Finding HMI Performance for the Long Run

By Linda Htay, IDEC Corporation

Equipment operators and maintenance personnel rely on robust control systems, and a key method for interacting with these systems is through human-machine interfaces (HMIs). Users must be able to visualize the available information, and issue selections and commands as required. Therefore, most HMIs have matured to provide a comprehensive suite of display and control objects that interface easily with underlying control platforms.

This operational functionality is crucial not just in the first few weeks of deployment, but for the lifetime of the equipment, which can be years if not decades. Any HMI that degrades over time, can’t withstand changing environmental conditions, fails outright or is difficult to maintain hinders important operations.

While it is true that an HMI needs the right technical capabilities, it is equally important for designers to examine how robust an HMI will be over the long run. This article looks at some of the quality, durability and environmental features designers should place high on their requirements list.

HMI Needs Versus Wants

When industrial automation professionals set out to specify an HMI, they typically develop a list of requirements. By necessity, mandatory “needs” include things like operating voltage, communication protocols, basic visualization functions and suitability for the expected environmental conditions. In addition, there may also be a secondary list of “wants”, including features such as ease of use, availability of handy apps and other attributes making it easier to integrate the HMI into the automation and higher-level computing systems.

Beyond these, system designers also want good overall value. This isn’t solely determined the day the HMI is first purchased, but it is instead realized over the lifetime of the device based on how well and how long it performs. Up front, designers want to minimize the effort required to deploy an HMI. Down the road, an HMI must remain reliably functional for operators, and if maintenance is needed the HMI should be easily and economically capable of being diagnosed, and then repaired or replaced, by technical personnel.

Therefore, designers considering an HMI platform need to keep a near-term focus of features and configurability, while maintaining a long-term view of reliable functionality and supportability. These two viewpoints can intersect when users select an HMI with the right programming and physical characteristics to perform on the first day of deployment, and to survive field conditions for years to come.

It Can Be Tough Out There

Because HMIs have an externally accessible touchscreen regularly used by operators, they are by nature exposed to the environment more than other automation system devices which can be better protected within enclosures. Additional care must therefore be taken with HMI selection as compared to other types of industrial automation components.

The two most common environmental factors are moisture and temperature. Ratings for IP and NEMA address the former, and rated temperature ranges define the latter. A third consideration concerns more specialized installations in hazardous areas. Users will not usually bench test HMIs to these extremes, but they will select HMIs to meet the known environmental conditions at the installation site.

End users commonly look for environmental ratings of IP66F, IP67F and NEMA 4X/12/13 to meet the needs of outdoor or washdown area installations (Figure 1). UL 61010, ULC and CE approval ratings are often necessary when installing HMIs into inspected equipment or systems. An HMI family that offers all these ratings for the product line is convenient for end users to avoid the need for picking through various makes and models to find the right approvals and specifications.

Figure 1:  Industrial automation devices used in many industries, such as food and beverage, must withstand harsh environmental conditions caused by temperature, moisture and airborne contaminants.

Many HMIs are rated for operating temperatures in the range of 0°C to +50°C, adequate for many applications. However, there are a select few HMIs able to operate at -20°C to +60°C. This expanded temperature range might enable use of the HMI in a remote oil pumping station, for example, without additional enclosures or heating provisions. The greater an HMI temperature range, the better chance that end users can consolidate models for consistency.

A similar concept applies for HMIs rated for use in Class I Division 2 hazardous locations. End users may not need this for most of their installations, but when the rating is required it is far better for the already familiar HMI family to carry it instead of needing to research other products.

In general terms, choosing an HMI family with ratings exceeding expected environmental conditions would tend to deliver a longer operating life and more installation options. Yet over-specifying can cost an unjustifiable amount, so designers must strike a balance. This task is made easier for designers when HMI products come standard with industry-leading ratings, in a family lineup offering a range of sizes.

Quality You Can Touch

Display size is just one key factor of the end user experience with an HMI. Operators and maintenance personnel will interact with and define the performance of an HMI by touchscreen responsiveness, good object appearance and usability and how bright the screen remains, even years after initial installation (Figure 2).

Figure 2: Process industry conditions often challenge the ability of HMIs to deliver a high quality operator experience.

Keep in mind that end users experience varying perceptions of an HMI and may be affected by wearing personal protective equipment, so designers may need to compensate on the high side of performance for the best result. Besides selecting the largest HMI display that is practical and economical, designers should dive into the details of the display resolution, color rendering and brightness. Many contemporary displays range from 640x480 to 1024x768 pixels depending on size, with a sufficiently vivid palette of many tens of thousands of colors or more (Figure 3).

Figure 3: This family of IDEC High Performance HMIs deliver visualization and control features, but also emphasize brightness and durability for operation over the long haul.

However, brightness is often the real distinguishing factor for an end user. A typical HMI can often only deliver 400 cd/m² or less, which might be barely adequate. However, HMIs specifically engineered to deliver up to 800 cd/m² are far easier to see, especially in challenging bright environments. Furthermore, even if the brightness degrades over time, it will likely remain above a useful level.

Direct sunlight is probably the toughest high-glare challenge. An exceedingly bright display will help, but for outdoor sun-exposed locations users should consider installing the HMI in a more protected shelter, or at least provide a sun shield for best results. These efforts will dramatically increase HMI life and usability.

HMI Durability

Over a normal HMI service life ranging from many years up to over a decade, durability is another important factor. From an operational standpoint, the ability of a display to continue operating at a good brightness can often be the most common and visible problem spot of a poorly specified HMI.

In many cases the backlight is the leading weak link, so it’s crucial to examine the backlight’s rated lifetime. Contemporary backlights tend to be integrated into the HMI and are not replaceable, so a failed backlight is solved by replacing an entire unit.

Each year has just under 9,000 hours in it, and many HMIs used for industrial applications would be expected to survive for a 10-year lifetime. Basic math and adding some margin equate to a desired backlight life rating of about 100,000 hours. This longevity is available but uncommon as many HMIs present far lower lifetimes and are therefore somewhat consumable.

Another leading indicator of HMI endurance is the manufacturer warranty. Many industrial automation electronics only offer one-year warranties, but HMI manufacturers offering two- or three-year warranties inspire better end-user confidence.

Recovering from Trouble

Sooner or later, any HMI might fail in service. At that point, the ability to quickly and economically recover is crucial. These time and financial costs are quantified not only in the material and labor to directly fix a problem, but also in expenses for operations due to downtime resulting in lost production.

The ability for maintenance to quickly remedy the situation is paramount. Today’s compact HMIs are usually replaced as a whole unit, although they may offer certain add-on modules that can be replaced independently. It is always prudent for an end user to maintain spares, but for an additional level of insurance users should look for an HMI vendor that tends to engineer newer models as direct replacements for legacy lines.

After physical replacement, it is usually necessary for technicians to configure certain aspects such as the IP address, and to then download the proper configuration. Best practice may be to pre-configure as much of this as possible since HMIs are generally easier to configure on a benchtop as opposed to out in the field. Furthermore, field configuration has traditionally meant using a PC with specialized software and cables. Assembling these items and potentially connecting them in the field can consume precious time.

A modern alternative is when HMIs are available with connectivity and removeable memory options, combined with corresponding mobile applications (Figure 4).

Figure 4: If there ever is a problem with an HMI, mobile help can help get a new unit back in service quickly with minimal disruption.

Therefore a mobile app can make the difference by reducing a potentially lengthy service into a quick event. It shortens service time by enabling users to connect to the HMI via Wi-Fi once it is installed, and then letting them use the app to trigger a configuration download. The mobile app can also offer other extended data viewing and diagnostic options once the HMI is running normally.

Reliability in Action

HMIs are considered a necessity for many types of equipment automation today, especially since end users are so familiar with visualization using their computers, tablets and phones. Many industrial HMI devices are available, in large part because of the proliferation of consumer display devices.

However, industrial applications are physically demanding, must have good operator usability in adverse conditions and should be easy to maintain in the relatively rare event they fail. Designers should therefore emphasize environmental ratings, display brightness and software connectivity options to find the HMI delivering the best functionality and value for their application.

About the Author

Linda Htay is a Product Marketing Specialist at IDEC Corporation, responsible for touchscreens and display products. She has experience working with automation and industrial products such as HMIs, PLCs and more. Linda holds a BS degree in Electrical Engineering from San Jose State University.

Figures all courtesy of IDEC

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