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A Device by Any Other Name…

April 26, 2011
When It Comes to Naming Points in a Control System, It Pays to Take the Time to Develop a Nomenclature Plan
By N. Lewis Bodden

"A rose by any other name still smells." Well, that doesn't sound quite right. When it comes to naming points in a control system, it pays to ponder the meaning.

Naming is how we distinguish between devices and hopefully know what each is. To do this we need to agree on some rules and definitions. The scheme must have common conventions of classifying devices into a hierarchical ordering concept. Think of postal addresses. They identify locations by the house, street, city, state and country. To carry the location analogy further, it implies the planet, solar system, galaxy and universe.

Everybody's seen the old PT-101 style references. They served us well, but lacked intuitiveness. You knew it was a pressure transmitter, but you had to refer to the drawings or a list to know which one. You could sort them by the reference and get them grouped by type. If you were careful in creating the associated descriptions, you could sort them by description to group them by associated device. With all the points associated with a device together, you quickly compare it to other similar devices to see if anything is missing or incorrect. This creates a pattern or structure.

We must systematically classify everything that is to be controlled now and in the future. It's a little like inventing a new language that can be clearly understood by engineers, managers, operators and technicians. A well-chosen naming convention aids the new user in navigating large control systems. Document the naming scheme and provide a means for all to have access to it with a method to maintain it.

The naming convention must be a simple, stable and acceptable system for naming devices. When we're naming a device, we don't care about many aspects of it. We don't care as much about where it is physically as we do the logical relation. We need to identify the unit, subsystem and component that it's part of and what function it serves. With multiple devices, we need to enumerate them. Numbers are a good way to do this, and use physical location—North, South, East and West, or left and right—if it makes sense. Consider any future expansion and how it would apply. Letters are a good enumeration, especially if numbers already have been used.

Name selection can be a frustrating and time-consuming task. Often, a name that satisfies some of the criteria will contradict others. Consistency can be especially difficult. Rules must be agreed on beforehand, and come from a common source. Disallow the overly long, comical or cute.

The name must be easy to remember. It should be suggestive, so that others can understand the name and relate it to the device.

Make a list of standard abbreviations and use them consistently. MIL-STD-12 has some good basic rules for selecting them.

We use spaces to separate words and periods to separate sentences (delimiters). To develop standard nomenclature, we need to decide how to separate the parts of a tag. The biggest contenders are dashes, underscores and periods. The dash is the traditional delimiter. Underscore is more difficult to type and is easy to overlook because it resembles an underline. The period is a newer delimiter and is heavily used in structured nomenclature like Rockwell's ControlLogix.

WonderWare InTouch has a super tag template structure that allows tags to be built as tag groups. There are some advantages to using super tag templates, but they are not very flexible. For most applications, it is sufficient to use spreadsheets to create sets of tags, following a structure, to be loaded into an InTouch application.

Tags can be subjected to consistency checks that are very similar to dimension checks in physics. A utility can perform many of these checks and improve standardization and documentation. A name picker often can be incorporated to allow the tag to be quickly picked from a list. A spreadsheet can be used to develop a list of tags and adapted to use a data view with a structured tag builder to quickly assemble the parts of a tag.

A good plan for naming devices could take a week or so to develop and get consensus on, but it will save time in the long run and make your job easier.

N. Lewis Bodden is a control systems consultant with 35 years of practical experience in all aspects of system integration and control system design.

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