New robot with sticky feet can climb vertical, upside-down surfaces

Scientists have created a robot whose adhesive foot pads, origami ankle joints, and specially engineered walking gait allow it to climb on vertical and upside-down conductive surfaces.

The new robot called HAMR-E is based on one of the existing micro-robots, HAMR, whose four legs enable it to walk on flat surfaces and swim through water, according to the study published in the journal Science Robotics.

While the basic design of HAMR-E is similar to HAMR, the scientists had to solve a series of challenges to get HAMR-E to successfully stick to and traverse the vertical, inverted, and curved surfaces that it would encounter in a jet engine.

"Now that these robots can explore in three dimensions instead of just moving back and forth on a flat surface, there's a whole new world that they can move around in and engage with," said Sebastien de Rivaz, a former Research Fellow at the Wyss Institute for Biologically Inspired Engineering at Harvard in the US.

"They could one day enable non-invasive inspection of hard-to-reach areas of large machines, saving companies time and money and making those machines safer," de Rivaz said.

Existing climbing robots can tackle vertical surfaces, but experience problems when trying to climb upside-down, as they require a large amount of adhesive force to prevent them from falling.

The researchers needed to create adhesive foot pads that would keep the robot attached to the surface even when upside-down, but also release to allow the robot to "walk" by lifting and placing its feet.

The pads consist of a polyimide-insulated copper electrode, which enables the generation of electrostatic forces between the pads and the underlying conductive surface.

The foot pads can be easily released and re-engaged by switching the electric field on and off, which operates at a voltage similar to that required to move the robot's legs, thus requiring very little additional power.

The electroadhesive foot pads can generate shear forces of 5.56 grammes and normal forces of 6.20 grammes -- more than enough to keep the 1.48-gramme robot from sliding down or falling off its climbing surface.

In addition to providing high adhesive forces, the pads were designed to be able to flex, thus allowing the robot to climb on curved or uneven surfaces.

The scientists also created new ankle joints for HAMR-E that can rotate in three dimensions to compensate for rotations of its legs as it walks, allowing it to maintain its orientation on its climbing surface.

The joints were manufactured out of layered fibreglass and polyimide, and folded into an origami-like structure that allows the ankles of all the legs to rotate freely, and to passively align with the terrain as HAMR-E climbs.

Finally, the researchers created a special walking pattern for HAMR-E, as it needs to have three foot pads touching a vertical or inverted surface at all times to prevent it from falling or sliding off.

One foot releases from the surface, swings forward, and reattaches while the remaining three feet stay attached to the surface.

At the same time, a small amount of torque is applied by the foot diagonally across from the lifted foot to keep the robot from moving away from the climbing surface during the leg-swinging phase.

This process is repeated for the three other legs to create a full walking cycle, and is synchronised with the pattern of electric field switching on each foot.

When HAMR-E was tested on vertical and inverted surfaces, it was able to achieve more than one hundred steps in a row without detaching.

It walked at speeds comparable to other small climbing robots on inverted surfaces and slightly slower than other climbing robots on vertical surfaces but was significantly faster than other robots on horizontal surfaces.

This makes it a good candidate for exploring environments that have a variety of surfaces in different arrangements in space, researchers said.