Since the days of the ancient Egyptians,
paper has
been the medium of choice for capturing data. Today, paper –
in the form of notebooks, maps, and photographs, to name a
few – is being rapidly supplanted by digital technology. The
computer has moved from the office into the field.
While rugged mobile computers were once used primarily by
military personnel and focused applications in the commercial
and industrial markets, today’s rugged computer is for people
who perform normal job functions, such as cleaning, maintenance
and inspections, in demanding environments.
The growing market for mobile computers is characterized
by a degree of confusion about the meaning of the term rugged.
There are some specific ratings, such as ingress protection (IP)
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. Specifically, these factors are: Protection
against water, dust, shock, vibration and temperature extremes.
Environmental Factors
Today’s tablet computers are built on lessons learned, often in
military environments, and applied to the design, manufacture
and service of commercial products. Critical areas where these
lessons led to environmental design improvements include keeping
out water and dust, blunting the effects of high and low temperatures,
and protecting against the harmful effects of shock
and vibration.
Dust and Water
Dust is a term used to define small particulate matter. Non-organic
matter can clog connections during mating of connectors
to port, or decrease the port’s useful life. Further, 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. If openings are not present to “back out” the
moisture, it collects in trapped areas and condenses, for example, on the LCD screen.
Thus, dust and water are two of a computer’s
worst enemies. 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
that specify the environmental
protection provided by an
electronic enclosure.
The IP rating normally has
two numbers, indicating the levels
of protection that the enclosure
provides against the
ingress of solid foreign objects,
including dust, and the
protection it provides equipment
inside against the
ingress of water.
These ratings are applied
to electronic enclosures
of all types,
including those intended
for continued submersion.
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.
Ingress Protection Ratings
First digit (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 degrees from the
vertical.
3 - Protected against direct sprays of water up to 60 degrees 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. In addition, overall thermal design establishes
a “cold boot” lower limit and maximum limit for hot
operation.
The ability of a computer to operate at high and low temperatures
also depends in part on the selection of materials
and how they relate to other materials in environments that
can cause differential expansion and contraction.
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.
Batteries function by chemical reaction to release stored potential
energy as electrical power. 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.
The spindles of rotating mass storage devices use lubricants
that are sensitive to temperature, again due to changes
in viscosity. At low temperatures, the viscosity increases,
requiring more energy from the motor to spin. At extremely
low temperatures, the drive can freeze up and stop spinning.
At extremely high temperatures the viscosity is so low that
using the drive increases the mechanical wear and thereby
reduces useful life.
Designs that leverage fans for heat convection 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, where heat-generating components
are connected directly to the housing via a path presenting
the least thermal resistance, has proven superior to an
internal fan.
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. While most
applications are typified by random vibration,
in some cases, such as onboard a ship,
there is an overlay of a strong periodic vibrating component
induced by engines.
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 to move the device’s natural
frequencies outside of excitation ranges. At the tablet
PC level, vehicle vibrations must be considered not to excite
internal natural frequencies leading to excessive wear.
Tablet spinning media is also highly sensitive to vibration
and shock. Isolation techniques are used to absorb shock
and vibration energy so the drive head does not crash leading
to loss of data. 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.
Shock testing involved repeatedly dropping the unit from
a height of 48 inches. Vibration testing involves attaching
the unit to a test fixture that represents actual conditions,
such as a vehicle being operated at a certain speed over a
rough road. Transducers measure and monitor vibration
throughout the test. When the prescribed time has elapsed
the computer is examined for failure, wear, looseness or
other changes attributed to vibration.
The development of next-generation rugged tablet computers
demonstrates how lessons learned through years of
experience with military rugged computers translate into design
improvements. The demands of today’s mobile worker
are such that it is no longer possible to wrap a hardened
cover around a regular computer and expect it to function in
the field.
Today’s rugged mobile computers are designed from the
inside out, and apply proven technology to deliver reliable
performance. ❑
Bill Glusing
is vice-president for Advanced Programs of
DRS Tactical Systems, a provider of ultra rugged, commercial-
off-the-shelf computers & laptops. For more, go to
www.drs-ts.com.
