Suction Cups Kinda Suck. A Wall-Climbing Robot Could Fix That

Wall-climbing robots that scale the walls of high-rise buildings to clean the glass aren’t new. Neither are the Spider-Man-like robots that travel about industrial areas and check boiler tubes, propane tanks, fuel silos, and more from the sides of the surfaces.

But while these bots have been around for years, they generally have one fatal flaw: If any of the environments they touch contain even just a trace of texture or roughness, the robots quickly fail. That’s because texturing disturb the vacuum seal that lies between the suction cups and the surface on which the robots work.

To construct robots that can better stick to tough facades, then, researchers from Zhejiang University in Hangzhou, China have reinvented the suction cups these bots utilize to hold to walls.

When a suction cup attaches to a rough surface rather than a smooth one, it produces gaps that a sealing ring alone can’t cover firmly enough, said Kaige Shi and Xin Li, the researchers behind the new work, which was published in Physics of Fluids. Air can then escape from the outside environment into the vacuum zone, altering the pressure within and reducing the vacuum seal until it’s totally broken.

Read More: How to Build a Robot

Shi and Li utilized that notion and constructed a robot that could climb even rough, unfinished outside walls of buildings. Using water and the basics of centrifugal force, the scientists were able to overcome any leakage by generating a high-speed, spinning ring of water that could maintain the vacuum, despite surface limits.

Exactly how realistic 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 level of a high-rise. But in the meanwhile, this notion of expanding the suction cup’s vacuum capabilities is a great technique.

Why Do Suction Cups Fall Off?

It’s irritating when a suction cup just won’t attach to a specific surface, whether that’s the textured tile in your shower or a coating on a pipe. In both circumstances, rough surfaces are to fault. That’s not because certain textures are intrinsically difficult for suction cups to initially cling onto—your waterproof AM/FM radio could flawlessly attach to the shower wall before finally tumbling to the ground—but rather because textured surfaces tamper with the seal that suction cups rely on.

Think about an outside brick wall on a home. You know a suction cup wouldn’t survive five seconds on that type of surface, but the reason why isn’t that simple. It all boils down to the flow of air from the outside environment into a vacuum zone, driven by pressure differences. Le Chatelier’s principle reminds us that changes in temperature, pressure, volume, or concentration will result in predictable opposing changes in a given system as it strives to establish a new state of equilibrium. Here, the pressure within the vacuum steadily adapts to the ambient pressure as more air leaks inside.

Text, Blue, Turquoise, Circle, Font, Line, Parallel,

In the image above, vacuum leakage is governed by the airflow from the atmosphere to the vacuum zone, driven by pressure differences at the boundary, which is depicted with a broken line. For vacuum leakage to occur, two criteria must be satisfied:

There must be a flow passage linking the atmosphere and vacuum zone (essentially a gap in the seal) (basically a break in the seal).
A pressure differential must exist at the edge of the vacuum zone.
While scientists have surely invented better seals that can slightly bend to cover the gaps between the sealing ring and the rough surface, that only marginally improves. When the surface you’re working on grows rougher and rougher, the gaps in the seal become greater, and the flow resistance of those gaps becomes smaller. That implies the flow rate of the outside environment grows and finally destroys the vacuum chamber.

The Zero Pressure Difference Method

This clip is imported from YouTube. You may be able to access the same content in another format, or you may be able to discover more information, at their website.

Shi and Li devised a suction cup that relies on a revolving jet of water to maintain a seal over any surface. This unique approach for avoiding vacuum leaking removes any pressure differential at the edge of the vacuum zone. The researchers term it the “zero pressure difference (ZPD) approach.”

“In order to eliminate the pressure differential 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 wrote in their research article. So the scientists intended to produce a steady pressure gradient around the borderline of the suction cup.

As seen in the graphic below, their ZPD approach employs a spinning layer of water on the outside of the vacuum zone to produce a pressure gradient. Inside the water layer, there’s a strong vacuum. Pressure grows radially and reaches the same level as the air pressure outside the water layer. Since there’s no pressure differential at the border anymore, the second criterion for vacuum leaking is thus violated.

With their zero-pressure difference approach in mind, Shi and Li came up with a novel suction cup design. The suction device and the regulator were both 3D printed. Nitrile foam rubber is embedded around the outside of the chamber in the same way as other suction units. A stationary fan in the chamber is powered by a motor. The original air in the chamber is removed using a micro-vacuum pump, resulting in a vacuum zone. The reservoir receives its water from an outside source.

Climbers of the Wall, Come Together!

A Spider-Man-style robot, a wall-climbing bot using the novel zero-pressure differential suction cups, and a robotic arm were all used to test Shi and Li’s innovative suction idea. When the suction cup is moved, you can see water being pushed out of the gadget. In the real world, a robot that uses this technology to climb walls will require a lot of water to move about.

Spider-like devices, such as this robot, can scale walls using suction cups, but that is not always the case. The firm Gecko Robotics, for example, utilizes its fleet of robots to examine thickness, cracking, pitting, and other types of deterioration in industrial settings, such as the insides of tanks, boilers, scrubbers, and pipelines. Rather than suction adhesion, these devices employ magnetic adhesion. This video was taken from YouTube and imported. On their website, you may be able to access the same content in a different format or extra information.

Van der Waals forces are used by another gecko-like robot developed at Simon Fraser University in British Columbia. On the bot’s tank-like tracks, a dry adhesive, a polymer that mimics silicon and permits adherence without the addition of chemicals or energy, is used. The substance’s molecules are transient dipoles, which means they have a positive and a negative charge on one side of them. Each of the robot’s charged poles is drawn to its matching opposite on the wall.

Researchers like Shi and Li must develop a means to allow the robots to store any necessary water inside the apparatus rather than attaching them to a water source. The business viability of such an idea is debatable. As of right now, though, we know that science holds up.

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