Getting the Message Across
Conveying understandable process information to an operator requires more than dazzling HMI graphics in real time.
..Four o'clock in the morning is a tough time for most operators to concentrate on tasks at hand. Ask anyone who has worked nights, whether as part of a swing shift or permanently, how the last couple of hours of the 'graveyard' shift can play havoc with the ability to accomplish simple chores or react to routine situations. Most would agree that they are not functioning at 100%during the wee hours. This can be a downright dangerous problem in many industries. And plants that process environmentally hazardous materials top the list.
come to replace many of the purpose-built hardwired control panels with their indicator lights, pushbuttons, gages, chart recorders, and annunciators, the problem of effectively communicating required process information to the operator--especially one rendered less alert by boredom or fatigue--remains the same.
Digging in the toolbox
..A control engineer charged with developing control screens has a wide array of software offerings available for that purpose. Although the tools are similar, differences in target industries do exist. Software packages that serve a wide variety of industries are often programmed freeform because of the intended broad diversity of applications.
leave it up to them. Most tend to match the previous layout of hardware controls, so we haven't bothered to provide guidelines.'
sistent ways of presenting information throughout its software products. For example, an on-screen button will be presented the same way in our software packages. RSI has also used results from usability tests, user input, and their application experience to suggest placement of alarms, buttons, trend charts, and other objects in a consistent manner.'
virtual controls, diagrams, readouts, and trend charts.
ome more important that process HMI applications are developed consistent with their expectations.'
sans serif fonts, with adequate background contrast, is best for control identification. On-screen language should be as close to natural dialog as possible in any screen view, and especially in dialog boxes. Avoid local jargon unless it is industry specific and contributes to quick understanding with all the users of the system.
hould look similar to give an indication of this. Consistency should be the goal at all levels of design, from the definition of tag names and descriptions to the appearance and behavior of individual controls in the process graphic. It should also extend to screen layout and overall navigation scheme if the operator has more than one screen to call up.
Don't 'paint it black'
Proper use of screen color can improve operator visualization, provide more information in less space, assist in creating priority in lists of alarms and messages, and reduce operator response time by drawing attention to a specific area of a screen. However, Sam Herb, PE and senior systems marketing specialist with Moore Products Inc. (Spring House, Pa.), cautions that color be used carefully. Mr. Herb offers these guidelines for color use on screens:
Define the meaning of each color and be consistent with this meaning throughout the system
Use color as a redundant indicator
Use neutral colors (gray, black, beige, tan, etc.) on large background areas
Use compatible color combinations
Use color to indicate quality, not quantity
Choose among any of the 256x256 colors available but avoid using more than seven colors. Use of too many colors, especially incompatible ones, will obscure, not enhance, on-screen information. Avoid low-contrast combinations like dark gray on light gray, and busy combinations like red and green.
ng a key to make an appointment for a screen view!' he adds.
er in all graphics and in all expected operation conditions ( see table ).
How much can you do at once?
..Consolidation of multiple control rooms along with reduction in operator staffing became part of the cost cutting/productivity enhancer methods employed over the last two decades. With process control computers and display technology fueling the changes, individual operators can now be given access to 'all the data' from any number of process units. Unfortunately, screen developers often miss the fact that while physical panel board space is no longer a restriction to the amount of information that can be communicated to an operator, that individual's ability to use data is still based on basic human cognitive abilities.
..Studies of human cognitive ability have determined the 'rule of thumb' for decision making is that a human can make a choice among 'seven, plus or minus two' items at one time. In other words, average operators are only able to maintain seven separate items in their heads (awareness) at one time. The best case that should be anticipated for human intervention in a process is to have an operator correctly chose which item to address between nine simultaneous choices. In short, it is easy to see that data-dense CRT screens have the ability to overwhelm operators, leading them to forget what they were originally doing especially when intervening events can keep occurring. Alarms are probably the best known example of cognitive choices that can mount quickly in a control room situation.
g an alarm system and its attendant displays. The guidelines (listed in order of precedence) are:
Use a formal alarm definition process
Always maintain an operator's situational awareness
Allow the operator's actions and responses to be consistent
Optimize actions requiring critical time response
Don't allow the media to distort reality
Strive for simplicity
Validate critical events before generating critical alarms
fixed in a highly visible area at the top or bottom of the screen.
Rest of the layout
..All on-screen information should be logically organized. Once the location of alarm indicators is placed, make sure that this area is free of other screen elements such as pop-up windows. At this point, using a template for screen design not only improves consistency but also makes design easier and quicker.
creen. A button bar near the bottom allows access to other processes, in this case A through E. The very bottom of the screen is used for global functions such as navigation. The very top of the screen provides a menu of advanced functions available to authorized operators. This same 'formula' can be repeated for other processes, using different background colors to help operators immediately identify where they are in the system.
an be required with keyboard-based control.
), 'Find out what the actual operators want to see. Their involvement in the planning will guarantee that they will like the finished product.'
For more on human-machine interface consistency see ' Optimum Human-Machine Interface Design '
ImprovingVisual Comfort at the Workstation
..Visual discomfort a user experiences when using a video display terminal (VDT) control interface occurs as visual demands of the task exceed the abilities of the individual. The problem can be eliminated by treating the user's visual condition and/or making the visual task less demanding. A thorough eye examination which includes analysis of an operator's eyes at near working conditions is required if visual problems are to be corrected.
..While quality of illumination in the viewing area is important, so is distribution of light. All objects in the operator's field of view should have equal brightness. Elimination or control of bright light from windows and auxiliary lighting fixtures, reorientation of workstations to eliminate bright lights from the operator's field of view, elimination of white reflective surfaces, and use of monitor hoods to shade overhead lighting are all helpful techniques. Light levels of 18 to 46 foot candles are recommended for most workstation environments.
he operator), or use of dark characters on a light background are helpful in eliminating screen reflections.
background is high. Also desirable is the use of 60 Hz or higher refresh rates to lessen the sensation of screen flicker and small dot pitches (less than 0.28 mm) on color monitors.
LCD terminal ergonomics
Liquid crystal displays (LCDs), which are rapidly becoming the display type of choice in many control applications, offer a number of advantages over CRTs. According to Steve Butzon, an occupational eye care consultant at Dupage Optical Inc. (Villa Park, Ill.), 'the main visual advantage of the LCD is that it maintains superior contrast under bright sunlight conditions. This attribute makes it more desirable for use in bright industrial and out-of-doors applications.'
corrected for display terminal viewing, provide a comfortable illuminance environment, and use backlit displays when possible, Dr. Butzon adds.
The best seat in the house
..Since control room and factory-floor monitors are usually not considered 'home' to users for a full eight hours per shift, comfortable furniture and viewing arrangements may not be considered important. In reality, allowing users to tailor their viewing geometry and working posture through the use of adjustable hardware and furniture is beneficial, especially for the user's alertness and productivity.
lexible, breathable, woven seat covering. A stable five-leg design with abbreviated arm rests that allows the chair to be moved under the table is also preferred. Forearm support for both fixed and movable units should be included if a keypad is used with it.
ane of the operator's face. Most important for sitting posture, the top of the screen should be just below the eyes. Screens located higher than these recommendations cause posture changes. Because so much information is taken in visually, it is said that 'the eyes lead the body.' This forces the operator to assume whatever posture is required to optimize viewing. Neck aches and backaches can result.
r vision syndrome.
The'Art' of the Hardwired Panel
..The birth of the software-configured, monitor-based control screen has not sounded the death knell for the hardwired control interface. And although electronic HMIs have found their way into much of the new plant construction, many hardwired control centers remain in existing process plants.
ed to drive the use of new technologies, abandoning carefully standardized engineering practices that have given control rooms corporation-wide that common 'look and feel' can be a difficult decision. In many of these installations, the anthropometric principles of human engineering (the basis of present-day ergonomics) were carefully followed.
llenge. In addition, adherence to correct placement of manual adjustments, lights, switches, and pushbuttons that emphasized ease of use, readability, and accessibility was also essential. Add the requirements for handling wiring and providing the control room with proper lighting and environmentalcontrol, and it is easy to see why this form of 'the window on the process' has been slow to disappear.
Resourcesfor the HMI Designer
..HMI-based control installations are every bit as application specific as hardwired ones. This allows for a good deal of creativity on the part of the control engineers who must develop the required displays. Besides guidance offered by HMI software suppliers, other sources are available. Human Factors and Ergonomics Society (Santa Monica, Calif.) publishes the Human Factors Perspectives on Human-Computer Interaction, that contain selections from proceedings of the society's annual meetings.
www.isa.org ) or by calling 919/990-9200.
, a different topic in the field of ergonomics will by explored by a foremost expert in the field. The website is www.ergoforum.com .
Suggested HMI Foreground Colors
Normal or invisible
(Courtesy: The Foxboro Company)