Manufacturing AUTOMATION

Features Columnbus Opinion
Afraid of fieldbus? Time to let go of your reservations


March 24, 2006
By Ian Verhappen

Topics

If you aren’t already using Foundation Fieldbus or another fieldbus technology, is it because the technology is still relatively new? If so, you are creating more barriers to adopting the technology than actually exist.

Fieldbus technologies do not change how you select a device, only how you connect it to your control system. It doesn’t matter whether it’s a transmitter, valve or flow element. As long as your device supplier supports fieldbus technology, you will be able to work with that supplier to take advantage of all the benefits associated with digital integration, including:

  • Increased accuracy. Each time a signal is changed from one format to another, there is a degree of degradation. However, all digital transmitters do not need to translate between analogue and digital. In fact, some devices are completely digital and use a frequency measurement as the means of sensing the process conditions.
  • Improved rangeability. Digital meters can be “reranged” to encompass any portion of, or the entire, design spectrum without loss of accuracy. As a result, it is possible to reduce the level of inventory needed to meet your measurement needs, and to more closely match those measurement needs to the process conditions. For example, if the desired range is zero to 275 inches water column (WC), this is now possible. In the past, the user may have “made do” with a zero to 250-inch WC or zero to 500-inch WC device and the resulting lack of range.
  • Enhanced diagnostics. Digital transmitters are smarter, and some of the extra “smarts” are being used by the device to continually monitor its own health with self-diagnostic routines. These diagnostics not only improve the reliability of the device because of the warnings provided as the device senses changes in its environment, but they also provide the technician charged with maintaining the equipment with an indication of the problem so that it can be resolved quickly.
  • Future proofing. If the device has available memory and power, new features can be added to digital fieldbus transmitters by downloading new software into the device, allowing it to be upgraded.

The physics on which the sensors are developed remains unchanged, as do all of the rules and sizing formulas used to select these devices. The same is true for all the criteria we use to meet the associated electrical classifications, though fieldbus systems offer more options on how to meet the needs of various area classifications to make more power available to the network. Perhaps someone should make the same ideas available to conventional loop powered devices so we can help them overcome their energy limitations and become smarter as well.

Fieldbus technology is not that different than what we’re use to. How to select the right equipment remains the same, and that is what requires time, training and experience. For example:
• Vortex meters still have a minimum cut-off flow rate;
• Pressure transmitters still need to be mounted so the impulse lines do not become plugged or the capillary fluid “baked out”; and
• Control valves need to operate in their correct range with the correct control profile selected.

Advertisment

Fieldbus is just another new skill to add to our repertoire, the same way we learned about sizing skills, Ethernet and even computers. Fieldbus is simply another communications technology with its own rules and limitations.

If you have any suggestions for this column, or topics you would like to see covered, please send me an e-mail and I will try to include it in the future as it fits with the editorial calendar.


Ian Verhappen, P.Eng. is an ISA Fellow and director of ICE-Pros, Inc., an independent instrument and control engineering consulting and training firm specializing in fieldbus, process analyzer systems and oil sands instrumentation and control. E-mail him at Ian.Verhappen@ICE-Pros.com, or visit his website, www.ICE-Pros.com.


BUS BITS
• Telecommunications Infrastructure Standard for Data Centers, TIA-942, from the Telecommunications Industry Association (TIA) provides requirements and guidelines for the design and installation of a data centre or computer room, and will affect the way control systems are designed.

• The OMAC (open, modular architecture control) Users Group has done some research on the suitability of Windows CE for use in the automation environment and determined that 95 per cent of systems require cycle times of one second or greater, with a tolerable variation of one millisecond. Therefore, with a 200 MHz x86 processor, Windows CE will meet or exceed this requirement.

• If you are interested in improving your knowledge about the HART protocol and how to use it, you may want to look into the free online offering from the HART Communication Foundation at hartcomm.org/etraining.

• Tri-State University, the Fieldbus Foundation’s regional training centre, will be offering its next certified Foundation Fieldbus class on May 8, 2006. To learn more or to register for the “Introduction to Fieldbus” course, visit www.tsutechcenter.org.