There are many ways to develop human machine interface (HMI) screens for machine automation and related applications, but effective implementation requires discipline in design. The look, feel and ease-of-use of an HMI can vary widely with all the tools, object libraries, animation and colors available with modern HMI software—but there are some HMI best practices and guidelines to improve HMI effectiveness. Read on to learn some HMI best practices.
Guidelines, standards and handbooks covering HMI best practices and design include those published by ISA, ASM, ISO, NUREG and others. All of these standards 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 used as 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 project to the next.
Story Boards are a Good Start
A good starting point for HMI design is a text-based outline documenting each screen’s content. With the operator’s point-of-view and ease-of-use in mind, detail the main screen, equipment status screens, set point or recipe screens, manual functions, message displays and fault displays. These text outlines can then be converted to a story board for each screen.
The story board should highlight dynamic graphics such as status indicators. It should also include repeated graphics, such as titles, at the top of the screen and navigation buttons at the edges of the screen. The screen buttons, indicators and numeric displays should be aligned and grouped as appropriate. For example, don’t mix screen selector buttons with start/stop pushbuttons.
Ask the Operator
Story boards are a good way to document the HMI style, graphics, user requirements and functional requirements for an early review of the basic HMI design. Comments from operators and other plant personnel during this review will help refine the HMI design and ensure an effective HMI.
While creating the story boards and subsequent HMI design, ask yourself what’s important to the operators. More data is not always better, so don’t overwhelm the operator with information. Instead, focus on the operators’ tasks and what is needed to understand the state of the machine or process. You don’t need to display every analog value, but instead you should present the information in a manner that allows equipment status to be understood at a glance.
Animation for the Sake of Animating
With the requirements defined, use caution when developing the graphics, as this is not the opportunity to express your inner Picasso. Some of the more artistically talented HMI designers enjoy using all the colors and graphic animations available in the HMI software. Showing the 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.
For example, using animation to show the position of a box on a conveyor is an efficient and quick indication of conveyor status. However, showing a pump motor rotating could be a distraction if the intent of the screen is to show high level fault indications. Only animate if it makes the operator more efficient.
Color is Not Always the Answer
Many of the HMI guidelines recommend limiting the use of color and using low contrast gray backgrounds. This makes for a much less cluttered screen. Consider a light gray screen background where a typical indicator would be dark gray in the OFF state and white in the ON state. This is easy on the eyes, and makes sense as a light bulb turns white when on.
Data display is a key component of HMI design, and different data will require different display types. A number on a screen may accurately display speed, but the engineering units may be in question and the acceptable range unknown. This can be solved by adding the units, inches per second for example, and the maximum and minimum values in a table fashion.
A Trend Graph Can be Useful
A line trend graph can display past and present data, and is a good indicator of future values as an operator can quickly see the values trending towards an upper or lower limit. In this case, displaying the actual numeric value is not necessary as the data is displayed graphically.
There’s a frustrated artist residing in many of us, but HMI screen design isn’t the time or place to express these urges. Instead, develop and follow internally developed HMI design standards to create screens consistent from one project to the next that will work well for operators. Your customers will thank you, and you can always take an art class to release your inner artist.