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Robotics work stalled by spacewalks, stuck spring

Posted: August 8, 2010

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The first maintenance task for a Canadian space robot is on hold until astronauts finish up urgent repairs to the International Space Station's external cooling system.

File photo of Dextre. Credit: NASA
The Canadian-built Dextre robot, outfitted with two 11-foot-long arms and a toolkit, was supposed to pull out a Remote Power Control Module, or RPCM, from one section of the space station's backbone structure and swap it with an identical unit from another part of the truss.

The changeout would overcome a problem with one of the module's power ports and restore the system to full functionality, a NASA spokesperson said.

Launched in 2008, Dextre's arms include tool-grasping grippers and arm joints to give the robot human-like skills for repair work outside the space station. Officials hope the robotic handyman can stand in for spacewalking astronauts for minor tasks.

RPCM units contain electrical switches and circuit breakers controlling the distribution of power across the orbiting complex. There are 72 such power controllers aboard the station, according to Kelly Humphries, a NASA spokesperson.

Each RPCM is about the size of a large dictionary and weighs more than 9 pounds.

But the robotic work was stalled when one of the power switchboards remained stuck inside its housing on the port side of the space station's truss. Engineers encountered the glitch July 20 as Dextre attempted to pull the unit out of the truss, then reinsert it during a rehearsal for the changeout, which was scheduled for the next day.

"We were a little surprised that the force to extract the RPCM was significantly more than what had been anticipated," said Mathieu Caron, the Canadian Space Agency's mission operations manager for the station's robotics systems.

NASA traced the problem to a spring used for electromagnetic interference shielding, according to Caron.

"Just to put things in context, we were approaching the operation expecting a 5-pound force extraction. But the design requirements levied on the RPCM say that extraction force must be limited to 20 pounds. What we did is we started pulling, pulled a little harder, and then when we reached the 20-pound (limit), we elected to stop," Caron said in an interview with Spaceflight Now.

Caron said engineers aborted the operation to ensure the power unit could withstand stronger pulling. The forces are well within the operating envelope of Dextre, which could apply up to 50 pounds of force.

"Dextre is performing exactly as it should be, per design," Caron said. "We were just thrown a curveball, and we're lucky because of the flexibility in the system, we can look at it and we can go beyond what the design was in the first place."

NASA says Dextre will have to apply about 34 pounds of force to overcome the snagged spring.

"The bottom line was that we had to pull significantly harder and presumably push harder in order to reinsert the RPCM," Caron said. "We were able to verify through analysis that Dextre is able to apply that force, and more, both in the extraction and insertion directions."

Ongoing analysis will determine just how much force the power controller can withstand. Another question is whether a specially-designed tool linking Dextre's grippers with the electrical box is strong enough for higher force numbers.

Engineers are also running figures on potential oscillations Dextre could impart on the station's robotic arm, also called Canadarm 2. For this work, Dextre is mounted on the tip of the arm, which operates from a rail car that slides up and down the length of the station's truss backbone.

The concern is the arm could swing back and forth when Dextre finally frees the power controller.

"We're going to have that twang effect when that energy is released," Caron said. "We want to make sure that, as result, we won't have too much oscillations and understand the loads that it will impart on the RPCM."

Engineers could soon be ready to clear Dextre for another try, but a series of unplanned spacewalks to replace a faulty ammonia pump takes top priority. After running into trouble during a spacewalk Saturday, astronauts will likely stage two more excursions to finish up the repairs and restart half of the station's external cooling system.

The spacewalks are gobbling up resources, Caron said, so Dextre's work is still several weeks away.

The station's six residents are not involved in the module swap, but flight controllers on the ground are crucial for Dextre's tasks.

A breakdown of Dextre's parts. Credit: CSA/MDA
"Dextre will be operated through supervised autonomy," a NASA spokesperson said. "The entire operation is broken up into small segments that Dextre executes on its own with the (robotics) team in Mission Control...initiating each sub-operation, monitoring its execution, and verifying its completion before authorizing Dextre to proceed."

Commands are sent to Dextre from Houston, while Canadian engineers in St. Hubert, a suburb of Montreal, stand ready for quick analysis.

If officials decide Dextre needs extra stability to pull out the electrical circuit breakers, the robot could use its other arm to grasp part of the station's structure.

"That way if you pull harder, you don't necessarily deflect the Canadarm 2," Caron said. "You're actually holding yourself steady with the other arm. In order to follow that course of action, we would have to completely revisit the sequence that we used for the RPCM swap, which is not impossible, but it would push it a little further. It may be where we end up, but before we get there, we want to ascertain the situation for the current plan."

Such a decision would require replanning the repair.

"The sequence would get much more complex," Caron said. That would take a little more time. I don't think we would be doing it later in August if we had to stabilize."

The power system work was the final step in a long series of tests to certify Dextre for service aboard the space station.

"Since Dextre's arrival on-orbit, we've had a number of commissioning sessions, where we have progressively asked more and more of Dextre, starting off with moving the arms in free space and moving into contact operations and so on. This was basically the final stage of its commissioning," Caron said.

If the RPCM swap gets delayed much further, officials may opt to shelve the plans and focus on planning for a more important task in early 2011, when Japan's second automated cargo ship arrives with supplies at the space station.

Dextre is required to remove a cargo container and flex hose rotary coupler from the ship and place them on the station, according to the Canadian Space Agency.