A ROBOTIC TESTBED FOR LOW-GRAVITY SIMULATION

Jakub Tomasek1, Hendrik Kolvenbach2, Marco Pagnamenta2, Kjetil Wormnes2

1Department of Control Engineering, Czech Technical University in PragueE-mail: jakub.tomasek@fel.cvut.cz

2Automation and Robotics Section, ESAE-mail: hendrik.kolvenbach@esa.int, marco.pagnamenta@esa.int, kjetil.wormnes@esa.int

ABSTRACT

This paper proposes a novel testbed for testing in amicrogravity-like environment. We describe the devel-opment of a prototype platform based on an omnidirec-tional mobile robot. When compared to conventional air-bearing facilities, the platform allows experiments withsignificantly lower inertia. We performed verification testssimulating contact between a chaser and a target in free-floating environment. We have demonstrated accurate dy-namics after impact at velocities ranging from 20 mm s−1

to 200 mm s−1 with payload mass of 4.7 kg and supportmass of only 2.9 kg.

1 INTRODUCTION

Over the past years the subjects of combined orbitalrobotics and advanced guidance, navigation, and control(GNC) have become increasingly important for Europeanspace missions. Automated docking has been successfullycarried out, and attention is now being focused on the cap-ture of uncooperative targets for Active Debris Removal inthe framework of Clean Space initiative. Additionally, thetwo subjects are capital for upcoming missions for land-ing and sampling on low-gravity bodies such as comets,asteroids and small moons.

To support the existing and upcoming missions, suchas e.Deorbit [2], and R&D activities [5, 6, 7, 12] in thesehigh-visibility technological fields, the Automation andRobotics (A&R) laboratory at the European Space Re-search and Technology Centre (ESTEC) has been up-graded to include an orbital robotics and GNC facility.This facility supports a large flat floor with several air-bearing platforms.

We start this paper with a short review of groundbased microgravity simulation in Section 1. Section 2illustrates the principle of the proposed testbed and de-scribes the robotic platform in detail. Section 3 describesthe experiments. Our experience and possible improve-ments are discussed in Section 4.

1.1 Ground-based Simulation of MicrogravityDifferent methods have been developed over the years

to emulate microgravity on the ground.Some are less suitable for simulating dynamics of

space robots than the others. The most accurate micro-gravity reproduction is a free fall in an evacuated droptower such as ZARM in Bremen, but microgravity isonly available for a very short time (4.74 s in the caseof ZARM). A slightly longer test can be achieved in aparabolic flight, such as those operated by Novespace,where reasonable micro-gravity can be obtained for a du-ration of around 20 s. To further extend the duration, wateris commonly used. By submerging astronauts or equip-ment in water together with buoyancy control devices onecan extend micro-gravity even further. However, in ad-dition to capital and operational expense of neutral buoy-ancy pools, the experiments suffer high drag forces whichmake contact dynamics and control tests infeasible.

Air-bearing facilities have been used since the begin-ning of spaceflight for such purposes [11]. Air bearingsuse compressed air to create a thin air cushion betweentwo surfaces and in this way minimise friction. Planar air-bearing facilities include extremely flat surface on whicha platform with compressed-air container floats using theair bearings. It provides three degrees of freedom; two intranslation and one in rotation. A large such facility wasconstructed as part of the Orbital Robotics and GNC lab-oratory in 2015 at ESTEC [8, 9].

Spherical air bearings provide low torque in attitudeand have been extensively used for attitude control tests.Spherical and planar air bearings can be combined to pro-vide up to 5 degrees of freedom.

A more recent development are facilities which useindustrial robotic arms commanded by a physical modelof a floating object [1]. External interactions can be sim-ulated using feedback control. These robotic arm facili-ties usually allow simulation of all 6 degrees of freedom.However, purely robotic facilities rely on the credibility ofthe introduced physical model and the performance of thehardware.

1.2 From Flat Surface to a Mobile RobotAir bearing facilities have several downsides.

Firstly, the object under test (hereafter referred to aspayload) must be mounted on an air-bearing platform.This platform has to have its own air and energy supply inorder not to suffer forces and torques from the connectedair hoses and electrical cables. As such the duration of theexperiments is limited by the energy and air storage.

Secondly, by placing the payload on such a platform,the inertia of the platform combines with that of the pay-load.

And thirdly, the flat surface is limiting the maximumdistance the platforms can operate over. Installing andmaintaining a permanent flat surface for experiments isexpensive and may be impractical for universities andsmaller research facilities.

This paper proposes a novel air-bearing robot thatovercomes all these three limitations.

2 ROBOTIC TESTBED

The robotic testbed, based on KUKA Youbot mobilerobot and named Robotic Testbed for Floating-DynamicsSimulation (ROOTLESS), allows to decouple the payloadfrom the platform, run experiments for extended dura-tions, and is scalable to do tests in larger areas.

We use air bearings to provide a low-friction move-ment in the horizontal plane similar to conventional air-bearing facilities. Three air bearings are mounted on amobile robot pointing upwards. The payload is mountedon a flat plate that floats on a thin air film on top of the airbearings. As the payload moves the mobile robot followsrespectively using the relative-position information froma contactless sensor.

Interface plate

Air Bearing

Frame

Ball transfer units

Spring/Damper system

Sensor

Holonomic robot

Air film

Payload

Figure 1: System overview.

2.1 System OverviewThe proposed platform consists of four core subsys-

tems: Locomotion, Pneumatic, Tracking, and Payload.Figure 1 gives an overview of the main hardware ele-ments.

The Kuka Youbot is a commercially-available holo-nomic mobile robot widely used by academic researchers

[3]. Figure 2 shows the robot. We opted for an existingsolution to benefit from the community, to shorten the de-velopment time, and to lower the price.

The Youbot is equipped with an onboard Linux com-puter. We chose the Robotic Operating System (ROS) asrobotic middleware for the software development sincethe drivers for the platform itself already existed. Weadded a Beaglebone Black microcomputer which pro-cesses data from the position sensor.

2.2 LocomotionThe core requirement for our application was uncon-

strained planar locomotion. The Kuka Youbot is equippedwith mecanum wheels so the robot can move in any direc-tion from any configuration.

Figure 2: Youbot from Kuka. It provides unconstrainedplanar movement with use of mecanum wheels.

Mecanum wheels feature series of rollers mountedat 45◦ angle around its circumference. By rotating eachwheel with different computed velocity one can move inany direction. The maximum velocity in the forward di-rection of the robot is 0.8 m s−1.

2.2.1 Vibration DampingThe drawback of mecanum wheels is the discontinu-

ous point of contact with the ground. There is a gap be-tween each roller and as the wheel rotates the rollers col-lide with the ground. This motion introduces significantvertical vibrations to the robot and disturbs the motion ofthe payload.

The vibrations are intolerable for the application andhence we added a damping element. In order to dampthe vibrations, we built a frame around the Youbot whichcarries the air bearings and the payload. The weight ofthe frame and payload is supported by ball transfer unitswhich allow holonomic movement with small rolling re-sistance. Vertical vibrations from the robot are damped bya spring-damper system between the frame and the robot.This proved to be sufficient to minimise the vertical vibra-tions.

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2.3 Pneumatic SystemThe pneumatic system provides a continuous and sta-

ble flow of air to the air bearings to provide frictionlessmovement of the payload. The platform has a standardquick coupling to connect oil- and water-free pressurisedair. The air is routed through a manual on/off switch and aparticle filter. Next, a pressure regulator with a connectedbuffer tank is attached to Youbot to provide a constantpressure to the air bearings. The tubing from the regula-tor to the air bearings is of equal length to avoid differentpressure levels. The main components of the pneumaticsystem are the three carbon air-bearing pucks. These airbearings have a porous carbon surface through which theair flows evenly and provide a steady air gap (Figure 3).

Figure 3: New Way air bearings.

2.4 PayloadThe air gap between the air bearings and the payload

is only a few micrometers wide. Therefore it is crucial thatthe interface surface between the payload and the air bear-ings is flat and smooth, or else the surfaces could comein contact. Additionally, the flat material must be stiff,so as not to deform when a heavier payload is used. Thematerial should also be light, so as not to constrain theminimum mass of the whole payload.

We tested several interface materials: aluminum,acrylic, and glass. Glass, while being delicate to handle,has the best floating properties. For the purpose of inter-face material we used an off-the-shelf circular mirror witha diameter of 50 cm and a thickness of 5 mm.

We attached the mirror to a 10 mm-thick honeycombpanel with equidistant M6 inserts to provide a generic andeasy to use mounting interface for the payloads. The totalweight of the interface plate is 2.94 kg.

The interface plate is the only addition to the payload.Because the interface plate is symmetrical it is easy toinclude it in simulation models. Unlike bulky platformswith pneumatic systems in conventional air-bearing facil-ities, it does not introduce any uncertainty in the model.

To reduce the weight even further, a dedicated interfaceplate for the payload which only consists of the smoothsurface can be used.

2.5 TrackingThe mobile robot moves by tracking the position of

the floating payload relative to the platform and by keep-ing itself aligned with the center of the interface plate.A PID controller commands the movement of the mobilerobot.

2.5.1 Robot Control

PID controller

Onboard computer Motor 1Trinamic TMCM

Motor 2Trinamic TMCM

Motor 3Trinamic TMCM

Motor 4Trinamic TMCM

RS485

Position sensor

Line CCD cameras

Beaglebone Black Board

Ethercat

Position change

Figure 4: Relative position control loop.

Figure 4 shows the feedback loop for the relativeposition control. The interface plate is circular so onlytranslation degrees of freedom, i.e. x and y position, aremeasured and controlled. The relative position is mea-sured by a custom-built contactless sensor especially de-signed for this purpose; see Section 2.5.2 for details. Theraw data from the sensor are processed by a BeagleboneBlack microcomputer. The Beaglebone Black Board feedsthe position information to the onboard computer throughRS485. The feedback loop is closed through a PID con-troller running on the onboard computer and commandsthe velocity vector of the platform.

The velocity-vector commands for the Youbot arethen converted into angular-velocity commands for eachmotor. The four independent motors are controlled by Tri-namic TMCM motor controllers which are connected withthe onboard computer via Ethercat.

2.5.2 Position Sensor

We built a custom contactless vison-based sensor tomeasure velocity and position of the payload relative tothe center of the mobile robot. It is composed of an activeand passive part. The active part is located on the mobilerobot; there are three parallel rows of line-scan camerasand LED backlights (Figure 5). The passive part is a fullblack circle printed on a bright background on the inter-face plate.

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Figure 5: Custom sensor for measuring the relative posi-tion between the payload and Youbot.

The gap between the sensor and the target is smallenough that the camera readings are not perturbed by am-bient light. The analog reading of each pixel is comparedin hardware to a threshold so the edges of the circle can beread directly by the microcomputer. The detection of twoedges of the circle is sufficient to estimate the position ofthe center of the target as long as the radius of the circle isknown.

This sensor provides the absolute position of the cen-tre of the interface plate relative to the centre of the mobilerobot with a resolution better than 0.5 mm and at a rate of2 kHz. It works over distance up to 20 cm, does not needcalibration, and only requires a passive target painted onthe payload.

2.5.3 Safety and Operation

An operator can control the platform remotely. Theoperator can start and stop the tracking of the payload andteleoperate the robot.

For safety, we added two Hokuyo laser scannerswhich detect the distance from the robot to his surround-ings. This is especially useful if, during an experiment,an unexpected object is detected in close proximity of theplatform. In such a scenario the feedback control is shut-down and the robot is stopped.

Additionally, we mounted bumpers to prevent the in-terface plate and the payload from sliding off the air-bearing support when tracking is stopped.

2.6 System Integration

Figure 6 shows the final version of the prototype.Rapid prototyping technologies, such as 3D printing, wereused for mechanical integration of the system. The pneu-matic system is mounted directly to the robot while the airbearings and position sensor are mounted to the damped

frame. The honeycomb panel and interface plate, onwhich a payload can be mounted, can be seen on top.

Figure 6: Final prototype of ROOTLESS.

3 EXPERIMENTAL VALIDATION

Figure 7: Setup of the experiments carried out on the OR-BIT flat-floor facility with Vicon motion capture system.On the left can be seen Mitsubishi PA10 robotic arm usedto impact the payload and in the center ROOTLESS witha mockup of a satellite launch adapter ring as payload.

3.1 Experiment SetupTo verify the system, we used very simple and pre-

dictable contact experiments. For this we placed a knowncompliance device, a spring-damper system, in the con-tact loop. The interface of the contact was designed asa sphere to closely emulate a single-point contact (Figure8). Additionally a load cell measured forces and torques.

We performed the experiments on the ORBIT flat-floor facility instrumented with a VICON motion track-ing system. The objects were tracked with a frequency of250 Hz with sub-millimetre precision. We placed a protec-tive foil on the floor to prevent damage to the floor from

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Figure 8: The robotic arm end effector used in the exper-iments. From left: load cell, compliance device, contactinterface. Spherical targets are attached for VICON mo-tion capture.

the wheel rollers. Figure 7 shows the setup of the experi-ments.

A Mitsubishi PA10 robotic arm was used to achieverepeatable and accurate contact. We commanded the lin-ear trajectory of the arm along the y dimension to ap-proach a satellite mockup with known parameters freefloating on ROOTLESS.

The experiments were performed as part of thecross validation of the ORBIT flat-floor facility withPLATFORM-ART, developed by GMV. A more detaileddescription of the experiments can be found in [10].

3.2 Experiment ResultsWe performed impacts with relative velocities of

10 mm s−1, 30 mm s−1, 50 mm s−1, and 100 mm s−1. Werepeated each experiment with different relative velocityfour times. Figure 9 displays the sample position evo-lution of free-floating satellite during the experiment foreach velocity except 10 mm s−1.

For the lowest velocities, the external disturbances,caused mainly by the imperfect inclination of the floatingplane, were dominant.

Higher velocities were limited by the maximum ve-locity of Youbot and by the reaction delay caused by thefeedback loop.

The error in the y direction (the direction of push) isbelow 10 % over a distance of 2 m. Figure 10 shows theerror between the expected and measured position of thepayload for multiple experiments for an approach veloc-ity of 50 mm s−1. In the x direction, the error is persistentbetween the experiments and can be explained by a con-stant force caused mainly by the inclination of the floatingplane.

0 2 4 6 8 10-2000

-1000

0

x[m

m] 30mm/s

50mm/s100mm/s

0 2 4 6 8 100

1000

2000

y [m

m]

Time [s]0 2 4 6 8 10

0

20

yaw

[°]

Figure 9: Position evolution comparison after contactwith different approach velocities in y direction. Thedashed line shows the expected trajectory after contactwithout any external disturbance.

We identified the force in the x direction by fitting themeasured error data with a parabola:

x =a2

t2 + v0t + x0

where t is the time, a the acceleration, x0 the initial posi-tion, and v0 the initial velocity. The fitted curve is shownin Figure 10: the estimated acceleration from the four ex-periments was 7.24 mm s−2. The acceleration correspondsto an inclination of 0.042◦.

Figure 11 compares the position of the payload andthe robot during the initial phase of the contact experimentwith approach velocity of 50 mm s−1. It shows how thefeedback controller tracks the floating payload. One cannotice reaction delay to the initial movement of the pay-load of approximately 0.25 s. After 0.7 s the robot reachesthe velocity of the payload. The errors, like slipping of thewheels, are corrected by the feedback controller, which re-sults in oscillations along the trajectory of the robot. Thereaction time, the acceleration of the robot, and the oscil-lations in position do not affect the motion of the payloadillustrating the advantage of this system.

4 CONCLUSION

We have demonstrated a novel testbed for free-floating dynamics in two dimensions. We have shownthat it can reproduce the dynamics during and after impact

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0 2 4 6 8 10-400

-200

0

∆ x

[mm

]

Estimated force

Time [s]0 2 4 6 8 10

0

200

400

∆ y

[mm

]

Figure 10: Error between expected position and real posi-tion after floating payload release for repeated contact ex-periments with approach velocity 50 mm s−1. The dashedline shows estimated constant force caused by inclinationof the floating plane.

accurately down to very low masses. Moreover we haveshown that it is possible to do this while decoupling theair-bearing platform from the payload thereby offering inpractice unlimited experiment duration and an operatingarea that is limited only by the length of your air supplyhose.

In the experiments we have demonstrated accurate dy-namics after impact at velocities ranging from 20 mm s−1

to 200 mm s−1, but the upper limit can be increased byfurther optimisation of the controller and the distance be-tween the air bearings.

The main limitation was the parasitic acceleration in-troduced by the inclination of the floating plane caused bythe unevenness of the floor and height misalignment of theair bearings. In our tests we measured parasitic accelera-tion to be below 8 mm s−2.

The mass of the interface plate was only 2.94 kg butit can be lowered by designing specific interface plate forthe experiment.

4.1 Future WorkThe most important future update will be the imple-

mentation of automatic levelling of the horizontal plane.The inherent inclination due to an uneven floor and themisalignment of the air bearings are the main limitationsof the platform. Using feedback control we will adjust theheight of the air bearings to compensate these effects.

Moreover, it is possible to introduce artificial gravityby slightly tilting the horizontal floating plane. The ca-pability to simulate low-gravity situations will be uniquecompared to other facilities, and allow for the validation

0 0.5 1 1.5 2 2.5

-40

-20

0

x [m

m] Robot

Payload

0 0.5 1 1.5 2 2.50

100

200

y [m

m]

Time [s]0 0.5 1 1.5 2 2.5

-2

0

2

4

yaw

[°]

Figure 11: Position evolution of payload and robot dur-ing simple contact experiment with approach velocity50 mm s−1. Time of contact between the chaser and thepayload was at tc = 0 s.

of robotic systems on asteroids, comets and small moons.The mechanical design could be further simplified by

replacing the current mecanum wheels with omnidirec-tional wheels specially designed to minimise the gap be-tween the rollers. It would allow to remove the frame withball transfer units. For instance [4] proposes such wheeldesign.

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