Solid HMI Connections, Sealed Edges Prove Tough Enough

The old saying about kitchens and heat applies, with a twist, to the process industries and rugged human machine interfaces (HMIs). Increasingly, these intermediaries between people and equipment can take the heat—not to mention the humidity and the vibration. They can do so thanks to advances in HMI design, manufacturing and technology.

By Hank Hogan for Control Engineering July 1, 2008

The old saying about kitchens and heat applies, with a twist, to the process industries and rugged human machine interfaces (HMIs). Increasingly, these intermediaries between people and equipment can take the heat—not to mention the humidity and the vibration. They can do so thanks to advances in HMI design, manufacturing and technology.

For one example of these innovations at work, consider mobile HMIs. David Krebs, director of the mobile and wireless practice at the technology analysis firm VDC, says that today mobile HMIs represent only a small part of the overall market. However, such innovations as bright but readable displays and the proliferation of wireless networks in manufacturing are changing that.

“The market for mobile HMI terminals is expected to increase at a double-digit clip over the next five years,” he says.

He notes that such devices must, at a minimum, meet IP54 standards and thus be dust protected and not harmed by splashing water. However, he adds that minimum is often not enough and greater protection is required for an HMI.

Small things, big effects

Ensuring the survival of an HMI, portable or otherwise, in a harsh environment starts with the basics, says Ed Brown, technical manager at the Tyngsboro, MA-based Monitouch HMI. Monitouch makes a variety of HMIs and runs them using a proprietary operating system. The company’s units are found in tire manufacturing plants, tunnel boring equipment, water jet textile processing, and street cars.

One of the characteristics of such harsh environments is vibration and temperature swings, with the latter requiring an HMI operating temperature range of 0 to 50 °C. The combination of temperature and vibration can lead to device failure unless care is taken.

“The real key, as far as operating in these rugged environments goes, is at the board level,” says Brown. “You tighten down the board. You make sure all the connections are solid connections. You make sure there are no loose wires.”

Wiring can be problematic even if the ends are firmly fixed. A free-to-move wire running along a board edge can be cut by repeated vibrations that rub it against the board. Correcting such little things leads to a long HMI life in a harsh environment. This knowledge, notes Brown, can only come with experience in producing rugged HMIs. At the same time, such practices can benefit equipment intended for less stressful settings.

The job of producing a rugged HMI can be made easier thanks to technological advances driven by the larger market. A case in point is solid state disk drives, says David Gardner, product manager for industrial computers, device integration, SCADA/alarm, and control at Wonderware of Lake Forest, CA. The company makes a rugged HMI tablet PC with a mobile Intel Pentium CPU that runs Microsoft Windows XP.

Gardner notes the impact of Flash memory in particular. “The price of the devices has been dropping and the technology has been spreading, too.”

The cost should continue to fall, dropping by half every few years if historic trends in semiconductor memory pricing hold true. Those price cuts, in turn, reduce the cost of implementing a solid state drive and make it less expensive to eliminate rotating media. Since rotating media is a common cause for failure in a harsh environment, getting rid of it makes an HMI more rugged.

Cheaper and larger solid state drives make it easier to do without a hard disk drive. Another added benefit is that a solid state drive looks like a standard drive to the operating system and thus requires no real change to the OS or applications.

The use of solid state storage pays other dividends. Rugged HMIs often operate in dusty, high humidity, or immersive environments. In such situations, a sealed unit is essential, therefore, eliminating the heat produced by a rotating drive is critical.

Sealing up a system also helps a rugged HMI meet other key requirements, notes David Kaley, product marketing manager for Omron Electronics of Schaumburg, IL. Rugged HMIs have to provide some moisture resistance, typically meeting such ratings as IP65. That requirement reflects conditions encountered by these units. “Hose down applications are typically what are being protected against. IP65 protects against low pressure, high volume streams of water,” notes Kaley.

Meeting such a requirement also impacts the choice of materials used in an HMI, says Eric Lai, product manager for the Cincinnati, OH-based Advantech Industrial Automation Group. He notes that systems intended for the harshest environments use anti-corrosive material like aluminum or stainless steel for the bezel, with the chassis often built with a rust-resistant coating.

Keep in touch

Even though an HMI may be sealed against the environment, it still has to handle its main mission: interfacing between people and machines. That requires input and output.

With regard to input, the two choices are touchscreens and keypads or keyboards, with the first category by far the most popular. Kerry Sparks, field marketing specialist for OI logic products at Eaton Corp. of Cleveland, OH, puts the ratio between touchscreens and keypads at five or six to one. Part of the reason for this is that touchscreens can be program specific and change as needed to support an interface.

The standard for an input device is for it to be rated for at least a million activations, with some specified for 10 or 50 times that number. Unfortunately, such specs don’t tell the whole story, since they often don’t detail how much oil or dirt can be on the device.

One solution is to make the touchscreen essentially a throw away by using a Mylar overlay, says Sparks. “You can protect the screen with these overlays and just peel them off when they get scratched.”

Those who need a keypad or even a full keyboard have a number of technological advances that promise to make life easier. Barentec of Clifton, N.J., touts its piezo module-based input technology. Diane Bersen, vice president of sales for Barantec Everswitch, points out that the company’s products offer more than 50 million activations and a temperature range of -40 to 125 °C. She notes that the devices have been installed in semiconductor clean rooms, oil fields, and chicken processing plants.

Bersen is aware of the move within the industry toward touchscreens but thinks other trends make tightly sealed input devices like those made by her company more favored. “With an increasing focus on food processing and drug safety, HMI devices will need to withstand greater washdown and sterilization processes,” she says.

The trend is toward fully-sealed, waterproof and dustproof keyboards, says Joel East, director of business development and technical design at iKey Industrial Peripherals of Austin, TX. The company makes products that can be used in explosive or hazardous environments. Often, this involves putting the computer or HMI in a sealed enclosure, with only the keyboard and a pointing device in the hazardous area.

Barriers between the input device and the HMI protect against potential problems, as does the design of the keyboard itself. “If the input devices are damaged, such as if the device or cables get cut or smashed, they are guaranteed not to spark and ignite the explosive atmosphere,” says East.

When it comes to output, HMIs run into some challenges. Liquid crystal displays (LCDs) are by far the most common output technology. The advantages of an LCD include the ability to do full color and low power consumption, which can be very important in a battery operated, lightweight, and fanless unit.

However, LCDs leave something to be desired when used in a harsh environment. For example, because they work by transmitting a backlight through the liquid crystal material, they don’t do well in bright sunlight. The problem can be overcome, but doing so runs up against other constraints.

“To design a sunlight-readable display requires boosting the current level to the display, which generates heat,” says Harold Muma, product marketing manager for human machine interfaces at Siemens Energy and Automation of Alpharetta, GA.

Another issue with LCDs is the temperature over which they can operate. The low end, in particular, can be a problem, says Tom Miller, executive director of the San Jose, CA-based Society for Information Display. One of the most common backlights used is a cold cathode fluorescent lamp, which can be difficult to start at temperatures approaching zero. On top of that, the liquid crystal material becomes unusable at just below that point.

One solution is to use heaters or even the waste heat from the electronics of a system. A longer term answer might be the use of a different display technology. One that holds promise is organic light emitting diodes, or OLEDs. Because they’re emissive, they don’t require a backlight and therefore don’t draw much as much power as an LCD. OLEDs might be able provide color over the full industrial temperature range, but Miller cautions that this possibility is based on preliminary and experimental results. However, the new display technology will soon be available in a high volume and demanding application: “You will see OLED displays in automotive dashboards very quickly,” says Miller.

Vendors aren’t waiting for the new technology but are instead taking steps to correct some LCD deficiencies. Rugged industrial computers are employing solid state LED backlights because they are inherently more robust and use less power than fluorescent backlights. They also allow systems to crank up the power for high brightness without running into heat constraints. Examples of this trend can be found in products from Industrial Computing of Waltham, MA, and DRS Tactical Systems of Melbourne, FL.

Ethernet connectivity

There is another form of output to consider. Like everything else on the factory floor, HMIs, even rugged ones, need to have Ethernet connectivity so that data can move easily into and out of the device.

That need to network is one reason cited for use of a Windows OS instead of a potentially more robust proprietary operating system. With Windows, many chores related to connectivity are handled by the OS and do not have to be developed, which would be the case with a proprietary OS. What’s more, with Windows, things like manuals can be put into standard electronic documents in a centralized location. Tools, including Web browsers and reader applications, make it easy for a user of a rugged HMI to navigate to and read such documents without a problem.

“The trend has been moving away from the proprietary packages to using more of an off-the-shelf package,” says Craig Resnick, a research director at the ARC Advisory Group analyst firm.

This doesn’t mean that rugged HMIs are running the latest version of Windows. Doing so might place a substantial hardware burden on the device, with more cooling and a beefed up battery required. Instead, vendors are opting to go with a lighter real-time OS, like Windows CE or Windows XP, or even XP embedded.

The other big trend involves wireless. This connects the HMI to the machines being interfaced to and does so without having to provide an opening in a sealed enclosure. It also makes a portable device truly mobile.

However, the deployment of wireless and other technologies is somewhat slower in the process industries than elsewhere, says Siemens’ Muma. This is partly due to safety and reliability concerns. But these issues won’t keep out these new technologies forever, he predicts: “The clear trend is to adopt these once the technology is proven.”

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Author Information
Hank Hogan is a contributing writer for Control Engineering.