Next-generation cockroach bot walks on water, swims


This bug-sized robot is at home on land, underwater, and skating on the surface tension in between.


Yufeng Chen, Neel Doshi, and Benjamin Goldberg / Harvard University

Cockroaches can survive underwater for up to 30 minutes, but researchers at Harvard University have built a robotic equivalent that can survive for even longer.

Harvard’s Ambulatory Microrobot, known as HAMR, can not only traverse land, it can paddle across the surface of water and walk underwater. This opens up new environments for robots to explore.

As described in a paper in Nature Communications, HAMR uses multifunctional foot pads that can allow the bot to walk on water thanks to the buoyancy induced by surface tension. Using a process called electrowetting, it can also apply a voltage to reduce the angle between the water and its feet to break the water’s surface when HAMR needs to sink.

The paddling motion is accomplished using four pairs of asymmetric flaps and custom-designed swimming gaits, not dissimilar to those used by a diving beetle. By travelling across the water’s surface, a microrobot can evade submerged obstacles. This also reduces drag by keeping the robot’s body above water.

“This research demonstrates that microrobotics can leverage small-scale physics – in this case surface tension – to perform functions and capabilities that are challenging for larger robots,” says lead author Kevin Chen.

A petite build is essential to a robot’s ability to swim. “If it were much bigger, it would be challenging to support the robot with surface tension,” co-author Neel Doshi explains.

However, there are limits to how small a robotic swimmer can be. “If it were much smaller, the robot might not be able to generate enough force to break it,” Doshi adds.

HAMR is, appropriately, a lightweight. It weighs only 1.65 grams, about as much as a large paper clip, and can carry 1.44 grams of additional payload without sinking involuntarily. It can paddle its legs 10 times a second, and is coated in a crystalline polymer film to keep it from short-circuiting under water. HAMR loses no mobility once it sinks, using the same gait to walk underwater as it does on dry land.

To return to terra firma, however, HAMR faces a stern test. A water surface tension force that is twice the weight of HAMR pushes down on the robot, and its hind legs are subjected to a dramatic increase in friction as a result of the induced torque.

The researchers remedied this by stiffening the robot’s transmission and installing soft pads to the robot’s front legs. This increases payload capacity and redistributes friction during climbing to ease the burden on its limbs. Finally, walking up a ramp at a slight incline, the robot is able breach the surface and return to dry land.

“This robot nicely illustrates some of the challenges and opportunities with small-scale robots,” says senior author Robert Wood. “Shrinking brings opportunities for increased mobility – such as walking on the surface of water – but also challenges, since the forces that we take for granted at larger scales can start to dominate at the size of an insect.”

The researchers’ next aim is to further improve HAMR's locomotion and find a way to return to land without a ramp. Gecko-inspired adhesives and impulsive jumping mechanisms have been proposed as possible solutions.

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  1. https://www.nature.com/articles/s41467-018-04855-9
  2. https://nature.mdc.mo.gov/discover-nature/field-guide/predaceous-diving-beetles-water-tigers
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