Industrial Ethernet: Powerlink, EtherNet/IP
At least eight organizations represent seven Ethernet protocols targeting industrial networking applications. Here are updates about two Ethernet protocols: Ethernet-Powerlink and EtherNet/IP. Information on other industrial Ethernet protocols can be viewed in the "Online Extra" addition to this article, below.
At least eight organizations represent seven Ethernet protocols targeting industrial networking applications. Here are updates about two Ethernet protocols: Ethernet-Powerlink and EtherNet/IP. Information on other industrial Ethernet protocols can be viewed in the 'Online Extra' addition to this article, below.
Marc Ostertag, vice president of B&R Industrial Automation, representing Ethernet-Powerlink, offers compelling reasons for using Ethernet for industrial automation. Ethernet is the most common network in IT; engineers are familiar with Ethernet, it offers transparent communication on all levels, it's an undisputed worldwide standard so it is 'future-proof,' it has compelling data rates, and offers a lot of software and many tools.
Ethernet Powerlink is a decentralized real-time communication system for automation and motion control, Ostertag says. Benefits are that it's an open protocol managed by the Ethernet Powerlink Service Group (EPSG), uses standard chips, has deterministic real-time data transfer, is easy to use for automation, allows any topology, and doesn't require a network guru, he says. Security is achieved via domain separation.
Ethernet Powerlink has been accepted by the IEC as a publicly available IEC specification (PAS), according to EPSG. The organization hopes EPL will become part of the IEC Standard IEC 61784-2 and IEC-61158—the fieldbus standard. Specifications were submitted by IAONA (Industrial Automation Open Network Association) to IEC SC65C as a real-time industrial Ethernet communication profile. IAONA, the platform organization for promotion of Ethernet in factory automation, works with ODVA (Open Device Vendor Association), EPSG, and three other groups to elaborate technical guidelines and recommendations in areas of mutual interest.
B&R resolves Ethernet's limitation for guaranteed data transmission by superimposing a 'time slicing' method over the carrier-sense multiple access/collision detection mechanism of Ethernet. It cuts the transmission interval into numerous slots, making a time slot available for each device linked to a network segment. This ensures that no network collisions occur. Ostertag says cycle times down to 200 microseconds and jitter below 1 microsecond can be obtained.
Bystronic glass uses Ethernet Powerlink for fully automatic cutting, grinding, and drilling with 450 ft/min cutting speed, 2/1000 in. accuracy. The company moved away from proprietary electronics; has improved cutting accuracy, quality, and throughput by scoring at higher speeds; and has 800
EtherNet/IP is a Common Industrial Protocol (CIP) supported by ODVA members, who also support the following CIP network technologies: DeviceNet, CIP Sync, CIP Safety, CIP Sync, and CIP Motion.
CIP Networks share a common, complete suite of services and device profiles plus seamless bridging and routing between multiple CIP sub-nets, says Katherine Voss, ODVA executive director. CIP and EtherNet/IP protocols are complementary to standard Ethernet technologies, she says, calling EtherNet/IP a pure Ethernet solution for control networks, enabling Internet and enterprise connectivity.
General Motors chose EtherNet/IP as its global standard for industrial Ethernet. With EtherNet/IP, General Motors expects to lower operating costs, leverage engineering resources, share information systems applications, and incorporate safety networks, according to Voss. EtherNet/IP is also in use at the CSIRO Exploration and Mining at the Queensland Centre for Advanced Technologies in Australia. Underground automation equipment, such as for coal mining, has similar concerns as factory automation and process control, Voss explains, such as:
Though application bandwidth requirements are modest, demands will increase, including addition of on-machine cameras.
Condition monitoring will collect vibration data, which involves large file transfers.
CSIRO plans to add wireless Ethernet for broadband communications with a 90-ton, 50-foot long automated coal shearer. Radio waves can withstand the environment better than cables.
CSIRO avoided a 'home-spun simple protocol in TCP/IP,' because of desire to keep the system open, expandable, and maintainable, Voss says.
Mark T. Hoske, MHoske@cfemedia.com
Industrial Ethernet: five more
After all the industry supposedly learned from the fieldbus wars, why are there at least seven in-dustrial Ethernet protocols from which to choose?
Leaders within ODVA and Ethernet-Powerlink Specification Group suggested, when asked this question, that diversity is good. While it was a goal at one point to have a single interoperable fieldbus, multiple versions eventually formed under the “standard,” serving various areas of the world. Some offer technological strengths for certain applications, and various manufacturers support some more than others.
Same thing has happened with Ethernet. Diverse applications and competition have lead to diverse protocols. A number of these are undergoing a formal process to become standards, perhaps a similar, multi-headed admission that there will not be a unified industrial solution.
There is evidence, however, of a certain level of willingness to work together—five protocols have joined an Ethernet organization, IAONA (Industrial Automation Open Network Association), which is collaborating on common issues “to prevent further incompatibilities of the existing solutions,” according to a statement on the IAONA Web site.
IAONA members are:
•Open Device Vendor Association (ODVA);
•EtherCAT Technology Group;
•Ethernet Powerlink Specification Group (EPSG); and
•Interests Group SERCOS Interface.
They have defined a common strategy for developing Ethernet-based products for industrial automation. As a medium-term goal, the groups are collaborating on the following topics: security; cabling and wiring; use of IEEE 1588, and systems.
EtherNet/IP and EPSG were mentioned in the main article. A bit of background about each of the others follows.
To advance Modbus protocols, an open Modbus TCP/IP specification was developed in 1999. Modbus TCP/IP protocol is said to combine the physical network (Ethernet) with a universal networking standard (TCP/IP) and a vendor-neutral data representation (Modbus). It is also said to be extremely simple to implement for any device that supports TCP/IP sockets. In January 2005, the Standardization Administration of China (SAC) launched the Modbus Protocol Implementation Guide over Serial Link and the Modbus Protocol Implementation Guide over TCP/IP as standards in the People’s Republic of China, according to the Modbus organization. More than 200 Modbus TCP/IP devices are available in the market, the group says. It is used to exchange information between devices, monitor, and program them. It is also used to manage distributed I/O devices, said to be the preferred protocol by manufacturers of this type of device. Modbus TCP/IP takes the Modbus instruction set and wraps TCP/IP around it. Those that have a Modbus driver and understand Ethernet and TCP/IP sockets, can create an Modbus TCP/IP driver within a few hours, the group says. Standard PC Ethernet cards are used. Modbus protocol is a trademark owned by Schneider Automation, but license and specification are available free, without licensing fees for Modbus or for Modbus TCP/IP protocols, the organization says.
High Speed Ethernet (HSE) , the Fieldbus Foundation’s backbone network, runs at 100 Mbit/second. HSE Field Device is a fieldbus device connected directly to a High Speed Ethernet (HSE) fieldbus. Typical HSE Field Devices are HSE Linking Devices, HSE Field Devices running Function Blocks (FBs), and Host Computers. An HSE Linking Device connects H1 fieldbus Segments to HSE to create a larger network. An HSE Switch is standard Ethernet equipment used to interconnect multiple HSE devices, such as HSE Linking Devices and HSE Field Devices to form a larger HSE network. In February 2005, the Fieldbus Foundation released the High Speed Ethernet Interoperability Test Kit (HSE ITK) Version 1.0. The kit delivers test cases that expand the scope of interoperability testing benefiting end-users of F OUNDATION fieldbus devices. The HSE ITK test engine executes over 400 test cases that exercise the device implementation. The test kit also includes a Device Description viewer. Upgrades are planned as new standard Function Blocks become available.
Ethercat is published by IEC, called open and real-time, and can use tree, daisy-chaining, drop lines, or switched star topologies. The organization for Ethercat reported in February 2005 that with a 96% approval rate, the IEC SC65C voting member countries have agreed to publish the Ethercat specification as IEC PAS (Publicly Available Specification). This means that the Ethercat Spec is acknowledged as official IEC document and will be made available through the IEC. Since the Ethercat specification was written with IEC standardization in mind, integration into the next edition of the fieldbus IEC 61158 is expected. Beckhoff developed the original specification. Ethercat processes 1,000 distributed I/O signals in 30 microseconds or 100 axis in 100 microseconds using twisted pair or fiber-optic cable. Ethercat gives a topology choice: it supports a simple low-cost line structure, a tree structure, daisy chaining or drop lines—no expensive infrastructure components are required. The more expensive switched-star topology can be used.
Profibus Trade Organization calls Profinet an integrated and open Industrial Ethernet standard for automation, using TCP/IP and IT standards; it offers real-time Ethernet; and can protect in-vestments with integration of any fieldbus protocols. The Profinet concept, said to meet all industrial automation requirements, is modular, enabling users to select the required functionality. Isochronous real-time (IRT) communication enables cycle times less than 1 microsecond and is thus well suited for use in motion control applications. Profinet has two “views.” Profinet IO for integration of distributed I/O connections, and Profinet CBA for creation of modular plants. Profinet CBA primarily consists of component-based architecture via TCP/IP and real-time (RT) communication with components. Profinet IO uses RT and IRT communication with distributed I/O devices. The designations RT and IRT describe the real-time properties. Profinet communication is scalable in three levels. Component-based communication uses TCP/IP and enables cycle times on the order of 100 microseconds. It is recommended for communication between controllers. Real-time (RT) communication enables cycle times on the order of 10 microseconds and can be used with distributed I/O devices. All these communication levels can co-exist on the same bus line together with IT communication.
SERCOS III has been defined such that any standard IP telegram (such as TCP/IP) can be transmitted in a non-real-time time slot, in parallel to the real-time transfer of the data required for motion control. SERCOS uses the IEEE 1588 synchronization, and is based on the established real time mechanisms of the original SERCOS interface, working on the principle of cyclic data transfer with an exact time pattern. Hardware-based synchronization is the indispensable prerequisite for the reliable implementation of motion applications, such as electronic line shafts in printing machines, packaging machines or multi-axis machine tools, the organization says. A SERCOS III controller can exchange telegrams with “superordinate” devices in the network. It will be compatible with the established SERCOS mechanisms to the greatest possible extent, to hold down cost and effort of upgrades, according to the governing organization. (A stated goal is the reduction of the pernode interface costs by offering powerful, lower-priced hardware components. Options being examined include an FPGA-based [field gate programmable architecture] controller and a multi-protocol communication controller. These considerations will be finalized in the few next months.) Rough technical concept of SERCOS III has been worked out and verified, and the SERCOS promotional groups are announcing that the first SERCOS III prototypes will be available in 2005.
Mark T. Hoske, Control Engineering , MHoske@cfemedia.com
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