New trends and old friends in network choices

Though he says DeviceNet still is used in applications requiring highly deterministic real-time control, the trend is toward localization of control tasks in small PLCs, such as ControLogix PLCs, which connect via Ethernet to existing networks.

Furduy adds that, “There are fieldbuses that will continue to be used for factory networking. For example, we occasionally use Profibus, but this is when Siemens PLCs are being used.” But it all winds up on Ethernet. “No matter what control bus is being used, factory data base connectivity typically drives the need to provide connections via Ethernet,” he concludes.

ETHERNET STILL GOING STRONG
Ethernet is probably the most popular network in the world, thanks to its long life and use by all WinTel (Windows/Intel) systems. Just about every industrial network has an Ethernet version, including Foundation fieldbus (HSE), Profibus (Profinet) and our old friend, Modbus (Modbus TCP/IP). Various Ethernet hardware is used by big vendors such as Emerson, Rockwell and Omron. All the various protocols can run on the same Ethernet cable at the same time, and OPC Data eXchange software lets various Ethernet devices read the other protocols.

Without OPC, connecting various Ethernets is a problem, because there are so many. “It’s important that users understand the differences between standards-compliant networks and standards-based networks,” says Doug McEldowney, Netlinx strategic marketing manager, Rockwell Automation. “Standards-compliant networks, like Ethernet/IP, HSE, and Modbus TCP/IP use existing, unmodified networking components and networking standards to define their solution.”

Berge puts it more bluntly: “If the product specification just says ‘Ethernet’ or ‘TCP/IP’ without specifying the application protocol, you can be certain it is proprietary.” Every vendor is jumping aboard the Ethernet train, and for good reason: it is dominating the industry.

A major factor pushing Ethernet lower in the factory is the increasing use of remote diagnostics and remote monitoring. As a recent Control Design web poll indicates, 47% of machine builders are incorporating web-based diagnostics.

“At a minimum, products connected to Ethernet provide some level of diagnostics about the product and/or system via Web servers today,” adds McEldowney. “Also, the ability to leverage web services, in general, opens opportunities to connect dissimilar speaking devices within the enterprise. This area will continue to expand over time with more functionality being embedded deeper in the control systems.”

OPC is working in that area, too. "We expect to release the new OPC Unified Architecture (OPC UA) spec by the end of the year," says Lee Neitzel, Emerson Process Management’s senior technologist and editor of the new OPC UA spec. "The new OPC UA will permit OPC to be used across the Internet. By early next year, OPC applications will begin to communicate seamlessly using Internet communications wherever they are in the world. This will be a significant advance in automation system communications."

For users and OEMs, this will be another feature to add to the Ethernet steamroller. For competing networks, it may be another nail in their coffin.

---

With Wireless, Who Needs Networks?

HERE'S ONE SOLUTION to the power and speed problems with wireless proximity sensors that requires no batteries, operates as many times and as fast as needed, and suffers no delays or interference from other transmitters. Chris Jeffrey, business development area manager at ABB explains that several customers, including robotics, powertrain and stamping companies in the automobile industry asked his firm to help reduce their huge, ongoing costs associated with broken wires and worn/loose connections.

Jeffrey says sensor cables are one of the most frequent sources of failure in factory automation. "One automaker was spending about $40,000 per month just fixing wires on one machine," he says. "Wires and connections on robot arms, portable assembly pallets, platforms and anything else that moves will flex and break connections on a regular basis." It sounded like a battery-powered sensor was just the ticket.

Alas, replacing batteries, which never wear out on schedule, is just as much trouble as fixing wires. "If you have a machine with 50 battery-operated sensors, they all wear out at different times, depending on how often they are actuated," he says. Perpetually replacing batteries introduces a new maintenance nightmare. "Our wireless proximity switch design target was 10 years of maintenance-free operation, which ultimately means no batteries."

ABB developed a system that uses an electromagnetic field inside or around the machine (see Figure 2 below) to power all these wireless sensors. Because power is no longer a problem, the sensors (see photo) can operate continuously at high rates. Each sensor has a wireless transmitter that sends on/off signals to a base station. "ABB uses standard 2.4-GHz radio transceivers in its sensors, such as those used in Bluetooth, but employs a tailor-made real-time communication protocol.”


FIGURE 2: ELECTROMAGNETICALLY ENCLOSED

Wireless proximity switches have a ferrite cube, rectifier and voltage regulator which supply power regardless of the sensor’s orientation in the magnetic field.


BLUETOOTH is a general-purpose radio, originally developed for computer peripheral communication in offices. It is not usually considered for industrial real-time use in control systems. Likewise, Jeffrey says, ZigBee will be suitable only for data collection and non-time-critical devices, like in building automation.