Not to be confused with user interfaces, human machine interfaces (HMI) are complex control systems that allow a human operator to control, manage and monitor the processes of a machine or system. Manufacturing companies, for instance, often use HMIs to control the operations of robotic assembly lines. A typical HMI features data regarding the system’s output as well as controls that allow a human operator to control the system.
There are dozens of different types of HMIs. Some HMIs are basic, consisting of nothing more than an electronic display and dials. Others, however, are more complex, featuring fully functional touchscreens, membrane keypads and more. The complexity of an HMI varies depending on the task for which it’s designed. A touchscreen interface, for instance, may not be needed on an HMI used to monitor a basic system.
Regardless of complexity, though, most HMIs are usually designed in three stages: interaction specification, software specification and prototyping. According to Wikipedia, common practices used in interaction specification include user-centered designs, persona, activity-oriented designs, resiliency designs and scenario-based designs. Practices for software specification designs include constrain enforcement and use cases. And practices for prototyping involves interactive designs using interface elements.
Of course, there are other things that manufacturers must consider when designing an HMI, including the environmental conditions in which it will be used. If the HMI is used in a particularly dusty or wet environment, for instance, it should be designed to resist or prevent the intrusion of foreign objects. Referring to the Ingress Protection (IP) rating system can help you choose an HMI that meets this criteria.
Some HMI also require the use of backlights. If the backlight is too dim, users may not be able to see the controls or readouts. On the other hand, if the backlight consumes too much energy, it may burn out and require replacing more frequently. Light-emitting diode (LED) backlights are a popular choice because of their ability to create ample illumination while consuming minimal power.
Responsiveness is another key metric used when designing an effective HMI. The HMI should offer a fast and fluid response to input. If the user touches an icon on a touchscreen panel, for instance, the HMI should respond by immediately opening the app or program. HMI with long delays are considered unresponsive, and because of their negative impact on usability, should be avoided.