernet-based solution for more than 30 control manufacturers and 15 drive manufacturers that offer SERCOS interface products. For new users, the new standard maintains software compatibility with previous versions and no changes to application software.
One of the biggest advantages that SERCOS holds is its use and adoption in a wide variety of motion applications.
Data Transfer: A key goal for Profinet IRT is TCP/IP compatibility with other devices on the network. During a 1-msec cycle time, approximately 60 percent of the cycle is used for real-time data transfer and 40 percent for regular TCP/IP data. IEEE 1588 is utilized to provide synchronization.