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Industrial field networking technologies as represented by the various flavors of fieldbus and industrial Ethernets represent a step change in the way automation and control systems are designed, installed and operated. They are still relatively new and unknown quantities. Even though the Fieldbus Foundation (FF) celebrated its 10th anniversary in 2004, a lot of confusion remains about how to effectively use the technology.
Nevertheless, there are compelling incentives for adopting the approach. A January 2003 Automation Research Corp. study, “Best Practices for Maximizing the Value of Fieldbus Implementations,” concluded that fieldbus enables superior return on assets and life-cycle cost savings. The report also stated that fieldbus requires intensive upfront planning.
The key point to “getting it done right” is managing the change. James Rhame of Shell Chemical Co. during his keynote address at the FF’s 2004 General Assembly emphasized that the biggest challenges in managing that change are:
- understanding the importance of people, and the fact that different people have different reactions to change;
- treating change as a mental, physical and emotional process, not as an event; and
communicating openly and honestly.
Fieldbus technology is more complex than traditional analog control systems with their point-to-point connections. It is also far more powerful. To capture that power requires that the decisions made when a project is being conceptualized and formulated are the right ones. In this respect, the project is just like any other one in which decisions made at the beginning can have a significant impact on final success or failure. It is for this reason that front-end engineering design (FEED) processes are used in most projects today. Of course, without sufficient knowledge or experience, it is almost impossible to be able to make informed decisions; consequently, one of the most critical components of a project’s success is another personnel issue: training.
Fortunately, a number of companies, technical societies, educational institutions and the fieldbus technology organizations themselves provide vendor-neutral training on fieldbus and industrial field-networking technologies.
Obviously, a one- or two-day course will not make people experts in fieldbus technology. It will, however, make them aware of some of the differences between a conventional analog system and a distributed digital control system as represented by fieldbus technology. It should convince them of the value of the approach for their project and also make them realize that they are not familiar enough with the technology to make the best decisions by themselves.
The team involved in the project probably is familiar, though, with the “decade rule,” which was proved by IBM in the 1970s and is still valid today. It states that $1 spent making a decision at any stage in the project is multiplied by an order of magnitude if the decision is delayed until the next stage. That means $1 spent during conceptual engineering by a small design team grows to $10 during FEED, $100 at procurement, $1,000 during construction and $10,000 at startup. So, spending a few extra dollars upfront will yield significant savings later. Yes, a consultant might be expensive, but that cost pales in comparison to the price of an idle plant, field rework or several idle field mechanics later in the project.
Now that we appreciate the importance of getting it right early, what are some of the FEED issues that need to be considered for a fieldbus project?
Power supply issues
Fieldbus network signals differ from analog ones because they alternate above and below a reference point to indicate a “1” or “0,” creating a ripple in the power supply. A power supply, however, is designed to maintain a steady signal on the wire pair, and so does its best to remove the ripple. It is for this reason that fieldbus systems have power conditioners, which, in some cases, are incorporated into the power supply as a single unit. These power conditioners can be either active or passive, depending upon their componentry and design. Active conditioners use solid-state components, such as diodes and transistors, to achieve the necessary signal conditioning, whereas passive conditioners employ induction coils and resistors. Many papers have been written on the relative merits of each type. With the reliability of silicon-based devices today, there is minimal difference between the two approaches.
In 2004, the FF issued a standard on power conditioners that provides a baseline for users. Units that comply with the standard will receive a FF “check mark.” Purchasers still will have to consider the level of local support and inventory, as well as the form factor and active versus passive design. What is more important, though, is the amount of power that remains on the output terminals of the conditioner — because more power means more devices can be placed on a network.
The size or output rating of a power supply largely depends upon the environment in which the devices will be placed. Just like in a traditional analog system, the area classification and the methods used to meet the electrical requirements of that classification drive a number of decisions. If the choice is to go with explosion-proof fittings for all devices, there really are no limitations on the power available to the network. However, if one of the other safety mechanisms — intrinsically safe (IS), fieldbus intrinsically safe concept (FISCO) or fieldbus non-incendive concept (FNICO) — is selected, it will have an impact on the design.
IS can be used in any area classification, but supplies the least amount of current. It can only provide about 85 mA to 90 mA to the network. Because each device takes anywhere from 15 mA to 30 mA, the network is limited to between four and six devices.