A squishy underwater robot with limbs that bend in every direction requires unusual control strategies

Up until recently, robots have mainly been used in factories, where their rigid arms are well suited for the repetitive tasks at hand and the accuracy required. Now, however, roboticists want to put their creations to work in more unpredictable settings where conventional robots often run into trouble.

Some researchers want to build flexible robots that can navigate irregular landscapes, like the ocean floor or the surface of Mars [pdf]. These robots must move over rough terrain without getting stuck and need manipulators that can grab whatever strangely shaped objects they encounter. Other researchers are focusing on soft robots that can be trusted not to hurt the people they come into contact with. Such soft robots could, for example, work as aides for the disabled or the elderly, and miniature soft robots could even serve as surgical tools inside the body.

In pursuit of these goals, robotics researchers are increasingly studying animals. That makes sense because the bodies of animals are composed mostly of soft materials, with pliable joints and tissue that can change shape without damage. Because their soft tissues absorb shocks and can conform to varied surfaces, animals can use simple control strategies that don’t demand great precision.

That, in a nutshell, is why I helped launch the Octopus Integrating Project. The effort brought together several labs from European and Israeli universities, which began working together in 2009 to build a robot replica of the fascinating animal. Some of the consortium members had worked on a previous effort that resulted in an “OctArm” attached to a tanklike robot, and they eagerly joined the new effort to copy the animal’s remarkable capabilities. We knew it wouldn’t be easy.

Illustration: Emily Cooper

Octo-anatomy: In a real octopus’s arm, a criss-cross arrangement of muscles provides all the movement. When the longitudinal muscles contract the arm gets shorter and fatter; when the transverse muscles contract the arm gets longer and skinnier.

The octopus has neither an internal nor external skeleton, and its eight arms can bend at any point, elongate and shorten, and stiffen to apply force. It can twist its arms around objects and manipulate them with great dexterity, as demonstrated in plenty of entertaining YouTube videos, including one where the animal steals a camera from an underwater photographer and another where it releases itself from a jar by unscrewing the lid from the inside. An octopus needs such dexterity to survive in the wild. When it crawls along the seafloor, for example, its arms must coordinate their movements in a complex rippling sequence to push and pull its body forward.

We wanted to build a robot that could replicate those agile motions. We started by studying the octopus arm’smuscular hydrostat structure, which allows the overall volume of the arm to remain constant while individual muscles contract and change shape. So when the diameter of an arm decreases, its length increases, and vice versa. To translate biology into engineering, we worked with marine biologists to take measurements of octopus arms and make computer models that could inform our designs. Then we began experimenting with soft actuators that could mimic the animal’s muscles.

One option was to make artificial muscles using materials known as electro-active polymers (EAPs). A layer of a soft material is sandwiched between two electrodes; when a voltage is applied, the EAP acts as a capacitor and the electrodes draw closer together, squeezing the soft material between them. Exploiting this phenomenon, researchers have created contractile units that can be arranged in stacks to generate significant forces. A research network in Europe is actively pursuing EAPs for artificial muscles.

Another possibility was to construct our robotic arms using fluidic actuators, in which liquids or gases fill soft chambers to change the shape of the larger structure. Clever design of the shapes and arrangement of the compartments allow a robotic arm to bend in the desired directions and may eventually enable more complicated movements.

Yet another interesting approach relies on filling a chamber with a granular material, such as sand or even ground coffee, instead of a fluid. With this technique, called jamming, the soft robot remains pliable until a vacuum is applied. Then the robot’s body stiffens into a hard shape—like a vacuum-packed brick of coffee on the grocery-store shelf. By applying vacuum to discrete sections in programmed sequences, researchers can make soft robots stiffen and move in specific ways.

My team was most interested in creating artificial muscles using materials called shape-memory alloys (SMAs). When heated, SMAs deform to a predefined shape, which they “remember.” We fashioned SMA wires into springs and ran electric current through them to heat them, causing the springs to scrunch up in a way that imitates muscular contractions. For the Octopus project, my team constructed a prototype arm using SMA springs to stand in for the longitudinal and transverse muscles found in the limbs of a real octopus. By sending current through different sets of springs, we made the underwater arm bend at multiple points, shorten and elongate, even grasp things.

Gif: The BioRobotics Institute/Scuola Superiore Sant’Anna

An octo-bot’s actuators mimic real octopus muscles.

Our work is primarily meant to demonstrate the potential of soft robotics, and much work remains before a robot octopus will be ready to crawl out of the lab. For example, a bot with sensors on its limbs could provide feedback about its position and the materials it encounters, which could lead to better control strategies. A team of researchers at the Worcester Polytechnic Institute, in Massachusetts, is addressing just that challenge by embedding proprioceptive sensors in a robotic snake [pdf].

It’s fun to imagine how an advanced robot octopus with eight dexterous arms could perform in the wild. Take the marine-energy industry, where there’s great interest in placing tidal turbines on the seabed to harvest power from the flowing water. But if the machinery breaks, repairs would be difficult and expensive: Workers would have to either haul turbines up to the surface or send human divers down. Maybe, one day, an octo-bot technician could be sent instead. With its agile limbs, it could manipulate tools and fix whatever is broken.

We roboticists aren’t interested in the octopus for its limbs and muscles alone—we also value its particular brand of intelligence. The octopus’s brain and peripheral nervous system are well developed compared with those of other mollusks, but they’re still fairly limited. It’s surprising, then, that they can control a huge range of movements in eight independent arms. So our next challenge under the Octopus project was to study how the animal controls its arms. We hoped the results would help us find ways to manage a flexible robot’s complex movements.

Biologists have determined that the octopus’s brain doesn’t issue top-down commands for every small movement of its twisty limbs. In Octopus vulgaris, the common octopus, the brain actually contains far fewer neurons than the peripheral nervous system. Biologists believe that the brain initiates motions, while lower motor centers control the precise neuromuscular activity. Experiments have shown that even if you sever the nerves descending from an octopus’s brain, its arms can still recoil from unpleasant stimuli and reach out as if to grab something.

And here’s what we found even more interesting: The octopus’s limbs don’t need comprehensive directions to produce the desired movement. Thanks to millions of years of evolution, their bodies are designed to respond to their environment in certain automatic and useful ways. This concept is often called morphological computation [pdf] by roboticists, while artificial intelligence researchers refer to it as embodied intelligence.

When translated to the robot world [pdf], this principle means we should design our robots so that the physical properties of their bodies automatically produce the desired movements. With this strategy, extremely simple commands can cause a robot to efficiently carry out complex tasks.

For further reading, go to:

http://spectrum.ieee.org/robotics/robotics-hardware/robot-octopus-points-the-way-to-soft-robotics-with-eight-wiggly-arms