Current satellite transportation sensors can provide a binary indication of the
acceleration or shock that a satellite has experienced during the shipping process
but do little to identify if significant structural change has occurred in the satellite
and where it may be located. When a sensor indicates that the satellite has
experienced shock during transit, an extensive testing process begins to evaluate the
satellite functionality. If errors occur during the functional checkout, extensive
physical inspection of the structure follows. In this work an alternate method for
inspecting satellites for structural defects after shipping is presented. Electro-
Mechanical Impedance measurements are used as an indication of the structural
state. In partnership with the Air Force Research Laboratory University
Nanosatellite Program, Cornell's CUSat mass model was instrumented with
piezoelectric transducers and tested under several structural damage scenarios. A
method for detecting and locating changes in the structure using EMI data is
presented.
KEYWORDS: Sensors, Wave propagation, Structural health monitoring, Satellites, Signal attenuation, Actuators, Finite element methods, Data modeling, Analytical research, Nondestructive evaluation
This work focuses on an analysis of wave propagation in isogrid structures as it
relates to Structural Health Monitoring (SHM) methods. Assembly, integration,
and testing (AI&T) of satellite structures in preparation for launch includes
significant time for testing and reworking any issues that may arise. SHM methods
are being investigated as a means to validate the structure during assembly and
truncate the number of tests needed to qualify the structure for the launch
environment. The most promising of these SHM methods uses an active wave-based
method in which an actuator propagates a Lamb wave through the structure; the
Lamb wave is then received by a sensor and evaluated over time to detect structural
changes. To date this method has proven effective in locating structural defects in a
complex satellite panel; however, the attributes associated with the first wave
arrival change significantly as the wave travels through ribs and joining features.
Previous studies have been conducted in simplified ribbed structures, giving initial
insight into the complex wave propagation phenomena. In this work, the study has
been extended numerically to the isogrid plate case. Wave propagation was
modeled using commercial finite element analysis software. The results of the
analyses offer further insight into the complexities of wave propagation in isogrid
structures.
This work focuses on the detection, localization, and quantification of damage in the form
of loose bolts on an isogrid satellite structure. In the process of rapid satellite development
and deployment, it is necessary to quickly complete several levels of validation tests.
Structural Health Monitoring methods are being investigated as a means for reducing the
number of validation tests required. This method for detecting loose bolts enables quick
confirmation of proper assembly, and verification that structural fasteners are still intact
after validation testing. Within this testing framework, feature selection is presented as well
as a localization methodology. Quantification of fastener torque is also developed. Locating
damage in an isogrid structure is complicated by the directionally dependent dispersion
characteristics caused by a propagating wave passing through ribs and holes. For this
reason, an actuation frequency with the best first wave arrival clarity is selected. A
methodology is presented in which a time map is constructed for each actuator-sensor pair which establishes times of flight for each location on the sample. Differences in time between healthy and damaged sensor signals are then extracted and used to create a map of possible damage locations. These resulting solution maps are merged yielding a final damage position. Fastener torque is correlated to a damage parameter, and the loose bolt position is calculated within 3 cm.
Integrity of bolted joints is critical for successful deployment and operation of space structures. Conventional
structural qualification tests span weeks if not months and inhibit rapid launch of space systems. Recent developments
in the embedded ultrasonic acousto-elastic method offer fast diagnosis of bolted joints and opportunities for locating
the fault. However, in current acousto-elastic measurement procedures, a baseline representing the healthy condition
of the joint is necessary. To mitigate a requirement of the baseline, a new methodology based on relative amplitude
and phase measurements is developed. The approach has been validated on laboratory specimens, and modifications
were suggested for applications in realistic structures. The paper discusses principles of the baseline-free acoustoelastic
method, its practical realization, and respective advantages and disadvantages. Comparison of baseline and
baseline-free approaches is presented showing the utility of the recently proposed methodology. Fundamentals of the
acousto-elastic response were studied in experiments involving guided wave propagation in a thin plate under tension.
The results indicate a difference between acousto-elastic responses collected using sensors oriented parallel and perpendicular to the applied stress. It is suggested that this effect may be used to infer stress orientation in the sample. Practical issues related to acousto-elastic measurements in realistic complex structures are discussed, damage diagnosis algorithms are presented, and potential extensions of the acousto-elastic technique are proposed.
KEYWORDS: Satellites, Structural health monitoring, Space operations, Data modeling, Visual process modeling, Sensors, Satellite communications, 3D displays, Defense and security, Aluminum
The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing Structural Health Monitoring
(SHM) technologies in support of the Department of Defense's Operationally Responsive Space (ORS) initiative. Such
technologies will significantly reduce the amount of time and effort required to assess a satellite's structural surety.
Although SHM development efforts abound, ORS drives unique requirements on the development of these SHM
systems. This paper describes several technology development efforts, aimed at solving those technical issues unique to
an ORS-focused SHM system, as well as how the SHM system could be implemented within the structural verification
process of a Responsive satellite.
KEYWORDS: Autoregressive models, Ultrasonics, Structural health monitoring, Microsoft Foundation Class Library, Algorithm development, Signal processing, Data modeling, Satellites, Sensors, Performance modeling
The Operationally Responsive Space (ORS) strategy hinges, in part, on realizing technologies which can facilitate
the rapid deployment of satellites. Presently, preflight qualification testing and vehicle integration processes
are time consumptive and pose as two significant hurdles which must be overcome to effectively enhance US
space asset deployment responsiveness. There is a growing demand for innovative embedded Structural Health
Monitoring (SHM) technologies which can be seamlessly incorporated onto payload hardware and function in
parallel with satellite construction to mitigate lengthy preflight checkout procedures. In this effort our work is
focused on the development of a joint connectivity monitoring algorithm which can detect, locate, and assess
preload in bolted joint assemblies. Our technology leverages inexpensive, lightweight, flexible thin-film macro-fiber composite (MFC) sensor/actuators with a novel online, data-driven signal processing algorithm. This
algorithm inherently relies upon Chaotic Guided Ultrasonic Waves (CGUW) and a novel cross-prediction error
classification technique. The efficacy of the monitoring algorithm is evaluated through a series of numerical
simulations and experimentally in two test configurations. We conclude with a discussion surrounding further
development of this approach into a commercial product as a real-time flight readiness indicator.
Responsive space satellites must be assembled and tested in extremely short times. Integrity of structural joints is one of
the major concerns during satellite assembly and qualification processes. A structural health monitoring (SHM)
approach based on nonlinear ultrasonics is suggested for rapid diagnostics of structural connectors and joints. Embedded
piezoelectric sensors are utilized to enable propagation of elastic waves through bolted aluminum panels. Signal
parameters indicative of the nonlinear behavior are extracted from the received waveforms and are used for assessment
of structural integrity. Experimental studies reveal variation of the nonlinear response of the joint due to applied
structural loads. These changes are explored as diagnostic features of the method. We discuss theoretical aspects of the
nonlinear wave propagation through joints and provide experimental data showing feasibility of the embedded nonlinear
ultrasonics method for monitoring of structural integrity.
Brandon Arritt, Steven Buckley, Jeffrey Ganley, Jeffry Welsh, Benjamin Henderson, M. Eric Lyall, Andrew Williams, Jeffrey Preble, John DiPalma, Greg Mehle, R. Roopnarine
The Air Force Research Laboratory/Space Vehicles Directorate (AFRL/RV) is developing a satellite structural
architecture in support of the Department of Defense's Operationally Responsive Space (ORS) initiative. Such a
structural architecture must enable rapid Assembly, Integration, and Test (AI&T) of the satellite, accommodate multiple
configurations (to include structural configurations, components, and payloads), and incorporate structurally integrated
thermal management and electronics, while providing sufficient strength, stiffness, and alignment accuracy. The chosen
approach will allow a wide range of satellite structures to be assembled from a relatively small set of structural
components. This paper details the efforts of AFRL, and its contractors, to develop the technology necessary to realize
these goals.
The Department of Defense is actively pursuing a Responsive Space capability that will dramatically reduce the cost and
time associated with getting a payload into space. In order to enable that capability, our space systems must be modular
and flexible to cover a wide range of missions, configurations, duty cycles, and orbits. This places requirements on the
entire satellite infrastructure: payloads, avionics, electrical harnessing, structure, thermal management system, etc. The
Integrated Structural Systems Team at the Air Force Research Laboratory, Space Vehicles Directorate, has been tasked
with developing structural and thermal solutions that will enable a Responsive Space capability. This paper details a
"symbiotic" solution where thermal management functionality is embedded within the structure of the satellite. This
approach is based on the flight proven and structurally efficient isogrid architecture. In our rendition, the ribs serve as
fluidic passages for thermal management, and passively activated valves are used to control flow to the individual
components. As the paper will explain, our analysis has shown this design to be structurally efficient and thermally
responsive to a wide range of potential satellite missions, payloads, configurations, and orbits.
KEYWORDS: Satellites, Structural health monitoring, Sensors, Space operations, Connectors, Satellite communications, Diagnostics, Signal processing, Analytical research, 3D displays
The United States is striving to develop an Operationally Responsive Space capability. The goal is to be able to deliver
tailored spacecraft capabilities to the warfighter as needs arise. This places a premium on the timespan between
generating that requirement and having a functioning satellite performing its mission on orbit. Although there is lively
debate regarding how to achieve this responsive space capability, one thing remains undeniable; the satellite flight
qualification and launch vehicle integration process needs to be dramatically truncated. This paper describes the Air
Force Research Laboratory's attempts to validate the use of Structural Health Monitoring (SHM) in lieu of traditional
structural flight qualification testing schemes (static and shock loads, random vibration, coupled loads analysis, thermal
vacuum testing, etc.) for potential Responsive Space (RS) satellites.
The Department of Defense is pursuing efforts to develop ever larger space-based apertures in an attempt to provide
better imagery and intelligence for national defense purposes. Unfortunately, due to their shear enormity, many of these
apertures will experience displacements and kinematics that work to degrade the resolution of incoming images.
Currently, there are multiple efforts to develop software-based compensation schemes, to "clean up the image;"
however, these schemes rely on high precision metrology systems to provide accurate information regarding the shape
and dynamics of the structure. This paper details a collaborative effort, between the Air Force Research
Laboratory/Space Vehicles Directorate (AFRL/VS), the Aerospace Corporation, CSA Engineering, and Jackson and Tull
Engineering, conducted to assess the potential of using a fiber-Bragg's system for the metrology of large, space-based
apertures.
We discuss work in progress on a near-infrared tunable bandpass filter for the Goddard baseline wide field camera concept of the Next Generation Space Telescope Integrated Science Instrument Module. This filter, the Demonstration Unit for Low Order Cryogenic Etalon (DULCE), is designed to demonstrate a high efficiency scanning Fabry-Perot etalon operating in interference orders 1 - 4 at 30 K with a high stability DSP based servo control system. DULCE is currently the only available tunable filter for lower order cryogenic operation in the near infrared. In this application, scanning etalons will illuminate the focal plane arrays with a single order of interference to enable wide field lower resolution hyperspectral imaging over a wide range of redshifts. We discuss why tunable filters are an important instrument component in future space-based observatories.
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