The Touchscreen That Was Too Small for the Operator
We used a 7-inch HMI on a machine. On the bench, the screens looked fine. On the floor, the operator (wearing gloves) couldn’t tap the small buttons. The screen’s buttons were 8 mm wide. With a work glove, the touch was unreliable — the operator tapped the wrong button. We switched to a 12-inch HMI with bigger buttons (15 mm). The operator tapped accurately. The mistake was sizing the HMI for the engineer (who works at a desk) rather than the operator (who wears gloves and stands at the machine).
HMI and panel PC selection isn’t the cheapest screen. The HMI is the operator’s interface. Size it for the operator, not the engineer. This article covers the choices.
HMI vs Panel PC
HMI (Human-Machine Interface)
A dedicated touchscreen (a “brick”) that talks to the PLC. It runs HMI software (screen pages, alarms, trends). Fixed function (you configure the screens). Cheap, reliable, fanless. Standard for most machines.
Best for: Standard machines (start/stop, alarms, simple setpoints). 7–15 inch.
Panel PC (Industrial PC)
A full PC (Windows or Linux) in an industrial enclosure. It runs the HMI software plus other applications (data logging, reporting, database). More flexible but more expensive. Has a fan (moving parts). Needs maintenance (updates, antivirus).
Best for: Complex machines (recipes, data logging, MES integration, multiple languages).
| Type | Cost | Flexibility | Best For |
|---|---|---|---|
| HMI (dedicated) | Low | Fixed screens | Standard machines, simple operation |
| Panel PC (industrial) | High | Full PC, apps, database | Complex machines, MES, data logging |
Step 1: Screen Size
The screen size depends on the operator’s use:
- 7 inch: Small machines (a few buttons, simple status). For a tight panel. Buttons are small (8–10 mm).
- 10–12 inch: Standard. Buttons 12–15 mm. Fits most machines.
- 15 inch: Complex machines (many screens, trends, alarms list). Buttons 18–20 mm.
The touchscreen that was too small was a 7-inch with 8 mm buttons. The operator (with gloves) couldn’t tap accurately. Use a 12-inch (15 mm buttons) for gloved operation.
The HMI rule: Size the screen for the operator, not the engineer. The operator stands at the machine, wears gloves, and views from arm’s length. Use 10–12 inch minimum, with buttons at least 15 mm. The HMI that was too small was a 7-inch for gloved operators.
Step 2: Touchscreen Type
- Resistive touch: Press anywhere (works with gloves, styli). Standard for industrial. Can be operated with gloves.
- Capacitive touch: Requires skin (like a phone). Doesn’t work with gloves. Consumer-grade. Avoid for machines (unless the operator is bare-handed in a clean room).
Industrial HMI uses resistive touch (or projected capacitive that works with gloves). Specify “glove-mode” touch.
Step 3: Communication Protocol
The HMI talks to the PLC. The protocol must match.
- Modbus TCP/RTU: Standard, universal. Most PLCs and HMIs support it.
- EtherNet/IP: For Allen-Bradley (Rockwell) PLCs.
- Profinet: For Siemens PLCs.
- OPC UA: Modern, for data exchange (MES, cloud).
The HMI and PLC must speak the same protocol. A Siemens HMI and an Allen-Bradley PLC won’t talk natively (you’d need a gateway).
Step 4: Brightness and Viewing Angle
The factory has overhead lights. The HMI must be readable.
- Brightness: 400–800 nits. For a bright factory floor, 800 nits.
- Anti-glare coating: A matte screen (not glossy) reduces reflections.
- Viewing angle: The operator views from various angles. Use a wide-angle screen (IPS or similar).
Step 5: Environmental Rating
The HMI is mounted on the machine. It needs the right IP rating (article 34) for the environment.
- Dust/oil: IP65 (front bezel). The front of the HMI (exposed to the operator) is sealed. The back (inside the cabinet) is IP20.
- Washdown (food/pharma): IP69K (high-pressure wash).
- Temperature: 0–50°C standard. For a hot factory (near ovens), use a wide-temp model (-20 to 60°C).
HMI Screen Design (Not Just Hardware)
The HMI hardware matters, but the screen design matters more. The operator sees the screens.
- Big start/stop buttons: On the main screen. Color-coded (green start, red stop).
- Alarm list: Visible. A red banner shows the active alarm. The operator taps to acknowledge.
- Status lamps: Green (running), yellow (idle), red (fault). At a glance.
- Setpoints: In a separate (password-protected) screen. Not on the main screen (avoid accidental changes).
- No tiny text: The operator views from arm’s length. Use large fonts (14–18 pt).
Mounting
The HMI is mounted on a swing arm (articulated arm) so the operator can position it. The arm lets the operator pull the screen close, view it at an angle, and push it back when done.
The mounting height is about 1.5 m (eye level for a standing operator). Not too high, not too low.
| Element | Selection |
|---|---|
| Type | HMI (simple) or Panel PC (complex) |
| Screen size | 10–12 inch minimum (15 mm buttons) |
| Touch type | Resistive or glove-mode capacitive |
| Protocol | Matches the PLC (Modbus, EtherNet/IP, Profinet) |
| Brightness | 400–800 nits, anti-glare |
| IP rating | IP65 front (washdown = IP69K) |
| Mounting | Swing arm, 1.5 m height |
An HMI Selection Checklist
- HMI or Panel PC? (Simple or complex?)
- What screen size? (10–12 inch minimum?)
- Touch type? (Resistive / glove-mode?)
- PLC protocol? (Modbus, EtherNet/IP, Profinet?)
- Brightness? (Factory floor lighting?)
- Anti-glare coating?
- IP rating? (Dust, washdown?)
- Temperature range? (Hot or cold environment?)
- Mounting? (Swing arm? Height?)
- Screen design? (Big buttons, alarm banner?)
- Setpoints password-protected?
- Is the font readable from arm’s length? (14–18 pt?)
The Bottom Line
HMI and panel PC selection is for the operator, not the engineer. The touchscreen that was too small was a 7-inch with 8 mm buttons — the operator wore gloves and couldn’t tap. Use 10–12 inch minimum, resistive (glove-mode) touch, 15 mm buttons, and IP65 front. Match the protocol to the PLC. Mount on a swing arm at 1.5 m. The HMI that the operator uses confidently wasn’t the cheapest screen — it was sized for gloved operation.
