The Mobile Servicing System was a further development of the highly successful NASA Space Shuttle RMS system, both provided by the Canadian Space Agency. The SSRMS was designed for use on the US segment of the ISS and is comprised of several parts assembled over many shuttle assembly flights. There were also two other robot arm systems planned for the ISS, one the European Robotic Arm planned for the Russian segment Science Power Platform, and the Japanese Experiment Module Remote Manipluator. In addition there are two types of manually powered cranes, one US and one Russian, for movement of people and equipment.
Components of the Mobile Servicing System are:
|SSRMS Arm Specifications:
Mass (approx.) 1,800Kg
Mass handling capacity 100,000Kg
Degrees of freedom 7
Peak power 2000 Watts
Average power 1360 Watts
Stopping distance (under max. load) 0.6M
The SSRMS has two identical grapple end points called Latch End Effectors that enable it to reattach either end to the station as its new base. It moves in inch worm fashion around the US segment by placing one end on a mounting point and disengaging its other end and using it to grapple something else or another mounting point. Each mounting point provided power and control to the SSRMS. The SSRMS was also able to be mounted on a movable base which could move back and forth on the US truss segments. Also part of the project was the dextrous manipulator which could be attached to an end of the SSRMS for more precise tasks of replacing small orbital replacement units without the need for an astronaut to do a spacewalk.
For delivery to the ISS, the SSRMS was built with a hinge in the middle of both booms of the arm reducing its folded length to half of the normal length. Astronauts were required to do a spacewalk to unfold the arm and bolt it in position and connect it to the US Lab. The SSRMS was carried on a pallet which was attached to the US LAB where power and data cables provided the ability for the arm to activate and latch itself to the ISS and detach from the US Lab. The pallet was then removed for return to Earth.
|The SSRMS is a 56-ft (17-m) symmetric manipulator that supports electronic boxes and video cameras. It is composed of several ORUs, including two Latching End Effectors (LEE), two booms, and seven joints that can be rotated +/- 270 degrees . A LEE at each end of the SSRMS creates a walking capability between attach points called Power and Data Grapple Fixtures (PDGFs). This walking ability is the only mode of transportation for the SSRMS prior to the arrival of the Mobile Transporter (MT) and the Mobile Remote Servicer Base System (MBS).
For redundancy, the SSRMS has two separate electrical and electromechanical strings which are functionally identical. An ORU failure in one string causes a loss of that string, however, the second string can still be used for operations. Additional redundancy is contained in the LEEs, which have both prime and redundant power and data connections between the SSRMS and the payload. When there is a critical failure involving the SSRMS, the Built-In Test/Equipment detects it, and the SSRMS is automatically entered into a safe state (brakes-on).
A Latching End Effector (LEE) that can lock on one of many special fixtures, called Power Data Grapple Fixtures (PDGF), then detaching its other end and pivoting it forward. The range of accessibility of the SSRMS will be limited only by the number of PDGF's strategically installed on the station. The LEE is designed to provide power and data signals at both ends of the arm.
Unlike the Canadarm, the SSRMS stays in space for its useful life. This requirement necessitates an innovative design feature which allows astronauts to repair it on-orbit. The SSRMS is built in sections called Orbital Replacement Units (ORU's) which are easily removed and then replaced by either an astronaut or the Special Purpose Dexterous Manipulator.
The SSRMS is equipped with four TV cameras that feed wide and close-up views to the operators of the Canadian-built robotics, and an advanced vision system which has the ability to track payloads, and can sense various forces and moments to ensure smooth movement of payloads. The station arm also has a collision-avoidance capability. Two sets of cameras are mounted on the booms, one set on each side of the elbow joint. The remaining two sets of cameras are on the latching end effectors (one set on each LEE).
The SSRMS in checkout in Canada
The SSRMS with dextrous manipulator as envisioned in 1997
|Latching End Effectors
To provide the increased mass handling capability and mobility of the SSRMS, Latching End Effectors (LEE) have been designed to provide power and data signals at both ends of the arm. Additionally, the increased requirement for mass handling necessitated the use of mechanical latches that will augment the three-wire snare mechanism (first developed for the Shuttles Canadarm) as the principal load bearing device. This electro-mechanical interface appears four times on each SSRMS LEE. The precision alignment of the mating connectors is achieved through the introduction of a matching pair of Curvic couplings.
|Power Data Grapple Fixture
The Power Data Grapple Fixture (PDGF) is designed for mechanical actuation and electrical, power, data transfer to and from a variety of devices and payloads through two pairs of umbilical connectors. Power and data connections will be supplied to and from either end of the SSRMS as well as to and from any payload equipped with a PDGF. The PDGF also services at the base for the SSRMS and SPDM.
Flight-Releasable Grapple Fixture
A second type, the Flight-Releasable Grapple Fixture (FRGF), is primarily used for handling payloads and does not provide any power, data, or video connections. These can be seen along the truss, as well as the elements (Pressurized Mating Adapter 2 (PMA2), Z1, PMA3, P6, Lab, Space Lab Pallet/Lab Cradle Assembly (SLP/LCA), MPLM, Airlock, S0, S1, Node 2, Cupola, P1, and P3/4).
Mobile Remote Servicer Base System
The MBS is an interface between the SSRMS, SPDM, ORUs, payloads, EVA, and the MT, the MT cannot transport anything until the MBS arrives. It functions both as a work platform and as a base for the arm. The MBS has several important components. The MBS Computer Unit (MCU) has various functions, including providing control and monitoring, and performing failure management functions for MBS equipment. A device called the Payload/ORU Accommodation (POA) acts as a spare Latching End Effector (LEE) and provides power and a temporary storage location for payloads and ORUs. The MBS Common Attach System (MCAS) also provides a temporary storage location for payloads, including structural interfaces and power and data interfaces through the UMA on the MT. Another interface with the MT is provided by the MT Capture Latch (MTCL) which attaches the MBS onto the MT. There are four PDGFs to support attachment of the SSRMS and SPDM. Attach points are also provided for EVA. The Canadian Remote Power Control Modules (CRPCMs) distribute and switch power to the MBS equipment and attached payloads. These CRPCMs are not interchangeable with other Station RPCMs.
Control of the MBS is also through the Robotic Workstation (RWS). No health status is available when the MBS is powered down. Similar to the rest of the MSS subsystems, the MBS has redundancy built in. Like the SSRMS, the MBS has dual electrical and electromechanical strings. It has a primary and redundant MBS Computer Unit (MCU) and three primary and three redundant CRPCMs.
The Mobile Transporter (MT) provides structural, power, data, and video links between the ISS and the Mobile Remote Servicer Base System (MBS). It also provides transportation for the SSRMS, SPDM, payloads, and even EVA crewmembers, but not until the MBS arrives. At its greatest velocity (1 inch/sec), the maximum automated translation time is 50 minutes from one end of the truss to the other. When the MT is transporting large payloads across the Station, there can be an impact to the Guidance, Navigation and Control (GNC) System due to the changing mass properties of the ISS. If the Control Moment Gyros (CMGs) are unable to handle the change in momentum, jets may be fired to compensate for the change.
The Trailing Umbilical System (TUS) provides the power, communication, and video connections between the MT and the ITS. The Umbilical Mechanism Assembly (UMA) provides the capability to transfer power at utility ports for stationary operation of the MBS/SSRMS/SPDM. The UMA can be connected only when the MT is at one of the ten MT worksites along the truss. The Remote Power Control Module (RPCM) provides power switching between appropriate MT power sources and loads. The Linear Drive Unit (LDU) provides for the translation of the MT along the truss rails, while the Roller Suspension Unit (RSU) constrains the MT to the truss. Finally, the Load Transfer Unit (LTU) firmly fixes the MT to the truss at predetermined worksites.
Operator interface for the MT is through a PCS Graphical User Interface (GUI) that can be located either at the RWS or connected to another PCS port. Since no switches are needed, total control from the ground is possible, although ground control capability is primarily for powerup and system checkout. The communication interface is given by the Trailing Umbilical System (TUS). The TUS transfers commands from the MT and data from the MBS when at a worksite. It also allows the MBS to send video to the truss. The power interface for the MT is also provided by the TUS, while the MBS receives power through the Umbilical Mechanism Assembly (UMA). After the MT arrives at a worksite, it locks itself down and connects the UMA so it can send power to the MBS. Loss of the ability to translate the MT when in between worksites is a time-critical situation. If the MBS, SSRMS, or SPDM are on the MT while it is stranded, they cannot receive power; the temperature constraints on the cameras limit the time the power can be off to about 4 hours. To address these possible problems, the MT has some redundant features. There are two functionally independent power and data path strings provided by the TUS and there are two Umbilical Mechanism Assemblies (UMAs). There is only one Linear Drive Unit (LDU), but it has redundancy built in.
Mobile Transporter Structure
|The MSS is controlled using the RWS from either the Lab or the Cupola. Until the Cupola arrives, there is no direct viewing; therefore, the MSS Video System and the Space Vision System (SVS) provide the main visual inputs. The Video System, combined with ISS Communication and Tracking (C&T) systems, provides video generation, control, distribution, and localized lighting throughout MSS elements. The SVS provides synthetic views of operations using cameras, targets, and graphical/digital real-time position and rate data.
The SSRMS can be operated from only one workstation at a time. When the SSRMS is unpowered, the arm software is not loaded, therefore, the health status of the SSRMS is not available to the crew or ground. Due to this software implementation, SSRMS-specific ground support is needed only when SSRMS activities are performed.
The Robotic Workstation (RWS) provides the operator interface to control and receive data from the SSRMS. On Flight 6A, both workstations were delivered and then placed in the US Lab. To provide operators with out-the-window viewing, the second RWS is planned to be moved into the Cupola when it is added to the station. The RWS has components which are either external or internal to the rack. The external components are portable and include three video monitors, a Translational Hand Controller (THC) and a Rotational Hand Controller (RHC), a Display and Control (D&C) panel, a Portable Computer System (PCS), and an Artificial Vision Unit Cursor Control Device (AVU CCD). Unlike the external components, which are moved between the Lab and the Cupola, the internal components are fixed into the Lab racks. The internal components include an AVU and a Control Electronics Unit (CEU) which houses the RWS software.
During operations, one workstation is active (prime), while the second is in monitor mode or powered down. The active RWS has primary control of MSS functions, while the backup only provides emergency stop, control/display of additional camera views, and feedback of function status. If the prime RWS fails, the second workstation can transition from monitor mode to active. The RWS interfaces with the MSS local bus, the PDGF local bus, and the Command and Control (C&C) bus (to C&C Multiplexer-Demultiplexer (MDM) and PCS). The workstation also provides various modes for operating the SSRMS and Special Purpose Dexterous Manipulator (SPDM), including Manual Augmented mode via hand controller input, Automatic Trajectory mode via prestored and operator input, and Single Joint Rate mode (joint-by-joint movement) via the Translational Hand Controller (THC) and the joint select switch.
Special Purpose Dexterous Manipulator
The Special Purpose Dexterous Manipulator (SPDM) is the final component of the MSS to arrive at the ISS. It is composed of two 11.5-ft (3.5-m) seven-joint arms attached to a central single-joint body structure. These joints help to create the dexterity of this system. Due to this manipulators ability to execute dexterous operations, its primary function is to perform maintenance and payload servicing. The SPDM can remove and replace ORUs and ORU subcarriers, as well as inspect and monitor payloads and ORU equipment. It can provide lighting and Closed Circuit Television (CCTV) monitoring of work areas for EVA and Intravehicular Activity (IVA) crews. SPDM can assist EVA by transporting and positioning equipment. Control of this manipulator is provided through the RWS with control modes and features common to the SSRMS. Only one SPDM arm may be used at a time; the other arm can be used for stabilization at a worksite.
Images from NASA, ESA, NASDA, CSA, RSA
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