The Draper Technology Digest - Draper Laboratory

operations. For a thorough discussion of the other modesalong with additional details of the ADCS, see Pong et al. [4].Three-axis coarse pointing control down to approximately60 arcsec (3σ) is provided by a set of orthogonallymountedreaction wheels. These are contained, along witha set of orthogonal electromagnetic torque coils, insidethe MAI-200 unit, manufactured by Maryland Aerospace,Inc. ExoplanetSat operates in a momentum-biased modeand uses the torque coils to periodically desaturate thereaction wheels. Fine image stabilization at the arcsecondlevel is achieved by actuating the two-axis piezo stage inthe manner outlined above. The star tracker CMOS, withits relatively small pixel pitch, tracks centroid movementson a group of guide stars. These motions are then fed backas part of an inner control loop that moves the piezo stageto compensate for image jitter on the detector that thereaction wheels cannot control.The primary image stabilization sensor for fine star trackingis the smaller, fine-pitch CMOS. Attitude determination isaccomplished using the larger star camera imager and starmatchingalgorithms. The ADCS sensor suite also includesa magnetometer and coarse sun sensors, which are usedtogether for attitude determination during orbit day. Themagnetometer is also used for detumble, and the coarsesun sensors are used to acquire the sun after deployment.The ADCS has been the subject of a large portion of thehardware testing completed to date on the project. Manyof the results are summarized by Pong et al. [4], includingdetailed characterization of the MAI-200 reaction wheelimbalances and guide detector centroid performanceunder flight-like imaging conditions. Figure 7 shows a finepointing hardware in the loop test bed assembled at MIT(a light-tight box has been removed for clarity).On one end of the optical bench is a flight-equivalent lens,plus a fine guidance detector mounted to a two-axis piezostage. This constitutes the spacecraft emulator. On the otherend of the bench sits a perforated back-lit screen mountedto a second two-axis stage. This screen constitutes the starfield emulator and recreates a sky image on the guidancedetector. Coarse pointing errors are precomputed usingmeasured MAI-200 reaction wheel imbalance data. Thesemotion trajectories are injected into the star field emulatorstage, mechanically simulating the expected motion ofthe spacecraft as viewed through the lens. The spacecraftemulator then closes a loop using only centroids from thefine guidance sensor—no a priori knowledge of the injectedmotion trajectory is used. The centroid trajectories withoutand with piezo stage motion compensation are shown inFigure 8. The test provided a successful proof-of-conceptdemonstration of the fine pointing loop. Pointing errorswith the piezo stage were 2.3 arcsec (3σ).Y Position (arcsec)403020100-10-20-30-40-40 -30 -20 -10 0 10 20 30 40X Position (arcsec)40ComputerStar field emulatorY Position (arcsec)3020100-10-20-30Spacecraft emulator-40-40 -30 -20 -10 0 10 20 30 40X Position (arcsec)FIGURE 7. Fine image stabilization hardware-in-the-loop (HWIL)test bed.FIGURE 8. HWIL test bed results showing centroid trajectorieswithout activating the piezoelectric stage (above) and after (below).26 The ExoplanetSat Mission to Detect Transiting Exoplanets with a CubeSat Space Telescope