Crossing the Chasm

Digital Fieldbus Networks Bridge the Gap from Field Devices to the Control Room

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For more on fieldbuses, go to www.controlglobal.com/fieldbusguide.html

By Dan Hebert, P E, senior technical editor

Experts juggle swords, but the rest of us are better off just watching.  Put another way, just because someone somewhere has successfully installed a digital fieldbus network doesn’t mean it can by done by you or anyone at your company without proper training.

Those who don’t look before leaping across the fieldbus chasm are destined to end up like these necessarily anonymous respondents from a major U.S.-based engineering firm. “We have been reluctant to specify Foundation Fieldbus, HART, Profibus and other digital fieldbus networks,” says the first engineer. “We have seen the presentations, attended the brown bags, said thank you, and I hope we will continue to wait on this.”

He has good reason to be hesitant. “I have a friend working for a systems integrator who has been tearing out what is left of his hair for about two years trying to get a fieldbus network to work properly. Maybe there is a place for magic wires and magic devices in industrial applications where advanced data acquisition is justified and where the owner’s on-site technicians can understand and maintain it. But in the industry we serve, we do not want it or need it yet,” concludes our first respondent.

Another engineer summarizes his firm’s general opinion of digital fieldbus technology. “Fieldbus is great in theory, but for whatever the reason, in practice it doesn’t yet seem to live up to the promise. Some difficulties seem to be related to training and to understanding of the technology by designers and installers. Other problems seem to be related to the complexity of configuring the systems. Overall it sounds like the benefits are offset by the associated headaches.”

More Complex Than PC Networks

Consider the analogy to PC networks. You have at least one PC at work and likely a few more at home. You may even know a little bit about networking if you have a home wired or wireless LAN. But that doesn’t make you a computer networking expert. Likewise, you probably also know quite a bit about instrumentation and control systems. You may have even designed and installed one or two small-scale digital fieldbus instrument networks. But that doesn’t mean you are a digital fieldbus expert.

You know that to manage a large scale office network, your company needs at least a small group of experienced and highly qualified computer networking professionals.

If you want to make the transition from 4-20mA to a digital fieldbus, you will also need a small group of experienced and highly qualified digital fieldbus professionals. “There is a big learning curve for someone to become fully digital,” says Richard McCormick, principal at Mick Automation, Levis, Quebec. “This is especially true for instrument techs that may have limited experience working with PCs.”

Don’t make the mistake of thinking that a digital fieldbus network is simpler than an office network just because it has fewer nodes. You may have only 50 instruments, but other factors can make even a small digital fieldbus network as complex as a PC network with hundreds of users.

These factors include multiple incompatible protocols, harsh installation conditions and multiple vendors who would often rather compete than cooperate. “Not only is there a learning curve associated with first-time evolution to a digital fieldbus network, but one must also deal with incompatible device drivers and with multiple vendors when you need problem resolution,” observes Thomas Toms, lead engineer at Progress Energy, headquartered in Raleigh, N.C.

Profinet Integrates Fieldbus Systems

End users must often deal with multiple fieldbus networks in their plants. This can complicate network design, installation, and maintenance. Courtesy of Profibus International.

Courtesy of Profibus International

Dealing with multiple vendors requires care. “The master unit of one vendor’s network may not be fully compatible with another vendor’s spur blocks,” relates Philip Daniels, an engineer with the Wastewater Treatment Division of King County in Washington.

“One vendor may have the intrinsic barrier located at the master unit, while another may have it at the spur block. If the wrong combination of vendor products is selected, there will be no intrinsic barrier in the circuits,” adds Daniels.

“Another problem is addressing of the units. The Foundation Fieldbus network is just like any other network, and the addresses of the various components must be unique. We ended up addressing field devices just like our office network intranet addressing. Another issue that caused us problems was individual component programming. Each device has unique programming needs, and vendors do not always provide proper documentation,” concludes Daniels.

So how do you get up to speed? One way is to hire a digital fieldbus expert who has prior experience with the design, installation and maintenance of your desired network and your existing control system. Good luck finding such a person.

This is not like the PC world with its true and few interoperable standards, its long track record and its throngs of qualified and certified network engineers. You instead have to deal with multiple incompatible technologies, relatively new products and a paucity of experienced professionals.

If hiring an expert is virtually impossible, the other avenue is for you and your colleagues to become experts. This means getting training, either from a vendor or from a service company.

Training Challenges

The next step is to train the right way. This end user learned the hard way. “Our main challenge with our digital networks was training instrument techs on the more complicated devices” reports Danny Cox, the director of manufacturing systems at AOC Resins, Collierville, Tenn. Cox’s company used Foundation Fieldbus, DeviceNet, AS-i and HART to connect field devices to an Emerson Process Management DeltaV control system.

“We tried vendor training, and it was a total disaster. We now have corporate engineering travel to the plants to provide hands-on training. At least for our situation, in-house training seems to work much better,” he concludes.

AOC Resins has to deal with four different networks and with instruments from 10 different vendors. Sending your entire staff to ten different vendor training sessions is not practical. Better to have an internal company expert learn as much as possible from vendors, from network organizations and from other end users; and then train others.

Another challenge that distinguishes a digital fieldbus network from an office network is the sheer number of people that have to be familiar with it. In an office, a small number of network experts know the network in detail. In a plant, the entire automation group will probably have to deal with at least some part of the network on an on-going basis. Engineers need to know how to set up the network initially. They also need to specify the instrument parameters of interest for the process control and information systems. They need to know how to configure the software packages that receive the network data.

Instrument techs need to know how to use the network for remote calibration. They need to be able to make sense of the instrument data for predictive maintenance. Instrument techs keep calibration records, so they need to know how where that data is stored.

If something goes wrong or if changes need to be made after initial installation, engineers and technicians often need to work together.

So we have a situation where many people within your company must get up to speed on multiple networks, various field devices and at least one control and information system. Proper training is essential so that you can pre-identify some of the gotchas. Read and heed how some of your fellow end users are winning their field bus battles.

Winning the Battle

The big problem with vendor information is that too rosy a picture is painted because vendors want to promote their particular networks. If you listen to vendors married to particular digital fieldbus technologies, you invariably will go in to your project with unrealistic expectations.

There are two main sources of straight dope. The first is other end users, system integrators and engineering firms who have been where you want to go. The is suppliers who make a living helping end users install networks from multiple vendors.

Let’s hear from an end user first. While Richard McCormick was at Ultramar, the company implemented Foundation Fieldbus (FF) on a grass-roots unit. The engineers connected field instruments to a Honeywell Experion PKS automation system through fieldbus interface modules. “We used Honeywell’s asset manager software to gather FF data from the Experion system. I consider such software a must if you want to fully exploit FF.  Because there is so much data available from FF instruments, you need a piece of software that will massage the data prior to presentation to instrument techs. This greatly simplifies their work by providing common root-cause information from different instruments vendors’ embedded diagnostics,” advises McCormick.

Now let’s hear from a vendor who works with many different fieldbus technologies and has no interest in promoting one over another. “Some of most baffling problems occur during startup and are because of improper installation,” reports Desmond Ho, fieldbus applications consultant at MooreHawke in Shanghai, China.

“In many cases, segment terminators are not set properly. All fieldbus segments need proper termination to prevent communications errors through uncontrolled signal reflections. Many subcontractors pay little heed to the terminators and either forget them completely or simply install them everywhere, neither of which allows the segment to operate properly. Often, physical inspection of junction boxes and field enclosures is the only way to locate and correct the terminator position, which is a significant delay to the commissioning process,” continues Ho.

“This problem can be avoided from the beginning simply by specifying newer device couplers with automatic segment terminators such as our device couplers. The automatic segment terminator greatly assists in segment commissioning by eliminating the issue of over or under terminating. If multiple field device couplers are used, the auto-terminator is always activated at the farthest unit and automatically migrates up the segment if that line is disconnected,” concludes Ho.

ARC Advisory Group research director Wil Chin gives some sound advice on dealing with multiple vendor protocols. “A best practice for end users is to pick a supplier with multi-protocol capability. As an alternative, FDT technology should be considered for plant upgrades, particularly when the objective is deploying a plant-wide asset management system.”

The Benefits of Getting it Right

Once AOC Resins figured out how to get their people up to speed, they began to reap the benefits. “The main advantages of digital data are online instrument health monitoring and communication of multivariable information. The best example is looking at the drive gain for our Coriolis meters. If one exceeds a limit, we assume that the meter has gas, and we switch our primary measurement to load cells. In a similar fashion, we verify radar level is not locked on a target by looking at signal strength and this prevents over filling,” relates Cox.

Preventing one over-fill incident with an associated spill, shutdown, and clean-up can go a long way to justifying a digital fieldbus system.

McCormick of Mick Automation says that other key benefits are decentralized control on the wire capabilities, reduced implementation costs, measurements with better precision and predictive maintenance.

“Process plants gain availability of historical data for process analysis, process improvement and real-time management of production,” says Dan Miklovic, the vice president of manufacturing research at Gartner. “Plants can also use simulation to configure soft sensors and thus avoid installing and maintaining hard sensors in hazardous locations.”

Are the benefits of fieldbus worth the costs and risks? It’s your call. Just make sure that you include all of the costs and benefits in your analysis based on good information from unbiased sources.

For more on fieldbuses, go to www.controlglobal.com/fieldbusguide.html


What Makes Digital Fieldbus Difficult to Implement?

  • Lack of industry wide expertise
  • Too many standards
  • Frequent requirement to work with multiple networks and vendors
  • Harsh environments
  • Huge amounts of data to be handled
  • Multiple users must learn how to work with the network
  • Vendors promote their favored networks instead of warning users about implementation issues 

How to get up to Speed

  • Designate one or more people in your company to become experts
  • Have your internal experts train others within your company
  • Make sure that operations personnel use the technology early and often
  • Find and attend vendor neutral presentations and seminars
  • Speak with other end users who have been where you need to go
  • Engage an independent system integrator

What you’ll get out of it

  • Reduced installation costs
  • Faster commissioning
  • Remote instrument maintenance
  • More instrument data
  • Easier to use multivariable instruments
  • Better measurement precision
  • Enable decentralized control

For more on fieldbuses, go to www.controlglobal.com/fieldbusguide.html

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