MOSAR-WM: Integration and Test Results of a Relocatable Robotic Arm Demonstrator for Future On-orbit Servicing Missions

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MOSAR-WM: Integration and Test Results of a Relocatable Robotic Arm Demonstrator for Future On-
orbit Servicing Missions
Mathieu Deremetza*, Pierre Letiera, Gerhard Grunwald b, Máximo A. Roab, Bernhard Brunnerb, Alvaro
Ferrán Cifuentesa, Mateusz Szydelkoa, Jeremi Ganceta, Michel Ilzkovitza
a Space Applications Services NV/SA, Leuvensesteenweg 325, 1932 Sint-Stevens-Woluwe (Brussels Area), Belgium,
firstname.lastname@spaceapplications.com
b Institute of Robotics and Mechatronics, German Aerospace Center (DLR), 82234 Wessling, Germany,
firstname.lastname@dlr.de
* Corresponding Author
Abstract
Existing commercial satellites and space platforms are traditionally the result of a customized monolithic design
with very limited or no capability of servicing and maintenance. To increase these capabilities while remaining cost
effective, high performing, reliable, scalable and flexible, key technologies need to be developed. Versatile and
advanced robotic systems can be considered currently as one of these key enablers, opening a new horizon of
possibilities to facilitate this fundamental shift of paradigm in designing and deploying satellites and spacecraft.
This paper deals with such a robotic system called MOSAR-WM. MOSAR-WM is a robotic manipulator, aka.
“walking” capable, developed in the context of the European Commission’s Space Robotic H2020 MOSAR project.
This robotic system aims at installing and releasing satellites modules (from servicer to client spacecraft and
inversely). It also has the capability to relocate itself over the spacecraft structure in order to perform tasks at
different locations. To do so, MOSAR-WM features 7 degree of freedom (DOF), a symmetrical and
anthropomorphic kinematics, and standard interconnects at each tip for mechanical, data and power connections to
the spacecraft. It also embeds its own power and data avionics as well as servo and robot control units.
Within the H2020 MOSAR project, a Technology Readiness Level (TRL) 4 ground demonstrator of MOSAR-WM
has been developed, built and tested. The overall length of the robotic arm is 1.6 meters for an approximate weight of
30kg and has a lifting capability of 10-kg payloads at 1g in its entire workspace. This paper describes the assembly,
integration and testing (AIT) activities related to the development of such a relocatable robotic arm, as well as the
demonstration results.
Keywords: Space robotics, Relocatable robot, Mechanism Design of Manipulators, Standard Interconnects
Acronyms/Abbreviations
Critical Design Review (CDR), European Space
Agency (ESA), Final Presentation (FP), Kick-Off (KO),
Preliminary Design Review (PDR), Standard
Interconnect (SI), System Requirement Review (SRR),
Test Readiness Review (TRR), Walking Manipulator
(WM), Walking Manipulator Controller (WMC).
1. Introduction
For now limited to geosynchronous equatorial orbit
interventions, the on-orbit market should expand in the
coming years to new low earth orbit missions:
monitoring, reparation, payload installation and
replacement, spacecraft reconfiguration, assembly,
manufacturing and recycling. Such an increasing
servicing demand is foreseen because of the ongoing
trend to shift spacecraft design paradigms. Existing
commercial satellites and space platforms are indeed in
most cases the result of a highly customized monolithic
Fig.1. MOSAR-WM: Relocatable robotic arm ground
demonstrator

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design with very limited or no capability of
maintenance, while upcoming spacecraft developments
will increasingly adopt reconfigurable structures
featuring spacecraft bus designed to support new
servicing missions.
To offer these capabilities while remaining cost
effective, high performing, reliable, scalable and
flexible, key enablers need to be developed. The
exploitation of standard interconnects (SI) as well as
more advanced and capable robotic systems, for
performing the operations, opens such a new horizon of
possibilities for future space missions, ranging from On-
Orbit Servicing of existing satellites (for refuelling,
orbital replacement unit or de-orbiting) to large On-
Orbit Assembly and reconfiguration of modular
spacecraft.
In such context, MOSAR-WM, a relocatable robotic
arm demonstrator introduced in [1], has been developed
in the frame of the H2020 MOSAR project (see Fig. 2).
This project, completed mid-2021, aimed at proposing a
modular alternative to spacecraft design and related key
technologies, such as robotic assets, to enable future
sustainable on-orbit assembly and reconfiguration of
satellites. More details about the MOSAR project can be
found in [2,3].
The outcome of the MOSAR-WM development is
described in this paper, focussing especially on the
integration, testing and experimental results of such a
robotic arm system.
Fig. 2. MOSAR-WM development history from KO to
FP.
The structure of this paper is as follows: Sec. 2
provides an overview of the MOSAR-WM system
including its kinematics and overall characteristics. Sec.
3 details the subsystem and system integration of the
robot while Sec. 4 describes the experimental results
achieved with the ground demonstrator. Sec. 5 finally
provides a conclusion on MOSAR-WM and its work
perspectives.
2. System overview
2.1 Concept of operations
Assuming a spacecraft structure and spacecraft
modules equipped with SIs, the two basic operations
considered for the walking manipulator (WM) are (1)
re-localization to a new attachment point on the satellite,
and (2) manipulation of a spacecraft module using the
SI attached to the arm (Fig. 3 & 4).
Fig. 3. Artist representation of the re-localization
operation.
Fig. 4. Artist representation of the manipulation
operation.
2.2 Robot description
The walking manipulator comprises the following
elements:
 Structure (Limbs): The walking manipulator is
composed by eight structural elements. The
overall configuration of the manipulator is
based on a human-like arm with asymmetric
joints (See Fig. 5 & 6).

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Fig. 5. Overview of the MOSAR-WM.
 Motorization (Robotic joints): The walking
manipulator is equipped with seven revolute
joints according to the following symmetric
configuration R┴R┴R┴R┴R┴R┴R, where R
indicates a revolute joint and ┴ the
orthogonality between two successive joint
axes (see Fig. 7). Motors have been sized for
lifting a 10kg payload at 1g across the entire
workspace of the manipulator.
Fig. 6. Limb description for the MOSAR-WM.
 End effectors (HOTDOCKs): Each extremity
of the walking manipulator is equipped with a
Standard Interconnect (SI), namely
HOTDOCK as illustrated in Fig. 6. These
interfaces allow the robot to relocate, since the
arm can be attached to a supporting structure
on both sides. More details about HOTDOCK
can be found in [4].
 Avionics: In order to perform motions, control
its robotic joints and receive/forward power
and data through its structure, the walking
manipulator is equipped with independent
avionics as illustrated in Fig. 8. Each motor is
driven by a dedicated joint controller (driver)
while the control of the WM displacement at
arm level is performed by an embedded
computer called Walking manipulator
controller (WMC) running a Linux operating
system with a real-time kernel patch.
Fig. 7. Axis definition for the MOSAR-WM.
Fig. 8. Integration of the avionics inside MOSAR-WM.
More details about MOSAR-WM architecture,
including avionics, software and control aspects, can be
found in [1].
2.3 Kinematics
2.3.1 Structural parameters
Table 1. MOSAR-WM’s Denavit Hartenberg
parameters.
a [mm]
[deg]
d [mm]
[deg]
80
90
345
0
-80
-90
0
0
75
90
455
0
-75
-90
0
0
80
90
455
0
-80
-90
0
0
0
0
345
0

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2.3.2 Joint limits
Table 2. MOSAR-WM’s joint limits.
Joints
Joint limits [deg]
min
max
J1
-175
175
J2
-175
45
J3
-175
175
J4
-175
45
J5
-175
175
J6
-175
45
J7
-175
175
Fig. 9. MOSAR-WM’s joint limit definition.
2.4 MOSAR-WM’s characteristics
Table 3. MOSAR-WM’s characteristics.
Features
Value/Details
Degrees of freedom
7
Payload
10kg (at the end-effector in
all workspace)
Weight
32.5 kg
Maximum reach
1.2m
Power Consumption
 Stationary (with or w/o
payload and holding
brakes): 35W
 When operational
without payload: ~80W
in average
 When operating a 10kg
payload at 1g: 150-200W
max
Joint torque
 Joint 1-3-4-5-7: 175Nm
capabilities
 Joint 2-6: 260Nm
Joint maximum
rotational speed
0.15 rad/s
Sensors
In each joints:
 Position
 Torque
 Temperature sensors in
each joints
Avionics
 Embedded dedicated
robot control unit (Intel
NUC)
 servo control units at
joint level
 CAN, WIFI and
SpaceWire
communication
interfaces
 USB interface to internal
INTEL NUC computer
Mechanical
structure features
 Aluminium based
 Symmetrical (for
connecting the WM from
one or the other
extremity without
changing the kinematics)
 Hollow shaft (enabling a
complete internal
harnessing)
End effectors
The two tips are equipped
with active standard
interconnects, namely
HOTDOCKs, allowing for
mechanical, power and data
transfers.
Safety
 Safety brakes on all
joints
 Software joint position,
velocity, torque limits
Power Input
48V DC
3. Integration and testing
3.1 Joint integration and testing
A unitary prototype and seven hollow shaft robotic
joints (two different types depending on the required

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torque) have been integrated and successfully tested
within the MOSAR project.
The joint integration and testing consists of the
following steps:
 Mechanical preassembly and assembly of the
motor chassis, input and output shafts as well
as integration of the sensors cables and related
connectors.
 Joint electronics integration.
 Preliminary test of the joint prior to its
integration inside the arm. This includes the
overall behaviour of the joint as well as the
sensor suite and safety brake functioning.
Some integration steps of the joints are depicted in Fig.
10.
Fig. 10. Joint integration: from part inspection to
preliminary testing.
3.2 Arm integration
A tailor made structure was designed to embed all
the required components on top of meeting the technical
requirements and to simplify the integration and
maintenance operations. A 48V power bus and three
data buses (SpaceWire / EtherCAT / CAN) are installed
inside and through the overall length of the WM. At
joint level, sensor cables are also locally passing
through the hollow shaft.
The arm structure integration consists of the following
steps:
 Joint mechanical installation in the dedicated
limbs;
 Inserting and routing power and data cables
through the arm structure and joint hollow
shafts;
 Fixing cables with cable clamps;
 Interfacing the joint electronics with the arm’s
power and data (EtherCAT) buses.
Some integration steps of the arm are depicted in Fig.
11.
Fig. 11. Arm integration: from part inspection to joint
electronic harnessing.

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3.3 Computer and communication interfaces
integration and testing
A whole computer and related communication
interfaces and software are embedded in the
manipulator body.
The avionics integration consists of the following steps:
 Mechanical integration of the preconfigured
walking manipulator controller (WMC) and
related power converter unit, connection to the
arm power bus and EtherCAT network;
 Mechanical integration of the communication
interfaces (SpaceWire and CAN), connection
to the OBC and to the arm data buses;
 Testing the functioning of the WMC when
supplied by the arm power bus;
 Testing of the communication buses
(EtherCAT, SpaceWire, CAN).
The avionics integration inside the MOSAR-WM’s
forearm is depicted in Fig. 12.
Fig. 12. Avionics integration inside one of the MOSAR-
WM’s forearm.
3.4 End-effector integration and testing
Two active HOTDOCK interconnects with WM
dedicated interfaces and a local power conversion unit
have been developed to equip the tips of MOSAR-WM.
The end-effector integration consists of the following
steps:
 Standard interconnect mechanisms assembly
and integration of the internal harnessing and
related connectors;
 Standard interconnects electronics integration:
mounting of the controller board, power
converter unit and preparation of the external
harnessing.
 Testing of the standard interconnect outside the
arm structure.
 Mechanical integration of the standard
interconnect at the tip of the arm structure.
 Interfacing the standard interconnects
electronics with the arm’s power and data
(SpaceWire, CAN) buses.
 Testing of the interconnects when controlled by
the embedded WM controller.
Some integration steps of the standard interconnects are
depicted in Fig. 13.
Fig. 13. End-effector integration: from subsystem to
system integrated interconnect.
3.5 Overall System integration and testing
Once all subsystem have been integrated, a last test
series is performed at system level to assess the correct
functioning of the overall functionalities.
Fig. 14 & 15 illustrate the fully integrated WM and
its first installation on the MOSAR’s ground
demonstrator structure.
Fig. 14. Fully integrated MOSAR-WM.

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Fig. 15. MOSAR-WM installed on the MOSAR’s
ground demonstrator structure.
4. Experimental Results
4.1 MOSAR ground demonstrator
The MOSAR ground demonstrator, depicted in Fig.
16, is composed of the following element:
 A Ground segment featuring:
 A mission control centre composed of two
computers. The first computer generates a
valid sequence of operations for the robot.
It also monitors and controls the space
segment. The second computer validates
the client spacecraft reconfiguration and
the related mission operations simulation
in communication with the first computer.
 A Space segment featuring:
 The servicer and client spacecraft
structures composed of rigid frames and
equipped, on the top, with standard
interconnects HOTDOCK (on a 40cm
grid). Both servicer and client spacecraft
are here rigidly fixed, simulating two
docked spacecraft. Both the servicer and
client spacecraft are equipped with
computers (communication with the
spacecraft modules and forwarding high
level commands to the robot), power
distribution units and two 48V/10A power
supplies (providing energy to the system
through the 48V power bus interconnected
between the standard interconnects).
 Five Spacecraft Modules (40x40x40cm,
10kg in average), two on the servicer and
three on the client, each of them,
representing a subsystem of the client
satellite. Each module is equipped with its
own instrument control unit, power
distribution unit, standard interconnects
and payload.
 The 1.6m long and 32.5kg mass
relocatable robotic manipulator, presented
in this paper, which can move along the
two spacecraft and manipulate the
modules. The robot is operating at 1g
without gravity compensation devices.
Fig. 16. MOSAR test setup.
4.2 Demonstration scenarios
Within the MOSAR project, a series of scenarios has
been performed to assess the WM concept during
operations:
 Scenario 1: the robot performs, in position
mode, the client spacecraft reconfiguration by
adding and substituting spacecraft modules. To
do so the robot has to manipulate modules and
relocate itself on the spacecraft bus to optimize
its workspace [5].
 Scenario 2: the robot performs, in position
mode, the manipulation of a spacecraft module
requiring simultaneous multiple connections,
then relocates itself on the horizontal panels of
the spacecraft modules, and finally returns on
the servicer spacecraft [6].
 Scenario 3: the robot performs, in position
mode, a relocation on the vertical panel of a
spacecraft module, then lifts itself on the top
panel of the same module and finally returns
on the spacecraft bus.
 Scenario 4: the robot performs SI connections
(robot end effectors and module manipulation)
using the impedance control mode for the
approach.

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4.3 Relocation results
As part of scenarios 1, 2 and 3, the robot performed
several relocation operations on the spacecraft structure
and on the panels of the spacecraft modules (horizontal
and vertical). Figs. 17, 18 and 19 illustrate the
successful motion sequences of the WM during the
different walking operations.
Fig. 17 illustrates in particular planar step
sequences, namely side-by-side and diagonal. In this
case, the WM is reaching SIs located respectively at
40cm and 56.6cm on the same surface from its
attachment point.
Fig. 17. Walking sequence on the spacecraft structures:
walk on side-by-side SIs (top), walk diagonally on SIs
(bottom).
Fig. 18 illustrates a greater walking sequence on
different structures levels, starting from the top panel of
a spacecraft module and stepping on a SI located on the
spacecraft structure. Here, the WM managed to reach
successfully an SI located at 80cm horizontally and
40cm vertically from its attachment point.
Fig. 18. Walking sequence on/from spacecraft modules.
Fig. 19 finally illustrates an even more demanding
walking operation where the WM relocates itself on the
top SI of a spacecraft module after relocating and lifting
itself from the vertical panel SI of the same spacecraft
module. In this situation, the WM is first reaching the
vertical panel SI located at (80cm,20cm,20cm) from its
initial attachment point and then the top panel SI located
at (20cm,0,-20cm) from the side panel SI.
Fig. 19. Walking sequence on the side and self-lifting
on the top of a spacecraft module.
The relocation results obtained during the test
campaign validated the ability of the WM to perform
successful relocation operations from one SI to another
in planar and 3D configurations.

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4.4 Manipulation results
As part of scenarios 1 and 2, the WM performed
manipulation operations of spacecraft modules to
reconfigure the spacecraft architecture. Figs. 20 and 21
illustrate some manipulation sequences performed
during the trials.
Fig. 20 illustrates first a manipulation sequence
where the WM grasps a module attached to a servicer
spacecraft SI and positions it above a client spacecraft
module. In this case, the robot grasps a module SI
located at (60cm,40cm,20cm) from its base and
positions it at (60cm,40cm,60cm) from its base.
Fig. 20. Manipulation sequence of a spacecraft module
from the client spacecraft.
Fig. 21 illustrates a second manipulation operation
where the WM is grasping from the client spacecraft a
client module and positions it above a client spacecraft
module. Here the robot grasps a module’s SI located at
(40cm,40cm,20cm) from its base and positions it at
(100cm,40cm,60cm) from its base.
Fig. 21. Manipulation sequence of a spacecraft module
from the servicer spacecraft.
The relocation results obtained during the test
campaign validated the ability of the WM to perform
successful manipulation operations of 10kg spacecraft
modules at 1g.
4.5 Impedance control results
During scenarios 1, 2 and 3, the control of the WM
motion has been performed in full position control mode
(either Joint or Cartesian), without visual feedback. The
nominal strategy was however to have a mix of position
and impedance control, the latter being used for SI
alignment and during connection, in order to provide a
level of flexibility in the contacts.
Fig. 22 illustrates the successful experiments of
impedance control for SI approach and connection,
which have been performed during the preparation of
the final MOSAR demonstration, for different
configurations of operations (vertical and horizontal
approach of the WM end-effector and manipulation and
alignment of a spacecraft module in double connection).
Fig. 22. SI mating sequences using robot impedance
control: vertical mating (top), horizontal mating
(centre), and spacecraft module mating (bottom).
5. Conclusion and Perspectives
This paper describes the MOSAR-WM demonstrator,
its integration and its experimental results performed in
the scope of the H2020 MOSAR project.
MOSAR-WM is a small size and mass robotic
manipulator (1.6m/32.5kg) with a redundant and
symmetrical anthropomorphic kinematic structure
(7DOF). MOSAR-WM has the main capability of
relocating and lifting itself using standard interconnects
as end-effectors. It has also the capability to lift
substantial payloads (10kg at 1g in the entire
workspace). It specifically embeds its own intelligence
and internal harnessing (WMC, servo control units, data
and power buses) and runs position and impedance
based control (since the framework is known, vision
system is not needed).
Experimental results performed during the MOSAR
project test campaign demonstrated the ability for
MOSAR-WM to perform successfully a selection of on-
orbit servicing, assembly and manufacturing relevant
operations, in a ground demonstration setup.
Thanks to these features and achieved results, the
MOSAR Walking Manipulator presages technologies

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that “new space” actors and applications may rely upon
in the coming decade. It is a unique robotic system w.r.t.
the existing state of the art in the category of
autonomous anthropomorphic small relocatable robotic
manipulator, currently developed at TRL4 and targeting
future on-orbit servicing applications.
Future work will mainly focus on the
characterization of the robotic performances and on
maturation of the WM concept. Maturation aspects
include TRL increase and further developments related
to modular robotics, in particular within the ESA
MIRROR project [7].
Fig. 23. Artist representation of the MIRROR concept.
Acknowledgements
This work was partially funded by the European
Commission Horizon 2020 programme under grant
number 821966 (MOSAR).
References
[1] M. Deremetz, P. Letier, G. Grunwald, M. A. Roa, B.
Brunner, B. Lietaer, M. Ilzkovitz. MOSAR-WM: A
relocatable robotic arm demonstrator for future on-orbit
applications. In 71th International Astronautical
Congress (IAC), The CyberSpace Edition, 2020.
[2] MOSAR. A Modular and reconfigurable Future for
Space Missions.
https://www.youtube.com/watch?v=cwqKEzwAVn8
(accessed 30.09.2021).
[3] P. Letier, X. Yang, M. Deremetz et al., MOSAR:
Modular Spacecraft Assembly and Reconfiguration
Demonstrator, ASTRA, 15th Symposium on Advanced
Space Technologies in Robotics and Automation,
Noordwijk, Netherlands, 2019.
[4] P. Letier, T. Siedel, M. Deremetz, E. Pavlovski, B.
Lietaer, K. Nottensteiner, M.A. Roa, J.S. Garcia-
Casarrubios, J.L.C. Romero, J. Gancet. HOTDOCK:
Design and Validation of a New Generation of Standard
Robotic Interface for On-Orbit Servicing. 71th
International Astronautical Congress (IAC), The
CyberSpace Edition, 2020.
[5] Final Demonstration of MOSAR Project
https://www.youtube.com/watch?v=kqwKEPRXEYU
(accessed 30.09.2021).
[6] MOSAR Demonstration
https://www.youtube.com/watch?v=8F4kJZAv09w
(accessed 30.09.2021).
[7] M. Deremetz, Modular Robotic Assets and
Technologies for Future on-orbit Applications, Robotics
in-situ servicing, assembly and manufacturing workshop
at the IEEE/RSJ International Conference on Intelligent
Robots and Systems (IROS), 2020.
https://wvrtc.com/iros2020/invited-speakers.shtml#
(accessed 30.09.2021)