Builder Beware

Planning and installing fieldbuses requires attention to detail.

Aw 3385 0812 Des36
Several thousand man-years of effort have gone into fieldbus technologies over the past decade or two. Though simplifying installation is a central aspect of this ongoing design effort, installing a network remains a difficult task. Meticulous planning can eliminate many problems, but that doesn’t mean everything will fall into place when the switch is finally turned on. For more than five years before the Dekalb County Department of Watershed Management opened a water treatment plant in Decatur, Ga., engineers made preparations that included network design.

The network is expansive: a dual-redundant fiber optic network that runs two miles, connecting more than 300 Profibus instruments and 300 DeviceNet points. But all that planning and close communication with the system integrator who installed the network didn’t matter when an intermittent failure struck one half of the network.

Phantom problems

“We had a phantom problem on the south side of our facility. Things would time out,” says Merat Zarreii, division manager for the project. Like many sporadic problems, finding a solution was no simple task. The troubleshooters kept escalating their efforts, finally bringing in equipment suppliers. “We took equipment from the north side, where everything worked fine, and attached it to temporary wiring, but it still didn’t work. We finally got instrument and PLC (programmable logic controller) people in, and they spent a week before finding out it came down to termination,” Zarreii says.

The network had too many terminations, which caused reflections under certain circumstances, so a fix was pretty straightforward: Some terminations were eliminated. That time-consuming problem occurred even though the network ran at a forgiving 1.5 megabits per second (Mbaud), Zareii notes. In a new facility now under construction, he predicts that speeds will be quadrupled to 6 Mbaud. “If you install terminations correctly and keep their numbers down, you can minimize reflections and push bus speeds higher,” Zarreii says.

These sorts of problems can occur regardless of whether designers and installers have experience with fieldbuses or are building their first system. Though fieldbus architectures have been in use for years, many companies are still using old analog wiring. When they move to networks, new issues pop up. “A lot of users, when they switch from 4 to 20 milliAmps wiring to fieldbuses, encounter situations they aren’t ready for,” says Moin Shaikh, fieldbus marketing manager at vendor Siemens Energy & Automation Inc., in Spring House, Pa. “Signals travel only so far, then you need to add repeaters.”

Equipment suppliers are responding to this ongoing need with a number of tools. They address a range of problems, with some focusing on the common problem that plagued Zareii’s installation. “People are making tools that help at commissioning time, providing ways to see if there are problems like the lack of a terminating resistor,” says Carl Henning, deputy director at the Profibus Trade Organization, based in Scottsdale, Ariz.

Diagnostic equipment continues to expand its reach. Managers can now search for many types of faults using one tool. That saves time and expense, compared to the days when dedicated tools could detect only one type of problem. “We monitor the bus, looking at noise, ground faults and whether everything is terminated correctly,” says Bernd Schuessler, business development manager at vendor Pepperl+Fuchs Inc., in Twinsburg, Ohio. “With this tool, you can commission a network from the comfort of the maintenance shop instead of running around with handheld tools.”

Other tools focus on day-to-day operation. Though networks often run for years without problems, there’s always a chance that something will happen, especially in harsh industrial environments. Products make it simple to monitor operations. “We’ve got a diagnostic repeater that constantly gives detailed diagnostic data, even detecting wire breaks,” Shaikh says.

Usage expands

The growing number of tools underscores a solid expansion in the use of various fieldbus architectures. Fieldbus architectures once battled each other. Now, they’re battling Ethernet, which is extending its reach beyond a backbone for the factory.

Ethernet proponents say their technology can play a role in input/output (I/O) and other areas where fieldbuses have been dominant. But many industrial managers find that fieldbuses are simpler to install, often saving money. Ethernet switches can add complexity and cost, but that’s not the only reason some managers avoid them.

Some of the Dekalb County facilities use Ethernet, but Zareii decided to drop it in a new facility. Profinet and Profibus will be the only networks used in a small pumping station that should come on line next summer. Avoiding potential turf wars was one of the main reasons for making this change.

“Using Profinet and Profibus separates industrial automation from IT (information technology), so there’s no confusion about what part IT is involved in and where industrial people take charge,” Zarreii says. Having Ethernet in industrial environments meant IT staffers had to help troubleshoot when problems arose. The IT specialists don’t understand the plant floor requirements, so it’s very time consuming to get them to devise fixes that meet those requirements, Zareii adds.

The newest Dekalb County facility will have Profibus in most areas, with a minimal amount of Profinet. Product availability is a key reason behind that decision. The PTO’s Henning says that’s a big factor in the continued growth of Profibus. “There are thousands of Profibus offerings, compared to hundreds for Profinet,” Henning notes. That is driving growth that many didn’t expect once Profinet started gaining momentum. “We continue to see Profibus growing like mad, 25 percent per year or so,” Henning says.

Though Dekalb County uses DeviceNet and Profibus, they aren’t the only architectures in its facilities. Today, those plants have many small components that are connected using complex analog wiring schemes. In newer networks, those components are being linked with a low-level networking architecture.

“No matter what you do, you’re still going to have a lot of little things that give you a lot of analog parts. We’re using the AS-Interface, which lets us go from 16 wires to two. That saves us copper wiring and conduit,” Zarreii says.

Broad expanse

The continued growth of fieldbus architectures is prompting changes in many segments of the broad electronics industry. Equipment makers aren’t the only ones who are racing to provide new features and functions for the various fieldbus architectures. Chipmakers are also making sure their microcontrollers help these suppliers get to market quickly. “We support any fieldbus anyone asks about,” says Alexandra Dopplinger, global industrial leader for factory automation at Freescale Semiconductor, a chipmaker in Austin, Texas. “We provide the interfaces.”

When engineers are linking fieldbuses to Ethernet, that means microcontrollers must have at least two Ethernet ports and more than eight UARTs (universal asynchronous receiver/transmitters), Dopplinger explained.

Yet another sign of ongoing growth for fieldbuses is that new consortia continue to address issues relevant to their use. In September, Cisco, Sun Microsystems, and chipmakers Atmel, Freescale and Jennic joined a number of others to establish the Internet Protocol for Smart Objects Alliance.

IPSO’s goal is to promote the use of Internet Protocols for sensors and other embedded devices. The group notes that IP can be used on a range of wireless and wired networks. Wireless communication is a growing area in industrial applications, with a number of different proprietary and open protocols competing for what’s expected to be a huge market.

That market may grow more rapidly if a repeat of the fieldbus wars is avoided. “Sensor arrays are moving to wireless. Companies also need to use technologies like IP. The alliance will develop code that will help promote the use of IP in these applications,” Dopplinger says.

While managers debate the pros and cons of various networking schemes and protocols, those issues will fade once the installations are up and running. Then, users will be trying to get the most out of the networks. Though that end goal can be overlooked, it’s always important to keep it in mind.

“The real advantage is not which bus technology people use, but the information they get,” Siemens’ Shaikh says. “You have access to all sorts of information in intelligent devices. You can also check the performance of devices and maintain them using this information.”

Sidebar - Bridging the Gap
To read the accompanying sidebar to this article, go to www.automationworld.com/feature-4875
More in Control