JPL uses metallic glass to make better robot gears
Bulk metallic glass, a metal alloy, doesn’t get brittle in extreme cold. That makes the material perfect for robotics operated in space or on icy planets. (Click to enlarge) Photo Credit: NASA/JPL-Caltech
NASA’s robotic spacecraft and rovers face a number of hazards as they move out into the Solar System on missions of exploration. Extremely low temperatures can make metal parts brittle, particularly gears, which are vital to the operation of robotic arms and other crucial moving parts. As the space agency plans future missions to frigid locales such as Jupiter’s icy moon Europa, new materials that can withstand these extreme conditions must be developed.
Douglas Hoffman, a technologist at NASA’s Jet Propulsion Laboratory (JPL), and his team are working to build better gears using bulk metallic glass (BMG), a specially crafted alloy with properties that make it ideal for robotics.
“Although BMGs have been explored for a long time, understanding how to design and implement them into structural hardware has proven elusive,” said Hofmann. “Our team of researchers and engineers at JPL, in collaboration with groups at Caltech and UC San Diego, have finally put BMGs through the necessary testing to demonstrate their potential benefits for NASA spacecraft. These materials may be able to offer us solutions for mobility in harsh environments, like on Jupiter’s moon Europa.”
The key to BMG’s unique properties is its atomic structure. Metals have an organized, crystalline arrangement; however, if you heat them until they melt, the atoms become randomized. If you cool the molten metal quickly enough, about 1,832 degrees Fahrenheit (1,000 degrees Celsius) per second, you can trap the atoms in their non-crystalline “liquid” form. This produces a random arrangement of atoms with an amorphous or non-crystalline microstructure. The material is technically a glass and can flow easily and be blow-molded when heated, just like windowpane glass. Metallic glasses were first developed at Caltech in 1960. Since then, they have been used to manufacture everything from cellphones to golf clubs.
One of the attractive properties of BMGs is their low melting points. This allows parts to be cast using injection-molding technology, similar to what is used in the plastic industry, but with much greater strength and wear-resistance. MBGs also do not get brittle in extreme cold. This quality makes them ideal for the kinds of robotics done at JPL.
Gears made from BMGs can be “run cold and dry”: preliminary testing has demonstrated strong torque and smooth turning without a lubricant, even at –328 degrees Fahrenheit (–200 degrees Celsius). This can be a power-saving advantage for robots sent to frozen landscapes. NASA’s Mars Curiosity rover, for example, must expend energy heating up grease lubricants every time it needs to move.
“Being able to operate gears at the low temperature of icy moons, like Europa, is a potential game changer for scientists,” said R. Peter Dillon, a technologist and program manager in JPL’s Materials Development and Manufacturing Technology Group. “Power no longer needs to be siphoned away from the science instruments for heating gearbox lubricant, which preserves precious battery power.”
Hoffman and his colleagues also investigated using BMGs to lower the cost of manufacturing strain wave gears. This type of gear, which includes a metal ring that flexes as the gear spins, is tricky to mass produce and very common in complex robots.
Hoffman’s research indicates that BMGs can be used to manufacture these gears at a fraction of the cost of their steel versions without sacrificing performance. This could dramatically reduce the cost of robots that use strain wave gears because they are often their most expensive part.
“Mass producing strain wave gears using BMGs may have a major impact on the consumer robotics market,” Hofmann said. “This is especially true for humanoid robots, where gears in the joints can be very expensive but are required to prevent shaking arms. The performance at low temperatures for JPL spacecraft and rovers seems to be a happy added benefit.”
An example of a strain wave gear, also known as a harmonic drive – one of the most expensive types of gears used in high-precision robotics. As the gear turns, the flexible ring inside it squeezes, becoming an oval shape. Photo Credit: NASA/JPL-Caltech
Jim Sharkey
Jim Sharkey is a lab assistant, writer and general science enthusiast who grew up in Enid, Oklahoma, the hometown of Skylab and Shuttle astronaut Owen K. Garriott. As a young Star Trek fan he participated in the letter-writing campaign which resulted in the space shuttle prototype being named Enterprise. While his academic studies have ranged from psychology and archaeology to biology, he has never lost his passion for space exploration. Jim began blogging about science, science fiction and futurism in 2004. Jim resides in the San Francisco Bay area and has attended NASA Socials for the Mars Science Laboratory Curiosity rover landing and the NASA LADEE lunar orbiter launch.
