1. ICRA ’07 Space Robotics Workshop A Form Based Control Algorithm for Reducing the Complexity of an Attitude Control System at ICRA 2007 Space Robotics Workshop April 14, 2007 Cleveland State University / Industrial Space Systems Lab Northrop Grumman Space Technology Brian Fast Dr E. bfast@ieee.org

2. ICRA ’07 Space Robotics Workshop Problem Description System Characteristics and ObjectiveSystem Characteristics and Objective –Investigation complexity related to a flywheel based power/attitude control system System Architecture ConsiderationsSystem Architecture Considerations –Hardware constraints Packaging limitationsPackaging limitations Mounting tolerancesMounting tolerances –Software constraints Computation complexityComputation complexity Performance MetricsPerformance Metrics –Complexity of the system Algorithm & tuningAlgorithm & tuning –Development time The focus of this work is to demonstrate that a well designed control algorithm can dramatically reduce: the hardware constraints, the time to test and debug, and the cost to develop a system.

3. ICRA ’07 Space Robotics Workshop Benefits & Objectives Increased life time compared to batteriesIncreased life time compared to batteries Improved system characteristicsImproved system characteristics –state of charge Increased charge and discharge capabilitiesIncreased charge and discharge capabilities Dual use systemDual use system –power and attitude control Reduces the need for propellantReduces the need for propellant for attitude control NASA-GRC

4. ICRA ’07 Space Robotics Workshop A Three Degree of Freedom Satellite Attitude Control System Four reaction wheels mounted in a tetrahedron configurationFour reaction wheels mounted in a tetrahedron configuration Assumptions:Assumptions: –Imaging spacecraft –Camera is mounted at the bottom of the spacecraft –Global reference frame is at the bottom of the satellite –Reaction wheels are not mounted in the global reference frame

5. ICRA ’07 Space Robotics Workshop The Systems Mathematical Description

6. ICRA ’07 Space Robotics Workshop The Systems Mathematical Description MIMO Cross Coupling Terms

7. ICRA ’07 Space Robotics Workshop A General Form of a Problem The ability to systematically design a control algorithm to account for complex system dynamics will dramatically speed up the development time and reduce the number of hardware constraints. By approximating the system dynamics the algorithm is capable of compensating for unknown and time-varying parameters, nonlinearities, and the cross coupling terms in MIMO systems.

8. ICRA ’07 Space Robotics Workshop Form Based Control Algorithm Observer: Approximates immeasurable signalsObserver: Approximates immeasurable signals Observer Gains: Determines the performance of the observerObserver Gains: Determines the performance of the observer Observer Outputs:Observer Outputs: –Filtered measurement –Approximation of the dynamics Control AlgorithmControl Algorithm –Inner loop compensates for the system dynamics –Output loop dictates tracking performance

9. ICRA ’07 Space Robotics Workshop Simulation Results 3DOF Satellite Attitude Control

10. ICRA ’07 Space Robotics Workshop Simulation Results 3DOF Satellite Attitude Control

11. ICRA ’07 Space Robotics Workshop Algorithm Summary & Conclusions Generates a good approximation of the system dynamicsGenerates a good approximation of the system dynamics –Approximates unknown parameters, states, disturbances, and coupling between MIMO systems Requires minimal information about the plantRequires minimal information about the plant Mathematically elegantMathematically elegant –Increased confidence in the design –Good for computationally starved systems or systems which require a large control frequency –Parallel nature allows multiple processes to be controller simultaneously ConstraintsConstraints –The available control effort is large enough to perform the desired operation –The system does not have a large time delay

12. ICRA ’07 Space Robotics Workshop Problem Summary & Conclusions Flexibility when designing the system architectureFlexibility when designing the system architecture –More flexibility packaging the system by not having to be as concerned about the specific alignment Decreased development timeDecreased development time –Less system parameters to identify –Accounts for cross coupling terms –Systematic approach to improving the performance Minimal computational overheadMinimal computational overhead