issues and solutions for testing free-flying robots

Acta Astronautica 60 (2007) 957–965www.elsevier.com/locate/actaastro

Issues and solutions for testing free-flying robotsCarlo Menon∗, S. Busolo, S. Cocuzza, A. Aboudan, A. Bulgarelli, C. Bettanini,

M. Marchesi, F. AngrilliCISAS-“G. Colombo” Center of Studies and Activities for Space, Department of Mechanical Engineering, University of Padova, Italy

Received 25 May 2005; accepted 13 November 2006Available online 20 February 2007

Abstract

Space robotics currently has an important role in space operations and scientists and engineers are designing new roboticsystems for space servicing missions and extra-vehicular activities. In particular, free-flying robots with extended arms havecompelling applications and several prototypes have recently been developed. Testing on Earth free-flying robots is a main issueas the unconstrained environment of free space must be simulated. From the experience acquired by testing a free-flying robotprototype both in a tethered facility and during a parabolic flight campaign, and after several years of experiments using air-bearing planar systems, the authors describe and discuss methods to test free-flying robots. A recent study aimed at designing afree-flying platform suitable for an under-water environment is also presented and discussed.© 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Free-flying robots have numerous potential appli-cations in future space missions and have receivedconsiderable research attention in recent years.In particular, they may be used in intra-vehicularactivities, extra-vehicular activities, and in spaceservicing operations. During intra-vehicular ac-tivities, a free-flying platform could manipulateobjects by means of its robotic arms or monitor inter-nal space environments as proposed in the PersonalSatellite Assistant project [1]. In the case of extra-vehicular activity, autonomous free-flying robots couldeventually take the place of astronauts thus sparinghumans from the hostile space environment. As re-gards space servicing operations, remarkable missionshave been designed, such as those for repairing the

∗ Corresponding author.E-mail address: [email protected] (C. Menon).

0094-5765/$ - see front matter © 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.actaastro.2006.11.014

Hubble Space Telescope [10–12]. In addition, the Na-tional Aeronautics and Space Administration (NASA),Canadian Space Agency (CSA) and many other spaceagencies have funded important programs for devel-oping space robotic arms [2–9]. Free-flying robots areexpected to successfully perform multipurpose opera-tions, taking advantage of their inherent capability offlying in free space.

In this paper, test-beds used to test free-flying plat-forms built in our center, CISAS-“G. Colombo” at Uni-versity of Padova, are presented and their pros and consdiscussed. The paper is organized as follows. Section2 introduces free-flying robot requirements. Section 3presents and discusses air-bearing planar systems usedfor testing two-dimensional free-floating robots. Sec-tion 4 describes a suspension system developed for tun-ing and testing a three-dimensional free-flying robot.Section 5 describes micro-gravity conditions achiev-able in parabolic flight tests. Section 6 presents devel-opments in the design of a new robot propelled by an

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958 C. Menon et al. / Acta Astronautica 60 (2007) 957–965

unconventional system suitable for an under-water en-vironment. The last section draws conclusions and dis-cusses developments for the future.

2. Free-flying robot test requirements

The main characteristics of a free-flying robot withan extended arm are briefly presented in this section inorder to infer the requirements that a test-bed shouldhave. In the framework of this research, a free-flyingrobot consists of a base, which could be a spacecraft,and an n-degree-of-freedom robotic arm. For saving fuelduring operations, the following phases are considered:

1. The robot uses the propulsion system of its base toapproach a target.

2. The propulsion system is turned off.3. The robotic arm performs manipulations.4. The propulsion system is turned on.5. The robot moves to a new target.

It is important to remark that the propulsion systemis turned off during arm movements in order to savefuel. In fact, refueling is usually not possible in spacemissions. In order to reduce disturbances induced on thebase by arm movements, a manipulator with redundantdegrees of freedom could be used and joint trajectoriescould be chosen to minimize base displacements.

For our analyses, it is assumed that:

• in the initial state the position and orientation of thespacecraft are well known in the inertial coordinatesystem;

• there are no external forces applied, the momen-tum conservation and the equilibrium of forces holdstrictly true;

• during arm movements, attitude control actuators,such as reaction wheels or thrusters, are turned off;the internal forces are generated only by motors thatactuate the robotic arm.

In Figs. 1 and 2 some simulation results of a free-flying robot with a four-degree-of-freedom arm areshown. In the simulation, the end-effector is supposedto follow a predetermined trajectory. Only the positionof the end-effector is taken into account, whereas itsrotation is neglected. By using a suitable differentialkinematic inversion algorithm, the base motion can bereduced. Results obtained using two sets of arm jointmovements are presented in Fig. 1. In the first case theyaw control is off (dashed line), whereas in the secondcase, the yaw control is enabled and the rotation of

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5s

rad

yaw control enabled

yaw control off

−0.05

−0.1

−0.15

−0.2

−0.25

Fig. 1. End-effector position for two sets of arm joint trajectories.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.50

0.5

1

1.5

2

2.5

3x 10−5

s

m

yaw control enabled

yaw control off

Fig. 2. Error of the end-effector position for two sets of arm jointtrajectories.

the base around its yaw axis is reduced. As shown inFig. 2, the error of the end-effector position is negligi-ble for both the two sets of arm joint movements.

The validation on Earth facilities of path planningand control algorithms designed for free-flying robotsis very challenging. Test-beds should guarantee that:

• Gravity effects are compensated.• The base and arm of the robot are free to move in

any direction.• The movements of the end-effector and base are mea-

sured and tracked at a suitable rate to reconstruct thewhole state of the system.

C. Menon et al. / Acta Astronautica 60 (2007) 957–965 959

3. Air-bearing planar systems

The air-bearing table is one of the most useful test-bed for testing space robots with extended arms. Thissystem is of great interest as the absence of gravityis suitably simulated by reducing friction forces at theinterface robot/table. Despite its inherent characteristicof being a planar test-bed, air-bearing tables have beenextensively used especially for studying flexible manip-ulators and free-flying robots with extended arms. Thissystem is typically used for testing small and low-masstest bodies.

Several groups have built air-bearing tables fortesting free-flying robots including Stanford Univer-sity [17–19], University of Victoria [20–23], TokyoInstitute of Technology [24,25], Naval PostgraduateSchool [26,27], Honeywell Space Systems and Vir-ginia Polytechnic Institute and State University [16],Massachusetts Institute of Technology [28], Carnegie-Mellon University and Texas Robotics and AutomationCenter [29], Georgia Institute of Technology, and manyothers [16].

CISAS has also developed a test-bed for planar sys-tems. This test-bed consists of a smooth granite tableand a vision system able to track floating robot move-ments (Fig. 3). In this test-bed robots must be equippedwith air-bearing supports and on-board compressed airtanks in order to float on the granite table.

Two robots were built to study free-flying robot ma-neuvers. The first robot, which has a three-degree-of-freedom extended arm, is aimed at grasping a target bytaking into account the effect of the floating base move-ments. The second one, shown in Fig. 4, has an air-jetthruster system able to control base movements. In thiscase, the robot is also able to reach and catch a farawaytarget.

Performed experiments made it possible to assesspros and cons of this test-bed and also to draw the fol-lowing considerations:

1. the test-bed is suitable for proving theory achieve-ments;

2. it permits long time tests;3. it is a robust test-bed;4. it efficiently simulates the micro-gravitational envi-

ronment in two dimensions;5. it does not allow balanced out of plain movements;6. it needs a custom-built robot prototype.

This test-bed has been extensively used at CISAS be-cause of its simplicity and suitability to test robot proto-types. However, additional facilities, presented in the

Fig. 3. Air-bearing facility at CISAS.

Fig. 4. Free-floating robot.

Table 1Characteristics of the experimental robot

Total weight 27.05 kgInertia for the robot in rest position 0.72 kg m2

Maximum electrical power 200 WMicro-gravity level 100.10 �gMass ratio vehicle/robot 22Dimensions 456 × 456 × 855 mm3

Compressed air capacity 6 lAir bearings pressure 0.3 barThrusters pressure 3.0 barSingle thruster force 0.4 N

Total thrust forces measured Fx 0.7 NFy 0.7 NT� 0.2 N m

following section, were also required to allow three-dimensional movements of free-flying platforms.

The main parameters of the two-dimensional robotprototype which we have developed are shown inTable 1; it is possible to achieve a good micro-gravity


level (between 10E − 4g and 10E − 5g) with the re-duction of friction by air bearings; the planarity of thesystem and the perpendicular direction of the joint axesrespect to the plane are critical for maintaining thisperformance.

4. Suspension system

Test-beds based on suspension systems can be usedfor testing three-dimensional free-flying robots beforetheir use in space missions. These on-ground test-bedsgenerate compensating forces of the same amplitudeand in the opposite direction of the weight force of eachrobotic subsystem (base and links). These compensat-ing forces should be applied in the center of mass ofeach subsystem while their values should remain con-stant during the three-dimensional motion of the roboticarm. This requirement is necessary as both actuators andcontrol algorithm of the free-flying robot are designedfor a 0g environment. In addition, test-beds based onsuspension systems should not exert additional forceson the free-flying platform, leaving free all its degreesof freedom.

Active robotic systems designed to suspend spacerobotic arms have been developed and tested [30,31].A free-flying robot may be suspended by wires attachedto mechanical arms which follow the free-flying robotmotion. The free-flying robot base is suspended by amultidegree-of-freedom mechanism, which emulatesthe free movements of the base. With this system amicro-gravity environment of 0.01g can be achievedby accurately tuning the control parameters [30]. Thissuspension system architecture shows two main issues.The first concerns identification of kinetic friction andits compensation. Instabilities may arise and distur-bances on the free-flying robot may be induced. Thesecond issue concerns vibration instability: the systemmay become unstable due to the coupled vibrationmodes of the space manipulator and suspension system.

Test-beds relying on completely passive suspen-sion systems can also be designed. We consideredthis solution for tuning control parameters of a three-dimensional free-flying robot [14]. The base of therobot is suspended by an inextensible cable fixed inthe vertical projection of its center of mass. This con-figuration allows the base to have only one constraineddegree of freedom along the vertical axis. The arm linksare hung by a system that (i) exerts a force equal inmodulus to and in the opposite direction of the gravityforce; (ii) allows vertical displacement (z-axis) with nosignificant variation of the force provided; (iii) allowsin plane (x–y plane) arm displacements while inducing

Fig. 5. Free-flying robot suspended by a passive spring suspensionsystem during a test session at CISAS.

negligible perturbations. In order to limit weight com-pensation error, the robot can be suspended by meansof elastic devices having low stiffness (the weightcompensation error is equal to the maximum verticaldisplacement of the considered link times the stiffnessof the elastic system). A hanging system with low stiff-ness can be obtained by using long rubber bands orsprings. An additional advantage of using long suspen-sion systems is to reduce disturbances induced to therobotic links in the x–y plane. In the tests performed atCISAS, a vertical hanging system of 5 m was used. Bychoosing an adequately low stiffness, a micro-gravityenvironment of 0.08g was reached. In the case of a20 m vertically long suspension system, 0.01g couldhave been reached using the same configuration. How-ever, the use of low stiffness suspension systems makesthe test-bed have low resonance frequencies. In orderto minimize disturbances induced on the free-flyingrobot, it is necessary to verify that the movement of therobotic arm does not excite the natural frequencies ofthe suspension system.

Fig. 5 shows a free-flying robot prototype suspendedby passive spring suspension system during a test ses-sion at CISAS.

Both passive and active suspension systems cansimulate a micro-gravity environment with an error of±0.01g. The main advantage of an active suspensionsystem is to induce negligible disturbances in the x.yplane. On the other hand, if the objective of the test is tovalidate the control algorithms implemented and tunethe control parameters of a free-flying robot, a passivetest-bed is a simpler and more economic solution. Thedrawback of the passive test-bed is that the cables donot remain exactly vertical during the movements ofthe manipulator. Due to the disturbances introduced inthe x.y plane, the correct reconstruction of the base


motion is difficult to compute.Both active and passive systems show resonance is-

sues that should be taken into account especially in thephases of path planning and post-processing.

5. Parabolic flight

Parabolic flights can be successfully used for shortduration technological experiments in reduced grav-ity conditions before on-orbit operations. Several spaceagencies organize parabolic flight campaigns suitablefor different research in Physiology [32], reactor proto-types [33], Neurology [34], Acoustics [35], space suits[36], etc. Even for robotic experiments, parabolic flightsare widely used when suspension systems induce ex-cessive perturbations as mentioned in Ref. [37].

We performed an experiment during an ESA cam-paign. The robot prototype designed for this test isshown in Fig. 6. The experiment that was carried out isbriefly presented here.

Fig. 6. Three-dimensional free-flying robot prototype.

Fig. 7. Parabolic flight phases (image reproduced with the kind authorization of the captain Gilles Le Barzio).

The campaign consisted of two flights of 30 parabo-las each. A reduced gravity environment was obtainedusing a modified Airbus A300 Zero-G. Approximately20 s of 0g acceleration were obtained. Each parabolalasted about 1 min and consisted of three phases asshown in Fig. 7: pull-up phase, micro-gravity phaseand pull-out phase. In order to protect people on boardof the aircraft, the free motion of the robot was con-strained using a metallic rack fixed to the aircraft.During the micro-gravity phase and before the start ofthe autonomous operations, the robot was positionedin the center of the rack by means of thin dyneemacables. At the end of the short micro-gravity phase, therobot was recovered in a safe position and fixed to theaircraft for the pull-out phase.

Based on the experiment performed, the followingconsiderations can be drawn:

1. Parabolic flights allow researchers to efficiently per-form three-dimensional tests of free-flying robotswith extended arms in unconstrained conditions.

2. Payloads fixed to aircraft experience micro-gravitational conditions (acceleration in the directionof the aircraft yaw measured during a micro-gravityphase is shown in Fig. 8), whereas free floatingpayloads inside the aircraft cabin can experiencezero-gravity.

3. Free-flying robots can be tested only if their volumeand weight are compatible with the available spaceon the aircraft.

4. A free-flying robot can freely float for only few sec-onds since displacements of the aircraft during themicro-gravity phase make the robot crash against air-craft sides.


Fig. 8. Aircraft normal accelerations during micro-gravity phase.

5. Crew operators induce considerable disturbances onthe robot attitude when they unlock the robot justbefore the micro-gravity phase.

6. An onerous post-experiment phase is necessary toelaborate data and correct drifts due to non-stationaryconditions of the robot at the start time of the micro-gravity experiment.The considerations mentioned above lead to the fol-

lowing improvements for future tests:

• An automatic releasing system should be built andused in order to reduce disturbances induced by crewoperators.

• A miniaturized prototype can increase the availabletime for tests.

• Redundant sensors could ease the post-experimentphase which is necessary to accurately reconstructthe robot motion.

Taking into account the considerations highlighted inthis section, it can be stated that parabolic flights aresuitable for testing floating robot prototypes in zero-gravity conditions. However, free-flying robots built forspace servicing missions are difficult to be tested be-cause of their dimension and weight. In addition, avery short time is available—only critical trajectoriescan therefore be tested. Due to the short time avail-able, robots should also be previously tuned in groundfacilities.

6. Under-water environment

The use of an under-water facility has the poten-tial advantage that experiments can last for an almostunlimited amount of time. However, space qualifiedrobots cannot generally be tested directly under water

Fig. 9. Initial design for handling robot.

as they can be seriously damaged. Custom-built proto-types, characterized by neutral buoyancy, must thereforebe designed and built. Testing under water a free-flyingrobot with extended arms is also challenging as reactionforces are exerted by the surrounding fluid. The effectof the water should be taken into account.

Recent studies performed by the authors havefocused on developing a free-flying robot prototypewith one extended arm suitable for under-water condi-tions (see Fig. 9). This robot should be capable to reacha pre-fixed position using the thrusters of the base,and then perform manipulations with its robotic arm.According to the operation phases described in Section2, the base should not be controlled by its thrusters dur-ing the movements of the robotic arm. Therefore, thedesign and fluid-dynamic analysis of the base, aimedat minimizing drag-force disturbances, have particu-lar importance. A new propulsion system concept wasdesigned in order to obtain base optimal shape.

6.1. Base propulsion system trade-off

Fluid-dynamic simulations showed that the optimalbase should: (1) be compact; (2) have a spherical shape;(3) be propelled by a system that minimizes distur-bances on the surrounding environment.

In order to achieve these results, the propulsion sys-tem, which determines the shape and dimensions of thebase, was carefully analyzed. Three propulsion systemswere studied: (1) screw propeller system; (2) gas-jetsystem; (3) hydro-jet system.

The first system was used for several under-waterrobot prototypes (e.g., ODIN robot [38], SCAMP


robot [39]) especially because of its simplicity andeffectiveness in the water environment. However,the hydro-dynamic impact of this propulsion systemwhen it is turned off (during arm monuments) makesthis kind of propulsion system unsuitable for ourapplication.

The analyzed gas-jet system has one main chamberin which air is kept compressed. Several jets controlledby electro-valves provide air-flux exit. Computationalanalyses showed a prohibitive rate of flux when anuninterrupted working time of 1000 s is assumed. In ad-dition, the variation in mass of the system during oper-ation requires an additional compensating system thatcomplicates and modifies the ideal spherical shape ofthe base.

The hydro-jet system was also taken into account andwas selected for our application. In its final design, ahydro-jet system could potentially propel the robot in-ducing low disturbances on the surrounding environ-ment while keeping the robotic base both spherical andcompact (see Fig. 10).

Differently from the other two propulsion sys-tems, the hydro-jet system needs suction doors forthe hydraulic pumps inside the robot base. In orderto investigate suction effects, fluid-dynamic simula-tions were carried out. Fig. 11 shows the wash of anideal spherical base in laminar flow (Reynolds number= 50 000, Diameter base = 0.4 m; velocity base =0.15 m/s).

In addition, analyses without, with one and with twosuction doors were performed. For these simulations,the fluid-dynamic characteristics of the system wereinvestigated by positioning the base inside a fictitiouspipe in which the water flowed with a velocity of0.15 m/s. The drag coefficients for the three differentcases are: (1) Cd =0.46 (spherical shape); (2) Cd =0.48(one suction door); (3) Cd = 0.50 (two suction doors).Since the above coefficients have almost the same val-ues, a base configuration with two suction doors waschosen. This solution guarantees a symmetric design ofthe base.

6.2. Base propulsion design

The design of the base propulsion system focused onthe optimization of the pump system and on the dis-tribution system. The final base design consists of twosymmetrical suction tubes, two hydro-pumps, two suc-tion doors, four pipes and eight nozzles. A represen-tation of a single propulsion system and water parti-cle trajectories is shown Fig. 12. Fig. 13 shows detailsof the system and pump design. In order to have a

Fig. 10. Detail of the ideal hydro-jet nozzle.

Fig. 11. Path lines of velocity magnitude around spherical base.

Fig. 12. Simulation results of robot pump: path lines of water throughimpeller and volute.


Fig. 13. Details of the base.

Fig. 14. Velocity vectors inside the propulsion system at maximumflux rate.

maximum linear velocity of 0.15 m/s and rotational ve-locity of 1.2 rad/s, analysis yields a propulsion systemhaving a thrust of 10 N. The design of the robot basehas a 0.4 m wide external diameter and weighs 60 kg.

Fig. 14 shows the velocity vectors inside the finalpropulsion system design through the impeller, voluteand four pipes which connect the pump to the nozzles.

The design of the volute is critical and its good de-sign can guarantee high efficiency of the propulsion sys-tem. The characteristics of the volute are presented inTable 2. A comparison of the drag coefficients for theideal spherical design (Cd =0.46), and final base designshown in Fig. 15 (Cd = 0.49), suggests that a perfectspherical shape is not required for our application.

Table 2Final volute

Inlet Outlet Variation

PSTATIC (Pa) 119 000 102 000 −17 000PDYNAMIC (Pa) 31 000 25 000 −6000PTOTAL (Pa) 150 000 127 000 −23 000

Output velocity (m/s) 7Nozzle thrust (N) 19Losses through volute (Pa) 12 000 (4%)Total losses (Pa) 24 000 (8%)

Fig. 15. Path lines of velocity magnitude around the vessel duringmotion.

7. Conclusions

The work presented in this paper investigated test-beds for free-flying robots with extended arms, empha-sizing pros and cons of the different test-beds built andused. In particular, tests performed by the authors werediscussed and analyzed. Air-bearing and under-watertest-beds require custom-built free-flying robot proto-types although they are widely used for assessing newconcepts and control algorithms. Suspension systemscan be used for flight hardware but they induce distur-bances on the attitude of free-flying robots which areoften not acceptable. Tests performed on parabolicflights are particularly suitable for their zero-conditionenvironment although the size of the robotic systemsshould be small. The main drawback concerns the shorttime available which can be not sufficient for testingseveral long realistic operations.

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issues and solutions for testing free-flying robots

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