Choosing an Industrial HMI: Panel PCs, Touch Monitors and Vehicle HMIs

An industrial HMI (human-machine interface) is the touch screen, computer and software an operator uses to monitor and control a machine, a production line or a vehicle, built to keep working in dust, water, heat, cold and vibration. The hardware is usually a panel PC, a touch monitor connected to a computer in the cabinet, or a dedicated operator panel, and it talks to the machine’s PLC over Ethernet or serial.

Choosing one is partly a hardware decision and partly a software one. The screen has to suit the operators and the place it is installed; the computer behind it has to run the HMI software the site uses and speak the PLC’s protocol; and the whole build has to stay available for as long as the machine is in service. This guide compares the five common forms of industrial HMI, explains how an HMI fits with PLCs and SCADA, covers protocols, software, environment, screen size and processor choices, and ends with a specification checklist.

HMI stands for human-machine interface: what HMI means in practice is the screen through which a person sees what a machine is doing and tells it what to do next. An industrial HMI shows live values, alarms, trends and recipes, and sends the operator’s commands to the PLC that runs the machine.

HMIs sit at every level of an automated site: on the door of a machine, at the end of a packaging line, in a forklift cab and on the wall of a control room. What makes an HMI industrial is where it has to work. A commercial all-in-one PC with a fan will not last long next to hot, humid, dusty or hosed-down machinery, so industrial HMIs are fanless, sealed at the front or all round, rated for wider temperatures and built for long availability.

Examples of industrial HMIs

• A 15 inch (15″) touch panel PC in the door of a packaging machine, showing speeds, counts and faults
• A stainless steel washdown HMI at a filling line, used to load recipes and acknowledge alarms
• A touch monitor on a pump skid, driven by a DIN rail PC in the cabinet
• A 7 inch (7″) vehicle mount terminal in a forklift, running the warehouse management client
• A sunlight readable screen on a fuel dispenser or loading bay, operated in gloves
• An operator panel on a small machine, running the PLC maker’s own HMI runtime

There are five common ways to build an industrial HMI, and the right one depends on the software, the machine and how the site is likely to change. The table is the quick reference.

Panel PCs are the most flexible: one sealed computer on the machine that can run the HMI, log data, connect to MES and host a vision application at the same time. Where the front of the machine is too hot or too tight for a computer, a sealed panel mount touch monitor on the door with a DIN rail PC inside the cabinet does the same job, and the computer can be upgraded later without touching the door.

For a recent example, Axiomtek’s rugged GOT321B and GOT315B touch panel PCs pair a PCAP touch screen with an IP65 front bezel and flush or VESA mounting.

An HMI runs one machine or process; SCADA supervises many. The difference between HMI and SCADA is mostly one of scope, and the two often share software and screens. The layers below are the usual way an automated site is organized.

An HMI does not need SCADA: a stand-alone machine with its own panel PC is complete on its own. But when a machine joins a SCADA system, the same panel PC often runs a SCADA client as well as the local HMI, which is one reason a general-purpose panel PC is chosen over an operator panel.

The HMI reads values from the PLC, known as tags, and writes setpoints and commands back. It does that over an industrial protocol that both ends support, and the protocol decides which ports the HMI hardware needs.

Our explainer on Modbus RTU vs Modbus TCP covers the most common choice, and the guide to OPC UA vs MQTT explains how HMI data reaches plant and cloud systems.

Plan the ports before choosing the hardware. Two separate Ethernet ports let the HMI sit on the machine network and the plant network without bridging them, isolated serial ports protect against ground loops on long RS-485 runs, and USB covers scanners and printers. The Winmate R17ITWS-MHM1 in the products block below, for example, has two Gigabit LAN ports, two RS-232/422/485 ports and a PCIe x16 slot wired with x4 signals for a fieldbus or I/O card.

The software decides as much about the hardware as the machine does. A vendor’s operator panel runs only that vendor’s runtime, while a panel PC can run any of the options below. The examples are common, not exhaustive.

For Windows builds, Windows IoT Enterprise is the usual choice because it can be locked down to the HMI application, and the LTSC editions are supported for ten years; our guide to Windows 11 IoT Enterprise LTSC covers editions and support dates. Embedded Linux suits web HMIs and soft PLC runtimes, and Android panel PCs run app-based HMIs with kiosk mode keeping operators inside the app.

Panel PCs are computers, so we configure, image and test them with the operating system, drivers and settings the HMI software needs. Touch monitors are configured and tested with the host computer they will run from.

Where an HMI is installed decides most of its specification. The table sets out typical requirements for the settings we build for most often; the cards below link to the matching ranges. The IP rating chart explains the sealing levels.

Two figures are worth checking on every datasheet. The operating temperature range should cover the coldest start and the hottest day inside the enclosure, not just the room; our explainer on the industrial temperature range gives the usual bands. And the brightness should match the light: our guide to choosing a sunlight readable display sets out the nits each setting needs.

Screen size follows from how much the operator needs to see at once and how far away they stand. Touch targets matter as much as the picture: a button large enough for a gloved finger takes more of a small screen.

Match the resolution to the HMI project rather than the other way round: an application drawn for 1024 × 768 looks stretched on a widescreen panel unless it is redrawn. The 15 inch panel PCs and 21.5 inch panel PCs are the two sizes we supply most often for machine and line HMIs.

Match the processor to the software, not the other way round. A single-machine HMI runs well on an efficient low-power processor; a SCADA client, MES terminal or vision station needs more cores and memory, and should still be fanless.

Lifecycle matters as much as performance. Machines stay in service for ten years or more, so choose a model with long availability, so that a replacement fits the same cut-out and runs the same software image without requalifying the machine. Our guide to MTBF explains how reliability figures are quoted. A TPM 2.0 chip is worth asking for on any HMI that connects to the plant network.

These projects show the choices above in practice: stainless steel HMIs for food and beverage manufacturers, intrinsically safe HMIs for wastewater treatment in hazardous areas, and a 10 inch HMI touch screen gateway collecting IIoT data in supermarkets.