Choosing the right HMI

Human-machine interfaces (HMIs) are available with many hardware and software features and users should work backwards from the target application to make sure all needs are addressed.

By Bill Dehner June 10, 2019

End users expect intuitive operator interfaces, and product designers have responded by building these interfaces into more products. For industrial automation, touchscreen human-machine interfaces (HMIs) are a mature technology and represent a valuable addition to many machine and process automation applications, extending the capabilities of switches, buttons and lights in a control panel. However, this familiarity should not discount the fact that every use case for an HMI should be evaluated carefully to ensure products are selected properly based on the application requirements.

It may be tempting to begin by looking at desired HMI features and choosing devices and software that seem to meet the needs. However, a more pragmatic and reliable approach is to examine the specific needs of the target application and work backwards to confirm all necessary options are selected.

Weathering the elements

Environmental conditions for the target application must be evaluated first because the HMI must be able to survive physically at a location to even be considered a viable choice (Figure 1). Most of these choices are intuitive, but some are not.

HMIs located in environments where they are subject to falling water or directed washdown will need compatible ratings in NEMA 250 enclosures for electrical equipment standard or UL 50 listings. This is critical when cleaning chemicals, process chemicals or other airborne contaminants could impact the HMI. A site pollution degree or material safety data sheet may be available to help define expected environmental conditions. Customer specifications, along with local installation codes or preferences, may drive the need for certain ratings or listings.

Another key concern is whether the HMI will be located indoors or outdoors. NEMA and UL ratings commonly are constrained as “indoor use only.” Even the brightest HMIs struggle with sunlight glare outdoors and direct sunlight can cause heat issues. Users should consider locating outdoor HMIs into more protected areas or outfitting them with sunshields.

HMIs typically contain sensitive display elements and backlights and are likely to have more restrictive allowable operating temperature ranges than other automation components. The display brightness (usually expressed in nits) and the average backlight lifetime should be evaluated carefully. Brightness is a key indicator of how easy it will be to view the display while the always-on backlight life will indicate how long the unit is likely to last. 

HMI physical display size and interface considerations

Choosing the right display size is often a balancing act (Figure 2). Most operators would prefer the largest display size possible, especially if they need to select touch links, but also if the application is view-only and if they want to observe the display from a distance. Bigger displays provide more real estate for programmers to develop graphics. HMI hardware product lines with a range of sizes can help users learn one system that is applicable to many different models.

Of course larger HMIs cost more and can chew up precious installation space for control panels. The display needs to be installed at a working height suitable for most employees, often considered to be around 66 inches from finished floor to the middle of the display.

Most HMIs offer many colors, but in addition to the display physical size, users should make sure to compare the resolution in pixels, which determines how detailed the graphics can be.

For some applications—particularly involving data entry—users would want to use an on-board physical keypad or an external keyboard and mouse. However, most designers are following the lead of mobile devices and looking for touchscreens.

It is important to note many consumer touch devices tend to use capacitive multi-touch. This technology usually won’t sense gloved fingers and can be affected by water, but it does have the capability to interpret multiple concurrent touches or gestures. On the other hand, industrial HMIs often employ resistive single-touch technology. Resistive touchscreens can be engaged with gloved fingers and only respond properly to one touch at a time, which often is preferred for industrial applications.

Don’t forget the memory

In addition to physical size, HMI offerings often are grouped with regards to memory and processor performance. They are rated for communicating a certain number of software tags and average displays. Careful designers will storyboard the anticipated graphics and do some rough estimates of tag counts to ensure they are within limits.

HMIs with expandable memory or multiple memory sizes may help to remove constraints. It’s also important to remember advanced features such as on-board calculations and trending and recipes will impact HMI performance, so it’s best to use conservative estimates.

It has become common for HMIs to support one or more SD Card slots so users can install their own removable storage for data logging. This feature is a must for any historizing and trending data applications.

Connectivity concerns

Connectivity is also a key factor for choosing an HMI. Most commonly, users will want an Ethernet port so the HMI can communicate on the programmable logic controller (PLC) network. However, in some applications, it may still be desirable to use serial RS232C or RS422/485 connections. Serial connectors are most commonly 15-pin D-sub type, but they also can be RJ-12 or even terminal block style. It is imperative the PLC connection requirements be defined prior to finalizing an HMI selection.

USB ports come in two versions: type A and type B. Type A ports (like on PCs) let users add accessories like a keyboard, mouse, memory stick, barcode scanner or other compatible interface devices. Type B (often found on printers) allows users to connect an upload/download programming cable from the PC directly to the HMI without impacting the Ethernet network.

Another feature that is becoming a must-have is an HDMI output port. An HDMI connection can drive a second off-board monitor such as a large-format TV located in the plant or facility. An operator can work locally on the HMI display while other operators can see the larger display from a distance.

More esoteric connections such as “audio line out” or “microphone in” may be needed for very specific applications. To ensure the necessary HMI connectivity is obtained, it is recommended designers list all required connections and perhaps even sketch a system architecture diagram.

Make a software checklist

Selecting an HMI with the right software features requires detailed investigation. HMI software should be evaluated in three areas:

  • Configuration environment (and simulation if applicable)
  • Runtime features
  • Remote access.

The HMI configuration environment is where programmers will spend much of their time (Figure 3). It is important for end users to find out the licensing requirements, or if the software is free. Typical users will be interested in the software’s ease of use and features such as drag-and-drop editing, clicks to create an object and if the most-used objects are predefined. Programmers will want to know if the software has good import/export support and database tools, offers efficient ways to replicate and duplicate work and if a built-in simulation or testing environment is included so users can test the project proof-of-concept and check the programming without actual hardware.

Programmers use configuration software to create tags that communicate with PLCs and other devices, to develop graphics linked to those tags, and to configure other built-in HMI features. Here are the 10 most important runtime features users should consider:

  • Basic objects: For drawing static graphics like pipelines and equipment
  • Static library objects: Examples include tanks, vessels and silos
  • Bitmaps capability: Allows users to import their own images
  • Animated objects: Buttons, number displays and bargraphs
  • Preconfigured library objects: More complex objects like a proportional-integral-derivative (PID) faceplate
  • Advanced objects: Examples include trending, recipe and alarm displays
  • Security: Ability to define users and access groups and assign privileges to objects
  • Language switching: Useful for manufacturers that sell equipment internationally
  • Alarming: To indicate discrete and analog alarm conditions
  • Data logging: For supporting on-board trending of off-board analysis.

Even though an HMI seems to offer the right features on a specification sheet, users need to realize the actual implementation is not always as expected. For users requiring a specific look and functionality, the best plan is to storyboard a few types of HMI configurations, make a list of necessary features, and obtain a version of the configuration software to test it out.

Advanced applications may call for investigating very specific HMI details before choosing a model. How well an HMI handles floating point tags and any associated math or calculations can be a concern. Logic usually is executed within PLCs; information is displayed on HMIs.

However, some HMIs offer on-board logic, trigger evaluation, action capability and event manager features. These often are used to make more elaborate displays, but they may be involved as part of the machine logic. When sophisticated features are needed, it is well worth the effort to pre-test each of them.

Extending an HMI’s reach

Some of the most exciting developments with HMIs in recent years have been with regards to remote access options. Today’s HMIs often include on-board web servers, which allow local displays to be viewed on any external devices capable of running a web browser such as a laptop, smartphone or tablet.

Embedded file transfer protocol (FTP) servers make it easy for developers to remotely update and backup HMI configurations. Finally, some HMIs can initiate outbound emails to remotely notify users of production status or warnings.

These remote access functions can extend equipment automation capabilities in new ways and may be a deciding factor when choosing an HMI.

Getting started

HMIs are an important addition for many industrial automation projects. Hardware and software features deserve close attention by designers as they select an HMI. Other commercial considerations include hardware and software pricing, warranty duration and if free support is available. For applications with complicated or custom needs, obtaining a test sample of the HMI hardware and software is recommended to prove it will meet the project needs. When the software includes a simulation mode available at low or no cost, it is far easier for end users to test-drive an HMI to ensure it meets their needs.

Bill Dehner, technical marketing manager, AutomationDirect. Edited by Chris Vavra, production editor, Control Engineering, CFE Media,


Keywords: Human-machine interface, HMI, mobile devices

Touchscreen human-machine interfaces (HMIs) represent a valuable addition to many machine and process automation applications.

Factors such as indoor and outdoor conditions, memory and physical display size should be considered when choosing an HMI.

Hardware and software features deserve special attention when choosing an HMI.

Consider this

When you choose an HMI, what is the key factor in your decision?

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About the author

Bill Dehner has spent the majority of his 14-year engineering career designing and installing industrial control systems for the oil & gas, power and package handling industries. He holds a bachelor’s degree in Electrical Engineering with an Associate’s in Avionics from the USAF. He is currently working for AutomationDirect as a technical marketing engineer.

Author Bio: Bill Dehner has spent the majority of his 14-year engineering career designing and installing industrial control systems for the oil and gas, power and package handling industries. He has a BSEE with an associate degree in avionics from the USAF and is currently working for AutomationDirect as a technical marketing engineer.