Gearing up space robots with metallic glass
In a quest to give robots human-like grace even in the frozen wastes of space, NASA's Jet Propulsion Laboratory (JPL) is looking at exotic gears made out of exotic materials. In a pair of papers, technologist Douglas Hofmann and his team describe how high-precision gears made of Bulk Metallic Glass (BMG) could lead to more graceful robots that cost less to build.
According to NASA, the key to these more dexterous robots is using a metal that's not really a metal – one that's technically not even a solid. In fact, it has more in common with glass than what we commonly think of as metal.
Metals have a crystalline structure, which is so common that such a structure is almost the definition of a solid. Glass, conversely, for all its hardness, isn't a solid. In engineering terms, it's what's known as a supercooled liquid. That is, when glass cools down from a molten state, it has little or no structure, and its atoms line up in random patterns instead of the geometric order of crystals. This lack of order gives glass many of its properties, including its strength and its tendency to shatter so spectacularly.
Discovered by Caltech in 1960, metallic glasses are special metals that are used in a variety of products and have a structure, or lack thereof, similar to glass. They're formed by melting metals until the atoms randomize, then suddenly cooling them by 1,832º F (1,000º C). This turns the metal into a solid so quickly that there isn't time for it to crystallize. The result is a metal that's a supercooled liquid like glass. When this amorphous metal, or metallic glass, is in batches larger than 1 mm long, it's called BMG.
Metallic glasses can flow like regular glass and can even be heated to a plastic state and blown like glass. They also have low melting points, so they can be cast with injection-molding technology like bottle glass or plastic, yet retain the strength and durability of metal.
The space agency is interested in BMGs because they don't become brittle in extreme cold and they can run without lubricants at -328º F (-200º C). This means robots made using gears fabricated from BMGs would work without the need for the power-hungry heaters needed on the Curiosity Mars rover to keep its lubricants warm.
BMGs are particularly useful for producing what are called "strain wave gears," which are a type of precision gear popular with roboticists. These harmonic gears are very compact, yet have very high gear ratios. However, they consist of a rotating elliptical core that, instead of simply turning, changes shape as it comes into contact with the circular outer section for brief periods.
This allows for making compact, very precise gears that are especially suited for humanoid robots, but the steel versions are tricky and very expensive to mass produce. The hope is that by using BMGs, it will be possible create strain wave gears that are not only much cheaper, but better able to handle extremely low temperatures.
"Although BMGs have been explored for a long time, understanding how to design and implement them into structural hardware has proven elusive," says 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."