Sizing Up Ruggedized Computers
March 26, 2009 The computer has moved from the office into the field. Today, people who perform normal job functions in demanding environments use rugged mobile computers that guard against water and dust, operate in high and low temperatures, and protect against shock and vibration. However, meeting these standards is not a "yes/no" or "pass/fail" proposition. Computers are designed with different tolerances. Understanding the working environment and how computers handle these conditions is a key part of selecting the correct unit.
Dust and Water
Dust and water are two of a computer’s worst enemies. Non-organic matter can clog connections. Organic matter such as mud can lead to fungal growth or the retention of moisture, leading to corrosion that could take months before intermittent operation occurs. Water, in the smallest amounts, turns to vapor upon operation of electronic equipment and thereby migrates throughout the entire unit. In order to prevent or minimize dust and water ingression, a computer’s external housing must be designed and manufactured to very tight tolerances, and all gaps must be carefully sealed.
Rugged computers are often ranked by ingress protection ratings which specify the environmental protection provided by an electronic enclosure. The IP rating normally has two numbers (see table), indicating the levels of protection against water and dust. Within the category of rugged computers, the highest levels of dust protection (5 or 6) are desired; a water protection level of 6 is considered to be water resistant.
| First digit (dust/solid objects) | ||
|---|---|---|
| 0 | No protection. | |
| 1 | Protected against solid objects up to 50mm. | |
| 2 | Protected against solid objects up to 12mm, e.g. fingers. | |
| 3 | Protected against solid objects over 2.5mm (tools and wires). | |
| 4 | Protected against solid objects over 1mm (tools, wire, and small wires). | |
| 5 | Protected against dust limited ingress (no harmful deposit). | |
| 6 | Totally protected against dust. | |
| Second digit (water) | ||
| 0 | No protection. | |
| 1 | Protected against vertically falling drops of water (condensation). | |
| 2 | Protected against direct sprays of water up to 15° from the vertical. | |
| 3 | Protected against direct sprays of water up to 60° from the vertical. | |
| 4 | Protected against water sprayed from all directions. | |
| 5 | Protected against low pressure jets of water from all directions. | |
| 6 | Protected against powerful jets of water. | |
| 7 | Protected against effects of immersion between 15cm and 1m for 30 min. | |
| 8 | Protected against long periods of immersion under pressure. | |
High and Low Temperatures
Mobile devices are subjected to a much broader range of temperatures than are office computers. Sound thermal design practices not only address electronic components, but also the liquid crystal display (LCD), battery, and spinning storage media.
Designs that use fans to remove heat run the risk of single point failures, may become clogged, or are difficult to clean and/or decontaminate. A computer designed for use in harsh environments uses mechanical design to transfer heat away from internal components, such as chip sets, to the housing where heat is dissipated into the ambient air. This "heat pipe" approach has proven superior to an internal fan.
The liquid crystal chemistry of LCDs is also affected by temperature. At low temperatures, the viscosity of the liquid increases, and if inappropriate vendor specification and/or design techniques are used, the display updates are slow, leading to "ghosting" of images. At high temperatures, the viscosity decreases to almost that of water, resulting in "brownout” of the screen.
A battery that is fully charged provides its stated capacity at room temperature. However, this capacity will decrease as temperatures rise or fall to extreme operational limits. Rugged designs leverage trades between weight and total available battery capacity, selection of battery chemistries, thermal design, and power management to optimize user performance over broad temperature ranges.
Shock and Vibration
Mobile devices are subjected to a broad range of intentional and non-intentional usage environments not present with office equipment. Non-intentional shock includes dropping and sliding off surfaces, while intentional shocks occur through operations such as a vehicle backing up to a loading dock, a rail car engaging, or a tow truck winch operating. Vibration, on the other hand, is specific to the vehicle or stationary equipment on which the device is mounted.
Careful attention to material properties such as malleability (too malleable leads to wear) and fragility (such as glass shatter) are required. All components, boards, brackets, and cables need to have rigid mounting. Spinning media are isolated so the drive head does not crash. For shock, materials that deform and then relax at a slow rate are used. For vibration, materials that absorb the vibration energy and dissipate as heat are used.
Conclusion
The growing market for rugged computers is characterized by a degree of confusion about the meaning of the term rugged. There are some specific ratings, such as ingress protection that may be used to gauge specific performance criteria. And there are categories like "semi-rugged," rugged" and "fully rugged" that are often used, but are not easily defined. Thus, users must evaluate levels of protection in light of the specific environment in which the computer will be deployed. For more information visit the DRS Tactical Systems web site.
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