Ground Control Concept for On-Orbit Robotic Maintenance by fkj92926


									          Ground Control Concept for On-Orbit Robotic Maintenance
                Operations on the International Space Station

                                           Nasreen Dhanji
                                           MSS Operations
                                            MD Robotics

                                      Canadian Space Agency
                                     6767 route de l'Aéroport,
                                  Saint-Hubert, Québec, Canada,
                                               J3Y 8Y9
                                        tel: (450) 926-6714,


The International Space Station (ISS), one of           This paper presents the ground control concept
mankind’s greatest collective achievements, is a        recently developed to reduce the crew time
unique international space research facility that       needed to perform SPDM based maintenance
orbits the Earth at an altitude of about four           tasks and payload operations. Specifically the
hundred kilometers. Keeping the station fully           paper discusses the concept, operational
functional requires a substantial portion of crew       constraints and safety concerns, as well as the
time. Early estimates indicate that the crew may        proof of concept demonstration.
spend up to 60% of their time on overall station
maintenance tasks, which is time that could be          INTRODUCTION
devoted to science research.
                                                        The Mobile Servicing System (MSS) aboard the
To minimize the need for extra-vehicular activity,      International Space Station (ISS) is comprised of
"spacewalks", many maintenance tasks and                the Space Station Remote Manipulator System
payload handling operations external to the ISS         (SSRMS), the Mobile Remote Servicer (MRS)
will be performed by the Special Purpose                Base System (MBS), and the Special Purpose
Dexterous Manipulator (SPDM). The SPDM,                 Dexterous Manipulator (SPDM). The MSS is used
designed and built by MD Robotics for the               for assembly and maintenance of the ISS.
Canadian Space Agency (CSA), is an external
robotic system that consists of two seven degree        The MSS can be commanded via both on-orbit
of freedom arms mounted on a one degree of              crew as well as from the ground via the Mission
freedom body. The present concept of operations         Control Center (MCC) at the Johnson Space
requires the on orbit operator to command and           Center (JSC). Ground commanding infrastructure
monitor any SPDM motion from one of two                 is also being developed at the Canadian Space
robotics workstations inside the ISS. Initial           Agency (CSA) and eventually ground
timeline estimates indicate that the equivalent of 3    commanding of the MSS will be possible from
crew days is required to replace a battery box on       Canada. However, currently ground operators are
the ISS. Minimizing this crew time by enabling          unable to send commands that initiate MSS
ground control of the on-orbit robotics, via a          motion. Thus, there has been a heavy reliance on
phased implementation approach, will allow the          on-orbit crew for all maintenance and assembly
crew to redirect resources toward scientific            tasks that utilize the MSS.
                                                   Presently, an average of only 25% of the timelines
                                                   can be commanded from the ground because
                                                   ground operators do not have the capability of
                                                   issuing all commands required to initiate MSS

                                                   MSS COMMUNICATIONS ARCHITECTURE

                                                   The MSS can be commanded via both on-orbit
                                                   crew as well as from the ground. Communication
                                                   between the ground and the ISS occurs via the
                                                   Tracking and Data Relay Satellite System
                                                   (TDRSS). Commands issued from the ground are
                                                   delivered to the Command and Control
                                                   Multiplexer/Demultiplexer (C&C MDM) on the
                                                   ISS via TDRSS. From the C&C MDM,
                                                   commands are then transmitted to the Workstation
                                                   Host Software (WHS) in the Control Electronics
                                                   Unit (CEU) contained within the Robotic
                                                   Workstation (RWS) via the CB-External Bus. The
                                                   WHS then processes the commands and transmits
                                                   them to the relevant systems
                                                   (MBS/SSRMS/SPDM). Figure 2 illustrates the
Figure 1. The Mobile Servicing System              MSS Command Path.

Analysis of SPDM operating procedures indicates                                                                                                                       Satellite
that the timeline required to perform most SPDM                                                     Command and Control                  SCU          ACBSP                                                  Ground
                                                                                                   Multiplexer / Demultiplexer
                                                                                         MSD              (C&C MDM)

operational scenarios is lengthy and would
constitute a significant portion of the on-orbit
                                                                                                                           C&C Local Bus (MIL-STD-1553)
                                                                                                                           RWS Local Bus (MIL-STD-1553)
                                                    Primary RWS

crew's time. Based on these operational timeline    RWS LAS5 External Rack

                                                                                                                                      RWS LAS5 CEU

studies performed by MDR, implementation of            AVU CCD
                                                                                                                                         WHS Master
                                                                                                                         Processors                         1553
MSS Ground Control will enable the ground                                                                                    (3)
operators to assume up to 75% of the timeline,         Translational
                                                                             Display and
                                                                                Panel                                                                                              Artificial

thus allowing that much more crew time to be                                    (DCP)
                                                                                                                                          OCS SLave
                                                                                                                                                                                  Vision Unit
devoted to scientific research.                       Handcontroller

                                                                             LCD Video

Implementation of MSS ground control aims to
allow the ground operators to accomplish the               RWS LAP5
                                                            Ext. Rack
                                                                             RWS LAP5
                                                                                                                      Secondary RWS

following tasks:                                                                                                      MSS Local Bus (MIL-STD-1553)
                                                                                                                      PDGF Local Bus (MIL-STD-1553)

    a) Maneuvering of payloads using                                                                         MBS                                SPDM                    SSRMS

                                                   Figure 2. MSS Command Path
    b) Berthing/Unberthing of payloads using
                                                   On-orbit, the crew may command the MSS via the
    c) Capture/Release of payloads using
                                                   Portable Computer System (PCS) Graphical User
                                                   Interface (GUI) and from the Display and Control
    d) SPDM Socket Extension Tool (SET)
                                                   Panel (DCP). Commands sent from the PCS are
                                                   transmitted to the WHS via the C&C MDM
    e) Positioning of SSRMS/SPDM
                                                   whereas commands sent from the DCP are
                                                   transmitted directly to WHS as illustrated in the
Figures 3 and 4. SSRMS/SPDM motion may also                                                        c) Capture/Release of payloads using
be initiated via on-orbit crew operated Hand                                                          SSRMS/SPDM
Controllers (HC) when the system is operated in                                                    d) SPDM Socket Extension Tool (SET)
Manual Mode.                                                                                          Operation
                                                                                                   e) Positioning of SSRMS/SPDM

                                                                                               In general, these tasks may be accomplished
                                                                                               through a series of auto-sequences or by Manually

        C Cm ad g
       P S o mnin                                                                              operating the SSRMS/SPDM using the Hand
                                                                                               Controllers. Table 1 provides the SSRMS/SPDM
                                                                                               control modes.

                                                                                               Manual Modes Manual Augmented Mode (MAM)
                                                                                                            Single Joint Rate Mode (SJRM)
                                                                                                             Arm Pitch Plane Change Mode (APPC)
                                                                                               Modes         Operator Commanded POR Mode (OCPM)
                                                                                                            Operator Commanded Joint Position Mode (OCJM)
                                                                                                            Pre-Stored POR Auto-sequence Mode (PPAM)
                                                                                                            Pre-Stored Joint Position Auto-sequence Mode (PJAM)
Figure 3. PCS Command Path                                                                     Table 1. SSRMS/SPDM Control Modes

                                                                                               The proposed design for MSS Ground Control
D PCm ad g
 C o mnin                                                                                      will allow ground operators to initiate MSS
                                                   E                                           motion using Auto-sequence Modes only.
                    DP          Cmadiss ntoCU
                                 o mn et E                                                     Operations requiring the use of Manual
                                  H) r a a n
                                (WSfo v lidtio
                                                        H                                      Augmented Mode (including the use of Hand
                                        2                            WSs nsH c mad (H
                                                                        H ed C o mns C         Controllers), Single Joint Rate Mode or Arm Pitch
   1 Oe to ete aco mn b p ssin
      pra r n rs mad y re g
                                                                         gr, e ie e g n T M
                                                                     trige Vrn rs ttin)adOC
                                                                 3 sw hsd ctlytoth mn u to
                                                                     itc e ire    e aipla r.
                                                                                               Plane Change Mode will not be incorporated into
       as itc o dfletin th hn c n lle
         w h r e c g e ad otro r                                     A o e c mad a fo a e
                                                                      ll thr o mns re rwrdd
                                                                     toOSfo v lidtio
                                                                         C raan
                                                                                               the initial design of MSS Ground Control since all
                                                        C                                      desired MSS tasks can be accomplished using
                                                                                               auto-sequences. Currently, MSS motion can only
                                                                       C ee te o mn
                                                                     OSgnra sc madto
                      5 Dpnin o co mn,oe to w b
                         eed g n mad pra r ill e            4          proria S otro r
                                                                     ap p teMSc n lle          be initiated on-orbit via activation of switches on
                         p mte toc n u (P So H trige
                          ro p d otine C r C gr)                        . C/MU
                                                                     (ieA U C)
                                                                                               the Robotic Workstation (RWS) Display and
                                                                                               Control Panel (DCP). Figure 5 illustrates the
                                                                MSCn lle
                                                                 S otro r
                                                                                               switches on the Display and Control Panel that are
                                                                                               used to initiate MSS motion.

Figure 4. DCP Command Path


Implementation of MSS ground control aims to
allow the ground operators to accomplish the
following tasks:
     a) Maneuvering of payloads using
         SSRMS/SPDM                                                                            Figure 5. Display and Control Panel
     b) Berthing/Unberthing of payloads using
 Table 2 provides a summary of the MSS                        received by the CEU from the ground via the
 operations that can currently be performed from              C&C MDM. With this software in place,
 the ground as well as those that cannot.                     commands to initiate MSS motion can be sent
                                                              from the ground using the existing Mission
     Ground                        Non-Ground                 Control Center (MCC) commanding
 Commandable MSS                 Commandable MSS              infrastructure. Minimal effort, in terms of
    Operations                      Operations                software modifications, is required in order to
MSS Power-up Procedures         SSRMS Stow/Unstow             introduce MSS Ground Commanding capability.
MSS Power-down Procedures       Latching End Effector (LEE)
                                                              The MCC commanding infrastructure consists of
MSS Configuration and State     SSRMS/SPDM LEE                a command inventory from which commands may
Transition Procedures           Capture/Release of Grapple    be selected and uplinked for on-orbit execution.
                                Fixture (GF) Procedures       Addition of MSS motion initiation commands into
SSRMS/SPDM Force                SSRMS Joint Brake
                                                              the command inventory will allow ground
Moment Sensor Calibration       Diagnostics
SSRMS Latching End              SSRMS/SPDM LEE                operators to achieve all desired MSS tasks.
Effector (LEE) and Joint Unit   Operating Procedures          Commands that initiate any kind of motion on the
Diagnostics                                                   ISS are deemed to be hazardous commands and
SSRMS and SPDM Base             SPDM On-orbit Replacement     are classified as such. Issuing of these commands
Change Procedures excluding     Unit (ORU) Tool Change-out
the actual capture of the       Mechanism (OTCM)              requires the operator to go through a three-step
PDGF by the LEE.                Capture/Release Procedures    "Ready-Arm-Fire" process. This process provides
Payload Activation and          SSRMS/SPDM Control Mode       the controls to ensure that the operator actually
Deactivation Procedures         Selection                     intends to send the commands that initiate motion.
Removal and Application of      Application of MSS Safing.
SSRMS Safing
                                                              Since the infrastructure needed to issue MSS
MBS Checkout Procedures         SSRMS/SPDM Manipulator        commands from the ground already exists, no new
excluding the POA               Control Mode Selection        hardware and/or facilities would be required in
Mobile Transporter (MT)         MBS Payload ORU               order to implement MSS Ground Control.
Translation                     Accommodation (POA)           However, new ground displays are required to
Activation of the Robotic       MBS Payload ORU
                                                              incorporate the new telemetry from the Ground
Workstation (RWS)               Accommodation (POA)           Control software including: Auto-sequence Target
                                Operating Procedures          Position Data and Ground Command Status and
Configuration of Cameras and    SPDM Brake Control            Parameters. Therefore, by enabling the required
                                                              MSS commands on the ground, making the new
Video System Activation         SPDM LEE/OTCM
                                Calibration                   telemetry available to ground operators and
                                SPDM Backup Drive             making minimal modifications to the on-orbit
                                Activation                    system to accept those commands from the
 Table 2. MSS Operations Commandable from                     ground, the MSS would be fully commandable
 Ground vs. Operations Not Commandable from                   from the ground.
                                                              OPERATIONAL CONSTRAINTS
 Through the implementation of minimal DCP
 switch throw commands including brake switches,              In order to safely command the MSS from the
 pause/proceed switch, SPDM torque drive                      ground, various operational constraints must be
 switches, safing switch, EE trigger and                      addressed. Major operational constraints include:
 coarse/vernier switch, the majority of MSS                        • Loss of Signal (LOS) between the
 operations can be performed from the ground. In                        Ground Segment and the International
 order to implement commands that initiate MSS                          Space Station
 motion from the ground, software changes to the                   • Adequate situational awareness for
 WHS will be made to duplicate relevant portions                        ground operators
 of the DCP functionality in the form of Consultive                • Timing and latency
 Committee for Space Data Systems (CCSDS)
 commands, that is, software commands that are
Loss of Signal (LOS)                                  purposes provides predictability of expected
                                                      system behaviour and eliminates the need for
Communications between the ground segment and         hand controller inputs. Furthermore, the trigger
the ISS occurs via Tracking and Data Relay            commands for manual LEE commands have been
Satellite System (TDRSS). Telemetry and               modified such that trigger duration is specified
commands are transmitted via a S-Band signal          within the command.
and video is transmitted via a KU-Band signal.
Hand-overs between TDRS satellites, antenna           Given that the ground operator may not be able to
blockage by structure, and communications needs       continuously monitor the system in the event of an
of other customers affect the availability of a       unplanned LOS, operations must be planned such
communication signal between the ground               that the risk of collision due to any reason other
segment and the ISS.                                  than SSRMS failure is mitigated before motion is
                                                      initiated. Since ground commanding utilizes auto-
Satellite communication coverage is scheduled a       sequence modes exclusively, all trajectories are
few days to a week in advance of a given on-orbit     planned and simulated before they are executed.
operation. Since satellite coverage is scheduled in   Furthermore, before the auto-sequence is
advance, it is possible to predict Loss of Signal     executed, a survey verifies that the model used in
(LOS) between the ground segment and the ISS.         the simulators to create the auto-sequence matches
The ground segment is responsible for developing      the real workspace.
a satellite communication coverage schedule that
indicates the name of the TDRS satellite that is to   Situational Awareness
provide communications coverage as well as the
duration of the coverage. The schedule is then        Whether the MSS operator is on the ISS or on the
used to determine the window within which the         ground, prior to initiating a command, he/she
ground operators may communicate with the ISS.        must understand what the resulting state of the
The average window of opportunity for                 MSS will be; this includes knowing the current
communication is approximately 80% per orbit          state of relevant MSS components including
for S-Band (telemetry) signal and 60% per orbit       manipulator configurations, clearances from
for KU-Band (video) signal.                           structure, and alignments with targets. The
                                                      operator must also be aware of how the command
Although MSS Ground Control operations will           will change these states, configurations,
always be coordinated in accordance with the          clearances, and alignments.
availability of communications and video
coverage, unplanned LOS can occur due to              The MSS operator currently achieves situational
equipment failure or for environmental reasons,       awareness from several sources. Before the
e.g. ratty communication in the Alaskan Zone.         operation is ready to be executed, the operator is
Unplanned LOS or ratty communication caused           expected to have reviewed and understood the
by environmental reasons lasts for only a few         operating procedures. In addition to providing
minutes; however, unplanned LOS caused by             information about the states of the various
equipment failure can last longer. Unplanned          systems, the procedures generally include
LOS is a concern for ground control of robotics in    diagrams illustrating expected camera views and
situations where the ground-based operator needs      bird’s eye views to help the operator visualize the
to give continuous input to the system, perform       big picture. When the operator is ready to
continuous monitoring of the system, or perform       proceed with the operation, he/she can use live
operations within a specified time limit.             camera views, data displays, video overlays and
                                                      station models to confirm that the situation is as
The two types of commands that require                expected.
continuous input from the operator include Hand
Controller commands and LEE Trigger                   At present, direct visual contact, i.e., through ISS
commands for manual end effector modes. The           windows, is not used as a primary cue for on-orbit
use of auto-sequences for ground commanding           operations. On-orbit operators rely on camera
views displayed on the RWS monitors for               In order to know how the system will respond to
situational awareness. The same camera views          commands, the operator must be aware of the
used by the on-orbit operators can be transmitted     state of the related sub-systems. This information
to the ground, thus providing ground operators        can be obtained from the Portable Computer
with the same level of situational awareness as the   System (PCS) Graphical User Interface (GUI)
on-orbit operators.                                   and/or the video overlays on the RWS monitors.
                                                      Video overlay information includes the target
Station models are used to plan MSS operations        position/orientation data as well as data used for
and provide situational awareness prior to            grapple or berthing alignment during auto-
performing on-orbit operations. Station models        sequence maneuvers. All of the information that
are an integral part of determining whether the       is displayed on the PCS GUI is also available on
required clearances between MSS components            the ground. Target position/orientation and
and the ISS structure are maintained as well as       alignment data is deduced from other available
determining whether the necessary alignment for       digital data.
contact operations is achieved. It is therefore
essential that the station models accurately          Timing and Latency
represent the configuration of the ISS. Station
models undergo a rigorous process of verification     The magnitude of the communication latency
and certification before being used to plan MSS       between the ground and the ISS is variable and
operations.                                           depends on several factors such as cable length
                                                      and computer processing time. It is difficult to
Station models are developed using CAD models         compute precise boundaries on the latency but
provided by partners furnishing ISS hardware.         experience indicates that the average delay in
Station models evolve in three phases:                system response to the time commands are issued
     • Best Available model                           is between 3 and 7 seconds round-trip.
     • As Designed model
     • Final model.                                   Manual maneuvering (including MAM, SJRM,
                                                      and APPC) is designed for use with minimal
The ‘Best Available’ model is developed from          latencies. Human-in-the-loop analysis indicates
preliminary design reviews. Hardware providers        that any latency greater than approximately 300
provide CAD models for the ‘As Designed’ and          ms begins to affect the operator’s ability to control
‘Final’ models. Where available, Digital Pre-         the manipulator effectively. However, there is no
Assembly (DPA) data (comprised of digital             real need to support manual maneuvers from the
measurements of actual hardware) is also used to      ground since all desired tasks can be carried out
develop station models. The CAD models are            using auto-sequence modes instead. OCPM or
validated against drawings and undergo a quality      PPAM could be used in place of MAM and
control process to ensure the models accurately       OCJM or PJAM could be used in place of SJRM.
represent the actual configuration of the ISS.
                                                      As long as operations are carried out using auto-
 Since the very same station models that are          sequence modes (OCPM, OCJM, PPAM, PJAM),
currently used to plan on-orbit MSS operations        latencies do not create any control problems.
will be used to plan ground based operations of       Even much larger latencies (such as the 40-minute
the MSS, ground operators will have the same          command-to-response time for commanding
level of situational awareness as on-orbit            robots on Mars) do not affect the behavior of
operators currently have. Furthermore, in order to    auto-sequence modes. As such, MSS ground
ensure that the station models accurately reflect     operations will be limited to those that do not
the workspace, a pre-mission survey of the path       require real-time, human-in-the-loop
that is to be traversed will be conducted prior to    commanding.
execution of any ground based operation.
                                                      The use of manual modes has not been ruled out
                                                      entirely. Future enhancements including the use of
an on-orbit safety monitor that incorporates          MSS GROUND CONTROL
computer-aided vision and target tracking             DEMONSTRATION
algorithms could make the use of manual modes,
including the use of Hand Controllers, feasible.      A successful demonstration of the MSS Ground
However, the initial implementation of MSS            Control operations concept was performed in July
ground control will be restricted to the use of       2003. The demonstration occurred prior to
auto-sequence modes only.                             implementing the required software changes;
                                                      therefore, the ground operators were able to
TRAJECTORY DEVELOPMENT AND                            completely set up all the steps required to execute
MISSION DESIGN                                        the demonstration but needed the on-orbit crew to
                                                      issue the final motion initiating commands (i.e.
Based on the current implementation of ground         throwing of DCP switches). With the
control, new operational and safety constraints       implementation of the required software changes,
will be placed on mission planning and execution.     the ground operators will be able to perform all
Trajectories will be designed to facilitate a pre-    desired MSS tasks from the ground with no on-
mission survey of the complete path before the        orbit crew involvement. The demonstration served
operation is executed. For those trajectories where   as a first step in allowing both the on-orbit and
a complete survey is not possible before motion       ground robotic operators to gain confidence in
commences, the trajectory will be broken up in        commanding the MSS from the ground.
order to allow for intermediate survey positions.
                                                      The primary objective of the demonstration was to
Camera views will be set up prior to initiation of    illustrate the ability to position the SSRMS and to
motion in order to observe expected manipulator       perform grapple and release operations using
motion. Trajectories will be designed such that       auto-sequences only and to demonstrate that auto-
movement starts and ends in the field of view of      sequences are sufficient to perform the required
the camera so that camera commanding during           tasks. The demonstration provided the opportunity
manipulator motion is not required.                   to:
                                                           • Illustrate proof of concept of the
Planned communication coverage will need to be                  proposed method of implementation of
taken into consideration when timelines for                     MSS Ground Control
ground controlled mission operations are                   • Obtain "early-in-design" operational data
developed because the amount of communication                   to determine if there were any
coverage available will limit how much time is                  deficiencies in the proposed design
available for ground commanding.                           • Assist the ground operators in
                                                                determining the additional workload
Procedural impacts of these new constraints                     associated with ground commanding
include:                                                   • Determine if any enhancements were
     • An additional procedure to be developed                  required to the existing ground
         for the pre-mission survey                             commanding infrastructure
     • Telemetry deemed critical for correct               • Allow ground operators and on-orbit
         arm positioning and operation must be                  crew to coordinate on-orbit operations
         listed in the procedure and verified to be
         correct by two independent ground            To demonstrate that auto-sequences are sufficient
         operators before motion is initiated         to control the SSRMS for positioning and
     • Procedures developed for ground control        grapple/release operations, the demonstration
         will include camera set up instructions,     included two types of maneuvers:
         Acquisition of Signal (AOS)
         requirements and steps for verification of       1.   A series of single joint maneuvers
         critical telemetry.                                   (executed as a series of auto-sequences in
                                                               OCJM mode) designed to perform free
                                                               space maneuvering to position the
         SSRMS. Successful execution of single
         joint maneuvers using OCJM mode
         demonstrated that auto-sequences
         provide sufficient functionality for the
         ground to perform free space motion.
    2.   Frame of Reference (FOR) OCAS mode
         was used to maneuver the SSRMS
         Latching End Effector (LEE) into the
         grapple envelop of a Power and Data
         Grapple Fixture (PDGF), and to back-off
         after release. Successful execution of the
         approach and back-off maneuvers
         demonstrated that auto-sequences
         provide sufficient functionality for the
         ground operators to perform
         grapple/release maneuvers.

The demonstration illustrated the ability to
successfully command the MSS from the ground.
The concept of using auto-sequence modes to
accomplish all desired tasks was successfully
proven. Furthermore, it instilled sufficient
confidence in the robotics operations community
in the ability to command the MSS from the
ground in a safe and efficient manner.


The implementation of MSS ground control
provides a potential 75% reduction of crew time
required for assembly and maintenance tasks on
the ISS. Extensive analysis of the ground control
concept has shown that minimal changes to on-
orbit software are required to enable complete
ground commanding capability of the MSS.
Demonstration of ground commanding has proven
that the concept is viable and can be accomplished
in a safe and efficient manner. With the ambitious
deadline for completing ISS assembly by 2010,
ground control not only has an essential role to
play in accomplishing this task but will also push
Canada’s tele-robotic expertise to new frontiers.


The Ground Control Operations Concept cited in
this paper was developed under CSA contract to
MDR Systems Engineering in conjunction with
the ISS Robotics Flight Control Team at the
Johnson Space Center in Houston, Texas.

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