Ruggedisation of Commercial Electronics for Defence Applications

One of the challenges facing the defence industry today is the adoption of state of the art technologies in harsh environments where equipment is expected to remain operational over periods of decades.

Background

Prior to the 1990’s, military customers were primary sponsors of technology development. Systems were designed in-house around proprietary interfaces and utilised military grade electronic components which could tolerate the extreme environmental conditions to which this equipment would be subjected.

With the acceleration of technology development in the civilian sector and defence budgets starting to experience limitations, it was becoming too costly for the defence sector to remain at the cutting edge of technology with development completely at its own expense. This prompted US Secretary of Defense William Perry in 1994 to challenge US defence procurement to consider Commercial Off-The-Shelf (COTS) products in order to benefit from the latest technologies while taking advantage of the reduced costs offered by the large volume commercial products. An additional benefit to be gained from using COTS components, products and technologies would be the reduction in development time and non-recurring engineering costs.

As the US alone contributes almost half of the world’s defence budget, this change in policy has had a ripple effect to the rest of the world. Nowadays, military systems often comprise a number of COTS products from different vendors which are relatively easily integrated as the products conform to standard form factors and open interface standards. Indeed the concept of open standards has proven its effectiveness with the continued success of the PC architecture, which has seen the PC evolve continuously over three decades with numerous technologies becoming incorporated into its design along the way. Military systems can now also allow for “technology insertion” upgrades, where parts of systems can be replaced by newer technologies to offer new functions, to improve performance or simply to replace products that have become obsolete.

Requirements of Military Systems

Systems that are intended for military use will typically be expected to comply with a fairly comprehensive list of specifications. Before a design is approved for “Production Readiness”, a prototype system will undergo exhaustive qualification testing to prove compliance.

Equipment is expected to remain operational over wide ambient temperature ranges. As a minimum this could be -40°C to +85°C, but this could be as much as -55°C to +125°C. In addition, the effects of extended exposure to the sun must be considered, not only for the effect of sudden changes of temperature, but also the effect of ultraviolet radiation on materials and paint.

Systems will be tested to ensure that they can survive the extreme vibrations experienced not only during normal operational use, but also during transportation. The levels tested are usually very severe and can result in larger components detaching themselves from circuit board assemblies unless sufficient care is taken during design and manufacture to provide additional mechanical support. Similar design principles apply in order to prevent damage from mechanical shock. Qualification testing for shock often not only involves subjecting the equipment to 40g shock in both directions in three axes, but also dropping the equipment onto concrete several times from a height of one meter.

As much military equipment is used in close proximity to radio and radar equipment, particular attention must be placed on electromagnetic compatibility (EMC) aspects. Radiated and conducted emissions are measured over a wide spectral range to ensure that they will not interfere with other equipment. The equipment is also subjected to radiated and conducted signals over a wide range of frequencies and monitored for ill-effects.

Equipment must also be shown that it can withstand the environmental effects of humidity, rain, salt, fog, sand, dust, wind and hail. In some cases it must also be tested to show that it can survive being underwater and still remain operational. Surfaces must be shown to be resistant to the effects of contamination by fluids such as aviation fuel, diesel fuel and others. Equipment must also be shown not to support fungal growth.

Figure 1: Equipment Before and After a Dust Test

Defence purchasers also expect the reliability and maintainability of the product to meet specified values. Reliability is quantified by Mean Time Between Failures (MTBF) measured in hours (usually tens or hundreds of thousands) and maintainability is quantified by Mean Time To Repair (MTTR).

There are many other non-functional requirements which are not mentioned here, but this article will be focussing particularly on the aspects just discussed.

Characteristics of Commercial Grade Products

Commercial grade electronic components, in particular integrated circuits, are typically rated for temperatures in the range 0°C to +40°C. These components are sometimes available in industrial grade (typically -20°C to +70°C) but military grade components (-40°C to +85°C) have generally not been available for quite some time.

Fortunately, with the increasing number of electronics and computer applications in the automotive industry, many components are now available in automotive grade, which for underhood applications require an operating ambient temperature range of -40°C to +125°C.

Few commercial computer systems and electronic circuit board assemblies will withstand the levels of shock and vibration specified for the military environment. Generally problems arise with non-latching connectors, poor quality edge connectors, unstable circuit module assemblies and components not securely attached to circuit boards. Common standard interfaces for PCs nowadays utilise PS2 and USB connectors for keyboards and screens, by way of example. These are non-latching and may loosen on shock unlike the D-type connectors used on serial interfaces which could be secured with screws. The standard edge connectors specified by the PCI expansion bus interface and its successors is a very lost cost solution appropriate for the competitive desktop PC peripheral market, but are notoriously unreliable and wholly unsuitable for portable equipment. The PCI cards themselves are poorly mechanically supported by a single screw at the top of its rear bracket which leaves the other top corner of the card entirely unsecured. Even the electronic components themselves are less securely attached to circuit boards with mounting technologies moving away from through-hole mounting components to surface mount devices.

Commercial products are often poorly engineered with regard to EMC characteristics. Some countries are stricter with radio interference policies than others (Europe requires a CE marking on products, for example), but generally the standards prescribed by organisations such as IEC are far less stringent than the corresponding military standard.

Many other attributes of commercial equipment are simply never tested or characterised. For example it would be extremely unlikely for a component datasheet to indicate whether a device is immune to aviation fuel spills or fungal attack. This is not to say that the product is not compliant in these regards, but it is difficult for a designer to determine compliance as the material composition of the product is information that the manufacturer does not disclose. Similarly, obtaining reliability figures from manufacturers can often prove to be impossible.

A last very significant characteristic of commercial products is that they are generally obsoleted within two to five years because the highly competitive market drives manufacturers to continuously replace their products with newer ones. This is typically the same time frame that it takes to design and test a complex integrated military system. This means that components that are designed into a system are no longer available by the time the system is ready for manufacture.

Overcoming the Limitations

The preceding section might indicate that commercial electronics products are wholly unsuitable for use in military systems. Unmodified and on their own, this would be true. However it is possible to construct military standard compliant systems from commercial products by selecting the right products and paying attention to enclosure design.

System complexity and volume requirements will largely dictate whether the system will be a COTS bus-based system, a COTS single-board computer or a custom circuit board integrating COTS components. The latter choice, which is probably most appropriate for simpler and physically smaller systems, will allow the greatest design flexibility and control but will also negate most of the benefits afforded by a true COTS system. However, the design guidelines presented here are generally applicable to all of these options.

Withstanding wide ambient temperature ranges requires careful choice of products. Computer motherboards and peripherals which are designed for desktop PCs simply will not work. Good industrial and embedded products will provide options for various degrees of ruggedisation. Certain form factors (such as VME, cPCI, PMC, etc.) allow for conduction-cooling which provides highly efficient thermal management. This can be further improved with the use of a Peltier heat pump. But even without conduction-cooling it is possible to create an adequate environment by designing or choosing an enclosure with good thermal design.

Figure 2: Typical Ruggedised Conduction-Cooled Enclosure

Whether you are using a single-board computer or bus-based system, make sure circuit boards are adequately supported mechanically, especially where heavier components are situated. All circuit boards and devices should be secured by at least four mounting points. Consider using shock mounts in your enclosure design to provide additional protection to circuit boards. Try to avoid using devices that have moving parts such as hard drives as they will generally not withstand shock and vibration. The effects of non-operational vibration can also be mitigated by providing protective transport and field handling containers for equipment.

Also check that the heavier components are securely mounted to their circuit boards. In some cases bought-out modules may need some attention by applying some glue (“staking”) to larger components to provide additional support. If one is modifying bought-out circuit boards in this way then it may also pay to have them conformal coated at the same time. This will give the circuit board and its components an extra layer of protection.

Figure 3: Enclosure with Military Grade D38999 Connectors

It is extremely important to choose computer modules with good connectors. Always use latching or screw down connectors internally. External connectors should be of excellent quality as this will affect system compliance in many ways (shock, vibration, water ingress, dust ingress, etc.). Although expensive, D38999 circular connectors remain the connector of choice for military use. Dust caps should also be fitted to external connectors.

Pay attention to the EMC characteristics of your equipment and enclosure. Shielding between your power supply and the rest of the system can help prevent internal interference. Make sure your chassis is designed to minimise emissions by using a minimal number of separate panels and EMC gaskets where these join. Connectors should also be chosen with EMC in mind. Power supplies should have adequate filtering to prevent unwanted voltage spikes and noise entering or leaving the system on the power lines.

Where data is required on products that do not appear in the datasheets (such as MTBF figures), it is prudent to request the additional information from the manufacturer before purchasing the item. If it is impossible to obtain the information from the manufacturer, and in some instances the manufacturers are not prepared to invest the effort to provide the information if the order quantities are small, it may be necessary to perform the tests oneself. Examples where this might be appropriate would be to measure electromagnetic radiation or the effects of spilt fluids on external surfaces.

Where there are problems with meeting reliability and availability requirements, redundancy can be utilised. This is particularly useful for power supplies which could otherwise be a problem single point of failure item.

Obsolescence of commercial products and components will always be a problem which will require constant management. This can be alleviated by choosing manufacturers that offer managed future upgrade paths for their products. If the system is designed to incorporate open standards throughout, then new generation parts can be inserted with a minimal amount of re-engineering.

By following these guidelines, one can indeed design and manufacture systems that comply with military standards using COTS products without the cost, extended development times and risk associated with a completely custom design effort.

- Alastair Knight

 
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