Why a robot that sweats is a big breakthrough

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At 51/2 feet tall, its face fixed between a grin and a grimace, the humanoid robot Kengoro is something of a synthetic Arnold Schwarzenegger. That is not to say the machine, created by the University of Tokyo's JSK laboratory, is a recursive time-traveling assassin. Rather, Kengoro is a sweaty beefcake among metal bipeds.

The average two-legged machine does not have Kengoro's stamina. For a svelte robot, Kengoro is strong, powered by 108 motors. Thanks to this large number of motors plus a unique metal frame, Kengoro can hammer out push-ups for 11 minutes straight.

But Kengoro's true superpower, so to speak, is that it can perspire.

Were Kengoro flesh and blood, its feat of upper-body strength would not merit a second glance from the keepers of the Guinness World Records. When American Charles Linster set a then-record for nonstop push-ups, in 1965 — 6,006 push ups in a row — his groundbreaking exercise lasted more than three hours.

For the average robot, however, 11 minutes of such exertion could spell hot, hot catastrophe.

Like humans, robots generate heat when they perform tasks. Overheating may lead to failure. As engineers from Carnegie Mellon University and Worcester Polytechnic Institute noted in 2015, in a postmortem analyzing the three-year DARPA Robotics Challenge, successful roboticists "put a lot of emphasis on heat dissipation and thermal management in their robot and control system designs."

To that end, robots are designed with heat sinks, fins, fans and other cooling systems, although these often require their own power sources or add bulk to a frame. Not Kengoro. The robot's passive water-based cooling system is three times more efficient than using air to cool, according to IEEE Spectrum. (A dedicated radiator might still do a better job than either, IEEE noted, but would add weight and expense while decreasing the space reserved for motors.)

The University of Tokyo lab — famous for its humanoid robots, like one that mimics human musculature or the one that does the dishes — presented the machine recently at the Institute of Electrical and Electronics Engineers/Robotics Society of Japan International Conference on Intelligent Robots and Systems, under the title "Skeletal Structure with Artificial Perspiration for Cooling by Latent Heat for Musculoskeletal Humanoid Kengoro." The artificial perspiration in question is a cup of deionized water, a purified type of water that lacks dissolved minerals and salts. This water bubbles through Kengoro's porous frame and must be replaced roughly every 12 hours.

To create Kengoro's aluminum skeleton, the roboticists used a laser to bond metal powder into bones of varying density. Some of those support structures had few holes, but others were riddled with tiny gaps. Filling the metal bones with water kept Kengoro chill. (As horses and humans can appreciate, when sweat evaporates as a gas from our skin, the vapor takes heat with it.) Smart layering of the holey structures allowed the water to run through channels within Kengoro's frame, pass to an outer level and evaporate, all without a moist robot dribbling on the floor.

"Usually the frame of a robot is only used to support forces," University of Tokyo's Toyotaka Kozuki said in an interview with IEEE. "Our concept was adding more functions to the frame, using it to transfer water, release heat, and at the same time support forces."

Scientists are exploring new methods to keep things chill with tiny holes, and those methods are not just for mechanical bipeds, either. As Stanford University researchers demonstrated in September, a new type of plastic, breathable fabric with nano-sized pores could wick out bodily radiation — the ideal cooling duds for increasingly oppressive summers.


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