HMI Guide
The HMI (Human-Machine Interface) is the operator's window into the process. Learn good HMI design principles, common mistakes, and how to make screens that improve operator effectiveness.
What is an HMI?
An HMI (Human-Machine Interface) is the screen through which operators monitor and control the process. It displays process graphics (pumps, valves, tanks, pipes), real-time data (temperatures, pressures, flows), alarms, and trends. Operators use it to start/stop equipment, change setpoints, and acknowledge alarms.
A well-designed HMI gives operators the information they need at a glance, without clutter. A poorly designed HMI buries important information under excessive detail and causes operators to miss critical events.
Good HMI Design Principles
- Hierarchy: Overview screen showing the whole process. Click to drill down to area, then to individual equipment. Each level shows more detail.
- Colour coding: Use colour consistently. Green = running, red = stopped, yellow = alarm, grey = out of service. Don't use colour as the only indicator (colour-blind operators).
- Alarm priority: Show high-priority alarms prominently. Don't flood the screen with low-priority alarms. Use audible alarms sparingly, only for critical events.
- Trends: Embed trends on process screens so operators can see the history of key variables without navigating to a separate trend page.
- Minimal detail: Show only the information relevant to the current task. Too much detail makes it hard to find what matters. Use pop-ups or drill-downs for detailed information.
- Consistent layout: Place navigation, alarms, and controls in the same location on every screen. Operators develop muscle memory.
- Large touch targets: If the HMI is a touchscreen, buttons must be large enough to tap accurately (at least 20mm). Small buttons cause mis-taps.
Common HMI Mistakes
- Too much animation: Spinning pumps, flowing pipes, and blinking lights look impressive but distract operators from important information. Use minimal animation.
- Inconsistent colour: Using red for 'running' on one screen and 'stopped' on another. Standardise colour meanings across all screens.
- Excessive alarms: Alarming every minor deviation creates alarm flooding. Only alarm conditions that require operator action.
- Tiny text: Text that operators can't read from their normal viewing distance. Use at least 10pt font for data, 12pt for labels.
- No trends: Showing only current values without history. Operators need to see trends to understand what the process is doing.
- Too many screens: burying equipment 5 or 6 clicks deep. Important equipment should be accessible in 2 to 3 clicks.
Design for the operator
The HMI is designed for the operator who uses it every shift, not for the engineer who designs it once. Involve operators in the design. Ask them what information they need and where they look for it. The best HMIs are designed by the people who use them.
Tip: Review your HMI screens with operators annually. As processes change, the HMI should evolve. Remove obsolete information, add new equipment, and simplify screens that have become cluttered. An HMI that was well-designed 5 years ago may no longer serve the current process.