Dextre, the Canadian robot, waits begins operational service

by Chris Bergin

Canada’s Special Purpose Dexterous Manipulator (SPDM) robot “Dextre” is making himself useful on the International Space Station (ISS), as he prepares to swap out a failed RPCM (Remote Power Control Module) on the P1 Truss. Having conducted his induction training on Tuesday, Dextre was set to clock in for work on Wednesday. However, operations are on hold, as engineers evaluate an obstruction with the RPCM.

Dextre’s Time On Orbit:

Dextre is capable of robotic execution of ISS external maintenance tasks, such as the removal and replacement of dexterous compatible Orbit Replaceable Units (ORUs), and the servicing of scientific payloads.

Supporting EVA-based maintenance is also part of its role, with the preposition of ORUs or Integrated Assemblies, the provision of lighting and camera support, and is it is also capable of actuating external mechanisms, performing inspection tasks, and extending the reach of the SSRMS (Space Station Remote Manipulator System).

Dextre arrived in Endeavour’s Payload Bay (PLB) during STS-123, although his Special Logistics Pallet (SLP) initially suffered from ‘Keep-Alive’ power supply issues, when the SLP was removed from the Payload Bay. Repeated attempts to activate the SPDM PSU (Power Switching Unit) for powering on the ‘keep-alive’ heaters were unsuccessful.

The first plan of action was to try a software patch, due to engineers suspecting an issue with the software timing between the RWS (Robotics Work Station) applying power to the PSU and establishing data communications. However, this failed to solve the issue.

“SPDM troubleshooting: The MSS software patch was uplinked with no joy on the SPDM power up,” noted MMT information at the time. “The plan is to grapple the SPDM on FD5 following OBSS (Orbiter Boom Sensor System) handoff and power the SPDM via the PDGF (Power Data Grapple Fixture).”

Thanks to further evaluations by engineers, the problem was pinpointed to a design error in a cable – unavoidable given the ground based test bed research wouldn’t of been able to be simulate the on orbit conditions.

Flight Day 6 saw STS-123 spacewalkers Rick Linnehan and Mike Foreman install Dextre’s 11 foot arms to the torso of the robot. During the EVA, one of the arms proved slightly troublesome, along with some issues with the Joint and Brake Tests during diagnostic checks.

All was well with Dextre – attached to the Destiny Lab – until the end of 2008, when engineers identified a potential failure mode in the SPDM Power Switching Unit (PSU), as they prepared to put the robot through a series of checks on the end of the Space Station Remote Manipulator System (SSRMS).

“We received data that MDA (MacDonald, Dettwiler and Associates Ltd) had identified a potential failure mode in the SPDM power switching unit (PSU) that could result in power flowing through from one power channel to another,” noted one of several memos at the time.

“Preliminary inputs we received from MDA and CSA (Canadian Space Agency) were that the failure should not affect the planned SPDM operations, but after asking a few questions it became clear that they had not fully assessed the impacts of the failure.”

Such checks were required to ensure Dextre wouldn’t fail in a position where it could become trapped on the end of the SSRMS or the Mobile Base System (MBS).

However, by the turn of the year, Dextre was deemed to be in good health, with the back up of contingency workarounds available in the event of issues with the PSU during future robotic tasks. This resulted in a milestone on January 20, 2009, with all three elements of the Canadian robotics system on the ISS (collectively known as the Mobile Servicing System) were working together.

“From its perch on the Mobile Base System, Canadarm2 successfully lifted Dextre off the ISS’s Destiny lab, and held it overnight until further operations on Wednesday,” noted the Canadian Space Agency (CSA) at the time. “This maneuver marks the first time ground controllers have moved Canadarm2 while carrying a payload at the end of the arm.”

With thermal covers installed on Dextre during Discovery’s STS-119 EVA-2 – conducted by Steve Swanson and Joe Acaba – engineers prepared for the robot to begin operational tasks, originally scheduled for the fall of 2009.

“The SPDM continues to be checked out by the mission control team in preparation for a R&R of a RPCM (Remote Power Control Module) in the fall,” added an 8th Floor ISS update early in 2009. “The team operated the Space Station Remote Manipulator System (SSRMS) and SPDM.

“Operations included motion of the dexterous arm 2 shoulder roll joint, which had a hardware problem during STS-123 in March 2008. Engineers were able to correct this with a software update in the MSS 5.2 load which was uplinked in December.

“Arm 2 motion, including the shoulder roll joint, was perfectly nominal. Operations also included SPDM dexterous arm motion in Frame of Resolution (FOR) Auto-Sequence mode, SSRMS FOR Auto-Sequence (and motion) using an SPDM FOR (at a dexterous arm tip), and SPDM dexterous arms ‘limped’.”

By the spring of 2009 another checkout marked the first time a dexterous arm was aligned with a dexterous grapple fixture and target on ISS, and was deemed to be a full success.

“These operations were successful, demonstrating that maneuvers over distances as small as 2mm can be performed with control and precision. System performance was nominal,” noted the Space Station Program Control Board (SSPCB) at the time.

On May 27, 2009, Ground Controllers (ROBO) conducted another SPDM On-Orbit Checkout Requirements (OCR), with the main objective to perform an automatic grasp operation.

“Ground controllers maneuvered the SPDM body and arm 1, while based on the Mobile Remote Servicer (MRS) Base System (MBS), and positioned the arm1 end-effecter over the Robot Micro Conical Tool (RMCT)-1. After calibrating the arm1 force/moment sensor, ROBO then maneuvered the arm over the micro fixture, and executed an auto-grasp with Force/Moment Accommodation (FMA) enabled,” noted information at the time.

“The team then opened the grippers, backed off the fixture, and the SPDM arm1 and body were re-stowed. This was the first time that we have actually grappled hardware with the SPDM so this is considered to be a major step on the road to our first R&R. Congratulations to Sarmad Aziz and the entire team for this significant achievement.”

In October of last year, robotics teams continued their work on checking out Dextre, as the SPDM Backup Drive Unit (BDU) was exercised to control both the ORU and Tool Changeout Mechanisms (OTCM), checking the status of the robot, following the uplink of the MSS 6.1 software load.

“The checkouts included: functionality for OTCM mechanisms; body joint maneuvering; maneuvering arms 1 & 2. The Robotic Micro-Conical Tools (RMCT) were grappled nominally and then the RMCT launch locks were released and the tool were released and maneuvered clear of the Tool Holster Assembly (THA) to a stow position.

“Everything is looking good for any future R&R activity. Due to the high activity, the Program has decided not to currently pursue the previous objective to perform a swap of two Truss RPCMs.”

So, with one RPCM to changeout, Dextre knows what he’ll be tasked with on Wednesday. Work began on Monday when Dextre was transported to the worksite, prior to a dress rehearsal on Tuesday.

“The SPDM “Dextre” will be used to swap out a failed RPCM (Remote Power Control Module) on the P1 Truss this week. This will be the SPDM’s first ever operational use,” noted status information.

“Operations began on Monday when the SSRMS grappled the SPDM and maneuvered it to the P1 Truss worksite. On Tuesday flight controllers at MCC-H began conducting a “dress rehearsal” of the actual replacement as they commanded the SPDM to partially remove and reinstall a RPCM on the P1 Truss. MCC-H plans to swap the failed RPCM with a spare from the P3 Truss on Wednesday.”

UPDATE: Engineers have noticed an obstruction related to the RPCM, postponing Wednesday’s task.

“Today’s operations have stopped. Robotics ground controllers have been operating the SPDM (Special Purpose Dexterous Manipulator) through a functionality & robotic maintenance demonstration program of grasping, unbolting, extracting, reseating & installing an RPCM (P11A),” noted on orbit status. 

“Yesterday’s (Day 2) extraction step jammed the RPCM in its soft dock position due to an underestimated pulling force applied by metal spring finger gaskets used for EMI (Electromagnetic Interference) shielding (34 lbf instead of expected 20 lbf). A plan has been proposed to reattempt the operations later with and without FMA (Force/Moment Accommodation) or line tracking enabled.

“Russian thrusters are disabled during SPDM operations. The test extraction may be reattempted tomorrow (22/07) but a new date for the RPCM replacement has not been set.”

UPDATE 2: Ops remain on hold (July 27) due to continued evaluations.

A timeline of additional tasks have been mapped out, pending the success of the RPCM R&R task.

With the ISS close to having its extension to 2020 confirmed via the ongoing political process – along with the potential of another extension to 2025, maybe even as far out as 2028 – Dextre can look forward to a large amount of job security.

He can also look forward to being joined by another of his Canadian relatives during the final shuttle mission, when the OBSS becomes a permanent fixture on the ISS.

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