Dextre demo paving the way for robotic EVA assistance
The Special Purpose Dexterous Manipulator (SPDM) has completed a series of tests aimed at reducing the human EVA workload involved with the installation of new batteries on the International Space Station (ISS). The impressive Canadian robot – more commonly known as “Dextre” – worked through three days of Main Bus Switching Unit (MBSU) demonstrations ahead of the arrival of new batteries on the Japanese cargo ship HTV-6.
Dextre is a permanent fixture on the outside of the ISS, ready to be translated to worksites where it can be tasked with external maintenance tasks, the removal and replacement of dexterous compatible Orbit Replaceable Units (ORUs) and the servicing of scientific payloads.
The SPDM was designed and built by MacDonald Dettwiler and Associates, (MDA), and financed by the Canadian Space Agency (CSA).
The robot rode to space in the payload bay of Shuttle Endeavour in 2008 (STS-123) and is now part of a trio of Canadian robotic assets that provide vital services to the ISS and a number of Visiting Vehicles.
Dextre was assembled during STS-123 EVAs, with spacewalkers Rick Linnehan and Mike Foreman installing the robots 11-foot arms to the torso of the robot on Flight Day 6 on the mission.
Despite a few teething problems, checkouts of the robot proceeded well over the coming months, eventually allowing the robot to enter into operational status in 2010.
However, Dextre’s opening operations proved to be troublesome, although the challenges provided a number of important lessons that have fed into the numerous successes that have followed over recent years.
The first major success for Dextre came during the HTV-2 mission, working in tandem with its compatriot the Space Station Remote Manipulator System (SSRMS) – or Canadarm2. The duo worked on removing payloads hosted in the Japanese vehicle’s Exposed Pallet.
Later in 2011, Dextre took on a marathon set of robotic tasks, in turn feeding into the approval to conduct tests on future technology advances.
Beginning in 2012, the first use of Dextre with NASA’s Robotic Refuelling Mission (RRM) tests marked the first ever attempt at satellite servicing demo tasks to be performed at the ISS.
The RRM is an ISS payload developed by the Satellite Servicing Capabilities Office (SSCO) at NASA’s Goddard Space Flight Center (GSFC), launched with Shuttle Atlantis during STS-135.
Numerous operations were carried out, marked by the successful conclusion of the initial test series in 2013.
Dextre has also worked with a number of SpaceX Dragon vehicles, following an initial meeting during the commercial spacecraft’s debut mission to the Station – testing out clearances and camera views ahead of the future payload removal role involving payload items in Dragon’s trunk section.
The first hardware to be removed from Dragon’s Trunk came during the CRS-2/SpX-2 mission.
However, this only involved the SSRMS, tasked with the removal of two Heat Rejection Subsystem Grapple Fixtures (HRSGFs) – which are essentially bars each featuring two Flight Releasable Grapple Fixtures (FRGFs).
The first payload removal task between Dextre and Dragon’s trunk came during CRS-3/SpX-3, an operation that was repeated again during the CRS-5/SpX-5 mission.
Dextre has also provided many additional roles on the Station, such as visual inspections of the ISS modules and its robotic friends.
However, it’s the role of supporting EVA-based maintenance that will provide a major boost to Station operations.
The recent test – known as the “Main Bus Switching Unit (MBSU) Demonstration” – was designed to validate the robotic transfer of battery style Orbital Replacement Units (ORUs) prior to the delivery of new ISS batteries on HTV-6.
The goal is to allow planners to reduce the number of EVAs required to install the batteries from around six to just two spacewalks.
The first task over the three-day test involved ground controllers tasking Dextre to move the spare MBSU Flight Releasable Attachment Mechanism (FRAM) from Express Logistics Carrier (ELC)-2 to the Enhanced ORU Temporary Platform (EOTP).
This demonstration was to be the first time the SPDM ORU Tool Changeout Mechanism (OTCM) and the Robotic Offset Tool (ROST) were used together.
Both of Dextre’s arms are terminated with OTCMs, which include “jaws” to grasp objects, a retractable socket drive, a camera and light, and an umbilical connector to provide and receive power, data and video to and from a gripped object.
The OTCMs also include Force/Moment Sensor (FMS) technology, giving the arms a “sense of touch”.
However, the milestone event on day one of the test suffered from a few teething problems.
“The ROST objective had to be deferred due to an issue with the SPDM OTCM grasping the ROST. The OTCM jaws came to ~0.1 mm from reaching the required grapple position. Several reposition and re-grasp attempts were made with the same result,” noted an L2 overview of the SPDM tasks.
“The team decided to delete the rest of the ROST activity and continue with MBSU prep activities using the OTCM. The ROST capability will be needed for some of the planned Battery operations during HTV-6 (late in 2016). The team is investigating the issue. A ROST demonstration will likely need to be re-planned for a future date.”
The secondary bolt was unfastened on the MBSU nominally by loosening it via the use of the OTCM. The SPDM was then un-grappled and moved to a safe overnight ahead of the second day of operations.
The second day’s tasks involved the removal of the MBSU from the FRAM structure and temporarily stowing the MBSU on EOTP side 3 before returning the MBSU back to the FRAM.
While maneuvering to the EOTP, a difference was noticed between the expected Frame Of Reference (FOR) in the procedure and the actual FOR. ROBO (controller team) was able to build a new FOR and finish the planned operations.
During the operations, more challenges were noted when attempting to back the MBSU away from the EOTP after unfastening the bolt, as Dextre did not initially move and loads started to build up.
“This was potentially due to the MBSU hanging up on the soft dock detents. Prior to timeout of the maneuver, the SPDM overcame the binding force and released. Forces applied were all within limits. When it released, SPDM motion stopped short of the intended back away position,” added the L2 notes.
“An additional maneuver was required to finish the back away maneuver. Both of these issues are being investigated.”
However, the operations were successfully concluded when the MBSU and FRAM were moved back to Express Logistics Carrier (ELC) 2 with no further issues reported.
The overall test was deemed successful, with the technical challenges aiding the knowledge base ahead of operations that won’t be required until HTV-6 arrives with the new batteries later this year.
A future requirement to conduct additional tests ahead of HTV-6 operations could be inserted into the schedule, given the operations are conducted from the ground and do not require the involvement of the ISS crewmembers.
For the interim, Dextre can look forward to being involved with upcoming SpaceX Dragon missions, as the Canadian robot continues to provide an important role from its vantage point on the outside of the orbital outpost.
(Images: via L2’s ISS Section – Containing presentations, videos, images and ongoing ISS status updates, with additional images NASA and ESA).
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