The science of supporting people within the work context

Optimizing the relationship between humans and machines ensures optimal safety and performance in a mission-critical, 24/7 environment.

Human machine interface (HMI) design is an area of human factors engineering (HFE) that focuses on optimizing the control system interface to ensure safe and efficient human performance. By analyzing the instruments and equipment people use to do their tasks—from software screens to control panels—HFE uses a scientific approach to ensuring the best possible interface between a human and the system.

BAW Architecture has pioneered HFE principles in control rooms for decades, incorporating best practices in control room design long before ISO 11064 codified them. Our HFE experience includes 10 years at the ASM® Consortium, and our trailblazing design principles eventually transformed control room design practices industry-wide. At the foundation of our operator-centric designs is a deep and nuanced understanding of the Human Machine Interface, using HFE specialist input.

People arrive at any work scene with a range of capabilities and limitations. It is these people who actually make a system work, often making up for deficiencies in design. They use specific tools and equipment to perform tasks, within the backdrop of the workspace, environmental and organizational context. An HMI analysis drills down into the requirements for these specific equipment items taking account of the context of use and different operational scenarios that the operators may face. The HMI analysis, alongside established design criteria, now embodied within comprehensive standards, is used to define the end user requirements for design. It includes identifying and controlling influences that can negatively impact the operators, such as noise, vibration, thermal variations, lighting, and proximity issues. It is an iterative design process which  requires structured end user involvement and user testing. It means engineering the interface to support the interaction between the person and the machine in every possible facet.


The development of HFE and HMI as formal disciplines began during World War II. The military was confounded at the less-than-expected human performance. They employed experimental psychologists to evaluate the cockpit layout, control panels, radar, and other equipment. These scientists determined that the design of the machine interface, including instrument panels, gears, levers and steering apparatus, did not accommodate the physical and cognitive capabilities and limitations of the operators. The team of scientists recommended console and equipment layout improvements, which were implemented by the military. With essentially the same equipment, but simply reorganized to complement the realities of the human pilot, the recommended solutions resulted in dramatically decreasing pilot error and attack strikes. Known as the “cockpit studies,” the HMI principles discovered during the war have been refined over the decades, within many different applications and industries, and are recognized as essential to equipment and software design.


We receive information through our senses, primarily visual and auditory (although tactile and olfactory play a part as well). We process that information, make decisions, then perform tasks based on that input, such as pressing a button, at which point the machine activity is modified. The result of the machine’s activity is displayed back to us again through our senses (such as through the control screen or alarm speaker). Trying to match the machine interface to how humans need to control and assimilate information is fundamental to effective HMI design.

Displays and controls need to meet detailed and specific criteria, such as the size, shape, spacing, positioning, color, and texture of controls. In terms of visual displays, there are also criteria related to information grouping, positioning, size, color, contrast with many more levels of detail. Auditory information also has criteria, which is discussed later. All of these criteria can greatly impact safety and efficiency, or the lack thereof. They are based on years of scientific research to ensure that the HMI is compatible with the way the human brain works. For example, the illustration below does not simply show 10 parallel lines. Our brains recognize the pattern of five groups of two lines each.

This is known as the principle of proximity such that if things are close to each other, we expect them to be functionally linked. This principle can be used to good effect in designing the panel, so that we position elements according to function and create separation between elements which are not linked. Effectively, this gives the brain a helping hand. It is, of course, much more complex than this as there are several principles that can be applied and sometimes need to be traded-off.          

HMI-influenced design seeks to be compatible with how our brain works, to support the tasks we are doing.


Auditory information is also very important. The intensity and frequency of sounds can be manipulated to optimally transmit information, and different sound patterns must be employed for different alarms to indicate different messages. Indeed, a human’s capacity to process auditory information is actually limited to five different sounds at any given time. There are real-world and sometime tragic implications when critical HMI information is overlooked, such as the terrible events that transpired at Three Mile Island. Within the first minute of the nuclear plant going into an emergency situation, no less than 500 alarms sounded; by the second minute, 800 alarms were sounding. Had a few very specific alarms sounded that indicated to operators the true nature of the problem, the most significant accident in U.S. commercial nuclear power plant history may have been averted.

Tactile information is also examined in an HMI design. The shape, texture and position of certain control elements provides cues to function. For example,  when sitting in a car you expect the steering wheel to be in front of you and the stick shift to be on the lower right. They have specific shapes, textures and sizes, all of which combine to indicate and confirm the function of a particular control element. HMI tries to play to the established norms that already exist in our daily lives.


Before the advent of computers, an operator walked around a plant reading dials, pulling levers, rotating valves and pushing buttons to keep processes in control. The advent of digital control systems and software made everything more flexible, but also more complex. A keyboard and mouse act as multi-purpose controls at the fingertips of an operator rather than multiple sizes and shapes of different types of control. The presentation and architecture of the information on display screens is crucial to the success of the operator to interpret and act on information on multiple screens. Four levels of screen hierarchy was established through the ASM® Consortium, namely plant overview (level one); main processes (level two); equipment (level three); and detailed data (level four). There are detailed and specific criteria in many different HMI standards which cover topics as wide reaching as error recover, navigation, information grouping and sequencing, as well as specific criteria such as screen colors and text size. So HMI considerations extend all the way through software, because the way people react, behave and think all need to be taken into account to optimize the interaction. It’s crucial that HMI factors are taken into account when software engineers are designing, so that they avoid designs that can lead to error.

Human factors engineering in general and Human Machine Interface in particular is an investment in safety and operator optimization that deserve particular attention in mission-critical industries. The highest level of attention to HFE best practice is a standard feature in all of our control building and control room designs. Contact BAW for more information.

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