HMI — Human-Machine Interfaces
The screen that separates the operator from the process. ISA-101 high-performance design (dominant gray, color reserved for abnormal, screen hierarchy), ISA-18.2 alarm management, industrial panels and web-native HMI, emerging AR/VR. Poor HMI design causes the worst incidents — Texas City 2005, Buncefield 2005.
ISA-101 principles
Standard ISA-101 (published 2015) inverted HMI logic: forget "beautiful and realistic", target operational legibility and cognitive load reduction. Four guiding principles.
Situational awareness
Operator must know within 5 s: "all good" or "something abnormal". ISA-101 sacrifices realism for legibility — gray dominant, color only for anomalies.
Screen hierarchy
4 levels: L1 plant overview (KPIs), L2 unit view (loops), L3 detailed equipment, L4 diagnostic (one sensor). Fast top-down navigation. No more than 7 ± 2 salient items per screen.
Useful alarms only
One alarm = one expected action. No informational alarms. ISA-18.2 target: < 6 alarms/h/operator in normal ops, < 144/day. Rationalization removes 50–80% of existing alarms.
Consistency and standards
Domain templates, standardized symbols (ISO 14617), identical behaviors across screens, unified site color code. An operator trained on one unit must work on another with no relearning.
HMI types
Machine HMI panel
4 to 22-inch screen dedicated to one machine or cell (Siemens Comfort/Unified, Pro-face, Beijer, Weintek). Robust IP65, fixed or Wi-Fi mobile. Programmed in TIA Portal, GP-Pro, EasyBuilder.
Supervisory HMI (SCADA)
Clients connected to a SCADA server — see SCADA hub. Multi-screen 24-32-inch per operator. Often virtualized (Citrix/VMware VDI).
Mobile HMI
Industrial tablets (Panasonic Toughbook, Getac), smartphones — secure apps with MFA lock. Very useful for rounds, maintenance, on-site intervention. Limit: no critical command from mobile.
Web-native HMI
Browser HTML5/SVG/JS rendering — no install, access from any authorized workstation. Ignition, WinCC Unified, AVEVA System Platform. The future of HMI.
AR / VR
RealWear glasses, HoloLens, Vuzix. Augmented maintenance (step-by-step overlay), VR training, remote expertise. Growing use cases, not yet mainstream.
ISA-101 do's vs anti-patterns
| Aspect | ❌ Avoid | ✅ ISA-101 |
|---|---|---|
| Background color | Navy or "techy" black | Light gray (#d0d0d0 to #e0e0e0) |
| Bright colors | Everywhere — green tanks, yellow pipes, constant red alarms | Only for abnormal states requiring action |
| 3D / realistic graphics | Perspective tanks, gradients, shadows | Flat 2D schematics, ISA-101 codes, clean process lines |
| Numeric data | Small, generic areas, no context | Large, next to symbol, with 30-min trend graph |
| Animations / blinking | Multiple (causes motion sickness) | Reserved for unacknowledged alarms |
Landmark incidents
These two accidents directly shaped ISA-101 and ISA-18.2.
Texas City
BP refinery — Texas, USA
15 dead, 180 injured during isomerization column startup. The operator did not see on the HMI that level exceeded maximum — display crowded out, alarms ignored due to flooding ("alarm fatigue").
Buncefield
Oil depot — UK
Massive explosion of an overflowing fuel tank for 40 minutes. The level gauge was broken, no HMI alarm dedicated to that sensor failure. No diagnosis visible to the operator. 43 injured, shockwave felt 200 km away.
Standards
- ISA-101.01-2015 — Human-Machine Interfaces for Process Automation Systems — global reference
- ISA-18.2-2016 — Management of Alarm Systems — rationalization, lifecycle
- EEMUA 191 — Alarm Systems — A Guide to Design, Management and Procurement
- IEC 62682 — Management of alarm systems for the process industries
- ISO 11064 — Ergonomic design of control centres — operator workplace view
- NUREG-0700 — US NRC — nuclear control room ergonomics reference