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, email: email@example.com ABSTRACT 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. research. 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 motion. 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 Personal RWS LAS5 CEU RS-170A RS-170A studies performed by MDR, implementation of AVU CCD Computer System (PCS) RGB Video Video WHS Master MIL-STD- Camera Signals Processors 1553 MSS Ground Control will enable the ground (3) ECB Interface Internal Video Switch operators to assume up to 75% of the timeline, Translational Handcontroller Display and Control Panel Artificial SYNC (THC) thus allowing that much more crew time to be (DCP) RGB Video Discrete Interface OCS SLave ECB MIL-STD- 1553 Vision Unit (AVU) Rotational devoted to scientific research. Handcontroller (RHC) Interface LCD Video Monitors Implementation of MSS ground control aims to allow the ground operators to accomplish the RWS LAP5 Ext. Rack RWS LAP5 CEU Emergency Stop Secondary RWS following tasks: MSS Local Bus (MIL-STD-1553) PDGF Local Bus (MIL-STD-1553) a) Maneuvering of payloads using MBS SPDM SSRMS SSRMS/SPDM Figure 2. MSS Command Path b) Berthing/Unberthing of payloads using SSRMS/SPDM On-orbit, the crew may command the MSS via the c) Capture/Release of payloads using Portable Computer System (PCS) Graphical User SSRMS/SPDM Interface (GUI) and from the Display and Control d) SPDM Socket Extension Tool (SET) Panel (DCP). Commands sent from the PCS are Operation 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) Auto-sequence 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 CU E motion using Auto-sequence Modes only. C DP Cmadiss ntoCU o mn et E Operations requiring the use of Manual H) r a a n (WSfo v lidtio WS 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 Ue sr 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 OS 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 MSS GROUND CONTROL CONCEPT 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 SSRMS/SPDM 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. Procedures MSS Power-down Procedures Latching End Effector (LEE) Calibration 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 Calibration 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 Overlays 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. 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. CONCLUSION 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. ACKNOWLEDGEMENTS 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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