The task analysis should identify the hierarchical information required for each of the displays from Overview, Unit View, Detail View and Diagnostic View. The philosophy and style guide will dictate how the information, based on priority and importance, will be displayed.
The philosophy also dictates how to take advantage of color, brightness, contrast, salience and line thickness, combined with graphical objects designed to transform data into information. In the past, HMI schematics have relied on operators interpreting lots of data and using high levels of cognition to put the data into context.
One of the tools control systems consultants, Lin & Associates of Phoenix, has developed is an analog gauge that supports easy identification of an abnormal condition (Figure 3). When the process being measured moves outside of predetermined or operator-set operating parameters, the gauge changes to a number, indicating that an operator action is required.
Once the schematic graphics have been developed, tested and reviewed by operations, they should be enhanced using an iterative process aimed at continuous improvement. Development of the displays is one activity; delivery of the displays to a console with screens is another important activity.
Console development is an important part of the high-performance HMI, and should be based on solid ergonomics and human factors defined in the International Standard ISO 11064. Most modern displays today use large-screen displays for overview information and four other screens for more detailed information, which includes change zones and diagnostic trending.
This change in approach to graphical schematic design has led to improvements in an operator's ability to detect problems before alarms are activated. It is easy to achieve a 50% improvement in operator performance, which can provide significant ROI and move today's operators from a "reactive" operating stance to a "predictive" or "proactive" operating stance.
Operator Response to an Abnormal Condition
When an operator leaves it until the alarm has sounded, often there is a domino effect, and many alarms initiate in sequence. The operator can only manage these alarms sequentially, so we often get a second alarm and a poor response as shown in Figure 4 by trend 1. The best possible response we can get is shown by trend 2. Either way, the more time we spend in this abnormal band, the more costly it is to our operations. It affects quality, equipment reliability, economics and production rates. Obviously, the more cost-effective response is to detect, diagnose and respond before the alarm condition occurs and being proactive by using trends and the analog meter object described earlier.
Waiting Too Long
This is why we call them high-performance graphics; they improve the operator's performance and ultimately the plant performance by avoiding abnormal conditions.