Techno Press
Tp_Editing System.E (TES.E)
Login Search


You have a Free online access.
aas
 
CONTENTS
Volume 5, Number 2, March 2018
 

Abstract
Jet Propulsion Laboratory has traditionally performed system level vibration testing of flight spacecraft. There have been many discussions in the aerospace community for more than a decade about spacecraft vibration testing benefits or lack thereof. The benefits and potential issues of fully assembled flight spacecraft vibration testing are discussed herein. The following specific topics are discussed: spacecraft screening test to uncover workmanship problems for launch dynamics environments, force- and moment-limited vibration testing, potential issues with structural frequency identification using base shake test data, and failures related to vibration shaker testing and ways to prevent them.

Key Words
random vibration and sine test; acoustic; shaker vibration; virtual shaker; force limited RV test

Address
Ali R. Kolaini, Walter Tsuha and Juan P. Fernandez: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Abstract
This paper deals with the idea to replace the usual high-level sine sweep test on shaker at system level, very severe, by a low level one completed by a transient test in the same configuration, in order to be more representative of the real environment, thus limiting over testing and improving the payload comfort. The problem of the transient test specification is first discussed. The proposed solution is to derive from LV/SC coupled analyses a shock response spectrum corresponding to two damping ratios. Then, the question of adequate shock synthesis is tackled. A new method with a given spectrum is considered for better potential and accuracy than the usual wavelets. A campaign on the Intespace bi-shaker devoted to system level showed its capability to perform the resulting test with one spectrum. First investigations to extend this approach to two spectra are in progress.

Key Words
SRS; shock synthesis; transient testing; fast sine sweep

Address
Alain Girard, Etienne Cavro and Paul-Eric Dupuis: INTESPACE, 2 Rond-Point Pierre Guillaumat, 31029 Toulouse Cedex 4, France

Abstract
The sine sweep base excitation test campaign is a major milestone in the process of mechanical qualification of space structures. The objectives of these vibration tests are to qualify the specimen with respect to the dynamic environment induced by the launcher and to demonstrate that the spacecraft FE model is sufficiently well correlated with the test specimen. Dynamic qualification constraints lead to performing base excitation sine tests using a sine sweep over a prescribed frequency range such that at each frequency the response levels at all accelerometers, load cells and strain gages is the same as the steady state response. However, in practice steady state conditions are not always satisfied. If the sweep rate is too high the response levels will be affected by the presence of transients which in turn will have a direct effect on the estimation of modal parameters. A study funded by ESA and AIRBUS D&S was recently carried out in order to investigate the influence of sine sweep rates in actual test conditions. This paper presents the results of this study along with recommendations concerning the choice of methods.

Key Words
sine sweep; FRF estimation; modal identification

Address
Nicolas Roy: Top Modal, 130 rue Galilee, 31670 Labege, France
Maxime Violin: Airbus Defence & Space, 31 rue des Cosmonautes, 31500 Toulouse, France
Etienne Cavro: Intespace, 2 rond-point Pierre Guillaumat, 31029 Toulouse, France

Abstract
During launch a spacecraft is subjected to a variety of dynamical loads transmitted through the launcher to spacecraft interface or air-born transmission excitations in the acoustic pressure field inside the fairing. As a result, spacecraft are tested on ground to ensure and demonstrate the global integrity of the structure against these loads, to screen the flight hardware for quality of workmanship and to validate mathematical models. This paper addresses the numerical modelling and simulation of the low frequency sine and random vibration tests performed on electrodynamic shaker facilities to comprise the mechanical-borne transmission loads through the launcher to spacecraft interface. Consequently, the paper reviews techniques and methodologies to derive a reliable and representative coupled virtual vibration testing simulation environment based on experimental data. These technologies are explored with the main objectives to ensure a stable, reliable and accurate control while testing. As a result, the use of the derived simulation models in combination with the added value of improved control and signal processing algorithms can lead to a safer and smoother vibration test control of the entire environmental test campaign.

Key Words
environmental spacecraft testing; multiphysics modelling and simulation; experimental system identification; structural coupling; vibration control

Address
Steffen Waimer:
1) Siemens Industry Software NV, Researchpark 1237, 3001 Leuven, Belgium
2) Acoustics and Vibration Research Group, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
Simone Manzato and Bart Peeters: Siemens Industry Software NV, Researchpark 1237, 3001 Leuven, Belgium
Mark Wagner: European Space Agency ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands
Patrick Guillaume: Acoustics and Vibration Research Group, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium

Abstract
The difficulties of satellite vibration testing are due to the commonly expressed qualification requirements being incompatible with the limited performance of the entire controlled system (satellite + interface + shaker + controller). Two features cause the problem: firstly, the main satellite modes (i.e., the first structural mode and the high and low tank modes) are very weakly damped; secondly, the controller is just too basic to achieve the expected performance in such cases. The combination of these two issues results in oscillations around the notching levels and high amplitude beating immediately after the mode. The beating overshoots are a major risk source because they can result in the test being aborted if the qualification upper limit is exceeded. Although the abort is, in itself, a safety measure protecting the tested satellite, it increases the risk of structural fatigue, firstly because the abort threshold has been already reached, and secondly, because the test must restart at the same close-resonance frequency and remain there until the qualification level is reached and the sweep frequency can continue. The beat minimum relates only to small successive frequency ranges in which the qualification level is not reached. Although they are less problematic because they do not cause an inadvertent test shutdown, such situations inevitably result in waiver requests from the client. A controlled-system analysis indicates an operating principle that cannot provide sufficient stability: the drive calculation (which controls the process) simply multiplies the frequency reference (usually called cola) and a function of the following setpoint, the ratio between the amplitude already reached and the previous setpoint, and the compression factor. This function value changes at each cola interval, but it never takes into account the sensor signal phase. Because of these limitations, we firstly examined whether it was possible to empirically determine, using a series of tests with a very simple dummy, a controller setting process that significantly improves the results. As the attempt failed, we have performed simulations seeking an optimum adjustment by finding the Least Mean Square of the difference between the reference and response signal. The simulations showed a significant improvement during the notch beat and a small reduction in the beat amplitude. However, the small improvement in this process was not useful because it highlighted the need to change the reference at each cola interval, sometimes with instructions almost twice the qualification level. Another uncertainty regarding the consequences of such an approach involves the impact of differences between the estimated model (used in the simulation) and the actual system. As limitations in the current controller were identified in different approaches, we considered the feasibility of a new controller that takes into account an estimated single-input multi-output (SIMO) model. Its parameters were estimated from a very low-level throughput. Against this backdrop, we analyzed the feasibility of an LQG control in cancelling beating, and this article highlights the relevance of such an approach.

Key Words
sine vibration testing; beating phenomena; control; LQG

Address
Alain Bettacchioli: Thales Alenia Space France, 5 Allée des Gabians - 06150 Cannes, France

Abstract
Physical tests are performed at various stages of the development cycle of a product, from prototype validation to product qualification. Although costly, there are growing demands for qualification tests like endurance vibration testing to be more representative of the real world. At the same time there are growing demands to assess the durability of these items based on FEA simulation. In this paper, we will explain how to set up a CAE-based test and how to correlate the results with some physical measurements. Specific assumptions will be explained and some advantages of using virtual tests will be highlighted such as the reduction of the number of prototypes needed, investigations on failures, evaluation of the level of reliability via sensitivity analysis, evaluation of the margins are at the end of a successful test. This presentation will therefore focus on explaining and showing how virtual tests can enrich the exploitation of physical tests.

Key Words
fatigue; endurance; shaker tests; FEA; modal analysis; mode shapes; virtual strain gages

Address
Frederic Kihm: HBM-Prenscia Products Division, France

Abstract
This work illustrates the progress of a TAS activity at exploring the challenges and the benefits of the Virtual Shaker Testing (VST) approach. The definition and the validation of new computational methodologies with respect to the state of the art were encouraged. The shaker Finite Element (FE) model in lateral configuration was built for the purpose and it was merged with the SpaceCraft (S/C) FE model, together with the S/C-Shaker adapter. FE matrices were reduced through the Craig-Bampton method. The VST transient analysis was performed in MATLABRnumerical computing environment. The closed-loop vibration control is accounted for and the solution is obtained through the fourth-order Runge Kutta method. The use of pre-existing built-in functions was limited by authors with the aim of tracing the impact of all the problems\' parameters in the solution. Assumptions and limitations of the proposed methodology are detailed throughout this paper. Some preliminary results pertaining to the current progress of the activity are thus illustrated before the conclusions.

Key Words
virtual shaker testing; vibration tests; structural dynamics; S/C mechanical testing

Address
Pietro Nali and Guglielmo Landi: Thales Alenia Space, Strada Antica di Collegno, 253, 10146, Turin, Italy
Alain Bettacchioli: Thales Alenia Space, 5 Allée des Gabians, 06150 Cannes, France
Marco Gnoffo: Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129, Turin, Italy

Abstract
In mechanical analysis of spacecraft structures situations appear where static and dynamic loads must be considered simultaneously. This could be necessary either by load definition or preloaded structures. The superposition of these environments has an impact on the load and stress distribution of the analysed structures. However, this superposition cannot be done by adding both load contributions directly. As an example, to compute equivalent Von Mises stresses, the phase information must be taken into account in the stress tensor superposition. Finite Element based frequency response solvers do not allow the calculation of superposed static and dynamic responses. A manual combination of loads in a post-processing task is required. In this paper, procedures for static and harmonic loads superposition are presented and supported by analytical and finite element-based examples. The aim of the paper is to provide evidence of the risks of using different superposition techniques. Real application examples such as preloaded mechanism structures and propulsion system tubing assemblies are provided. This study has been performed by the Structural Engineering department of Airbus Defence and Space GmbH Friedrichshafen.

Key Words
load superposition; structural dynamics; phase; static preload; spacecraft

Address
Xavier Vaquer-Araujo, Florian Schöttle, Andreas Kommer and Werner Konrad: Airbus DS GmbH Friedrichshafen, Claude-Dornier-Strabe, 88039 Friedrichshafen, Germany

Abstract
In the space industry, structures undergo several vibration and acoustic tests in order to verify their design and give confidence that they will survive the launch and other critical in-orbit dynamic scenarios. At component level, vibration tests are conducted with the aim to reach local or global interface loads without exceeding the design loads. So, it is often necessary to control and limit the input based on a load criterion. This means the test engineer should be able to assess the interface loads, even when load cannot be measured. This paper presents various approaches to evaluate interface loads using measured accelerations and by referring to mass operators. Various methods, from curve fitting techniques to finite element-based methods are presented. The methods are compared using signals with known imperfection to identify strengths and weaknesses of each mass operator definition.

Key Words
vibration; mass operator; acceleration measurement; load identification; neural network

Address
K. R. Olympio, M. Holz and A. Kommer: Structural Engineering Dept., Airbus DS GmbH, Claude-Dornier Strasse, 88090 Immenstaad, Germany
F. Blender: 3Mechanical Systems Germany Dept., Airbus DS GmbH, Claude-Dornier Strasse, 88090 Immenstaad, Germany
R. Vetter: Mechanical AIT Dept., Airbus DS GmbH, Claude-Dornier Strasse, 88090 Immenstaad, Germany


Techno-Press: Publishers of international journals and conference proceedings.       Copyright © 2018 Techno-Press
P.O. Box 33, Yuseong, Daejeon 34186 Korea, Tel: +82-42-828-7996, Fax : +82-42-828-7997, Email: subs@techno-press.com