Out with the Bad, In with the Good
EP Editorial Staff | March 26, 2019
In which category do your plant’s human-machine-interface layouts fall?
By Bill Dehner, AutomationDirect
Human beings are visual creatures, and most of us would choose a pretty picture over boring text any day. Popular image-based apps, such as Instagram and Snapchat, are a good example.
On the scientific side, it’s believed that our brains can process visual information 60,000 times faster than text, with 90% of information transmitted to the brain being visual, and more than half of the brain being devoted directly or indirectly to vision. Knowing that, one can conclude that using a visual representation of a machine or automated system would allow faster recognition and understanding of system variables. That’s where well-designed HMIs (human machine interfaces), such as those shown on this page, pay off.
An HMI allows operators to “interface” with automated equipment and processes they oversee by providing a visual overview of a system’s status and direct control of its operation. An HMI’s graphical screens can be programmed to display important status and control information to the operator. Pictures, icons, sounds, and colors can all be used by HMIs to visually represent different operating conditions. Many HMIs deploy touch-screen technology for user interaction with elements displayed on the screen.
HMIs have become vital control components that offer many advantages over old-school mechanical pilot devices. For one, instead of wiring numerous signal wires from a controller to indicator lights, wiring an HMI into a control system with a PLC is simply a matter of connecting one communications cable, such as serial or Ethernet, among others, between the HMI and PLC.
What about system modifications? For example, what would it take in terms of wiring and mounting to install three new pushbuttons on a control panel? With an HMI, it’s all in the program.
Adding or removing objects, such as buttons, on a screen takes no time at all and doesn’t cost anything extra.
In fact, most HMIs have object libraries that consist of numerous buttons, switches, dials, and other graphic items that can be easily inserted into position on the HMI screen—usually with a simple drag-and-drop motion. The many graphical objects in one small HMI could replace hundreds of mechanical control devices around a plant.
Although their benefits are numerous, the advantages that HMIs offer over simple pushbuttons and indicator lights can be nullified if their screens aren’t developed to best serve the operator.
The sidebars below and on the next page (p.27) offer specific examples of poorly and well-designed HMI layouts, as well as some of the reasons those screens fall into the “bad” and “good” categories.
BAD HMI DESIGN
• Don’t overwhelm operators with too much data. Focus only on what’s needed for them to understand the state of the machine or process.
• Don’t let layout and design factors detract from situational awareness. Data overload and focus on the wrong information are two of the most common culprits.
• Use caution when developing graphics. Limit colors and animated features. It’s not necessary to use all color and animations available in the software.
• Remember that, although spinning pumps, valves that open, and fluids flowing through pipes may be cool, such features can be distracting. Only use them if they make the operator more efficient.
GOOD HMI DESIGN
• Use color judiciously, with subdued colors or gray shades and positioning to group “like” items. Use bright, saturated colors only to indicate abnormal conditions.
• Be prudent when adding graphics. Since excessively large graphics and animation effects can slow screen performance, use them appropriately or isolate them on alternate screens to ensure fast response on primary screens.
• Display appropriate images. Resist, however, the temptation to model an entire P&ID piping diagram of the plant (or other process). The goal is to provide an intuitive display of relevant information.
• Keep important controls, i.e., Stop, Start, and Setpoints, among other things, available at all times. Reserve part of the screen for such items, and make sure that area is completely consistent everywhere it is displayed.
• Offer situational awareness by making sure relevant data is displayed clearly, i.e., so that operators can grasp the current state of the machine or process at a glance (or as quickly as is practical).
• Keep all information and controls within two to three clicks of the Home, or Main, screen. Consider the application’s workflow and optimize the layout for efficiency. Keep screen menus and/or change actions consistent, and ensure a clear and consistent “back path.”
• Use “pop-up” alarms or dialog boxes sparingly, if at all. Nothing’s worse than a cascade of error messages that have to be acknowledged individually before an operator can get to a screen where the condition can actually be resolved. EP
For more information on important HMI design principles, visit automationdirect.com, or click on these links:
Bill Dehner is an engineer with AutomationDirect, based in Cumming, GA. Visit automationdirect.com.