Metallic Glass Coated Gears Making Cobots a Reality
By Strategic Research Institute on Sep 07, 2021 09:00 am
For all the space that robots have occupied in the popular imagination, the number of real world automatons remains limited due to cost and safety. In the mid 1990s, two Northwestern University professors patented an alternative concept under a new term cobots, collaborative robots, smaller, smarter, more responsive with tighter self-control and better manners all around. In the years since, leaps in artificial intelligence and sensors have made these friendlier robots a reality, but cost still prevents their widespread adoption. A strain wave gear converts the fast, low-torque rotation of an engine into slow, precise, forceful motion. As the oblong wave generator at the center spins, it deforms the flexspline around it, which engages with the teeth of a fixed outer spline. The interaction causes the flexspline to rotate in the opposite direction of the wave generator, moving only two teeth for each turn of the motor. The biggest cost drivers are gears.
Now, Pasadena California based Amorphology hopes to drop the price of cobots with advances originally made for NASA’s planetary rovers. Gears on NASA’s rovers, like most gears on Earth, are made of steel, which is both strong and wear resistant. But steel gears need liquid lubrication and oils don’t work well in frigid environments like the lunar or Martian surface. So, NASA’s Curiosity rover spends about three hours warming up lubricants every time it prepares to start rolling, using energy that could be used otherwise. With an eye toward solving this and other materials-related issues, Jet Propulsion Laboratory hired Mr Douglas Hofmann in 2010, then a research scientist at Caltech with a background in materials science and engineering. NASA funded a new metallurgy facility at Jet Propulsion Laboratory to explore alternatives for gears and develop new metal alloys.
Most metallic glass alloys form a hard, smooth surface. This gives metallic glass gears a long lifetime without the need for liquid lubricants. An emerging class of specially engineered materials called bulk metallic glass, also known as amorphous metals, are rapidly cooled from liquid to solid before their atoms form the crystalline lattice structure that is common to all other metals. Instead, the atoms are randomly arranged like those of glass, giving the materials properties of both glass and metal. Depending on their constituent elements, often including zirconium, titanium, and copper, they can be very strong, and because they aren’t crystalline, they’re elastic. Most compositions also form a hard, smooth ceramic oxide surface
Currently, the Cold Operable Lunar Deployable Arm COLDArm, a collaborative effort between Jet Propulsion Laboratory and the company Motiv Space Systems for lunar missions, is expected to use bulk metallic glass gears to operate in temperatures down to minus 290 degrees Fahrenheit without the need for a heating source.
But amorphous metals alloys are designed to have low melting temperatures, because to make a metallic glass you have to cool the alloy faster than it can crystallize. This low melting point, together with their native strength and the fact that their volume hardly changes upon solidifying, makes bulk metallic glasses easy to use in injection molding, which can dramatically reduce the cost of making parts like gears.
Most high-strength metals have high melting points. They can’t be cast with molds because, in molten form they would simply melt the mold. And steel needs to be rolled or forged to strengthen it, which also precludes molding. So, gears typically start as steel billets that are machined into their final shape. Tiny gears, like those for small cobots, are especially challenging and costly. Flexsplines are thin, flexible, cup-shaped gears integral to strain wave gears common in robotics. They’re typically cut, ground, and drilled from steel billets in a process that is long and costly. The flexspline on the right was injection molded from metallic glass in a cheaper, simpler process. The most difficult, expensive gear component to machine from a steel block is one of the most common in robotic arms: the flexspline, an extremely thin-walled, flexible cup with a toothed rim. This is the centerpiece of what’s known as a strain wave gear assembly, which offers better precision, higher torque, and lower backlash than other gear sets. This eliminates positioning errors that would be compounded in a robotic limb with multiple joints.
This is where molding with amorphous metals promises the greatest savings: it costs about half as much as machining strain wave gears from steel. Molding small, high-performance planetary and strain wave gears became the central business plan for Amorphology.