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As Atlantis touched down for the final time at the Kennedy Space Center, it marked the poignant end of the Space Shuttle program. However, the precious cargo it left behind at the International Space Station could lead the way to a new wave of future missions. The first Robotic Refueling Mission, a collaboration between the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center and the Canadian Space Agency, has been set in motion to prove the process of in-orbit robotic refueling in space.
The Satellite Servicing Capabilities Office at NASA Goddard was anxious to prove its robotic capabilities were ready for prime time. In-orbit robotic refueling was a good place to start, as it could breathe new life into numerous multimillion-dollar satellite assets already in space. Demonstrations onboard the International Space Station would validate its tool designs, including cameras and sensors, the fuel-pumping system, and robotic task planning.
The Satellite Servicing Capabilities Office painstakingly researched and developed the payload, a 550-lb, cube-shaped robot refueling module that is roughly the size of a washing machine. The module breaks down each step of the refueling process into separate, testable tasks to access a satellite’s fuel valve. The valve, which is triple-sealed and covered with a protective blanket, was initially designed never to be accessed in space. The robot refueling module provides the components (e.g., protective thermal blankets, caps, valves, and simulated fuel), activity boards, and tools to practice those steps and remove any barriers. Once aboard the space station, the Robotic Refueling Mission would demonstrate the end-to-end refueling process during a period of two years.
Brian Roberts manages the robotic demonstration and test lab at NASA Goddard, and steers several projects, including robotic programming, building mockups, scheduling tests, and reporting the results. “The standard operating procedure has been to launch a satellite and let it go,” says Roberts. “We want the chance to prove these satellites can be revived when running low on fuel. Once on the International Space Station, we will fire up the robot three to five days a month. It will go over to the box, grab some tools, cut wires, move insulation, cut tape, and remove caps—all the things we believe can be successfully achieved on a satellite.”
Intense testing for the Robotic Refueling Mission lasted many months, and the team employed an articulating arm and laser trackers to provide the muscle needed for their various measurement and inspection requirements. Because every task for the mission requires a high level of robotic precision, building a mockup was the first order of business. The team had an engineering model of the robotic gripper used on the International Space Station, and it was placed at the end of their in-house industrial robot. Based on this setup, prototype tools were created to perform work on the Robotic Refueling Mission payload.
The robot has one camera in the nose of the gripper, so extra cameras were needed because more than one view is required to accomplish most of the tasks. When the project started, the team used simple manual calculations to determine the camera angles. Once the ROMER articulating arm was in-house, the measuring process changed dramatically. The lightweight, carbon-fiber arm duplicates the movement of a human arm, as it uses zero gravity counterbalance to offset the weight of the arm and probe. This system enables one-handed data acquisition from any position in the arm’s reach. Using the portable coordinate measurement machine (CMM), operators gathered 3-D data via the probe, even in hard-to-reach areas, due to the arm’s patented infinite rotation of the principal axes.
The portable arm was put to use immediately to measure the angle of the camera brackets and calculate the lens position relative to the tool tip and to the robot. The resulting data were accurate down to three decimal points. Additional evaluations were performed, and the tool designers were directed to set the adjustable camera brackets for all four tools, with two brackets and two cameras per tool, a total of 16 measurements.
When the flight tools arrived, the adjustable brackets were set and verified with the portable ROMER CMM, then drilled and pinned in the machine shop. Once the cameras returned to the lab, a technician inspected them again. “When trying to troubleshoot an issue with the operation on the space station, we must have identical tools to the flight unit so we can conduct the same testing,” says Roberts. “We had to ensure those cameras were perfectly placed. They are your eyes in space. In some cases, we did not have the luxury of building the tools at the same time, so the spare came later. The brackets ensured those matched.”
Before the flight unit was shipped to the Kennedy Space Center, every critical feature was digitized, measured, and documented, including the clearance between tools and structure, as well as the spacing between bolts, openings, and targets. Once the spare came, the same inspection routine was used to give the engineers full confidence the spare was truly identical to the version heading into space.
Laser trackers are used by many departments at NASA Goddard for dimensional control of their work. The Leica laser tracker, a portable coordinate measurement system that maintains high precision over large distances, is used for inspection, analysis, and component alignment. Typically used in harsh industrial environments, these particular laser trackers are well-known for their durability, a factor that ensures consistent and repeatable measurement results.
Brad Lotocki, a mechanical engineer for NASA contractor Jackson and Tull, was new to laser-tracking technology. A robot operator in the Robotic Refueling Mission test lab, he tested and developed robotic techniques and was heavily involved in metrology as applied to various tasks, measurements, alignments, and tool-development support for the Robotic Refueling Mission payload. A trainer from Hexagon Metrology arrived on site to explain the basic concepts of laser tracking and created a special course-measurement plan with the inspection tasks required by the Robotic Refueling Mission. The team would later expand and refine the program with more information for user-friendly interactions.
“Alignment is absolutely critical for robotic operations,” says Lotocki. Case by case, the team realized many potential areas where portable metrology helped. A robot operator does his best to align by eye, but the laser tracker is in the background streaming 3-D data down to four decimal points. There were many cases where the laser tracker helped in developing procedures that eventually would be used in orbit.
“The ROMER arm alone saved a serious amount of time,” says Lotocki. “It traveled all over NASA Goddard to the clean tent, darkroom, test lab, and other locations. The on-demand nature of the CMM played right into our rapid tool development. If the tools went into a vibration test, we would get them for 20 minutes after the session and quickly verify the camera brackets. We would then hand the tools off for testing in the thermal vacuum chamber, then check them again. The immediate accuracy—a quick setup and getting precision measurements—allows us to get tasks completed within our tight timelines.”
One of the robotic servicing tasks is to pull off a cap, roughly the size of a whiteboard marker cap. This exercise would entail the robot’s use of a tool with three spring-loaded fingers to move, turn, and capture the cap. To get it out of the tool, the arm moves up to a receptacle that spreads the fingers, and the cap falls into a little trash bag.
Sounds simple, right? “We have to guarantee that when the robot turns the cap, it does not lose hold of it,” explains Roberts. “There are many ‘what ifs.’ The folks in the space station safety team wanted more detail to be sure the module is safe to fly. This robot is in space, and there are a couple of seconds of time delay. The tool is at the end of an 11.5-ft arm, attached to a 4-ft robot body, which in turn is attached to the end of a 58-ft robotic arm. There is not a lot of motion out there, but the tool is not as precisely controlled as in an industrial setting.”
A series of tests were conducted using a Leica T-MAC (Tracker-Machine control sensor) attached to the side of the robot’s end effector. This 6 DOF (degree of freedom) tracking device works in tandem with a Leica laser tracker for remotely controlled measurement of XYZ coordinates and rotation angles. During the testing, the robot operator would watch precision data streaming from the laser tracker, which revealed the misalignment of the robot with the object being worked on. Team members calculated that if they were off by 2.5 degrees, the cap-turning task could be safely accomplished. If they were off the mark by 3 degrees, the tool might internally bind, which means there is a chance that the task cannot be accomplished.
“Using the Leica T-MAC, we were able to characterize robot performance such as straight-line deviation,” says Lotocki. “The laser tracker tells us how straight the line is and other data like linear draw length. We also use it to verify parameters of our robot machine-calibration routine. We could not perform these tasks accurately without the tracker. Our 12-ft robot sits on a giant stand, and we are not allowed to go into the robotic work space during tests. The laser tracker can perform in that environment as long as there is line of sight.”
The robotic refueling module in all of its precision glory arrived safely at its destination. As we wave a sentimental goodbye to the Space Shuttle, new history is being made as this story is written. In July 2011, astronauts Mike Fossum and Ron Garan spacewalked the robotic refueling module from the Atlantis cargo bay to the temporary platform on the International Space Station. We await news of the mission, eager for a new chapter in space innovation. You can watch the progress here.