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7th ESA Workshop on Advanced Space Technologies for Robotics and Automation ASTRA 2002

Robotic Assembly of Large Space Structures:

Application to XEUS

R. Licata(1), M. Parisch(1), I.J. Ruiz Urien(2), M. De Bartolomei(3), G. Grisoni(4), F. Didot(5) (1)ALENIA Spazio,Turin, Italy; rlicata@to.alespazio.it (2)SENER, Bilbao, Spain; ignacio.ruizurien@sener.es (3)Tecnospazio, Milan, Italy; mdebartolomei@tecnospazio.it (4)Media Lario, Lecco, Italy: qa@media-lario.it (5)ESTEC, The Netherlands; frederic.didot@esa.int

INTRODUCTION In-orbit robotic assembly and/or maintenance is an enabling technology for large space structures that, due to their substantial mass or dimensions, cannot be assembled on ground and hence be transported into space with conventional space vehicles The objectives of the Robotic Assembly of Large Space Structure (RALSS) ESA-Study, conducted by Alenia Spazio in the years 2001-2002, as Prime Contractor, with SENER and Tecnospazio as Sub-contractors, are related to this important technology or topic development, starting with some detailed analysis of the first important example of this type of activity, that is the on-going International Space Station (ISS) in-orbit build-up or assembly. During the ESA RALSS study development, detailed investigation has been conducted for the design and the application of the robotic assembly on the ISS of the required Mirror Sectors (MS) for the large Mirror Spacecraft needed for the second phase of the XEUS mission, called XEUS-2. The XEUS mission, in fact, is planned as a long term X-ray observatory to be grown in orbit, from an initial configuration of a mirror spacecraft, MSC-1, with a 1 keV mirror sectors effective area of 6 m2 to a second phase configuration of the mirror spacecraft, MSC-2, with an effective area of 30 m2, by the assembly of extra sectors to be firstly transported into orbit at the end of a first mission phase, while the Mirror Spacecraft is docked at the ISS. Some of the main development activities performed during the presently illustrated study may be summarized as: • to provide an overview of in-orbit assembly techniques

for large payload guiding and fixation, in particular considering constraints imposed by robotics means of the Space Station and the ERA manipulator.

• to review in detail of the in-orbit assembly scenario of XEUS Mirror Spacecraft at ISS.

• to establish essential requirements for the XEUS-2 mirror installation interfaces associated in particular with the mirror sector removal from the Transportation Container by the ERA arm, while manipulated by the SSRMS arm (hand-over operation) and its installation or mounting onto the mirror spacecraft MSC-1, while is docked at the ISS Service Module docking port.

Figure 1: XEUS Mission Illustration • to perform cinematic simulations of these XEUS hand-

over and assembly operations at ISS. • to formulate preliminary design concepts for the mirror

sector mechanism(s), to trade-off and selected the attachment mechanism design, before manufacturing a representative prototype.

• to perform dynamic simulations for proximity and contact operations of the mirror sector removal from its Transport Container and its installation onto the docked MSC.

• to demonstrate the feasibility of the robotic assembly of the XEUS-2 mirror with the attachment mechanism prototypes on representative mock-ups and a laboratory demonstration of proximity and contact assembly operations

• to perform mission feasibility analysis, including system configuration, sizing, launcher accommodation and mass and power budgets.

XEUS MISSION The XEUS mission will build upon the European leadership in X-ray astronomy that XMM will provide by supplying unique opportunities for studying the distant universe, hot plasmas, material under extremes of density, temperature and pressure and subjected to the effects of strong gravity.

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XEUS Scientific Goals The XEUS mission is a potential follow-on to the already launched ESA Cornerstone X-ray Multi-Mirror (XMM mission) and is envisaged by the ESA Horizon 2000 Survey Committee as a major astrophysics facility within the Space Station Utilization Programme. The mission is planned as a long term X-ray observatory to be grown in orbit, from an initial configuration, called XEUS-1.

Figure 2: XEUS Spacecraft Approach to the ISS XEUS is planned as a long term X-ray observatory designed to meet a sensitivity to measure a broad-band spectra of sources as faint as ~ 10-17 erg cm2 s-1 in the 0.5 - 2.0 keV energy range and a photometry limiting sensitivity of 10-18 erg cm-2 s-1, in order to address some fundamental questions in astrophysics. Currently, X-ray astronomy can only detect the most luminous of AGN, quasars, to a redshift of about 4. With comparable energy resolution, the sensitivity of XEUS will be much dramatically better than provided by the AXAF and XMM dispersive spectrometers. As well as allowing the detection of much more distant objects, XEUS will pioneer the field of detailed X-ray spectroscopy of these distant AGN. To reach a signal-to-noise ratio of about 30 with the above stated spectra in 10 6 sec (flux) requirements on mirror area and angular resolution are imposed (i.e. an effective mirror area of tens of square meters around 1 keV and a high enough resolution, ideally as high as 2 arcsec HEW). XEUS-2 Assembly Scenario The initial configuration (XEUS-1) consists of a mirror spacecraft, MSC-1, with a 1 keV effective area of 6 m2 and a separate detector spacecraft, DSC1, to be aligned by active control to provide focal length of 50 m with an accuracy of 1 mm3, as depicted in Figure 3 below. After several years of operations with this first configuration, XEUS-1 will visit the ISS, where from the MSC-1 the second mirror spacecraft configuration MSC-2 will be assembled by the addition of extra mirror sectors, as illustrated by the picture in Figure 4 to obtain an effective area of 30 m2.

Figure 3: XEUS Spacecraft Configuration A new detector spacecraft, DSC-2, will also replace the detector spacecraft, DSC-1, with next generation instruments and technology.

Figure 4: XEUS MSC-2 Spacecraft Assembly After assembly at ISS, the second mission XEUS2 will initiate by the MSC-2 returning back to observation orbit FTO to continue its astrophysical program. The robotic assembly operations are assumed to be carried out at the ISS, by means of the available robotics facilities, comprising the Shuttle Arm, the ISS SSRMS arm and the ERA manipulator to be installed and available mounted onto the Russian Service Module. ISS facilities, in particular, will allow the performance of the robotic transportation of mirror sectors delivered in a container by the Shuttle, on one side of the ISS, to the Russian Sector side of the ISS, as illustrated in the above Figure 5, where the XEUS mirror spacecraft will be docked and the ERA arm installed.

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Figure 5: MS Delivery and Assembly Locations on ISS Mirror Sector Design While the configuration of the XEUS MSC-2 Mirror Sector was depicted in the Figure 4 above, the eight Mirror Sectors to be integrated on the external rim of the basic mirror spacecraft (MSC-1) are in principle similar and with the preliminary structural configuration given in Figure 6. The mirror sector core elements are the mirror plates, which are integrated in a total of 12 petals, containing 45 to 122 plates each. The mirror petals are attached to the mirror support structure via mechanisms in order to enable the in-orbit alignment of the individual petals. In addition the mirror petals are foreseen to be equipped with entrance and exit doors, serving in-orbit also as baffles. The mirror support structure connects the complement of mirror petals and provides the interface to the mirror spacecraft. Additional equipment will be attached to the mirror support structure such as the solar panel sub-system and the RCS and balance sub-system (replacing mirror petals at specific locations). During launch the solar panels are folded and stored at the outer rim of the Mirror Sectors and will be deployed in orbit. The deployed panels serve as sun shield and baffle at the same time. The present baseline of the structure is a Nickel sandwich structure in order to achieve a uniform thermal expansion of the entire petal assembly, avoiding misalignments and image errors due to thermally induced deformations. The baseline concept foresees that the mirror plates are attached to the radial plates of the petal structure along the corresponding edge of the mirror plates. The first petal of the Mirror Sector (Ring 3 of the fully integrated telescope) consists of 122 mirror plates, the second petal (Ring 4) consists of 62 mirror plates and the third petal (Ring 5) consists of 45 mirror plates. Each Sector contains 4 petals of each ring, so that the 8 sectors together are building up rings of 32 petals each. Although the mirror plates are as thin as possible (a referenced thickness of 0.14 mm is foreseen for MSC 2 plates), the weight of a mirror petal is significant (up to 180 kg each) and correspondingly that of the Mirror Sectors. The individual mirror plates will be integrated in the corresponding petal structures with the necessary optical quality and alignment.

Figure 6: MS Preliminary Structural Design The individual petals will be mounted into the mirror support structure via specific actively controllable alignment mechanisms allowing adjustment of the alignment as necessary.

Dimensions Weight

MSC-1 Φ = 4.5 m (outer diameter)

12,400 Kg

MSC-2 Φ = 10 m (outer diameter)

25,000 Kg

TC (loaded)

12.9 x 4.6 x 4.6 m 15,735 Kg

TC (empty)

12.9 x 4.6 x 4.6 m 3,135 Kg

Table 1: XEUS Vehicles Mass & Envelope Budgets Transport Container Design and Requirements The XEUS-2 Mirror Sectors Transport Container (TC) shall transport into orbit the eight mirror sector elements just illustrated and shall incorporate a Grapple Fixture (FRGF) on the topside to allow the NSTS SRMS to grapple and extract the TC from the Shuttle Cargo Bay, as shown in Figure 7 below. This Grapple Fixture shall not prevent the extraction of any Mirror Sector. This means that this GF shall be removable (or has to be tilted) and shall be re-installed in place to allow the TC relocation in NSTS Cargo Bay when all MS’s have been extracted. No EVA shall be necessary to support this operation. When the Transport Carrier FRGF is being grappled by the NSTS SRMS no power is available to the TC and to the

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Mirror Sectors: the NSTS SRMS has only mechanical interfaces with the Transport Carrier Flight Releasable Grapple Fixture. If, for any reason, the Transport Carrier hand off operations fail (i.e. the TC can’t be grappled by the SSRMS) then the TC needs to be relocated in the NSTS Cargo Bay, so that power resources can be provided to the Mirror Sectors again.

Figure 7: MS's Transport Container (TC) Design At least one Power and Data Grapple Fixture (PDGF) needs to be present in the TC to allow for the hand-off operations between the SRMS and the SSRMS and to allow the SSRMS to grapple the TC during all the XEUS assembly phases. SSRMS will also provide data and power resources to the TC through the PDGF's installed on the TC itself. This allows the TC to remain grappled to the SSRMS, without time constraints.

Figure 8: TC when located at Z1 An additional PDGF (the third Grapple Fixture) may be required. From the 5th (TBC) MS onwards SSRMS grapples this 2nd PDGF on the TC; this is in favor of shifting the CG of the TC and of the elbow angle of the SSRMS.

ISS ROBOTICS FACILITIES & CONSTRAINTS The International Space Station Mobile Servicing System, (MSS) comprises of different components: • the Space Station Remote Manipulator System

(SSRMS) • two Robotic Work Stations (RWS) • the Mobile Transporter (MT) • the Mobile Remote Servicer Base System (MBS) • The Special Purpose Dexterous Manipulator (SPDM). All these facilities are located externally onto the ISS, excluding the two RWS that are accommodated internally. The main purpose of the MSS is the assembly of the ISS large elements, large payload and ORU handling, the performance of maintenance operations, EVA support and transportation. The MSS is commanded and controlled by means of the Robotic WorkStation (RWS) from either the Lab Module or the Cupola. Until the Cupola becomes part of the ISS, there is no direct external viewing and therefore the MSS Video System and the Space Vision System (SVS) will provide the main visual capability. The ISS Video System, combined with ISS Communication and Tracking (C&T) systems, provides video generation, control, distribution and localized lighting throughout MSS elements. The SVS provides synthetic views of operations using cameras, targets and graphical/digital real-time position.

Figure 9: SSRMS facility on ISS The Space Station Remote Manipulator System (SSRMS), illustrated in Figure 9, is a 56-ft (17-m) symmetric manipulator mounted onto the ISS and that supports electronic boxes and video cameras. The main components of the 7-DoF SSRMS facility are:

• two Latching End Effectors (LEE) • two booms • seven joints that can be rotated to +/- 270 °.

A LEE at each end of the SSRMS allows a “walking” capability between different Power and Data Grapple Fixtures (PDGF) on the ISS.

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A LEE unit is illustrated in the following Figure 10 and a PDGF is also pictured in the next Figure 11.

Figure 10: SSRMS End Effector (LEE) This “walking” ability is the only mode of transportation for the SSRMS prior to the arrival of the Mobile Transporter (MT) unit and the Mobile Remote Servicer Base System (MBS) unit.

Figure 11: SSRMS Power and Data Grapple Fixture (PDGF) The European Robotic Arm (ERA) will be the second Robotic Facility to reach the International Space Station. The ERA facility is being designed and built by the European Space Agency (ESA) for the Russian Space Agency (RSA) to use on the ISS Russian segments. It is a 36.7-ft (11.2-m) symmetric manipulator arm, consisting of two booms and seven joints, as shown in the Figure 12 above. During long non-operational periods the ERA arm can be stowed on the Russian Sector, making use of two base-points. This is the so-called hibernation configuration, where both EE units are clamped to base-points and thus protecting the EE connectors and cameras against environmental degrading and damage. During very long periods of hibernation of ERA, the EMMI units can be usually disconnected and stored inside the ISS modules.

SSRMS Constraints The following are some of the major constraints on the robotic assembly imposed by the SSRMS facility on the ISS: • The SSRMS facility requires that the perpendicular

distance from the GF centreline to the Payload CG is within 11 feet and that the total distance from the Payload CG to the GF base be within 13 feet.

• SSRMS must be able to provide power resources to the TC (needed for Mirror Sectors thermal conditioning) through the PDGF's installed on the TC itself. This allows the TC to remain grappled to the SSRMS, without time constraints.

• A PDGF (for SSRMS) shall be available on FGB, in such a position that is compatible with the required TC and MS operations.

• The required PDGF will be launched on ISS mission 10A. That PDGF will be on a side wall carrier on the Shuttle. Currently where that PDGF will be installed is undecided

• Determine which is the SSRMS Arm position that best fit with ERA movement capability on one side and that allows the highest possible stiffness of the system SSRMS+ERA. In fact the SSRMS flexibility needs to be minimized during the ERA operations for grappling each individual Mirror Sector.

• All SSRMS operations have been assessed in detail with respect to the ISS hardware, SSRMS speed and breaking distance, Transport Carrier dimensions and weight.

ERA Constraints The following are some of the major constraints on the robotic assembly imposed by the ERA facility to be installed on the ISS: • An optimized ERA base-point shall be installed (an

EVA is required) on the ISS Russian Service Module. After the MSC1 docking to the Service Module, in fact the ERA needs to relocate from SPP, where it is stored to the SM base-point. The necessary fixation interface is available on the external diameter of the Service Module and can be used to connect the ERA base-point.

• Present XEUS assembly scenario is based on the assumption that the ERA can operate based on the Service Module. If this can't be accomplished the complete XEUS assembly sequence must be revised.

• As the Mirror Sector Grapple Fixture is currently located on the front surface of each individual Mirror Sector, the ERA is required to operate (both for the extraction and the insertion of the MS) in a direction that is perpendicular to the direction of the main axis of the ERA arm. This means that additional capabilities shall be provided to the ERA, such as the visibility in a direction that is perpendicular to the ERA arm (i.e. the EVA installation of a mirror may be required) and new operational procedures shall be developed and assessed to operate the ERA in this new scenario.

• Mirror Sectors extraction operation from the TC has been assessed in detail from the dynamic standpoint,

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taking into account that the TC is being held by the SSRMS (with its own flexibility) and that the MS is being grappled by the ERA (with its own flexibility).

• Currently ERA design is qualified to manipulate payloads on a stiff carrier structure.

• The flexibility of the SSRMS, the length of the TC requires ERA to operate to manipulate objects under new circumstances, have been studied. In particular the capability of ERA to grapple objects located on a flexible carrier needs has been assessed in detail, being this task the "core" of this scenario.

Figure 12: MS's TC Transported by the SSRMS Facility Constraints to ISS XEUS mirror sector assembly also impose some constraints on the ISS during its robotic operations, such as: Constraints to movement of other objects on ISS-RS. 1. Movement or deployment of objects in the

neighbourhood of ERA should not take place during an ERA operation.

2. Exemptions are only allowed if ERA mission planning has taken the worst case conditions with respect to the environment into account.

3. An object that is deployed should have its largest contours included in the supplied initial condition world model.

4. An object that is moved shall have both its initial and final position present as elements in the supplied initial condition world model.

5. Before ERA enters the zone of potentially moving ISS objects (solar arrays, radiators etc.), these must be stopped.

6. Moving or deploying of objects without ERA mission planning involvement invalidates the World Models used for on-line Collision Avoidance and IMMI display. This has major safety-related consequences.

Constraints to space vehicle docking/undocking Similarly, there are some special constraints on the ISS docking/undocking operations, such as: • When vehicle docking or undocking has to take place,

ERA shall be in hibernation pose. • Forces and torque on ERA due to vehicle

docking/undocking may be too great for ERA’s brakes to sustain.

• The Hibernation pose is a safe attitude that has been mechanically verified.

*

*

*

*

*

*

*

***** **

Figure 13: RS Forbidden Zones (Cones) Constraints to RCS/ACS of ISS In particular, there are some special constraints on the ISS RCS/ACS operations, such as: 1. When ERA is in dynamic mode, ISS thruster firing shall

be postponed until ERA’s brakes are engaged 2. Forces and torques on ERA due to ISS thruster firing

may be too great for ERA’s control to sustain, dependent on the ERA pose, ERA’s load and the characteristics of the thruster pulses. Normally, operating with ERA should be interrupted and ERA put in a safe attitude during periods of thruster firing.

3. Simultaneous thruster firing and ERA operation may be considered on a case-by-case base.

MIRROR SECTORS ASSEMBLY SEQUENCE Detailed analysis has been performed for the robotic MS's assembly sequence represented by the following phases: Phase 1. MSC1 Docking.

The XEUS mission is expected to operate at 600 Km above the Earth, in the so-called Fellow Traveler Orbit (FTO), directly above the ISS, with an orbit inclination of about 51.6o. At the end of the planned XEUS1 observation mission phase, in order to perform the mirror upgrade MSC1 is docked at the SM of the ISS.

Phase 2. ERA re-location on SM. Once the MSC1 vehicle has docked to the Service Module, the European manipulator arm ERA will be relocated from the SPP, where it is stowed, to a dedicated base-point that shall be installed on the SM.

Phase 3. Mirror Sectors upload. Eight Mirror Sectors (MS), required to upgrade the MSC1 configuration, will be brought to the Shuttle.

Phase 4. NSTS docking to ISS. The Shuttle will be launched shortly after the MSC1 is docked to the Service Module and it will dock to the PMA2, ISS Forward Side.

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Phase 5. Transport Carrier extraction from NSTS. The Transport Carrier will be extracted from the Shuttle Cargo Bay by the NSTS SRMS and will be handed off to the Space Station RMS (that is located on the Mobile Base System) using the nominal payload hand-off procedure. Initial grappling of the TC has to be done with the SRMS. A first Grapple Fixture, on the topside of the TC, allows SRMS grappling and a second PDGF, installed on the TC wall side, is used by the SSRMS for grappling the TC itself.

Figure 14: ISS view showing Z1 Truss location Phase 6. TC storage on Z1 Truss.

The SSRMS transfers the TC on the top of the Z1 Truss in upright position. The Z1 Truss is the only area on ISS where a container of such size and mass can be stored. The Z1 Truss is empty after retraction of the P6 truss in stage 13A of ISS Assembly Sequence.

Phase 7. SSRMS relocation on FGB. Once that the TC is mechanically and electrically mounted on Z1, the SSRMS will relocate itself to the Zarya module (FGB). The SSRMS will release the MBS base-point and transfer to the PDGF installed on the FGB module. All the subsequent SSRMS operations will be conducted with the SSRMS based on the FGB.

Figure 15: ERA grapples one Mirror Sector Phase 8. TC relocation on the Russian side.

From the base-point on Zarya (FGB), the SSRMS will grapple again the TC stored on Z1 Truss and transfer it to near the Service Module, at a hand over pose. Once

arrived the SSRMS drives will be disabled and its brakes applied.

Phase 9. Mirror Sector Extraction. The ERA manipulator, based on the Service Module, will move to the hand over pose and get a Mirror Sector from the TC held by the SSRMS (see Figure 19).

Phase 10. Mirror Sector Installation. The Mirror Sector is extracted by ERA from the TC, transferred to and installed on the MSC itself. Dedicated mechanisms will allow the mechanical latching between the MS and the MSC and the mating of the required electrical connectors. Once the safety of the mechanical fixation of the Mirror Sector has been verified, ERA manipulator will release the Mirror Sector. The Mirror Spacecraft, mounted on a rotary structure, will then be rotated until a new Mirror Sector slot is presented, for the next Mirror Sector assembly.

Figure 16: XEUS Mirror Sectors Assembly Phase 11. TC during night period.

After the MS extraction, the TC is kept by the SSRMS close to the operations area for the night period. The SSRMS, with its brakes on, will maintain the TC close to the MSC, waiting for the continuation of the assembly operations. The SSRMS holds the TC grappling the TC PDGF; the SSRMS is able to provide power resources to the TC through the PDGF itself so that the TC may remain grappled to the SSRMS for extended period of time. Current baseline foresees that one Mirror Sector per day is installed on the MSC.

Phase 12. MSC built up continuation. The next day, XEUS assembly operations will start again: SSRMS moves the TC in the proper position for the new Mirror Sector installation. This procedure will continue for the full eight Mirror Sectors installation.

Phase 13. MSC built up completion. Once the last Mirror Sector is installed on the MSC (and becoming MSC2) Thermal Baffles and Sunshade are deployed by EVA with support of the ERA facility. The same EVA operations will perform an overall inspection of the new assemblies of MSC2 spacecraft.

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Phase 14. TC downloading. After the last Mirror Sector is extracted from the TC, the empty container is stored back in the NSTS Cargo Bay for return to ground. This will occur according to the following operational steps: • SSRMS relocates the empty TC on Z1 Truss • SSRMS relocates to the MBS • SSRMS (based on MBS) transfers the TC from Z1

Truss to a NSTS SRMS hand over position • SSRMS hands off the TC to the NSTS SRMS • NSTS SRMS re-installs the TC inside the NSTS

Cargo Bay • Shuttle un-docks and leaves the Station

Phase 15. MSC2 Checks and undocking. The upgraded and completed MSC2 is overall checked and then un-docked from the Service Module.

Phase 16. XEUS2 observation phase. At this stage of the XEUS mission a new Detector Spacecraft (DCS2) is launched. The rendezvous and docking with the MSC2 is performed and the two XEUS2 spacecraft is transferred to the proper XEUS mission observation orbit.

Figure 17: MSC-2 Un-docking from ISS Assembly Time-Line ROBCAD simulations have been performed which led also to the following conclusions on the assembly daily operations splitting: • the operations involving SSRMS and ERA take 4 days. • In this period, starting from the full TC on Z1 and

ending with empty TC on Z1, all the MS are assembled onto MSC.

Table 2 below summarizes such time splitting organization. Recovery Strategy & Off-nominal Events The fully manual mode would be used only in response to contingencies or anomalies. In this case the operator will place the ERA or the SSRMS in a safe configuration and wait for the ground to analyze the situation and decide on trouble shooting and recovery operations. The following ground-rules and assumptions are considered on the basis of all the scenarios considered below: • The Mirror Sectors require power resources for thermal

conditioning purposes. Power resources are provided to the Mirror Sectors either through the Transport Container or from the MSC.

• Any time the Transport Carrier is grappled by the SSRMS through the PDGF the TC can receive power resources from the chain ISS-SSRMS-PDGF.

• The MSC continuously receives power resources from the ISS through the Interface with the Russian Service Module.

• The Transport Container can not receive power when grappled by the NSTS SRMS through the FRGF. The FRGF in fact only provides mechanical interfaces to the SRMS.

• The Transport Container can receive power resources when it is located in the NSTS Cargo Bay.

• When Temporary Stored on Z1 the Transport Container can not receive power resources, as the interface between Z1 and the TC is assumed mechanical only. If the TC is required for any reason to remain stored on Z1 for longer periods, then the SSRMS is required to grapple one of the TC PDGF in order to provide power resources to the TC.

• The MSC is able, at any time of its assembly to be able to un-dock from Station and to survive for a limited period of time and to re-dock to ISS to allow build-up operation continuation.

Day Initial condition

Final condition Time (h,min)

3

SSRMS and ERA in home pose; full TC on Z1

3 MS installed; ERA in home pose; SSRMS holding TC.

5,44

4

3 MS installed; ERA in home pose; SSRMS holding TC

5 MS installed; ERA in home pose; SSRMS holding TC.

6,45

5

5 MS installed; ERA in home pose; SSRMS holding TC.

7 MS installed; ERA in home pose; SSRMS holding TC.

6,50

6

7 MS installed; ERA in home pose; SSRMS holding TC.

8 MS installed; SSRMS and ERA in home pose; empty TC on Z1.

2,56

Table 2: daily operations splitting • EVA tasks are not expected to be nominally performed

in order to support MSC build-up operations, but it assumed that an EVA could always be performed to recover from an off-nominal event.

• The maximum period of time the NSTS is allowed to remain docked on Station, waiting for XEUS Assembly completion is 9 days including contingencies. Regardless of any possible off nominal event the NSTS is required to leave ISS with the Transport Carrier after 9 days maximum.

• All the off-nominal events precluding the MSC docking

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on ISS will imply a delay of the NSTS flight carrying the additional hardware to upgrade the MSC1 into MSC2.

Figure 18: Mirror Sector Extraction from TC by ERA DYNAMICS SIMULATIONS Dynamic Modelling and Simulation activities associated with the Mirror Sector robotic assembly have been performed utilizing the DCAP simulation software package, including ERA model validation. ERA Modelling ERA modelling was performed. All reference frames at joint level and the rotation rules correspond to the rules of the reference documents, as well as the physical characteristics of the arm as reported in the Study documents. The DCAP modelling of ERA consisted of 12 rigid bodies connected through hinges. Of these bodies, 4 represent actual components of the arm, separated by the actuated joints, while 8 are used to setup a lumped mass representation of the flexible limbs 1 and 2.

ELBOW PITCH

WRIST

WRIST YAW

ECC

W RI

-185o

SHOULDER YAW

SHOULD ER PIT CHER ROLL+185o

-185o

-120o

+120o

-120o+120o

-176o+30o

-120o

+120o-120o+120o

EE1

EE2

limb 2

limb 1

Z

Y

X

Z

X

Y

Z

XY

Z

X

Y

X

Z

Y

+185°

X

YZ

J1

J2

J3

J4

J5

J6

fully streched arm: all joint angles are 0

Roll: rotation about X

Pitch: rotation about Y

Yaw: rotation about Z

WRIST PITCH

WRIST ROLL

X

YZ

J0SHOULDER ROLL

+185°

-185°

Arm base frame

Arm tip frame

Figure 19: ERA Components ERA Controller The following Figure 20 below shows an overview of the ERA controller implemented on DCAP for the dynamics simulation of the Mirror Sector assembly operations. The Robcad simulation block in the Figure 20 diagram describes the overall mission in terms of a Motion Command sequence from an initial position and is an input

for ERA Control; the Plant block on the other hand represents the DCAP modelling of ERA and environment dynamics; the Sensor simulation block represents the modelling and simulation of the CLU, the Force/Torque sensors, position sensors and motor resolvers of ERA and they were implemented in ANSI C; the ERA Control block represents the simulation of ERA Control SW and also implemented in ANSI C. ERA Control SW was scheduled by DCAP simulator at 300Hz rate. ERA control SW was in charge of the scheduling at the proper rate of its internal processes. DCAP simulator provided also Sensor simulation with “ideal” motor and joint positions, Forces and Torque sensed in ERA F/T sensor frame, ERA TIP frame and Grapple Fixture frame (for CLU simulation). The Sensor simulation block provides information for the ERA control SW. ERA Control SW output are motor current set-points which lead to ERA motion.

Figure 20: ERA Controller Model With this type of modelling ERA may work in several control modes: • free motion in the joint space: the joint set-points QDES

are directly provided by the interpolator to the joint control loops at 20Hz rate;

• free motion in the Cartesian space: the Cartesian set-point XINT (=XCOM) is provided by the interpolator inverse kinematics to generate joint set-points QDES at 20Hz rate;

• proximity control: to the Cartesian set-point XINT is applied a correction ∆XPROX coming from the proximity control block after processing of CLU data. The outcome XCOM is then provided to the inverse kinematics;

• F/T control: to the Cartesian set-point XINT is applied a correction ∆XTF coming from the F/T control block after processing of F/T sensor data. The outcome XCOM is then provided to the inverse kinematics.

Regardless of the active control mode, each joint movement is regulated by its own control loop, which runs at 300Hz rate. In the following, the components of the ERA Control model will be analyzed with more detail.

Sensor simulation

ROBCADSimulation

Motion CmdPlanner

InverseKinematics

XCOM QDES Joint Control Loop(300 Hz)

Plant

I

Q*MOT

Q*JOINT

JPS andresolver

simulation(300 Hz)

F/T sensorsimulation

(20 Hz)

QMOT

QJOINT

FT*

CLU simulation(10 Hz)

TIP_pos

GF_pos

Interpolator(20 Hz)

XINT

XDES

ProximityControl

F/T Control

∆∆XTF∆∆XPROX

FTSENS

XVIS

ERA Control

FTC,DES

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SSRMS Modelling The SSRM arm, illustrated in Figure 23, has been modelled as an equivalent flexibility on which the TC and considered as mounted on the ISS. This choice has been driven by the following considerations: • the mass of the TC carrying the MS is about 10 time the

full SSRMS mass; • during the MS manipulation by ERA, SSRM will be

passive namely the SSRM control system is inactive e.g. the arm has brakes applied. This condition guarantees that the main SSRM flexibility effects are due to the limbs flexibility;

• the overall scenario to be simulated is very cumbersome from the computational point of view; moreover the integration step must not exceed 1.e-3 [s] for stability reasons; anyway the upper threshold is dictated by the ERA controller (300 [Hz]).

ISS Dynamics Model The ISS dynamics was modelled as two rigid bodies considered locked one each other for the time being. If needs arise to account for ISS flexibility, a rotational spring/damper device can be foresees between the two bodies. The Space Station is considered in orbit and effects due to the gravity gradient are taken into account. At the base of the ERA arm linear and rotational acceleration noises are applied. MS ASSEMBLY MECHANISMS DESIGN Preliminary design concepts and main requirement analysis for attachment mechanism design have been performed. Identification of other potential candidates and conceptual design was selected. Basic Concept The final selected concept for the detailed design activity was based on the following concept, also illustrated in Figure 21, represented by three fixation points and one central latch with two synchronized claws in which: • One central latch mechanism mounted on the MS • Two connected claws for reduction of dimensions. • Three guides in V shape located at 120º on the MS for

final positioning (Figure 22below). • Damping system on guides not required as no contact

occurs at berthing. • Four passive handles on the MSC for attachment

Figure 21: MS Attachment Mechanism Basic Concept

Therefore the basic configuration of the selected Attachment Mechanism design included also: • Drive with an electrical actuator with redundant winding • Proposed redundancy approach: one failure tolerant

(double winding actuator) for XEUS mission operations. • Unlatching from TC and latching to MSC. • Proposed redundancy approach for two failure tolerant

design in safety critical operations: EVA manual override for second failure

• EVA manual override approach: • Unlatching from MSC and latching back to TC for

contingency: one axis connected to the drive motor and locate a compatible coupling with EVA tools at the border of the MS

The central latch is responsible for providing the final pre-loading. The guides must make the alignment of the sector and remain pre-loaded once the mechanism has been latched. Mechanism Components The following are the main attachment mechanism electro-mechanical components, represented in particular by the passive and active latching devices and the associated elecytronics unit. Latch mechanism The passive elements are represented by handles and guides, as illustrated in Figure 21 above. The attachment active parts are represented by the mechanism illustrated in Figure 23 below.

Figure 22: MS V-Shape Guiding System These elements are manufactured in Titanium and such that • their dimension is related to capture range of the robotic

assembly operation • the guide angle is maintained higher than 45º to

minimise friction loads during alignment • the handle stiffness defined for final frequency

requirement of the complete MS. A value of 3.5 Hz has been adopted with the current design.

• some special coatings may be used to reduce friction. • the mechanism has two over-center positions

‘connected’ to obtain a considerable mechanical advantage.

• in the final position, both over-centers guarantee the

Passive G.

Active

11

latched configuration. • a spring maintain a positive latching for the worst

thermal conditions • an end-stop in the driving bar provides the final latched

position.

Figure 23: MS Attachment Mechanism Active Device There is a shaft connected to the stepper to make a manual override of the mechanism in case of 2 failure. The shaft is normally de-coupled of the stepper so that there is no energy lost because of the bearings Stepper and harmonic drive assembly For the mechanism active part, a metallic cylinder supports the harmonic drive and the stepper, also illustrated in the section view of following Figure 24. High torque and resistance requirements are associated with the active part design.

Figure 24: Section View of Mechanism Active Part Electronics The following are functions associated with the attachment mechanism electronics box : • provide the right motor sequencing for the berthing

mechanism and the electrical connection mechanism. • provide the means for correct speed/current control, for

every mechanism motor. • allow for mechanism monitoring useful for FDIR

purposes. • allow the required commandability and observability of

the entire assembly from higher level. There is no need to be an “intelligent” unit, since it has to respond only to external commands and hence to act solely as a driving unit. For each XEUS Mirror Sector, electronics configuration with two redundant circuits, as the one illustrated in Figure 25, with the following main characteristics:

• Envelope Volume: 1.5 litres (150 x 150 x 70 mm) • Mass: 1 kg • Power requirement: 10 W.

Figure 25: Mechanism Electronics Mechanism Mass & Power Budgets From the performance of Mirror Sector attachment mechanism detailed design activity, details of mass and power budgets have been provided for the single sector to be assembled. Mass Budget Mirror Sector attachment mechanism mass budget, comprising of electromechanical passive and active parts and associated electronics, amounts to about 17 Kg with the components split-up provided by the following Table 3.

Table 3: MS Attachment Mechanisms Mass Budget Power Budget Similarly, the Mirror Sector attachment mechanism power budget, associated in particular with its electromechanical active part and electronics, is illustrated by the following Table 4 and amounts to a total power demand for each sector of about 42.5 Watt.

Table 4: MS Attachment Mechanism Power Budget

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Structural Analysis Also for the performance of the detailed design, functional and operational analysis to verify compliance of the attachment mechanism concept with the requirements, including guiding and capture range, capture speed, contingency operations, etc. have been performed. For the same purpose, to support the such design activities, mechanical / structural analyses including Static analysis for establishing load capability, stiffness and Dynamic Analysis, to verify natural frequencies and modes have also been performed and some of the results are illustrated in the following. As a starting point of these analysis activities, the following assumptions or exemplification on the attachment mechanism have been made:

üFor the latched position model, only the elements loaded have been considered. For this reason, the four gears and the second claw have not been modelled.

üIn the latched position, the force is decomposed and applied in the claw thickness, in particular in the location where the handle is locked, but only over an element edge line. Some Analysis Results The following are some of the results obtained by the performance of the attachment mechanism structural analysis activity: • The main claw is the element with higher stress levels. • The stresses in the rest of the elements are considerably

lower. • The maximum stress is 595 Mpa at the claw contact

point with the handle. • The critical part of the mechanism is the support

cylinder of the harmonic drive and the stepper, due to the high mass and dimensions of this two elements

• The maximum stress results to be of 391 Mpa.

Figure 26: Mechanism Active Part Structural Model Thermal Analysis Attachment mechanism thermal analysis has been also performed, with main objectives of ensuring that all components will be maintained within their qualification temperature range, that temperature gradient between active and passive components of the mechanism were below the

requirement, etc. The procedure of mechanical latching can take into account a waiting time after the two surfaces are in contact before final latching is performed Calculations of extreme temperatures for the sensitive components of the mechanism, such as Motor, Reducer, bushings, etc. were performed, together with evaluations of thermal gradients just before final latching of Mirror Sector onto the mirror spacecraft (MSC). It is assumed that heaters, thermistors and thermal control covers shall also be employed to limit maximum extreme temperatures. The following are some of the conclusions of the performed named thermal analysis activity. Active latch motor:

• According to the present analysis, heater with 10 (W) of nominal electrical power is required for each active latch motor, to maintain them within cold qualification temperature limits.

• As for design cases DC1H and DC2H, margins of 1 (ºC) and 4 (ºC) respectively have been calculated. For DC1H, the reason is that boundary temperature indicated in AD1 (65 ºC) plus acceptance and qualification margins (10 ºC), is 75 (ºC) which is only five degrees below the allowable operating temperature limit of the motor.

Harmonic drive temperatures: Considering ten degrees for acceptance plus qualification margins, temperatures calculated for design cases DC3C and DC3H for instant after 3600 seconds are, -26 (ºC) and 52 (ºC) respectively.

Waiting time for temperature attenuation: At least, a waiting time of 3600 seconds should be necessary, to reach acceptable qualification temperatures for motor components of the active latch, to start MS mechanical latching to MSC.

Final definition of the required waiting time should be based on the thermal evolution of the motor & reducer, as well as latch active and passive gradients: < 100ºC after 3600 sec. ASSEMBLY MECHANISM PROTOTYPE AND TESTING A 1:2 scaled down prototype of the MS attachment mechanism is being manufactured for mechanical testing and for a robotic laboratory demonstration. Prototype Mechanical and Laboratory Testing During the ESA RALSS Study development, for the attachment mechanism prototyping and testing it was proposed: 1. To design and manufacture of a complete attachment

and latching subsystem model in the 1:2 scale, taking into account • To keep in this reduced model the same stresses

than in the full-scale real mechanism. • In order to have the same stresses, or invariability

of them, we need to reduce the pre-loads or forces

13

to 25% of those required full-scale model. This since stresses are inversely proportional to the section of the bars of the mechanism and when all the geometric dimensions are reduced by 50%, the resulting cross section areas are four times smaller, or reduced to 25 %.

• The contact stresses may also be representative, and the required motor-reducer must give a torque that is eight times smaller than that of the full scale model.

2. To perform the mechanical testing of the prototype on the manufactured scaled down model. During the tests the following parameters will be measured: • Pre-load values on the trunnion. • Deformations of the mechanism. • Torque required by the driving bar.

3. To deliver a complete scaled-down attachment mechanism model with supporting flanges for functional demonstration in the Robotics Laboratory.

References [1] In-orbit assembly of large spacecraft: the XEUS

mission, Didot F. et al., iSAIRAS symposium, June ‘99, ESTEC, SP440

[2] ERA, the flexible robot arm; Ph. Schoonejans et al., iSAIRAS symposium, June ‘99, ESTEC, SP-440

[3] The European Robotic Arm: Control Performances, J. Kouwen et al., ASTRA workshop, December ’98, ESTEC, WPP-154

[4] ERA, the flexible robot arm; Ph. Schoonejans et al., iSAIRAS symposium, June ‘99, ESTEC, SP-440

[5] The European Robotic Arm: Control Performances, J. Kouwen et al., ASTRA workshop, December ’98, ESTEC, WPP-154

[6] BD-TN-ML-006, Technical Note, Inputs to RALSS Study, XEUS Mirror Sector Characteristics, Media Lario, November 2001

[7] NSTS 1700.7b ISS Addendum, Safety Policy and Requirements for Payloads Using the International S. Station; ftp://ftp.estec.esa.nl/pub/robotics/docs

[8] SSP42004; Mobile Servicing System to User I/F; ftp://ftp.estec.esa.nl/pub/robotics/docs/

[9] D684-10503-02, "Support External Robotic Operations Architecture Description Document", Volume II, Extravehiclular Robotics

[10] SSP 41167E, Mobile Servicing System Segment Specification for the ISS Program

[11] KHB 1700.7.b, Space Shuttle - Payload Ground Safety Handbook; ftp://ftp.estec.esa.nl/pub/robotics/docs/

[12] NSTS 1700.7b, Safety Policy and Requirements for Payloads System Safety Requirements; ftp://ftp.estec.esa.nl/pub/robotics/docs/

[13] NSTS 1700.7b ISS Addendum, Safety Policy and Requirements for Payloads Using the International Space Station

[14] Requirement Specification Torque-Force Control Algorithm, FS Doc. N. HS-ST-ER3-015-FSS

[15] ERA Simulation Facility – Model Specification CLU, FS Doc. N. HS-ST-ER3-009-FSS

[16] ERA motion control kinematics algorithms specification”, FS Doc. N. HS-NT-ER-093-FSS

[17] ERA Simulation Facility – Model Specification TFS FS Doc. N. HS-ST-ER3-010-FSS

[18] Requirement specification Proximity Control Algorithm, FS Doc. N. HS-ST-ER3-013-FSS

[19] ERA Simulation Facility – Model Specification JCE, FS Doc. N. HS-ST-ER3-010-FSS

[20] Manipulator Joint Subsystem (MJS), SABCA Doc. HS-NT-ERA-202-SABC

[21] ERA Model on Telegrip: simulations and performances: Final Report” ESA 26 Apr, 2001

[22] Flight Operation Manual and Procedures Issue 5, FS Doc HS -MU-ER-001-FSS

[23] ISS Assembly Flight 8A: Simulation of SSRMS Dynamic, G. Bilodeau et al., MacDonald Dettwiler, ISR2000, Montreal, May 2000

[24] XEUS Assembly Requirements and ROBCAD Simulation, Alenia Spazio,SD-TN-AI- ,RALSS, TN2, Issue 1, Jan. 2002

[25] Space Engineering; Part 3 : Mechanisms, Draft ECSS- E- 30A Part 3, October 1999; ftp://ftp.estec.esa.nl/pub/robotics/docs/

[26] XEUS Mirror-Sector Attachment Mechanism Requirements Specification, Alenia Spazio, SD- RQ-AI-0042 Issue 01, March 2002.

[27] ESABASE/THM-UM-043 Thermal Application Manual (January 93). Version 93.1.

[28] Cluster project. Measurement of thermo-optical properties for machined Titanium (Ti-6Al-4V). Annex 1 of document CL-DOR-MN-0651.

[29] Aerospace Structural Metal Handbook. Batelle Columbus Laboratories.

[30] Du Pont Catalogue on Vespel. SAGEM catalogue on motors.

[31] SHELDAHL catalogue on thermal control material and films.

[32] Teleoperation: From the Space Shuttle to the Space Station, Nguyen, P.K., Hughes P., AIAA, 1994

[33] SSP 50075-SYS, Book 12, Vol I "Assembly and Operations Support Plan Systems Data Report Structures and Mechanisms", Book 12, Vol I