The theaters of operation for marine, air and land vehicles continue to redefine the term “rugged environment.” No longer bound by on-board human operators, autonomous and unmanned military vehicles are now free to operate within unparalleled temperature, vibration and shock regimes. The limitation on survivability of such vehicles is defined primarily by the robustness of their internal electronics. To meet these challenges, suppliers of hardened electronic systems must employ various ruggedization techniques.
To appreciate ruggedization innovations moving forward, it is important to note some prior advances. In the early 90s, the IEEE organization standardized the conduction-cooled Eurocard, which was then applied to the VME-bus standard. The concept of the mechanically-interchangeable conduction-cooled card continued to be utilized and applied to newer backplane bus standards including CompactPCI (cPCI) and VPX. But the mechanical interchangeability standard did more than ensure proper fit of COTS circuit cards within a chassis; it provided a valuable boundary separation between the chassis and the plug-in circuit card. This eventually led to the establishment of ruggedization levels — boundary conditions that allow the decoupling of ruggedization efforts required for the chassis and the circuit cards. Given the card-to-chassis boundary conditions, the enclosure designer is given the minimum performance that the chassis must deliver for the circuit cards, and the circuit card designer is given the minimum environment within which the circuit card must survive.
Given that the boundary between the chassis and circuit cards is well defined, from a vibration ruggedization perspective it is necessary to maintain that separation with respect to mutual interaction of the enclosure and the circuit cards housed within it. With respect to shock and vibration ruggedization, this ultimately means that the chassis should not amplify the input shock and vibration levels at frequencies critical to the enclosed circuit card assemblies.
The solution to this is what is often referred to as the 1-octave rule, which defines that, for the chassis, the natural frequency of the mode with greatest mass participation be no closer than one octave from the natural frequency of any internal circuit card. Considering that a typical ruggedized 6U VME circuit card assembly resonates between 200Hz and 450Hz, and a 3U card can resonate at frequencies greater than 1000Hz, the stiffening required of the chassis assembly to satisfy this rule is substantial.
Stiffening ribs and other stiffening geometries are typically machined or cast into the walls of the chassis to reduce deflection while reducing weight, and ultimately increase the enclosure’s resonant frequency. For enclosures that are assembled from separate machined walls, dip brazing can be used to join the chassis walls in an almost seamless fashion. However, in cases where intricate chassis wall geometry does not permit the use of dip brazing because of the possibility of salt entrapment and potential latent corrosion, conductive epoxy is used at the wall-to-wall interfaces to provide a sturdy, permanent assembly. Vacuum brazing can also be utilized to create laminated honeycomb-like wall assemblies, which are then bonded together using a variety of readily available adhesives.
In addition to stiffening geometries and construction methods, the use of high strength, lightweight materials is necessary to reduce the inertial response of the chassis walls to the vibration imparted on the enclosure from the vehicle. This is also beneficial in reducing overall enclosure weight as part of a larger supplier initiative to maintain size, weight and power (SWaP).
However, weight reduction must not ultimately affect the chassis’ ability to transfer and discard heat from the circuit cards. Regardless of the overall heat rejection mechanism used to expel heat from the enclosure, the chassis must rely on thermal conduction to move heat from the circuit cards.
In general, maximizing conduction requires added material volume or optimization of thermal conductivity through careful selection of materials. For this reason, aluminum alloys are generally regarded as the material of choice for ruggedized enclosures, having a weight three times lighter than copper and a thermal conductivity 20 times that of titanium alloys. This allows for increased wall thicknesses and improved thermal conductivity without significant weight increases. Additional advances in blended composites are being developed to provide enhanced thermal and structural properties within a single amalgamated raw material that will ultimately provide thermal conductivity greater than copper with a weight less than that of aluminum.
Chassis Heat Rejection Methods
Added to the careful juggling act of managing weight, thermal conductivity and stiffness is the need to ultimately expel heat from the chassis, either to the ambient environment or to some other supplied cooling media. Natural convection is always a desirable choice for the vehicle integrator as it provides passive cooling with no additional cooling systems needed from the vehicle. To allow for natural convection, cooling fins or pin fin arrangements are added to the external machined or cast surfaces of the chassis walls. These added fins increase the exposed surface area of the chassis which allows for a lower temperature rise from ambient air to the chassis for a given amount of heat. Careful placement and orientation of the fins is required to allow for greater thermal convection in areas of high heat concentration.
Noting that natural convection is driven by air density differential and thus inherently gravity-dependent, fins must be oriented such that convective air flows between the fin patterns in a vertical direction. For this reason it is essential to know beforehand the intended orientation of the ruggedized electronic system within the vehicle. Ultimately, the fin length and fin population densities are driven by the allowable temperature rise from ambient air to the chassis, as well as the ambient temperature that the electronic system will be subjected to, and the proximity of other obstructions mounted around the electronic system within the vehicle.