HMI screens that help at three in the morning
The design question is not how it looks on the handover day but whether it helps somebody tired and under pressure.
Short answer
Design for the abnormal case, not the demonstration. High-performance HMI practice uses a grey background with colour reserved for abnormal conditions, so anything coloured means something. An overview screen should let an operator see whether the plant is healthy in one glance, with detail one level down.
The screens that get praised at handover and the screens that help during an upset are usually not the same screens.
Colour is a scarce resource
High-performance HMI practice puts everything normal in grey and reserves colour for the abnormal. It looks austere and it works, because a coloured thing on a grey screen is visible from across a room.
The opposite, a mimic in full plant colours, means an alarm has to compete with a green pipe and a blue tank for attention.
Layers, not one big screen
- Overview. Is the plant healthy? One glance, no reading.
- Unit. What is this section doing?
- Detail. Individual values, trends, and controls.
Most operators live on the overview and descend only when something needs attention.
Trends beat numbers
A number tells you where a value is. A trend tells you where it is going, which is what an operator actually needs to decide anything.
A tank level shown as 62 percent is less useful than the same value shown falling steadily for ten minutes.
Design for the tired
The person using this at four in the morning has been woken up. Everything that requires interpretation is a place where a mistake will happen.
Common questions
- Why do modern HMI screens use grey backgrounds?
- So that colour carries meaning. If the normal state is grey and colour appears only for abnormal conditions, an operator can spot a problem without reading anything. On a screen where everything is already coloured, an alarm colour is just one more thing competing for attention.
- What belongs on an HMI overview screen?
- Enough to answer 'is the plant healthy' in one glance, and nothing else. Key process values, equipment states, and active alarms. Detail belongs one navigation level down, because an overview crowded with detail is an overview nobody can read quickly.
Keep reading
- Safety
SIL or PL: which one does your machine need?
Two standards, two scales, and one machine. Which one applies, how they map to each other, and why the answer is usually ISO 13849.
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Categories B, 1, 2, 3 and 4, in plain terms
Five architectures, what a single fault does to each, and the practical wiring that goes with them.
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Safety relay or safety PLC: how to decide
One is a wiring decision, the other is a programming one. The count of safety functions, not the size of the machine, is what settles it.