Dextre grounded – engineers evaluating potential failure on ISS robot

by Chris Bergin

An issue with Canada’s Special Purpose Dexterous Manipulator (SPDM) robot “Dextre” is being evaluated, after engineers identified a potential failure mode in the SPDM Power Switching Init (PSU). As a result, the robot will remain in its current position attached to the Destiny Lab until further notice.

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).

The robot arrived on the International Space Station (ISS) during STS-123.

The preliminary information into the latest issue with Dextre – which was noticed last Friday – relates to a possible single-point failure mode with the SPDM Power Supply Unit.

“On Friday afternoon we received data that MDA 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 on L2 on Tuesday.

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

Dextre was due to be put through its paces this month – as early as this week – via the SSRMS. However, this is now postponed until further notice, due to a concern the issue may lead to Dextre being trapped on the end of the Station’s robotic arm. Such an issue would impact on February’s STS-119 mission.

“During a teleon, it was determined that this failure could result in trapping the SPDM on the SSRMS, or trapping the SPDM on the MBS (Mobile Base System),” added an opening memo. “Both of which would present significant impacts to 15A (STS-119).

“In addition, our current flight rules prohibit us from using the SSRMS to maneuver SPDM unless we are able to release the SPDM (so that the SSRMS is free to perform higher priority operations).

“Given the uncertainty associated with this failure its potential impacts and recovery actions, it seemed to me that taking the SPDM off the Lab PDGF (Power and Data Grapple Fixture) exposes us to risk of trapping the SPDM on the SSRMS.”

As a result of this potential concern, engineers agreed that keeping Dextre on its current location would be prudent, until they can be assured the failure does not affect the ability to relocate the SPDM within their current set of rules.

“So for now, until we are directed otherwise, we expect that the SPDM operations planned for next week will be deferred,” added the memo. “We will start looking at what that does to the checkout plan and the road-to-RPCM (Remote Power Control Module) R&R over the next few days.”

Another memo, written on Tuesday, explained the issue relating to Dextre’s PSU – in more technical detail.

“SPDM PSU has two strings, prime and redundant. The concern is that the redundant power supply relay could fail closed. It’s a FET controller, so I presume we’re talking a FET Controller Hybrid (FCH) type failure,” added another memo on L2.

“If this happens, you’ve tied two non-parallel DDCUs together and could potentially damage the RPC on the redundant power leg due to high current flowing backwards through it. Best case is the RPC sees the high current and trips off. Or other equipment could be damaged as the two DDCUs tied together are both grounded to structure.

“It gets worse when SPDM is based on the SSRMS since here you could fry SSRMS components plus potentially have a hot LEE causing sparking and arcing when you try to grapple. Could end up with SPDM stuck at the end of the arm to avoid this.

“Based on this possibility, the robotics community has recommended cancelling planned SPDM OCR session #2. I concur with that recommendation. Disappointing for everyone involved in this effort, but prudent.”

Further notes of the forward plan were mentioned, although engineers are still working on the evaluations.

It may yet be found that the issue is workable, and will not result in any negative impact on operations. However, due diligence in understanding the potential fault path will continue until engineers are confident Dextre can begin its important role on the orbital outpost.

More will follow on this story as information arrives.

L2 members: All documentation – from which the above article has quoted snippets – is available in full in the related L2 sections, now over 4000 gbs in size.

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