NEXRAD in Space (NIS) Workshop

1. High Precision Shape Control of the 35-m NIS Reflector Houfei Fang Jet Propulsion Laboratory California Institute of Technology NEXRAD in Space (NIS) Workshop University of Miami/Rosenstiel School for Marine and Atmospheric Science April 10-11, 2007

2. Outline • Introduction • State-of-the-Art Sensing Approaches • State-of-the-Art Actuation Approaches • Numerical Results and Recommendations

3. Introduction Overall Goal • Have performed a feasibility study of high precision control of the 35-M spherical reflector to assess development efforts to achieve NIS reflector requirements Problem Statement • The 35-M Spherical Reflector has extremely stringent requirements – • Large size (35 m diameter), high surface accuracy (0.17 mm), light weight, etc. • Exceed current state of the art of large space reflectors • Without active control, the NIS requirements may not be feasible to achieve

4. In-Space Thermal Disturbances Different Orbital Positions Different Temperature Distributions

5. Major Components

6. Outline • Introduction • State-of-the-Art Sensing Approaches • State-of-the-Art Actuation Approaches • Numerical Results and Recommendations

7. State-of-the-Art Sensing Approaches: Photogrammetry • Photogrammetry • Provides global surface deformations • Absolute surface measurement • Accuracy depends on surface targets (number, size and contrast) • Up to 0.06 mm resolution in gossamer membrane experiment [Black & Pappa, 2004] Photogrammetry Setup

8. State-of-the-Art Sensing Approaches: Focal-plane Metrology Focal-plane Metrology • Focal-plane Metrology • Phase retrieval at or near the focal plane • It is simple, only a probe light and a receiving image detector are required • A fiber imaging bundle can be used to convey the focal plane image to a camera installed elsewhere in the system

9. State-of-the-Art Sensing Approaches: Focus Diversity Phase Retrieval Focus Diversity Phase Retrieval • Focus Diversity Phase Retrieval • Uses a number of defocused images to extract the phase information • This concept has been demonstrated and employed by NGST • This concept requires moving the camera in and out of focus

10. State-of-the-Art Sensing Approaches: Shack-Hartmann Sensor Shack-Hartmann Sensor • Shack-Hartmann Sensor • It uses a lenslet array to subdivide the wavefront • Each subset of the wavefront forms a focus on the detector behind the microlens, • It provides information about the local tip/tilt of the wavefront • A Shack-Hartmann sensor produces 1:1 mapping of the wavefront under investigation

11. Outline • Introduction • State-of-the-Art Sensing Approaches • State-of-the-Art Actuation Approaches • Numerical Results and Recommendations

12. State-of-the-Art Actuation Approaches: Boundary Control Displacement from boundary inputs on 0.5m Membrane [Lindler & Flint, 2004] • Very low bending stiffness gives transverse boundary inputs large shape-control authority (radial, tangential & transverse boundary inputs) • Attractive for large 35m diameter application since number of actuators needed scales linearly with circumference, not area • Experiments have been performed on 0.5-m plate with jack-screw boundary inputs [Lindler & Flint, 2004]

13. Piezoelectric Bimorph Membrane Charge deposited on front by electron-gun cathode ray Distributed Anode on back side Electron Gun Applied Potential Deflection Plates State-of-the-Art Actuation Approaches: Piezoelectric Polymer Films Electron gun shape-controlled PVDF thin-film reflector concept [Main et. al., 2000] • Utilizes continuous piezoelectric polymer film patches such as PVDF applied on one side of reflector to enable direct actuation of surface • Piezoelectric circuit completed via a non-contact electron gun • Cathode ray generates localized strain in PVDF and reflector surface in vicinity of electron-beam flux • Electron-beam steering enable global shape control • Eliminates extensive wiring needed for standard distributed piezo arrays

14. Large Scale Net Shape Membrane Deployable Thin Film Electrode Elements High Voltage Power Supply and Control System Lightweight Astromesh Structure State-of-the-Art Actuation Approaches: Out of Plane Electrostatic • Out of Plane Electrostatic (SRS Technologies) • Successfully Demonstrated 5-Meter Lightweight Deployable Electrostatic Backing Structure Under SRS/Northrop Research Program • High Voltage Power Supply and Control Demonstrated • Large Dynamic Range Shape Control

15. State-of-the-Art Actuation Approaches: Gore/Seam with Shape Memory Alloy Cables Gore/seam cable actuated shape control of inflated precision gossamer reflectors – Assessment study [DeSmidt, Wang & Fang, 2006] • Fabrication of seamless 35-m gossamer reflector very challenging, most likely will be fabricated with gore/seaming • This strategy takes advantage of seams to facilitate better spatial distribution of actuators across the membrane to allow greater control authority without affecting deployablity • Proposes to utilize tendon-like actuators, such as Shape Memory Alloy (SMA) based cables, to adjust tension at the gore/seam boundaries to implement shape control

16. State-of-the-Art Actuation Approaches: Electroactive Polymer Patch Shape Control with EAP Patch [Fang, Pattom, Wang, & Im, 2007] • Electroactive polymer (EAP) actuators are attached to the back of the reflector to produce contraction/expansion forces • EAP actuator is very thin and flexible, it can be implemented to the membrane reflector without significant weight penalty and packaging difficulty • EAP exhibits an exceptionally high electrostrictive strain with low hysteresis

17. Outline • Introduction • State-of-the-Art Sensing Approaches • State-of-the-Art Actuation Approaches • Numerical Results and Recommendations

18. Z F d Sensor Measurements Integrated Reflector/Actuator Y Sensors System Vcntrl V Least-Squares Shape Control Saturation Vmax weff Analytical Model Sensor Measurement Locations • Control inputs, patch actuator voltages, V • Sensor measurement vector, Y,(grid of surface measurements) • Least-Squares shape control minimizes objective function,J • Saturation block accounts for PVDF voltage saturation effects Control Law Objective Function System Transfer Matrix

19. 4 Loading Condition: Uniform Temperature Deviation, T0=20 K Plus Temperature Gradient, T=20 K 3.5 3 2.5 wRMS, mm 2 1.5 1 RMS Error Tolerance 0.5 0 312 P 480 P 624 P 52 C 104 C 312 P 480 P 624 P 312 P 480 P 624 P 52 C 52 C 52 C 104 C 104 C 104 C Actuator Configuration, (P = Patches, C = Cables) Patch Control Cable Control Patch+Cable Control Performance Comparison Thermal Loading: Uniform + Gradient Only Hybrid approach can satisfy error tolerance

20. 40 624p 104c Actuator Configuration: P = Patches C = Cables 35 480p 104c 30 624p 52c 25 312p 104c |T0|max , K 480p 52c 20 624p 0c 312p 52c 15 480p 0c 10 312p 0c 5 30 40 50 60 70 80 Control System Mass, kg Performance Comparison Mass of Control System Max Allowable Uniform Thermal Load, K • From the control system weight point-of-view, the hybrid patch/cable approach outperforms “patch only” and “cable only” approaches • Results are similar for Gradient and W-error cases

21. Pros and Cons of different Actuation Technologies • Boundary Control • Not effective for controlling shape at intermediate radial locations • EAP Patch Control • High precision (higher modal controllability compared to cable control) • Limited force capability (limited by saturation) • Higher control voltages compared to cable control • Gore-Seam Cable Control • Large force capability • Lightweight compared with patch control • Precision limited • Hybrid Patch/Gore-Seam Cable Control (Overall the Best Approach) • High precision (due to patches) • High force capability (due to cables) • Most weight efficient design • More complex than patches and cables alone

22. Recommended Follow-On Studies • Light-weight in-space deployable reflector and reflector material • Reduce the material Coefficient of Thermal Expansion (CTE) • Implementation of gore/seam cable control • Develop shape memory alloy (SMA) based cable technology • Development of EAP patch control • Study of new P(VDF-HFP) copolymers for better actuation • Explore new P(VDF-TrFE) terpolymers for order of magnitude better performance • Investigate the possibility of multi-layer (2 to 3 layers) actuator • Control scheme and analytical model advancement • Develop advanced control scheme to accommodate advanced actuator and metrology technologies • Design tool advancement • For optimized spatial distribution of actuators for maximum performance and minimum weight • Metrology system • High resolution, large dynamic range and easy to implement

23. End