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AE256 Satellite Electrical Power Systems EPS of KUTEsat Pathfinder Leon S. Searl April 5, 2006 University of Kansas KUTEsat Pathfinder EPS • Pathfinder • Pico size satellite, 10cm x 10cm x 10cm cube – < 1kg mass • Communications – Dipole Antenna – Handheld Ham Radio • Attitude Determination and Control – Sun sensors and 3 axis Magnetometer (attitude determination) – 3 axis Electromagnet Torquer coil (attitude coil) • Payload – CCD Camera – Dosimeters University of Kansas Pathfinder Diagram University of Kansas Pathfinder EPS Requirements • Do nothing for 20 minutes after Deployment • No CPU clock allowed • Go into low power wait mode • Power CTDH and Communication system • Allows: – Transmit Morse code ID every few seconds – Listen for known signal from ground station to start full powerup • Full power mode • Allows CTDH to command power to ADCS, and Payloads • Deliver 1.5W maximum power • Emergency Power mode • Automatic power down of all systems when primary power lost and secondary power is low • When primary power available recharge secondary power to minimum for Power Wait Mode University of Kansas Pathfinder EPS Components • Primary Power • Dual solar cells on 5 faces of satellite • Each solar cell delivers 2.05V @ 285mA • Solar cells wire in series – 4.1 volts @ 285 mA – Max Solar Power 4.1V * 0.285 * 1.7 (3 adjacent faces exposed) = 1.98Watts • Each cube face wired in parallel • Secondary Power • Two Lithium-Ion batteries – 4.2V to 3.3V, 1800 mAH • Wired in Parallel • Primary and Secondary power connected by ORed diodes. University of Kansas Pathfinder EPS Components (cont) • Batter Charger • Takes Solar power, converts to 4.2V output to battery – Controls charge current – On/Off controlled by EPS PIC • Power Converters • DC-DC converter takes Solar and Battery and converts to regulated 5V • 5V is converted to 3.3V regulated output by DC-DC converter • 5V is converted to 12V regulated output by DC-DC converter University of Kansas Pathfinder EPS Components • Power Buses • 3.3V and 5V delivered to CTDH continuously – EPS has signal to CTDH to tell it to be powered down for Emergency Power Mode • 3.3V, 12V and 5.5V delivered to Secondary Systems by switch – Powered off by default – Only powered on by command from CTDH • 3.3V, 12V and 5.5V delivered to Secondary Systems by switch – Powered off by default – Only powered on by command from CTDH University of Kansas Pathfinder EPS Power Bus University of Kansas Pathfinder EPS Power Bus Backplane University of Kansas Pathfinder EPS Power Bus Backplane • 50 lines • 19 Power • 31 Signal • Each trace can carry 1.4A • Each pin can carry 1.5A University of Kansas Pathfinder EPS Comonents • Power Management • Power is managed by a micro controller (MicroChip PIC) – Onboard flash and ram – Holds control program – Analog to Digital Converters – Used to measure voltages – General Purpose IO lines – Used to control charger, DC-DC converters and switches • Handles decisions for Emergency Power Mode • Responds to commands from CTDH (SPI interface) • Obtains telemetry and delivers it to CTDH (SPI interface) University of Kansas Pathfinder EPS Components • Telemetry • • • • • • • • • • • Measured by PIC and delivered to CTDH via SPI interface Solar cell voltage combined Solar cell current (for each panel) Battery voltage Battery current Battery temperature 5V DC-DC output voltage 3.3V DC-DC output voltage 12V DC-DC output voltage Current into 5V DC-DC Board temperature University of Kansas Pathfinder EPS Components • Radiation Effects Handling • Single Event Upset – PIC has internal WatchDog timer that resets PIC if it stops running • Single Event Latchup – PIC power runs through a self resetting current limiting switch – When current increases to SEL limit (25mA) the power switch turns off power to PIC then turns it back on • No extra shielding added University of Kansas Pathfinder EPS Components • Heat Handling • Nothing done to distribute waste heat • Heater added to batteries to keep them warm when temperature drops below -10C. University of Kansas Support Slides University of Kansas Radiation • Any component with transistors is susceptible to: • Single Event Upset – Change the binary value from 1 to 0, visa-versa – Can be handled with Error Correction Codes – Can be detected with CRC or Parity – Can often be handled with software • Single Event Latch-up – Can cause very high current draw that destroys transistors – Can be detected with load sensor – Can be remedied with power cycle of hardware effected University of Kansas