Flat panel displays (FPDs) are rapidly replacing CRTs as the primary human-machine interface (HMI) in the industrial world because of their thin form factor, light weight, low power consumption, outstanding image quality, and distortion-free operation.  In its recent LCD Monitor Strategy Report, DisplaySearch predicts that on a revenue basis, LCD monitors will grow 353 percent faster than CRTs between 1999 and 2005, and 278 percent faster for the same period on a unit basis.

It’s true that CRTs have one major advantage over FPDs – low cost.  You can get a 17-inch CRT for as little as $200, whereas an LCD of an equivalent size will cost you five or six times as much.  In addition, CRTs have wide viewing angles, provide clear images, and have excellent response times.   

In addition, a CRT typically consumes twice as much power as a comparable-size LCD, and so costs more to operate.  Over time, this can be a very substantial amount for a factory with numerous monitors. More power means more heat and in turn, that means more money must be spent on a system to cool it off, as well.   

Industrial monitors have to be pretty bright to operate in high ambient light conditions, requiring 250 or 350 nits.  (A nit is the metric unit of emitted or total surface luminance, and is expressed in candelas per meter squared [Cd/m2].)  Displays to be used in direct sunlight can require as much as 1200 nits.  CRTs are not capable of that much brightness.  And because the backlight on an LCD can be replaced, a brighter version can be installed so that the display can be seen even in direct sunlight conditions.  That’s not possible on a CRT.

Space is another important consideration.  An LCD is typically one-fourth as thick as a CRT.  In some embedded applications, it would be impossible to use a CRT because the space is simply not available.  In addition, designing or buying a larger enclosure for a CRT adds cost.

A CRT eventually burns out, and when it does, the whole CRT must be replaced at a cost of at least $200.  The  newer LCDs have a backlight whose life is roughly equivalent to that of a CRT.  When it burns out you only have to replace the backlight, at a cost of roughly $25.

LCDs are not affected by magnetic fields, and degaussing is not necessary.  CRTs require bulky and expensive purging systems not needed for LCDs.

Designing for the industrial environment

LCDs designed for the PC and laptop environments are not rugged enough to withstand the harsh conditions found on the factory floor, which include heat, moisture, vibration, shock, electromagnetic fields (EMI), and dust.  In addition, an ordinary LCD usually does not have a wide enough viewing angle so that images are clear from wherever an operator may be standing.  The viewing angle in a standard LCD typically is 50 or 55 degrees.  The industrial LCD has to provide a view angle of as much as 170 degrees. 

Nor are standard LCDs bright enough for high ambient light conditions.  The LCDs in the PC marketplace are typically 150 nits.  As already stated, as much as 350 nits may be needed in an LCD for the industrial market.

Today a wide variety of high-quality color FPDs are available that are designed specifically for industrial environments.  These displays are rugged enough to hold under severe environmental conditions, and have the viewing angles and brightness required.  They can range from five-inch class LCDs to full-size displays of 20 inches or more.

The specific type of FPD you choose for your industrial automation and/or control system depends on a variety of factors, and care must be taken to make choices that fit your particular application.

For example, except for the fact that they are dedicated to a single purpose, embedded systems have acquired so much intelligence, bandwidth, and processing power that they are much like any other computer, including having the capacity for almost universal connectivity and communication.  This added complexity means that the one- or two-line flat panels formerly used to display simple information for embedded devices are no longer adequate.  Taken together, these developments mean that the designer of industrial embedded systems must now make choices not only as to whether or not to incorporated a large screen display, but which technology to employ, and how to do so.

Whether you are designing an embedded or a standard system, the following choices must be considered.

  Types of LCDs  

The two primary types of LCD are active matrix and passive matrix. Although passive matrix displays are much cheaper than active matrix (TFT) and operate in a similar manner, they generally are not a good choice for industrial environments. The images are not as sharp as active matrix images, they do not have as broad a viewing angle, and response time and contrast ratio are not as good. All these are major issues in an industrial environment. 

A variety of viewing modes and backlighting technologies in active matrix LCDs (AMLCDS) are available.  Choices are usually determined by ambient viewing conditions, including viewing angle, brightness, and contrast ratio.

  Reflective LCDs have a mirror or other reflective material adhered to the backside of the panel so that when light is reflected off the surface of the material, the selected pixels are visible. This reflective type of backlighting is best suited for small panels for viewing indoors or outdoors in high ambient light conditions.

Transmissive LCDs usually have cold cathode fluorescent lamps as backlights.  These lamps provide polarized lighting, and need to be complemented with light guides that include polarizers, defusers, and enhancement film to spread the light evenly.  They are suitable for viewing in either well-lit indoor conditions or dark environments.  CCFL backlights offer high brightness, long life, and low cost.  They provide an efficient source of illumination in varying strengths.

CCFL lifetime ratings are expressed as thousands of hours, but that figure refers to when the bulb reaches its half-life mark.  For example, if a bulb is rated at 30,000 hours, the light output will decrease by 50% when it has been operating for that many hours.  Lamp current will affect CCFL longevity. By making adjustments to the supplied lamp current, the life span of a CCFL can be increased considerably.

Luminance and brilliance affect the overall performance of a backlight system as well. Luminance refers to the brightness of the light being emitted by the CCFL bulb.  Changes in chromaticity may become an issue for certain operator display applications where light purity is essential.

Touch screens

Touch screens are increasingly popular for machine and process control, but have their own design issues.  Transmissivity needs to be considered when considering using touch screens. Transmissivity in a touch screen refers to how much light passes through the LCD screen from the backlighting system. The issue is that when a touch screen is added to an LCD, the light emitted from the backlight system has an additional layer of material to pass through, which can reduce light intensity. The designer needs to pay attention to the touch screen’s transmissivity rating.  The higher the transmissivity percentage rating is, the more light being transferred through the panel. Resistive and capacitive touch screens have transmissivity values ranging from 60 to 80 percent, while surface acoustic wave (SAW) and infrared scanning technologies can achieve transmissivity ratings of from 90 to 100 percent. Transmissivity degradation in touch screens can be compensated for by selecting flat panels with higher backlight brightness ratings.

Analog vs. digital LCDs

The designer also must decide whether to choose an analog or a digital display.  To drive a digital LCD from a PC that outputs analog video signals, an interface card is necessary to convert the signal and image quality is degraded in the process.  NEC makes analog LCDs as well as digital ones.  These displays are virtually plug-and-play with PCs that output analog signals because no conversion is necessary, and signal and image quality are preserved.

Eventually all PCs will be capable of outputting digital video signals.  Using a digital LCD with the digital-output PC will make conversion unnecessary and thus will save the cost of designing an interface card, as well as preserve image quality.  Therefore, the type of video signal from the PC should be an important factor in making a decision about the type of LCD to be designed into the system.

Other Design Issues

When extra protection is needed for extremely harsh environments, enclosures must be designed that provide the needed protection without affecting electronic performance.  Care must be taken when buying enclosures, however.  The degree of protection against liquids, dust, and ice varies, and some enclosures offer little protection against dust and moisture.  Whether you design your own enclosure or buy one from a vendor, it’s important to be certain that the enclosure is capable of protecting your display from the worst-case conditions of the environment.

The National Electrical Manufacturers Association (NEMA) and the International Electrotechnical Commission (IEC) publish standards for the design and testing of enclosures for protection of electronic equipment in harsh environments. However, NEMA does not provide any verification testing, nor does it certify conformance.  It’s the designer who must give careful attention to determine the level of protection needed, and to verify that the vendor’s product claims are met. 

Standards for commercial and industrial displays are NEMA 12 and NEMA 4/4X (roughly equal to IEC levels IP55 and IP66, respectively). NEMA 12 covers products that are used primarily indoors and subject to circulating dust and dripping non-corrosive liquids. NEMA 4 is for products that must resist windblown dust and rain, splashing liquids, and the formation of ice on the enclosure. NEMA 4X is basically the same as NEMA 4, but with added protection against corrosion.

The best flat-panel displays and enclosures also provide protection against heat, vibration, and electromagnetic interference (EMI). These conditions are not covered in the NEMA standards, so you will need to ask the manufacturer to provide specifications of heat, vibration and electromagnetic interference (EMI) thresholds to verify that the display you choose has the protection you need.