WELCOME TO this newest addition to the Digital Fieldbus Network world! Its my intention that every month well discuss at least one aspect of whats happening in the world of industrial fieldbus. Since there are more than 30 different fieldbuses in use in the industrial marketplace, this should not be too great a challenge, though to make this column successful it will require input from you, the readers, on topics that you would like to learn more about.
Being from the wet process industries, my natural tendency will be to focus on the fieldbus networks used in this realm, including Foundation Fieldbus, Profibus, DeviceNet, AS-i, HART, and Modbus.
So what makes a fieldbus? This question has been debated for some time with purists saying a fieldbus must be all digital, while others profess it need only be able to communicate digital information. It appears that the consensus is leaning toward the latter definition. This is why the HART communications protocol, which uses a Frequency Shift Key (FSK) signal superimposed on an analog signal, is normally included on the fieldbus list.
Of course, incorporating fieldbus technology in a device and control system is not done simply to please engineers and give them something new to play with. A planned fieldbus system must demonstrate an economic incentive and payback. This payback must appeal to people approving the project initially, such as project managers and others concerned with capital savings. More importantly, however, it must also appeal to operations people, who will operate and maintain the fieldbus equipment of +15 years after the project has been turned over to them.
The first fieldbus installations were justified based on wiring savings. Since then, experience has shown that cabling costs, or more precisely physical layer costs, often are about the same. This is partly because fieldbus systems require added components not found in traditional analog loops. Where the savings are found, however, is in field labor and installation. Because fieldbuses consist of multiplexed signals, they have far fewer terminations per signal than traditional loops, as much as 50% less. At todays pay rates and due to a shortage of skilled technical professionals, labor savings from using fieldbuses can add up quickly.
Another opportunity for fieldbus is to use it as a standard multiplexer-demultiplexer. It always seems that the location that first runs out of spare cables is the one furthest from the control center, for example, at the far end of a tank farm. Fieldbus technology offers an easy way to at least triple the number of devices in the field. By converting the field instruments and I/O card in the host to fieldbus technology, the multiplexing capability of fieldbus will allow installation of more than one device on a single wire. The downside is that the control system is now a mixed signal, and so a way must be found to differentiate one signal and cable type from the other(s). Maintenance, engineering and operations personnel also will have to learn about multiple technologies, and the host system will have to support the fieldbus too. In the case of some older control systems, this may require grafting newer technology onto the legacy system, perhaps through a gateway.
The more significant economic component of any installation is the Operational Expense phase, which is where more than 80% of a projects lifecycle costs are incurred. Day-to-day operation of a facility over +10 years also has the advantage that any savings realized here are captured every day throughout that period, which is more than the typical one-time-only savings for each phase of the programs capital portion. Despite the fact that there is little documentation in the literature about lifecycle savings attributable to fieldbus technologies, there are references to savings equivalent to 1-3% of plant capacity. This extra capacity is captured without any physical changes to the process, but is instead realized from the diagnostic capabilities of the field devices. These enable a facility to better predict true equipment health, and thus operate a facility longer, reduce the number and impact of outages, and effectively result in true prognostic maintenance. This means working only on devices that need maintenance when they are closest to failure and not before.
The above gives you an idea of the breadth of topics we can cover in this column, so please contact me with ideas on what aspects of fieldbus technology you would like to explore from the nuts and bolts mechanics, through to operational and maintenance concerns, to economic analyses and justifications. As control folks, all of you know that it is through feedback that control improves. So, please provide me the feedback I need to help you better.
Next column we will cover the thorny issue of why many Engineering Procurement Construction, (EPC) companies are not adopting fieldbus technologies as part of their offerings.
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