RRM Sets High Bar for Accuracy

Images above: A variety of looks throughout the process of NASA's robotic refueling mission, from its preparation for flight to a view of the Dextre robot transferring and installing the robotic refueling module onto its permanent home on the International Space Station on September 2, 2011. Image credit: NASA

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 (ISS) could lead the way to a new wave of future missions. The first Robotic Refueling Mission (RRM), a collaboration between 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.

Breaking Barriers
The Satellite Servicing Capabilities Office (SSCO) at NASA Goddard was anxious to prove their 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 multi-million dollar satellite assets already in space. Demonstrations onboard the ISS would validate their tool designs including cameras and sensors, the fuel pumping system, and robotic task planning.

The SSCO painstakingly researched and developed their payload, a 550 lb cube shaped RRM 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, with an initial design to never be accessed in space, is triple sealed and covered with a protective blanket. The RRM module provides the components (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 RRM would demonstrate the end-to-end refueling process spanning 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, and scheduling tests and reporting the results. He discusses the RRM mission, “The standard operating procedure has been to launch a satellite and let it go. We want the chance to prove these satellites can be revived when running low on fuel. Once on the ISS, 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.”
 

All About Accuracy
Intense RRM testing 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 RRM task 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 ISS, and placed it at the end of their in-house, industrial robot. Based on this setup, the next step was the creation of prototype tools to perform work on the RRM payload.

The robot has one camera in the nose of the gripper, so there was a need for extra cameras, as it requires more than one view to accomplish most of the tasks. When the project started, the team used simple, manual calculations to determine the camera angles. Once their 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-G 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 3D data via the probe, even in hard to reach areas due to the arm’s patented infinite rotation of the principal axes.

Put to use immediately was the portable arm, in order 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 was 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 they returned to the lab, a technician inspected the cameras once 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,” Roberts states. “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 shipping the flight unit to the Kennedy Space Center, every critical feature was digitized, measured, and documented, including the clearance between tools and structure, the spacing between bolts, openings, targets, etc. Once the spare came, use of the same inspection routine gave the engineers full confidence the spare was truly identical to the version heading into space.

Before the flight unit was shipped to the Kennedy Space Center, every critical feature of the RRM was digitized, measured, and documented, including the clearance between tools and structure, the spacing between bolts, openings, targets, and more. Image Credit: NASA

Portable Metrology Stands Up
Many departments at NASA Goddard, for dimensional control of their work, utilize laser trackers. The Leica laser tracker, a portable coordinate measurement system (PCMM) that maintains high precision over large distances, is 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 RRM 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 RRM Payload. A trainer from Hexagon Metrology arrived on-site to cover the basic concepts of laser tracking, and created a special course measurement plan with the inspection tasks required by the RRM project. The team would later expand and refine the program with more information for user-friendly interactions.

“Alignment is absolutely critical for robotic operations,” states 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 3D data down to four decimal points. There were many cases where the laser tracker helped in developing procedures that would eventually find use in orbit.

“The Romer arm alone saved a serious amount of time. It traveled all over NASA Goddard to the clean tent, dark room, 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,” Lotocki concludes.
 

Safety with Precision
One of the robotic servicing tasks is to pull off a cap, roughly the size of a white board 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?

Roberts explains, “We have to guarantee that when the robot turns the cap, it does not lose hold of it. 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 is a couple of seconds of time delay. The tool is at the end of an 11-½ft arm, attached to a 4ft robot body, which in turn attaches to the end of a 58ft 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 tracking device works in tandem with a Leica laser tracker for remotely controlled measurement of X-Y-Z coordinates and rotation angles. During the testing, the robot operator would watch precision data streaming from the laser tracker, revealing misalignment of the robot with the object requiring work. The team calculated if they were off by 2.5 deg, they could still safely accomplish the cap-turning task. If they were off the mark by 3 deg, the tool may internally bind which means there is a chance that the task can not be accomplished.

“Using the Leica T-MAC, we were able to characterize robot performance such as straight line deviation. 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 12ft robot sits on a giant stand, and we are not allowed to go into the robotic workspace during tests. The laser tracker can perform in that environment as long as there is line of sight,” Lotocki states.
 

A Sentimental Salute
The RRM module in all of its precision glory arrived safely at its destination. As we wave a sentimental good-bye to the Space Shuttle, new history begins. In July 2011, astronauts Mike Fossum and Ron Garan spacewalked the RRM 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. To watch the progress, visit ssco.gsfc.nasa.gov.

NASA Goddard Space Flight Center
Greenbelt, MD
ssco.gsfc.nasa.gov

Hexagon Metrology Inc.
Elgin, IL
hexagonmetrology.us