CASTOR: Structural dynamics in the MIR station

Acta Astronautica

Vol. 39,

Pergamon

1996

Printed in Great Britain 0094-5765/96 %15.00+0.00

PII: SOO94-5765(%)00160-9

CASTOR: J. P. VIALANEIX, Centre

No. 7. pp. 507-515,

0 1997ElsevierScience Ltd. All rights reserved

National

STRUCTURAL DYNAMICS MIR STATION? P. BOUSQUET,

Y. DANCET,

P. GUAY

IN THE

and F. MERCIER

d’Etudes Spatiales, 18, avenue Edouard Belin, 31055 Toulouse Ckdex, France (Received 31 January 1996)

Abstract-Prediction and reduction of space structure vibrations have become critical issues over the last ten years. Motivating factors include optical systems with high stability requirements, large antennas, sensitive micro gravity payloads, compatibility of flexible appendages with attitude control, and large orbital infrastructure surviving. The experiment “CASTOR” (French acronym for ChAracterisation of STructures in ORbit) is dedicated to the identification of the structural dynamic modes of the MIR station and to the investigation of the dynamic behaviour-in zero g conditions-of a truss mock-up equipped with various passive and active damping technologies. The measured modal parameters of MIR are to be compared with the results of a finite element model analysis. The differences between in-flight and on-ground dynamics of the truss will be thoroughly analysed. This project has been conceived and is managed by CNES. The flight hardware will be delivered by the end of 1995, and the experimental work will be performed by a French cosmonaut within the framework of the CASSIOPEE mission in June 1996. In the first place, a comprehensive overview of CNES activities in the prediction of in-flight structural dynamics, and of similar experimental efforts found in literature will be discussed. Afterwards, the motivations behind the CASTOR experiment will be shown, followed by a full description of its equipment. The results of the ground tests and analyses, and the in-orbit test plan will then be presented extensively. Particular emphasis will be laid on the performances of the active damping systems which will be validated in flight. In conclusion, the potential re-utilisation of the experiment material within the framework of later flights will be highlighted. 0 1997 Elsevier Science Ltd

1. INTRODUCITON

may reduce the accuracy lations: l

I. I. Technical context

The efforts dedicated to predicting or overcoming the vibrations of structures in space have increased tremendously over the last ten years. Motivating factors are optical systems with high stability requirements, large antennas, sensitive micro gravity payloads, compatibility of flexible appendages with attitude control, and large orbital infrastructure survivability. The requirements on dynamic transfers often lead to specific design constraints such as frequency decoupling with noise sources in the spacecraft, adding passive or active damping in the structural path, or setting up suspension devices. Although the active control techniques look attractive with regard to their high potential efficiency, they make the issue of structural dynamic prediction more critical, for an unreliable estimate of the dynamic behaviour of the structure in orbit may challenge the stability of control loops. Even if these predictions rely on ground test results, many factors TPaper IAE95-14.08 presented at the 46th International Astronautical Congress, Oslo, Norway, 2-6 October 1995.

l

l

l

l

l

of mathematical

simu-

the very low level of the excitations in orbit, which makes representative ground tests difficult, is particularly important because it usually induces very low damping values, the presence of air during ground tests, which adds damping, particularly on panel modes, the long term impact of space environment (thermal cycles, humidity description, outgassing, radiation, UV exposition, . . .) which modifies the performance of certain structural materials, the need for suspension systems to simulate free-free conditions on the ground, which perturb specimen behaviour, the non-testability on the ground of complete large specimens with deployable appendages, the presence of gravity on the ground, which may have a direct stiffening effect, modifies non-linearity (by loading the joints for example), and induces large displacements on low stiffness structural members.

The experiment “CASTOR” (French acronym for ChAracterisation of STructures in ORbit), which is described in the present paper, is dedicated to the study of the last three differences between ground and space. It will be performed during the fourth flight of 507

J. P. Vialaneix et a/

508

a French cosmonaut in the Russian station MIR (CASSIOPEE mission planned in June 1996). Two structural specimens will be investigated: l

l

a truss construction, with and without active damping devices, the MIR station itself.

I .2. CNES

in--ight

passive and

l

l

l

l

reduced gravity experiment aboard KC- 135, demonstration test of an actively damped 12 m long truss during parabolic flights[4], MODE experiment, test of a truss in different configurations during space shuttle flight in 1991 [S], MACE experiment, test of centralised and localised control of-a beam structure, space shuttle flight in February 1995[4,6,7], ACTive structure flight Experiment, test of an actively damped structure with reconfigurable control electronics, technological package on a NAVY spacecraft[8], general work performed in the 1980s on the Freedom station (behaviour of structures with very high modal density, simulation of dynamic identification tests in orbit, analysis of the evolution of modal parameters during construction, .) [9 or lo].

In Europe, ESA has performed a dynamic identification of the OLYMPUS satellite with the PAX system, worked on the identification of platform EURECA vibration measurements[ 111, measured structural transfers in SPACELAB-D2 with impact hammer excitation[l2], and is planning the development of a spacecraft dedicated to sloshing investigation (SLOSHSAT)[13]. 1.4. Contents qf this paper

l l

objectives of the experiment description of the hardware

results.

Then an overview of the potential regarding utilisation of the hardware for new experiments during later MIR flights will be given, before a global conclusion.

The main motivations CASTOR are: l

l

l

context

This paper will describe the CASTOR according to the following topics:

in-orbit test plan on-ground tests and analysis

2. OBJECTIVES OF THE EXPERIMENT

Similar activities have been realised or are planned in the US. Some significant examples found in the literature are: l

l

experiments

CNES has run several dynamics oriented experiments in the MIR station. ERA (dynamic identification of a truss deployed outside the station) took place in 1988, together with AMADEUS (deployment of a three dimensional arm with Carpentier joints mechanism). MICROACCELEROMETRE[ l] measured the micro gravity environment in 1992 and 1993, and was also used to perform the dynamic identification of a simple free-floating specimen[2]. With respect to non-manned missions, the experiment MICROMEDY will investigate the dynamics of the SPOT 4 spacecraft in 1996[3]. 1.3. International

l

experiment,

l

l

behind

the

experiment

complete validation of analysis and test procedure for advanced space structure dynamic simulation (ground testing at elementary and global level modelling of passive and active damping, model updating, modelling of gravity influence, performance prediction, comparison of ground and flight results), demonstration of the efficiency and of the complementary nature of passive and active damping elements, characterisation of the main global dynamic modes of the MIR station, including the damping values, and updating of a basic finite element model, characterisation of the spectra of the perturbations generated by life (astronaut exercises, . .) and mechanisms (air fans, attitude control equipment, ) on board, analysis of the evolution of the MIR station, and evaluation of the practical applicability of dynamic identification techniques to the monitoring of large orbital infrastructures.

In parallel, three technical objectives

will be achieved:

application of an infra red wireless data link for free-floating specimen dynamic testing, 0 application of an acceleration acquisition system based on network technology, l installation of a dynamic testing facility in the MIR station. l

3. HARDWARE DESCRIPTION

CASTOR l

l

is composed

of two main parts:

the acquisition system, including the accelerometers dedicated to the MIR station study (which will be called “DYNALAB” during the rest of this paper); the truss specimen and its various damped bars and associated command electronics (which will be called “TRUSS”).

3.1. DYNALAB The main functions of both flight and ground segments of DYNALAB are presented in Fig. 1. Its most noteworthy capabilities are: l

acquisition of the measurements in the station by a network link, making possible the installationwith one cable only-of seven blocks scattered in

509

CASTOR

l

l

the different modules of MIR (each of them including three axis accelerometers and one impulse hammer input); command and supervision of the configurations, and acquisition of the measurements on the truss via an infra red link, guaranteeing an almost perfect free floating configuration; reconfigurable user friendly interface, including a command screen.

The major technical goal performances l

l

l

l l l

are:

simultaneous acquisition of 22 channels digitised at 1000 Hz, station accelerometer ranges: 20 pg/40 mg and 1 w/2 g, frequency range of the station accelerometers: 0.01 to 300 Hz, bit rate capacity of the infra red link: 500 kbytes/s, resolution of the truss accelerometers: 100 pg, frequency range of the truss accelerometers: 5 to 400 Hz.

The total mass of DYNALAB (including cables, flight containers, accessories, but excluding the acquisition elements which will be mounted on the truss in test configuration) is 37 kg. The maximum power consumption is 160 W. 3.2. Truss A global view of the truss specimen is shown in Fig. 2. The total mass in test configuration is 16 kg. Its

various components are presented below. For more details on the justification of the passive and active element design and location, one should refer to Ref. v41. 3.2.1.Truss stracrure.The

truss structure is composed of three modules of two cells each, made with 300 mm and 420 mm dismountable bars. The total length is 1900 mm. The shape of the module was chosen for its simplicity (only two bar lengths) and for its isostaticity. The length and diameter of the tubes, and the mass of the connection nodes were adjusted so that there would be at least 3 global modes between 50 and 100 Hz, and a high density of local modes (node rotations) between 100 and 200 Hz. There was also a minimum target length for the bars to make possible the implementation of damping elements. The shape of the nodes was designed to accommodate easily accessible elements such as sensors, electrical connectors and exciter. The principle of individual dismount ability for all bars was adopted for on-ground and in-orbit practical reasons. The connection between the bars and the nodes is shown in Fig. 3. This system is simple; it can be manipulated by the experimenter without any special tools and provides a stiff consistent joint. Considering the limited amount of time available, and the number of electrical elements on the specimen, the truss is transported in three modules of two cells, each module being totally pre-equipped. The nodes between each module are cut into halves during transport; their assembly is straightforward and consistent. Stiff plates were designed to support

3 axis sensors

I

Y Video

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r-----1

--t;-4

TMbox I--; TM I_____J ,

I

: Clock

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i

) Power 28 V I.------.

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iv

Digitised data

I------------

t

CADMOS Toulouse

,a4 6 4

17 Sen

nfra red link 1 bars _-----

’

Telecommand

4 Simulation (sensors, excitation)

Data files Fig. 1. DYNALAB:

functional

MIR station

diagram.

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J. P. Vialaneix et al.

Fig. 2. Truss test configuration.

the various electronic boxes, in order to avoid local modes. The total structural mass is 8 kg. 3.2.2. Passive damping bars.The three passive bars include a damping element developed by METRAVIB RDS and shown in Fig. 4. The dissipation is provided by a viscoelastic cylinder solicited in shear. The passive bar can be locked to facilitate its integration in the truss. The stiffness of the passive bar is 200 kN/m and its structural damping coefficient is 0.8 at 25°C and 25 Hz. 3.2.3. Acrive damping.Two different types of active damping elements have been selected for demonstration. The criteria here were their potential for

space application, and the maturity sponding technologies: l

l

three active bars use PHYSIK INSTRUMENT low voltage piezo-electric members, mounted with a collocated force measurement sensor. They behave like plain structural bars when de-activated. These bars can be used to generate excitations in the truss, one reaction mass actuator uses an electrodynamic system driven by a collocated velocity sensor.

3.2.4. Exciter.This shaker will produce 1 N and 3 N forces at point 2 (cf. Fig. 5). It is based on the same electrodynamic system as the reaction mass actuator.

Insert Blocked

configuration

Open configuration Fig. 3. Bar/node

of the corre-

connection

principle.

CASTOR

tl-_-_-_-_-_-_-_---_-_-_ Fig. 4. Passive damping element, unlocked configuration.

3.2.5. Electronics. In agreement with the transport configuration, the electronics will be mounted on three plates located at the two ends and in the middle of the specimen. The main functions in each module are presented in Fig. 6. 4. IN ORBIT TEST PLAN

4.1. MIR station dynamic characterisation The planned position of the 3-axis sensors is shown in Fig. 7. The two sensors located on each side of KRISTALL interface will be used to determine the rigidity of the corresponding joint. Measurements will be taken after impacts produced by the experimenter with an impact hammer, and also during “natural” excitations of the station (booster torques, cosmonaut jogging, . . .). 4.2. Truss measurements The time limitation (one preliminary period of an hour and two of three and a half hours) is of prime importance in the elaboration of the test plan. Between periods, the truss may have to be dismounted. The actual time available for the experimentation is estimated to be five hours altogether. Four configurations are to be investigated: l l l l

non-damped truss, passively damped truss, active damping with piezoelectric bars, active damping with reaction mass actuator.

The excitation of the truss will take place along both flexion axes. It will be of burst random type; two peak to peak levels are used: 1 N and 3 N. Each test will consist of 20 elementary runs of 10 s. The excitation will be provided by the electrodynamic shaker, or by each active bar. An impact hammer will also be. available for redundancy. The acquisition system and the processing method are designed to accommodate

to repositioning

of the specimen after a possible shock with the station walls. 5. ON-GROUND 5.1.

TESTS AND ANALYSIS

RESULTS

MIR dynamic behaviour

Using a simplified (340 elements) finite element model of the whole station (see Fig. 8) provided by RKK-ENERGIA (Russia), CNES has performed a complete modal analysis in order to determine the most appropriate locations for the accelerometers. The first modes appear below 0.1 Hz which is a severe constraint for the cut-off frequency of the accelerometers. The transfer functions between the excitation points (hammer impact zones) and the accelerometers’ location nodes have been evaluated considering different damping factors according to the type of the elements concerned by the mode (solar arrays, antennas, main structure, secondary structure, . . .) in agreement with RKK-ENERGIA recommendations. It appears that the solar array modes are hardly noticeable; the structural torsion ones appear clearly but the flexion ones are more difficult to identify because they are deeply inter-coupled. These analyses will have to be updated at the time of the flight because the station configuration has already undergone modification (docking of the spectra module in May 1995) and may well be subject to further change (docking of PRIRODA at the end of the year). 5.2. Truss A complete set of dynamic tests has been performed on the truss in the CNES microdynamic laboratory. Suspended from a frame with four soft elastic springs, the truss is equipped with all the electronic boxes and the cables, and has been successively tested in the configurations defined for

Fig. 5. Diagram of electrodynamic actuator.

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J. P. Vialaneix et al.

Fig. 6. Functional schematic of the truss electronics

the flight: no added damping, passive damping, active damping using piezo-electric elements or the reaction mass actuator. An acquisition system uses LMS software records and processes the test data. Although the laboratory model of the truss is identical to the flight model, the latter will be tested just before launch since the comparison between ground and in-orbit behaviour must be made using exactly the same reference model. The excitations generated by either the shaker or the piezo elements of the active bars are of burst random type. Each sequence comprises 5 seconds during which the exciters are active, followed by 5 seconds of free dynamic energy release. This gives the most efficient balance between the injected energy level and the structure dynamic free decay. The sequence is repeated several times to allow an estimation of transfer functions.

The results of these characterisations show the very high efficiency of both passive and active damping systems and the robustness of the active control concept which has been developed for this experiment. The locations of the damping bars are the same for the passive and the active cases. The choice of these locations, as well as the stiffness and damping characteristics of the bars, have been calculated (using the finite element model of the structure) so as to concentrate the deformation energy where it can be dissipated. 5.2. I. Pussioe dumping. Neither the outgassing rate in vacuum nor the resistance to space radiations have been criteria for the choice of the viscoelastic damping material because the tests will always be performed in the air and within the protection of the stations’ metallic walls. Therefore, the efficiency of

Fig. 7. Accelerometer location in MIR station.

CASTOR

Kvant-2

513

Rapana

EEU

HGA

Derrick

Fig. 8. Finite element model of the MIR station.

the bars described below is likely to be higher than that which could be expected for a similar configuration in the space environment, but the main conclusions can be extrapolated to an actual mission. The efficiency of the passive damping technology can be seen in Fig. 9.

50 40

-

30

-

I

I

I

I

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I

52.2. Active damping optimisation. Active damping bars. A first test was performed using a direct excitation with each active bar individually. The bars are in open loop configuration and the forces are measured with the transducers. This test allows a verification of the stability

I

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I ~

-..-..

Iln-

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I

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I

I

15 trei: 2:+2 force bpa123 cas_2 1 trei: 2:+Z force bpa123cas_l

IO

F

2

3 2

0.7

’

E 0:4 0.6

IO

I 20

I 30

I 40

I 50

I 60

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u I10

Fig. 9. Transfer function with and without passive damping.

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J. P. Vialaneix

514

et nl.

0. I 0.07

Fig. 10. Transfer

function

with and without active damping bars

margin with regard to the positive system approach[l4]. The same data are employed to identify an open loop state model of the system. This state model can be used to compute the optimal gain coefficients for each bar (up to six tuning coefficients are used). In this approach, the structure of the controller is determined by the positive controller characterisation which guarantees stability for a wide range of gain

coefficients. These coefficients are then tuned to get the best performance. Either the performance objectives or the structural characteristics can be changed without affecting stability. Figure IO shows the performance obtained in laboratory. Reaction mass actuator. As shown in Fig. 1I, the reaction mass actuator has only a local effect. Located on one edge node of the truss (see Fig. 2) along the Y axis, it only has an influence on the bending mode around the Z axis.

I

- -

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0 trei: 2:+Y force pub1 i bo 1 trei: 2:+Y force pub1 i bf

I 2 a

0.7

z

8:4 0.4

5

0.3

2

0.2

0.1 8:E 0.05 0.04 0.03 0.02 0.01 0

IO

20

30

40

50

60

70

80

90

100

II0

HZ Fig.

II. Transfer

function

with and without

reaction

mass actuator.

120

130

140

150

CASTOR

Nevertheless, this damping principle looks interesting for well identified local mode attenuation.

515 REFERENCES

I.

J. P. Granier and P. Fauchet, MIR microgravity

Microac<romttre experiment. 5th European Symposium on Space Environmental Control Systems, Friedrichsafen, Germany, 20-23 June 1994.

environment, 6. PERSPECTIVES

FOR LATER

EXPERIMENTATION

CNES envisages flying other experiments in later missions on the MIR station. The next one should take place in April or September 1997. The CASTOR hardware will remain on board after the CASSIOPEE flight and will thus be available for the next one. The dynamic characterisation of the station will probably be complemented using the accelerometers and DYNALAB. Concerning the truss, new experiments are envisaged: passive damping bars using fluid systems developed within the framework of a current CNES research project, semi-adaptive tuning of the control loop gains in order to optimise the efficiency of the active system, global control methods. Even if all these experiments cannot be performed during the flight, they will be carried out at CNES on the laboratory model of the truss. Moreover, CNES is organising coordinated activities dealing with the tuning of the truss finite element model using laboratory test data. 7. CONCLUSION

The French mission CASSIOPEE provides CNES with a very good opportunity to acquire knowledge on the behaviour of structures in zero g conditions. Several methods and technologies to reduce the effect of dynamic perturbations on high stability payloads by adding artifical damping in the structure will thus be evaluated in actual conditions. The results of the on-ground tests performed in the micro-dynamic laboratory at CNES show that both passive and active damping methods are efficient and robust with regard to the shift of the dynamic response of the structure. This makes us confident in the success of the CASTOR experiment.

2. P. W. Bousquet, Y. Daneet and J. M. Le Duigou, Structure characterization by a cosmonaut in 1993. Colloque structures des vehicules spatiaux et essais mecaniques, Paris, France, 21-24 June 1994. 3. J.

M. Le Duigou, Acceleration measurements on SPOT4: MEDY and MICROMEDY exneriments.

Colloque structures des vehicules spatiaux’ et essais mkaniques, Paris, France, 21-24 June 1994.

4. M. Aswani. B. K. Wada and J. A. Garba. US perspectives’on technology demonstration experiments for adaptive structures. 30th IEEE Conference on Decision and Control, Brighton, UK, December 1991. 5. E. F. Crawley, M. S. Barlow, M. C. Van Schoor and A. S. Bicos, Variation in the modal parameters of space structures. AIAA 92-2209~CP. 6. D. A. Rey, E. F. Crawley, H. L. Alexander, R. M. Glaese and P. Gaundenzi, Gravity and suspension effects on the dynamics of controlled structures. AIAA-1993-1662CP. 7. S. R. Hall, In-flight experiments on flexible space structures. INFAUTOM’95-I lth Annual Sup’Aero Symposium, 9-10 March 1995. 8. R. A. Manning, R. E. Wyse and S. R. Schubert, Development of an active structure experiment. AIAA 93-l 114. 9. C. Flanigan, J. Habermeyer and D. Hunt, Test and analysis challenge for large space structure. IAF 92-0316. 10. J. P. Wetzel and D. M. Curtis, Methods for evaluation of the microgravity environment aboard Space Station Freedom. AIAA-93-1663~CP. 11. D. Eilers, EURECA microgravity and microdynamics in-orbit measurements. IAF 93-1.2.219. 12. H. R. Stark, In-orbit determination of satellite structural transfer functions. Colloque structure des vehicules spatiaux et essais micaniques, Paris, France, 21-24 June 1994. 13. H. R. Stark and C. Stavrinidis, ESA microgravity and microdynamics activities-an overview. IAF 93-1.2.218. 14. F. Mercier, P. W. Bousquet, P. Guay and G. Thomin, Passive and active damping, application to a truss structure. Colloque structure des vthicules spatiaux et essais mecaniques, Paris, France, 21-24 June 1994.