Transactions on the Built Environment vol 19, © 1996 WIT ... · connections are used. Experimental...

Spacecraft dynamic behaviour in microgravity

conditions

T. Olbrechts, D. Vandepitte, W. Heylen

Katholieke Universiteit Leuven, Departement Werktuigkunde,

Afdeling PMA, Celestijnenlaan 300B, B-3001 Haverlee, Belgium

Abstract

Spacecraft dynamic behaviour is an important parameter in microgravityresearch experiments. Experiments and specimens must not be exposed tostructural vibrations.Traditional design methods of spacecraft for dynamic environments mainly relyon finite element models. Validation of these models is done throughcomparison of modal parameters that are measured in an experiment onground, in Ig gravity conditions. Real spacecraft dynamic behaviour inmicrogravity conditions is supposed to equal behaviour in gravity conditions.It is unknown whether conventional finite element models truly representmicrogravity conditions.This research is a follow-on to experiments that were performed on ESAmission D-2. Structural vibration experiments are carried out by astronautshitting the Spacelab structure with a hammer. Due to structural complexity ofSpacelab, results of this experiment are not quite well suited for correctinterpretation.This paper discusses another simplified investigation on dynamic behaviour inmicrogravity conditions. This research is done in th framework of anESA/PRODEX project. A test structure is defined that is representative for realspacecraft components. Different types of materialsand different types ofconnections are used. Experimental modal analysis is performed in Ig gravityand in microgravity conditions. Microgravity research will be done inparabolic flight with a research aircraft. Finite element models are built foreach of both conditions, for all different types of structures. Global

Transactions on the Built Environment vol 19, © 1996 WIT Press, www.witpress.com, ISSN 1743-3509

458 Structures in Space

conclusions compare structural behaviour in both conditions. Guidelines forappropriate finite element modelling will be derived.

1. Introduction

The determination of structural transfer functions form a vital part tocontrol and limit vibrations which are generated by various disturbance sourcesonboard of earth orbiting spacecraft. Such spacecraft, especially their payloads,may be sensitive to inertial vibrational forces or relative deflections whichmight be experienced in microgravity missions and very high pointingtelecommunication and scientific satellites. The structural transmissibility ofvibrations from a disturbance source to a sensitive receiver can be computed byFinite Element Models or other analytical methods. Partial or completeexperimental measurements are needed to validate and support the analyticalresults.

During the German Spacelab D2 mission structural transfer functionmeasurements, using hammer excitation, have been performed under orbitalconditions (ref.l). The limiting cirumstances of these tests yielded noinformation about possible variations in dynamic behaviour, due to the changeof gravity environment. Therefor, a more simple and reliable experiment hasbeen defined, aiming specifically at identifying these variations. If the designersof equipment and test set-ups know these variations, they can take them intoaccount in an early stage of the design. This increases the chance that theequipment will meet the specifications.

2. Micro-G environment

The spacecraft and its payloads need to be designed and verified tomeet given limit acceleration levels. Individual disturbance sources must notintroduce excitations which create spacecraft accelerations in excess of thespecified limits.

The vibration response of a flexible spacecraft structure due to force,torque and noise excitations can be described with frequency response andvibro-acoustic transfer functions for single or multiple input/outputtransmission paths. As a result the knowledge of the source to receivertransmission path frequency response functions is a key element in the controlof the microdynamics environment of satellite systems.

The transfer functions are derived by analysis and tests. Analyticalpredictions need to be validated with test measurements when spacecrafthardware become available. Ground test configurations of spacecraft systemsdo not completely simulate the orbital conditions. On the other hand in-orbittransfer function measurements are limited by available resources and thepresence of background noise.


Structures in Space 459

3. Spacelab D2 MMA transfer function experiment

The Microgravity Measurement Assembly (MMA) was flown in theSpacelab D2 Mission as a centralized microgravity measurement system forcontinuous and time correlated (real time) monitoring of broadbandacceleration responses in the Spacelab module. The MMA is using dedicatedmicro-mechanical acceleration detectors which can be installed very close toscientific experiments due to their small dimensions and mass. The MMAfunctions were extended by connecting an impact hammer which enabled themeasurement of structural dynamics transfer functions under on-ground and in-orbit conditions.

The force input locations in the Spacelab module were selected for asystematic investigation of disturbance transmissions in payload racks, fromracks to racks, across the module, from the floor to racks, and overheadstructure to racks. The structural vibration transfer functions in terms ofacceleration responses due to force input excitation over the frequency rangefrom 0.1 Hz to 100 Hz, which can be evaluated from the measurement set,form the basis for the Spacelab D2 MMA Transfer Function Experiment.

Due to the fact that in-orbit measurements of such transfer functionsare not always possible and that analytical predictions have to be supported bynon-complete (partially integrated) ground configuration measurements, the in-orbit Spacelab Transfer Function Experiment was complemented with on-ground reference measurements.

4. Experimental data analysis

On-ground and in-orbit measurements have been set up forcomparison purposes. The test conditions have to be accepted as far from ideal.On-ground as well as in-orbit, the hammer operator is moving along thestructure under test, thus changing the mass distribution. Furthermore, in orbit,while the test is going on, other astronauts are stowing away material or pursueother activities which create additional background noise. Since all applianceslike pumps, ventilation fans and other subsystems together with the in-flightmeasurement instrumentation have to operate during the flight they contributeto the background noise. The selected hammer type causes accuracy problemsat low frequency. Typical time responses for on-ground and in-orbitmeasurements show a short and clear response to the hammer blow. However,this response quickly decays due to the system damping and is lost in thebackground noise.

When analysing the recorded time data, compromises had to be maderegarding the type of frequency response function estimator, the number ofaverages and spectral lines, the use of correction windows, the accelerometersensitivity correction procedure an other data processing parameters.

Given the limiting circumstances, the accuracy of the resultingfrequency response functions is acceptable: they were useful for estimating



maximum possible responses in different frequency ranges. On the otther handthe accuracy is insufficient to derive useful estimates of the system dynamicchracteristics in terms of resonance frequencies, damping factors and modeshapes (modal parameters). Therefore, these data are not suitable for detectingpossible differences in the dynamics due to the decrease in gravity. Hence, amore dedicated test is designed.

5. The PRODEX project

The measurement conditions of the Spacelab D2 MMA Transfer FunctionExperiment were far from ideal. Keeping this experiment in mind, a new moresimple and more reliable experiment was defined. This investigation will try tomeasure a difference in the dynamic behaviour of structures in gravity andmicrogravity and involves experimental and numerical analysis of teststructures.

Two series of experiments will be performed on different teststructures. Modal parameter estimation techniques will analyse themeasurements on each structure, tested in two different environments: eachstructure undergoes tests both in Ig gravity and in microgravity conditions.Optimum measurement conditions in both gravity conditions minimise externalsources of error in data acquisition. Free-free boundary conditions are selected.On ground, suspension of the structure at one soft spring approaches thiscondition. Parabolic flights of an aircraft will make it possible to simulatemicrogravity in the aircraft cabin for short intervals of time. During theseintervals of time, the structures will be free floating inside the aircraft cabin.

Independent of measurement, finite element models of each structurewill be constructed. Commercially available finite element software willanalyse the models of the test structures (MSC/NASTRAN). Both normalmodes analyses and frequency response analyses will be run on each structure.Different options for modelling damping in structures will be utilised. Modalparameters and frequency response functions will be calculated. Results will becompared to experimental results. Finite element models will be updated foroptimum correlation with measured data.

5.1 Test definition

The test structures comply with the following basic requirements:• The test structures will be free floating during microgravity tests.• Reducing the complexity of the test structures makes it possible to do the

measurements in a straightforward way. This will avoid problems like noiseentering the structure.

• Comparison of gravity tests with microgravity tests needs a high degree ofrepeatability. The structures have to be the same in the two tests.



• The structures should be designed in such way that its dynamic behaviourexhibits a full range of structural properties, typical for current aerospace

structures.• Structures will have a sufficiently high modal density in the frequency range

of 0 up to 500 Hz. The structure should have at least 25 modes in the

frequency range of interest.• The finite element models have to be simple and accurate.• Parabolic flights require easy-to-handle structures.

5.1.1 Design of the test structuresAll these requirements and different numerical simulations with finite elementnormal mode analysis on different designs, resulted in the following basic

design:

600

230

40

270

20

40

r

jFigure 1 :Basic design of test structures

The basic design consists of an aluminium rectangular frame, size800 x 600 mm with a thickness of 4 mm. Slightly out of centre, there's a mass,size 160 x 160 mm and a thickness of 20 mm. This mass is connected to theframe with 4 spoke-like members (width 20 mm, thickness 4 mm), rectangular



to the frame. Finite element normal mode analysis shows that this structure has26 modes in a frequency range from 0 up to 500 Hz.

The aim of the experiments is to check whether gravitation has arelevant influence on the dynamic behaviour of a structure. Connectionsbetween members of the structure can possibly induce differences in dynamicbehaviour. The extended design of the test structures will contain threedifferent types of interchangeable joints.

The frame of the basic design is not very well damped. To measureother damping ratios in this experiment, tests will include two other frames:one with an intermediate damping and another with large damping.

The combination of 3 interchangeable frames and 3 interchangeablejoints will make it possible to do 9 different tests. The different testcombinations will show the possible influence gravity has on the dynamicbehaviour of structures.

Figure 2 : Aluminium frame with bolt joints

Frames: Three frames of different materials with different levels of dampingwill undergo the tests: a low damping frame (aluminium), a mediumdamping frame (composite material: impregnated 3D glassfibre textile) anda high damping frame (laminate of two layers aluminium glued togetherwith Neoprene foam tape). All frames will have the same dimensions andare cut with water jet. Each side of the frame contains 2 holes where thespokes are bolted onto the frames.Mass: The mass is the same in all experiments.



• "Spokes-Joints-Mass" sets: Three different sets containing Spokes, Jointsand Mass, will be produced. Three different kinds of joints will connect themass to the spokes: bolted joints, glued joints and ball joints. The spokes arenearly the same in all cases. The material is aluminium, thickness 4 mm,width 20 mm. The spokes are bolted to the frame in all experiments. Thesebolts facilitate the interchangeability of the "Spokes-Joints-Mass"sets.

Figure 2 shows the combination of the aluminium frame with the bolt joints:

5.1.2 Measurement setupExperimental modal analysis gets its information through the measurement ofFRF's (ref.2). Two inertial shakers excite the test structures. Two forcetransducers measure the force input into the structure. Accelerometers measurethe response.

Preliminary tests showed that 2 excitation points (2 inertial shakersconnected to the structure) gives more accurate and repeatable tests, resulting inbetter estimations of the modal parameters. The front-end produces burstrandom signals and feeds it through amplifiers to the shakers. Burst random isthe best excitation signal because it minimises leakage.

82 accelerometers (PCB structcel) measure the structural response. A24-channel SCAD AS front-end conditions the accelerometer and force signalsand sends them to a HP-workstations where LMS CADA-X software does theprocessing of the measurements.

Figure 3 : Location of shakers and accelerometers



5.1.3 Boundary conditionsIn parabolic flight, microgravity conditions will be effective for short intervalsof time (15 to 20s). During these intervals of time, the structures will be free-floating inside the aircraft cabin.

On ground, a suspension of the structure at one or several soft springsapproaches this condition. Two significant differences should be mentionedthough. First, the weight of the structure has to be compensated for inattachment springs. Connection of these springs to the structure inevitablyintroduces damping that is not present in flight. Second, the weight of theshakers has to be compensated for. In ground tests, springs suspend the shakers.The shakers are connected to the structure in such way to minimise forces andmoments exerted on the structure.

To cover the full range of gravity effects on the structure, the structureswill be tested in different positions relative to the earth's gravity (+x/-x, +y/-y,+Z/-Z).

5.1.4 Measurement parametersPreliminary tests on the simplified structure helped to determine the optimalmeasurement parameters.

Each measurement contains some noise. Averaging measurementsminimises the effect of noise. Preliminary tests demonstrated that 8 averagesare sufficient. Analysis of the Frequency Response Functions (FRF's) showedlittle difference between measurements with 8 averages and those with 100averages. Blocksize determines the resolution of the measurement. Results andanalysis of previous tests showed little difference between a blocksize of 2048or 4096. The frequency band of interest is 0 up to 500 Hz. Because of filters,measurements have to be done from 0 up to 625 Hz (500 Hz + 20%). Samplingfrequency is f%= 1250 Hz.

With a blocksize of 2048, the measurement with 8 averages takes about15s. This fits just in the time interval where microgravity takes place duringone parabolic procedure. Usually microgravity takes place during 15 to 20s.When each test is repeated once, 16 averages are available.

On ground the measurement parameters will be very similar to the in-flight parameters.

5.2 Preliminary tests

This section discusses the first results of the preliminary tests. These tests haveto prove that the test structures comply with the requirements. At this time,only the aluminium frame with the bolt joints was tested in gravity condition.Being in the gravity condition, 4 different positions relative to the gravity areconsidered. Figure 4 shows three of the four configurations that are tested.



5.2.1 Measurement resultsFirst test results are satisfying. Coherence and FRF's are very good except inthe frequency band from 0 up to 45Hz. In this frequency band theautopowerspectra of the force cells drop. It seems that the inertial shakerscannot put enough energy into the structure below 45Hz. This results in uselessmeasurements in this band.

The structure proofs to be linear. Other tests checked the force inputs tobe sure that no important other forces enter the structure. The eigenfrequenciesof the suspension are well below the frequency band that will be used formodal analysis.

Figure 4: Different test configurations relative to gravitation vector

5.2.2 RepeatabilityRepeating the same tests over different periods of time shows that themeasurements are repeatable. Also tests with different ambient temperature



confirm the repeatability. To be sure that the structure is the same in bothgravity tests, we will reduce the time span between the tests on ground and thetests in parabolic flight.

Unfortunately accelerometer calibration values tend to change in time.Using new and more stable accelerometers will solve this problem.

5.2.3 Configurations in gravityIt is possible that gravity has an influence on the dynamic behaviour of thestructure e.g. on the preloading of joints. Rotating the structure in differentpositions relative to the gravitation vector can change this preloading. Forexample turning the structure upside down changes the gravitation with 2grelative to the structure.

Three tests were performed: one with the structure placed horizontallyand two tests with the structure placed vertically. Figure 4 shows theseconfigurations.

10"

fo-

lio'

10-50 100 150 200 250 300

frequency (Hz)350 400 450 500

-2

50 100 150 200 250 300 350 400frequency (Hz)

Figure 5: Comparison between test4 [ ] and test6 [_._]

450 500

Figure 5 shows the driving point (point 101) FRF's of the two verticaltests. Test4 is the test with the long side of the frame in the vertical position,test6 is the test with the long side placed horizontally. Some peaks in the FRF'sshift in frequency, others do not change. Slight differences in damping can beobserved. The frequency band from 330Hz up to 380Hz is very sensitive to



configuration changes. The test with the structure placed horizontally shows thesame behaviour.

The reason of this is until now not very clear. It is possible that theaccelerometer cables have an influence on the dynamic behaviour. Further testswith less accelerometers have to find out whether this effect is caused bygravitation or by the weight of the accelerometer cables.

5.2.4 Modal analysisModal analysis on the measurements of previous tests test4 and test6 show theeffect configuration changes have on the dynamic behaviour of the structure.Table 1 gives the results of these modal analyses.

Mode

1234567891011121314151617181920212223242526272829303132

test4freq.[Hz]57.9563.9277.9888.3397.56130.37137.19137.39148.65156.29166.65174.65187.29212.02224.73231.92253.60259.68265.29277.15296.10308.73312.40357.88361.30365.10376.29406.87460.07474.48493.09504.65

damping[%]0.460.550.450.350.360.850.420.670.860.480.590.480.640.570.450.660.500.440.400.530.420.330.460.830.620.520.330.320.410.780.510.42

test6freq.[Hz]58.0666.7077.8288.2599.19130.28137.28138.87145.32156.68164.57177.23194.34211.74224.94229.54252.89260.04264.99276.55297.02310.09312.02356.28363.70367.39376.10408.01462.15475.75493.127504.88

damping[%]0.470.510.450.400.551.090.680.541.040.440.530.530.640.650.500.680.610.410.380.470.530.420.410.510.690.450.440.330.500.820.520.45

test9freq.[Hz]58.1366.7578.1488.2299.88130.62137.34139.67145.21156.32163.93176.86195.68210.03225.02228.30251.39259.60265.33276.30297.43310.45312.26352.41362.51367.10376.15407.12462.06474.97491.90503.31

damping[%]0.540.490.540.480.430.950.480.631.420.550.530.440.730.590.470.640.590.460.350.390.410.370.470.890.850.920.410.350.460.560.470.47

Table 1: Comparison eigenfrequencies and damping



As table 1 shows, damping is very sensitive to changes in configuration.This is also caused by the fact that the aluminium frame with bolt joints is avery low damped structure.

10"

10

10"'0 50 100 150 200 250 300 350 400 450 500

frequency (Hz)

2

! 0L

-2

50 100 150 200 250 300 350 400 450 500frequency (Hz)

Figure 6: test9 with cables [ ], test 10 without cables [-.]

5.2.5 Influence of accelerometer cablesThe purpose of this investigation is to look for differences in dynamicbehaviour. The dynamic behaviour of a structure consists of theeigenfrequencies, damping and mode shapes. With the measurement setup usedin previous tests, 82 accelerometers hang on the structure. This big amount ofaccelerometers give a good estimation of the mode shapes but have aninfluence on the eigenfrequencies and the damping of the structure. A test withonly two accelerometers proofs this. Test9 is a test with the structure placedhorizontally and completely instrumented with 82 accelerometers. Using thesame test setup after replacing 80 accelerometers with dummy masses, gives ustest 10. Comparison of test9 FRF's with test 10 FRF's (figure 6) demonstratesthe effect the accelerometer cables have on the dynamic behaviour of thestructure. Damping increases dramaticaly by putting accelerometer onto thestructure. This structure is very sensitive to changes in damping because thestructure itself has very low damping. The relative effect of this damping on thehigher damped structures has to be investigated in other tests. The frequency



shift can be caused by the fact that the dummy mass and accelerometer masswere not perfectly the same.

This means that when a good estimation of the damping is needed, asecond test with only a minimum of accelerometers has to be performed. It isnot possible to estimate all modal parameters in a repeatable way with only onetest.

6. Conclusions

Keeping in mind the experience of the MGDDl-test on Spacelab, a moresimple test set-up has been designed to detect possible shifts of dynamicbehaviour due to a change of gravity conditions. The base structure is a simpleframe type structure, representative for typical space structures, with differentkinds of connections, relatively easy to model and measure. Preliminary onground tests show nevertheless that these tests have to be carried out verycarefully: some shifts of dynamic behaviour have been detected due to thestructure's relative position with respect to the earth's gravity and due to theloading effect of accelerometers and cables. The on ground test boundaryconditions are designed to mimick as good as possible the free-free boundaryconditions of the tests to be carried out in parabolic flights, apart, of course, ofthe gravity.Further on ground tests will give a first idea of the possible influence of gravityon the dynamics of this structure and establish exactly the tests to be carried outduring the parabolic flights.

References

(1) H.Stark, C.Stravinidis, D. Eilers, W.Heylen, L.Bregant, "Inorbitdetermination of satellite structural transfer functions",Porceedings ISMA 79,, Vollll Leuven, September, 1994, p. 1273-1286

(2) W.Heylen, P.Sas, S.Lammens, "Modal analysis theory and testing",PMA - K.U.Leuven, September, 1995, pp.340



r»Ud Bap

Figure 7 : Comparison between test4 and test6



Figure 8 : Comparison between test with accelerometer cables and without


Transactions on the Built Environment vol 19, © 1996 WIT ... · connections are used. Experimental...

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Transcript of Transactions on the Built Environment vol 19, © 1996 WIT ... · connections are used. Experimental...