Who is building your HMI?
Who is building your HMI system? If you are a vendor, then who are the designers of your standard HMI screens? If you are an owner-operator, then who are the designers of your operational screens? If the answers to these questions are your programmers, engineers, or web designers that you have hired out of college, then you probably have a low performance HMI (human machine interface). If your graphical displays look like P&IDs (piping and instrumentation diagrams) covered with hundreds of numbers and multiple colors, then you definitely have a low-performance HMI. Operational screens are not like Web pages where flash and glitter are used to draw attention to text and where the user can move a mouse over the screen to discover active links. Operational screens are used by operational staff to monitor and interact with a supervisory control and data acquisition (SCADA) or manufacturing execution system (MES) software. They provide situational awareness of the process.
Poor HMI design, higher risk
HMI screens are used to manage the operation and supervise the process. They are not intended to wow the staff with fancy distracting graphics, or to show the power of the embedded graphics processor by animating near realistic images of the equipment. These HMI designs are not really realistic because they usually reflect the equipment design image. They do not show the holes in the tanks and fluid spilling out, or the broken valves or stopped motors. Unfortunately, poor performance of the HMI system has been cited numerous times as a significant contributing factor to major industrial accidents. The U.S. Chemical Safety Board (CSB) has estimated the total loss due to operator error as $10 billion per year, and poor performing HMIs are a major cause. If your HMI was designed over five years ago, then you may now be running a multimillion-dollar operation from HMIs created when there was little knowledge of proper HMI practices and principles. Newly designed interfaces follow the principle of high-performance HMIs. A high-performance HMI is one that is designed with a consideration of user and functional requirements, with good human factor engineering, and that supports all normal, abnormal, startup, shutdown, and switchover modes of operation.
Usable and safe, not pretty
Designing a high-performance HMI (HP-HMI) is one case where it is important to follow good software engineering practices used in user interface design, and not just copy existing designs. Otherwise, you can end up with screens that resemble badly designed mobile and web designs with hidden hot-spots, small text, inconsistent color use, and too much or little information on each screen. Good software engineering practices involve usability labs and usability studies. In usability studies, users are given a minimal amount of training, usually commensurate with the minimal job skills, and then asked to perform specific tasks using the user interface. All user interactions are recorded, including mistakes and repeats to discover the good and bad aspects of the interface. The studies are often performed in a usability lab where the users can give a running commentary on the mental activity; all conversations, mouse movements, and keystrokes are recorded; and sometimes even eye movements are recorded to see where on the screen users are looking for information. All of the recordings are then analyzed to reduce user confusion, changes are made to the interface, and tests are rerun. This may sound like a lot of work, but it is actually only a small percentage of the total effort required in designing HMIs. Usability studies are often short, involving only a few hours of testing, and typically involve only one or two usability experts.
The closest analogy to high-performance HMI screens are stock market tracker screens. Normally there are only a few items that are watched all the time. In these cases, only the current values are needed to decide if any actions are required. Most items are watched only when they reach limits. These can be actual limit values, or when the trend indicates that an item is approaching a limit. Then the user may drill down to see the trend, maybe look at a longer term trend, or link to related information. The purpose of the interface is not to control the process, but to provide situational awareness and supervision of automated activities.
HMI design standards
Fortunately, there is help in designing high-performance HMIs, both in formal standards and in general rules. Formal standards include: ISA 101 Human Machine Interfaces for Process Automation Systems (DRAFT), NUREG-0700 Rev. 2-2002 Human-System Interface Design Review Guidelines, ISO 9241 Ergonomic requirements for office work with visual display terminals, ISO 11064 Ergonomic design of control centers, EEMUA 201 Process plant control desks utilizing human-computer interfaces: A guide to design, operational and human-computer interface issues, and ASM Consortium Guidelines: Effective Operator Display Design.
Despite these guidelines, there is no standard recipe for designing a good display. There are too many variables, such as knowledge of the operators, local rules for colors and symbols, scope of control, and complexity of the process, to define a prescriptive set of rules that apply in all cases. The general rules are: the design should be functional for up 15 years, it needs to accurately depict the process, it should be simple in design, it must be designed for both normal and abnormal situations, it must differentiate between safety system requirements and process control requirements, and it must allow for supporting information sources, such as CCTV, web cameras, and other visual sensors.
Some of the worst errors in HMI design involve the use of colors and information location. There are very specific rules for the use of color and font size on displays in the standards listed. Background color should be selected to optimize color differentiation and avoid eye strain or eye fatigue, usually resulting in a gray background. Recent studies suggest that the chromatic aberration that occurs with the aging of the human eye happens sooner than thought, and that it tends to make blue and purple tend to gray. This means that blue and purple letters should be avoided. Don’t use only color to indicate an important item; redundant coding of important information with symbols or icons can help address color limitations. If possible, code information with spatial positioning and set grouping patterns. Where color is used for emphasis, it should be employed conservatively and consistently. Once colors are assigned a specific use or meaning, no other color should be used for the same purpose, especially when indicating alarm functionality. The human mind is great at detecting patterns, so color coding, grouping, and symbols should all be used to improve situational awareness and direct users’ attention to the important items and not flashy graphics.
Training creates savings
Designing high-performance HMIs is an acquired skill, which requires continual feedback from usability labs and usability studies. If your HMIs are being designed without general rules, or without usability studies, then you may have interfaces that would be more appropriate for programmers than operators, or worse for websites and not production operations. Train your development staff in the rules of high-performance HMIs and you will be improving operator performance, reducing operational errors, and potentially saving millions per year due to missed critical information.
– Dennis Brandl is president of BR&L Consulting in Cary, N.C., www.brlconsulting.com. His firm focuses on manufacturing IT. Contact him at firstname.lastname@example.org. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, email@example.com.
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