What Does HMI Mean?

HMI stands for human-machine interface: the part of an automated system that a person actually uses. It turns the signals inside a machine into screens an operator can read, and turns the operator’s touches and key presses into commands the machine can act on. The same idea is also called an operator interface terminal (OIT) or a man-machine interface (MMI).

In everyday use the term covers three things at once: the hardware (a touch screen panel PC or operator panel), the HMI software that draws the screens, and the screens themselves. When an engineer says “the HMI” on a factory floor, they normally mean the touch screen mounted on or beside the machine. Behind it, a PLC usually runs the control logic, and our FAQ on how an HMI works with a PLC explains how the two exchange data.

A good HMI screen gives the operator what they need to run the machine and nothing more. Most HMIs handle the same core jobs:

Industrial HMIs come in a few forms. The choice depends on how much software the HMI must run, where it is mounted and how many machines it serves.

For positions where the screen must stand apart from the computer, our touchscreen monitors mount in the panel and connect to a computer in the cabinet.

The two most common kinds of HMI hardware are the HMI PC and the operator interface terminal (OIT). Both are often just called an HMI panel, but they work differently. An OIT is a closed terminal: a touch screen with an embedded processor that runs only its maker’s HMI runtime, set up in that maker’s software and connected through built-in drivers for popular PLCs. An HMI PC is a panel PC, with a full industrial computer behind the touch screen, running Windows or Linux and whichever HMI or SCADA package the project chooses.

An OIT suits a machine with a single PLC brand, simple screens and a builder who already uses that maker’s tools. An HMI PC suits a screen that must talk to several controllers, store data, run other software or outlast one HMI vendor’s product range. On an HMI PC, the lockdown features of Windows IoT Enterprise keep it behaving like an appliance, and our panel PCs cover HMI work with resistive or PCAP touch, sealed fronts and stainless steel housings.

HMIs are everywhere people operate equipment. A touch screen on a packaging machine, the overview screen of a water treatment plant, a terminal on a forklift, the controls of a lift or escalator and a self-service kiosk are all HMIs. In a hygienic food plant the HMI is often a stainless steel waterproof panel PC that survives daily washdown. In a control room it is a bank of large monitors showing a whole site, which is where HMI meets SCADA.

Screen design affects how quickly operators spot and fix problems. The ISA-101 standard for high-performance HMIs sets out the main principles: grey or muted backgrounds, with colour kept for abnormal states and alarms; consistent navigation across screens; values shown with context, such as normal ranges; and a clear hierarchy from plant overview down to individual equipment.

The hardware matters too. Touch technology decides whether the screen works with gloves, water on the glass or a stylus; brightness decides whether it can be read outdoors or near windows; and the front-panel IP rating decides whether it survives washdown. Our guide to choosing touch technology for industrial displays compares the options.

Start from the installation: screen size and viewing distance, panel cut-out or VESA mount, sealing, temperature and the ports the machine network needs. Then size the computer to the software, since a panel PC that also runs data logging or a vision application needs more processing and memory than one that only draws screens. Lifecycle matters as well: a machine builder shipping the same design for years needs a panel PC that stays available for that time, or a planned successor that fits the same cut-out. Our guide to choosing an industrial HMI explains how to specify the hardware for each type.

We supply panel PCs and touch monitors for HMI use with the operating system, drivers and mounting fitted as standard, and test each system before it ships. Tell us the machine, environment and software, and we will specify the HMI hardware to match.