1. OCULUS - ASR M ICHIGAN T ECHNOLOGICAL U NIVERSITY Andrew Mauragis Joshua Ehlert Lars Ramseyer SHOT II P RE- L AUNCH J UNE 11, 2010

2. SHOT II M ISSION O VERVIEW: Background: Magnetic torquer components were run in a vacuum chamber, and the H-bridges controlling current to the magnetic torquers rapidly approached their thermal maximum. Purpose: To test the function of Oculus-ASR’s magnetic torquer system, specifically the H-bridges on the power board, for proper function in the hostile ambient temperature and low pressure of a near-space environment.

3. C ONNECTION TO O CULUS- ASR M ISSION -Magnetic Torquers – “Magtorquers” -Function -Uses onboard earth magnetic field model and magnetometers to generate a counteracting field. -Generated magnetic dipole and Earth’s magnetic field create torque. -3-axis control -Relevance to Mission -De-tumble – Right satellite after launch -De-saturate – Allow reaction wheels to slow from max rotation -Power, Communication, Imaging, ASR Current iteration of Oculus-ASR magnetic torquer >>

4. SHOT II D ESIGN & T EST D ETAILS: -Pulse-Width Modulation (PWM) -Convenient method for digital/analog conversion -H-bridges programmed to 100% duty cycle -H-bridge function -Change current direction (reverse magnetic field of magnetic torquer) -Current amplification for use in magnetic torquers -Electrically identical LR model magnetic torquer, -8 Ω Resistor in series with 33 mH inductor -Inductor affects PWM switching characteristics, produces realistic loading on the H-bridges -Thermistors -Experimental: Mounted close to H-bridge, logs temperature of H-bridge -Control: Thermally isolated on far side of board, logs environmental temperature

5. SHOT II D ESIGN & T EST D ETAILS: -Sparkfun Logomatic v1 Board converts analog thermistor current to ASCII -Equations for ASCII  Temperature Conversion -ASCII output (N) to voltage (V): V = (N*3.3)/1000 -Voltage (V) to Resistance (R): R = 1300*(5-V)/V -Resistance (R) to Temperature (T °C) T = -24.45*ln(R/1000) + 84.718 -RC circuit low-pass filter applied to reduce fluctuations in dataset -Fully compliant with SHOT II User Guide -Mass – 750 g -Flight string attachment

6. E XPECTED R ESULTS: -H-bridge temperature increased rapidly during vacuum tests. This mission will test the thermal properties of the H-bridge in low pressure combined with low temperatures (-40 °C).

7. D EMONSTRATION: -Demonstration: Test functionality of temperature data collection from H- bridge. -H-bridge subjected to a wide range of temperatures (heat gun and aerosol can) -ASCII data stored on SD card -ASCII data translated to temperature in MATLAB program -Graphs display temperature vs. time -Payload Inspection -Questions? Magtorquer board layout with thermistors attached >>