There are many ways to develop human-machine interface (HMI) screens for machine and process automation applications. Effective implementation requires discipline in design. The look, feel, and ease of use of an HMI can vary widely given all the tools, object libraries, animations, and colors available with modern HMI software—but there are guidelines to improve HMI effectiveness.
Guidelines, standards, and handbooks covering HMI design include those published by ISA, ASM, ISO, and NUREG. All these resources discuss a wide range of design, build, operation, and maintenance methods for effective HMIs. Many also discuss safety, quality, reliability, and efficient control of the equipment or process under normal and abnormal conditions.
These standards can be a basis to create internal company HMI design guidelines, which in turn can be used to create consistent and effective HMI screens from one machine or process to the next. Standards are important, and users should consult them before final implementation of an HMI design, but they can be difficult to read and interpret. The tips below are provided as design aids.
A good starting point for HMI design is a text-based outline documenting each screen’s proposed content. With the operator’s point of view and ease of use in mind, the designer creates storyboards to flesh out the details of the main screen, equipment status screens, set point or recipe screens, manual functions, message displays, fault displays, and other ancillary screens. The designer can then convert these text outlines to a storyboard for each screen. The storyboards should highlight dynamic graphics, such as status indicators, and should include repeated graphics such as titles at the top of the screen, along with navigation buttons at the edges of the screen. Align and group screen buttons, indicators, and numeric displays as appropriate.
While creating the storyboards, ask yourself what is important to the operators, or better yet, ask the operators if they are available for consultation. Having more data on the screen is not always an improvement, so do not overwhelm the operators with information. Instead, focus on their tasks and what is needed to understand the state of the machine or process. You do not need to display every analog value, but screens should instead present information so that overall operating status can be understood at a glance.
Have an experienced operator review the proposed storyboards. Many machines and process skids are built by original equipment manufacturers (OEMs) and sold to customers. If this applies to your situation, now is the time to engage your customers and their operators in the HMI design to get their input and incorporate their specific requirements.
Deploy a prototype of the HMI as early as practical in the design cycle, and watch how the operator uses the interface. If it is not possible to develop and test a prototype, use the storyboards for a mock-operation session with the operators.
Look for awkward situations where an operator may be struggling to interpret what he or she is seeing, and look at the number of actions necessary to carry out a common task. Are there additional features you can add to relieve operators of needless button presses or otherwise make them more efficient?
Ask the operators for their advice and opinions, but filter their comments. They may not always be tuned into the big picture. At the same time, realize that your detailed knowledge of the machine or process may cloud your perspective of what is actually required to operate the machine efficiently. Never forget that you are building the HMI for their use, so their input is very valuable.
Many HMI guidelines recommend limiting the use of color and using low-contrast gray backgrounds to create less cluttered screens. For example, a light gray screen background where a typical indicator would be dark gray in the off state and white in the on state is easy on the eyes, and makes sense intuitively, as a light bulb turns white when on. In general, use subdued colors or shades of gray, and reserve bright, saturated colors to indicate abnormal conditions.
Be cautious when developing graphics, as this is not the opportunity to express your inner Picasso. Some more artistically talented HMI designers enjoy using all the colors and graphic animations available in the software, but this should be avoided. Showing pumps and fans spinning, valves opening and closing, and fluid moving in the pipes is cool—but may be excessive and can distract the operator. Only animate if it makes the operator more efficient.
For example, using animation to show the position of a part on a transfer line might be an efficient and quick indication of production status. However, showing a pump motor rotating is a distraction if the intent of the screen is to show only high-level fault indications.
Source: ISA News