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What makes a good HMI?

One of the many major benefits of automation is the elimination of human error through pre-programmed repeated sequences, safety interlocks, and process control. However, for the moment, most automated systems still rely on an operator to tell the system how much, how fast, and what type of product should be made. This human element component of process control introduces the potential for error and failure, most often due to a breakdown in communication between human and machine. Therefore, it is critical that the human-machine interface (HMI) between operator and equipment be intuitive, informative, and practical in order to improve plant safety, increase production and profit, and decrease operator reaction time.

Automation provides many advantages including increased productivity, improved predictability of quality, and reduced long-term costs and expenses. The plant operator is responsible for maximizing these advantages by operating securely, safely, and efficiently, and the HMI is the tool they use to accomplish this. Good HMI screens excel at turning data into meaningful information and drawing attention to the most important information first. Giving the operator easy access to critical information allows them to quickly and accurately anticipate and react to changing plant conditions. A well-optimized HMI will increase the safety, production, and profit of your operation.

However, at some point we have all encountered unhelpful, frustrating, or perhaps infuriating interfaces. A lot of these bad interfaces might “look good” at a glance, but are too flashy and lacking in information for daily use. Their main problems are clutter, too much detail without meaningful information, lack of context for values, and a focus on aesthetics over usability. Some offenders use animation that distracts the eye from the more important information being presented on the screen, but the worst omit, obscure, or misrepresent data. Sometimes they require the operator to take action in a certain manner, but don’t expressly demonstrate that functionality. Together or separate these elements make up a bad interface, which can lead to operator mistakes, reduced productivity, or reduced response time.

While most problems with bad user interfaces amount to no more than annoyances, the consequences of poor design can be costly, and on rare occasion fatal. In 1988, the crew of the USS Vincennes shot down a civilian airliner, a tragedy at least partially attributed to the poor interface design that caused them to identify it as a hostile inbound aircraft. In 1979, the nuclear meltdown at Three Mile Island was in part related to a valve status light which showed closed when the valve was commanded closed, rather than actual position. And in 1992, Air Inter Flight 148 crashed in France after the descent input screen failed to notify the pilots it was in an alternate mode (feet per minute vs angle) which caused them to descend four times faster than expected. While these issues are the extreme case, they illustrate the critical importance of good HMI design.

So what makes a good HMI? According to one whitepaper, a well-designed HMI is one that puts data in context, makes tasks easy for the operator, and uses an easily navigated information hierarchy; and when we start looking across the industry, we see these same principles applied. Well-optimized HMIs make use of analog displays to give the operator indication of current state versus their desired state. They use trends to show where conditions have been, are now, and will be. And they use emphasis such as color, size, and position to make important information stand out.

At NorthWind, we apply these same design principles to our HMIs. The focus is on providing meaningful process state information to the operator in an intuitive and informative way in order to improve their effectiveness. The goal is to make the interface between operator and process as seamless as possible to promote safe, reliable, and optimized production.