Robotic Technologies for In-Space Assembly Operations

Maximo A. Roa, Korbinian Nottensteiner, Armin Wedler, Gerhard GrunwaldGerman Aerospace Center (DLR), 82234 Wessling, Germany, Email:firstname.lastname@dlr.de

Abstract— On-orbit robotic assembly is a key technologythat can increase the size and reduce costs of construction oflarge structures in space. This document provides an overviewof existing or emerging robotic technologies for space-bornassembly, including also the development of standard interfacesfor connectivity that combine mechanical connections with elec-tronic and power signals. Technologies that can enable on-orbitassembly demonstrations in the near future are currently underdevelopment at the Institute of Robotics and Mechatronics inthe German Aerospace Center - DLR, as showcased in a setupfor autonomous assembly of structures made out of standardaluminum profiles.

I. INTRODUCTION

Large scale space mission scenarios for the Post-ISS erasuch as the Moon Village (ESA), Mars exploration (NASA),and Orbital Hub in LEO (DLR) [1], require the availabilityof dexterous robot systems capable of performing complexassembly tasks. In contrast to the construction of ISS, whichwas performed by astronauts and took over a decade, neworbital structures are expected to be assembled by roboticsystems. In the last two decades, robotized On-Orbit Ser-vicing (OOS) missions have increasingly gained importance.OOS addresses the maintenance of space systems in orbit,including repairing and refueling, using technologies that canbe extended to on-orbit robotic assembly. In-space assemblycan enable the construction of much larger orbital structures,for instance for setting up more sensitive radar imagingor for obtaining much higher speed of communications byassembling bigger and more sensitive antennas.

In the future, robotic construction capabilities could alsobe applied to create space infrastructure, standing structuressuch as refueling depots, in-space manufacturing facilities,space-tourism complexes, and asteroid mining stations [2].Current manufacturing and technological limitations areevident in the construction of antennas and mirrors thathave to be deployed from a single launch with a singlesatellite. However, a dexterous and capable space robot couldassemble pieces from multiple launches to construct antennasand other structures of enormous size, thanks partially to theexpected reduction of launch costs from private companiessuch as SpaceX and Blue Origin.

Operations required for servicing, maintenance and as-sembly missions, such as autonomous rendezvous, dock-ing and undocking, are relatively common nowadays, butautonomous on-orbit robotic servicing is currently underdevelopment [3]. Robot-based assembly in the absence ofgravity addresses fundamental technical questions that do notexist for terrestrial applications. The robot is mounted on anactively regulated platform, and the motion of the robot has

dynamic effects on the platform itself. The dynamic effectsdepend on the mass of the objects and the velocity of themotion. When the robot works on a platform different to itsown floating base, the required physical contact affects bothplatforms. The contact can also affect the perception system,as minor changes in position might lead to significant visualocclusions. These effects must be considered for planningmotions of the robotic manipulators.

Some approaches for robotic on-orbit servicing and assem-bly are being developed, but they are mostly at conceptuallevel [4]. Current missions such as NASA’s Restore-L [5]and DARPA’s RSGS [6] are expected to demonstrate theenhancement of functionalities well beyond servicing ex-isting satellites. It is expected that within five years somesystems in orbit will be able to demonstrate the autonomy,dexterity, and delicacy needed to work on orbital assembly.This work provides an overview of robotic technologiesaiming to provide such capabilities (Section II).

Main in-space assembly technologies used so far byastronauts include the deployment of truss and beam sub-assemblies, with revolute joints or special latches for easysnap-on, and recently, in-space manufacturing using additivemanufacturing techniques [7]. Independently of the tech-nology used for the mechanical frame, one of the mainchallenges for assembling a large functional structure is thecombination of power and data connectors together with themechanical latch. Possible alternatives for solving this issueinclude magnetic latching and reversible joints that allowdisassembly for repairing or for module replacement. A briefoverview of standard interfaces that allow full functionalityfor modular assemblies is also presented here.

While teleoperated or partially assisted assembly oper-ations are possible on ground, autonomous assembly ofstructures has been recently proven feasible through thecombination of adaptable perception, integrated assemblyand grasp planning, and compliant control of the manipu-lators [8]. A description of such ground-based demonstratoris presented in this work (Section III). The paper concludeswith a discussion on some open topics in the field ofautonomous on-orbit robotic assembly (Section IV).

II. OVERVIEW OF EXISTING IN-SPACE ROBOTICASSEMBLY TECHNOLOGIES

In-space assembly has been studied for more than threedecades now [9] due to the great potential applications forperforming tasks such as assembling systems larger thatcurrent cargo areas in commercial transportation vehicles,and can also be applied to more commercially interesting

Fig. 1. Current robotic demonstrations of in-space assembly. From left to right: Dextre approaching the RRM module (Courtesy of NASA), Robonautassembling truss structures on ground (Courtesy of NASA), Trusselator for SpiderFab (Courtesy of Tethers Unlimited), and initial ground demonstrationof Phoenix with the FREND arms (Courtesy of DARPA).

activities such as repair, refuel and upgrade of currentsatellites, although the economic benefit of those operationsis controversial [10], [11]. Possible applications of in-spaceassembly include artificial gravity or asteroid redirect ve-hicles, space transportation hubs, space telescopes, in-situresource utilization for construction, solar electric powerand propulsion, sun shields, and atmospheric deccelerators[7]. An analysis of the potential applications shows that thedemanded cross-cutting (non application-dependent) capa-bilities are robotic assembly, standardized interfaces, mod-ular design with high stiffness, deployable subsystems, anddocking and berthing. This survey is focused on the topicsof robotic assembly and standardized interfaces, which aremore relevant for roboticists working on space applications.The design of deployable structures with high stiffnesscorresponds to the subfield of space architecture; autonomousdocking and berthing are maneuvers that have been tested inmultiple occasions and are currently being tested for on-orbitservicing operations [3].

A. Technologies for robotic assembly

Several on-orbit servicing missions have been carriedout, and have created the technological basis for on-orbitassembly. The first robotic on-orbit servicing mission wasperformed by NASDA (now JAXA, Japan) with the ETS-VII (KIKU-7) mission [12]. It used the first satellite equippedwith a robotic arm, a 6 DoF, 2 m long manipulator, whichwas employed mainly to perform experiments of rendezvousdocking between a chaser (Hikoboshi) and a target (Ori-hime), and also to prove the feasibility of doing an ORU(Orbital Replacement Unit) exchange. Experiments were per-formed in 1999 [13]. The next relevant mission was OrbitalExpress, a joint effort of DARPA and Boeing. Experimentstook place in 2007 [14], and included rendezvous, docking,fluid transfer and ORU transfer between two satellites, aservicing satellite equipped with a 6 DoF arm (ASTRO),and a serviceable satellite (NEXTSat) [15].

Different technologies have been proposed for roboticassembly in space, although real experiments have beenperformed only at the level of servicing and maintenancetasks, mainly with the Dextre manipulator (Fig. 1). Dextre,also known as the Special Purpose Dexterous Manipulator(SPDM), was assembled in 2008 onboard the ISS. It wasbuilt by MacDonald, Dettwiler and Associates (MDA) and

maintained by the Canadian Space Agency (CSA). Theoriginal goal of the robot was to relieve astronauts from ex-ternal servicing and maintenance tasks on the ISS, includingremoval and installation of batteries, opening and closingcovers, or reconnecting cables [16]. It has also been recentlyused for removing cargo payload from supply ships like theDragon capsules. Dextre is a dual arm manipulator, each armwith 7 joints and a length of 3.35 m, and a total mass of 1560kg [17]. It has a power fixture at the “wrist”, which can begrasped by the Canadarm for repositioning at different worksites, or to be attached directly to the ISS. For performingoperations, Dextre moves one arm at a time, teleoperatedeither from the ISS or from a ground control center. In2009 NASA initiated the Robotic Refueling Mission (RRM),aiming to use Dextre for performing robotic servicing tasksin orbit, including refueling and maintenance [18]. The firstphase was developed between 2012 and 2013, and employedfour different tools: a wire cutter, a multifunction tool, asafety cap removal tool, and an EVR nozzle tool. It provedthat the robot was capable of performing on orbit refuelingof existing satellites. The second phase was launched in2013 to further mimic servicing tasks, which were performedbetween 2015 and 2016. The tasks included the installationof two task boards onto an existing module, the visual inspec-tion of Canadarm using a specialized tool, and assessmentof performance of a solar cell. The mission ended in March2017 with the removal of the RRM payload.

Since 2011, Robonaut 2 (Fig. 1) became the first humanoidrobot in Space [19]. It was co-developed by NASA andGeneral Motors (GM) to serve as an assistant to humanworkers [20]. The initial Robonaut included only the torso,but in 2014 the mobility platform for Robonaut was de-livered, to augment the robot with two legs for maneu-vering inside the ISS. The robot has human-like arms andhands, which enables it to use the same tools as the crewmembers. On-ground tests have demonstrated the ability ofRobonaut to perform different tasks, including assembly oftruss structures [21]. On-orbit uses of Robonaut so far havebeen limited to cleaning and monitoring tasks inside the ISS,although the robot has the potential for becoming a usefulassistant for instance in EVA tasks.

Traditionally, satellites are fabricated and tested on ground,and then launched as payload on rockets. This demands a

Fig. 2. Different robotic concepts for in-space assembly. From left to right: SpiderFab (Courtesy of Tethers Unlimited), Archinaut (Courtesy of Made inSpace), CIRAS (Courtesy of Orbital ATK), and Dragonfly (Courtesy of Space Systems Loral).

large effort and cost in the preparation of the system tosurvive the launch process, while the size of the satelliteis limited by the cargo capabilities of current spacecrafts.To survive these restrictions, Tethers Unlimited has pro-posed the SpiderFab architecture [22], a satellite “chriysalis”,combining manufacturing techniques with robotic assemblytechnologies (Fig. 2). This effort was supported through aNASA Innovative Advanced Concepts (NIAC) grant [4]. Thearchitectural concept included:

• Techniques for processing suitable materials to createstructures: while most rapid prototyping techniques onground rely on gravity to facilitate bonding of thematerials, the microgravity environment in space callsfor a different approach. However, the lack of gravityalso means that structures can be fabricated in anydirection without the distortions induced by gravity. Thesystem proposes the combined use of Fused FilamentFabrication to 3D print high-performance reinforcedpolymers. An initial test of the manufacturing of sup-porting structures was performed with the “Trusselator”(Fig. 1), for creating large truss structures by usinga continuous fiber reinforced thermoplastic yarn [23].This technology was recently selected for a new project,the MakerSat, aiming to endow small satellites with theability to grow its size in space.

• Mechanisms for mobility and manipulation of toolsand materials: involves the use of multiple dexterousrobot arms to position the fabrication heads, positionthe robot along the constructed structure, and positionthe structural elements for assembly. As initial concept,the “Kraken” 7 DOF, 1 m long arm was developed fornanosatellite applications, and delivered on Feb 2013.

• Methods for assembly and joining structures: bond-ing can be accomplished in multiple ways, includingwelding, mechanical fasteners, adhesives, and snap-onlatches. Fusion bonding can also be exploited by com-bining heat and pressure to fuse thermoplastic materials.

• Thermal control: one big challenge is the control oftemperatures and heat gradients on the manufacturedparts, especially considering the influence of the sunaccording to the position on orbit. The materials must beprocessed such that they cool and solidify in a mannerthat does not induce undesired stresses in the structure.

• Metrology to enable closed-loop fabrication: required tomeasure the overall shape of the structure, and to enableaccurate positioning of the fabrication heads.

• Methods for integrating functional structures: the con-cept of SpiderFab was focused on creating supportstructures, but it required also suitable mechanismsto integrate functional elements that are produced onground, such as solar cells, antenna panels, sensors,wiring, reflective membranes, and payload packages.

In 2012 DARPA started the Phoenix program, aiming todemonstrate the capabilities of a multi-arm satellite with toolchanging capabilities (Fig. 1). The short term mission goalsare repairing of satellites, such as fixing a component ordeploy an antenna. Medium term goals include performingupgrades on orbit, and the long term goal is the on orbitconstruction of a geostationary satellite capable of servic-ing an operational spacecraft by 2020 [24]. The arms forthe satellite were developed under the FREND program(Front-End Robotics Enabling Near-Term Demonstration) forgrappling and repositioning Resident Space Objects (RSOs);they have 7 DoF, a weight of 78 kg, and a total length of2.5 m [25]. As part of the Phoenix program the arms areupgraded, new components like tools and tool changer are tobe designed, and novel algorithms for control, teleoperation,mission sequencing and fault detection are also expected. Avideo with some of the on-ground results of the project isavailable in [26]. Two parallel missions under developmentand with similar goals are NASA’s Restore-L and DARPA’sRSGS. Restore-L (Refuel Government-owned client satellitein Low Earth Orbit) aims to create a spacecraft that will ex-tend satellites’ lifespans by refueling and relocation in LEO,mostly through teleoperation for critical actions [5]. RSGS(Robotic Servicing of Geosynchronous Satellites) aims forreplacing and repairing faulty hardware relying mostly onautonomous operations [6].

At the end of 2015, NASA announced new Public-PrivatePartnerships to advance “Tipping Point”, Emerging SpaceCapabilities. Three projects were selected to explore thefuture of robotic in-space manufacturing through manipula-tion of structural trusses: Archinaut, CIRAS and Dragonfly(Fig. 2). Phases A/B will be developed during 2 years, beforeentering the potential flight demonstration phase in 2019.

The first project, Archinaut (officially called VersatileIn-Space Robotic Precision Manufacturing and AssemblySystem) is under development by Made In Space Inc. Itaims to construct, assemble and integrate larges structuresin space [27]. It is constructed around a space-capablemanufacturing unit based on an Additive ManufacturingFacility, already tested on the ISS, and capable of producing

structures larger than itself. The setup for testing additivemanufacturing of structures is called ESAMM (ExtendedStructure Additive Manufacturing Machine), and is intendedto test manufacturing in a simulated space (thermal-vacuum).To provide the assembly robotic component, the 3D printerwill be complemented with three manipulator arms builtby Oceaneering Space Systems, which help to conform the“Ulisses” spacecraft for in space manufacturing. NorthropGrumman will provide systems engineering, control electron-ics, software, testing and assistance with Archinaut’s spacestation interface.

The second project is called CIRAS (Commercial Infras-tructure for Robotic Assembly and Services), and is carriedout by Orbital ATK [28]. It aims to attach and detach a solararray, demonstrating assembly and reuse capabilities on anintegrated ground test using an air-bearing laboratory. Thesystem will use two upgraded 15-meter TALISMAN (Ten-sion Actuated Long Reach In-Space Manipulator) togetherwith precision EBEAM welding using reversible quick-connects, and a special jigging precision assembly robot. Theproject builds upon existing capabilities of the LightweightSurface Manipulator System (LSMS) [29].

The third project is called Dragonfly, or On-Orbit RoboticInstallation and Reconfiguration of Large Solid RF Reflec-tors [30], [31]. It is developed by Space Systems Loral,and aims to modify existing antenna and robotic equipmentto perform a flight demonstration of antenna assembly fora GEO satellite. It intends to use existing assembly in-terfaces with robot manipulators, and demonstrate stowagetechniques for larger than traditional solid reflectors fora potential launch. Tethers Unlimited is a partner in thisproject, contributing with the in-space truss manufacturingfacility developed with SpiderFab.

Concepts for an on-orbit servicing mission that can bepotentially expanded for on-orbit fabrication have also beendeveloped at DLR, with the DEOS (Deutsche OrbitaleServicing Mission) [32]. As part of the preparation forthese activities, DLR developed the OOS-SIM, an On-OrbitServicing simulator (Fig. 3) that tries to replicate conditionsof operations in micro-gravity environments.

Fig. 3. OOS-SIM, an on-orbit servicing simulator developed at DLR.

B. Connectivity and interfaces

Fig. 4. Existing standard interfaces: Top left: EE on SRMS (Courtesy ofNASA), Top right: LEE on SSRMS (Courtesy of NASA), Bottom left: JEMEE (Courtesy of JAXA), Bottom right: OTCM (Courtesy of NASA).

A standard interface is defined as “a combination ofdevices that allow to couple active payload modules (APMs)to a manipulator, amongst themselves, and to spacecraft.It shall allow transferring of mechanical loads, electricalsignals and data, as well as thermal flux between the coupledmodules” [33]. A recent review on standard interfaces canbe found in [34]. This section focuses mainly on detachableinterfaces for orbital robotics; several of them have alreadybeen implemented in space missions (Fig. 4). The roboticarms often possess an End Effector (EE) that interfaceswith a grapple fixture. Examples are the Shuttle RemoteManipulator System (SRMS), and the latching end effector(LEE) on the Space Station Remote Manipulator System(SSRMS). The JEMRMS (Japanese Experiment Module Re-mote Manipulator System) on the ISS Japanese experimentalmodule (JEM), has a similar mechanism to interface withseveral kinds of common grapple fixtures placed on variouscomponents of ISS modules. Their ability to grasp and movemassive objects in space environment has been significantlydemonstrated so far in different ISS projects. Also, theORU/Tool Changeout Mechanism (OTCM) of the SPDMcan attach at the tip several tools to conduct various works.A typical interface such as LEE is a detachable electro-mechanical interface (DEMI) designed to possess gender,i.e. an EE side and a grapple fixture side. All the interfacespossess active mechanisms on the arm side, an exclusiveactuator used only for the docking operation.

Besides those practically implemented interfaces, there areseveral interfaces developed on ground aiming for futureimplementation. As part of space-borne manipulator, JAXAhas developed an EE (Fig. 5) and grapple fixtures aiming tobe used for construction of large space structures. However,as the majority of interfaces in space, it is designed to provideonly electro-mechanical connection between components.Another example is the Compact Tool Exchange Device(CTED, Fig. 5), developed for connecting DexArm (ESA)

Fig. 5. Different standard interfaces: Top left: JAXA EE (Courtesy ofJAXA), Top right: CTED (Courtesy of ESA), Bottom: RSGS interface(Courtesy of MDA).

and Dexhand (DLR) [35]; it was initially developed as partof the EUROBOT project. The RSGS interface (Fig. 5) wasdeveloped by MDA and adopted to interface the FRENDarm and different EEs. Also MDA has developed a passiveinterface mechanism (PIF, Fig. 6) for the Next GenerationCanadarm (NGC) project of CSA, to interface mechanicallybetween the NGC arm and tool tip. Another interface namedDWIM (Fig. 6) was developed by Tokyo Tech for a manipu-lator of an astronaut-supporting robot. While DWIM and PIFare detachable mechanical interfaces (DMI), the other three(JAXA EE, CTED, RSGS) are DEMI. Also, all interfacespossess gender. In addition, all of them are active, exceptfor the MDA PIF (has no actuator on both sides). One recentproposal that combines mechanical coupling, power, heat anddata transfer in one interface is the iSSI (Intelligent SpaceSystem Interface) [36].

Fig. 6. Different standard interfaces: Left: MDA PIF (Courtesy of MDA),Right: DWIM (Courtesy of Tokyo Tech).

For comparison, standard industrial manipulators use in-dustrial adapters like the SWS series from Schunk for

changing tools (Fig. 7). It is an electro-mechanical interfacewith diameter from 40 mm to several hundred mm. As anexample, the SWS-060 Schunk adapter is a DEMI, and itpossesses gender to be able to use an active mechanism onthe arm side that adopts pneumatic actuation to dock/undockthe tool [37].

Fig. 7. Standard industrial adapter: Schunk SWS-060 (Courtesy of Schunk).

III. AUTONOMOUS ROBOTIC ASSEMBLY

Common robotic systems in space applications have asmall degree of autonomy. The execution of tasks usually re-lies on teleoperation, which requires an appropriate feedbackchannel for the operator, typically affected by substantialtime delays. The concept of shared autonomy increases thedexterity of such systems and reduces the effort for theoperators in complex tasks [38]. Nevertheless, teleoperationapproaches only have limited use when it comes to theassembly of complex structures. Because of the fine gran-ularity of assembly tasks, classical teleoperation becomesunfeasible as it consumes substantial amounts of time forthe synchronization of operator commands and manipulatoractions. Therefore, a robotic assembly system should becapable of performing a sequence of operations or even thecomplete assembly task autonomously.

A. Robotic Assembly System

A prototypical system for autonomous assembly of alu-minum structures was recently developed at DLR [8]. It wasdeveloped for a terrestrial use case; however, it demonstratesthe potential of such systems in space applications. Thestructures are made up of a modular building kit sys-tem typically used in industrial facilities for the setup ofproduction equipment, composed by aluminum profiles ofvarious lengths and angle bracket connectors. The systemautomatically decomposes a given assembly specification,i.e. a list of parts and their relative configuration, into a tasksequence, which is then mapped to a sequence of appropriaterobotic skills. Fig. 8 shows the workflow starting with anassembly planning unit, followed by the task to skill mappingand finally by the skill execution.

Although the first humanoid robot is already in spaceoperation [19], the general dexterity and sensing capabilitesfor the autonomous execution of complex assembly tasksare not yet reached. For our system, we combine generaldexterous manipulators with specialized tools and gripperssuitable for the target building kit. The initial demonstrationof autonomous assembly was performed with a single arm

AssemblyPlanning

Mapping Tasks to Skills

SkillExecution

tasksequence

skillsequence

assemblyspec.

PickUpProfile

PlaceProfile

...

positionprofiles

add anglebracket

Fig. 8. Workflow of the robotic assembly system. An assembly specificationis automatically decomposed into a task sequence and then mapped toappropriate robotic skills.

and a number of specific fixturing units conveniently locatedin the robot work cell. The setup was recently extendedwith a second robot arm (Fig. 9) to completely remove therequired fixtures, thus increasing the flexibility of the sys-tem and enabling the assembly of more complex structuresthanks to the implementation of multi-arm collaboration andmanipulation. The system consists now of two KUKA LBRiiwa R800 manipulators, each arm with 7 DoF and equippedwith joint torque sensors. The manipulators are the industrialversion of the light-weight robot arm (LBR) technologydeveloped at DLR [39]. The robotic components of the LBRwere verified for space applications with experiments on theISS in the ROKVISS mission [40]. The LBR’s sensitivityand the impedance control mode allow robust and stable finemanipulation especially in contacts, which is essential in theautonomous execution of assembly tasks.

Fig. 9. Two light-weight robot arms (KUKA LBR iiwa) collaborate in theassembly of aluminum structures.

Fig. 10 shows the selected end effector, an industrialSchunk WSG 50 gripper, with a specific finger design for theparts and tools of the building kit. The modified geometryof the fingers allow form-closure grasps on the parts andthe required tools (screwdriver). In this way, grasping un-certainties are reduced significantly, and grasp stability can

be guaranteed directly by the mechanical design. The twoactuated fingers are mainly used for power grasps, while anadditional third finger is designed for grasping angle bracketsand slot nuts; it also contains a small pin for the positioningof slot nuts inside the profiles.

grasp slot nuts

slide slot nuts

grasp angle brackets

profiles screwdriver tool

Fig. 10. Adapted mechanical gripper (Schunk WSG 50), modified toprovide stable grasps and actions for the given building kit.

B. Robust Assembly SkillsModularity is an important requirement for building large

structures. An infinite number of structures can be con-structed with the building set used in this demonstrator.The square-shaped profiles are customizable in length, andconnections can be freely placed along the four sides. Asingle connection between two profiles is made with an anglebracket, which is fastened with screws and nuts in the slotsof the profiles. The subtasks that the system must combineduring the construction of a given assembly are [8]:

• insert slot nuts in the profiles• position profiles• add angle bracket• add screwsAll subtasks require robust assembly strategies. These

strategies are implemented in basic skills that encapsulate therobot capabilities in a parametrizable and reusable way. Skillscan be adapted to the current task, and the desired behaviorcan be guaranteed to solve a given task through a certificationprocess [41]. Fig. 12 shows an example of sequence ofactions for the slot nut insertion. Open-loop robustness on ahigher level is achieved here thanks to the sensitivity of therobot arm and the impedance controller. A robust strategy forsuch insertion tasks can be chosen considering the passivealignment properties and the compliance of the robot in thecontact [42]. Furthermore, observation algorithms based onthe joint torque measurements are currently developed toreduce uncertainties and monitor the execution [43].

C. Planning for Autonomous ExecutionFor complex and large structures it might not be possible

to engineer and pre-compute every assembly step in detail, as

Fig. 11. Realization of various subtasks. Clockwise: slot nut insertion,placement of an angle bracket, pickup of a profile from the parts depot, andfastening the angle bracket with screws using the screwdriver.

the system might need to adapt the execution according to thepresent state of the environment. In order to achieve the re-quired autonomy, our system uses a set of dedicated plannerslocated in the planning unit. The main objective is to selectand parametrize the skills for the present tasks. Threrefore, anassembly sequence planning, a grasp planning and a motionplanning component are tightly connected through a runtimeworld model for symbol grounding according to the presentstate of the assembly, as depicted in Fig. 13. The sequenceplanning considers geometrical constraints in an assembly-by-disassembly approach to find a feasible sequence of tasks.Grasps are generated and checked for feasability for allsubassemblies [44]. The selected skills access data from theplanners in order to find the appropriate configurations, e.g. aspecific grasp is selected amongst the available grasps in thedatabase according to reachability criteria and available pathsprovided by the motion planner [8]. The motion plannertherefore synchronizes its internal geometrical representationwith the runtime world model in which all objects are regis-tered with type and pose, and are also labeled semanticallyto connect with the planning units and skills. So far, thesystem is capable of planning automatically sequences forplanar aluminum structures by only providing a specificationof the desired assembly goal. In future work, we planto add more flexibility to the system by using advancedreasoning algorithms and more complex structures based onan expanded set of connectors.

IV. FINAL DISCUSSION

New technologies like additive manufacturing of compo-nents have been recently introduced for space applicationsby companies such as Made in Space. In a medium term,

however, the construction of large scale structures will stillrely on robotic manipulators for performing delicate on-orbitassembly operations. Several challenges remain to prove thefeasibility of this ambitious idea, but initial tests on groundhave tested the capabilities and versatility of autonomousassembly using robotic manipulators. These terrestrial ap-plications typically rely on the use of fixtures that providemechanical support during the assembly operation. However,the use of fixtures in space can be limited due to the dynamiceffects of zero gravity conditions. Feeding of parts for theassembly, size of the parts to be handled, feasibility andlimits of usage of one or two arms for the operations, oruse of assembly kits are still open questions in the field.The extension and testing on relevant environments of zero-gravity autonomous robotic assembly is a current topic ofinterest at DLR.

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approach align nutestablish contact push in move to position- detect contact- switch controller mode- impedance controller- start alignment

- tilt slot nut- use compliance to align- oscillate to support alignment

- use compliance- get relative position reference

- move down- position controller - switch controller mode

- position controller- slide slot to relative position in profile

Fig. 12. Implementation of open-loop robustness in the insertion of the slot nut in a profile. The actions exploit the sensitivity of the LBR robot arm todetect contacts, and the compliance control for the compensation of position uncertainties.

parts robotstools

SequencePlanning

GraspPlanning

Motion Planning

Runtime World Model

Assembly Planning

Fig. 13. Components of the planning unit for the autonomous executionof assembly skills. The sequence planner, the grasp planner and the motionplanner interact with a runtime world model to select and parameterize anappropriate sequence of skills.

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