Modeling and Control of a 2-D Membrane Mirror With a PZT Bimorph

The future of space satellite technology lies in ultra-large mirrors and radar apertures for significant improvements in imaging and communication bandwidths. The availability of optical-quality membranes drives a parallel effort for structural models that can capture the dominant dynamics of large, ultra-flexible satellite payloads. Unfortunately, the inherent flexibility of membrane mirrors wrecks havoc with the payload's on-orbit stability and maneuverability. One possible means of controlling these undesirable dynamics is by embedding active piezoelectric ceramics near the boundary of the membrane mirror. In doing so, active feedback control can be used to eliminate detrimental vibration, perform static shape control, and evaluate the health of the structure. In the present work, a piezoceramic wafer was attached in a bimorph configuration near the boundary of a tensioned rectangular membrane sample. A finite element model of the system was developed to capture the relevant system dynamics from 0 – 500 Hz. The finite element model was compared to experimental results with fair agreement. Using the validated finite element models, structural control using Linear Quadratic Regulator (LQR) control techniques were then used to demonstrate effective vibration control. Typical results show that less than 12 V of actuation voltage is required to eliminate detrimental vibration of the membrane samples in less than 15 ms. The functional gains of the active system are also derived and presented. These spatially descriptive control terms dictate favorable regions within the membrane domain to place sensors.

Design of a Multifunctional Aircraft Skin With Energy Harvesting via Entropy Generation Minimization

The Entropy Generation Minimization (EGM) approach is applied to the design of a new integrated radar aircraft skin, which both meets requisite aircraft structural needs and provides a pathway for the waste heat from structurally integrated power devices. Thermoelectric (TE) devices, sandwiched between a heterogeneous skin layer and the radar devices for the purpose of harvesting waste heat rejected to the ambient, are considered in the analysis. A heterogeneous skin layer is designed using the EGM approach, which is then applied to the overall mission of the aircraft to determine the optimal skin thickness and volume fractions of the matrix and inclusions in the composite skin.

Functionally Graded Nanocomposites: Manufacturing and Characterization

Multi-walled carbon nanofiber (MWCN) composites having tailored internal structure are created using Field Aided Micro Tailoring (FAiMTa) technology. FAiMTa is a technique that relies on the application of an electric field to a suspension while it cures. The particles in the suspension align in the direction of the electric field while the matrix material hardens, locking the aligned particles in place. The outcome is an orthotropic micro-tailored composite. Three 1% by volume MWCN/epoxy composite systems are manufactured and characterized: (a) random orientation, (b) fibers aligned through the thickness of the sample, and (c) half-aligned through the thickness and half random orientation. Electronic Speckle Pattern Interferometry (ESPI) and Dynamic Mechanical Analysis (DMA) are used to evaluate mechanical material properties as a function of particle alignment. The half aligned sample demonstrates the ability of FAiMTa to locally tailor a material.

Experimental Design and Analysis of Bimorphs as Synthetic Jet Diaphragms

Synthetic jet actuators are promising Active Flow Control (AFC) devices which could lead to saving millions of dollars in fuel consumption each year. The Bimorph piezoelectric actuators are an attractive alternative to other type of actuators as active diaphragms and are the focus of this work. Among the properties of a Bimorph actuator, a number of geometrical and physical external factors may have an effect on its performance as a synthetic jet actuator. Using statistical tools some of the physical and geometrical factors are evaluated as independent variables that may have an effect on the synthetic jet peak velocity, the dependent variable. Among the factors studied are the geometry of the synthetic jet cavity, the driving signal used to operate the active diaphragm, and the effect of a pressure gradient on the device. Among the six factors considered, the driving signal was found to have the highest effect on the peak jet velocity, and the factor of frequency proved to have a smaller effect. The cavity geometrical parameters were also relevant, a smaller orifice and a smaller cavity produce higher peak jet velocities. An adverse pressure gradient was also found to have a significant effect on peak jet velocity, diminishing its magnitude with increasing pressure.

Self-Powered Wireless Corrosion Detection System

Corrosion affects the maintenance of metal aircraft. Because the onset of corrosion is unpredictable, sensing corrosion is a challenge and scheduled inspections are mandated by corrosion prevention and control programs. Visual inspection is the most common method of corrosion detection. Visual inspections of aircraft structures that are difficult to access are costly and invasive. Beyond visual inspection, several non-destructive corrosion detection methods exist, such as ultrasonic scanners and pulsed eddy current systems. The functionality of these systems, however, does not minimize the invasiveness of inspections. Access to the structure under inspection is required to use these systems or to perform visual inspections. This paper describes a self-powered, wireless corrosion detection system which could enable modification of existing inspection schemes in difficult-to-access areas where corrosion is expected to develop, for example, on structure beneath an aircraft galley or lavatory. The system consists of an energy harvester, an energy storage and conditioning circuit, a corrosion sensing element, and a wireless transceiver network. Advances in energy harvesting and low-power wireless transceivers have enabled the design. The system allows users to download corrosion data from a sensor through a wireless connection, without the need for costly structural disassembly. Because the device is self-powered and wireless, it operates indefinitely without battery replacement, and does not require power or data wiring from the aircraft.

Anisothermic Oxidation of Carbon/Epoxy Laminates

This work deals with the ageing of a carbon epoxy composite material for aeronautic and supersonic applications. One of the main parameters which governs the durability of this kind of materials is the matrix oxidation, which is limited to surface layers. The long-term behaviour of organic matrix composites includes combined effects of ageing: matrix oxidation occurring at high temperature and matrix cracking due to thermo-mechanical ply stresses induced by differential expansion between matrix and fibers or between the various plies. For some years ENSAM has developed for isothermal conditions a kinetic model of radical chain oxidation coupled with the equation of oxygen diffusion. This model is based on a "close-loop" oxidation mechanistic scheme and gives access to the concentration profile of oxidation products in the sample thickness. In this work we expressed the temperature by a Fourier series and we simulate the oxidative behaviour of samples exposed to the following thermal cycles: -50°C/+180°C, -50°C/+150°C and +50°C/+180°C. The weight loss of the oxidised samples was chosen as indicator of oxidation. Numerical results are compared to experimental ones to check the validity of the model. Good agreement between experimental and numerical results is obtained.

Nonlinear Analysis and Optimization of Diamond Cell Morphing Wings

In this work, a design optimization procedure is developed to maximize the energy efficiency of a scissor mechanism for the NextGen's Batwing application. The unit cells are modeled using a finite element approach. The model considers elastic skin, modeled as linear springs, as well as actuator and aerodynamic loads. A nonlinear large displacement analysis is conducted, and the position of the actuator is optimized using Matlab's gradient based optimization algorithm FMINCON. This optimization procedure is used to investigate the effect of different constraints and load cases. The model is expanded to include multiple unit cells and actuators. A two stage optimization process using a Genetic Algorithm and traditional gradient based optimization (FMINCON) is also developed. The two stage optimization is used to optimize actuator position and placement for different constraints and load cases. Results show that placement and position optimization produce small gains in maximizing energy efficiency; morphing using a soft isotropic skin is more efficient than stiff isotropic and anisotropic skin. In addition, the GA did not use the all of the available actuators to maximize energy efficiency.

Phase-Dield Simulation of Rhombohedral and Tetragonal Phases in Ferroelectric Single Crystals

Ferroelectric materials exhibit spontaneous polarization and domain structures below the Curie temperature. In this work the phase field approach has been used to simulate phase transformations and the formation of ferroelectric domain structures. The evolution of phases and domain structures was simulated in ferroelectric single crystals by solving the time dependent Ginzburg-Landau (TDGL) equation with polarization as the order parameter. In the TDGL equation the free energy of a ferroelectric crystal is written as a function of polarization and applied fields. Change of temperature as well as application of stress and electric fields leads to change of the free energy and evolution of phase states and domain structures. In this work the finite difference method was implemented for the spatial description of the polarization and the temporal evolution of polarization field was computed by solving the TDGL equation with an explicit time integration scheme. Cubic to tetragonal, cubic to rhombohedral and rhombohedral to tetragonal phase transformations were modeled, and the formation of domain structures was simulated. Field induced polarization switching and rhombohedral to tetragonal phase transition were simulated.

Tuning Criterion of Variable Piezoelectric Circuitry for Frequency-Shift-Based Damage Identification Enhancement

A new concept of using piezoelectric transducer circuitry with tunable inductance to enhance the performance of frequency-shift-based damage identification method has been recently proposed. While previous work has shown that the frequency-shift information used for damage identification can be significantly enriched by tuning the inductance in the piezoelectric circuitry, a fundamental issue of this approach, namely, how to tune the inductance to best enhance the damage identification performance, has not been addressed. Therefore, this research aims at advancing the state-of-the-art of such a technology by proposing guidelines to form favorable inductance tuning such that the enriched frequency measurement data can effectively capture the damage effect. Our analysis shows that when the inductance is tuned to accomplish eigenvalue curve veering, the change of system eigenvalues induced by structural damage will vary significantly with respect to the change of inductance. Under such curve veering, one may obtain a series of frequency-shift data with different sensitivity relations to the damage, and thus the damage characteristics can be captured more effectively and completely. When multiple tunable piezoelectric transducer circuitries are integrated with the mechanical structure, multiple eigenvalue curve veering can be simultaneously accomplished between desired pairs of system eigenvalues. An optimization scheme aiming at achieving desired set of eigenvalue curve veering is formulated to find the critical inductance values that can be used to form the favorable inductance tuning for multiple piezoelectric circuitries. In the numerical analyses of damage identification, an iterative second-order perturbation-based algorithm is used to identify damages in beam and plate structures. Numerical results show that the performance of damage identification is significantly affected by the selection of inductance tuning, and only when the favorable inductance tuning is used, the locations and severities of structural damages can be accurately identified.

Energy Efficient Mobile Wireless Sensor Networks

Wireless sensor networks (WSN) have tremendous potential in many environmental and structural health monitoring applications including, gas, temperature, pressure and humidity monitoring, motion detection, and hazardous materials detection. Recent advances in CMOS-technology, IC manufacturing, and networking utilizing Bluetooth communications have brought down the total power requirements of wireless sensor nodes to as low as a few hundred microwatts. Such nodes can be used in future dense ad-hoc networks by transmitting data 1 to 10 meters away. For communication outside 10 meter ranges, data must be transmitted in a multi-hop fashion. There are significant implications to replacing large transmission distance WSN with multiple low-power, low-cost WSN. In addition, some of the relay nodes could be mounted on mobile robotic vehicles instead of being stationary, thus increasing the fault tolerance, coverage and bandwidth capacity of the network. The foremost challenge in the implementation of a dense sensor network is managing power consumption for a large number of nodes. The traditional use of batteries to power sensor nodes is simply not scalable to dense networks, and is currently the most significant barrier for many applications. Self-powering of sensor nodes can be achieved by developing a smart architecture which utilizes all the environmental resources available for generating electrical power. These resources can be structural vibrations, wind, magnetic fields, light, sound, temperature gradients and water currents. The generated electric energy is stored in the matching media selected by the microprocessor depending upon the power magnitude and output impedance. The stored electrical energy is supplied on demand to the sensors and communications devices. This paper shows the progress in our laboratory on powering stationary and mobile untethered sensors using a fusion of energy harvesting approaches. It illustrates the prototype hardware and software required for their implementation including MEMS pressure and strain sensors mounted on mobile robots or stationary, power harvesting modules, interface circuits, algorithms for interrogating the sensor, wireless data transfer and recording.