A Robotic Approach to Refueling Satellites

In February 2014, as part of NASA's Remote Robotic Oxidizer Transfer Test (RROxiTT) project, the US space agency placed a seven-axis robot at the Kennedy Space Center. But the engineer controlling the robot sat more than 800 miles away at the Goddard Space Flight Center in Greenbelt, Maryland. The goal was to demonstrate the ability to robotically transfer oxidizer to a satellite valve in flight-like conditions with the teleoperated commands being sent over long distances.

Satellites operate on a limited amount of fuel once launched and in orbit. The impetus for putting together a test like RROxiTT was equal parts innovation and cost-savings. If NASA could bring hazardous fuel to satellites already in space and safely and successfully transfer that fuel, then the space agency could potentially extend an orbiting spacecraft's operational life.

Controlling Motion From Afar

"The latency introduced by control from this distance adds complexity to the task," said Alex Janas, the lead robotic operator for RROxiTT. The approximately 800-mile distance between Greenbelt, Maryland, and the Kennedy Space Center simulated the time delay between Earth and a spacecraft.

NASA's operations required a very low level interface to the robot's servo system. The control architecture provided NASA with the necessary performance to carry out this important mission. As a result, each of the robots that NASA is using for these projects is being controlled via a low-level, PC-based control platform. This controller allows NASA to update the robot trajectory at a much faster rate than traditional robot controllers. The SIA robots are not flight hardened and will not be deployed in space but instead the robots are intended as earthbound simulators and development platforms.

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NASA chose these particular robots because they offer kinematically redundant arms, much like the flight arm the agency already has deployed in space. A robot is kinematically redundant if it possesses seven or more degrees of freedom. For example, think of a human's arm from shoulder to wrist. The human arm possesses seven degrees of freedom (7DOF) with each degree of freedom being a direction in which independent motion can occur. The shoulder and wrist each offer 3DOF; the elbow has 1DOF. That means if you hold a pencil with your hand, you can keep the point in one place while changing where your arm is in space. With a 7DOF robot, you maintain a given position and orientation of the tool tip independent of configuration, or location of the elbow. And this is what NASA desired in simulating the dexterity needed for a satellite refueling effort.

In contrast, with a 6DOF robot, like the kind you might find on an industrial assembly line, the person using the robot cannot choose the configuration of the arm. It is a result of the choice of the position and orientation of the tool tip. This limitation of 6DOF robots means that they are often hampered by interference with the environment or with other robots. As a system automates a series of processes using 6DOF robots, the opportunity for interference grows geometrically. The 7DOF robot, on the other hand, snakes its way to where it needs to be.

This SIA50 robot employed by NASA for its RROxiTT project can handle a 50 kg payload. The robot has 2,597 mm of vertical reach and 1,630 mm of horizontal reach. At its widest point, the robot is 633 mm. And the robot's dexterity enables it to reorient its elbow without affecting its hand position or causing "self-interference," which is when its arm may attempt to move in a path that crosses part of its own structure.

Another reason NASA opted for the SIA50 was cost. Building a duplicate flight arm that was rated for earth gravity would have cost considerably more. Instead, NASA obtained the SIA50 for a six-figure sum and could successfully develop and perfect techniques on the ground.

As for the intersection of engineering and control theory, which is a hallmark of mechatronics, the control architecture provided NASA with the necessary performance to carry out the 7DOF robot's refueling demonstration test. In support of these efforts, engineers constructed a new robot controller that could communicate with other upper level controls at a higher frequency. NASA was interested in visual servoing (VS) technology which taps feedback from a vision sensor to control the motion of the robot. NASA wanted to use visual servoing at a high speed because, like catching a pen as it rolls off a desk, connecting a refueling arm to an orbiting satellite means hitting a moving target. To do that, the robot has to react to visual feedback very quickly. No standard robot controller could accomplish that. With NASA's implementation, all the VS work was performed on an external Linux PC and the software provided gave the robot commands at the joint level.

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NASA wanted to apply its thorough understanding of the dynamics in the satellite refueling system to the robot control scheme. The low level interface gave NASA the ability to see what was happening within the parameters of the system, enabling the NASA engineers to have a better understanding of how the SIA50 behaved during each task. If the NASA team saw the robot jittering, they could see the root source of that delay. For instance, if an engineer was streaming positions into the robot to put the SIA50 arm into position for refueling, the engineer could see at what point the streaming slowed.

NASA called the February test a success. And NASA's Satellite Servicing Capabilities Office (SSCO) is developing technologies and capabilities for ongoing maintenance of on-orbit satellites. In fact, NASA has a number of SIA robots of differing payloads. The agency has SIA10, SIA20, and SIA50 robots deployed both at the West Virginia Robotics Technology Center (WVRTC) and at the Servicing Technology Center at Goddard Space Flight Center. NASA is using the robots for development of tools and methods associated with satellite servicing.

"While the RROxiTT is aimed at solving a problem in space, the implications of this technology for remote teleoperation technology for Earth-bound operations should not be overlooked," said Brian Roberts, robotic demonstration and test manager at the Goddard Space Flight Center. "There are many applications handling biohazards, fissile material, or other substances where robotic teleoperation offers the promise of improved human safety."

The same year that humans first set foot on the Moon, in 1969, Yaskawa engineer Tetsura Mori coined the term mechatronics. Although Mori did not help with the first moonshot, the heritage of mechatronics is very much helping NASA to develop robotic refueling technologies for satellite missions.

Erik Nieves is technology director for Yaskawa Motoman.

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