Human Machine Interface Engineering

Incorporating HMI and user interface in user-friendly and maintainable PLC control systems.

An example of custom-developed HMI screen. (Image: Wayne Schaefer)

An example of a custom-developed HMI screen. (Image: Wayne Schaefer)

The base design of the modern Programmable Logic Controller (PLC) in automation has been around since at least 1968 when General Motors designed a specification for a ‘Standard Machine Controller’. This lead innovation and the eventual creation of the modular digital controller, better known as the Modicon 084. Incorporation of ‘ladder logic’ was significant, as it quickly became the accepted standard industrial machine programming language.  Regardless of other languages being adopted over the years, ladder logic has always been a favorite of controls engineers and technicians. Although the automation industry has seen many technological advances over the past few decades, however, the PLC continues to be the backbone for these systems.

When PLCs were first introduced, the interface between the operator and the machine primarily consisted of lights for status indication and pushbuttons or switches to convey commands from the operator to the machine. By the 1990’s, the first the Human-Machine Interfaces (HMI) started to garner some interest. The eventual inclusion of the HMI brought the importance of an intuitive user interface to the forefront of effective machine operation and maintenance. Unfortunately, good HMI design largely depends on the personal preference of the programmer. The first draft of an HMI specification was not released until 2015 with ISA’s first human-machine interface standard.

Symbiotic Relationship: PLCs and HMIs

A PLC is the brain of an automation system. The PLC program is tailored to make real-time decisions based on inputs and then executing corresponding actions. Switches and other various sensors allow the PLC to determine machine status and monitor process variables. Outputs from the PLC allow actions to be taken based on a set of rules corresponding to the specific sequence. Due to the physical nature of processes, the monitoring and controls of machine states and actions can be defined and predictable.  An HMI, on the other hand, is the part of this system and that must communicate with the human element and is therefore less well defined. The HMI offers a visual representation of a process and allows human operators to interact, oversee and intervene when necessary.

Modular and Hierarchical Design for HMIs

From a technical perspective, an HMI application’s effectiveness lies in its ability to present the vast amounts of available data in a meaningful manner . By adopting a modular and hierarchical design, developers segregate processed data into “chunks,” allowing users to start from a general overview of the system and then drill down to reveal detailed process information and the current machine state. However, just creating a user interface with a logical arrangement of the necessary inputs and outputs may not be the best solution. Designs, although technically correct, could overburden the operator with a cluttered display, incorrect or overuse of graphics and colors or even non-intuitive placement of buttons and indicators.

Humans Elements: Operators do not Make Decisions Based on Logic

One mistake designers make when creating interfaces is not integrating insights on emotional decision-making into HMI design. Studies have demonstrated even the most logical decisions can end up being emotional.  Creating an intuitive, user-centric system that aligns with natural human processes can have a positive influence on training, operations, useability and operator retention. For example, by understanding that humans naturally relate to certain shapes, colors and movements, HMI developers can correctly use emotionally expressive interface elements.

Rounded shapes, for instance, are often perceived as more friendly and safe, while sharp, angular shapes might indicate caution or danger. Colors also have strong relationships with emotional triggers. Color-coding and logical layout design play pivotal roles in HMI effectiveness. By assigning specific colors to certain states (e.g., red for emergencies), developers can ensure instant operator recognition. Moreover, a clutter-free, logically segmented layout ensures optimal performance.

The Language of Symbols: Adopting Universally Recognized Icons

Consistency in symbology across the HMI can drastically reduce the learning curve for operators. By using universally recognized icons and symbols, operators can intuitively understand processes, reducing training time and potential errors.

Solution: Interactive Diagnostic Tools and Feedback

Feedback is central to any control system. While PLCs manage feedback at a process level, HMIs do so at a human level. By visually confirming user commands and displaying the real-time system states, HMIs can reinforce operator confidence and ensure command continuity. Remember that the HMI does not have to be just a passive display tool. Modern HMIs come equipped with interactive diagnostic utilities. For example, when an alarm is triggered in the PLC, the HMI can guide the operator through a troubleshooting process, even suggesting potential remedies. This can be done within a PLC subroutine or even by running scripts on the HMI itself.

Solution: Customizable User Profiles on HMI

Not all operators require access to all system functionalities. Modern HMIs allow for customizable user profiles depending on who is logged in thus ensuring that operators only access the features relevant to their roles. This not only enhances security but also declutters the UI. This type of design can reduce the possibility of an operator either getting confused after discovering a screen designed for maintenance personal or an operator accessing maintenance functions they have not been trained to use.

Solution: Integrating Voice Commands and Augmented Reality

The real future of HMI lies in integrating cutting-edge Artificial Intelligence (AI) technologies. Voice commands allow hands-free operation, while Augmented Reality (AR) overlays crucial data on real-world machinery, offering an immersive troubleshooting experience. Leveraging the AI component can allow the user to have a conversation with the interface no different then asking a human “what’s wrong with the machine?” AI can also direct the operator to the next step in the sequence or even detect if the operator tries to invoke a command out of the normal order. This type of design is already being tested for in-cabin automobile systems where the driver no longer has to look at or touch the interface.

User Acceptance Testing: Simulating Real-World Scenarios

User feedback is the cornerstone for iterative HMI development. By regularly gathering end user feedback, developers can continually refine the interface, ensuring it aligns with evolving needs. During development, HMI operation can be tested and refined by using simulated industrial environments. This pre-emptive measure can help in identifying potential UI issues or data representation inaccuracies. In post development, HMIs should be rigorously field tested with each of the stakeholders.

Documentation and Annotations

While PLC programs have comments embedded in code, HMIs offer hover tooltips, help sections, and interactive tutorials. Such features make the onboarding process smoother and serve as quick references during operations.

Seamless System Integration 

The union of PLCs and HMIs is one of the pinnacles of modern industrial automation. While PLCs drive decision-making processes, HMIs make these processes transparent, interactive, and intuitive for human operators. By holistically designing PLC programs with their HMI counterparts in mind, developers can ensure seamless system integration, ease of diagnosis, and optimal maintainability. As technology continues to evolve, the boundaries between human operators and machines will blur, with HMIs acting as the bridge to this symbiotic future.