Designing COTS LCD Displays For Rugged Applications
- Thursday, 01 March 2012
Commercial electronics and LCD displays typically operate at temperatures above freezing and below 70°C (known as the clearing temperature). At temperatures greater than the specified clearing temperature, liquid crystal can go from the nematic state to an isotropic state where the display “clears” or turns black. As discussed previously, protection from environmental hazards can adversely affect the ability of the enclosure to transfer heat out of the display. For instance, a large LCD drawing 80 Watts within a sealed enclosure could have a thermal resistance of 0.6°C/Watt resulting in an internal rise of 48°C. Added to a 23°C room temperature, the display is running at its high extreme of 71°C before the ambient environment rises above room temperature.
In addition to isotropic display clearing, long-term reliability is adversely affected by running panels at or near the clearing temperature. Depending on the application, enclosures can be de signed to circulate air through filtered and louvered vents. This can prevent dust and water ingress while providing a cooling mechanism capable of keeping the panel within specified operating temperatures.
The transition of display backlights from CCFL to LED has also helped reduce the amount of energy in a panel, which has been a great benefit to thermal management. Displays that are used in direct sunlight, however, have to deal with solar gain which can add as much as 1000W /m2 to the problem on a sunny, cloudless day at high noon. The amount absorbed depends on the enclosure’s material and color, but typically blocking IR films or a laminar flow of air over the display are used to prevent the display from “blacking out”. In sub freezing environments, such as outdoor, or non temperature controlled areas, supplemental heaters may be required to prevent slow response of the LCDs due to low temperature.
Shock and Vibration
The deployment of large screen LCDs in control rooms, ships, industrial areas, or public venues requires consideration of tampering, vibration, and shock. It is important to understand the nature of the vibration or shock in magnitude and frequency to which the screen may be subjected. Sources can be motors, conveyors, engines, propeller blades or even seismic events. In many industries, there are published standards, which represent shock and vibration experienced by the display in both transit and operation. Some of the component considerations when designing a display requiring ruggedization are listed in Table 3.
Touch, gesture, and motion sensing has added an additional dimension to displays used for mission critical applications, allowing the user to interact with the graphical display of data without the use of a standard keyboard and mouse. The most popular method is the touch screen but there is an emergence of new touchless technologies using cameras and intelligent vision, which should be viable for use in the future.
There are several touchscreen technologies available, each having its own set of strengths and weaknesses. It is important to understand the end use and user to choose the best solutions. For instance, using an infrared touch screen in an outdoor location at night can attract insects which can actually cause false touches if they land on the screen and break the IR light beam. Other touch screen technologies such as capacitive are sensitive to metal enclosures making them difficult choices for very rugged applications. Some of the more popular technologies and their strengths and weaknesses are listed in Table 4.