The HMI (Human Machine Interface) is the layer through which the operator talks to the machine. It shows what the machine is doing, allows setting changes, and explains what happened in the event of a fault. A properly selected HMI shortens commissioning time and reduces fault-related stoppages; a poorly selected HMI, on the other hand, constantly produces problems on the user side even if the rest of the machine is flawless.
Selection has two legs: hardware and software environment. The second is often underestimated but is more decisive in the long run.
1. Screen size and resolution
What determines the screen size is not aesthetics but the amount of information that must be shown at the same time. A small panel is sufficient for a few settings and status readings; if recipe tables, production counters and diagnostic screens are needed on a multi-axis machine, a large screen is required.
Resolution determines the information density. On a low-resolution screen, small text cannot be read; to fit the information, the design has to be compromised.
The mounting location is also taken into account: the operator’s viewing distance and angle to the screen determine the requirements for text size and viewing angle of the screen.
2. Screen technology and touch type
Resistive touch: Works by pressure. It can be used with gloves and wet hands. It does not support multi-touch, and its image quality is lower than capacitive.
Capacitive touch: Works by finger contact. It offers a clearer image and multi-touch support. Its use with thick gloves and on wet surfaces is limited — for this reason it must be carefully evaluated in food and wash-down environments.
Brightness: In areas exposed to direct sunlight or with very high ambient light, high brightness and an anti-reflective surface are required. For outdoor installations, UV resistance and a wide temperature range are also sought.
3. Need for physical keys
In some applications touch alone is not sufficient. In gloved operation, vibrating environments, or situations where the operator must perform an action without looking at the screen, models with physical keys (or hybrid models) are preferred.
Safety-related commands such as emergency stop, safety functions and manual/automatic selection are not left to the touch screen; these remain as hardware elements.
4. Processor, memory and performance
Three things determine HMI performance: screen complexity, the number of data points (tags), and the communication load. Screens with many live values, trend graphs and animations run slowly on a panel with low-end hardware — page transitions lag, and values update late.
On the memory side there are two separate needs: project memory (screens, images, fonts) and data memory (recipes, alarm history, trend records). If long-term data is to be stored, support for external memory (SD card, USB) is sought.
5. Communication
This is the most critical checkpoint: Can the HMI talk to the PLC and other devices to be used?
Things to check:
- The list of supported drivers/protocols (Modbus TCP, Modbus RTU, Profinet,
- How many different devices it can connect to at the same time — an HMI may be required to read both a PLC and
- Number of physical ports: Ethernet, RS-485/RS-232, USB
- If remote access is needed, support for VNC / web-based access
EtherNet/IP, manufacturer-specific protocols)
a drive and a power analyzer
If remote access will be used, the security side must be planned from the outset; the panel is not opened directly to the internet.
6. IP class and environmental conditions
The IP class of the front surface is selected according to the environment in which the panel is mounted. In food and wash-down production areas, a high IP class and a stainless-steel front frame are preferred. On vibrating machines and mobile equipment, vibration/shock resistance is checked from the catalog.
The operating temperature range is evaluated together with the in-cabinet temperature calculation. Heat build-up behind the panel is one of the most common causes that shorten screen life.
7. Software environment — the most decisive criterion in the long run
Hardware can be selected from a catalog; the software environment, however, is something to be lived with throughout the life of the machine.
License and accessibility: Is the development software paid, and is it licensed by tag count? Will the team that maintains the machine be able to access this software? If not, even a simple text change becomes dependent on an external party.
Library and object support: Ready-made alarm, trend, recipe and user management objects shorten the project. If these are missing, the same functionality has to be built manually.
Multi-language support: If the machine will be exported or operators working in different languages will use it, the ability to change language at runtime is sought.
User authorization: Operator, maintenance and engineer levels must be definable with separate permissions. Changing of critical parameters by the operator is prevented.
Data transfer: If production data needs to be exported as a file, reported, or sent to a higher-level system, this capability is checked from the outset.
8. HMI or Panel PC
An HMI panel is a single-purpose, stable device running an embedded operating system. Its boot time is short and it requires no maintenance.
A Panel PC, on the other hand, is a general-purpose computer. It is preferred when a database, image processing, complex reporting, or running of third-party software is required. In return, it also brings along matters such as operating-system updates, disk life and boot time.
The decision point is simple: an HMI panel for a standard operator interface need, a Panel PC for applications requiring a computing load.
9. Screen design — the real work after selection
Good hardware does not save a poorly designed interface. A few principles that work in practice:
- The main screen should show the overall status of the machine at a glance
- Alarm messages should explain what happened and what to do; an error code
- Color should carry meaning and be used consistently; for color blindness, color should not be the only
- Frequently used operations should be accessible with a small number of touches
- The diagnostic screen should be considered separately for the maintenance team: input/output states,
alone is not sufficient
distinguishing feature
communication status, counters
Selection checklist
- [ ] Screen size and resolution according to the information density to be shown
- [ ] Touch type and brightness suitable for the environment
- [ ] Need for physical keys evaluated
- [ ] Processor/memory according to tag count and screen complexity
- [ ] Protocol compatibility with the PLC and other devices verified
- [ ] Number of ports and simultaneous connection capacity sufficient
- [ ] IP class, temperature and vibration resistance suitable
- [ ] Software license and maintenance team’s access clarified
- [ ] Multi-language and user authorization needs met
- [ ] Data logging and export need planned
Related product families: HMI, HMI Panel, Touch HMI, Panel PC, HMI Accessories, PLC
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