The Touchscreen That Was Too Small for the Operator: HMI Sizing for the Shop Floor
A 5-inch HMI on a machine. The operator stood 1.5 m away. He couldn’t read the fault messages. The buttons were small — he pressed the wrong one. The customer asked for a bigger screen. This is about HMI sizing and design, not about the HMI hardware being defective.
The sizing rule
| HMI size | Viewing distance | Use for |
|---|---|---|
| 3.5 inch | 0.5 m | Setup panels only |
| 5 inch | 0.8 m | Simple machines, single function |
| 7 inch | 1.5 m | Standard machines, operator interface |
| 10 inch | 2.5 m | Complex cells, multiple stations |
| 15 inch | 4 m | Andon, overview screens |
What we did
1. Upgraded to 7 inch. The operator stood 1.5 m away. The 5-inch screen was too small. The 7-inch screen has 14-point text (not 10-point). The fault messages are readable from the working position.
2. Big buttons. The start/stop buttons are 50 mm on screen. The operator doesn’t need to look closely. Emergency stop is a physical button (not on the screen).
3. Plain-language faults. The HMI shows “Clamp not extending — check air pressure” instead of “E1023.” The operator knows what to check. No manual lookup needed.
4. Physical buttons for safety. Start, stop, and e-stop are hardwired physical buttons. The HMI is for status and setpoints. Safety-critical actions aren’t on the touchscreen.
The viewing angle
The HMI is mounted at eye level (1.5 m from floor), tilted 15° toward the operator. The anti-glare coating reduces reflections. In direct sunlight, we added a sun hood. The customer can read it from the normal working position.
The HMI rule: Size the screen for the viewing distance. 7-inch for 1.5 m. Use big buttons and plain-language faults. Keep safety actions as physical buttons. A 5-inch screen in a noisy plant is a frustration, not an interface. The HMI that was too small wasn’t defective — it was the wrong size for the distance.
