Eric D. Swenson

Automated detection of delamination and disbond from wavefield images obtained using a scanning laser vibrometer

The paper presents signal and image processing algorithms to automatically detect delamination and disbond in composite plates from wavefield images obtained using a scanning laser Doppler vibrometer (LDV). Lamb waves are excited by a lead zirconate titanate transducer (PZT) mounted on the surface of a composite plate, and the out-of-plane velocity field is measured using an LDV. From the scanned time signals, wavefield images are constructed and processed to study the interaction of Lamb waves with hidden delaminations and disbonds. In particular, the frequency–wavenumber (f–k) domain filter and the Laplacian image filter are used to enhance the visibility of defects in the scanned images. Thereafter, a statistical cluster detection algorithm is used to identify the defect location and distinguish damaged specimens from undamaged ones.

A Comparison of 1D and 3D Laser Vibrometry Measurements of Lamb Waves

This paper compares and contrasts one-dimensional (1D) and three-dimensional (3D) scanning laser Doppler vibrometer (LDV) measurements of Lamb waves generated by lead zirconate titanate (PZT) transducers. Due to the large cost and capability differences between the previously mentioned systems, this study is provided to highlight differences between these systems. 1D measurements are defined here as measurements of only out-of-plane velocities which are well-suited for studying anti-symmetric Lamb wave modes. 3D measurements provide both in- and out-of-plane velocities, which are especially important when studying both symmetric and anti-symmetric Lamb wave modes. The primary reason for using scanning LDVs is that these systems can make non-contact, accurate surface velocity measurements over a spatially-dense grid providing relatively high resolution image sequences of wave propagation. These scans can result in a clear understanding of Lamb waves propagating in plate-like structures and interacting with structural variations.

Computational Lamb wave model validation using 1D and 3D laser vibrometer measurements

Lamb waves are being explored for structural health monitoring (SHM) due to their capability of detecting relatively small damage within reasonably large inspection areas. However, Lamb wave behavior is fairly complex, and therefore, various computational techniques, including finite element analysis (FEA), have been utilized to design appropriate SHM systems. Validation of these computational models is often based on a limited number of measurements made at discrete locations on the structure. For example, models of pitch-catch of Lamb waves may be validated by comparing predicted waveform time histories at a sensor to experimentally measured results. The use of laser Doppler vibrometer (LDV) measurements offers the potential to improve model validation. One-dimensional (1D) LDV scans provide detailed out-of-plane measurements over the entire scanned region, and checks at discrete sensor locations can still be performed. The use of three-dimensional (3D) laser vibrometer scans further expands the data available for correlation by providing in- and out-of-plane velocity components over the entire scanned region. This paper compares the use of 1D and 3D laser vibrometer data for qualitatively and quantitatively validating models of healthy metallic and composite plates.

Prototype Development and Dynamic Characterization of Deployable CubeSat Booms

Abstract : The current barrier to CubeSat proliferation is their lack of utility depth. These small satellites are exceptionally well suited for specific space missions such as space weather observation and other scientific data gathering exploits, however, they are not suited for every mission. The 10cm-cube form factor that gives the CubeSat its unique advantage is also its greatest hindrance. A potential bridge over this gap is the successful integration of deployable booms onto the CubeSat structure. With this research, the Air Force Institute of Technology (AFIT) explored the parameters of deployable tapespring booms using the triangular retractable and collapsible (TRAC) cross-sectional geometry developed by Air Force Research Labs (AFRL) and used on NASAs CubeSat, Nanosail-D These booms were augmented with reflective membranes and specifically designed to deploy on orbit for the purpose of ground observation, observations that could later be used to determine the deployed dynamics of the booms from optical data gained passively by solar illumination.

Delamination Detection in Composite Structures using Laser Vibrometer Measurement of Lamb Waves

In this study, the feasibility of using a scanning laser vibrometer for detecting hidden delamination in multi-layer composites is explored. First, Lamb waves are excited by Lead Zirconate Titanate (PZT) transducers mounted on the surface of a composite plate, and the out-of-plane ultrasonic velocity field is measured using a 1D scanning laser vibrometer. From the scanned time signals, wave field images are constructed and processed to study the interaction of Lamb waves with hidden delamination. In order to highlight the defect area in the image, the performance of different image processing tools were investigated. In particular, the Laplacian image filter was found to accentuate the visual indications of the ultrasound-defect interaction by suppressing the presence of incident waves in the wave field images. The performance of the proposed scheme is investigated using experimental data collected from a 1.8 mm thick multilayer composite plate and a 10 mm thick composite wing structure.

Near Real-Time Closed-Loop Optimal Control Feedback for Spacecraft Attitude Maneuvers

Abstract : Optimization of spacecraft attitude maneuvers can significantly reduce attitude control system size and mass, and extend satellite end-of-life. Optimal control theory has been applied to solve a variety of open-loop optimal control problems for terrestrial, air, and space applications. However, general application of real-time optimal controllers on spacecraft for large slew maneuvers has been limited because open-loop control systems are inherently vulnerable to error and the computation necessary to solve for an optimized control solution is resource intensive. This research effort is focused on developing a near real-time optimal control (RTOC) system for spacecraft attitude maneuvers on the Air Force Institute of Technologys 2nd generation simulated satellite, SimSat II. To meet the end goal of developing a RTOC controller, necessary preliminary steps were completed to accurately characterize SimSAT IIs mass properties and attitude control system. Using DIDO, a pseudospectral-based optimal control solver package, to continuously solve and execute a sequence of optimized open-loop control solutions in near real-time, the RTOC controller can optimally control the state of the satellite over the course of a large angle slew maneuver. In this research, simulation and experimental results clearly demonstrate the benefit of RTOC versus other non-optimal control methods for the same maneuver.

Adaptive Estimation of Nonlinear Spacecraft Attitude Dynamics with Time-Varying Moments of Inertia Using On-Board Sensors

For spacecraft conducting on-orbit operations, changes to the structure of the spacecraft are not uncommon. These planned or unanticipated changes in structural properties tightly couple with the spacecraft’s attitude dynamics and require estimation. For dynamic systems with time-varying parameters, multiple model adaptive estimation (MMAE) routines can be utilized to provide a probabilistic parameter and state estimate. This research applies MMAE routines that employ a parallel bank of unscented attitude filters to analytical models of spacecraft with time varying moments of inertia (MOI). The objective of this new application of adaptive estimation to the attitude determination problem is to estimate the time-varying MOI of the spacecraft and also to probabilistically classify spacecraft behavior. The results presented in this work use on-board three-axis magnetometers and gyroscopes for measurements and investigate the extension of sensor boom. This work lays the foundation for future research using ground-based measurements to estimate time-varying MOI and spacecraft behavior to further enhance the toolbox of the space situational awareness mission.

Evaluation of Formal Methods Tools Applied to a 6U CubeSat Attitude Control System

Exhaustive test of complex and autonomous systems is intractable and cost prohibitive; however, incorporating formal methods analysis throughout the system design process provides a means to identify faults as they are introduced and drastically reduce the overall system development cost. Software errors on fielded spacecraft have resulted in catastrophic faults that could have been prevented had formal methods been applied to the system design. In this research, formal methods, such as model checking and limited theorem proving, are applied to the requirements, architecture, and model development phases of the design process of a reaction wheel attitude control system for a 6U CubeSat. The results show that while feasible, several gaps exist in the capability of formal methods analysis tools. The tools are capable of expressing and analyzing some of the properties of the system, but more work is needed to properly address inherent nonlinearities in complex systems.

PZT behavior in cyclic strain environments

This paper presents results of an experiment designed to determine the impact of repeated strain cycles on lead ziconate titanate (PZT) transducers affixed to an aluminum test specimen. The goal of this research effort is to determine the impact of three cyclic strain levels on PZTs affixed with two different glue types. PZT transducers are evaluated because they are one of the leading health monitoring technologies used in aircraft structures due to their ability to transmit and receive Lamb Waves. Analysis of changes in the received signals can indicate the presence of structural damage. This monitoring paradigm can only be successful if signal changes due to exposure to aircraft environmental factors (temperature/strain/pressure cycles, etc) over time can be clearly identified and characterized. This paper presents the results and initial analysis of experiments to determine the changes in signal responses due to cyclic mechanical strain. Results indicate cyclic strain at 800 με has no effect to 510K cycles, while cyclic strain at 1700 and 2600 με both cause signal loss to varying degrees.

Finite Element Model Tuning with 3D Mode Shapes from FalconSAT-5

The US Air Force Academy constructed the second of two structural engineering models of FalconSAT-5 (FS-5) to validate design modifications resulting from a change in payload requirements. Accurate predictions of the dynamic responses of space launch payloads are required by launch vehicle integrators, but not achieved easily. The goal of this research effort is to develop a process for extensive modal testing and finite element (FE) model tuning to create a finite element model whose dynamic response closely matches the measured response of FS-5. The first step in the process involves measuring and tuning models of the satellite structure panels individually. In tuning the structural panels, material stiffness is the major design variable. The tuned panel FE models are integrated into the full satellite model which is then tuned, where the spring constants of panel connection elemetns are the primary tuning variables. The first eight modes of each side panel, six modes of the top panel, and five modes of the base panel were tuned with eigenvalues matching measured natural frequencies within 2%. Next, the first five modes ranging through 154 Hz were tuned on the full satellite FE model. Predicted natural frequencies were within 3% of measured values for most cases and modes. Modal assurance criterion values comparing tuned FE model eigenvectors and measured mode shapes decreased with increasing numbers of modes tuned, but remained above 0.75 through tuning five modes.