The Eye for Plant Operators' Eyes

Best Practices for Operator Interface Make Sure We See What We Need to See

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This article was printed in CONTROL's August 2009 edition.

By Doug Rothenberg

The plant operator has an extremely valuable and important responsibility: managing the real-time performance of a capital enterprise easily worth hundreds of millions of dollars. We ask operators to shoulder the burden of everything that goes wrong during their watch, all without any recognition when nothing does, and precious little—and that mostly blame—when it goes wrong, and they manage to manage. Within their area of responsibility and authority, they must be able to view every control loop, most sensors, most pieces of equipment and most of the supporting utilities, and then adjust as appropriate.

The failure to maintain situational awareness has been present in almost every disaster event that was not the result of a spontaneous, complete surprise. No one wants an accident. But accidents and disasters happen. We now know to a high degree of certainty that they happen because those in charge of ensuring that they don't aren't aware of critical events as they are happening. They fail to know the situation. They are unaware of what is really going on, what is likely to happen, or what isn't happening that they think is. As pointed out in my forthcoming book, Alarm Management for Process Control, from Momentum Press, this situation can be significantly improved by good operator interface design.

Evolution of the Technology

The video display unit seemed to represent a significant step forward when it replaced panel boards. Actually, it was a step forward in technology and a step backward in operator support. But it was the evolution of displays, not their intrinsic faults or limitations, that led us down the wrong early paths. Once the faceplate barrier was broken, so to speak, the world of graphic design opened up. As soon as color made the scene, all of the process control system (PCS) manufacturers started a race to see who could use the most appealing and exciting colors to flash in front of prospective buyers. What was missing from all of this new technology was the the answer to the question of what the video display should do and how best to do it.

Physical Display Architecture

We know that more screens are as necessary as more information is displayed. Figure 1 shows a recommended architecture. There are enough screens so that most tasks, including monitoring, can be viewed at the same time without requiring switching displays on a screen, much like a panel wall. The locations are arranged so that related information is naturally located.

The choice of which resident displays to locate on which screens is made so that those requiring close interactions are center and lower. Note that in Figure 1 the two working screens (1 and 2) are located at the bottom center and right. Here is where the operator would be monitoring specific control points and related variables, or he or she may be intervening to manage an abnormal situation or other event, say by altering a controller setpoint or moving a valve. Close at hand (bottom row at left) would be other screens with advisories to assist the operator. For example, the screen could provide assistance as needed to augment the operator's current activities. This assistance would show procedures, provide relevant background analytical data, alarm diagnostic assistance data and the like.

The screens that provide more global information are located above. As a package, the displays on these screens complement the operator's role of observing and managing. At top left, would be the screen dedicated to the alarm system. The screen at top center provides overview information on how well the process is working. This aids the operator working to ensure the plant doesn't go astray. The operator may also be working on process improvements. The screen that supports improvements is located at upper right.

New Operator Screen Design

What you see here depicts a best practice for graphical operator screens. It was developed based on the work of the authors listed in the sidebar, "More About HMI Best Practices."  Note that color is used only for information—to point out the unusual or abnormal.

  1. Shaded grays are used to delineate the operator's arena of responsibility.
  2. Contextualized icons focus on what's normal and what's not.
  3. A hierarchy of view provides
          a. Critical information only at overview,
          b. Robust details and causality at the secondary view(s),
          c. Support and guidance at the tertiary view(s).

Consequently, what the operator now gets is a powerful tool that focuses attention and encourages efficient navigation to what's he or she needs to see and attend to without the overhead of "clicking to oblivion" that we always find in conventional personal software.

 

It's All About the Bars
In the 1970s, VideoSpec from Foxboro used deviation diagrams to illustrate complex, interrelated situations. These diagrams consist of a series of bars representing production steps, process sequences or the processing order of the plant, from material (or energy) entry into the operator area to exit. The height of each bar shows how far away it is from a proper target—the expected value of the particular attribute needed for long-term, effective production. The beauty of such a display is that, even if this is the first time you have seen one, you can easily spot abnormalities anywhere and everywhere in the plant. For more on how deviation diagrams work, go to the extended version of this story at www.controlglobal.com/0908_HMI_DevDiag.html.

 

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