Flat Panels: Thin Is In
They're everywhere! In your living room, at the ATM, on your desk, and most especially as part of your automation and control system. Taking form most commonly as LCDs (liquid crystal displays), flat panels have finally made it to the big time, replacing with ever-increasing frequency those bulky, heavy CRTs (cathode ray tubes) with sleek and slender units that boast many benefits beyond being ...
They're everywhere! In your living room, at the ATM, on your desk, and most especially as part of your automation and control system. Taking form most commonly as LCDs (liquid crystal displays), flat panels have finally made it to the big time, replacing with ever-increasing frequency those bulky, heavy CRTs (cathode ray tubes) with sleek and slender units that boast many benefits beyond being fashionably thin.
LCDs have been around for many years. In fact, the phenomenon of liquid crystals was identified more than 100 years ago. Not until the 1970s, however, did the technology develop to the point of being applied to display products. And only in the past decade have these products begun to proliferate, as quality increases and mass production drives down costs and makes them an attractive choice for both new and replacement human-machine-interface (HMI) applications.
In the 1990s, flat panel computers and monitors were very expensive, observes Bobby Dixon, strategic accounts manager for GE Fanuc Embedded Systems. "We had to explain what an LCD was, why its features were better than a CRT. Now it's the de facto standard. It used to be difficult to interface an LCD with your electronics. There was little standardization in terms of connectors, types of interface signals, etc. Today, there's more quality and more consistency."
Flat panel LCDs offer several benefits over CRT technology. They are lightweight, offer higher resolution, and use about a third of the power of CRT deplays. Touchscreen capabilities are easily incorporated due to the flat nature of digital displays. "Viewing text on a flat panel display is the closest thing you'll achieve to actually reading printed text on a piece of paper," observes Jim Muta, engineering manager for Samsung.
Flat panels come in a variety of shapes, sizes, and types. At this time, the most common appears to be the TFT (thin-film transistor), or active matrix LCD using twisted nematic (TN) or super-twisted nematic (STN) technology. Other types include IPS (in-plane switching), MVA (multiple domain vertical alignment), and PVA (pattern multiple domain vertical alignment).
"Each sub-pixel in an LCD is capable of turning on or off ," explains Muta, "to provide clarity, extremely high contrast, and improved sharpness/focus compared to a CRT. Those features are the driving factors of the flat panel. They substantially improve front-of-screen performance over the CRT. The only area in which CRTs are still superior to flat panels is with motion, in video, and that's because of their very fast response time."
LCD front-of-screen performance, or how well a device displays an image, embraces the four primary characteristics—brightness, contrast, viewing angle, and response time—that distinguish one LCD from another. "The demand for front-of-screen performance is becoming more and more of an issue," says Muta. "Manufacturers are striving for optimum performance in all four areas. Until now, that hadn't been achieved. There were always tradeoffs. Now all the best attributes of these four factors are finally coming together in a single product."
Simply, flat panels are easier to see. Explains Dale Maunu, director of channel sales and customer support for Optrex America, "It is easier to control ambient light with an LCD. A traditional CRT is actually spherical. In terms of the optics, if you watch a CRT-based television, a light bulb in the room will be reflected back to the viewer perfectly in the TV screen. Because of its spherical shape, there is always some angle at which the light will be reflected back to the eye. If the glass were truly flat, you'd have to be at a very specific angle for that to happen. The amount of reflection in a flat panel is much less. A viewer, or operator, can more easily find a comfortable place to view the screen."
Light and lighter
Transflective backlighting uses both a backlight and surrounding light to display an image. Portable devices such as color-display cellular phones and PDAs are typically transflective. (Illustration courtesy of GE Fanuc Embedded Systems.)
Front-of-screen performance characteristics are not the only factors influencing the growing popularity of flat panel displays. Progress in backlighting has also been significant. Early flat panels had very short backlight life. Today's technology offers backlights that last 50,000 hours or more. In addition, backlight replacement kits are typically available and can be retrofit into a display.
"Brightness and readability as well as lamp life can be a problem with displays," says Brian M. Spahnie, mechanical/development engineer for Lumitex Inc., electronic business unit. "Lamps burn out too quickly. With older systems, both light source life and power consumption were issues. Today, progress is being made on creating brighter displays and displays that are easier to read in the sunlight."
Several technologies are available for backlighting an LCD. Common methods include molded light pipes, electroluminescence (EL), fiber optics, and cold cathode fluorescent tubes (CCFT). Some panels have no light source. A reflective display has a mirror to capture and use the surrounding light source, as opposed to transmissive displays that use a backlight. Color cell phones and PDAs are typically transreflective , using both a backlight and surrounding light to display an image (see illustration).
Strong backlighting is one way to overcome sunlight and improve readability. Today, coatings added to the front of the display reduce glare, explains GE Fanuc's Dixon. "Films inserted inside the display actually make use of sunlight and reflect it back to the display surface, making the image brighter in the sunlight than it would be inside. These measures increase readability without adding heat-generating, power-consuming backlights."
"What is the longevity of the backlighting?" is a legitimate question to ask when specifying an LCD, suggests Dixon. "Backlighting longevity," he says, "has increased dramatically." Adds Shalli Kumar, president and CEO of AVG/EZAutomation, "There are backlight panels that last 100,000 hours and researchers are still looking for more longevity. In 1995, backlight life was around 5,000 hours. Ten years later, minimum life is up to 40,000 hours, a significant increase. To expect more increases is not unrealistic."
Beyond the panel and backlighting, a flat panel display requires a third element: a method for converting the analog signal from either a PLC or PC to a digital signal that the LCD can understand. "Our controller cards work with just about any flat panel," says Dusty Perryman, director of sales for Digital View, a manufacturer of flat panel controllers. "Controller manufacturers are typically brand-independent and therefore can provide relatively unbiased information about the best way to drive a flat panel."
Advantages of an LCD to an HMI are obvious to Perryman. "A CRT is still somewhat cheaper," he notes, "but the advantages of a flat panel are tremendous. Ability to package an LCD to fit environmental constraints is tremendous." He cautions, however, scaling images to fit a specific panel is totally different with LCD technology than with CRTs, unless steps are taken to accommodate a change. "A 15-in. LCD with a native resolution of 1,024 by 768 pixels must operate within that resolution to maintain that specified clarity," he says. "However, we can equip a panel so that one can input a higher resolution, say 1,600 by 1,200 pixels, than the technology on the circuit board is designed for and scale those pixels to fit into the 1,024 by 768 native mode. Such flexibility extends a product's life span."
'In every nook and cranny'
"You can't mount a CRT on the side of a forklift, but you can put an LCD there," says Digital View's Perryman. His words sum up one of the primary reasons for the increasing application of flat panel displays. Adds Rob Rawlyk, application manager at Beckhoff, "The flat panel has freed the HMI system to go where it hasn't been able to go before. Now we can put a display screen wherever an operator needs one."
That's quite a change from what Tom Holden, president of Strongarm Designs, recalls. His company started out mounting bulky, unwieldy CRTs on columns and beams. Now, "everything is flat panel. It really has opened up the market for us. Our goal is to install a monitor where the customer didn't think it could be installed. We focus on making the most efficient use of space possible. We've had customers say to us: 'Boy, it would be ideal if the display could be right here!' Thanks to flat panels, we now can answer, 'it can be.'"
B+R Industrial Automation started using flat panels because of their space saving features, says John Roberts, the company's distribution sales manager. "Flat panels are a wise move for just about any application. They offer a lot more flexibility and portability. They give off less heat, are low-voltage, and lighter in weight. You can put them on swing arms or make them into mobile panels. You can walk around a machine and see what's going on in every nook and cranny. That wouldn't be possible with a CRT."
Flat panels have promoted portability and mobility, as well as simplifying thin client applications by allowing displays to operate apart from the intelligence. Clients, typically running Microsoft Windows CE operating systems, are networked over Ethernet TCP/IP to a server running the application. PCs can be kept in a clean, secure environment, while a thin client flat panel display on the machine acts as an input/output device.
"The driving force was to get the display as far from the computer as possible," says Beckhoff's Rawlyk. "Controls can be placed in a cabinet and the flat panel display easily mounted on a machine. Or multiple displays may be placed on a machine or along a process line."
A brighter, clearer future
Flat panels have few drawbacks. Their use is already commonplace and continuing to increase. They incorporate the best features of a CRT (fast response time, color uniformity) with the best of the LCD (high clarity, contrast, sharpness). And, the technology is still changing and improving. "We're seeing panel improvements every couple of years or so in all the critical areas: resolution, viewing angle, brightness, response time," says AVG/EZAutomation's Kumar.
The biggest advancements in flat panels have probably been with contrast ratio, in the opinion of GE Fanuc's Dixon. Contrast is a measure of image clarity. Contrast ratio is the measurement of the difference in light intensity between the brightest white and the darkest black. "Brightness levels have stayed around the 300 nits level [a nit measures luminance and equals one candela per meter squared; a candela is a measure of luminous intensity]. Contrast ratios, however, have gone from 100:1 to 500:1, even 600:1. This produces a crisper picture."
"CRTs are reaching the end of their lifespan," in the estimation of Ann Ke, marketing manager for touch panel computers and industrial tablets at Wonderware. "It is easier, and more cost effective, to replace a CRT with a flat panel than it is to fix the old technology. Industrial flat panels are ruggedly built and can last longer on the plant floor. Most withstand up to 50 °C. Colors are more vibrant now; quality of the graphics is so much better."
Further, more types of flat panels are coming. Passive-matrix displays with competitive front-of-screen performance characteristics, including response times, are making their way to market. Plasma screen televisions are already available. "Plasma screens are good for motion," says Samsung's Muta, but adds "They are passive devices with difficulty in linear color tracking. The technology will probably improve, but plasma screens will likely find application only as presentation devices, not as desktop displays."
Still in the experimental stage, OLED (organic light emitting diode) technology is very expensive and not yet practical. But prototypes have already been built, and Muta expects OLED technology to be an area of growth. "OLED displays will have the look and feel of an LCD," he predicts, "but we're a few years away from seeing these devices in the mainstream. You'll see more activity in LCD backlighting improvement before you'll see OLEDs on the market."
Optrex' Maunu agrees that OLED has a long way to go and forecasts a bright outlook for LCDs. "OLED materials are maturing," he observes. "We're seeing 10,000 hour lifetimes now. But there aren't any good manufacturing tools for making these devices, and according to recent estimates there won't be any before 2006 or 2007. The volume isn't there. OLEDs will show up in areas where they make sense, in automotive and some handheld applications. Vendors have some $35 billion invested in LCD TFT technology and production. And LCD technology keeps improving. Performance is getting better and better. Neither vendors nor users are going to walk away from that."
What is an LCD?
Among the most common type of flat panel, the TN TFT LCD is an active matrix technology that provides a clear, sharp image. (Illustration courtesy of Samsung.)
In simplest terms, a liquid crystal display, or LCD, is a light valve. Two sheets of polarizing material sandwich a solution of liquid crystal molecules. An electric current passing through the liquid causes the crystals to align so that light cannot pass through. Manipulating the molecules in this way permits or blocks the passage of light.
There are various types of LCD screens. Among the most common and popular are twisted nematic thin-film transistor (TN-TFT), or active-matrix, technology. Three transistors control every pixel to achieve high-resolution RGB color.
Beyond the screen, every display includes backlighting and a controller. Controller circuit boards convert analog signals from a PLC or PC to a digital signal that the LCD can understand. Backlighting brings the image to the screen surface and may be done in any of a number of ways, among them electroluminescence (EL), molded light pipes, fiber optics, and cold cathode fluorescent tubes (CCFT). Backlighting must be intensely bright, as only a small amount of light—in some cases as little as 6%—finds its way through the panel to the display screen surface. Transmissivity in a typical flat panel is 4% to 8%.