How to design a good HMI screen
A good HMI lets an operator see in two seconds whether the machine is fine, and what to do when it is not. A bad one is a colourful picture that hides the one alarm that matters. These rules come from the ISA-101 high-performance HMI guidelines and years of control-room experience.
1. Calm screens, colour only for problems
Use a grey or neutral background and grey equipment. Save strong colours for abnormal states: red for alarms, yellow/amber for warnings. If everything is bright green and red all the time, operators stop noticing colour, and a real alarm gets lost. Running/stopped can be shown with fill and text instead of green/red.
2. Consistent meaning everywhere
- The same colour, symbol and position always means the same thing on every screen.
- Do not use colour alone: add text or a shape (many people are red-green colour blind).
- Show units with every number (bar, °C, %, rpm) and sensible decimals.
3. A clear hierarchy of screens
| Level | Shows |
|---|---|
| 1. Overview | the whole line on one screen: status, key values, alarms |
| 2. Machine / area | one machine with its main controls |
| 3. Detail | one unit: all sensors, motors, valves, setpoints |
| 4. Diagnostics | I/O status, drive parameters, PLC diagnostics for maintenance |
The same navigation bar on every screen, and never more than about three touches to reach anything.
4. Alarms that help
- Every alarm needs a clear text that says what happened and what to do: "Tank 1 level HIGH: stop filling, check the level switch".
- Give alarms priorities and show the most important first.
- Avoid alarm floods: one root cause should not trigger 30 alarms. Suppress consequential alarms.
- Alarms must be acknowledged, and the history kept for fault finding (ISA-18.2).
5. Safe operator input
- Set min / max limits on every numeric input.
- Ask for confirmation before critical actions (start, delete recipe), and protect them with user levels.
- Show the actual value next to the setpoint.
- Show feedback: a "start" button that does nothing visible confuses operators. Show "Starting..." until the PLC confirms.
6. Made for touch and the shop floor
- Touch targets at least 10-15 mm, with space between them; operators may wear gloves.
- Large, readable fonts; the screen may be read from two metres away.
- No important buttons in corners where they can be pressed by accident.
- Test the screens with the people who will use them.
Checklist
- Can an operator tell in 2 seconds if the machine is running normally?
- Is every number labelled with a unit?
- Does every alarm say what to do?
- Are all inputs limited and protected?
- Is the E-stop a real button on the machine, not on the HMI?
Start with what an HMI is, then try designing a screen with the HMI panel in the PLC Sim Lab Playground.
Try it yourself. Wire sensors, contactors and a PLC, write ladder logic and press RUN in the free PLC Sim Lab Playground. Nothing to install.
Open the Playground →Guided lessons →