Station chief optimistic about Dextre software fix
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 13, 2008
Space station Program Manager Mike Suffredini said today he's optimistic a software patch will resolve a timing problem that engineers believe has prevented them from successfully routing power to the Canadian Space Agency's Dextre robot. Similar problems involving an electronic component common to the station's Canadian-built robot arm and its mobile transporter were resolved by similar patches that adjusted when power-up commands sent from the lab's robotic work station timed out.
Suffredini said the patch will be ready for installation Friday morning, after an overnight spacewalk by Richard Linnehan and Garrett Reisman to prepare a Japanese logistics module for attachment to the space station and to begin assembly of the special purpose dextrous manipulator, or SPDM, known affectionately as Dextre. The spacewalk was scheduled to begin at 9:23 p.m.
Dextre was launched disassembled, its arms, hands and central torso bolted to a Spacelab pallet. That pallet was moved from Endeavour's cargo bay and mounted on the station's robot arm transporter earlier today. Tonight's assembly work involves attaching hand-like grippers to each of the robot's 11-foot-long arms.
Without power, engineers will be unable to activate heaters to keep the robot's joints warm. But Suffredini said it would take at least five days without power to cause any problems and that he was confident the problem would be resolved long before then.
"From a time standpoint, we're just not real critical right now," he said.
Engineers believe the power-up problem involves the time needed for commands sent from the station's robotic work station to make their way through three amplifiers, multiple connectors and associated wiring before reaching a device known as a power distribution unit, or PDU, in the robot's torso.
"This PDU is a common component in the SSRMS (station robot arm) system and the mobile servicing system," Suffredini said. "And in fact, in both of those cases we had a timing issue and we went and made a patch to the RWS, the robotic work station, to fix the problem. The problem is, it sends out a command and says 'turn on' and then waits five seconds and starts commanding. If the thing hasn't warmed up and been ready to accept it so it can react, then (the RWS) goes 'oh, you didn't do what I asked you to do so I'm going to stop talking to you.'
"The problem is, all these things are wired differently, they go through a number of connectors, they go through a number of amplifiers. The clock starts ticking when the RWS sends out a command. In this case, it goes through three amplifiers, a bunch of wires, it gets to the PDU, the PDU starts thinking about it and meanwhile, the clock's ticking. At that point if it doesn't react right, the RWS says 'you're sick.'"
Suffredini said telemetry from the Spacelab pallet Dextre's components are mounted on shows the PDU is on and drawing power as expected. The problem, engineers believe, is in the software controlling the robotic work station. If the patch works, the issue will be resolved and the astronauts will press ahead with Dextre asssembly during a second spacewalk overnight Saturday.
In other developments, LeRoy Cain, chairman of NASA's Mission Management Team, told reporters this afternoon that analysis of launch imagery, an inspection of Endeavour's nose cap and wing leading edge panels and a preliminary assessment of the shuttle's heat shield tiles based on photographs shot by the station crew before docking Wednesday night indicate the ship's heat shield is in good shape.
No major problems have been identified and commander Dominic Gorie was told today there is no need for any additional, "focused" inspections. Cain said he expects the MMT to clear the heat shield for re-entry, after a few remaining, relatively minor questions are resolved.
One such open item is a very small depression, or ding, in the carbon composite material around the fitting that helps attach the shuttle's nose to the external fuel tank. Cain said the pit might represent a debris impact, but it is very small and not expected to require any additional attention.
"The team worked through the night, poring over the data from the scans that we get from the orbiter boom system as well as the data from the rendezvous pitch maneuver that we did. All of that data looks really good. The team has determined that we don't need any focused inspection, that is to say, we don't have any areas that we have any concern about that would require us to do a focused inspection."
He said NASA's Debris Assessment Team would continue to examine the data "and we anticipate being able to clear the vehicle for entry within a day or two."