The Return of the Blob … As a Robot

The Return of the Blob … As a Robot

In developing his WSL robots, Dr. Hong took inspiration from amoebas, microscopic single-celled organisms that move by extending “pseudopods.”

No one knows if there is life on Mars, but if Dennis Hong has his way, the Red Planet could some day sport a creature that moves a lot like an amoeba. The assistant professor of mechanical engineering and director of Virginia Tech’s Robotics & Mechanisms Laboratory (RoMeLa), has developed a Whole Skin Locomotion (WSL) device that may be one of the world’s most unusual robots. It can move as long as any portion of its surface is in contact with the ground, and it can even squeeze through small spaces that are narrower than its own diameter.

Robotic explorers on extraterrestrial worlds may be called upon to navigate a wide variety of terrain types, from rubble to steep inclines, to ice, to labyrinthine cave networks. Current locomotive methods include wheels, tank-like tracks, and legs, but Hong felt there was a need for an alternative. “Robot locomotion is still a big problem for practical applications. How can you reach the science rich sites on Mars? Wheels don’t cut it and legs are too complex and slow. Tank treads seem to be the best solution so far, but [they suffer from] low efficiency due to bulk and internal friction between the many components that make up the tank tread. They also cannot turn precisely,” says Hong.

The WSL device uses expandable rings embedded in the surface of a hollow tube to pull the tube forward. The rings then contract and retreat through the center of the tube, reemerging at the ‘rear’ of the robot, where they expand to reach the outer surface, and take part once more in pulling the tube forward.

An experiment to test the theory of using expanding and contracting rings to generate the novel motion of the amoeba robot.
Credit: RoMeLa: Robotics & Mechanisms Laboratory

To envision how it works, picture a shirt sleeve that has been cut off from the elbow. To turn it inside out, you could just push the sides of one end of the sleeve through the center, stuffing them in until the end of the sleeve emerges through the other opening and spreads out. Push far enough, and the shirt sleeve has inverted entirely.

Now imagine that, instead of your fingers, you use expandable rings that are embedded in the sleeve. They move along the outer surface of the sleeve, pulling the surface along with them. When they reach the end of the sleeve, there’s nowhere to go but over the rim of the cuff and into the hollow center, so they contract and slip over the cuff and then reverse direction, traveling along the interior surface until they reach the far end. There, they expand once more, reverse direction again, and travel the surface of the sleeve. Unlike a sleeve, the WSL device is amorphous. It constantly turns itself inside out as the rings flow through their cycle.

One of the most promising properties of the WSL mechanism is that it can squeeze into openings that are significantly smaller than its normal width, due to the elastic nature of the skin and the compressible rings. That could allow it to wedge itself through rock crevices or explore tight underground caves that might shelter life from an inhospitable extraterrestrial environment. “It is the only locomotion method that can go through a hole smaller than its nominal cross section area – our current prototype can travel through a hole one-third its size,” says Hong.

The WSL device could move as long as some part of its surface remained in contact with its physical environment. But where would its sensors and computing equipment be stored? Hong envisions these components residing in an inner tube, with protrusions at each end to give it the shape of a dog’s chew bone. The shaft of the “bone” would fit snugly inside the sleeve, with the protrusions on either end acting to ensure that the sleeve didn’t slide off. The actuating rings would travel over the outer surface of the sleeve, then contract and run inside the sleeve over the surface of the “bone.” The elastic nature of the outer skin and contracting rings would allow the robot to squeeze through any space larger than the diameter of the bone.

An early prototype built from measuring tapes and specialized springs known as shape-memory coils.
Credit: RoMeLa: Robotics & Mechanisms Laboratory

Hong expects to have a working prototype in early 2008 that will be powered by an electric motor, but that will work only as a temporary solution. Because the rings that drive the motion must contract and expand, Hong is investigating other means of locomotion besides conventional electric motors, gears, or pulleys. One possibility is the use of electro-active polymers (whose shape change when subjected to an applied voltage), but Hong stresses that power mechanisms are at an early stage of development while the team concentrates on analyzing the fundamental mechanics of its motion and perfecting the movement mechanism.

Yoseph Bar-Cohen, senior research scientist at NASA’s Jet Propulsion Laboratory, agrees that the WSL’s key advantages are its ability to squeeze through tight spaces and to use its entire surface area for traction in unstructured environments. But he also sees its surface material as a potential vulnerability. “The reliability and durability of this robot will depend on the quality of the surface material,” says Bar-Cohen. The surface would sustain a lot of wear and tear and probably would need to be able to self-repair. “Following the approach that is used in automobile tires may be helpful.” Self-sealing tires contain a liquid polymer in the lining that flows into a puncture, then solidifies.

Hong and his team at RoMeLa are also working on a number of other novel modes of robot locomotion methods, including a leg-wheel hybrid mobility platform called IMPASS, a unique walking robot with three legs called STriDER, and a robotic snake named HyDRAS that can climb scaffolding structures.

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