You built the HMI with the right buttons in the right places, tested it with the engineering team, and the operators still press the wrong one within a week. The problem isn’t the operators. It’s that the screen was designed by people who already understand the machine.
The expert blind spot
When you know the process, a screen with 16 buttons on one page looks fine. You can find the jog button in two seconds. An operator who sees the machine once a shift cannot. They scan the screen for familiar shapes, not for labels. If two buttons look the same and sit next to each other, they will press the wrong one under time pressure.
The fix isn’t bigger fonts. It’s grouping by what the operator does, not by what the engineer thinks is neat. A normal startup needs four buttons in sequence. Put those four in a row on one side, large and color-coded. Put the parameters, diagnostics, and settings on a second page that requires a deliberate navigation. Don’t put them on the same screen as the run and stop buttons.
Red is for stop, not decoration
When every alarm and every caution and every warning is red, red stops meaning anything. The operator starts ignoring the whole column. Reserve red for one thing: e-stop and critical faults. Use amber for warnings that need attention but don’t stop the machine. Use green for running status. If you find yourself using five colors on one screen, collapse them.
The same goes for button size. The start button should be the largest active control on the screen. The stop button should be larger still, and physically separated from the start button by a gap, not adjacent. If start and stop sit next to each other in the same row, someone will hit the wrong one with a gloved hand.
Status text that tells you what to do
“System fault” is not a useful HMI message. The operator reads it and thinks: what do I do now? A useful message says what happened and what the next step is: “Lubrication pressure low. Check pump filter at location 3B.” The operator still has to go look, but they know where to walk and what to check.
Don’t hide the cause in a sub-screen. The alarm list can have the long description. The main status bar should show the short version, in plain language, not a fault code. Fault codes are for maintenance. Operators need sentences.
The 2 AM test
Design the screen for the operator at 2 AM, half awake, on their third shift. Can they tell at a glance whether the machine is running, stopped, or faulted? Is the stop button reachable without scrolling? If the power drops and comes back, does the screen come back to the same page, or does it jump to a diagnostics screen nobody expects?
Walk the floor with the screen. Watch an operator who has never seen it find the start button. If they hesitate for more than two seconds, the layout is wrong. You already know where everything is. You can’t see it anymore. That’s why you need someone who doesn’t.
Bottom line
An HMI is not a data dump. It’s a control panel. If the screen shows more information than the operator needs at that moment, you’ve added noise, not usefulness. Simplify the main page, move detail to sub-screens, and reserve the loud colors for the things that actually stop the machine.
