Michael J. Shepherd

Quasi-static Optical Control of In-plane Actuated, Deformable Mirror: Experimental Comparison with Finite Element Analysis

Future space telescopes may use large diameter (20-meter plus), ∞exible, lightweight membrane-like optical quality mirrors to overcome launch weight and packaging limitations. In-plane actuators, such as piezoelectric elements embedded within the mirror’s structure, could be used to actively shape these deformable mirrors. Thus, the active deformable mirrors could form commanded conjugate surfaces necessary to remove atmospheric distortions in an incoming wavefront, as well as countering any manufacturing imperfections or space-atmospheric disturbances. A stepping stone to realizing these large scale structures is the creation of laboratory test articles and investigating their behavior. To this end, a flve-inch in-plane actuated, tensioned, deformable mirror was constructed and evaluated for optical level responses. The research presented herein is a comparison of experimental data with advanced flnite element modelling techniques using MSC.Nastran software, and the implementation of control algorithms developed from the results of the flnite element modelling used to achieve optical precision quasi-static shaping of the experimental mirror’s surface.

Low-order actuator influence functions for piezoelectric in-plane actuated tensioned circular deformable mirrors

Future space-based optical reflectors will be driven by the desire for large apertures versus the constraints of low weight and compactness of packaging. One possible way to satisfy these competing factors is through the use of piezoelectric in-plane actuated, tensioned, deformable mirrors. These configurations typically exhibit both plate-like and membrane-like behavior. Proposed is a new approximation method for the solution to this class of mirror, where the normalized plate stiffness to tension ratio is small. The approximation function is based on the exact analytical solution to this class of problems. The approximation method allows the problem to be reduced to a simple pressure forced membrane equation, a geometry which may be more readily analyzed. A case study comparing the results of the approximation method to a high fidelity finite element model constructed in MSC.Nastran is provided.

Quasi-Static Optics-Based Surface Control of an In-Plane Actuated Membrane Mirror

Future space telescopes may use large-diameter, flexible, lightweight membrane mirrors to overcome current launchweight and packaging limitations. In-plane actuated tensionedmembranemirrors may be a possible solution to maintaining the precise surface required of a telescope mirror. For this experiment a 0.127-m diam in-plane actuated deformable tensioned membrane mirror was constructed and tested. The research presented herein implements control algorithms based on the results of a nonlinear finite element MSC.Nastran model. The control method used a least-squares approach to create an influence function matrix which was implemented as a proportional plus integral controller. Precision shaping of the test article’s mirror surface, expressed in terms of a Zernike coefficient basis set obtained from an optics-based Shack–Hartmann wave front sensor, was demonstrated in a series of quasi-static closed-loop control tests. Micron-level sinusoidal control inputs representing the defocus Zernike coefficient were successfully tracked with an average absolute accuracy of 0:16 m. For multimode tracking, commanded tip, tilt, and defocus modes were tracked with an absolute average error of 0.14, 0.09, and 0:18 m, respectively, indicating that increasing the dimension of the control system did not significantly degrade its performance. This experimental demonstration illustrates the use of optics-based precision surface control for inplane actuatedmembranemirrors, however, significant technological challenges still exist whichmay impede the use of this technology for large space-based telescope applications.

Scaling Analyses for Large-Scale Space-Based Membrane Optics

To meet future requirements, space telescopes are envisioned to require primary mirrors that will be on the scale of ≥ 10 m in diameter. Packaging restrictions of current and foreseeable launch vehicles prohibit the use of a single rigid monolithic mirror of that size. Membrane-optics research seeks to create large-diameter apertures out of thin flexible filmlike reflective material. In this analysis, structures with embedded in-plane-actuated piezoelectric elements for active surface shape control were examined. By analyzing the nondimensional form of the governing differential equation, relative effects of linear and nonlinear terms are apparent. Then, through a series of MSC Nastran finite element models, scalability issues are explored to include the effects of nonlinear terms, existing membrane pretension, and unimorph-versus-bimorph actuation. Results show that although small-scale (existing) test articles may respond in accordance with linear models, they may mask the nonlinear characteristics that dominate large full-scale membrane optics in the proposed applications.

Limited aerodynamic System Identification of the T-38A using SIDPAC software

The T-38A is the primary training aircraft at the USAF Test Pilot School. The aircraft used was fully instrumented for all aerodynamic flight parameters including angular rates, accelerations, and control surface positions. Flight test data were obtained over a series of sub-sonic and supersonic test points in the clean aircraft configuration. The flight test data were reduced using the System Identification Programs for AirCraft (SIDPAC) toolbox for MATLAB resulting in an aerodynamic model of the T-38A. The investigation identified several considerations when conducting a shortterm, limited scope model identification test. The lessons learned from this application may be applied to further studies of aircraft dynamics.

Development and Flight Test of a Reconfigurable Avionics Research Pod for the USAF Test Pilot School

Considerable time and money are spent on the modification of fleet-support test aircraft in order to enable carriage of novel research experiments that require in-flight test and demonstration. In many cases, the cost of aircraft modification exceeds the cost of the flight test hardware and flight hours. The USAF Test Pilot School (TPS) has worked closely with the Air Force Institute of Technology (AFIT) to develop and flight test the Reconfigurable Airborne Sensor, Communication and Laser (RASCAL) pod. The pod concept will revolutionize the way that USAF TPS conducts Test Management Project training, and will enable rapid transition of cutting-edge technology under development at AFIT and national laboratories to the demanding airborne flight environment. The development and flight test of RASCAL are discussed, as are future concepts of operation expected to be conducted at the USAF TPS.

Scaling Analysis for Large Membrane Optics

To meet future requirements, space telescopes are envisioned to require optics tens of meters in diameter. Packaging restrictions of current and foreseeable launch vehicles prohibit the use of a single rigid monolithic mirror of that size. Membrane optics research seeks to create large diameter apertures out of thin flexible Aim-like reflective material. For our analysis, we examine those structures with embedded in-plane actuated piezoelectric elements for active surface shape control. By analyzing the non-dimensional form of the governing differential equation, relative effects of linear and non-linear terms are apparent. Then, through a series of MSC.Nastran finite element models, scalability issues are explored to include the effects of non-linear terms, existing membrane pre-tension, and unimorph versus bimorph actuation. Results show small-scale (existing) test articles may respond in accordance with linear models, but may mask the nonlinear characteristics which dominate large full-scale membrane optics in the proposed applications.