Modular approach helps specify design criteria for rugged enclosures

Paul Boughton

Pressures to reduce development time, costs and project risks have generated major interest in commercial off-the-shelf (COTS) approaches for defence applications. In place of defence-rated components and sub-assemblies, the search is on to achieve the same reliability and compliance using commercial equipment.

To survive military environments, a commercial electronics payload requires a rugged enclosure that has been designed not only to withstand harsh environments itself, but also to protect its payload from the military requirements of vibration, shock, heat, EMC etc.

By absorbing or blocking the defence environment, the enclosure presents internal electronics with virtually a commercial environment, while the sub-system delivers defence reliability levels.

The variety of defence systems has traditionally forced compromises between standard products, or the costs and risks of a custom solution. This article is aimed at providing an overview of some of the issues to consider when specifying or evaluating rugged enclosures for your next project. It also shows how a modular system can meet multiple requirements by using configuration customisation rather than custom design.

Shock and vibration

Each deployment requires different solutions, since the predominant vibration can be random (such as a land-based vehicle over rough terrain) or regular (such as a helicopter rotor). The equipment’s natural frequency is that which causes the maximum displacement, so the fundamental goal is to isolate the equipment from excitation at that frequency using isolation systems to shift the natural frequency as far away from the excitation frequency as possible.

The natural frequency for simple spring mass systems can be calculated from the mass and the spring rate of the system, but a good isolator has two components: a spring to support the load; and a damper to dissipate the incoming energy. Damping reduces the ratio of output response to input excitation (transmissibility), with 20 per cent transmissibility meaning that the damper is absorbing 80 per cent of the excitation.

Isolator technologies

The main isolator technologies offer different characteristics, suiting them to different applications:
Air springs suit low frequency applications of just a few Hz.
Wire rope isolators are ideal for applications with both shock and vibration requirements since they offer large deflections compared to their size, are unaffected by temperature extremes and resist solvents, chemicals, ozone etc.
Elastomeric isolators offer economical solutions when only vibration isolation is required.

The wire rope is very attractive for isolating a card cage with circuit cards, backplanes, disk drives and power supplies from both vibration and shock, since the stranded construction provides damping as the strands rub together. By varying size, shape, construction and number of coils it is possible to adjust the damping and spring rate, while orientation and use in compression or tension provide further flexibility. However, all isolation methods require the enclosure’s external frame or shell to be of sufficiently rigid construction to withstand the expected shocks without buckling or distorting. In addition, all mating parts must be bolted or welded so as to provide the structural integrity to withstand vibration.

Traditional problems with structural integrity of joints have favoured custom-designed welded frameworks, but advanced manufacturing is enabling vendors to offer strength with a modular approach. The 12R2 enclosure from Elma is an example of how combining extrusion, spot-welding and a special mating flange offers a range of ruggedised side plate sizes and extrusions that deliver structural integrity with modularity.

Pre-characterised subassemblies can further simplify the customised enclosure specification for individual projects, for example with simplified calculations of displacement (sway space) in all three axes.

Electromagnetic compatibility

It is essential to consider both susceptibility to external Electromagnetic Interference (EMI), and the emission of EMI, for both radiated and conducted EMI. The design essentials of enclosure Electromagnetic Compatibility (EMC) are:

  • Suppression to reduce EMI at its source.
  • Isolation of offending circuits by filtering, grounding and shielding.
  • Desensitisation by increasing the immunity of any susceptible circuits.

Typical isolation approaches include enhancing the shielding, eg providing continuous, conductive ground contact gaskets for all removable panels. However, each gasket has its own attenuation, RF impedance and corrosion control compatibility performance, so target specifications must be considered during the selection process. Extrusions can be designed with a locating channel for a braided wire and elastomer gasket, with closely spaced screws to ensure enclosure shielding effectiveness and structural integrity. Note that grounding a panel at one point will prevent it from becoming an antenna, but will not eliminate leakage from the seam, as gaps can act as RF apertures, potentially reducing the attenuation from 80–100 dB to as little as 20 dB.

Line filters can provide a high degree of attenuation to common mode and differential mode conducted EMI, while air inlet and exhaust openings based on a honeycomb of parallel waveguides can offer up to 97 per cent open area, and up to 110 dB of radiated EMI attenuation.

Suppression approaches should include selecting high-grade power supplies, eg to FCC part 15 class B, combined with proper grounding techniques and sufficient ground points in convenient locations. Similarly, all components should be vetted for EMI generation relative to MIL-STD-461D, especially through-panel mounted items such as fans, indicator LEDs, and switches.

Cable runs, I/O connectors and cable assemblies also need care, ensuring effective shielding to external cables, and separating primary ac power and dc wiring inside the enclosure, with all ground wires of minimum length.

Thermal management

As available space reduces and power density increases, thermal management becomes further complicated by the need for protection from harsh environments. Dust and air filtering, together with honeycomb EMC filters effectively reduce intake and exhaust openings.

Forced-air cooling requires the selection of suitable fan air-flow rates to dissipate the heat generation of the working system, where airflow in CFM, heat to be dissipated in Watts and the allowable temperature rise in ¢ªC is determined under free air conditions.

Since the payload electronics will provide obstructions to airflow, it is important to select a fan designed to provide high airflow into high static pressures for optimal performance even under hostile operating conditions. Manufacturers can provide curves of flow against static loading, as well as providing fan life data for the expected operating temperature range.

Once the designed airflow is sufficient, consider the path of the air, and ensure that it is directed over the heat-generating components, by planning inlet and exhaust location, and by providing baffles to guide the airflow. Otherwise local hot-spots can reduce lifetime significantly. Removable and washable dust and air filters not only protect the internal electronics from dust build-up that can reduce cooling effectiveness, but also help preserve the operating life of the fan(s).

Fans with tachometer or fan failure output can initiate preventative maintenance warnings, or safe system shutdown before permanent damage occurs to the payload, especially when combined with temperature sensors in the exhaust airflow. In addition to force cooling, new heat transfer technology is enabling a new generation of high performance conduction cooled enclosures, but space prevents discussing details here.
Internal cables must minimise obstructions to airflows, as obstructions shield components and add to the static air pressure, making cable management, and cable bend radius important considerations. A patch-panel approach to mounting connectors on the rear panel lends flexibility and ease of maintenance, especially if human issues such as connector spacing and access are considered, and adequate shielding is provided for external connections.

Reliability and maintainability

Mean time between failures (MTBF) and mean time to repair (MTTR) together define reliability, with overall MTBF being a function of the MTBF of each component, making it vital that ALL elements have good MTBF, backed up either by empirical or analytical data. MIL-HDBK-217F, for example, specifies the calculations to convert component MTBF to equipment MTBF for ‘ground benign’, ‘ground fixed’ or ‘naval sheltered’ environments at a specified temperature.

Careful planning of location and mounting of key components such as power supply, fan, disk drive and air filter can all reduce MTTR by simplifying access to core sub-assemblies, not forgetting that plug-in cards should also be easy to replace.

By considering design requirements (MIL-STD-461D) and test methods (eg MIL-STD-462D) with isolation technologies, it is practical to develop a rugged enclosure that not only survives defence requirements, but ensures that its electronics payload effectively sees only a commercial environment. This is ideal for maximising COTS content.
Further, advanced technology and innovative design can reduce traditional custom development to a faster, cheaper approach based on customising the configuration of a versatile modular system.

Martin Blake is managing director of ELMA in the UK. For more information,