The window between people and machines
A human-machine interface, or HMI, is whatever lets a person see what a machine or process is doing and tell it what to do. In industry this usually means operator interface screens: a touch panel mounted on a machine, a monitor in a control room, or a ruggedized tablet. Behind the screen is a controller doing the actual control. The HMI does not run the process; it shows its state and passes along requests such as “start,” “stop” or “change this setpoint.”
Because operators make decisions based on what they see, the quality of the screen affects both productivity and safety.
What an HMI typically does
Shows status. Machine states (running, stopped, faulted), counts, temperatures, levels, valve positions and other values are shown as numbers, bars, trends or graphic symbols.
Accepts commands. Operators start sequences, switch between automatic and manual modes, select recipes or products and adjust setpoints within allowed limits.
Presents alarms. When something is abnormal, the HMI tells the operator what, where and often when, and records acknowledgments.
Shows trends and history. Charts of recent values help operators see whether a temperature is climbing slowly or jumped suddenly.
Supports maintenance. Many screens include diagnostics, such as the state of inputs and outputs, so technicians can see which sensor or interlock is blocking a start. Good fault text reduces guesswork and downtime.
Local panels and larger systems
A machine-level panel is usually connected directly to one controller and focused on that machine. A control-room display may cover an entire process area and pull data from many controllers. Supervisory systems that span multiple sites, discussed under SCADA systems, include HMI screens as one part of a larger data collection and alarm framework. Roughly speaking, the machine panel answers “what is this machine doing?”, while the supervisory view answers “what is happening across the operation?”
What makes a display clear
Several general principles are widely recommended, though details vary by organization.
- Show what matters first. The operator’s main questions are usually: Is it running normally? If not, what needs attention? A screen crowded with every available value hides the important ones.
- Use color sparingly and consistently. If many items are bright, nothing stands out. Many designers use muted tones for normal conditions and reserve strong colors for abnormal states. Never rely on color alone; add shape, text or position, since some users have color vision deficiencies.
- Be consistent. The same symbol, color and placement should mean the same thing on every screen. Navigation should follow a predictable pattern.
- Show values in context. A number alone says little; showing it next to its normal range or target makes problems easier to spot.
- Make controls hard to misuse. Sensitive actions may require confirmation, a password or a particular user level. Buttons should be large enough for gloved hands where relevant.
- Keep the screen readable in real conditions. Glare, distance and lighting matter, and so does text size.
- Limit clutter and animation. Decoration adds little. Plain graphics tend to be easier to read quickly.
Alarm design
Alarms deserve special attention. When a screen produces constant nuisance alarms, operators learn to ignore them, and a real problem can be missed. Good practice generally includes making each alarm meaningful, ensuring it requires a response, prioritizing the most important ones and providing guidance on what to do. Alarm lists should be reviewed from time to time, and unneeded alarms removed or retuned. Organizations often follow established alarm management standards for this.
Access, change control and security
HMIs are part of the control system, so access matters. Different user levels allow operators to run the machine while limiting who can change settings or limits. Screens and their programs should be backed up like controller programs, and changes recorded. Panels connected to plant networks need to be included in security planning, as outlined in industrial cybersecurity.
Practical tips for people working with HMIs
- Involve operators when designing or changing screens; they know which information they need and which they never use.
- Test a new screen with real users before committing it.
- Keep a record of tag names and addresses to ease troubleshooting.
- Do not treat the screen as a substitute for safety devices. Emergency stops and guarding are normally hardwired or built with safety-rated components, independent of the display.
Screens, controllers and safety devices should be installed and changed by trained personnel following the manufacturer’s manuals and local regulations. This article is general information, not engineering advice.