The HMI That Was Too Small: Operator Interface Design on the Shop Floor
A 5-inch HMI on a machine in a noisy plant. The operator couldn’t read the screen from 1 meter away. The fault messages were tiny. The buttons were small — the operator pressed the wrong one. The customer asked for a bigger HMI. This is about HMI sizing and design for the shop floor, not about the HMI hardware.
The sizing rule
| HMI size | Viewing distance | Use for |
|---|---|---|
| 3.5 inch | 0.5 m | Small panels, setup 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 |
Our machine’s operator stood 1.5 m from the HMI. The 5-inch screen was too small. We upgraded to a 7-inch. The text was readable from 1.5 m.
The screen design
Even a big HMI is useless if the screen is poorly designed. The operator needs:
- A main screen: machine status (running/stopped/fault), cycle count, part counter. No sub-menus for the primary info.
- A fault screen: clear fault message in plain language. “Air pressure low at station 3” not “E1023”. A reset button.
- A setpoint screen: cycle time, temperature, speed. Accessible but not on the main screen (prevent accidental changes).
- A maintenance screen: PM intervals, alarm history, sensor diagnostics. Locked with a password.
What we changed
1. Bigger HMI. 7-inch, not 5-inch. The text is 14-point (not 10-point). The fault messages are red on yellow (high contrast).
2. Big buttons. The start/stop buttons are 50 mm on the 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 cylinder not extending — check air pressure” instead of “I:0/0 fault.” The operator knows what to check.
4. Physical buttons for safety. Start, stop, and e-stop are physical, not on the HMI. The HMI is for status and setpoints. Safety-critical actions are hardwired.
The viewing angle
An HMI in direct sunlight needs a bright display (500+ nits). An HMI in a dark machine shop can be dimmer. We mounted the HMI at eye level (1.5 m from floor), tilted 15 degrees toward the operator. The anti-glare coating reduces reflections. The customer could read it from the normal working position.
The HMI rule: Size the HMI for the viewing distance (7-inch for 1.5 m). Use big buttons and plain-language faults. Keep safety actions (start, stop, e-stop) as physical buttons. The HMI is for information, not for replacing hardwired safety. A 5-inch screen in a noisy plant is a frustration, not an interface.
