What is an HMI?
An HMI (Human-Machine Interface) is the operator screen used to monitor and control a machine or industrial process: it shows measured values, statuses and alarms, and passes commands to the underlying PLC or DCS. An HMI can be a touch panel on a machine, but also a workstation in a control room. Without an HMI, an operator could only work with the installation through switches, indicator lamps and gauges.
🧠 What does an HMI do?
- Visualise — show process graphics with tanks, pipes, motors and their current values
- Operate — start and stop machines, adjust setpoints, select manual mode
- Alarm — report faults and deviations and let operators acknowledge them (alarm management)
- Show trends — the course of measured values over time (trending)
- Manage recipes — load product settings, for example in batch processes
The HMI does not perform control itself. If the screen fails, the PLC simply keeps controlling the process.
🔧 What types of HMI are there?
| Type | Location | Example |
|---|---|---|
| Operator panel | On or next to the machine | 7 to 22 inch touch panel, e.g. SIMATIC HMI |
| Operator station | Control room | Workstation running SCADA or DCS client software |
| Web HMI | Any browser on the network | Ignition Perspective, WinCC Unified |
| Mobile HMI | Tablet or smartphone | Read-only dashboards or limited control |
Well-known HMI software includes WinCC and TIA Portal from Siemens, FactoryTalk View from Rockwell Automation, AVEVA and Ignition.
🔄 What is the difference between HMI and SCADA?
| HMI | SCADA | |
|---|---|---|
| Scale | One machine or part of an installation | Entire plant or multiple sites |
| Data | Live values from one or a few PLCs | Data from tens to thousands of controllers |
| History | Limited, local | Central historian |
| Users | Operator at the machine | Control room, maintenance, management |
A SCADA system contains HMI screens itself: the HMI is then the visualisation part of the wider SCADA system.
🎯 What makes a good HMI design?
For a long time HMIs showed colourful 3D pictures of installations. Investigations after incidents in the process industry showed that this made operators notice deviations too late. The ISA-101 standard (2015) therefore describes the high-performance HMI:
- Grey background, muted colours — use colour only for abnormal situations and alarms
- Hierarchy of displays — from an overview of the whole installation down to the detail of a single device
- Analogue values in context — bars showing the normal range instead of bare numbers
- Consistent navigation and symbols throughout the installation
Good design shortens operator response time and helps prevent alarm floods.
🔐 How do you secure an HMI?
HMIs often run on Windows and are therefore vulnerable to the same attacks as office PCs, yet they are rarely patched. HMIs are regularly found directly reachable from the internet with a default password or via VNC.
- No direct internet access — only through secure remote access
- Replace default passwords and set up roles (RBAC)
- Kiosk mode and application whitelisting on operator panels and stations
- Patch management agreed with the vendor
- Lock down USB ports against malware on removable media
🛠️ How do you design an HMI display step by step?
- Analyse the operator’s tasks — which decisions do they make, and what information do they need for them?
- Build a display hierarchy — level 1 shows the whole installation at a glance, level 2 a process area, level 3 a device, level 4 diagnostics and detail
- Define a style guide — colours, symbols, font sizes and navigation apply to every display
- Design for abnormal situations — normal conditions are a calm grey, only deviations stand out
- Link alarms to actions — every alarm points to the display where the operator can act
- Test with operators — have real users walk through scenarios before commissioning
| Level | Content | Purpose |
|---|---|---|
| 1 | Overview of the whole installation | Situational awareness (Situational Awareness) |
| 2 | Process area or unit | Normal operation |
| 3 | Device or control loop | Detailed operation and tuning |
| 4 | Diagnostics and documentation | Troubleshooting |
❓ Frequently asked questions
What does HMI stand for?
HMI stands for Human-Machine Interface. It is the screen or panel an operator uses to monitor and operate a machine or industrial process. In industry, HMI almost always refers to a graphical operator panel or an operator station in a control room.
Is an HMI the same as a PLC?
No, an HMI and a PLC are different devices. The PLC performs the control and keeps working if the HMI fails. The HMI is the operator screen that shows the PLC’s state and passes on the operator’s commands. Some compact devices combine both in a single housing.
What software is used to program an HMI?
HMIs are usually configured with vendor software, such as WinCC in TIA Portal for Siemens or FactoryTalk View for Rockwell. Vendor-neutral packages such as Ignition and AVEVA are also widely used. Design is graphical: display objects are linked to tags in the PLC.
What is a high-performance HMI?
A high-performance HMI is a design approach based on the ISA-101 standard in which displays are grey and calm and colour only indicates deviations. Measured values are shown in context, for example with bars showing the normal range. The goal is that operators notice deviations sooner and respond better.
Why is an HMI a security risk?
An HMI often runs on Windows, is rarely patched and can send commands directly to the controller. An attacker who takes over an HMI can therefore influence the physical process. HMIs reachable from the internet with default passwords or via VNC are regularly abused.
📌 In summary
An HMI is the operator’s window on the process: it visualises, raises alarms and passes operating commands to the PLC, which performs the actual control. A clear design in line with ISA-101 and proper network isolation make the difference between an HMI that helps and an HMI that becomes a risk.