Operator interfaces (HMIs) are the bridge between people and processes. A poorly designed screen results in information clutter, alarms noticed too late, and incorrect interventions. Industrial HMI design should target situational awareness before aesthetics.
Core Principles
1. Show Information, Not Data
Do not let operators get lost among hundreds of values. Only information critical to the current task should be on screen. Detailed data belongs on optional drill-down screens.
2. Use Status Colors Correctly
Follow the ISA-101 standard's color recommendations: gray/blue tones for normal states, yellow for warnings, red for alarms. Reserve green for "running"; coloring everything destroys the meaning of colors.
3. Manage Alarms Intelligently
Alarm floods prevent operators from seeing the real problem. Apply alarm prioritization, grouping, and deadband rules. The goal is not to reduce the number of alarms but to increase the rate of alarms that lead to action.
4. Consistent Navigation
Use the same layout, same button positions, and the same return logic on every screen. The operator should not have to think about the screen instead of the process.
Reducing Operator Errors
- Confirmation steps: Add a second confirmation step for critical commands.
- Reversibility: Make commands reversible where possible.
- Touch target sizes: On touchscreens, targets should be at least 44x44 pixels; the risk of accidental touches must be reduced.
- Feedback: Every command must be acknowledged with visual and/or audible feedback.
ISA-101 and Continuous Improvement
ISA-101 treats HMI design not as a one-time project but as a lifecycle process: design, implementation, operation, and continuous improvement. Organizations that regularly collect operator feedback and update screens accordingly improve interface quality over time.
Implementation Steps
- Review existing screens with operators; identify frequently used flows.
- Design a "main screen" for each process: status, alarms, and core controls visible at a glance.
- Document color, font, and symbol standards.
- Test with operators in a simulation environment; apply feedback.
- Measure after commissioning: alarm rate, intervention time, operator error count.
The measure of HMI design is not how beautiful the screen is, but how quickly and confidently the operator makes the right decision.
Conclusion
Operator-centered HMI design improves safety, efficiency, and job satisfaction together. Small but consistent improvements bring the biggest gains in the long run.
