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Building a Better Wall Climber

A new kind of robot can cling to walls and relax its grip.

Researchers have designed a robot that uses a novel form of electrically activated adhesion to enable it to scale any kind of vertical surface. The robot can even climb surfaces that are dusty or wet, be they concrete, glass, or drywall.

Spiderbot: An electrically driven form of adhesion (above) could make it easier for robots–and eventually people–to scale walls.

“What’s really unique about this is the technology, not the robot,” says Harsha Prahlad, senior mechanical engineer at SRI International, a nonprofit research organization based in Menlo Park, CA. There are other robots that can climb walls. But these have usually involved using microscopic fibers designed to mimic the function of the hairlike setae that give geckos their remarkable sticking power, Prahlad says.

In contrast, SRI’s robot works by inducing electrostatic charges in the surface of a wall. The advantage here is that the adhesive climbing surfaces of the robot can be turned off, making movement much simpler, says Prahlad. It also makes the robot’s adhesive surfaces self-cleaning, he says, thereby avoiding any gradual buildup of dust and dirt that would ultimately reduce the adhesion.

Tests have shown that the robot is capable of generating 1.5 newtons of sticking force per centimeter square of contact with a wall. Presenting his results at this year’s International Conference on Robotics and Automation, in Pasadena, CA, Prahlad showed that the robot was able to scale walls while carrying weights of up to 75 pounds.

“It’s an interesting and robust approach,” says Metin Sitti, a mechanical engineer at Carnegie Mellon University, in Pittsburgh, who has been working on wall-climbing robots for some time. However, he says, the forces generated are just one-tenth as strong as is currently being seen when the gecko-inspired approach is used.

On the plus side, however, the simplicity of Prahlad’s approach should make it easier to apply to human wall-climbing applications, says Nicola Pugno, a professor of structural mechanics at Turin Polytechnique, in Italy, who has been working on a sort of Spiderman suit using nanotube-covered adhesive surfaces.

“There is no fundamental reason why you can’t scale this up to, say, 200 pounds,” says Prahlad. So with a suitable interface, it should be possible to allow a human to use this technology to climb walls, he says. However, such a system would require large pads to increase the surface contact of a person’s hands. Otherwise, there would not be enough sticking power to support his or her weight, says Prahlad.

The attractive forces that create the adhesion come from electric fields generated by positive and negative electrodes within the surface pads of the robot, says Prahlad. When a high voltage is applied to these electrodes, positive and negative charges build up, which, in turn, attracts opposite charges from the surface of a wall near the electrodes.

These charges are similar to the electrostatic charges generated when a person walks across a carpet, says Prahlad. The difference is that with the robot, the charges are generated by a power supply, and the electrodes are well insulated so that the field strength between the electrodes is great. What’s more, there is no neutralization, or grounding, of this charge when contact with the wall is made (and there is therefore no spark).

Individually, the attractive forces between opposite charges within the robot and the wall are tiny, says Prahlad. But cumulatively, they are sufficient to hold the robot to the wall.

In its current form, the robot, featuring off-the-shelf components, moves pairs of tracks, like a tank, in which each tread is made up of strips of flexible sheets containing the pairs of electrodes. These sheets look a bit like Post-it notes and make contact with the wall as the tread moves around, peeling off again as the tread leaves the wall. However, this design is merely for convenience, says Prahlad. The beauty of electroadhesion is that you don’t have to peel the pad off, which can’t be said for the geckolike adhesion approach.

The flip side to this, says Pugno, is that you need a power supply to maintain adhesion. If your power supply fails, then so does your stickiness. Even so, what is most remarkable about this approach is just how little power is needed to maintain adhesion. For although 5,000 volts are applied to the electrodes, a current of just 50 nano amps drives this, meaning that only microwatts of power are used. The vast majority of the power is spent driving the motors that turn the robot’s sticky, tank-like tracks. In theory, if the same technology were used to stick a picture on a wall, a small battery could keep it hanging for decades before running out.

Prahlad says that some of the robotic applications are for military purposes, such as to allow robots to set up ad hoc networks by scaling structures in urban or rural battlegrounds, or for surveillance applications.

Electroadhesion is not a new concept and has even been proposed as a means of lifting silicon wafers during chip manufacturing, says Sitti. “However, its application to climbing robots is new.” What would be interesting now is to see how a hybrid approach works using setae-like fibers that also have electroadhesive properties, he says. This could potentially provide the robot with a stronger grip.

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