HMI Design: Operator Screens Without Confusion

What Is an HMI Screen?

An HMI (Human-Machine Interface) is the window through which an operator monitors and interacts with an industrial process. From small touchscreens on individual machines to large multi-monitor control rooms in oil refineries, HMI design directly impacts how quickly operators detect abnormal conditions and the quality of their decisions.

Why Good Design Matters

Major industrial incidents have been traced to poor HMI design:

• Operators missed critical alarms buried under hundreds of nuisance alarms
• Misleading color schemes suggested normal operation during dangerous conditions
• Cluttered screens made it impossible to distinguish normal from abnormal values

The ISA-101 standard and the High-Performance HMI philosophy emerged to address these problems.

ISA-101: The Industry Reference

ISA-101.01-2015 is the international standard for high-performance HMI design. Its core principles:

1. Task-based design: Screens must help operators perform their tasks, not serve as artistic displays
2. Hierarchy: From overview to detail in 3-4 levels
3. Situation awareness: The operator should understand the process state at a glance
4. Alarm management: Integrated with screen design from the start

Screen Hierarchy

ISA-101 organizes screens into four levels:

Golden rule: Operators should spend 80% of their time on Levels 1 and 2. If they are constantly navigating to Levels 3 and 4, the design has failed.

Color Usage: Less Is More

The most common mistake in HMI design is excessive use of color. Traditional screens were filled with bright greens, reds, and blues, making it impossible to identify what actually requires attention.

High-Performance Color Philosophy

Background: Light gray or medium gray (never black, never bright white). Reduces eye strain during long shifts.

Normal state: Neutral colors (dark gray, muted white). When everything is normal, the screen should appear calm and subdued.

Abnormal states: This is where vivid colors are used. Red, orange, and yellow are reserved exclusively for alarms and abnormal values.

The rule: If the screen is colorful when everything is normal, color will not draw your attention when something goes wrong.

Alarm Management

Alarm flooding is one of the most dangerous design problems. ISA-18.2 defines acceptable limits:

• Normal operation: No more than 6 alarms per hour per operator
• During upsets: No more than 10 alarms in the first 10 minutes

Alarm Priority Levels

Common Alarm Mistakes

• Alarming every value without analyzing its importance
• No shelved alarm mechanism for planned maintenance
• Chattering alarms from noisy signals -- use deadband and delay timers
• No periodic alarm rationalization to review and prune the alarm database

Screen Navigation

An operator should reach any screen in 3 clicks or fewer from any location.

Effective Navigation Principles:

• Persistent navigation bar at the top of every screen showing the current level and providing a link back to the overview
• Navigation hotspots on the process schematic -- clicking equipment navigates to its detail screen
• Always-accessible alarm summary reachable from any screen
• Site map showing all available screens and their relationships

Avoid excessive popup windows -- they obscure information and disorient the operator.

Effective Screen Layout

Fundamental Rules:

1. Flow direction matches the process: Left-to-right (or right-to-left for Arabic interfaces) following actual process flow
2. Important information at the top: The eye naturally starts at the top of the screen
3. Adequate white space: Do not fill every pixel -- empty space helps the eye organize information
4. Readable font size: Minimum 3 mm on the actual display
5. Consistency across screens: Same colors, same symbols, same positions

Displaying Numeric Values:

• Always show setpoint alongside process value
• Use bargraphs to represent values -- faster to read than numbers alone
• Place alarm limits on bargraphs with clear markers
• Color the bargraph only when deviating from normal -- no color in normal state

Common Design Mistakes

Trend Graphs

Consider this: a heat exchanger currently reads 180 C. Is that good or bad? It depends. If the temperature is falling from 200 C, there may be a problem. If it is rising from 150 C, the system is working correctly.

Trend graphs provide this temporal context:

• Short-term trends (1-4 hours) embedded next to the numeric value
• Medium-term trends (24-48 hours) on detail screens
• Historical trends (weeks to months) for performance analysis

HMI Design Tools

Summary

Good HMI design is not about aesthetics -- it is about situation awareness. A well-designed screen lets an operator detect problems in seconds rather than minutes. Remember: neutral colors are the default, vivid colors are reserved for abnormal conditions only, and simplicity is the highest form of professionalism.