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About this book

Sensors and Instrumentation, Aircraft/Aerospace and Energy Harvesting, Volume 7: Proceedings of the 38th IMAC, A Conference and Exposition on Structural Dynamics, 2020, the seventh volume of eight from the Conference brings together contributions to this important area of research and engineering. The collection presents early findings and case studies on fundamental and applied aspects of Shock & Vibration, Aircraft/Aerospace, Energy Harvesting & Dynamic Environments Testing including papers on:

Alternative Sensing & Acquisition

Active Controls

Instrumentation

Aircraft/Aerospace & Aerospace Testing Techniques

Energy Harvesting

Table of Contents

Frontmatter

Chapter 1. Orion MPCV E-STA Nonlinear Dynamics Uncertainty Factors

Abstract
NASA vibration testing of the European Service Module (ESM) Structural Test Article (E-STA) for the Orion Multi-Purpose Crew Vehicle (MPCV) program demonstrated significant nonlinear behaviors and response deviation from pre-test finite element analysis (FEA). A linear FEA correlation effort, previously performed in 2017, resulted in the creation of two finite element models (FEM) – one correlated to high-load level swept sine responses and one correlated to low-load level swept sine responses. Additional work was required to quantify the uncertainty introduced when applying these linear models to non-sinusoidal flight load cases. To do this, an additional nonlinear dynamics model was developed and correlated with sine sweep test responses for low load level and high load level load cases. Results showed that, when the appropriate linearized model was selected for each specific Coupled Loads Analysis (CLA) loading type (i.e. Liftoff, Transonic, etc…), the linearized models closely matched predicted nonlinear responses with modest error (uncertainty factor). Results of this investigation have established a physics-based nonlinear dynamics approach that uses empirical test data to establish the credibility assumption for usage of linear FEM(s) in CLA. It is anticipated that the methodology employed can be extended for usage in correlation and flight loads analysis of subsequent spacecraft with major joint nonlinearities.
Matt Griebel, Adam Johnson, Brent Erickson, Andrew Doan, Chris Flanigan, Jesse Wilson, Paul Bremner, Joel Sills, Erica Bruno

Chapter 2. The Vibration and Acoustic Effects of Prop Design and Unbalance on Small Unmanned Aircraft

Abstract
The vibration and acoustic effects due to prop design, damage, and unbalance on a popular small unmanned aircraft systems (sUAS) is presented. The use of sUAS or drones is becoming ever more popular for hobbyists, as well as in commercial and military operations, and the influence of props on the vibration and acoustic environment are of interest. Many types of props are available for use on sUAS that promise extended run times, increased performance, and quieter operation. While many systems promise these results, few studies have been conducted to measure and evaluate their true performance. Therefore, this review provides data obtained by experimentally measuring the vibration levels onboard the host aircraft as well as the acoustic levels produced. The data is analyzed to gain further understanding of the vibration and acoustic properties and make predictions on prop performance. The fundamental frequencies of the aircraft are found along with the acoustic signature. These two outcomes are compared and studied to find the correlation between them. The analysis will utilize an airframes that is commonly used in the UAS community along with frequently used props that are balanced and unbalanced and with and without damage. The data is obtained from aircraft fully powered and airborne in a hovering or level flight configuration. This study provides sUAS operators the information required for choosing the most effective prop design to effectively reduce vibration and acoustic sound levels.
William H. Semke, Djedje-Kossu Zahui, Joseph Schwalb

Chapter 3. A Deformed Geometry Synthesis Technique for Determining Stacking and Cryogenically Induced Preloads for the Space Launch System

Abstract
The Space Launch System (SLS) stacking and Core Stage (CS) fueling induce significant preloads that contribute to the liftoff pad separation “twang”. To accurately capture this, an approach is required that can replicate the physics of all SLS physical stacking steps, CS cryogenic shrinkage, associated geometric nonlinearities, and the transient behavior and decay of the preloads with changing boundary conditions as the vehicle separates from the pad.
The Deformed Geometry Synthesis (DGS) approach presented here satisfies the above requirements. DGS determines induced preloads by modeling components in their deformed geometry states and then enforcing compatibility by closing the resulting “deadbands”. DGS seamlessly integrates into the multibody modal synthesis framework and does not require the use of artificial external loads to enforce preloads or post-processing steps to remove their influence. Since DGS iterates to solve for the deformed state inclusive of geometric nonlinearities, running linearized parametrics to exercise different potential orientations of ball jointed struts that connect the CS to Boosters for cryo-shrinkage analyses is entirely avoided.
Relative to the transient behavior and decay of stacking and cryo-induced preloads with SLS liftoff pad separation, this is an area of considerable interest to the SLS program. To capture this in the most accurate way possible, DGS algorithms are designed to work with Henkel-Mar nonlinear pad separation algorithms which operate on the separating longitudinal and lateral degrees of freedom (DoFs) between the vehicle and the pad. As the separating DoFs release, in whatever manner as dictated by the interface geometries, interface loads and interface flexibilities as well as the external loading on the vehicle, the subject preloads generate a complex twang/decay time-trace as dictated by the physics of the problem.
This paper presents DGS numerical verification against the closed-form solution for Timoshenko’s 3 ball-jointed strut preload problem. This problem is then extended by the authors to the geometric nonlinear case where DGS is compared to the Newton-Raphson solution of the nonlinear equations. Next, DGS is utilized to solve the SLS stacking and cryogenic shrinkage coupled loads analyses. Finally, Henkel-Mar pad separation simulations are executed that isolate the impact of the induced preloads’ twang and decay characteristics.
Joel Sills, Arya Majed, Edwin Henkel

Chapter 4. End-to-End Assessment of Artemis-1 Development Flight Instrumentation

Abstract
As the space industry continues to strive for more efficient launch vehicles they must rely on increasingly accurate predictive models. Verification of models typically requires physical testing. Flight data measurements offer the most real and therefore the most accurate data for model correlation. As NASA prepares for the inaugural launch of their new Space Launch System (SLS), Artemis-1, they must rely heavily on predictive system models to ensure flight safety. Artemis-1 will be an unmanned scientific mission with the intent of blazing a trail for future manned missions. NASA has implemented a system of Development Flight Instrumentation (DFI) in the hopes of recovering useful flight data during liftoff and ascent to aid in correlating their predictive models to ensure human safety in future missions. An end-to-end assessment of the DFI system was performed to verify data acquired during Artemis-1 would be adequate for the targeted flight test objectives (FTOs). This was accomplished using a computational simulation of all sensors and Data Acquisition (DAQ) parameters to investigate any potential problem areas in the current architecture. Input nominal signals were approximated and injected into the system model. Synthesized acquired signals were recovered to verify FTO success.
Andrew Doan, Adam Johnson, Tony Loogman, Paul Bremner, Joel Sills, Erica Bruno

Chapter 5. Space Launch System Mobile Launcher Modal Pretest Analysis

Abstract
NASA is developing an expendable heavy lift launch vehicle capability, the Space Launch System, to support lunar and deep space exploration. To support this capability, an updated ground infrastructure is required including modifying an existing Mobile Launcher system. The Mobile Launcher is a very large heavy beam/truss steel structure designed to support the Space Launch System during its buildup and integration in the Vehicle Assembly Building, transportation from the Vehicle Assembly Building out to the launch pad, and provides the launch platform at the launch pad. The previous Saturn/Apollo and Space Shuttle programs had integrated vehicle ground vibration tests of their integrated launch vehicles performed with simulated free-free boundary conditions to experimentally anchor and validate structural and flight controls analysis models. For the Space Launch System program, the Mobile Launcher will be used as the modal test fixture for the ground vibration test of the first Space Launch System flight vehicle, Artemis 1, programmatically referred to as the integrated vehicle modal test. The integrated vehicle modal test of the Artemis 1 integrated launch vehicle will have its core and second stages unfueled while mounted to the Mobile Launcher while inside the Vehicle Assembly Building, which is currently scheduled for the summer of 2020. The Space Launch System program has implemented a building block approach for dynamic model validation. The modal test of the Mobile Launcher is an important part of this building block approach in supporting the integrated vehicle modal test since the Mobile Launcher will serve as a structurally dynamic test fixture whose modes will couple with the modes of the Artemis 1 integrated vehicle. The Mobile Launcher modal test will further support understanding the structural dynamics of the Mobile Launcher and Space Launch System during rollout to the launch pad, which will play a key role in better understanding and prediction of the rollout forces acting on the launch vehicle. The Mobile Launcher modal test is currently scheduled for the summer of 2019. Due to a very tight modal testing schedule, this independent Mobile Launcher modal pretest analysis has been performed to ensure there is a high likelihood of successfully completing the modal test (i.e. identify the primary target modes) using the planned instrumentation, shakers, and excitation types. This paper will discuss this Mobile Launcher modal pretest analysis for its three test configurations and the unique challenges faced due to the Mobile Launcher’s size and weight, which are typically not faced when modal testing aerospace structures.
James C. Akers, Joel Sills

Chapter 6. Feasibility Study to Extract Artemis-1 Fixed Base Modes While Mounted on a Dynamically Active Mobile Launch Platform

Abstract
There are several challenges associated with the scheduled integrated modal test (IMT) of the Space Launch System (SLS) Artemis-1 flight vehicle mounted on the mobile launcher (ML). While the goal of the test is to characterize the Artemis-1, the inclusion of the ML as the support stand for the test means that the entire system must be well characterized. A considerable amount of effort and schedule will have to be devoted to understanding both the test stand (the ML) and the Artemis-1 flight vehicle, and there is a risk that the effort may not be completed in time for a successful launch.
NASA has requested that alternative methods be investigated to generate test results that can remove the effects of the ML from the test. ATA Engineering, Inc., (ATA) was given a reduced model of the Artemis-1 flight vehicle’s IMT configuration containing the candidate set of accelerometers and interface degrees of freedom (DOF). The model was used to determine how well ATA’s fixed base correction technique is able to estimate fixed base modes from test data collected on the IMT. This paper presents the fixed base correction method results.
Kevin L. Napolitano

Chapter 7. Challenges to Develop and Design Ultra-high Temperature Piezoelectric Accelerometers

Abstract
Piezoelectric accelerometer sensors are widely used for testing and monitoring vibrations in automotive, aerospace or industrial applications. The temperature limitation of most piezoelectric accelerometers is below 500 °C, which meets the requirements of typical vibration measurement applications. However, in extreme cases such as engine monitoring, measurements are required up to 900 °C, where the ultra-high temperature accelerometers are needed. This research work focuses on the development of ultra-high temperature piezoelectric accelerometers using bismuth layer-structured ferroelectric piezoceramics as sensing elements. The challenges to develop these materials and create the corresponding ultra-high temperature accelerometers will be discussed in this paper.
Chang Shu, Neill Ovenden, Sina Saremi-Yarahmadi, Bala Vaidhyanathan

Chapter 8. Application of Quasi-Static Modal Analysis to an Orion Multi-Purpose Crew Vehicle Test

Abstract
Bolted structural joints often exhibit load-dependent stiffness and energy dissipation that leads to nonlinear, amplitude dependent frequency and damping in the structure. As an alternative to direct integration of the nonlinear equations of motion, quasi-static modal analysis (QSMA) determines the dependence of frequency and damping on response amplitude using loading behavior from nonlinear static analyses. QSMA has previously been demonstrated to substantially reduce computational cost and maintain accuracy relative to full nonlinear dynamic simulation. This work explores the applicability of QSMA to a complex, large-scale aerospace structure. QSMA is employed to analyze a nonlinear model of test hardware developed to support the Orion Multi-Purpose Crew Vehicle program, which exhibited nonlinear behavior during dynamic testing at flight-like load levels. In addition to the extraction of amplitude-dependent frequency and damping curves, a Bouc-Wen hysteresis model was used in conjunction with the quasi-static results to develop nonlinear, uncoupled, time-domain modal equations of motion for the structure. Excellent agreement was observed between the reduced and full-order nonlinear models, encouraging future employment of QSMA to support accurate and efficient model reduction of structures with bolted joint nonlinearities.
Matthew S. Allen, Joe Schoneman, Wesley Scott, Joel Sills

Chapter 9. Using BB-gun or Acoustic Excitation to Find High Frequency Modes in Additively Manufactured Parts

Abstract
Additive manufacturing (AM) considers parts that are produced at a low volume or with complex geometries. Identifying internal defects on these parts is difficult as current testing techniques are not optimized for AM processes. The resonant frequency method can be used to find defects in AM parts as an alternative to X-ray or CT scanning. Higher frequency modes at approximately 8000 Hz and above cannot be tested with a traditional modal hammer or shaker since they do not provide enough excitation. The goal of this paper is to evaluate creative testing techniques to find internal defects in parts with high frequency modes. The two types of testing methods considered are acoustic excitation provided by two speakers and high velocity impact testing produced by a BB – gun. Although the frequency ranges of interest are part dependent, these techniques were able to reach up to 16,000 Hz, which is an additional 8000 Hz above what the traditional modal hammer is able to reach. This work was funded by the Department of Energy’s Kansas City National Security Campus which is operated and managed by Honeywell Federal Manufacturing Technologies, LLC under contract number DE-NA0002839.
Aimee Allen, Kevin Johnson, Jason R. Blough, Andrew Barnard, Troy Hartwig, Ben Brown, David Soine, Tristan Cullom, Douglas Bristow, Robert Landers, Edward Kinzel

Chapter 10. Parametric Analysis and Voltage Generation Performance of a Multi-directional MDOF Piezoelastic Vibration Energy Harvester

Abstract
Piezoelectric vibration energy harvesting has been extensively investigated in recent years and the majority of results focus on using the cantilever beam model under base driven motion. The main focus of this paper is to perform a parametric analysis of multi degrees of freedom piezo-elastic energy harvester to optimize the capability curve exploiting crossing/veering between modes. The structure under test consists of a combination of slender beams with one or more orthogonal beam segments placed on it. The resulting combined structure exhibits bending vibration modes in orthogonal planes. The cross and veering phenomena are studied in deep, attempting to improve the resulting mechanical-electrical energy conversion of the combined structure. A numerical model of the system under investigation is developed considering also non-classical damping. A parametric analysis of the system’ s performance due to geometrical and electrical properties variations are investigated to design a broadband harvester. An experimental analysis is performed on a test rig specially built to investigate the crossing and veering phenomena effects on the resulting output voltage from the energy harvester. Numerically simulated and experimental data are compared to provide information for updating the model as well as to address the efficiency of the harvester in terms of voltage generation.
Paulo S. Varoto, Elvio Bonisoli, Domenico Lisitano

Chapter 11. Are We Nearly There Yet? Progress Towards the Fusion of Test and Analysis for Aerospace Structural Dynamics

Abstract
The dual roles of Test and Analysis (experimentation and simulation) in Structural Dynamics have become increasingly inter-dependent for several years, and we are now approaching a level of interaction referred to as Fusion. Today, analysis and test can serve to minimise the effects of the inevitable uncertainties which arise in real engineering circumstances where assumptions, approximations and selections must be made. Here we review the status of this development and identify some specific situations where further enhancements can still be made.
David Ewins

Chapter 12. Feasibility Study of SDAS Instrumentation’s Ability to Identify Mobile Launcher (ML)/Crawler-Transporter (CT) Modes During Rollout Operations

Abstract
The Space Launch System (SLS) and its Mobile Launcher (ML) will be transported to the launch pad via the Crawler-Transporter (CT) system. Rollout (i.e., transportation) loads produce structural loads on the integrated SLS/Orion Multi-Purpose Crew Vehicle (MPCV) launch vehicle which are of a concern with respect to fatigue. As part of the risk reduction process and in addition to the modal building block test approach that has been adopted by the SLS Program, acceleration data will be obtained during rollout for use in modal parameter estimation. There are several occurrences where the ML/CT will be transported either into the Vertical Assembly Building (VAB) or to the launch pad and back without the SLS stack as part of the Kennedy Space Center (KSC) Exploration Ground Systems (EGS) Integrated Test and Checkout (ITCO). NASA KSC EGS has instrumentation installed on both the ML and CT to record data during rollout, at the launch pad, and during liftoff. The EGS instrumentation on the ML, which includes accelerometers, is referred to as the Sensor Data Acquisition System (SDAS). The EGS instrumentation on the CT, which also includes accelerometers, is referred to as the CT Data Acquisition System (CTDAS). The forces and accelerations applied to the ML and CT during a rollout event will be higher than any of the planned building block modal tests. This can be very beneficial in helping identify nonlinear behavior in the structure. Developing modal parameters from the same test hardware in multiple boundary conditions and under multiple levels of excitation is a key step in developing a well correlated FEM.
The purpose of this study was three fold. First, determine the target modes of the ML/CT in its rollout configuration. Second, determine if the test degrees of freedom (DOF) corresponding to the layout of the SDAS/CTDAS accelerometers (i.e. position and orientation) is sufficient to identify the target modes. Third, determine if the Generic Rollout Forcing Functions (GRFF’s) (“Development of Generic Crawler/Transporter Rollout Forcing Functions for Coupled System Dynamics Analysis,” NASA Exploration Systems Directorate/Cross-Program Systems Integration Technical Assessment Report, ESD 20038, July 31, 2018) is sufficient for identifying the ML/CT target modes accounting for variations in CT speed, modal damping, and sensor/ambient background noise levels.
The finding from the first part of this study identified 28 target modes of the ML/CT rollout configuration based upon Modal Effective Mass Fractions (MEFF) and engineering judgement. The finding from the second part of this study showed that the SDAS/CTDAS accelerometers (i.e. position and orientation) are able to identify a sufficient number of the target modes to support model correlation of the ML/CT FEM. The finding from the third part of this study confirms the GRFFs sufficiently excite the ML/CT such that varying quantities of the defined target modes should be able to be extracted when utilizing an Experimental Modal Analysis (EMA) Multi-Input Multi-Output (MIMO) analysis approach. An EMA analysis approach was used because Operational Modal Analysis (OMA) tools were not available and the GRFFs were sufficiently uncorrelated. Two key findings from this third part of the study are that the CT speed does not show a significant impact on the ability to extract the modal parameters and that keeping the ambient background noise observed at each accelerometer location at or below 30 μgrms is essential to the success of this approach.
Even though this study relies heavily upon the accuracy of both uncorrelated ML and CT FEM’s and unconfirmed rollout forcing functions, all of which will most likely differ from actuality, it provides important insights into the ability to extract modal parameters from the upcoming rollout events.
James P. Winkel, James C. Akers, Erica Bruno

Chapter 13. The Integrated Modal Test-Analysis Process (2020 Challenges)

Abstract
The prevailing approach to the integrated test analysis process (ITAP) in the U.S. aerospace community involves execution of seven successive tasks, namely (1) model development, (2) modal test planning, (3) measured data acquisition, (4) measured data analysis, (5) experimental modal analysis, (6) test analysis correlation, and (7) model reconciliation. Persistence of closely spaced body and shell breathing modes in launch vehicle and spacecraft structures presents a “many modes” challenge to selection of analytical and experimental target modes. Well established coupled loads analysis decomposition appear to provide relief to the “many modes” challenge. Strategies aimed at prioritizing the multitude of sensitivities and uncertainties offer further relief. Established government orthogonality criteria are difficult to satisfy due to the multitude of closely-spaced measured complex modes. Recent introduction of a left- hand eigenvector, experimental modal analysis strategy appears to eliminate these difficulties. Unavoidable nonlinearity in structural joints threatens to invalidate many aspects of the established ITAP scheme. Recollection of space shuttle era nonlinear coupled loads methodologies and incorporation of modern, nonlinear hysteretic joint models offers promising relief to the structural joint difficulties. All of the above noted techniques and strategies may lead to more effective test analysis correlation and reconciliation outcomes.
Robert N. Coppolino

Chapter 14. Roadmap for a Highly Improved Modal Test Process

Abstract
Difficulties encountered in modal test planning and recent advances in experimental modal analysis of aerospace systems have resulted in a new paradigm for experimental modal analysis. The Simultaneous Frequency Domain (SFD) method, weighted complex linear least squares correlation methodology, and state space model left- and right- hand eigenvector properties combine to produce the following benefits that are independent of an explicit TAM mass matrix: (1) verification and validation of experimental modes via isolation of individual experimental modes, (2) automatic self- orthogonality of experimental modes, (3) test- FEM cross orthogonality, and (4) experimental complex mode kinetic energy distribution. The new approach directly employs complex experimental modes (rather than real mode approximations) and frees experimental data from a potentially flawed TAM mass matrix. A roadmap for incorporation of the new paradigm into NASA and USAF standards and continued progress are outlined in this paper.
Robert N. Coppolino

Chapter 15. Using Low-Cost “Garage Band” Recording Technology for Acquiring High Resolution High-Speed Data

Abstract
The Oak Ridge National Laboratory (ORNL) has developed and tested a novel system architecture for acquiring high fidelity high-speed data. The approach uses a consumer grade audio recording device that is normally associated with “garage band” recording of music. ORNL has coupled this low-cost data acquisition hardware with computing technology running open-source software. The main advantage of this approach is per-channel cost; an instrument grade data acquisition system typically costs between $800 to $2000 per channel compared to less than $50 per channel for these consumer grade components. Three systems, each featuring four channels, have been deployed for acquiring data from geophones and the electrical supply system that supports the High Flux Isotope Reactor (HFIR) and the Radiochemical Engineering Development Center (REDC) at ORNL. Each channel samples at 96 kHz at 24-bit resolution. The deployed systems operate continuously 24/7 and produce about 4 terabytes of data per month per system. This paper provides a technical overview of this approach, its implementation, and some preliminary results from qualification testing. This work was conducted in support of the Multi-Informatics for Nuclear Operations Scenarios (MINOS).
Randall Wetherington, Gregory Sheets, Tom Karnowski, Ryan Kerekes, Michael Vann, Michael Moore, Eva Freer

Chapter 16. Hybrid Slab Systems in High-rises for More Sustainable Design

Abstract
Greenhouse gases trap heat within our atmosphere, leading to an unnatural increase in temperature. Carbon dioxide and its equivalent emissions have been a large focus when considering sustainability in the civil engineering field, with a reduction of global warming potential being a top priority. According to a 2017 report by the World Green Building Council, the construction and usage of buildings account for 39 percent of human carbon emissions in the United States, almost one third of which are from the extraction, manufacturing, and transportation of materials. Substituting wood for high emission materials could greatly reduce carbon if harvested and disposed of in a controlled way. To investigate this important issue, San Francisco State University and University of South Carolina partnered with Skidmore, Owings & Merrill LLP, a world leader in designing high-rise buildings, through a National Science Foundation (NSF) Research Experience for Undergraduates (REU) Site program, to investigate and quantify the embodied carbons of various slab system designs using a high-rise residential complex in San Francisco as a case study. Three concept designs were considered: a concrete building with cementitious replacement, a concrete building without cementitious replacement, and a concrete building with cementitious replacement and nail-laminated timber wood inlays inserted into various areas of the superstructure slabs. The composite structural slab system has the potential to surpass the limitations of wood-framed structures yet incorporate the carbon sequestration that makes wood a more sustainable material. The results show that wood substitution could decrease overall emissions from the aforementioned designs and reduce the environmental footprint of the construction industry.
Katherine Berger, Samuel Benzoni, Zhaoshuo Jiang, Wenshen Pong, Juan Caicedo, David Shook, Christopher Horiuchi

Chapter 17. Ground Vibration Testing of the World’s Longest Wingspan Aircraft—Stratolaunch

Abstract
The record-setting Stratolaunch (Roc) carrier aircraft first took to the skies on April 13, 2019, staying aloft for 149 minutes before successfully landing back in Mojave, California. Since 2012, Stratolaunch Systems Corporation, a space transportation venture created in part by Scaled Composites, has been designing, building, and testing the world’s largest composite aircraft. The goal of this mobile launch system is to make orbital access to space more convenient, reliable, and routine. To achieve the first successful flight of Roc, several ground vibration tests (GVTs) were necessary to characterize the modal properties of the composite aircraft and its subassemblies. ATA Engineering, Inc., (ATA) completed two partial GVTs and a full-scale GVT to help Stratolaunch Systems Corporation engineers achieve their successful first flight. The results of the GVTs were used to update the finite element models (FEMs) used for flutter and dynamic stability predictions. Testing an aircraft of this size imposed a number of challenges not encountered in most GVT programs; to efficiently conduct the tests, a distributed data acquisition system approach was used, and seismic accelerometers characterized the aircraft’s low-frequency rigid body modes. The distribution of shakers and sensors around the aircraft was addressed by the implementation of a new sensor cable system and the adaptation of multishaker excitation methods using temporary support structures.
Douglas J. Osterholt, Timothy Kelly

Chapter 18. Using Recorded Data to Improve SRS Test Development

Abstract
The Shock Response Spectrum (SRS) approach is an effective and widely used method for analyzing mechanical shock phenomena. Synthetic waveforms are commonly used in the development of an SRS; unfortunately, there is often no synthetic pulse with a frequency response that matches well to a given real-world transient event.
This presentation describes a unique approach, where recordings from a field environment are modified to meet or exceed a specified SRS. Comparisons are made between a modified user waveform developed from this field-based SRS and waveforms developed with commonly applied synthetic pulses.
Joel Minderhoud

Chapter 19. Distributed Acquisition and Processing Network for Experimental Vibration Testing of Aero-Engine Structures

Abstract
Detailed vibration testing of large assembled structures, such as aeroengines, leads to significant requirements on data acquisition and processing. This can lead to high system cost and long post processing times, which often limit the amount of data that can be acquired. A novel hardware-software acquisition system combination is proposed here to overcome some of the challenges of large scale data acquisition, based on the idea to distribute the acquisition and data processing load between a network of specialized acquisition nodes. The nodes work in parallel and are independent of each other, while sharing a synchronization clock. Each node has the capability to process the data being acquired on-line. The network allows for testing of novel data analysis methods and its modular nature enables an easy expansion of the system when required.
Michal J. Szydlowski, Christoph W. Schwingshackl, Andrew Rix

Chapter 20. Modal Test of the NASA Mobile Launcher at Kennedy Space Center

Abstract
The NASA Mobile Launcher (ML), located at Kennedy Space Center (KSC), has recently been modified to support the launch of the new NASA Space Launch System (SLS). The ML is a massive structure—consisting of a 345-foot tall tower attached to a two-story base, weighing approximately 10.5 million pounds—that will secure the SLS vehicle as it rolls to the launch pad on a Crawler Transporter, as well as provide a launch platform at the pad. The ML will also provide the boundary condition for an upcoming SLS Integrated Modal Test (IMT). To help correlate the ML math models prior to this modal test, and allow focus to remain on updating SLS vehicle models during the IMT, a ML-only experimental modal test was performed in June 2019. Excitation of the tower and platform was provided by five uniquely-designed test fixtures, each enclosing a hydraulic shaker, capable of exerting thousands of pounds of force into the structure. For modes not that were not sufficiently excited by the test fixture shakers, a specially-designed mobile drop tower provided impact excitation at additional locations of interest. The response of the ML was measured with a total of 361 accelerometers. Following the random vibration, sine sweep vibration, and modal impact testing, frequency response functions were calculated and modes were extracted for three different configurations of the ML in 0 Hz to 12 Hz frequency range. This paper will provide a case study in performing modal tests on large structures by discussing the Mobile Launcher, the test strategy, an overview of the test results, and recommendations for meeting a tight test schedule for a large-scale modal test.
Eric C. Stasiunas, Russel A. Parks, Brendan D. Sontag, Dana E. Chandler

Chapter 21. Using Deep-Learning Approach to Detect Anomalous Vibrations of Press Working Machine

Abstract
In recent years, there has been a demand for advanced maintenance in factories. Data collection from factory equipment is being carried out, and the collected sensor data is widely used for statistical analysis in quality control and failure prediction by machine learning. For example, if it is possible to detect an abnormality using vibration data obtained from an equipment, increase in the operation rate of the plant can be expected. In this research, we aim at early detection of equipment failure by finding signs of abnormality from vibration data, using a deep-learning technique, particularly an autoencoder.
In this paper, the following two methods were tested. The first scheme is based on the reconstruction error in an autoencoder. An autoencoder is trained using normal data only. Looking at the difference between input data and reconstructed data, we can regard the data having higher difference as abnormal. In the second approach, given the input data, values of the middle layer of the autoencoder are extracted, and we calculate the degree of abnormality using a Gaussian Mixture Model (GMM), representing a data set by superposition of a mixture of Gaussian distributions. In this framework, regarding an autoencoder structure, we tested both full-connection networks and convolutional networks.
In this work, we chose a press machine. Frequency characteristics were acquired from the data in production mode of a press machine. Then using each method, we evaluated whether abnormality could be found by calculating the degree of abnormality. We employed two-day data without failure as training data, and another data set was prepared as forecast data obtained on the following days; on one of the days the machine stopped due to a sudden abnormality. Similar to time-series signal processing, we applied framing processing so that we can analyze data even in the case we can only get a small amount of data. As a result, our method succeeded in finding the day when the abnormality occurred and the machine stopped. In addition, the degree of abnormality became higher before the abnormality occurs, indicating we can detect signs of abnormality.
In conclusion, the degree of abnormality could be calculated using the reconstruction error using an autoencoder from the vibration data during production, and the method using GMM from the middle layer of autoencoder. We consequently conclude it is possible to detect a sudden abnormality in which the device stopped, from actual vibration data. These results provide new solutions for equipment failure estimation.
Kazuya Inagaki, Satoru Hayamizu, Satoshi Tamura

Chapter 22. DAQ Evaluation and Specifications for Pyroshock Testing

Abstract
Pyroshock events contain high-amplitude, extreme rise-time accelerations that can be damaging to electronics and small structures. Due to their extreme nature, these events can be difficult to capture, exceeding the performance limits of transducers, signal conditioning, and data acquisition (DAQ) equipment. This study assesses the ability of different data acquisition systems to record quality pyroshock data. Using a function generator and voltage input, different tests were performed to characterize the data acquisition systems’ anti-alias filter, out-of-band energy attenuation, number of effective bits, in-band gain, and slew rate. These tests include a shorted-input noise test, a sine sweep test, and a high amplitude low frequency square wave test. Although the data acquisition systems evaluated have similar specifications, their ability to record quality pyroshock data varied. Some of these data acquisition systems do not appropriately handle the rapid transient content and may have inadequate fidelity to record pyroshock data. Data acquisition system performance for pyroshock testing cannot be evaluated by the specification sheet alone.
Erica M. Jacobson, Jason R. Blough, James P. DeClerck, Charles D. Van Karsen, David Soine

Chapter 23. Optimal Replicator Dynamic Controller via Load Balancing and Neural Dynamics for Semi-Active Vibration Control of Isolated Highway Bridge Structures

Abstract
During the past few decades, major structural damages due to natural disasters like earthquakes has led bridge engineers to develop structural control systems to mitigate damage and improve vibration reduction in real-time. Among different kinds of vibration control systems, base isolation is one of the most commonly used passive control strategies for civil structures. However, base isolators have their own limitations due to the lack of real-time adaptability and lower energy dissipation. In order to overcome this limitation, semi-active damping devices are installed between the deck and piers. In the present study, a semi-active control system comprised of magneto-rheological (MR) dampers is proposed for vibration mitigation of isolated bridge structures. Recently, inspired by evolutionary game theory, a replicator dynamic control algorithm was developed to allocate the input voltage of MR dampers. In this paper, a load balancing strategy is studied to reallocate additional resources and improve the power distribution over semi-active MR dampers. In order to achieve a high-performance design of the replicator controller, a modified patented Neural Dynamic (ND) model of Adeli and Park is used to optimize the load-balanced replicator control parameters. The ND model incorporates a penalty function, the Lyapunov stability theorem, and the Karush-Kuhn-Tucker conditions to guarantee the global convergence of the solution. The objective function is then defined to minimize the dynamic response of the bridge. The proposed methodology is evaluated using a benchmark control problem that is based on Interstate 5 overcrossing California State Route 91 bridge in Southern California subjected to near-field earthquake accelerograms. The performance of the proposed controller is evaluated and compared with conventional Lyapunov and fuzzy control algorithms in terms of 16 different performance criteria describing the reductions in dynamic response of the bridge structure.
Sajad Javadinasab Hormozabad, Mariantonieta Gutierrez Soto

Chapter 24. Forcing Function Estimation for Space System Rollout

Abstract
American crewed spaceflight systems have used a significant tracked transportation system to rollout the stacked vehicle and launch platform from the Vehicle Assembly Building (VAB) to the launch pad. This system has been shown to produce specific narrow-band harmonic forces that can excite the complete system, including the ability to force a resonance for long periods of time. This work reports on the efforts to develop tools and processes to reconstruct rollout forcing functions to support advanced needs for the extraction of dynamic properties. The multi-step processes provided in this work include a hybrid version of a traditional force reconstruction approach; transfer of expected CG forces to the assumed force input locations; expansion of the forces to a full-rank set of input forces; application of known dynamic constraints; and joint updating of FRFs and the estimated forces.
Three levels of success criteria are suggested for these processes as applied to analytical, laboratory, or measured field data: synthesizing output accelerometer data; reconstructing input forcing functions; and estimating modal properties. A dynamically simple system, limited instrumentation, and a simple single-speed forcing function are used to provide measured and analytical data to exercise and assess the tools of interest. Useful comparisons supporting the first success criteria are provided. Data and insight are also provided for the second success criteria. The simplicity of the example system supporting the work reported in this paper does not yet support the development of significant insight on the third success criteria. Follow-on activities are recommended to add additional insight into the process success with all three criteria.
This work not only addresses engineering development work of specific and unique interest to spaceflight systems but also suggests general applicability for force estimation, dynamic property estimation, and operational testing situations. Specifically these tools are to enable the estimation of dynamic properties when the loading and constraint environments complicate traditional OMA techniques. Unique contributions of this process include: expanding inputs and constraints of traditional force reconstruction techniques; using null space vectors to complete the basis set for full rank force reconstruction; and constraining linear solutions to filter estimated forces for targeted force updating.
George James, Robert Grady, Matt Allen, Erica Bruno
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