|We designed a general purpose 1 ATR VME avionics chassis as part of an avionics upgrade for the Cobra helicopter. The chassis employs aluminum and high thermal conductivity epoxy graphite core 6U VME modules with integral wedge locks and a general purpose 64x backplane with the standard VME signal complement embedded in PC. The backplane also utilizes wire wrap connectors for implementing the I/O wiring between the front panel and the various modules. Rigid flexible circuitry with controlled impedance brings all standard I/O wiring to the backplane.A full complement of modules in the chassis dissipates 450 watts, requiring forced air heat exchangers in the side walls of the chassis and two high performance fans in a custom fan tray. The heat exchanger is designed with a barrier to allow diminished cooling with one fan failure. Unit is required to operate at 15,000 feet and at 40 degrees C, and with a 30 minute 71 degree C hot start.
Chassis design is a “screw and glue” approach with the vacuum brazed side wall heat exchangers functioning as major structural elements and providing mounting hard points to the tray. Side walls are attached to front and rear panels with internal EMI gaskets and conductive structural adhesive at the interface to maintain 90dB shielding integrity for the 3000v/meter EMI requirement.
The helicopter application requires broadband low frequency random spectra with sinusoidal and gunfire peaks superimposed on top, requiring a very stiff chassis to prevent unwanted coupling to the PC modules. Remainder of the environment is full MIL spec, i.e. salt spray, human factors, etc.
Chassis and tray is designed, documented and fabricated within tight eight month development schedule.