Human machine interfaces (HMIs) are the graphical gateway operators use to control and monitor complex equipment systems in manufacturing and other industrial environments. Modern equipment often arrives at the facility with mediocre HMI graphics. All the functionality is present once you find it, but the interface looks like something from a late 1980s sci-fi movie: non-intuitive and visually unappealing.
The secret to elevating HMI graphics is to get past the automation engineers’ focus on the application of technology and apply a heuristic method to evaluate the usability of the interface by comparing it against a set of established usability principles. Some of these principles should be applied during initial development, while more advanced applications require a bit more planning and are typically included in post-install optimization efforts.
Initial development usability principles are easy to self-diagnose and apply using a simple checklist:
- Recognition over recall: Display information in a way that makes it easy to recognize and understand, rather than requiring a user to recall a set of submenus or context.
- Status visibility: Clearly indicate the current status of the equipment and systems being controlled. Critical process parameters such as temperature, pressure, or time should not be buried or obscured by irrelevant data.
- Mirror the real world: Use terminology, symbols, labels, and relative positions that are familiar to users and match the systems being controlled.
- Consistency and standards: The HMI’s layout, navigation, commands, and feedback should be consistent while complying with industry standards.
- Aesthetic and minimalist design: A clean, simple, and uncluttered design with a minimalist navigation scheme is best.
A minimalist design is the most difficult to self-identify during development, but it is critical to the success of advanced heuristic evaluation applications. Human psychology is susceptible to cognitive bias known as anchoring, where an individual grabs hold of the one piece of information they heard first to the detriment of other information. In design, it often manifests as an object that provides no measurable value but remains because “it has always been there.”
True elevation to elite status occurs when effort, time, and value is invested on advanced heuristic evaluations. The goal is to quantify seemingly subjective metrics, such as usability. Following are the evaluations that lead to exceptional results:
- Cognitive walk through: Conduct a cognitive walkthrough by simulating a user’s mental process while interacting with the interface to identify potential usability issues. If you cut back too far for a minimal design, this is where you’ll catch it.
- Time on task: Measure the time it takes for users to complete tasks with the interface. Compare multiple users’ times to evaluate the efficiency of the layout, drill-down menus, and navigation. Get feedback from the users on items to improve.
- Error rate: Parallel to time-on-task measurements, measure error rate of operator inputs for data, navigation, and recognition. Note where entering large amounts of data may be replaced by scanning a label, a common control requires multiple clicks to access, or the same navigation mistake is being made across users.
- Conduct A/B testing: Leverage the insights gained from time on task and error rate evaluations to modify the interface. Keep the original screens and have your users compare tasks between the original and optimized versions. Compare the metrics between the variations.
These evaluations are focused on the graphical layout for operator control, but the same principles apply to the data systems as well. Visibility into data for process monitoring, maintenance troubleshooting, and quality control are prime candidates for this level of evaluation. While not typically grouped under HMI design, augmenting simple trends with pre-configured pen sets, selecting reports based on batch event frames, or dashboarding critical site-wide data are examples of data visualization that would benefit from this level of evaluation.
While a graphical designer can add to the aesthetics, this process of applying heuristics can be used by anyone to identify areas for improvement of your existing equipment HMIs.
Bill Mueller, is the founder and senior engineer of Lucid Automation and Security, an integrator member of the Control System Integrators Association (CSIA). For more information about Lucid Automation and Security, visit its profile on the Industrial Automation Exchange.
