William Wilkie

Low-cost piezocomposite actuator for structural control applications

The design, manufacture, and testing of a low-cost, flexible, planar composite piezoceramic actuator device will be presented. The actuator uses interdigitated electrodes for poling and subsequent actuation of an internal layer of machined piezoceramic fibers. The fiber sheets are formed from monolithic piezoceramic wafers and conventional computer controlled wafer-dicing methods. The fabrication and use of fiber sheets allows precise handing and alignment of piezoceramic fibers during subsequent phases of actuator assembly. Test show that the actuator is capable of producing large, directional in-plane strains; on order of 2000 parts-per-million under a 4000 V peak-to-peak applied voltage cycle. Preliminary endurance testing indicates that the device is relatively durable, with no reductions in free-strain performance up to 90 million electrical cycles.

Anisotropic Piezocomposite Actuator Incorporating Machined PMN-PT Single Crystal Fibers

The design, fabrication, and testing of a flexible, planar, anisotropic piezoelectric composite actuator utilizing machined PMN-32%PT single crystal fibers is presented. The device consists of a layer of rectangular single crystal piezoelectric fibers in an epoxy matrix, packaged between interdigitated electrode polyimide films. Quasistatic free-strain measurements of the single crystal device are compared with measurements from geometrically identical specimens incorporating polycrystalline PZT-5A and PZT-5H piezoceramic fibers. Free-strain actuation of the single crystal actuator at low bipolar electric fields (± 250 V/mm) is approximately 400% greater than that of the baseline PZT-5A piezoceramic device, and 200% greater than that of the PZT-5H device. Free-strain actuation under high unipolar electric fields (0-4kV/mm) is approximately 200% of the PZT5A baseline device, and 150% of the PZT-5H alternate piezoceramic device. Performance increases at low field are qualitatively consistent with predicted increases based on scaling the low-field d33 piezoelectric constants of the respective piezoelectric materials. High-field increases are much less than scaled d33 estimates, but appear consistent with high-field freestrain measurements reported for similar bulk single-crystal and piezoceramic compositions. Measurements of single crystal actuator capacitance and coupling coefficient are also provided. These properties were poorly predicted using scaled bulk material dielectric and coupling coefficient data. Rules-of-mixtures calculations of the effective elastic properties of the single crystal device and estimated actuation work energy densities are also presented. Results indicate longitudinal stiffnesses significantly lower (50% less) than either piezoceramic device. This suggests that single-crystal piezocomposite actuators will be best suited to low induced-stress, high strain and deflection applications.

Piezocomposite Actuator Arrays for Correcting and Controlling Wavefront Error in Reflectors

Three reflectors have been developed and tested to assess the performance of a distributed network of piezocomposite actuators for correcting thermal deformations and total wave-front error. The primary testbed article is an active composite reflector, composed of a spherically curved panel with a graphite face sheet and aluminum honeycomb core composite, and then augmented with a network of 90 distributed piezoelectric composite actuators. The piezoelectric actuator system may be used for correcting as-built residual shape errors, and for controlling low-order, thermally-induced quasi-static distortions of the panel. In this study, thermally-induced surface deformations of 1 to 5 microns were deliberately introduced onto the reflector, then measured using a speckle holography interferometer system. The reflector surface figure was subsequently corrected to a tolerance of 50 nm using the actuators embedded in the reflectors back face sheet. Two additional test articles were constructed: a borosilicate at window at 150 mm diameter with 18 actuators bonded to the back surface; and a direct metal laser sintered reflector with spherical curvature, 230 mm diameter, and 12 actuators bonded to the back surface. In the case of the glass reflector, absolute measurements were performed with an interferometer and the absolute surface was corrected. These test articles were evaluated to determine their absolute surface control capabilities, as well as to assess a multiphysics modeling effort developed under this program for the prediction of active reflector response. This paper will describe the design, construction, and testing of active reflector systems under thermal loads, and subsequent correction of surface shape via distributed peizeoelctric actuation.

An Active Composite Reflector System for Correcting Thermal Deformations

A meter-scale piezoelectrically active composite re ector has been developed and tested. The active re ector consists of a spherically curved, graphite face sheet, aluminum honeycomb core composite panel augmented with a network of distributed piezoelectric composite actuators. The piezoelectric actuator system may be used for controlling the structural dynamic response of the re ector, or for correcting low-order, thermally-induced quasistatic distortions of the panel. In this study, thermally-induced surface deformations of 1 to 5 microns were deliberately introduced onto the re ector, then measured using a speckle holography system. The re ector surface gure was subsequently corrected to a tolerance of 100 nm using a lattice of 90 piezoelectric composite actuators distributed across the re ector’s back face sheet. Initial experimental investigations consisted of open-loop gure control to determine in uence functions and control authority for each individuallyaddressable actuator, followed by a closed-loop gure control implementation. This paper will describe the design, construction, and testing of the active composite re ector system under thermal loads, and subsequent correction of thermal deformations via distributed piezoelectric actuation.

Structural Feasibility Analysis of a Robotically Assembled Very Large Aperture Optical Space Telescope

This paper presents a feasibility study of robotically constructing a very large aperture optical space telescope on-orbit. Since the largest engineering challenges are likely to reside in the design and assembly of the 150-m diameter primary reflector, this preliminary study focuses on this component. The same technology developed for construction of the primary would then be readily used for the smaller optical structures (secondary, tertiary, etc.). A reasonable set of ground and on-orbit loading scenarios are compiled from the literature and used to define the structural performance requirements and size the primary reflector. A surface precision analysis shows that active adjustment of the primary structure is required in order to meet stringent optical surface requirements. Two potential actuation strategies are discussed along with potential actuation devices at the current state of the art. The finding of this research effort indicate that successful technology development combined with further analysis will likely enable such a telescope to be built in the future.

Controlling Wavefront in Lightweight Reflector Systems using Piezocomposite Actuator Arrays

As part of a three-year research task, we have investigated active reflector systems as a less costly, lightweight alternative for future missions needing high surface precision reflectors. An active reflector can correct thermally-induced deformations, which reduces the structural constraints and gives the required thermal performance with a lighter system. Active reflectors can also correct manufacturing errors that dominate large composite reflector fabrication, which in turn reduces the manufacturing cost required to meet a given surface tolerance. As an added benefit, an active reflector adds robustness to the mission performance, with the ability to correct for deployment misalignments, mechanical creep in the structure, and other unpredicted disturbances. The benefits of an active reflector are balanced against increased complexity. The original Active Composite Reflector (ACR) testbed was developed to assess the authority of flat piezoelectric patches integrated into the back facesheet of a large (meter-scale) microwave reflector. Based on the success of the initial results, a control and metrology testbed was developed to pursue open-loop and closed-loop thermal distortion compensation in the meter-scale reflector and achieved sub-micron RMS surface quality under 10 microns of thermally-induced distortion. Smaller bench-scale reflectors at the 150-250 mm scale were also tested to assess surface performance as a function of substrate mechanical properties such as thickness and modulus. Final testing pushed the limits of the distributed actuator array concept to reach optical performance. A parallel multiphysics model was developed to support orbital performance simulations. This report describes the final experimental and modeling results for lightweight, low-cost, reflector systems with piezocomposite actuators.