IF ROBOTS are ever going to be welcome in the home they will need to become a lot quieter. Building them with artificial muscles that run on hydrogen, instead of noisy compressed-air pumps or electric motors, could be the answer.
Kwang Kim, a materials engineer at the University of Nevada in Reno, came up with the idea after realising that hydrogen can be supplied silently by metal hydride compounds.
Metal hydrides can undergo a process called reversible chemisorption, allowing them to store and release extra hydrogen held by weak chemical bonds. It's this property that has led to the motor industry investigating metal hydrides as hydrogen "tanks" for fuel cells.
To make a silent artificial muscle, Kim and his colleague Alexandra Vanderhoff first compressed a copper and nickel-based metal hydride powder into peanut-sized pellets. They then secured them in a reactor vessel and pumped in hydrogen to "charge" the pellets with the gas. A heater coil surrounded the vessel, as heat breaks the weak chemical bonds and releases the stored hydrogen.
The next step was to connect the vessel to an off-the-shelf artificial muscle, which comprises an inflatable rubber tube surrounded by Kevlar fibre braiding. Two of these placed either side of a robotic joint can mimic the push/pull action of muscles by being alternately inflated and deflated (see diagram).
Turning the heater on and off controls the flow of hydrogen into the rubber tube, causing the muscle to move silently. Even better, the pair say, the muscle performs as well as those that run on compressed air systems (Smart Materials and Structures, DOI: 10.1088/0964-1726/18/12/125014). Importantly, the gas didn't leak.
"The system has biological muscle-like properties for humanoid robots that need high power, large limb strokes - and no noise," says Kim.
Yoseph Bar-Cohen, an engineer specialising in artificial muscle technology at NASA's Jet Propulsion Lab in Pasadena, California, says this is "a novel approach" for controlling artificial muscles. "It is an important contribution and increases the arsenal of potential actuators that may become available in the future," he says.
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