Fieldbus Technology How a Fieldbus Infrastructure Ensures Effective Process Management

Author / Editor: Andreas Hennecke / Dr. Jörg Kempf

Nobody doubts that digitalization is the key to comprehensive process control. That is why Fieldbus H1 and Profibus PA are at the core of any new system. They not only enable digitalizing process control from the PCS all the way down to each individual device. They also pave the way for the big data approach of a new generation of production control.

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Trunk-and-spur topology. One spur for each field device provides transparency and operational safety.
Trunk-and-spur topology. One spur for each field device provides transparency and operational safety.
(Picture: Pepperl+Fuchs)

Journey back in time to the 1950s and1960s: Three black and white television channels are a sought-after luxury. There are simple adjustment dials for both volume and brightness. The resolution is 30 lines — around 1280 pixels — with a refresh rate of 12.5 Hz. Electronics is finding its way into the process industry: Instead of compressed air, a thin two-wire cable provides power supply and instrument control.

The resistance is high: Maintenance people must now carry a device with them all the time: the multimeter! The counterargument: “With compressed air, I can hear whether it is working if I pull out the hose.” The planning engineer says: “Now I have to switch everything to the new technology and cannot just create a carbon copy of the previous system.” (The photocopier has already been invented, but has not yet been universally introduced.)

At this point, we will avoid repeating the advantages of digital communication using Profibus PA or Foundation Fieldbus H1. As is also the case with conversion from compressed air to electrical signals, the benefits can be reaped when all of the tasks from planning and commissioning to operation and maintenance are adapted to fully utilize the features. Process automation with fieldbus already starts today for itself at commissioning time.

The fieldbus is essential for all those who are serious about the “Internet of Things” and Big Data. In this respect, the following instructions apply to all digital transmission systems, including Hart and Wireless Hart.

250 automation specialists attended the 2015 PI conference in Speyer. This conference is organized every two years by Profibus and Profinet International. Workshops demonstrated the practical application, showing how a fieldbus infrastructure ensures effective process management. The workshop on planning and explosion protection showed, using the example of Profibus PA, how planning needs to be adjusted, so that the first “profit” from a higher level of efficiency and shorter project duration can be seen at commissioning time.

What to do? Tips from the expert – please continue reading ...

Use typical variables, models, and templates! For Profibus, the application profile PA3.02 describes a standardized template example independent of manufacturer and software revision for each instrument type, which governs the devices and control technology. In practical terms, this means: configure the address, connect, and run. The Profibus PA profile also handles device changes without any problems. The maintenance engineer no longer has to worry about compatibility between device and control technology. The new instrument is automatically compatible: configure the address, replace the device, connect, and run.

Prototyping can take care of 80 % of the loops with efficient preparation and testing, allowing to fine-tune the parameters at this early stage. The purchasing department orders the devices with the optimized parameters, and factories preset these values during production. This step also minimizes issues of DCS integration.

Take advantage of the added value of devices! Measuring instruments with multiple variables eliminate the need for additional compensation calculations or supply values for energy, density, and mass in one telegram. The manufacturers will be happy to show how it works and how digital technology helps increase accuracy it is accurate even for internal billing purposes.

Plan the installation in advance! Trunk-and-spur topology is the industry standard for installation. This topology is simple to understand and corresponds conceptually to the old terminal board. Each field device is individually accessible. Free tools such as Segment Checker (www.segmentchecker.com) check cable length and voltage drop against specifications regarding geographic dimensions or device count.

Plant documentation also validates intrinsic safety in accordance with FISCO at practically no additional costs. FISCO compliance is required for all components and just needs to be documented. With the high-power trunk concept, cable distances over 1000 m present no problems, even in hazardous areas up to Zone 0/Div. 1.

Check and define the shielding and grounding concept as part of planning activities! There are no standards for this. A specialist develops a suitable concept depending on conditions, such as the electrical installation, the geographic conditions, and so on. Don’t skip this step. It is crucial to establishing reliable communication.

Take special care of shielding! Even though a fieldbus device essentially connects in the same way, there are some differences. Installers may be accustomed to connecting one of the two leads to ground. This is wrong! Training installation personnel takes no longer than a one-day session.

Decide on the need for lightning protection at an early stage! Space requirements typically prohibit retrofitting in the control cabinet or in the fieldbus junction box at a later stage. State of the art lightning protection is self-monitoring and can automatically report to the plant asset management system when the end of the useful life is reached. This is real savings for the maintenance team as repetitive checks after an electrical storm are obsolete.

Time Saved, Satisfaction Secured

Well-thought-out planning will be rewarded with a significant reduction in commissioning time — especially when compared with conventional 4…20 mA technology. A reduction of several weeks is possible for larger systems, e.g. one with 1200 instruments. The production management will certainly be pleased about this. A technical white paper discusses best operating practices for commissioning of fieldbus systems made efficient through diagnostics. It illustrates the potential efficiency and time saving in the context of a case study.

* The author is Product Marketing Manager, Process Automation Division, Pepperl+Fuchs GmbH, Mannheim/Germany.

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