Canadian engineers focus on suspect cable
BY WILLIAM HARWOOD
STORY WRITTEN FOR CBS NEWS "SPACE PLACE" & USED WITH PERMISSION
Posted: March 14, 2008
After extensive troubleshooting, Canadian robotics experts now believe a suspect data cable - not a software timing issue - is preventing computer commands from powering up a $209 million maintenance robot under construction aboard the international space station. Covering the bases, they uplinked a software patch early today to adjust the timing of the computer commands as originally planned, but, as most expected, the patch didn't work.
The special purpose dextrous manipulator, or SPDM - also known as Dextre - was launched disassembled, its arms, hands and torso mounted on a Spacelab pallet in the shuttle Endeavour's cargo bay. After the shuttle's docking with the space station Wednesday night, the pallet was pulled from Endeavour's cargo bay and mounted on the mobile base normally used to move the station's robot arm from one work site to another.
All of that went well. But fight controllers were unable to route electricity to the robot's components to power so-called keep-alive heaters. Engineers initially thought a software timing issue was to blame, believing power-up commands were timing out before the robot's electronics could fully respond. Similar problems have occurred in the past with the station's robot arm and transporter, which feature similar electronic components.
"We've had some experience in the past with timing issues and we thought, well, we can fix that with software," said Pierre Jean, acting space station program manager for the Canadian Space Agency. "We kicked that off right away, that was something we're very comfortable with.
"As we looked through it and we really delved into the design, it became apparent to our engineers across the team that the actual design of the cable was wrong. It passes the power, but the way it handles the data, it doesn't guarantee that data will be passed properly and returned. So it basically boils down to a design issue in the cable that was not discovered prior to this moment.
"I guess it's somewhat related to the fact that the hardware that Dextre is trying to mate to is the MBS (mobile base system), which was launched in 2002, so we don't have existing hardware on the ground. Some of the testing that was done was done in the simulation environment. There was lots of testing done, but it wasn't until the last day, really, that we looked at the design and we realized there was an issue there. That's why we're fairly confident, we have strong confidence, this is going to be resolved and shown to be the way it is, to be the smoking gun, if you will."
The cable in question runs between Dextre's latching end effector and the power and data attachment fitting holding the Spacelab pallet in place on the mobile base system. Engineers now plan to lock the station's arm onto Dextre late today, bypassing the pallet cable and supplying power and data directly to the robot's electrical systems.
"At this point in time, we're pretty confident that by 10 o'clock (Central Time) tonight we should have the answer to this particular question," Jean said.
During an overnight spacewalk, the first of five planned for Endeavour's mission, astronauts Richard Linnehan and Garrett Reisman attached gripper-like hands to Dextre's 11-foot-long arms. The arms, in turn, will be attached to the robot's torso during a second spacewalk overnight Saturday.
Jean said Dextre can survive the extreme temperatures of space without keep-alive power for 100 to 125 hours, much longer than engineers believe they will need to resolve the problem.
Today's spacewalk lasted seven hours and one minute, successfully preparing a Japanese module for attachment to the space station and beginning assembly of Dextre. The spacewalk began at 9:18 p.m. Thursday and ended at 4:19 a.m. today.
"Boy, I can't wait to take a shower," one of the spacewalkers said from inside the Quest airlock.
"You're going to be waiting a long time," his companion laughed.
This was the 105th spacewalk devoted to station assembly and maintenance since construction began in 1998 and the first of five planned by Endeavour's crew.