Friday , October 23 2020

Weirdly enough, it's going to take some water.

Suction Cups Kinda Suck. This Wall-Climbing Robot Could Make Them Better

Wall-climbing robots that scale the sides of high-rise buildings and clean the glass aren’t new. Neither are the Spider-Man-like robots that drive around industrial environments and inspect boiler tubes, propane tanks, fuel silos, and more from the sides of the surfaces.

But while these bots have been around for years, they usually have one fatal flaw: If any of the environments they touch have even just a hint of texture or roughness, the machines instantly fail. That’s because texturing interrupts the vacuum seal that sits between the suction cups and the surface on which the robots operate.

To build robots that can better adhere to challenging facades, then, researchers from Zhejiang University in Hangzhou, China have reimagined the suction cups these bots use to grip to walls.

When a suction cup sticks to a rough surface rather than a smooth one, it leaves gaps that a sealing ring alone can’t close tightly enough, say Kaige Shi and Xin Li, the researchers behind the new study, which was published in Physics of Fluids. Air can then escape from the outside environment into the vacuum zone, changing the pressure inside and decreasing the vacuum seal until it’s completely destroyed.

Shi and Li took that concept and built a robot that could climb even rough, unfinished exterior walls of buildings. Using water and the basics of centrifugal force, the scientists were able to overcome any leakage by creating a high-speed, rotating ring of water that could maintain the vacuum, despite surface restrictions.

Exactly how practical is it for wall-climbing robots to use water this way? The jury is still out. After all, imagine if the robot ran out of water supply while cleaning the 60th floor of a high-rise. But in the meantime, this idea of extending the suction cup’s vacuum ability is a clever approach.

It’s frustrating when a suction cup just won’t stick to a given surface, whether that’s the textured tile in your shower or a coating on a pipe. In both cases, rough surfaces are to blame. That’s not because certain textures are inherently difficult for suction cups to initially grip onto—your waterproof AM/FM radio might perfectly stick to the shower wall before ultimately falling to the ground—but rather because textured surfaces mess with the seal that suction cups rely on.

Think about an exterior brick wall on a home. You know a suction cup wouldn’t last five seconds on that kind of surface, but the reason why isn’t so intuitive. It all comes down to the flow of air from the outside atmosphere into a vacuum zone, driven by pressure differences. Le Chatelier’s principle tells us that changes in temperature, pressure, volume or concentration will result in predictable opposing changes in a given system as it tries to achieve a new state of equilibrium. Here, the pressure inside the vacuum slowly adjusts to the atmospheric pressure as more air leaks inside.

In the figure above, vacuum leakage is determined by the air flow from the atmosphere to the vacuum zone, driven by pressure differences at the boundary, which is illustrated with a broken line. For vacuum leakage to occur, two conditions must be satisfied:

While scientists have certainly created better seals that can slightly deform to close the gaps between the sealing ring and the rough surface, that only marginally helps. When the surface you’re working on gets rougher and rougher, the gaps in the seal become larger, and the flow resistance of those gaps becomes smaller. That means the flow rate of the outside atmosphere increases and eventually destroys the vacuum chamber.

Shi and Li created a suction cup that relies on a rotating stream of water to maintain a seal over any surface. This new method for preventing vacuum leakage eliminates any pressure difference at the boundary of the vacuum zone. The researchers call it the “zero pressure difference (ZPD) method.”

“In order to eliminate the pressure difference at the boundary of the vacuum zone, the pressure at the boundary must be equal to the atmospheric pressure, while a high vacuum is maintained in the zone,” Shi and Li write in their research paper. So the scientists wanted to create a stable pressure gradient near the boundary line of the suction cup.

As shown in the image below, their ZPD method uses a rotating layer of water on the outside of the vacuum zone to create a pressure gradient. Inside the water layer, there’s a high vacuum. Pressure increases radially and reaches the same level as the atmospheric pressure outside the water layer. Since there’s no pressure difference at the boundary anymore, the second condition for vacuum leakage is then broken.

Shi and Li designed a new suction cup with their zero-pressure difference method in mind. Both the regulator and the suction unit were 3D-printed. Similarly to other suction units, a nitrile foam rubber ring is embedded on the outside periphery of the chamber. A motor drives a fan that is fixed in the chamber. A micro-vacuum pump is used to evacuate the initial air, creating a vacuum zone in the chamber. The water in the reservoir is supplied by an outside water source.

Shi and Li tested their new suction idea through three apparatuses, including a Spider-Man-style robot (shown in the video above), a bot that climbs walls with the new zero-pressure difference suction cups, and a robotic arm. In each instance, you can see water is pushed out of the device when the suction cup is moved. In practice, that means a wall-climbing robot using this technique will need a whole lot of water to roam around.

Meanwhile, another kind of gecko-like robot out of Simon Fraser University in British Columbia uses Van der Waals forces to stick to walls. This bot’s tank-like tracks are covered with a dry adhesive, a polymer that resembles silicon and allows adhesion without added chemicals or energy. Molecules in the substance are temporary dipoles, meaning they have both a positively charged side and a negatively charged side. The charged poles are attracted to their corresponding opposites on the wall that the robot is climbing.

For the new class of wall-climbers to flourish, researchers like Shi and Li must find a way to let the robots store any necessary water inside the apparatus, rather than connecting the machines to a water supply. Whether or not that will ever be commercially viable is unclear. But for now, we know science sticks.

This Article was first published on popularmechanics.com

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