John A. Main

Experimental Modeling of Spinal Cord Injury: Characterization of a Force-Defined Injury Device

We examined the ability of a novel spinal cord injury (SCI) device to produce graded morphological and behavioral changes in the adult rat following an injury at thoracic level 10 (T10). The injury device uses force applied to the tissue as the control variable rather than tissue displacement. This has the advantage of eliminating errors that may arise from tissue movement prior to injury. Three different injury severities, defined by the amount of force applied to the exposed spinal cord at T10 (100, 150, and 200 kdyn), were evaluated at two different survival times (7 and 42 d). Unbiased stereology was employed to evaluate morphological differences following the injury. Quantitative behavioral assessment employed the Basso, Beattie, and Bresnahan locomotive rating scale. There was a significant force-related decline in locomotive ability following the injury. Animals subjected to a 200-kdyn injury performed significantly worse than animals subjected to a 100- and 150-kdyn injury. The locomotor ability at different days post injury significantly correlated with the amount of force applied to the spinal cord. Statistical analysis revealed several significant force-related morphological differences following the injury. The greatest loss of white and gray matter occurred at the site of injury impact and extended in both a rostral and caudal direction. Animals subjected to the greatest force (200 kdyn) displayed the least amount of spared tissue at both survival times indicative of the most severe injury. The amount of spared tissue significantly correlated with the locomotor ability. This novel rodent model of SCI provides a significant improvement over existing devices for SCI by reducing variability with a constant preset force to define the injury.

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.

Smart Materials in Inflatable Structure Applications

The focus of this work is to investigate the use of smart materials for vibration testing and control of inflated satellite components. Lightweight inflatable structures are a viable alternative in aerospace structure design. These structures, however, pose special problems in testing and in controlling vibrations due to their extremely lightweight, flexible, and high-damping properties. The smart materials offer the required flexibility with very high electromechanical coupling and, hence are logical elements for the use in the dynamics and control of inflated structures. The aim of the new concept presented in this work is to provide a refined methodology in ground testing for verifying predictive modeling efforts, and to design sensor/actuator systems to actively control unwanted vibrations of an inflated space object. Multiple sensors/actuators and modern state-space based controllers have been implemented to study the various performance of the proposed concept.

Vibration Testing and Finite Element Analysis of an Inflatable Structure

Vibration testing and e nite element analysis of an ine ated thin-e lm torus are presented. Ine atable structures show signie cant promises for future space applications. However, their extremely lightweight, e exible, and high damping properties pose dife cult problems in vibration testing and analysis. Smart materials are used as sensors andactuatorsforperformingvibrationtestsofanine atedtorus.Inaddition,apredictivemodelhasbeendeveloped, which can be compared with experimental results. A commercial e nite element package, ANSYS, is used to model a prestressed ine atable structure. Both experimental and e nite element results are in reasonable agreement with each other. The predictive model can be used for analyzing the dynamics of ine ated structures and for designing control systems to attenuate vibration in an ine ated torus.

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.

Noncontact Electron Gun Actuation of a Piezoelectric Polymer Thin Film Bimorph Structure

Noncontact electron gun actuation of a bimorphmirror structure composed of two polyvinylidene fluoride layers is demonstrated. The electric field is applied to the piezoelectric structure by controlling the potential of a nickel-copper electrode on one side of the bimorph mirror and subjecting the opposite, bare side of the bimorph to an electron flux. The electrode also serves as the reflecting surface of the thin film mirror structure. Significant characteristics of this actuation approach are demonstrated, including the ability to address discrete areas of the continuous mirror without affecting the global mirror shape and the persistence of the shape changes after the electron beam is turned off.

Survey of piezoelectric material strain response to electron gun excitation

A plate of PZT5h was prepared with a single electrode on one face connected to a power amplifier. The opposite face was left as bare ceramic material which was then exposed to an electron beam. Sixteen strain gages were attached atop the electrode to measure the strain response and as a function of electrode potential (backpressure voltage). A range of sinusoidal voltage inputs were applied to the electrode and the strain response and current draw through the PZT were recorded. Electrode potentials between -15 and 100 V yield very predictable strain response and extremely small currents (approcimately 10-7 - 10-6 microamperes) which appear to be independent of the electrode potential. Below -15 V the current through the PZT suddenly increases to 10 (mu) a. At -15 volts level the strain response is still predictable but, as the electrode voltage decreases the strain signal begins to display significant drift. The root cause of this phenomenon is examined with the aid of the deBroglie-Einstein postulate and the Schr*dinger wave equation.

Time response of electron gun strain control of piezoelectric materials

In this paper the dynamic strain response of a piezoelectric material subjected to an electron beam charge input is examined. A piezoelectric material plate (PZT5h) was prepared with a single distributed electrode on one face and the second face subjected to a variety of electron beam inputs. Strain gages were attached atop the electrode to measure the strain response due to the combined effects of the electrode potential and the charge from the electron gun. When the electrode potential is stepped from 0 to 100 volts the strain needs only 1 second to reach steady state position, but when the electrode potential is stepped down it needs almost 1 minute to reach steady state. This phenomenon can be explained as follows: raising the electrode potential increases the energy of the electrons, so the secondary electron yield falls well below one and negative charge builds up quickly. Dropping the electrode potential decelerates the incoming beam, so the secondary yield becomes only slightly higher than one, so the negative charge decreases at a much lower rate, thus it takes longer to reach steady state.

Electron gun control of smart materials

Smart material patches are currently an impractical choice in applications requiring fine spatial resolution or control of complex areas. The static nature of electrodes, the conventional choice for control signal application to many smart materials, makes them unsuitable in these instances. To address this issue the use of electron guns as charge sources for smart material control is investigated in this paper. 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 the system behavior.

Strain and current responses during electron flux excitation of piezoelectric ceramics

The electric field induced strain in piezoelectric materials subjected to an electron flux is examined in this paper. An analysis using quantum mechanics indicates that stable and controllable strains with very low current draw should be achievable over a range of positive and negative control potentials. The model also predicts instability in the internal electric field at larger negative potentials. The model was evaluated by observing the strain output of PZT5h plates subjected to an electron flux on one face and voltage inputs from a single electrode on the opposite face. The strain response and current flow were measured as a function of electrode potential and electron energy. All of the significant predictions of the model were verified by the experimental results. Further experiments were performed to examine the time response of the strain induced in the plate. It was found that the location and potential of the electron collector dramatically influences the dynamic response of the system.