EE Times Under the Hood - June 23, 2008 - (Page U8)

UTH0623pg4-10.qxd 6/13/08 5:45 PM Page 8 under the hood: FEATURE Main controller board Switch controller box Kenwood amateur radio TH-K2 Dc/dc converter/EMI filter Heat sink MCP9800 SPI temperature sensor MAX3232 RS-232 MCP6022 Op amp Level shifter 4-KHz low-pass filter PIC18F8722 Microcontroller SST25LF080A 8-Mbit SPI serial flash MC14541 (x3) Programmable timer implement three interlocks to prevent the transmitter from sending out messages before a 30-minute timeout, thereby giving the cosmonauts time to reenter the space station after pushing away the SuitSat. To keep from exposing the astronauts to toxic fumes, all the components had to be built from materials that could pass a thermal-vacuum test without outgassing. But that wasn’t the only selection constraint: Tantalum and ceramic capacitors were chosen instead of electrolytic, since electrolytics are basically a can and are also potential leakers. All mounting hardware had to be stainless steel, and no lock washers could be used, because they produce microscopic metal shavings that could short out fine-pitch SMT parts. Due to the vibration associated with takeoff, all wires were laced together and glued down. All bare-metal joints were covered with RTV to prevent any debris from shorting connections, and all bolts were glued to prevent loosening. Even the glue itself had to be specially approved by NASA. One of the more interesting considerations when preparing for a vacuum is the Fermi effect, whereby operation at lower oxygen pressure can cause arcing or a corona effect at voltages of around 40 V. To prevent this hazard, Bible said, the enclosures had extra holes to ensure rapid venting to vacuum. While ionizing radiation and its associated bit-flips and latchups were somewhat of a concern, the low-earth orbit at about 400 to 500 km, and the short life expectancy of the satellite, gave this problem low priority. In the end, it proved to not be an issue. Thermal, power management Thermal management did become an issue, however. While convection or forced-air cooling can be used on earth, the vacuum of space meant the designers had to rely on blackbody radiation to keep the system cool. To maximize heat 8 Electronic Engineering Times, TechOnline | June 23, 2008

Table of Contents Feed for the Digital Edition of Under the Hood - June 23, 2008

Under the Hood - June 23, 2008 Extreme Design: SuitSat Pushes Desigers' Limits Evolution of the Smart Phone Mature Devices get Rolly Rocking GPS: Garmin Nuvi 750 vs. HP iPaq 310 Inside the Sony OLED TV Multizone Dgital Audio Flip Ultra Camcorder - An Ode to Clean Design Robot Guitar Tunes Itself E-book is a Sight for Sore Eyes Scientific Calculator Boils Down to Two ICs $100 BOM Eludes First OLPC Laptop 45 nm: What Intel Didn't Tell You Next Step in NAND Flash Evolution Surveillance on a Shoestring Hot 3G Phone Owes Debt to Analog SecurID Fob: Single-Chip Safety Net

Under the Hood - June 23, 2008

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