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; [email protected] (2)SENER, Bilbao, Spain; [email protected] (3)Tecnospazio, Milan, Italy; [email protected] (4)Media Lario, Lecco, Italy: [email protected] (5)ESTEC, The Netherlands; [email protected]
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 Figure 1: XEUS Mission Illustration needed for the second phase of the XEUS mission, called · to perform cinematic simulations of these XEUS hand- XEUS-2. over and assembly operations at ISS. The XEUS mission, in fact, is planned as a long term X-ray · to formulate preliminary design concepts for the mirror observatory to be grown in orbit, from an initial sector mechanism(s), to trade-off and selected the configuration of a mirror spacecraft, MSC-1, with a 1 keV attachment mechanism design, before manufacturing a mirror sectors effective area of 6 m2 to a second phase representative prototype. configuration of the mirror spacecraft, MSC-2, with an · to perform dynamic simulations for proximity and effective area of 30 m2, by the assembly of extra sectors to contact operations of the mirror sector removal from its be firstly transported into orbit at the end of a first mission Transport Container and its installation onto the docked phase, while the Mirror Spacecraft is docked at the ISS. MSC. Some of the main development activities performed during · to demonstrate the feasibility of the robotic assembly of the presently illustrated study may be summarized as: the XEUS-2 mirror with the attachment mechanism · to provide an overview of in-orbit assembly techniques prototypes on representative mock-ups and a laboratory for large payload guiding and fixation, in particular demonstration of proximity and contact assembly considering constraints imposed by robotics means of operations the Space Station and the ERA manipulator. · to perform mission feasibility analysis, including system · to review in detail of the in-orbit assembly scenario of configuration, sizing, launcher accommodation and XEUS Mirror Spacecraft at ISS. mass and power budgets. · to establish essential requirements for the XEUS-2 mirror installation interfaces associated in particular XEUS MISSION with the mirror sector removal from the Transportation The XEUS mission will build upon the European leadership Container by the ERA arm, while manipulated by the in X-ray astronomy that XMM will provide by supplying SSRMS arm (hand-over operation) and its installation or unique opportunities for studying the distant universe, hot mounting onto the mirror spacecraft MSC-1, while is plasmas, material under extremes of density, temperature docked at the ISS Service Module docking port. 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 3: XEUS Spacecraft Configuration Figure 2: XEUS Spacecraft Approach to the ISS A new detector spacecraft, DSC-2, will also replace the XEUS is planned as a long term X-ray observatory designed detector spacecraft, DSC-1, with next generation instruments to meet a sensitivity to measure a broad-band spectra of and technology. 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). Figure 4: XEUS MSC-2 Spacecraft Assembly After assembly at ISS, the second mission XEUS2 will XEUS-2 Assembly Scenario initiate by the MSC-2 returning back to observation orbit The initial configuration (XEUS-1) consists of a mirror FTO to continue its astrophysical program. spacecraft, MSC-1, with a 1 keV effective area of 6 m2 and a The robotic assembly operations are assumed to be carried separate detector spacecraft, DSC1, to be aligned by active out at the ISS, by means of the available robotics facilities, control to provide focal length of 50 m with an accuracy of 1 comprising the Shuttle Arm, the ISS SSRMS arm and the mm3, as depicted in Figure 3 below. ERA manipulator to be installed and available mounted onto After several years of operations with this first configuration, the Russian Service Module. XEUS-1 will visit the ISS, where from the MSC-1 the ISS facilities, in particular, will allow the performance of the second mirror spacecraft configuration MSC-2 will be robotic transportation of mirror sectors delivered in a assembled by the addition of extra mirror sectors, as container by the Shuttle, on one side of the ISS, to the illustrated by the picture in Figure 4 to obtain an effective Russian Sector side of the ISS, as illustrated in the above area of 30 m2. 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 Figure 6: MS Preliminary Structural Design mirror petals are foreseen to be equipped with entrance and The individual petals will be mounted into the mirror support exit doors, serving in-orbit also as baffles. The mirror structure via specific actively controllable alignment support structure connects the complement of mirror petals mechanisms allowing adjustment of the alignment as and provides the interface to the mirror spacecraft. necessary.
Additional equipment will be attached to the mirror support Dimensions Weight structure such as the solar panel sub-system and the RCS and F = 4.5 m balance sub-system (replacing mirror petals at specific MSC-1 12,400 Kg locations). During launch the solar panels are folded and (outer diameter) F = 10 m stored at the outer rim of the Mirror Sectors and will be MSC-2 25,000 Kg deployed in orbit. The deployed panels serve as sun shield (outer diameter) TC and baffle at the same time. 12.9 x 4.6 x 4.6 m 15,735 Kg The present baseline of the structure is a Nickel sandwich (loaded) structure in order to achieve a uniform thermal expansion of TC 12.9 x 4.6 x 4.6 m 3,135 Kg the entire petal assembly, avoiding misalignments and image (empty) errors due to thermally induced deformations. Table 1: XEUS Vehicles Mass & Envelope Budgets The baseline concept foresees that the mirror plates are attached to the radial plates of the petal structure along the Transport Container Design and Requirements corresponding edge of the mirror plates. The XEUS-2 Mirror Sectors Transport Container (TC) shall The first petal of the Mirror Sector (Ring 3 of the fully transport into orbit the eight mirror sector elements just integrated telescope) consists of 122 mirror plates, the illustrated and shall incorporate a Grapple Fixture (FRGF) second petal (Ring 4) consists of 62 mirror plates and the on the topside to allow the NSTS SRMS to grapple and third petal (Ring 5) consists of 45 mirror plates. Each Sector extract the TC from the Shuttle Cargo Bay, as shown in contains 4 petals of each ring, so that the 8 sectors together Figure 7 below. This Grapple Fixture shall not prevent the are building up rings of 32 petals each. Although the mirror extraction of any Mirror Sector. This means that this GF plates are as thin as possible (a referenced thickness of 0.14 shall be removable (or has to be tilted) and shall be re- mm is foreseen for MSC 2 plates), the weight of a mirror installed in place to allow the TC relocation in NSTS Cargo petal is significant (up to 180 kg each) and correspondingly Bay when all MS’s have been extracted. No EVA shall be that of the Mirror Sectors. The individual mirror plates will necessary to support this operation. be integrated in the corresponding petal structures with the When the Transport Carrier FRGF is being grappled by the necessary optical quality and alignment. NSTS SRMS no power is available to the TC and to the
3 Mirror Sectors: the NSTS SRMS has only mechanical ISS ROBOTICS FACILITIES & CONSTRAINTS interfaces with the Transport Carrier Flight Releasable The International Space Station Mobile Servicing System, Grapple Fixture. If, for any reason, the Transport Carrier (MSS) comprises of different components: hand off operations fail (i.e. the TC can’t be grappled by the · the Space Station Remote Manipulator System SSRMS) then the TC needs to be relocated in the NSTS (SSRMS) Cargo Bay, so that power resources can be provided to the Mirror Sectors again. · 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 Figure 7: MS's Transport Container (TC) Design the Space Vision System (SVS) will provide the main visual At least one Power and Data Grapple Fixture (PDGF) needs capability. to be present in the TC to allow for the hand-off operations The ISS Video System, combined with ISS Communication between the SRMS and the SSRMS and to allow the SSRMS and Tracking (C&T) systems, provides video generation, to grapple the TC during all the XEUS assembly phases. control, distribution and localized lighting throughout MSS SSRMS will also provide data and power resources to the elements. The SVS provides synthetic views of operations TC through the PDGF's installed on the TC itself. This using cameras, targets and graphical/digital real-time allows the TC to remain grappled to the SSRMS, without position. time constraints.
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 Figure 8: TC when located at Z1 · seven joints that can be rotated to +/- 270 °.
An additional PDGF (the third Grapple Fixture) may be th A LEE at each end of the SSRMS allows a “walking” required. From the 5 (TBC) MS onwards SSRMS grapples capability between different Power and Data Grapple this 2nd PDGF on the TC; this is in favor of shifting the CG Fixtures (PDGF) on the ISS. of the TC and of the elbow angle of the SSRMS.
4 A LEE unit is illustrated in the following Figure 10 and a SSRMS Constraints PDGF is also pictured in the next Figure 11. 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
Figure 10: SSRMS End Effector (LEE) Shuttle. Currently where that PDGF will be installed is This “walking” ability is the only mode of transportation for undecided the SSRMS prior to the arrival of the Mobile Transporter · Determine which is the SSRMS Arm position that best (MT) unit and the Mobile Remote Servicer Base System fit with ERA movement capability on one side and that (MBS) unit. 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
Figure 11: SSRMS Power and Data Grapple Fixture (PDGF) available on the external diameter of the Service Module and can be used to connect the ERA base-point. The European Robotic Arm (ERA) will be the second · Present XEUS assembly scenario is based on the Robotic Facility to reach the International Space Station. assumption that the ERA can operate based on the The ERA facility is being designed and built by the Service Module. If this can't be accomplished the European Space Agency (ESA) for the Russian Space complete XEUS assembly sequence must be revised. Agency (RSA) to use on the ISS Russian segments. · As the Mirror Sector Grapple Fixture is currently It is a 36.7-ft (11.2-m) symmetric manipulator arm, located on the front surface of each individual Mirror consisting of two booms and seven joints, as shown in the Sector, the ERA is required to operate (both for the Figure 12 above. extraction and the insertion of the MS) in a direction During long non-operational periods the ERA arm can be that is perpendicular to the direction of the main axis of stowed on the Russian Sector, making use of two base- the ERA arm. This means that additional capabilities points. This is the so-called hibernation configuration, where shall be provided to the ERA, such as the visibility in a both EE units are clamped to base-points and thus protecting direction that is perpendicular to the ERA arm (i.e. the the EE connectors and cameras against environmental EVA installation of a mirror may be required) and new degrading and damage. During very long periods of operational procedures shall be developed and assessed hibernation of ERA, the EMMI units can be usually to operate the ERA in this new scenario. disconnected and stored inside the ISS modules. · Mirror Sectors extraction operation from the TC has been assessed in detail from the dynamic standpoint,
5 taking into account that the TC is being held by the · The Hibernation pose is a safe attitude that has been SSRMS (with its own flexibility) and that the MS is mechanically verified. 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. *
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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: Figure 12: MS's TC Transported by the SSRMS Facility 1. When ERA is in dynamic mode, ISS thruster firing shall
be postponed until ERA’s brakes are engaged Constraints to ISS 2. Forces and torques on ERA due to ISS thruster firing XEUS mirror sector assembly also impose some constraints may be too great for ERA’s control to sustain, on the ISS during its robotic operations, such as: dependent on the ERA pose, ERA’s load and the Constraints to movement of other objects on ISS-RS. characteristics of the thruster pulses. Normally, 1. Movement or deployment of objects in the operating with ERA should be interrupted and ERA put neighbourhood of ERA should not take place during an in a safe attitude during periods of thruster firing. ERA operation. 3. Simultaneous thruster firing and ERA operation may be 2. Exemptions are only allowed if ERA mission planning considered on a case-by-case base. has taken the worst case conditions with respect to the environment into account. 3. An object that is deployed should have its largest MIRROR SECTORS ASSEMBLY SEQUENCE contours included in the supplied initial condition world Detailed analysis has been performed for the robotic MS's model. assembly sequence represented by the following phases: 4. An object that is moved shall have both its initial and Phase 1. MSC1 Docking. final position present as elements in the supplied initial The XEUS mission is expected to operate at 600 Km condition world model. above the Earth, in the so-called Fellow Traveler Orbit 5. Before ERA enters the zone of potentially moving ISS (FTO), directly above the ISS, with an orbit inclination objects (solar arrays, radiators etc.), these must be of about 51.6o. At the end of the planned XEUS1 stopped. observation mission phase, in order to perform the 6. Moving or deploying of objects without ERA mission mirror upgrade MSC1 is docked at the SM of the ISS. planning involvement invalidates the World Models Phase 2. ERA re-location on SM. used for on-line Collision Avoidance and IMMI display. Once the MSC1 vehicle has docked to the Service This has major safety-related consequences. Module, the European manipulator arm ERA will be relocated from the SPP, where it is stowed, to a Constraints to space vehicle docking/undocking dedicated base-point that shall be installed on the SM. Similarly, there are some special constraints on the ISS Phase 3. Mirror Sectors upload. docking/undocking operations, such as: Eight Mirror Sectors (MS), required to upgrade the · When vehicle docking or undocking has to take place, MSC1 configuration, will be brought to the Shuttle. ERA shall be in hibernation pose. Phase 4. NSTS docking to ISS. · Forces and torque on ERA due to vehicle The Shuttle will be launched shortly after the MSC1 is docking/undocking may be too great for ERA’s brakes docked to the Service Module and it will dock to the to sustain. PMA2, ISS Forward Side.
6 Phase 5. Transport Carrier extraction from NSTS. arrived the SSRMS drives will be disabled and its The Transport Carrier will be extracted from the Shuttle brakes applied. Cargo Bay by the NSTS SRMS and will be handed off Phase 9. Mirror Sector Extraction. to the Space Station RMS (that is located on the Mobile The ERA manipulator, based on the Service Module, Base System) using the nominal payload hand-off will move to the hand over pose and get a Mirror Sector procedure. Initial grappling of the TC has to be done from the TC held by the SSRMS (see Figure 19). with the SRMS. A first Grapple Fixture, on the topside Phase 10. Mirror Sector Installation. of the TC, allows SRMS grappling and a second PDGF, The Mirror Sector is extracted by ERA from the TC, installed on the TC wall side, is used by the SSRMS for transferred to and installed on the MSC itself. grappling the TC 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 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 Figure 16: XEUS Mirror Sectors Assembly FGB module. All the subsequent SSRMS operations Phase 11. TC during night period. will be conducted with the SSRMS based on the FGB. 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. Figure 15: ERA grapples one Mirror Sector Phase 13. MSC built up completion.
Once the last Mirror Sector is installed on the MSC (and Phase 8. TC relocation on the Russian side. becoming MSC2) Thermal Baffles and Sunshade are From the base-point on Zarya (FGB), the SSRMS will deployed by EVA with support of the ERA facility. grapple again the TC stored on Z1 Truss and transfer it The same EVA operations will perform an overall to near the Service Module, at a hand over pose. Once inspection of the new assemblies of MSC2 spacecraft.
7 Phase 14. TC downloading. · Any time the Transport Carrier is grappled by the After the last Mirror Sector is extracted from the TC, the SSRMS through the PDGF the TC can receive power empty container is stored back in the NSTS Cargo Bay resources from the chain ISS-SSRMS-PDGF. for return to ground. This will occur according to the · The MSC continuously receives power resources from following operational steps: the ISS through the Interface with the Russian Service · SSRMS relocates the empty TC on Z1 Truss Module. · SSRMS relocates to the MBS · The Transport Container can not receive power when · SSRMS (based on MBS) transfers the TC from Z1 grappled by the NSTS SRMS through the FRGF. The Truss to a NSTS SRMS hand over position FRGF in fact only provides mechanical interfaces to the · SSRMS hands off the TC to the NSTS SRMS SRMS. · NSTS SRMS re-installs the TC inside the NSTS · The Transport Container can receive power resources Cargo Bay when it is located in the NSTS Cargo Bay. · Shuttle un-docks and leaves the Station · When Temporary Stored on Z1 the Transport Container Phase 15. MSC2 Checks and undocking. can not receive power resources, as the interface The upgraded and completed MSC2 is overall checked between Z1 and the TC is assumed mechanical only. If and then un-docked from the Service Module. the TC is required for any reason to remain stored on Z1 Phase 16. XEUS2 observation phase. for longer periods, then the SSRMS is required to At this stage of the XEUS mission a new Detector grapple one of the TC PDGF in order to provide power Spacecraft (DCS2) is launched. The rendezvous and resources to the TC. docking with the MSC2 is performed and the two · The MSC is able, at any time of its assembly to be able XEUS2 spacecraft is transferred to the proper XEUS to un-dock from Station and to survive for a limited mission observation orbit. period of time and to re-dock to ISS to allow build-up operation continuation.
Initial Time Day Final condition condition (h,min) SSRMS and 3 MS installed; ERA in home ERA in home pose; 3 5,44 pose; SSRMS holding full TC on Z1 TC. 3 MS installed; 5 MS installed; ERA in home ERA in home pose; 4 pose; 6,45 SSRMS holding SSRMS holding Figure 17: MSC-2 Un-docking from ISS TC. TC 5 MS installed; Assembly Time-Line 7 MS installed; ERA in home ROBCAD simulations have been performed which led also ERA in home pose; 5 pose; 6,50 to the following conclusions on the assembly daily SSRMS holding SSRMS holding operations splitting: TC. · the operations involving SSRMS and ERA take 4 days. TC. 7 MS installed; · In this period, starting from the full TC on Z1 and 8 MS installed; ERA in home ending with empty TC on Z1, all the MS are assembled SSRMS and ERA in 6 pose; 2,56 onto MSC. home pose; SSRMS holding Table 2 below summarizes such time splitting organization. empty TC on Z1. TC. Table 2: daily operations splitting Recovery Strategy & Off-nominal Events · EVA tasks are not expected to be nominally performed The fully manual mode would be used only in response to in order to support MSC build-up operations, but it contingencies or anomalies. In this case the operator will assumed that an EVA could always be performed to place the ERA or the SSRMS in a safe configuration and recover from an off-nominal event. wait for the ground to analyze the situation and decide on · The maximum period of time the NSTS is allowed to trouble shooting and recovery operations. The following remain docked on Station, waiting for XEUS Assembly ground-rules and assumptions are considered on the basis of completion is 9 days including contingencies. all the scenarios considered below: Regardless of any possible off nominal event the NSTS · The Mirror Sectors require power resources for thermal is required to leave ISS with the Transport Carrier after conditioning purposes. Power resources are provided to 9 days maximum. the Mirror Sectors either through the Transport Container or from the MSC. · All the off-nominal events precluding the MSC docking
8 on ISS will imply a delay of the NSTS flight carrying for ERA Control; the Plant block on the other hand the additional hardware to upgrade the MSC1 into represents the DCAP modelling of ERA and environment MSC2. 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 18: Mirror Sector Extraction from TC by ERA ERA Control X Motion Cmd ROBCAD DES Planner Simulation DYNAMICS SIMULATIONS
Interpolator XINT XCOM Inverse QDES Joint Control Loop Dynamic Modelling and Simulation activities associated (20 Hz) Kinematics (300 Hz) with the Mirror Sector robotic assembly have been QMOT Sensor simulation QJOINT I DXPROX DXTF performed utilizing the DCAP simulation software package, JPS and resolver simulation including ERA model validation. (300 Hz) * Q MOT * Q JOINT
F/T sensor FTSENS FT* ERA Modelling F/T Control simulation (20 Hz) Plant ERA modelling was performed. All reference frames at joint FTC,DES level and the rotation rules correspond to the rules of the TIP_pos Proximity XVIS CLU simulation Control (10 Hz) reference documents, as well as the physical characteristics GF_pos of the arm as reported in the Study documents. The DCAP modelling of ERA consisted of 12 rigid bodies Figure 20: ERA Controller Model connected through hinges. Of these bodies, 4 represent actual With this type of modelling ERA may work in several components of the arm, separated by the actuated joints, control modes: while 8 are used to setup a lumped mass representation of · free motion in the joint space: the joint set-points Q the flexible limbs 1 and 2. DES are directly provided by the interpolator to the joint
control loops at 20Hz rate; J6 WWRISTRI ROLL · free motion in the Cartesian space: the Cartesian set- -185o X +185° point XINT (=XCOM) is provided by the interpolator X X EE2 Y limb 2 Z Arm tip frame inverse kinematics to generate joint set-points QDES at Y -120o Z 20Hz rate; Z +120o Y WWRISTRIST PITCH o +120 -120o J4 ECC · proximity control: to the Cartesian set-point XINT is Z limb 1 Z Y o X -176 WRIST YAW +30o Y applied a correction DXPROX coming from the proximity ELBOW PITCH J5 EE1 J3 X control block after processing of CLU data. The X o SHOULDER ROLL X -120 ER ROLL o o o fully streched arm: all joint angles are 0 J0 -185°o +120 -120 +120 SHOULD ER PIT CH outcome X is then provided to the inverse +185 Z Y J2 Roll: rotation about X COM -+185°185o SHOULDER YAW J1 Pitch: rotation about Y kinematics; Z Y Yaw: rotation about Z Arm base frame · F/T control: to the Cartesian set-point XINT is applied a Figure 19: ERA Components correction DXTF coming from the F/T control block after processing of F/T sensor data. The outcome XCOM is ERA Controller then provided to the inverse kinematics. The following Figure 20 below shows an overview of the Regardless of the active control mode, each joint movement ERA controller implemented on DCAP for the dynamics is regulated by its own control loop, which runs at 300Hz simulation of the Mirror Sector assembly operations. rate. The Robcad simulation block in the Figure 20 diagram In the following, the components of the ERA Control model describes the overall mission in terms of a Motion will be analyzed with more detail. Command sequence from an initial position and is an input
9 SSRMS Modelling The SSRM arm, illustrated in Figure 23, has been modelled Therefore the basic configuration of the selected Attachment as an equivalent flexibility on which the TC and considered Mechanism design included also: as mounted on the ISS. This choice has been driven by the · Drive with an electrical actuator with redundant winding following considerations: · Proposed redundancy approach: one failure tolerant · the mass of the TC carrying the MS is about 10 time the (double winding actuator) for XEUS mission operations. full SSRMS mass; · Unlatching from TC and latching to MSC. · during the MS manipulation by ERA, SSRM will be · Proposed redundancy approach for two failure tolerant passive namely the SSRM control system is inactive e.g. design in safety critical operations: EVA manual the arm has brakes applied. This condition guarantees override for second failure that the main SSRM flexibility effects are due to the · EVA manual override approach: limbs flexibility; · Unlatching from MSC and latching back to TC for · the overall scenario to be simulated is very cumbersome contingency: one axis connected to the drive motor and from the computational point of view; moreover the locate a compatible coupling with EVA tools at the integration step must not exceed 1.e-3 [s] for stability border of the MS reasons; anyway the upper threshold is dictated by the The central latch is responsible for providing the final pre- ERA controller (300 [Hz]). loading. The guides must make the alignment of the sector ISS Dynamics Model and remain pre-loaded once the mechanism has been latched. The ISS dynamics was modelled as two rigid bodies considered locked one each other for the time being. Mechanism Components If needs arise to account for ISS flexibility, a rotational The following are the main attachment mechanism electro- spring/damper device can be foresees between the two mechanical components, represented in particular by the bodies. The Space Station is considered in orbit and effects passive and active latching devices and the associated due to the gravity gradient are taken into account. elecytronics unit. At the base of the ERA arm linear and rotational acceleration Latch mechanism noises are applied. The passive elements are represented by handles and guides,
MS ASSEMBLY MECHANISMS DESIGN as illustrated in Figure 21 above. The attachment active parts are represented by the mechanism illustrated in Figure 23 Preliminary design concepts and main requirement analysis below. 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. Figure 22: MS V-Shape Guiding System · Four passive handles on the MSC for attachment 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 Passive G. · 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 Active ‘connected’ to obtain a considerable mechanical advantage. · in the final position, both over-centers guarantee the Figure 21: MS Attachment Mechanism Basic Concept
10 latched configuration. · Envelope Volume: 1.5 litres (150 x 150 x 70 mm) · a spring maintain a positive latching for the worst · Mass: 1 kg thermal conditions · Power requirement: 10 W. · an end-stop in the driving bar provides the final latched position.
Figure 25: Mechanism Electronics Mechanism Mass & Power Budgets Figure 23: MS Attachment Mechanism Active Device From the performance of Mirror Sector attachment There is a shaft connected to the stepper to make a manual mechanism detailed design activity, details of mass and override of the mechanism in case of 2 failure. The shaft is power budgets have been provided for the single sector to be normally de-coupled of the stepper so that there is no energy assembled. lost because of the bearings Mass Budget Stepper and harmonic drive assembly Mirror Sector attachment mechanism mass budget, For the mechanism active part, a metallic cylinder supports comprising of electromechanical passive and active parts and the harmonic drive and the stepper, also illustrated in the associated electronics, amounts to about 17 Kg with the section view of following Figure 24. components split-up provided by the following Table 3. High torque and resistance requirements are associated with the active part design.
Table 3: MS Attachment Mechanisms Mass Budget Power Budget Figure 24: Section View of Mechanism Active Part Similarly, the Mirror Sector attachment mechanism power Electronics budget, associated in particular with its electromechanical The following are functions associated with the attachment active part and electronics, is illustrated by the following mechanism electronics box : Table 4 and amounts to a total power demand for each sector · provide the right motor sequencing for the berthing of about 42.5 Watt. 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: Table 4: MS Attachment Mechanism Power Budget
11 Structural Analysis requirement, etc. Also for the performance of the detailed design, functional The procedure of mechanical latching can take into account and operational analysis to verify compliance of the a waiting time after the two surfaces are in contact before attachment mechanism concept with the requirements, final latching is performed including guiding and capture range, capture speed, Calculations of extreme temperatures for the sensitive contingency operations, etc. have been performed. components of the mechanism, such as Motor, Reducer, For the same purpose, to support the such design activities, bushings, etc. were performed, together with evaluations of mechanical / structural analyses including Static analysis for thermal gradients just before final latching of Mirror Sector establishing load capability, stiffness and Dynamic Analysis, onto the mirror spacecraft (MSC). to verify natural frequencies and modes have also been It is assumed that heaters, thermistors and thermal control performed and some of the results are illustrated in the covers shall also be employed to limit maximum extreme following. As a starting point of these analysis activities, the temperatures. following assumptions or exemplification on the attachment The following are some of the conclusions of the performed mechanism have been made: named thermal analysis activity. Active latch motor: üFor the latched position model, only the elements loaded · According to the present analysis, heater with 10 have been considered. For this reason, the four gears and the (W) of nominal electrical power is required for each second claw have not been modelled. active latch motor, to maintain them within cold üIn the latched position, the force is decomposed and qualification temperature limits. applied in the claw thickness, in particular in the location · As for design cases DC1H and DC2H, margins of 1 where the handle is locked, but only over an element edge (ºC) and 4 (ºC) respectively have been calculated. line. For DC1H, the reason is that boundary temperature Some Analysis Results indicated in AD1 (65 ºC) plus acceptance and The following are some of the results obtained by the qualification margins (10 ºC), is 75 (ºC) which is performance of the attachment mechanism structural analysis only five degrees below the allowable operating activity: temperature limit of the motor. · The main claw is the element with higher stress levels. Harmonic drive temperatures: · The stresses in the rest of the elements are considerably Considering ten degrees for acceptance plus lower. qualification margins, temperatures calculated for · The maximum stress is 595 Mpa at the claw contact design cases DC3C and DC3H for instant after point with the handle. 3600 seconds are, -26 (ºC) and 52 (ºC) respectively. · The critical part of the mechanism is the support Waiting time for temperature attenuation: cylinder of the harmonic drive and the stepper, due to At least, a waiting time of 3600 seconds should be the high mass and dimensions of this two elements necessary, to reach acceptable qualification · The maximum stress results to be of 391 Mpa. 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: Figure 26: Mechanism Active Part Structural Model 1. To design and manufacture of a complete attachment and latching subsystem model in the 1:2 scale, taking Thermal Analysis into account Attachment mechanism thermal analysis has been also · To keep in this reduced model the same stresses performed, with main objectives of ensuring that all than in the full-scale real mechanism. components will be maintained within their qualification · In order to have the same stresses, or invariability temperature range, that temperature gradient between active of them, we need to reduce the pre-loads or forces and passive components of the mechanism were below the
12 to 25% of those required full-scale model. This [14] Requirement Specification Torque-Force Control since stresses are inversely proportional to the Algorithm, FS Doc. N. HS-ST-ER3-015-FSS section of the bars of the mechanism and when all [15] ERA Simulation Facility – Model Specification the geometric dimensions are reduced by 50%, the CLU, FS Doc. N. HS-ST-ER3-009-FSS resulting cross section areas are four times smaller, [16] ERA motion control kinematics algorithms or reduced to 25 %. specification”, FS Doc. N. HS-NT-ER-093-FSS · The contact stresses may also be representative, and [17] ERA Simulation Facility – Model Specification the required motor-reducer must give a torque that TFS FS Doc. N. HS-ST-ER3-010-FSS is eight times smaller than that of the full scale [18] Requirement specification Proximity Control model. Algorithm, FS Doc. N. HS-ST-ER3-013-FSS 2. To perform the mechanical testing of the prototype on [19] ERA Simulation Facility – Model Specification the manufactured scaled down model. During the tests JCE, FS Doc. N. HS-ST-ER3-010-FSS the following parameters will be measured: [20] Manipulator Joint Subsystem (MJS), SABCA Doc. · Pre-load values on the trunnion. HS-NT-ERA-202-SABC · Deformations of the mechanism. [21] ERA Model on Telegrip: simulations and · Torque required by the driving bar. performances: Final Report” ESA 26 Apr, 2001 [22] Flight Operation Manual and Procedures Issue 5, 3. To deliver a complete scaled-down attachment FS Doc HS -MU-ER-001-FSS mechanism model with supporting flanges for functional [23] ISS Assembly Flight 8A: Simulation of SSRMS demonstration in the Robotics Laboratory. Dynamic, G. Bilodeau et al., MacDonald Dettwiler, ISR2000, Montreal, May 2000 [24] XEUS Assembly Requirements and ROBCAD References Simulation, Alenia Spazio,SD-TN-AI- ,RALSS, [1] In-orbit assembly of large spacecraft: the XEUS TN2, Issue 1, Jan. 2002 mission, Didot F. et al., iSAIRAS symposium, June [25] Space Engineering; Part 3 : Mechanisms, Draft ‘99, ESTEC, SP440 ECSS- E- 30A Part 3, October 1999; [2] ERA, the flexible robot arm; Ph. Schoonejans et al., ftp://ftp.estec.esa.nl/pub/robotics/docs/ iSAIRAS symposium, June ‘99, ESTEC, SP-440 [26] XEUS Mirror-Sector Attachment Mechanism [3] The European Robotic Arm: Control Performances, Requirements Specification, Alenia Spazio, SD- J. Kouwen et al., ASTRA workshop, December ’98, RQ-AI-0042 Issue 01, March 2002. ESTEC, WPP-154 [27] ESABASE/THM-UM-043 Thermal Application [4] ERA, the flexible robot arm; Ph. Schoonejans et al., Manual (January 93). Version 93.1. iSAIRAS symposium, June ‘99, ESTEC, SP-440 [28] Cluster project. Measurement of thermo-optical [5] The European Robotic Arm: Control Performances, properties for machined Titanium (Ti-6Al-4V). J. Kouwen et al., ASTRA workshop, December ’98, Annex 1 of document CL-DOR-MN-0651. ESTEC, WPP-154 [29] Aerospace Structural Metal Handbook. Batelle [6] BD-TN-ML-006, Technical Note, Inputs to RALSS Columbus Laboratories. Study, XEUS Mirror Sector Characteristics, Media [30] Du Pont Catalogue on Vespel. SAGEM catalogue Lario, November 2001 on motors. [7] NSTS 1700.7b ISS Addendum, Safety Policy and [31] SHELDAHL catalogue on thermal control material Requirements for Payloads Using the International and films. S. Station; ftp://ftp.estec.esa.nl/pub/robotics/docs [32] Teleoperation: From the Space Shuttle to the Space [8] SSP42004; Mobile Servicing System to User I/F; Station, Nguyen, P.K., Hughes P., AIAA, 1994 ftp://ftp.estec.esa.nl/pub/robotics/docs/ [33] SSP 50075-SYS, Book 12, Vol I "Assembly and [9] D684-10503-02, "Support External Robotic Operations Support Plan Systems Data Report Operations Architecture Description Document", Structures and Mechanisms", Book 12, Vol I 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
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