Sunday, June 28, 2009

Advances in Ethernet Technology Boost Industrial Networking Capabilities by Chris Muldowney

An assembly line, or any highly automated, complex industrial process, typically relies on a three-layer system of controllers. At the top is a human machine interface (HMI), where a human operator monitors the system. The middle layer of the hierarchy consists of programmable logic controllers (PLC) working with a communications system (such as Ethernet). The bottom layer is the fieldbus, which serves as a local area network (LAN) for instruments used in process control and manufacturing automation applications. It links the PLCs to devices such as sensors, actuators, electric motors and electric speed controllers.

During the 1990s, fieldbus networks flourished on the factory floor. These applications save time and money by simplifying machine wiring, but the low bandwidth communications they provide lack the speed needed to synchronize positions between multiple axes. In the last few years, Ethernet has received considerable attention as an alternative to these dedicated industrial buses, thanks to advances in its high-speed data network functionality.

Ethernet first ran at 10MB, then at 100MB and will soon operate at 1GB data-transfer rates, meaning it’s now fast enough to be used for a motionbus (a fieldbus for motion control purposes). An Ethernet motionbus takes advantage of high consumer volumes of Ethernet components to keep hardware costs down.

Ethernet POWERLINK is one of the most promising advances in motionbus technology. A real-time protocol for standard Ethernet, POWERLINK is managed by the Ethernet POWERLINK Standardization Group (EPSG). Austrian automation company B&R debuted this protocol in 2001. Ethernet POWERLINK acts as the nervous system, communicating to and from servo and stepper motor systems, drives and the other components that make up the system. As the mastermind of the HMI, Ethernet POWERLINK can oversee functions such as electronic ac motor control and electric motor speed control.

Ethernet POWERLINK solves the issue of determinism — the certainty of a system responding within a certain amount of time — which has been the historical drawback to using Ethernet as a motion-control solution. Standard Ethernet is non-deterministic. That means there is no guarantee when a data packet will arrive at its intended destination. Two different packets may collide on the network, and when they do, standard Ethernet resolves the collision by randomly assigning a wait time to each packet before it can be resent.

Synchronized timing isn’t a major issue for some everyday operations, but that precision is critical for multi-axis motion systems. Ethernet POWERLINK solves the determinism issue by assigning each important motion a dedicated, isochronous time slot when it can send and receive data.

Ethernet POWERLINK also solves the issue of elaborate cabling. Fieldbus networks used to rely on a tangled network of wires and cables that were both complicated and expensive. With Ethernet POWERLINK, there’s just one plug to connect everything, and that simple wiring results in significant cost savings.

Ethernet POWERLINK also offers off-the-shelf networking hardware and chipsets, making Ethernet an open and “future-proof” solution for motion control. Other solutions rely on closed, proprietary hardware designs accessible only to them and their licensees. They are using Ethernet cabling, and may support TCP/IP packets, but the chip hardware is unique. There are typically patent and intellectual property rights associated with these designs, adding cost and complexity along with support issues.

As a result of these advances in Ethernet technology, motion controllers on the factory floor can be linked right to the front office — meaning that an engineer can monitor a machine process without leaving his desk. As Ethernet technology continues to advance, it will improve on its ability to combine the benefits of fieldbus network wiring, Ethernet speeds and visibility, consumer volume hardware costs and open platforms.

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