Jeffrey W. Martin

Distributed Sensing and Shape Control of Piezoelectric Bimorph Mirrors

Meeting the long term needs of the remote sensing community requires the development of large aperture space-based optical systems to achieve dramatic improvements in resolution and sensitivity. It is possible that ultralarge apertures will be obtained using deployable thin film mirror technology, yet many technological barriers must be overcome to make this approach viable. This paper summarizes an initial research effort into the development of piezoelectric thin film mirrors that can be actively shaped using electric fields applied by an electron flux at selected locations. Recent progress is described in the key areas of mirror figure sensing methods, electron gun excitation, and shape control algorithm development.

Selection and Optimization of Piezoelectric Polyvinylidene Fluoride Polymers for Adaptive Optics in Space Environments

Various piezoelectric polymers based on polyvinylidene fluoride (PVDF) are of interest for large aperture space-based telescopes. Dimensional adjustments of adaptive polymer films depend on charge deposition and require a detailed understanding of the piezoelectric material responses which are expected to deteriorate owing to strong vacuum UV, γ -, X-ray, energetic particles and atomic oxygen exposure. We have investigated the degradation of PVDF and its copolymers under various stress environments detrimental to reliable operation in space. Initial radiation aging studies have shown complex material changes with lowered Curie temperatures, complex material changes with lowered melting points, morphological transformations and significant crosslinking, but little influence on piezoelectric d33 constants. Complex aging processes have also been observed in accelerated temperature environments inducing annealing phenomena and cyclic stresses. The results suggest that poling and chain orientation are negatively affected by radiation and temperature exposure. A framework for dealing with these complex material qualification issues and overall system survivability predictions in low earth orbit conditions has been established. It allows for improved material selection, feedback for manufacturing and processing, material optimization/stabilization strategies and provides guidance on any alternative materials.

MAINTENANCE OF INFLATED STRUCTURE SHAPE USING ELECTRON GUN CONTROLLED PIEZOELECTRIC MATERIALS

This paper examines one method for actively controlling the shape of an aerospace inflated structure. The basic concept is to fabricate the structure from a piezoelectric thin film, and then use an electron gun as a charge source for shape control. In the electron gun control method the need for separate electrodes and wire leads is eliminated by depositing the control charges directly on the surface of the piezoelectric material. Since piezoelectric materials are dielectrics the charges remain where deposited by the electron gun. The spatial resolution of this control method is as small as the spot size of the electron beam, which in a focused beam can be as small as tens of microns. Large areas can be covered by a single electron gun simply by scanning the beam using deflection plates. Some practical aspects of electron gun control are presented in this paper. A description of an experimental test bed assembled to evaluate electron gun control of PZT-5H is presented, as are results and conceptual models of system behavior.

Analytical and Experimental Studies of Orthotropic Corner-Supported Plates With Segmented In-Plane Actuators

This paper outlines a model for a corner-supported, thin, rectangular bimorph actuated by a two-dimensional array of segmented, orthotropic PVDF laminates; it investigates the realization and measurement of such a bimorph. First, a model is derived to determine the deflected shape of an orthotropic laminate for a given distribution of voltages over the actuator array. Then, boundary conditions are realized in a laboratory setup to approach the theoretical corner-supported boundary condition. Finally, deflection measurements of actuated orthotropic PVDF laminates are performed with Electronic Speckle Pattern Interferometry and are compared to the model results.

Optomechanical Design for Cost Effective DEMVAL Systems.

Sticker shock for optomechanical hardware designed for advanced optical DEMVAL systems can lead to program loss. In optomechanical design it is important to manage this risk through easily manufacturable and inexpensive hardware to meet demands of lower budget programs. The optical and optomechanical design teams must work closely to optimize system design for ease of manufacture, and assembly, while at the same time minimizing the impacts to system performance. Effective teaming often results in unique/creative design solutions which enable future system development. Outlined are some novel optomechanical structure concepts, with 5 degrees of freedom (DOF), used to design a low cost DEMVAL optical system. The concepts discussed include inexpensive repeatable magnetic kinematic mounts, flexure rings for lens preloading, simplistic drop-in lens housing designs, and adjustable tooling ball metering rods which accommodate alignment in 5 DOF.

Deflection model for corner-supported plates with segmented in-plane actuators

The shape control of flexible mirrors has been studied mainly for edge-supported thin-plate configurations. For applications such as optical reflectors, corner-supported configurations, which allow for paraboloidal geometries, prove more applicable. Moreover, the use of corner supports enables more flexibility and larger achievable deflections than with edge supports. This paper discusses a corner-supported thin rectangular plate actuated by a two-dimensional array of segmented piezoelectric laminated actuators. First, a model determining the deflected shape of a laminate for a given distribution of voltages over the actuator array is derived. Second, the results of the model are shown to agree well with a finite element simulation of the structure. Finally, paraboloidal deflection of a rectangular PVDF laminate is investigated.

Deflection Control of a Corner-Supported Plate Using Segmented In-Plane Actuators

This paper describes an array of in-plane piezoelectric actuator segments laminated onto a corner-supported substrate to create a thin bimorph for reflector applications. An electric field distribution over the actuator segments causes the segments to expand or contract, thereby effecting plate deflection. To achieve a desired bimorph shape, the shape is first expressed as a two-dimensional series expansion. Then, using coefficients from the series expansion, an inverse problem is solved that determines the electric field distribution realizing the desired plate shape. A static example is presented where the desired deflection shape is a paraboloid.Copyright

Nonlinear Deflection Model for Corner-Supported Thin Laminates Shape-Controlled with Moment Actuators.

The shape control of thin, flexible structures has been studied primarily for edge-supported thin-plates. For applications such as electromagnetic wave reflectors, corner-supported configurations may prove more applicable since they allow for greater flexibility and larger achievable deflections when compared to edge-supported geometries under similar actuation conditions. Models of such structures provide insight for effective, realizable designs, enable design optimization, and provide a means of active shape control. Models for small deformations of corner-supported, thin laminates actuated by integrated piezoelectric actuators have been developed. However, membrane deflections expected for nominal actuation exceed those stipulated by linear, small deflection theories. In addition, large deflection models have been developed for membranes; however these models are not formulated for shape control. This paper extends a previously-developed linear model for a corner-supported thin, rectangular laminate to a more general large deflection model for a clamped-corner laminate composed of moment actuators and an array of actuating electrodes. First, a nonlinear model determining the deflected shape of a laminate given a distribution of actuation voltages is derived. Second, a technique is employed to formulate the model as a map between input voltage and deflection alone, making it suitable for shape control. Finally, comparisons of simulated deflections with measured deflections of a fabricated active laminate are investigated.

Deployable large aperture optics system for remote sensing applications.

This report summarizes research into effects of electron gun control on piezoelectric polyvinylidene fluoride (PVDF) structures. The experimental apparatus specific to the electron gun control of this structure is detailed, and the equipment developed for the remote examination of the bimorph surface profile is outlined. Experiments conducted to determine the optimum electron beam characteristics for control are summarized. Clearer boundaries on the bimorphs control output capabilities were determined, as was the closed loop response. Further controllability analysis of the bimorph is outlined, and the results are examined. In this research, the bimorph response was tested through a matrix of control inputs of varying current, frequency, and amplitude. Experiments also studied the response to electron gun actuation of piezoelectric bimorph thin film covered with multiple spatial regions of control. Parameter ranges that yielded predictable control under certain circumstances were determined. Research has shown that electron gun control can be used to make macrocontrol and nanocontrol adjustments for PVDF structures. The control response and hysteresis are more linear for a small range of energy levels. Current levels needed for optimum control are established, and the generalized controllability of a PVDF bimorph structure is shown.

Shape control of a flexible mirror using an electron gun.

Mirrors made of PVDF film are being considered for lightweight transportation and deployment in space. An array of electrodes can be used to distribute charges over the PVDF film for active shaping of the mirrors. This paper presents the derivation of a matrix that enables calculation of the shape of the two-dimensional mirror for any given electron distribution. Finite element simulation shows good agreement with a theoretical example. Furthermore, if a desired shape is given, the required voltage distribution can be computed using the singular value decomposition. Experiments were done in a vacuum vessel, where an electron gun was used to actuate a PVDF bimorph to a desired shape. Dynamic shape control is attainable at low frequencies. At higher frequencies, still significantly below structural resonance, actuation lag and parasitic DC offset can be significant problems that require future research to solve. NOMENCLATURE