Comment

01 July 2004

Hostile surroundings bedevil machines

By Diana Bouchard

Measures can improve the odds of the computer's survival.

The electronic components of digital computers operate under certain conditions of temperature, humidity, and cleanliness and by electrical current that meets certain specifications. Failure to respect these conditions may cause components to fail or function erratically.

General-purpose computers are for use in homes and offices, where the environment offers few threats to electronic components. Their design, therefore, provides minimal protection against adverse conditions.

One way to increase the level of protection is to change the design of the computer to incorporate more rugged components. An extreme example of this is the "battle hardening" of computers in military equipment. Water- and dust-tight cases, shock-absorbent mountings, high-temperature electronics, and other such measures can improve the odds of the computer's survival in a hostile environment.

This approach, however, entails a substantial increase in cost, especially because solving some environmental problems may create others. For example, sealing up the outer case of the computer to exclude water, dirt, and harmful vapors will also cut off air circulation, resulting in excessively high temperatures inside unless one adds on large heat sinks and cooling systems.

For this reason, many industrial computer installations use the approach of enclosing unmodified general-purpose computers in a protective room where an environment suitable for the computers is maintainable. However, the latest distributed control systems are for use in most industrial environments.

Note that for programmable logic controllers (PLCs), the situation is somewhat different. PLCs were designed from the outset to function on the plant floor, and their environmental requirements are, therefore, less stringent. This does not mean, however, that PLCs do not need any protection. They are vulnerable to the same environmental threats as general-purpose computers, only somewhat less so.

Dust and dirt

Dust and dirt will quickly form a blanket over all accessible surfaces in a computer. The layer of dirt may prevent physical contacts from closing (as in keyboards) or may clog ventilation and other openings. If the dust is conductive, it may cause short circuits; if it is insulating, it may lead to heat buildup and component failures.

The obvious preventive for dust infiltration is to minimize the number of openings through which dust can enter. This approach works for many PLCs and industrially hardened computers, although it may create heat dissipation problems. The other approach is to filter the air, either at the inlet to the computer room or as it goes in through particular openings in the computer case. This method works well if filter media are appropriate to the contaminants and if one swaps out the filters regularly. Another simple though possibly unpopular measure is to ban smoking in the vicinity of computers.

Chemical vapors

Chemical vapors, if corrosive, can eat away at contacts and conductive traces inside a computer, eventually breaking their electrical continuity. They can also damage data recording surfaces on disks.

The usual defense against chemical vapors is either to make the computer or the control room tight against the vapors in question or to install appropriate filters. It may be necessary to set up the computer room at a distance from sources of chemical vapors to ensure they remain at a sufficiently low concentration.

Water shorts circuit

Water can get into a computer in two ways: by direct splash (installation near process equipment using water or accidental spillage) and by condensation. Obviously, water can cause a catastrophic short circuit inside a computer, and a chronic water problem can also lead to corrosion and failure of components. The operating specifications of most computers include a humidity range within which the computer should function.

Designing an entire computer to be watertight is difficult because of heat dissipation requirements, but watertight keyboards or key pads frequently serve on the plant floor and connect to a computer nearby in a protected room. Keeping coffee cups and other sources of nonprocess liquids at a safe distance from computers is a sensible preventive measure.

Condensation is usually the result of low temperature or extremely high humidity in the ambient air. If the problem is local, one may use a heater to raise the temperature, which lowers the relative humidity for a given air moisture content per unit volume. In an environmentally controlled room, a humidifier or dehumidifier may also be used. An air conditioning system will control both temperature and humidity.

Vibration data loss

The vibration that often accompanies operating process equipment can break contacts and solder joints inside computers and cause malfunctions and data loss in disk drives. The simplest remedy is to isolate the computer from the vibration source, either by moving it away from the vibrating equipment or by mounting it on a shock-absorbent base.

Electrical disturbances

Electronic computer components require a relatively unvarying supply of low-voltage, low-current electrical power. Standard distribution voltages (110–120 volts or greater) must be stepped down to supply these components, usually by a step-down transformer (often called the computer power supply), which outputs the voltage used on the system bus (typically 5 volts). Further voltage reductions may be required for specific components.

Voltage and current transients: An overvoltage or overcurrent condition, if severe or prolonged, will simply burn out electronic components. Less severe transients may register as a spurious change of state, causing erratic program function or corruption of data. With too low an initial voltage, the computer will simply fail to function, because the voltage will be insufficient to raise electronic devices above their threshold values. A severe voltage drop while the computer is in operation may trigger an unplanned restart (warm boot). The most serious condition is fluctuating low voltage, which may cause repeated partial restarts, leaving memory and data in unpredictable states and possibly hanging up the computer altogether. The effects of a complete power loss are actually less drastic, except that the contents of volatile RAM are lost and the contents of a file may become corrupt if a write operation is in progress when the power loss occurs.

Interference: Noise in the form of radio frequency interference (RFI) or electromagnetic interference (EMI) can be harder to track down and correct. Radio frequency interference arises most often from lengths of cable acting as antennae or from mobile communications sources in the plant such as walkie-talkies and lift truck radios. Electromagnetic interference can come from any piece of equipment that generates an electrical or magnetic field (most commonly, motors). Do not neglect to check possible temporary sources of interference; arc welding is a notorious culprit. Note also that computer components such as cables, long communication lines, or disk drive motors may themselves generate interference. Normally, however, computers are required to meet government specifications for RFI and EMI emissions so that they do not interfere with the operation of other electronic equipment.

The effect of electrical interference is similar to that of fast transients from the power line: the generation of spurious signals that the computer mistakes for data. These may cause erratic operation, or in extreme cases, total computer failure.

Power failure: If the continued operation of a computer is critically important, an uninterruptible power supply (UPS) may be a necessary addition to the incoming power line. During normal operation, the line power charges a battery, which, in turn, feeds the power supply and the computer. In case of a blackout, the power stored in the battery can maintain the computer in operation for a limited time, typically one to several hours. If one only needs enough time to copy the contents of memory to disk and shut down the computer in an orderly fashion, twenty minutes to half an hour should be enough; otherwise weigh the cost against the importance of the process and the normal length of power outages. The wattage rating of the UPS should be sufficient to support the memory, display, and disk drives (typically the biggest power consumer).

Static electricity: A static discharge is typically low current but can reach extremely high voltages that can damage computer equipment. Components such as memory chips and integrated circuit boards typically ship in static-proof bags to protect them from accidental discharges during shipment. Once one installs these components in the system, the computer's grounding subsystem protects them. During installation, however, they are vulnerable, and personnel should take appropriate precautions, such as wearing ground straps and static-free clothing and using antistatic mats. Maintaining adequate humidity levels helps to control static electricity.

Heat: Electronic components work within an operating temperature range. Find this range in the specifications. Outside this range, their characteristics change beyond design limits, their behavior becomes erratic, and their life lessens. In extreme cases, a fire hazard may arise.

The usual method of cooling computer components is by convection to the surrounding air, often assisted by a fan that draws air over the components. Other devices such as monitors (screens) have no fans and cool themselves only by convection. For this method to work, the surrounding air has to be substantially cooler than the components, which is one reason computer rooms are often air conditioned. Computers have vents to draw cooling air in and out; they must remain clear even if they seem to provide a nice shelf for manuals, drawings, and bag lunches. IC

Behind the byline

Diana Bouchard has a master's degree in computer science. She is an ISA member and leader as well as a member of TAPPI and IEEE Computer. She recently retired from the process control group at the Pulp and Paper Research Institute of Canada (Paprican) and now works as a statistical data analysis consultant. This piece is an excerpt from Fundamentals of Industrial Control, ISA Press, 2004. Write the author at dianab@aei.ca.