Developing high-performance HMIs: Evolution, improved usability
Inside Process: This two-part series examines the development of high-performance human-machine interface (HMI) methodology. Part 1 covers HMI evolution, security, improving usability, and consistent use of color.
In the past few years, we have witnessed an evolution in interface design, from terminals controlled by command prompts, to eye-movement-controlled interfaces, where learning is almost instantaneous. This evolution is the result of numerous studies and researches in many different fields of knowledge. But how can industry benefit from this breakthrough?
Many industrial interfaces are developed under certain short lead-time and constrained budget scenarios, which are outside influences that affect how interfaces are designed. This makes development driven by effectiveness, efficiency, and user satisfaction much more difficult.
High-performance human-machine interface (HMI) methodology is posed to bridge this gap by combining state-of-the-art interface design with aviation industry experience. The most visible features of such interfaces include (see Figure 1):
- Optimized colors
- Highlighted alarm signals
- Minimalistic-drawn objects
- Extensive use of charts and bar graphs
- Values displayed within a context
- Grid-based screens
- Efficient use of space.
The advantages of this approach extend far beyond aesthetics. They include:
- Fault prevention
- Reduction of operational errors
- Increased operational efficiency
- Greater ease of learning
- Improved information quality
- Lower cognitive-processing cost.
Tests that compare high visual performance HMIs to traditional HMIs have been conducted, and the results are so remarkable that they cannot be ignored (see Figure 2). However, when we consider that this methodology can be applied gradually—or even to just a few parts of an already existing system instead of starting from scratch—the cost-benefit relationship becomes even more attractive.
Focus on security
A safe industrial interface must have good usability. When developing an industrial interface, users must be considered first and the process second. HMI usability must be good in normal and critical situations. Therefore, it must be developed with users in mind.
ISO 9241-11-1998: Ergonomic requirements for office work with visual display terminals (VDTs) defines usability as a combination of effectiveness, efficiency, and user satisfaction in a system. This same standard indicates usability measurements:
- Easy to learn
- Easy to memorize
- Low error rate.
Many accidents happen due to human errors. However, if an interface is properly designed, it can help avoid many tragedies. This is done by enhancing the situational awareness of an operator. This way, an HMI may effectively contribute to an increase in security and safety. "Situational awareness" means being fully conscious of our surroundings. It is the perfect tuning between a perceived situation and a real situation. Becoming situationally aware involves perception, comprehension, and projection (see Figure 3).
First, we perceive a fact. Next, we must comprehend it in its context; that is, whether that situation is normal or not, or whether it is near an abnormality or not. Then, we must project the consequences of that perceived fact in its context to decide which action to take and its urgency. How do we implement these three aspects in an interface?
Raw data gives us perception about a value. However, it does not show us if this value is inside a normality or on the brink of an alarm. It does not offer a context.
Comprehension can be achieved by indicating alarm limits to make the range of operation in normal conditions of a monitored variable clear to an operator. This indication of context can be displayed in several objects, such as graphs, radar charts, or bar graphs. However, these limits still do not allow a projection of the consequences of a monitored behavior; they do not point to a variable’s trend through time.
A projection can be achieved in a number of distinct ways. Maybe the most intuitive way is by using a chart, which can estimate the behavior of a variable through time (see Figure 4).
An industrial HMI’s main purpose is to allow users to interact with a process. This includes efficiently alerting an operator when something is wrong on a plant. To accomplish this task efficiently, an interface must alert the operator about what is effectively important. It must not catch user’s attention to something irrelevant while signaling a failure, for example. Using pre-attentive design techniques may increase this efficiency. But what is this?
Our brain processes and stores visual information received from our eyes. Researchers have created a model to simulate this functionality. This model contains a memory called iconic or sensorial. During this step, there is a very fast processing of the received visual information (see Figure 5). This is when we perceive something, even if it is a glimpse, or when something catches our attention.
Next, information is transferred to our short-term memory (STM), where it can either be discarded or stored in our long-term memory. STM is limited and temporary. Information does not last very long in this type of memory. If we overload an operator with unnecessary visual information, STM is saturated, and it must discard old information so that new information can be stored. For example, information is lost, and the discarded information could be essential to solve an emergency. In an industrial process, this is not recommended.
This is the reason we must always avoid screens with too many colors and objects. This prevents the efficiency of an interface’s informative function, thus diminishing its usability. Therefore, when we do not need to know the exact value for each point, it is preferable to represent similar or correlated analog values in a graphical way (see Figure 6). Another object that offers a good view on correlated variables or even key performance indicators is a radar-type chart (see Figure 7).
In this chart, the continuous line indicates the status of the process, at every instant. The dotted line indicates the ideal situation. Thus, we can compare how close to the ideal situation each variable on the chart is at every instant.
Consistent use of color
The most important data must always stand out from the less important data. To do so, we must use muted colors for screens and avoid color gradients. To indicate critical and relevant statuses in a process, use colors that grab the attention of operators (see Figure 8).
Objects rendered in a very realistic way may present two types of problems. The first problem is using bold colors; the second problem is using too many pixels for decorative purposes. Therefore, instead of using a representation such as the one shown in Figure 9, opt for a simpler, more efficient display as the one shown in Figure 10. Note that in the representation in Figure 10, there are no decorative pixels or color gradients. The goal is to value the use of data pixels and drastically reduce the use of decorative, or nondate, pixels.
Another issue to avoid is the use of color gradients on screen backgrounds because context affects our color perception. In the example shown in Figure 11, both rectangles have the same color, but we perceive one to be lighter than the other one due to background color variation.
Part 2 will focus on accuracy versus context, accessibility, visual hierarchy, animation versus static indication, and 2D versus 3D screens.
Helcker Goetz is an analyst and designer for Elipse Software, where he develops HMI user improvement solutions, discovers new resources, and conducts research. He has worked for 22 years as a developer, graphic designer, and interface designer and has dedicated 13 years to the development of applications and interfaces for the industrial automation market. Edited by Jack Smith, content manager, CFE Media, Control Engineering, email@example.com.
- Using charts and graphs can enable operators to estimate the behavior of a variable through time.
- Analog values are better understood when presented graphically.
- Use colors that grab the attention of operators to indicate critical and relevant statuses in a process.
How effective are the HMIs in your plant?
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