A Robot that Sweats? Engineers Try Bio-Inspired Heat Management

Researchers have designed a humanoid robot which sweats water through its metal frame to cool itself.

Despite all the amazing advancements that are achieved in robotics over the years, there are still many unsolved problems ahead. We have witnessed that many of these advancements have been achieved by stealing nature’s solutions. From light-activated robots based on stingrays to soft robotics inspired by octopi that are helping us understand robotic movement, engineers are borrowing from biology to achieve astounding results.

And it is amazing that these bio-inspired methods outperform those dreamt up by humans in many cases. A Japanese humanoid named Kengoro, which imitates sweating to cool its high-torque motors, is another example of these efficient bio-inspired techniques. It hopes to improve heat management, which is vital to advancing robotics.

Kengoro is a strong and durable humanoid that uses "sweat" for self-cooling. Image courtesy of JSK Lab/University of Tokyo via IEEE Spectrum.

Humanoids: Even More Vulnerable to Heat

When an electric current goes through a circuit, each element of the circuit produces some heat. Since the environment temperature can adversely affect the performance of an electronic circuit, we need to get rid of the produced heat by utilizing fans, heat sinks, etc.
Humanoid robots usually require high-torque motors. Handling the heat produced by these motors is so challenging that it can totally limit the performance of the whole system. The infrastructure of conventional cooling methods, such as fans and heat sinks, makes the robot too heavy. As a result, scientists need innovative solutions which can keep the system cool without compromising its weight.

Passive Water Cooling

Researchers at the University of Tokyo’s JSK Lab have proposed an interesting cooling technique. Inspired by nature, they have added the capability of sweating to their robot.
The idea is to incorporate a porous skeletal structure from which water can seep out. The water can cool components as it flows through tubing throughout the robot, but really provides a cooling effect when it evaporates off the surface of the structure.
This is quite similar to how the body sweats to make tissues cooler when working out. The conventional active water cooling has a big drawback: it requires fans, tubes, and radiators and hence takes up space on an already incredibly complex Kengoro (which is already filled with structural components, circuit boards, gears, and 108 motors). On the other hand, the new technique, called passive water cooling, eliminates fans and radiators and provides Kengoro with a much more efficient way of cooling.

A Frame Filled with Water (Just Like a Sponge)

To build a skeleton capable of sweating, researchers needed to manufacture permeable metals. They employed a manufacturing process which applies a laser beam to sinter aluminum powder. With this method, it is possible to arbitrarily choose the permeability of the manufactured metal by controlling the energy density of the laser.
The laser-sintering technique gives you enough control to manufacture a seamless piece of metal which offers different amount of permeability in different parts. This allows the researchers to direct the water to the parts of the robot which require more cooling. Moreover, they can choose to make the outer layer of the metal more porous than the inner region. This technique helps Kengoro to more easily evaporate the water into the air rather than just leaking it onto the floor.

The frame obtained by laser-sintering aluminum powder offers controlled permeability. Image courtesy of JSK Lab/University of Tokyo via IEEE Spectrum.

With a cup of deionized water, Kengoro can run for half a day. The new cooling technique allows Kengoro to continuously do push-ups for as long as 11 minutes. Experiments show that, although the new method is not as efficient as the conventional radiator-based active cooling, it is three times more efficient than air cooling. The design is also much more successful than simply flowing water through the interior channels of the frame.

The Tokyo lab has previously unveiled daily assistive humanoids which can do the dishes. These robots imitate the operation of body parts such as shoulders and knees. The 1.7-metre tall, 56-kilogram Kengoro, which is the sixth bio-inspired robot of this lab, goes further and mimics the sweating system of humans. Other designs of the lab focus on robots which can help the elderly.
The detailed explanation of this technique was presented at IROS 2016 in Daejeon, Korea: “Skeletal Structure with Artificial Perspiration for Cooling by Latent Heat for Musculoskeletal Humanoid Kengoro”.