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ARE MODE ERRORS BUILT INTO THE OPERATOR INTERFACE OF YOUR CONTROL SYSTEMS?

NEWSLETTER ARTICLE

Without question, advanced control can increase the efficiency and productivity of a chemical processing plant. However, like all new technologies, advanced control is not without its problems. One area of potential problems lies in the man-machine interface and the operator being aware of the mode of operation of the advanced control. Errors have occurred because the operator was unaware of: (1) what mode of operation the advanced control was in, and (2) what it was doing, and made inappropriate control actions as a result.

Source of mode error has been summarized by suggesting if one wishes to create or increase the possibilities for erroneous actions, one way is to "change the rules. Let something be done one way in one mode and another way in another mode," (Norman, D.A., (1988). The psychology of everyday things. New York: Basic Books.). Mode error problems can occur when the control system has several different modes of operation and the system requires the operator to adjust the system differently in the different modes. Problems arise when the operator loses track of which mode of operation the system is in and makes incorrect adjustments as a result.

One recent study, (Sarter, N.B., Woods, D.D., "How in the World Did We Ever Get into This Mode? Mode Awareness in Supervisory Control," (1995), Human Factors, 37, (pp. 5-20)), explored the use of supervisory control and the related mode error problems. The study's findings can help control system designers create better advanced control operator interfaces.

The researchers stated that there are two basic contributing factors to mode awareness problems. The first is due to operators having poor mental models of the automation. The second is due to poorly displayed information concerning the automation's status and behavior.

Recommendations were suggested in the study to alleviate mode error problems. One of the suggestions called for expanded automation training. The training needs to be more than just facts about the control system. The training should include not only the normal functions of the automation, but also the off-normal/infrequently occurring situations and how the automation reacts under those circumstances. The training needs to be more cognitive/conceptual, and less skill/motor response oriented.

Another suggestion for improving mode error problems was through better interface design. Researchers in the study described the current display systems as being "opaque," in that (1) what exactly the control system was doing, and (2) which mode of operation it was in were not readily apparent from the display system. The most frequently asked questions operators have of the advanced control systems are, "What is it doing?", "Why is it doing that?" and "What is it going to do next?" The display system needs to make the answers to these questions readily apparent to the operators.

Advanced control, with its performance enhancement potential as well as its operator interface problems, is here to stay. Control system designers need to be aware of potential man-machine interface traps, such as mode error problems, that can inadvertently be built into the systems.

Copyright © 1995 Beville Engineering, Inc. , All Rights Reserved

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