Spacecraft Attitude and Power Control Using Variable Speed Control Moment Gyros A Thesis Presented to The Academic Faculty by Hyungjoo Yoon In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the School of Aerospace Engineering Georgia Institute of Technology December 2004 Spacecraft Attitude and Power Control Using Variable Speed Control Moment Gyros Approved by: Dr. Panagiotis Tsiotras, Advisor Dr. Eric Johnson Dr. J.V.R. Prasad Dr. Robert Braun Dr. David G. Taylor November 12. 2004 TABLE OF CONTENTS LIST OF TABLES ................................... vi LIST OF FIGURES .................................. vii SUMMARY ........................................ ix CHAPTER I INTRODUCTION ......................... 1 1.1 Motivation.................................... 1 1.2 Control Moment Gyros (CMGs) and Variable Speed CMGs (VSCMGs) . 2 1.3 Adaptive Attitude Tracking Control of Spacecraft . ......... 3 1.4 Integrated Power and Attitude Control System (IPACS) . ........ 4 1.5 Singularity of CMGs and VSCMGs . 8 1.6 Angular Velocity and Line-of-Sight Control Using A Single VSCMG . 11 1.7 Adaptive Spacecraft Attitude Tracking Control with Actuator Uncertainties 13 1.8 OutlineofthisThesis............................. 15 1.9 Contributions .................................. 16 CHAPTER II SYSTEM MODEL ......................... 18 2.1 Dynamics .................................... 18 2.2 Kinematics.................................... 20 CHAPTER III ATTITUDE AND POWER TRACKING CONTROL OF A SPACECRAFT ................................... 22 3.1 Model-Based Attitude Tracking Controller . ...... 22 3.1.1 Lyapunov Stability Condition for Attitude Tracking . ........ 22 3.1.2 Velocity-Based Steering Law for Attitude Tracking . ...... 23 3.1.3 Acceleration-Based Steering Law for Attitude Tracking....... 24 3.2 Adaptive Attitude Tracking Controller . ...... 25 3.2.1 Adaptive Control with VSCMGs . 25 3.3 PowerTracking ................................. 29 3.4 Solution of Velocity Steering Law for IPACS . ...... 29 3.5 WheelSpeedEqualization . 32 3.6 NumericalExamples .............................. 34 iii CHAPTER IV SINGULARITY ANALYSIS OF CONVENTIONAL CMG SYSTEM ....................................... 45 4.1 CMGs/VSCMGs System Modelling . 45 4.2 Review of a Conventional CMG System . 47 4.2.1 SingularitiesofaCMGSystem . 47 4.2.2 The Angular Momentum Envelope of a CMG . 49 4.2.3 Escape from Singularity using Null Motion . 51 CHAPTER V SINGULARITY ANALYSIS OF VSCMG SYSTEM ... 55 5.1 Singularity Analysis of VSCMGs Without Power Tracking . ........ 55 5.1.1 Singularity Avoidance using Null Motion of VSCMGs Without Power Tracking................................. 58 5.2 Singularity Analysis of VSCMGs With Power Tracking . ....... 60 5.3 The Angular Momentum Envelope of a VSCMG Cluster . 63 5.3.1 The Momentum Envelope for Given Kinetic Energy . 63 5.3.2 A Geometric Picture of the Inescapable Singularity Case...... 65 5.3.3 A Condition for Singularity Avoidance . 67 5.4 NumericalExamples .............................. 69 CHAPTER VI SPACECRAFT ANGULAR VELOCITY AND LINE-OF- SIGHT CONTROL USING A SINGLE-GIMBAL VARIABLE-SPEED CONTROL MOMENT GYRO ......................... 75 6.1 EquationsofMotion .............................. 75 6.2 Parametrization of the Spacecraft Orientations at Rest ........... 76 6.3 Linearized System Analysis and Controller Design . ......... 81 6.3.1 ControllabilityAnalysis. 82 6.3.2 LinearControlDesign ......................... 84 6.4 Nonlinear System Analysis and Controller Design . ........ 84 6.4.1 AngularVelocityStabilization . 85 6.4.2 Instability of the Nontrivial equilibria . ....... 87 6.4.3 Stabilization of ω, γe and Ωe ...................... 89 6.4.4 Characterization of the Nontrivial Equilibria . ........ 92 6.4.5 Nonlinear control design for stabilization of ω, γe and φe ...... 94 6.5 NumericalExamples .............................. 95 iv CHAPTER VII ADAPTIVE SPACECRAFT ATTITUDE TRACKING CONTROL WITH ACTUATOR UNCERTAINTIES ........... 103 7.1 ProblemStatement ............................... 103 7.2 AdaptiveControllerDesign. 106 7.3 NumericalExample ............................... 110 CHAPTER VIII CONCLUSIONS AND FUTURE WORK ........ 115 8.1 Conclusions ................................... 115 8.2 FutureWork................................... 116 8.2.1 Singularity analysis and avoidance of CMGs/VSCMGs with consid- erationofthespacecraftdynamics. 116 8.2.2 Feedback Control for Power Tracking . 117 8.2.3 DegenerateNullMotionProblem . 118 8.2.4 Reduction Of The Number Of Parameter Estimates In The Adaptive ControlDesign ............................. 118 APPENDIX A — DERIVATION OF THE EQUATION OF TOTAL AN- GULAR MOMENTUM OF A SPACECRAFT WITH VSCMGs ... 120 APPENDIX B — PROOFS FOR ANALYSIS OF THE SINGULARITIES OF VSCMGs .................................... 124 REFERENCES ..................................... 131 v LIST OF TABLES Table1 SimulationParameters. 35 Table 2 Nontrivial Equilibria of the System Under Controller Eq. (163). 90 Table 3 Spacecraft Model Parameters for LOS Control . ........ 96 Table 4 Control Design Parameters and Initial Conditions For LOS Control . 97 vi LIST OF FIGURES Figure1 TwoTypesofCMGSystem. 3 Figure 2 Typical Chemical Battery Used in a Satellite System ............ 5 Figure3 TypicalOrbitofaSatellite . ...... 5 Figure 4 Examples of Singularity and Null Motion. ......... 10 Figure 5 Examples of Infeasible Rest-to-Rest Maneuvering via a Single VSCMG. 13 Figure 6 Spacecraft Body with a Single VSCMG . ..... 18 Figure 7 A VSCMGs System with Pyramid Configuration . ...... 38 Figure8 AttitudeErrorTrajectory.. ....... 39 Figure 9 Desired (circle) and Actual (solid line) Power Profiles............ 39 Figure 10 Angular Wheel Speeds with Speed Equalization: Method 1 (top) and Method2(bottom). .............................. 40 Figure 11 Gimbal Angles, Control Inputs and Condition Number of Matrix C (Method 1)......................................... 41 Figure 12 Gimbal Angles, Control Inputs and Condition Number of Matrix C (Method 2)......................................... 42 Figure 13 Attitude Error Trajectories. Without Adaptation (top) and With Adap- tation(bottom). ................................ 43 Figure 14 Parameter Convergence for Coning Motion: Jxx, Jyy, Jzz (top) and Jxy, Jxz, Jyz (bottom)..................................... 44 Figure 15 Vectors at a Singular Gimbal State. ........ 48 Figure 16 Singular Surfaces of CMGs in Pyramid Configuration ........... 50 Figure 17 Angular Momentum Envelope of CMGs . ..... 51 Figure 18 Singular Surfaces of CMGs ( (i) ǫi = +1 for all i, and (ii) ǫi’s = +1 except one i ) ..................................... 52 Figure 19 Escapable Singularity of VSCMGs . ....... 66 Figure 20 Inescapable Singularity of VSCMGs . ........ 67 Figure 21 Reference and Actual Attitude Trajectory. ........... 70 Figure22 DesiredandActualPowerProfile. ....... 71 Figure 23 Simulation Without Singularity Avoidance. ............ 72 Figure 24 Simulation With Singularity Avoidance. ........... 73 Figure25 InescapableSingularity . ....... 74 vii Figure 26 Axes Definition of a Spacecraft with a VSCMG and an Antenna. 77 Figure 27 A Desired Inertial Direction nˆ in the Inertial Frame . .......... 79 H Figure 28 Desired Attitudes with ω = 0 for Given H0 and nˆ.............. 80 Figure 29 Flow Chart of Entire Control Procedure. ......... 95 Figure 30 Spacecraft Angular Velocity History ω(t)................... 99 Figure 31 Lyapunov Function Candidate History V2−(t)................. 100 Figure 32 Spacecraft Attitude Trajectories. ........... 100 Figure33 GimbalAngleandWheelSpeed. 101 Figure34 ControlInputs.. 101 Figure 35 Snapshots of the Spacecraft Orientation During the Maneuver. 102 Figure 36 Θ˜ T Θˆ 0inCase(iii). ........................... 109 Gi Gi ≥ Figure 37 The Reference Attitude Trajectory . ......... 112 Figure 38 The Attitude Error Trajectory Without Adaptation (q = q q) . 113 e d − Figure 39 The Attitude Error Trajectory With Adaptation (q = q q)...... 113 e d − Figure 40 Parameter Estimation Θˆ F ........................... 113 Figure 41 Square of Norms of Parameter Estimation Θˆ 2 ............. 114 k Gik Figure 42 Gimbal Angle γ and Wheel Speed Ω ..................... 114 viii SUMMARY A Variable Speed Control Moment Gyro (VSCMG) is a recently introduced actuator for spacecraft attitude control. It generates a torque by exchanging angular momentum with the spacecraft body. As its name implies, a VSCMG is essentially a single-gimbal control moment gyro (CMG) with a flywheel allowed to have variable spin speed. In other words, it is a hybrid between two types of internal torque generators; namely, a conventional control moment gyro and a reaction wheel. Thanks to its extra degrees of freedom, a VSCMGs cluster can be used to achieve additional objectives, such as power tracking and/or singularity avoidance, as well as attitude control. In this thesis, control laws for an integrated power/attitude control system (IPACS) for a satellite using VSCMGs are introduced. The gimbal rates of the VSCMGs are used to provide the reference-tracking torques, while the wheel accelerations are used for both attitude and power reference tracking. The power tracking objective is achieved by storing or releasing the kinetic energy in the wheels. The proposed control algorithms perform both the attitude and power tracking goals simultaneously. A model-based control and an indirect adaptive control for a spacecraft with uncertain inertia properties are developed. Moreover, control laws for