SPHERES MIT Space Systems Laboratory Cambridge, MA 2006-Aug-08 Synchronized Position Hold, Engage, Reorient, Experimental Satellites ISS Test Session 1 Results

SPHERES 2 Outline Test Session Objectives Timeline Summary Setup Results Analysis –Test 1: Quick checkout –Tests 2 & 3: Open-loop rotations using different mixer algorithms –Test 6: Closed-loop tests involving gyroscopes only –Tests 4, 8 & 8.3: Closed-loop tests involving the ultrasonic navigation system Consumables consumption Conclusions Lessons Learned Future Actions Points of Contact Revision History Appendix A. Converted IMU data Appendix B. Tests Operational Results

SPHERES 3 Test Session Objectives Primary objective: ”Hardware Checkout” –Powering the satellite –Establishing communications –Uploading the test session program –Running tests –Collecting data Science objective: demonstrate estimation and control algorithms leading to autonomous docking –Thruster mixers performance - to demonstrate the ability to perform open- loop maneuvers –3DOF quaternion rotations - to initialize the satellite pointing towards its target –Formation flight - to validate closed-loop control with the estimators –Docking - to demonstrate the full docking process

SPHERES 4 Timeline Summary Scheduled start: 15:05GMT (10:05CDT) Thursday 18-May-2006 Actual start: 15:20GMT Hardware locate: ~30m GUI Configuration: ~40m Program plan review & program upload: ~20m First test: ~17:00GMT Tests: 15 total, 1h 12m operating time, 4m 27s avg. time per test

SPHERES 5 Setup Setup for the first test Session took approximately 90 minutes before the first test was executed –30 minutes used to locate all SPHERES hardware The SPHERES beacon tester, previously believed misplaced, was found –40 minutes used to resolve a problem with the SPHERES GUI configuration The configuration file (gui.ini) was saved as “read-only”, which prevented the GUI from starting The SPHERES team found the problem and determined a solution in real-time The crew executed the changes in that specific SSC –20 minutes used to read the test plan and load the satellite The satellite loaded successfully within five minutes

SPHERES 6 Results Analysis Overview The tests ran during this session correspond to the mission objectives as follows: –Hardware Checkout  Test 1: Quick Checkout –Thruster Mixers  Tests 2 & 3: Open-loop rotations using different mixers –3 DOF Rotations  Test 6: Closed-loop rotations using only gyroscopes –Formation Flight, Docking  Tests 4, 8, 8.3: Closed-loop tests using ultrasound navigation system

SPHERES 7 Test 1: Quick Checkout Objectives –Test all thrusters & IMU sensor –Collect data for mass-identification algorithms Quick checkout was run twice, at the start and at the end of the test session Successfully collected necessary data, demonstrated thruster operations, and closed-loop control using the gyroscopes False positive on demonstration of closed-loop rotation control… more on section for Test 6 Initial quick checkout experiment resultsFinal quick checkout experiment results

SPHERES 8 Tests 2 & 3: Open-Loop Rotations Overview Objectives –Allow comparison between a basic mixer algorithm and an advanced mixer algorithm (mixers determine the length of thruster pulses for a requested force/torque) –Basic mixer assumes all thrusters are identical and pressure is constant –Advanced mixer accounts for thruster differences and drop in pressure when multiple thrusters are open Both tests performed a series of three open-loop 180 degree rotations, one about each body axis –Calculated desired torque to obtain a  of rad/s (rotate 180 degrees in 10s) –Commanded that torque to the mixer

SPHERES 9 Test 2: Open-loop Rotations Basic Mixer: Data Test ran two times –During the initial tests the tank was partially inserted –The tank was fully inserted for the second run –No noticeable differences in the resulting , therefore concluded that the first test provided valid data X and Y axis rotations had a smaller  than requested Z axis rotation had a larger  than request –Behavior makes sense physically, as the inertia is smallest about the Z axis –Indicates that the models used for the basic mixer do not estimate the inertia and/or torques appropriately First run of Test 2 Results Second run of Test 2 Results

SPHERES 10 Test 2: Open-loop Rotations Basic Mixer: Plots The data and crew notes show that the X & Y rotations did not complete to 180 degrees; the Z rotation overshot –This can be seen from the larger magnitude of the Z pulses in the plots Some coupling is noticeable on the Y axis during X & Z rotations During the second test the -X pulse (stop rotation) did not stop the rotation completely; reason under investigation Second run of Test 2 ResultsFirst run of Test 2 Results

SPHERES 11 Test 3: Open-loop Rotations Advanced Mixer The mixer commanded larger pulses, as expected The Z axis performance improved noticeably –The start and stop pulses were different due to the differences in thruster strength The X & Y performance did not improve as much as expected

SPHERES 12 Test 6: Closed-Loop Rotations Objective: demonstrate closed-loop attitude control Same 180 degree sequences as for tests 2 & 3 Tests did not complete successfully due to FLASH memory corruption –FLASH was only partially corrupted, therefore checkout test succeeded, but this test failed –Closed-loop path differences between checkout and full tests: The FLASH corruption error was found after the test session using telemetry data made available within six hours of the test session GyroscopeA/D Bias Conversion Factor PlantCommandMixer Checkout path FLASH Gain

SPHERES 13 Tests 4, 8, 8.3: Closed-loop Translations Objectives: –Test the ultrasound navigation system –Demonstrate control algorithms to maneuver the satellite –Demonstrate maneuvers for autonomous docking Closed-loop control was not possible due to the FLASH corruption Ultrasound navigation system provided encouraging results –Range, range rate (velocity), and quaternion (direction) calculated to within desired performance –Convergence within 5s –Range/quaternion curves correlated with motion of beacon or satellite seen in video Results for tests 4 and 8 (including corrupted rotation rates)

SPHERES 14 Consumables Consumption Efficient battery usage: –0h 13mSetup and program load –1h 12mRunning Tests Tank usage was not as efficient: –15%Running Tests –39%Inadvertent tank vents Tank vent maneuver entered by depressing Enable pushbutton for more than 10 seconds Since LED flashes for five seconds, the video shows the crew usually depressing the pushbutton for extended periods of time Need to improve feedback to the crew on SPHERES control panel Available resources after Test Session 1 –Approximately one hour of battery –Approximately 50% of tank

SPHERES 15 Conclusions Primary objective “Hardware Checkout”: SUCCESS –Powering satellite: Success –Establishing communication: Success (nominal packet drops) –Uploading test session program: Success –Running tests: Success –Collecting data: Success Science Objectives: Partial –Thruster mixer performance: Success Need further algorithm development –Closed-loop rotations: Failed due to FLASH corruption –Formation Fight and Docking: Partial Demonstrated estimator performance Unable to perform closed-loop control due to FLASH corruption

SPHERES 16 Lessons Learned ISS environment is highly benevolent for SPHERES –No air flow effects –No infrared or ultrasound interference –Crew highly skilled to deploy satellites Quick Checkout procedure not sufficient to test closed-loop control of both hardware and software The crew needs more feedback when the satellite downloads data The “Enable LED” operation must be simplified Pre-mature test termination due to communication failure must be minimized –Satellite should continue to download data unless commanded to stop by the crew or because of safety requirements

SPHERES 17 Future Actions All the critical values in the closed-loop path which were stored in FLASH are now uploaded every time a program is loaded All the programs uploaded to the ISS have been reviewed; none of them contain the FLASH corruption bug The internal MIT/PSI procedures for delivery of a satellite to NASA have been updated to check all FLASH values SPHERES GUI executable will be updated (potentially by Exp 14) to indicate to the crew when data download is taking place Unclear what actions to take to solve communications issues after this session SPHERES GUI executable updated to allow to function even if the files it uses are read-only when starting the program –Modified the code to check all files, not just gui.ini, for this issue SPHERES Core software has been updated so that the “Enable LED” turns solid on during all times after the “enable” command (pushbutton > 0.5s) has been received

SPHERES 18 SPHERES Points of Contact Payload Integration: John Merk Payload Systems Inc. (617) x524 Principal Investigator: Prof. David Miller Director, MIT Space Systems Laboratory (617) Space Test Program (Code WR1): Maj Matthew Budde, USAF, (281) Mark Adams, SAIC, (281) Science Lead: Dr. Alvar Saenz-Otero MIT Space Systems Lab. (617) Graduate Students: Simon Nolet (PhD) Swati Mohan (MS) Nicholas Hoff (MS)

SPHERES 19 Revision History

SPHERES 20 The raw (bit counts) data collected during tests 1, 2, and 3 is converted into metric units –metric = (raw - bias) * scaling factor Results: –Accelerometer biases need correction –Unable to validate accelerometer scaling factors without further analysis –Gyroscope data with reasonable margins Appendix A: Converted IMU Data Overview

SPHERES 21 Appendix A: Converted IMU Data Plots First Quick Checkout ResultsSecond Quick Checkout ResultsFirst Test 3 Results Second Test 3 ResultsTest r Results

SPHERES 22 Appendix B: Operational Results Summary of results from the GUI and crew notes