The HMI That Was Too Small
We picked a small HMI (a 4″ touch screen) for a machine. On the floor, the operator couldn’t read it (the screen was small, the plant lighting was bright). The operator had to lean in (close) to see the text. The maintenance couldn’t read the alarm (it was tiny). The problem: the HMI was too small (and the brightness was low). We upsized to a 7″ HMI (brighter, larger). The operator read it. The mistake was picking a 4″ HMI (for a complex machine with many alarms).
HMI screen selection matches the screen to the operator’s use. This article covers the sizing.
The Size (Diagonal)
- 4″ (small): For a simple machine (a few buttons, one value). Good for a panel mount (small). Not for many alarms.
- 7″ (medium): For a typical machine (a few screens, alarms). Readable from 1 m. The most common.
- 10″ (large): For a complex machine (many screens, trends). Readable from 1.5 m.
Step 1: The Viewing Distance
How far is the operator from the HMI? If 0.5 m (close), a 4″ is OK. If 1 m (normal), a 7″. If 1.5 m (far), a 10″. The operator shouldn’t lean in (to read). Pick the size for the viewing distance.
The HMI rule: For a typical machine (1 m viewing), use a 7″ HMI. The HMI that was too small was 4″ on a complex machine. Use a 7″ (or 10″ for complex). The operator reads from 1 m.
Step 2: The Brightness (cd/m²)
A dim screen (200 cd/m²) is hard to read in bright light (a plant with windows). A bright screen (500 cd/m²) reads in sunlight. For a bright plant, pick a high-brightness HMI (500 cd/m²). For a dark machine shop, 300 cd/m² is fine.
Step 3: The Touch Type
- Resistive (pressure): Operates with a glove (the operator wears gloves). Good for a dirty/glove application.
- Capacitive (touch): Operates with a finger (not a glove). Nice, but fails with gloves. For a clean, bare-hand application.
For a machine shop (gloves, oil), use a resistive HMI (works with gloves). A capacitive HMI needs a bare finger (the glove doesn’t work).
Step 4: The Number of Screens
A complex machine (many I/O, alarms, trends) needs many screens. A 4″ HMI (low memory, slow) can’t handle many screens (it lags). A 7″ (or 10″) handles many screens. For a simple machine (3 screens), 4″ is OK. For a complex machine (20 screens), use a 10″.
| Size | Viewing | Use |
|---|---|---|
| 4″ | 0.5 m (close) | Simple (few screens) |
| 7″ | 1 m (normal) | Typical (alarms) |
| 10″ | 1.5 m (far) | Complex (trends) |
An HMI Checklist
- How far is the operator? (m?)
- Is the machine complex? (Many screens?)
- Is the plant bright? (Needs brightness?)
- Does the operator wear gloves? (Resistive?)
- Is it readable from 1 m? (Test?)
- Are the alarms visible? (Text size?)
- Is the response fast? (Lag?)
- Is the mounting right? (Article 167?)
- Is the IP rating OK? (Article 77?)
- Does the operator use it? (Training?)
The Bottom Line
HMI screen selection matches the size to the viewing distance. The HMI that was too small was 4″ on a complex machine. Use a 7″ (normal). For gloves, use resistive. For bright light, high brightness. The operator that read the HMI wasn’t the cheapest one — it was the right size.
